18175581 lte-bell-labs-2009

◆ LTE and HSPA�: Revolutionary andEvolutionary Solutions for Global MobileBroadbandAnil M. Rao, Andreas Weber, Sridhar Gollamudi, and Robert Soni

Universal Mobile Telecommunications System (UMTS) with its high speedpacket access (HSPA) enhancements is currently being deployed as theprimary mobile broadband solution by operators worldwide. To ensurecontinued competitiveness of the Global System for Mobile Communications(GSM) family of technologies in the world market, the 3rd GenerationPartnership Project (3GPP) is rapidly standardizing the long term evolution(LTE) of UMTS, with significant performance improvement targets comparedto HSPA. The fact that LTE is not backwards compatible with HSPA spurnedthe introduction of the HSPA evolution (HSPA�) effort in 3GPP to protectcurrent operator investments in HSPA. HSPA� provides a framework forHSPA enhancements with the goal of providing performance similar to LTE ina 5 MHz carrier, while at the same time offering the advantage of backwardscompatibility with earlier releases. In this paper, we identify the keytechnology features of LTE which allow it to meet the desired performanceimprovements compared to HSPA, and then describe the key features ofHSPA� which allow it to remain competitive with LTE. We examine featuresat the physical layer, medium access control (MAC) layer, and networkarchitecture layer, as well as provide detailed air interface performancestudies. © 2009 Alcatel-Lucent.

packet access (HSDPA) in March 2002, which marked

a significant evolution of UMTS into the so-called 3.5G

space, providing not only significant improvements in

spectral efficiencies, but greatly enhancing the end-

user experience. HSDPA has already become widely

available with over 180 operators in service in almost

80 countries [12]. Operators deploying UMTS today

are typically doing so with HSDPA. To complement

the downlink improvements offered by HSDPA, the

IntroductionBuilding on the tremendous success of Global

System for Mobile Communications* (GSM*) for sec-

ond generation (2G) deployments, 3rd Generation

Partnership Project (3GPP*) Release 99 introduced

Universal Mobile Telecommunications System (UMTS)

in March 2000, and it has become the dominant third

generation (3G) technology in the world with over

200 operators in service in almost 90 countries [12].

3GPP Release 5 introduced high speed downlink

Bell Labs Technical Journal 13(4), 7–34 (2009) © 2009 Alcatel-Lucent. Published by Wiley Periodicals, Inc. Publishedonline in Wiley InterScience (www.interscience.wiley.com) • DOI: 10.1002/bltj.20334

8 Bell Labs Technical Journal DOI: 10.1002/bltj

Panel 1. Abbreviations, Acronyms, and Terms

2G—Second generation3G—Third generation3GPP—3rd Generation Partnership Project4G—Fourth generationACK—AcknowledgementAMR—Adaptive multi rateBPSK—Binary phase shift keyingCDF—Cumulative distribution functionCDMA—Code division multiple accessCLTD—Closed-loop transmit diversityCP—Cyclic prefixCPC—Continuous packet connectivityCQI—Channel quality indicatorCS-RS—Channel sounding reference signaldB—DecibelDCH—Dedicated channelDFT—Discrete Fourier transformDL—DownlinkDM-RS—Demodulation reference signalDPCCH—Dedicated physical control channelDSCH—Downlink shared channelDRX—Discontinuous receiveEDGE—Enhanced data rates for GSM evolutionE-DPCCH—Enhanced DPCCHeNB—Enhanced node BEPC—Evolved packet coreEPS—Evolved packet systemE-UTRAN—Evolved UTRANEV-DO—Evolution data optimizedFACH—Forward access channelFDD—Frequency division duplexFDMA—Frequency division multiple accessFFR—Fractional frequency reuseFTP—File Transfer ProtocolGERAN—GSM/EDGE radio access networkGGSN—Gateway GPRS support nodeGPRS—General packet radio serviceGSM—Global System for Mobile CommunicationsGW—GatewayHARQ—Hybrid automatic repeat requesths—High speedHSDPA—High speed downlink packet accessHSPA—High speed packet accessHSPA� —HSPA evolutionHSUPA—High speed uplink packet accessID—IdentificationIDFT—Inverse discrete Fourier transformIFFT—Inverse fast Fourier transformIoT—Interference over thermalIP—Internet ProtocolIPv4—IP version 4ISI—Inter-symbol interferenceIST—Information Society Technologieskm/h—Kilometers per hourLMMSE—Linear minimum mean square errorLTE—Long Term EvolutionMAC—Medium access controlMC—Multi-carrierMCS—Modulation and coding scheme

MIMO—Multiple input-multiple outputMME—Mobility management entityMMSE—Minimum mean square errorMRC—Maximum ratio combiningms—MillisecondsNACK—Negative acknowledgementNGMN—Next-generation mobile networkOFDM—Orthogonal frequency division multiplexOFDMA—Orthogonal frequency division multiple

accessPAPR—Peak to average power ratioPARC—Per antenna rate controlPCH—Paging channelPDN—Packet data networkPMI—Precoding matrix indicatorPS—Packet switchedPSD—Power spectral densityPSTN—Public switched telephone networkPUCCH—Physical uplink control channelQAM—Quadrature amplitude modulationQoS—Quality of serviceQPSK—Quadrature phase shift keyingRACH—Random access channelRAN—Radio access networkRLC—Release completeRNC—Radio network controllerRoHC—Robust header compressionRoT—Rise over thermalRRC—Radio resource controlRTP—Real Time Transport ProtocolRx—ReceiveSA—System architectureSAE—System architecture evolutionSC—Single carrierSCCH—Shared control channelS-CCPCH—Secondary common control physical

channelSDMA—Spatial division multiple accessSFBC—Space frequency block codingSGSN—Serving GPRS support nodeSIC—Successive interference cancellationSINR—Signal-to-interference-plus-noise ratioSMS—Short message serviceSRS—Sounding reference signalTA—Timing advanceTS—Technical specificationTTI—Transport time intervalTU—Typical urbanTx—TransmitUDP—User Datagram ProtocolUE—User equipmentUL—UplinkUMTS—Universal Mobile Telecommunications

SystemUTRAN—UMTS terrestrial radio access networkURA—UTRAN registration areaVoIP—Voice over Internet ProtocolWINNER—Wireless World Initiative New Radio

DOI: 10.1002/bltj Bell Labs Technical Journal 9

high speed uplink packet access (HSUPA) technology

was introduced in 3GPP Release 6 in March 2005. The

combination of HSDPA and HSUPA is called simply

high speed packet access (HSPA), and is strongly posi-

tioned to become the dominant high speed wireless

data technology for many years.

Even before the standardization of HSUPA had

been completed, in December 2004, 3GPP initiated a

feasibility study regarding the long term evolution

of UMTS, in order to ensure that the GSM family of

technologies maintained a competitive position in the

world market. The introduction of LTE was seen as a

way to provide a smooth migration to the yet-to-be-

defined fourth generation (4G), and to take advantage

of new spectrum allocations with wider bandwidths

that would become available (e.g., in the 2.6 GHz 3G

extension band). During the LTE feasibly study, an

aggressive set of performance targets and require-

ments were agreed upon to form the basis for LTE

standardization work. To justify the introduction of a

new technology, LTE would be required to provide

very large performance gains compared to HSPA in

3GPP Release 6 and to fully take advantage of new

spectrum allocations as wide as 20 MHz. In order to

satisfy these requirements, it was clear that LTE would

have to be built from the ground up, and could not

offer backwards compatibility with UMTS/HSPA.

The fact that LTE would not be backwards com-

patible with HSPA was not necessarily received well

by the large number of operators who had already

made significant investments in UMTS/HSPA and had

not yet begun to realize the benefits of those invest-

ments. This spurned the introduction of the HSPA

evolution (HSPA�) effort in 3GPP in March 2006.

While 3GPP had already started working on Release 7

enhancements as early as 2005, there was no general

framework to these enhancements. HSPA� formally

defined a broad framework and a set of requirements

for the evolution of HSPA; the primary goal being to

provide performance similar to LTE in a 5 MHz carrier,

while offering backwards compatibility with Release

99 through Release 6. HSPA� would then provide a

compelling alternative to LTE for operators who were

already deploying UMTS/HSPA, allowing them the

flexibility to introduce LTE in new spectrum while

enjoying enhancements which would protect their

existing investments in UMTS/HSPA.

In this paper, we will describe the requirements set

forth in standardization for both LTE and HSPA�, and

then give an overview of the key features of each tech-

nology which allow them to achieve their performance

requirements. We will see that many of the features

introduced in HSPA� closely parallel the innovations

developed in LTE. Where applicable, we provide

detailed system performance studies which illustrate

how close the technologies come to meeting the desired

performance targets. The remainder of the paper is

organized as follows: we start with the performance

requirements set forth in standards, give an overview of

the system architecture enhancements, describe the key

features in the downlink, describe the key features in

the uplink, describe the features in LTE and HSPA� that

enable efficient transmission of Voice over Internet

Protocol (VoIP), and offer our conclusions.

Requirements and Performance TargetsDuring the initial study item phase for both LTE

and HSPA�, 3GPP agreed upon a set of requirements

and performance targets to form the basis of the stan-

dardization work, and to determine what key features

or enhancements should be included as part of the new

technology. In this section, we discuss the requirements

and performance targets for both LTE and HSPA�.

LTE Requirements and Performance TargetsWhile 3GPP understood in early 2005 that the

HSPA technology—the uplink component of which

had just been standardized—would provide a highly

competitive mobile broadband solution for several

years, potential threats from other technologies cre-

ated a desire to ensure competitiveness in an even

longer time frame (i.e., for the next 10 years and

beyond). This formed the justification for opening the

LTE study item in 3GPP very quickly.

Important considerations for the long term evo-

lution of 3GPP included reduced latency, higher user

data rates, improved system capacity and coverage,

and reduced cost for operators. In order to achieve

this, it was seen that both an evolution of the air

interface as well as the network architecture would

need to be taken into account. Looking to the future,


the desire for even higher data rates also needed to

factor-in future additional 3G spectrum allocations,

and hence, LTE would need to include support for

transmission bandwidths greater than 5 MHz. At the

same time, support for transmission bandwidths of

5 MHz and less than 5 MHz would be needed to allow

for flexibility in whichever frequency band the system

might be deployed in. The requirements and per-

formance targets for LTE were agreed upon in [2],

and it should be noted that these performance targets

were decided when HSPA Release 6 was still being

finalized. Hence, the targeted improvements are in

many cases set relative to HSPA Release 6. The points

relevant to this paper are summarized here:

• Packet-only technology. LTE would support only the

packet switched (PS) domain of the core network,

and would be optimized to provide a high data

rate, low-latency, packet-optimized radio access

technology.

• Scaleable bandwidth. Support for bandwidths of

1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and

20 MHz. Originally, the smallest bandwidth allo-

cation was going to be 1.25 MHz to fit existing

CDMA2000*/evolution data optimized (EV-DO)

spectrum, but this was later changed to 1.4 MHz

which would fit an integral multiple of GSM

carriers.

• Improved peak rates. 100 Mbps peak rate in the

downlink in 20 MHz (5 bps/Hz) and 50 Mbps

peak rate in the uplink in 20MHz (2. 5bps/Hz). Peak

rates should scale linearly with the spectrum allo-

cation.

• Improved spectrum efficiency targets.

– Three to four times improvement in the

downlink spectral efficiency compared to

Release 6 HSDPA. This assumes a 1�2

antenna configuration for HSDPA but a 2�2

antenna configuration for LTE.

– Two to three times improvement in the uplink

spectral efficiency compared to HSUPA

Release 6. This improvement assumes a 1�2

antenna configuration for both HSUPA Release

6 and LTE.

• Improved user throughput. Target improvements are

placed on both average user throughput as well as

user throughput at the edge of the cell. The cell

edge user throughput is defined as the fifth per-

centile of the user throughput cumulative distri-

bution function (CDF); this quantity is important to

ensure broadband rates can be achieved through-

out most of the cell coverage area. The target

improvements below use the same assumptions

described above for the improved spectrum effi-

ciency:

– Three to four times improved average user

throughput per MHz in the downlink and two

to three times improved user throughput per

MHz in the uplink compared to Release 6.

– Two to three times improved cell edge user

throughput per MHz compared to Release 6.

• Improved latency. LTE targets significantly improved

control plane and user plane latency, including a

– Control plane supporting transition time of less

than 100 milliseconds (ms) from a camped state

(i.e., idle) to an active state (i.e., CELL_DCH),

and a transition time of less than 50ms from a

dormant state (i.e., URA/CELL_PCH) to an

active state.

– User plane supporting latency of less than

5 ms should be possible on the user plane in

an unloaded condition. The user plane

latency is defined as the one way transit time

between the user equipment (UE) and the

radio access network (RAN) edge node.

• Co-existence and inter-working with UMTS and GSM.

Given that LTE would co-exist with both

UMTS/HSPA terrestrial radio access network

(UTRAN) and GSM/EDGE radio access network

(GERAN), requirements were placed on inter-

working with these legacy systems. An interrup-

tion time of less than 500 ms is targeted for a

handover of a non-real time service between LTE

and either UTRAN or GERAN, while an interrup-

tion time of less than 300 ms is targeted for real-

time services.

• Given the scope of these requirements for evolu-

tion work, 3GPP agreed upon a work split—the

evolution of the radio access network would take

place in the 3GPP RAN working groups, and in

parallel, work on an evolved packet core (EPC)


would take place in the system architecture (SA)

working groups. At this point, it is useful to clarify

some terminology: the radio access network

enhancements are referred to as either evolved

UMTS terrestrial radio access network (E-UTRAN)

or LTE; the names are used interchangeably. The

evolution work for the EPC is referred to as sys-

tem architecture evolution (SAE). For some time,

the combination of these enhancements was

referred to as LTE/SAE, but more recently it has

become known as the evolved packet system

(EPS).

HSPA� Requirements and Performance TargetsIn order to protect operator investments in HSPA

and provide a smooth evolution path towards LTE,

which would not offer backwards compatibility with

earlier releases of UMTS/HSPA, a study item on HSPA

evolution was opened in 3GPP in March 2006. While

development of HSPA Release 7 enhancements was

already underway in 2005 with open work items

regarding HSDPA multiple input-multiple output

(MIMO), continuous packet connectivity (CPC), and

the “one tunnel” solution for optimization of packet

data traffic, there was no general framework in place

to guide the evolution of HSPA. The HSPA� effort

provided a broad framework for HSPA evolution with

a clear set of requirements and performance targets,

with the intent of identifying what performance bene-

fits could be achieved with the existing Release 7

work items and what gaps still existed.

The goal of HSPA� is not to replace LTE, but

rather to enhance HSPA by providing an incremental

evolution path for both the RAN and core network

which will enhance performance while leveraging

existing infrastructure. In addition, HSPA� aims to

enable co-existence with the EPS since it will be part

of future 3G systems. As described in [3, 8], the guid-

ing principles behind HSPA� are as follows:

• HSPA spectrum efficiency, peak data rates, and

latency should be comparable to LTE in a 5 MHz

bandwidth.

• The inter-working between HSPA� and LTE

should be as smooth as possible and facilitate joint

technology operation; the possibility of reusing

the evolved packet core defined as part of the sys-

tem architecture evolution should be analyzed.

• HSPA� should be able to operate as a packet-only

network, based on the utilization of shared chan-

nels only (i.e., HSDPA and HSUPA).

• HSPA� shall be backwards compatible in the sense

that legacy terminals compatible with Release 99

through Release 6 are able to share the same

carrier with terminals implementing the latest

HSPA� features, without any performance degra-

dation.

• Ideally, existing infrastructure should only need a

simple upgrade to support the features defined as

part of HSPA�.

As we will see in later sections, the framework

provided by the HSPA� effort initiated the develop-

ment of several new HSPA enhancements beyond

what was already being considered in early 2006, in

Release 7.

Network Architecture ImprovementsGiven the requirements and performance targets

described in the previous section, it was clear that not

only were enhancements to the radio interface

required, but in addition, the network architecture itself

needed enhancements. In the next section, we describe

the network architecture enhancements for both LTE

and HSPA�.

Evolved Packet System Architecture for LTEThe goal of the system architecture evolution

effort in 3GPP is not just to define an efficient packet

core network and RAN architecture for LTE to meet

the requirements described in [2], but rather to

develop a framework for an evolution and migration

of current systems to a high data rate, low latency,

packet-optimized system that supports mobility and

service continuity across heterogeneous access net-

works, since it is envisioned that Internet Protocol

(IP)-based services would be provided through vari-

ous access technologies.

In its simplest form, the EPS architecture consists

of two basic nodes in the user plane: a single node

called the enhanced node B (eNB) comprises all radio

access functions and a single node called the EPS

gateway comprises the entire bearer plane (i.e., user


plane) in the core network. In the control plane, the

mobility management entity (MME) node is logically

separated from the user plane EPS gateway with an

open interface between them. Figure 1 provides a

comparison of the EPS network architecture and the

UMTS network architecture. EPS offers a flatter net-

work architecture than UMTS, especially as far as the

user plane is concerned, which reduces latency.

The clean separation of the user plane and control plane

is a key feature of the EPS architecture, as it allows for

independent scaling of control plane functionality and

user plane functionality. This is very important from

a technical viewpoint because the scaling of the two

depends on different factors: the capacity of the con-

trol plane functionality typically depends on the num-

ber of mobile devices and their mobility patterns,

whereas the capacity of the user plane depends on

aggregate data throughput required to be supported.

Drawing a parallel between the EPS architecture and

UTRAN, the enhanced node B absorbs all radio access

functions that were contained in the node B and radio

network controller (RNC) elements in UTRAN. Note

that the eNBs are directly connected to each other via

an interface called X2; this facilitates seamless mobility

and interference management.

Figure 2 presents a more detailed view of the EPS

architecture, with the interfaces that exist to support

mobility between 3GPP and non-3GPP networks. The

EPS gateway may be split into two separate logical

nodes with the optional S5 interface: the serving gate-

way (GW), and the packet data network (PDN)

gateway. The serving GW terminates the core network

interface towards 3GPP radio access networks and

serves as the local mobility anchor point for inter-eNB

handover within the EPS, as well as mobility anchor-

ing for inter-3GPP mobility (i.e., between EPS and

UTRAN/GERAN). Note the direct control plane inter-

face (S3) and user plane interface (S4) between the

EPS network and the serving GPRS support node

(SGSN) in the UMTS/GSM networks; such an inter-

face allows for a packet session to be maintained in a

way that is seamless to the user of a multimode ter-

minal that migrates across LTE, UMTS/HSPA, and

GSM/EDGE coverage areas. This meets the requirements

Node BNode B Node BNode B

Packet corenetwork

Radio accessnetwork

eNode B eNode B

Internet Internet

UMTS EPSU-planeC-planeGGSN

SGSN

RNCRNC

MME

EPSgateway

EPS—Evolved packet systemGGSN—Gateway GPRS support nodeGPRS—General packet radio serviceHSPA—High speed packet access

MME—Mobility management entityRNC—Radio network controllerSGSN—Serving GPRS support nodeUMTS—Universal Mobile Telecommunications System

Figure 1.Comparison of UMTS/HSPA network architecture and EPS network architecture.


for co-existence/inter-working and gives the operator

the flexibility to roll out LTE gradually, starting with

the areas of highest demand first. The PDN gateway

provides access to the packet data network through

the control of IP data services and allocation of IP

addresses; it also serves as an anchor for mobility

between 3GPP and non-3GPP access systems, which is

sometimes referred to as the SAE anchor function.

Note that the specification of logical nodes does not

mandate a mapping to physical entities. For example,

the serving GW, PDN GW, and MME may be imple-

mented in the same physical entity, or the MME may be

integrated into the eNB. The mapping of logical nodes

to physical entities may follow a highly integrated

approach or a more distributed approach, based on ven-

dor implementations and deployment scenarios.

HSPA� Network ArchitectureFor the HSPA� network architecture, we begin

with a description of the one tunnel enhancement

(also referred to as “direct tunnel”) that was already

being discussed as part of Release 7 prior to the

HSPA� initiative. 3GPP recognized that the amount of

user plane data would significantly increase in the

near future because of the introduction of HSPA. With

the existing system illustrated in Figure 1, packet data

traffic must traverse both the gateway GPRS support

node (GGSN) and serving GPRS support node in the

UMTS core network (through the use of two tunnels),

independently of how the data traverses the UTRAN.

A more scalable architecture is possible with the one

tunnel solution, which permits direct tunneling of the

user plane data between the GGSN and the RNC, as

illustrated in Figure 3. In this way, a cleaner separa-

tion between the control plane and the user plane is

achieved, the advantages of which were discussed

previously for LTE. The new SGSN controller performs

all the control functions of the SGSN, and the

enhanced GGSN takes over all data transport func-

tionality that resided in the previous GGSN and SGSN.

To further flatten the network architecture,

HSPA� introduced the option of integrating the RNC

serving GW PDN GW

EPS gateway

E-UTRAN

UTRAN

GERAN

Non-3GPP† access

S1-MME

S1-U

S11

S4S3

S5Internet

SGSN

MME

3GPP—3rd Generation Partnership Project EDGE—Enhanced data rates for GSM†† EvolutionEPS—Evolved packet systemE-UTRAN—Evolved UTRANGERAN—GSM/EDGE radio access networkGPRS—General packet radio serviceGSM—Global System for Mobile Communications††

†Trademark of the European Telecommunications Standards Institute. ††Registered trademarks of the GSM Association.

GW—GatewayMME—Mobility management entityPDN—Packet data networkSGSN—Serving GPRS support nodeUMTS—Universal Mobile Telecommunications SystemUTRAN—UMTS terrestrial radio access network

Figure 2.Detailed view of the EPS network architecture, with interfaces to support mobility across 3GPP and non-3GPP access.


and node B functionality into a single node for packet

switched services (denoted here as node B� ); this is

also shown combined with the one tunnel solution

in Figure 3. The flatter architecture reduces latency

and could be useful in deployments in which a high

level of integration is desirable (i.e., HSPA femtocells).

From Figure 3, it quickly becomes apparent how simi-

lar the HSPA� network architecture is to the EPS net-

work architecture shown in Figure 1, which allows

easy integration of HSPA� and EPS networks.

Key Features in the Downlink of LTE and HSPA�

The downlink (DL) of a RAN bears a higher

amount of data traffic compared to the uplink (UL)

due to the increasing demand for unbalanced data

services like, for example, FTP download or video

streaming. A number of features have been intro-

duced in order to support increasing data rates.

LTE Downlink Key FeaturesIn the following subsections, we will discuss key

features of the LTE DL such as OFDM transmission,

MIMO, the possibility of using higher order modulation

schemes, time and frequency selective scheduling,

and fractional frequency reuse. Details about the

LTE DL channel structure can be found in [7]

and [12].

Orthogonal Frequency Division MultiplexThe LTE DL air interface is based on orthogonal

frequency division multiplexing (OFDM) which is a

technique that avoids inter-symbol interference,

exploits the scarce frequency resource nearly opti-

mally, combines the advantages of broadband and

narrowband transmission, and, at the same time,

avoids their disadvantages. OFDM is further described

below.

In conventional high bit rate air interfaces, the

data symbols are transmitted sequentially over the air

interface. According to the Nyquist theorem, the mini-

mum required bandwidth B is related to the symbol

duration Tsym with B � 1/Tsym. In real systems,

guard bands are required at both ends of the used

spectrum due to the application of non-ideal filters. In

multipath environments, broadband channels show

Node-B

Packet corenetwork

Radio accessnetwork

Internet

HSPA�one tunnel

HSPA�one tunnel

withintegrated

RNC/node B

GGSN

SGSN

RNC

Internet

GGSN

SGSN

Node-B�

GGSN—Gateway GPRS support nodeGPRS—General packet radio serviceHSPA�—High speed packet access evolution

U-planeC-plane

RNC—Radio network controllerSGSN—Serving GPRS support node

Figure 3.HSPA� network architecture.


a frequency selective behavior with several deep fades

in the frequency domain. In the time domain, this

behavior corresponds to an overlapping of symbols,

which causes the so called inter-symbol interference

(ISI), illustrated in Figure 4. The smaller the symbol

duration, i.e., the higher the symbol rate, the more

symbols experience ISI. In broadband transmissions,

an inversion of the channel transmission function is

required which corresponds to an equalization of the

received signal in order to cope with inter-symbol

interference caused by the relatively short symbol

duration (compared to the delay spread of the chan-

nel echoes).

One means to reduce inter-symbol interference

is to extend the symbol duration so that it is longer

than the difference between the delays of the earliest

and latest channel echo. A further improvement is to

extend the symbol duration by a guard time, during

which transmission of the new symbol has already

started but which is discarded by the receiver. This

feature is called cyclic prefix (CP). The total symbol

duration in this case is the sum of the original sym-

bol plus the CP duration, which should be longer than

the difference between the delays of earliest and lat-

est echo in the multipath channel.

The reason for the low ISI in the time domain is

because of a flat channel in the frequency domain. In

a multipath environment, this corresponds to a nar-

row bandwidth used for the transmission of the sym-

bol. Many parallel narrowband transmissions are

required to obtain a high bit rate channel. OFDM

avoids the guard band between the so-called subcar-

riers by a modulation of these subcarriers with rec-

tangular pulses, using the rect(t) function. In the

frequency domain, the spectrum of a pulse with dura-

tion Tsym corresponds to the sinc(x) � sin(x)/x func-

tion with zero crossings at k/Tsym, k � . . . � 2, � 1,

1, 2, . . . . Consequently, if these pulses modulate a

number of subcarriers, the inter-subcarrier interfer-

ence is zero with a subcarrier spacing of 1/Tsym which

is, according to the Nyquist theorem, the optimal

value, as shown in Figure 5. Furthermore, this opti-

mal value is reached without any filter.

The modulation of the equally-spaced subcarri-

ers with rect pulses corresponds to an inverse discrete

Fourier transform (IDFT) in the time domain. At the

receiver, the original symbols are reconstructed using

the opposite function, namely a discrete Fourier trans-

form (DFT). Figure 6 shows a schematic view of the

OFDM transmission chain.

Transmitted signal

Echo 1

Echo 2

Echo 3

Received signal

Symbol n Symbol n�1ISI

ISI—Inter-symbol interference

Figure 4.Inter-symbol interference caused by channel echoes.


In OFDM the bandwidth can be easily adapted to

the needs of the network operator. LTE FDD, for

example, offers bandwidths of 1.4 MHz, 3 MHz ,

5 MHz , 10 MHz , 15 MHz , and 20 MHz with a sub-

carrier spacing of 15 kHz. The total bandwidth

includes guard bands at both ends of the spectrum,

so that 72, 180, 300, 600, 900, and 1200 subcarriers

are conveyed in the respective bandwidth, as outlined

in [10].

The inverse fast Fourier transform (IFFT) and

corresponding FFT enable a very efficient calcula-

tion of the transmitted signal and the correspon-

ding reconstruction of the symbols. FFT and IFFT

require that the number of subcarriers N is N � 2n

with an integer value of n, although not all of these

subcarriers have to be transmitted. The granularity

in which the transmitter has to calculate the IFFT

and in which the receiver has to sample the

1

0.8

0.4

0.6

0

0.2

�0.2

Am

plit

ud

e/m

axim

um

am

plit

ud

e

�0.4�10 �5 0 5 10

Frequency (1/Tsym)

OFDM—Orthogonal frequency division multiplexing

Figure 5.Frequency domain of seven subcarriers of an OFDM signal.

Mapper IDFT DFT DemapperChannel

Bits Symbols s(t) s’(t) Symbols Bits

DFT—Discrete Fourier transform IDFT—Inverse discrete Fourier transformOFDM—Orthogonal frequency division multiplexing

Figure 6.OFDM transmission chain.


channel is Tsym/N and, consequently, depends on

the FFT size.

Multiple Antenna AlgorithmsMIMO, a synonym for a technique that uses at

least two antennas at the transmitter and at least two

antennas at the receiver for the transmission of signals

over the air interface, is depicted in Figure 7. The

antennas can be used to obtain an array gain, i.e., a

diversity gain, to reduce co-channel interference, or to

enable multiplexing of several data streams to the

same or to different receivers. Consequently, MIMO is

able to increase quality of service (QoS), coverage,

spectral efficiency, and peak data rate.

One or several data streams are, after channel

coding and modulation, multiplied with a precoding

vector and mapped on the different transmission

antennas. The precoding vector describes the phase

shifts of the data symbols and the mapping of these

data symbols on the antenna ports. The transmit (Tx)

and receive (Rx) antennas, respectively, are either

widely-spaced (or alternatively cross polarized) or

closely spaced in a linear array. In the first case, the

channel state at the antennas is uncorrelated. For

widely-spaced antennas, the antenna pattern gener-

ated is frequency dependent and has an irregular

shape. In the case of a linear array, the channel state

at the antennas is correlated; the generated antenna

pattern is frequency independent over the used band-

width and shows a regular shape with main and side

lobes. A special case for the uplink is virtual MIMO,

shown in Figure 8. In this case, two or more widely-

spaced mobile terminals are multiplexed on the same

resources. In contrast to the base stations, these

devices do not require multiple antennas. In general,

the maximum number of data streams corresponds

to the minimum of Tx and Rx antennas. In the case of

virtual uplink MIMO, the number of antennas in the

base station is the limiting factor.

In a closed loop transmission procedure, the pre-

coding vector is chosen so that a constructive super-

position of the signals is obtained at the receiver, or

that different data streams conveyed over the same

resources can be easily separated. In the LTE closed

loop mode, the receiver chooses the precoding vector

out of a limited set of possible precoding vectors in

order to reduce feedback signaling load. In this case,

the precoding feedback is reduced to an index in a

table of predefined precoding vectors, known as the

precoding matrix indicator (PMI). In the open loop

mode of LTE DL, space frequency block coding (SFBC)

is used under bad to medium channel conditions,

while per antenna rate control (PARC) is used under

good channel conditions and enables two stream

transmissions. Feedback from the receiver is still

required in order to signal the supportable rank, i.e.,

the number of supportable streams and the channel

quality.

The LTE DL works with orthogonal pilots for the

different transmission antennas in order to enable

the receiver to calculate the channel transmission

function, i.e., the transmission function from every

CodingModulationWeightingMapping

Channel

Data stream(s)

WeightingDemappingDemodulationDecoding

Data stream(s)

MIMO—Multiple input-multiple output

Figure 7.MIMO transmission and reception.


transmission to every receive antenna. The following

antenna algorithms can be applied with multiple

antenna systems:

• Transmit and receive diversity (with widely-

spaced or cross polarized antennas),

• Beamforming, or beamswitching (with closely-

spaced antennas),

• Spatial multiplexing (with widely-spaced or cross

polarized antennas), and

• Combinations of the previous algorithms.

Transmit diversity generates a rich number of

channel echoes at the receiver, which arrive from vari-

ous directions. By leveraging maximum ratio com-

bining (MRC), the receiver may use this receive

diversity in order to combine the signal of the differ-

ent receive antennas so that the received signal level

is optimized.

For beamforming and beamswitching, the anten-

nas have to be closely spaced, with a spacing of half

the wavelength of the carrier frequency. In an open

loop algorithm, the antenna spacing has to be cali-

brated. For a closed loop algorithm with channel feed-

back from the receiver, calibration is not mandatory

since the receiver chooses the optimal beam. Several

data streams can be conveyed over the same resources

to different users, if the beams have sufficient angular

separation, possible via spatial division multiple access

(SDMA).

Spatial multiplexing is an antenna algorithm

based on widely-spaced or cross polarized antennas.

Several data streams are mapped on the transmit

antennas. The receiver, e.g., a minimum mean square

error (MMSE) receiver, repeats to combine the sig-

nals of the different receive antennas so that the

signal-to-noise-ratio of one data stream is optimized

while the other data streams are suppressed, until all

data streams are reconstructed. Successive interfer-

ence cancellation (SIC) is an enhanced receiver algo-

rithm that subtracts the interference of successfully

received data streams from other data streams with

low signal-to-interference-plus-noise ratio (SINR).

A combination of spatial multiplexing and beam

forming or beam switching is enabled by a number

of closely spaced, cross polarized antennas at the

transmitter. This antenna allows the transmission of

up to two data streams and, at the same time, it allows

a beam to form in order to optimize the signal level at

the receiver.

CodingModulationWeightingMapping

Channel

Data stream(s)WeightingDemappingDemodulationDecoding

Data stream(s)

Data stream(s)

WeightingDemappingDemodulationDecoding

MIMO—Multiple input-multiple output

Figure 8.Virtual MIMO in the uplink.


Other Performance Enhancing TechniquesLink adaptation is a state of the art technique that

allows the adjustment of channel protection to chan-

nel quality by choosing the best suited modulation

and coding scheme (MCS). LTE allows coding rates,

i.e., the ratio of data bits and transmitted bits, close to

1 for excellent channel quality. Quadrature phase shift

keying (QPSK), 16 quadrature amplitude modulation

(QAM), and 64 QAM are possible modulation

schemes and can be combined with any code rate.

QPSK conveys 2 bits in every data symbol (resource

element), 4 bits in 16 QAM, and 6 bits in 64 QAM.

Consequently, in good channel conditions, three times

more bits can be conveyed using 64 QAM than under

bad channel conditions when QPSK is applied.

However, only those MCS that have the best throughput

performance for a given channel quality will be

applied, i.e., those which are part of the hull curve of

all MCS as shown in Figure 9.

Frequency selective scheduling improves spectral

efficiency and cell border throughput for OFDM by

choosing only the best set of physical resource blocks

for a transmission. In a frequency division duplex

(FDD) system, for frequency selective scheduling, the

mobile terminal has to feed back the channel quality

for either all resources or for a subset of resources

with the best channel qualities, i.e., channel quality

indicator (CQI). The scheduler evaluates the individ-

ual sets of CQI values in combination with the indi-

vidual throughput of the mobile devices and

SISO AWGN, 630 resource elements per transport block

0

1

2

3

4

5

6

�10 �5 0 5 10 15 20 25

SNR (dB)

Thro

ug

hp

ut

(bit

s p

er s

ymb

ol)

MCS 1MCS 2MCS 3MCS 4MCS 5MCS 6MCS 7MCS 8MCS 9MCS 10MCS 11MCS 12MCS 13MCS 14MCS 15MCS 16MCS 17MCS 18MCS 19MCS 20MCS 21MCS 22MCS 23MCS 24MCS 25MCS 26MCS 27

AWGN—Additive white Guassian noise dB—DecibelMCS—Modulation and coding scheme

OFDM—Orthogonal frequency division multiplexingSISO—Single input-single outputSNR—Signal-to-noise ratio

Figure 9.Throughput versus SNR for different MCS for the OFDM downlink.


calculates a priority for every resource of every device.

The target of the scheduler is to work as close as pos-

sible at a predefined operating point which corre-

sponds to a compromise between fairness and sector

throughput. Measured over a certain period of time,

a proportional fair scheduler gives every mobile

device approximately the same number of resources.

These are the best resources possible compared to an

average channel quality in frequency and in time.

However, the scheduling strategy can also be modified

gradually towards higher cell throughput or fair

mobile throughput.

Fractional frequency reuse (FFR) is a technique

that uses most of the downlink resources for every

sector without any restriction. A small portion of these

resources, e.g., one third or one seventh, are either

never used, or are used with reduced power by the

base station. Network planning is necessary in order to

assign a different set of these resources to different

base stations in such a way that every pair of neighbor

sectors either do not use, or reduce the power of a dif-

ferent set of resources. In the latter case, due to power

reduction, these resources can still be used close to the

base station, i.e., by mobile terminals which have good

channel conditions. For a device at the cell border

towards a neighbor sector, some resources are avail-

able that have an artificially increased quality, namely

resources that are either not used or are used with

reduced power by the neighbor sector.

HSPA� Downlink Key FeaturesThe code division multiple access (CDMA)-based

HSDPA in 3GPP Release 5 and Release 6 already pro-

vided an efficient high speed downlink air interface

through the use of a short subframe length (2 ms),

hybrid automatic repeat request (HARQ), and fast,

channel-sensitive scheduling on a shared channel,

facilitated by the use of channel quality feedback and

the addition of a new advanced scheduling entity,

MAC high speed (MAC-hs) located in the base sta-

tion. HSDPA, in Release 5 and Release 6, supports

QPSK and 16 QAM modulation, and offers a peak of

14. 4 Mbps. Several enhancements have been intro-

duced for HSDPA in Release 7 as part of HSPA� in

order to improve spectral efficiency and cell border

throughput [9].

Higher Order ModulationHSPA� allows up to 64 QAM modulation in the

downlink, which conveys 6 bits per symbol instead of 4

bits in the case of 16 QAM and consequently increases

the peak data rate by 50 percent to 21.6Mbps. 64 QAM

can be applied under good channel conditions. Due to

this fact, the possibility of using 64 QAM will enhance

spectral efficiency but will not have a high impact on

cell border throughput. For backward compatibility,

new terminal types have been defined that support

64 QAM.

MIMOHSPA� allows closed loop 2x2 MIMO with two

transmit antennas and two receive antennas. Under

good channel conditions, dual stream transmissions

are possible that can double the peak bit rate to

28.8 Mbps. As already described for LTE MIMO, the

mobile terminal chooses the best precoding vector out

of a set of predefined precoding vectors together with

CQI values for one or two streams. In order to enable

the device to measure the signal quality separately for

both antennas, the antennas carry orthogonal pilot sig-

nals. In case of dual stream transmission, both streams

can have different modulation and coding schemes

according to their channel quality. In case of low chan-

nel quality, the scheduler can decide to switch back to

single stream transmission, which then takes place on

the two antennas via closed-loop transmit diversity

(CLTD). The MIMO scheme, precoding vector, and

MCS signal the mobile device via the high speed

shared control channel (HS-SCCH). Dual stream

MIMO in HSPA� supports improved system capacity

rather than improved cell border throughput.

However, the fallback mode of CLTD for single stream

transmission will increase cell border throughput com-

pared to the case of transmitting with a single antenna

only. The combination of MIMO with 64 QAM is not

foreseen for Release 7, but will be part of Release 8 of

HSPA�, increasing the peak rate to 43. 2 Mbps.

Enhanced Receiver TypesOne common means to increase downlink sys-

tem capacity and cell border throughput is to enhance

the requirements for the mobile receiver. For Release 5,

requirements are based on a single antenna rake

receiver. Release 6 defined requirements based on a


rake receiver with dual antenna receive diversity

(enhanced receiver type 1) and on a single antenna

receiver with equalization, e.g., an MMSE receiver

(enhanced receiver type 2). Release 7 defines require-

ments for the combination of dual antenna receive

diversity and equalization (enhanced receiver type 3).

The introduction of so-called interference aware

receivers further improves performance. Using this

feature, the receiver reduces the interference from

neighbor cells, which works to enhance cell border

throughput. This feature is used in conjunction with

equalization for single antenna (enhanced receiver

type 2i) or dual antenna Rx diversity (enhanced

receiver type 3i).

Downlink Performance ComparisonTable I and Table II show the performance com-

parison of HSDPA Release 6 with 5 MHz bandwidth

and LTE DL with 5 MHz and 10 MHz bandwidth,

respectively. For the basic assumptions we used the

HSDPA Improvement Release 6 LTE LTE compared(5 UEs/cell (5 UEs/cell (10 UEs/cell to HSDPAin 5 MHz) in 5 MHz) in 10 MHz) 1 � 2

Case 1 (1 � 2) 0.47 1.33 1.52 2.8 – 3.2x

Case 1 (2 � 2) NA 1.47 1.60 3.1 – 3.4x

Case 1 (4 � 2) NA 1.73 1.85 3.7 – 3.9x

Case 3 (1 � 2) 0.44 1.24 1.40 2.8 – 3.2x

Case 3 (2 � 2) NA 1.37 1.50 3.1 – 3.5x

Case 3 (4 � 2) NA 1.60 1.70 3.6 – 3.9x

Table I. Average downlink spectral efficiency (bps/Hz/cell) with NGMN assumptions.

HSDPA—High speed downlink packet accessLTE—Long term evolutionNA—Not applicableNGMN—Next-generation mobile networkUE—User equipment

HSDPARelease 6 LTE LTE Improvement

(5 UEs/cell in (5 UEs/cell in (10 UEs/cell in compared to5 MHz) 5 MHz) 10 MHz) HSDPA 1 � 2

Case 1 (1 � 2) 195 223 321 1.1 – 1.6x

Case 1 (2 � 2) NA 257 345 1.3 – 1.8x

Case 1 (4 � 2) NA 337 462 1.7 – 2.4x

Case 3 (1 � 2) 170 140 209 0.8 – 1.2x

Case 3 (2 � 2) NA 186 262 1.1 – 1.5x

Case 3 (4 � 2) NA 257 323 1.5 – 1.9x

Table II. Five percent CDF downlink user throughput (kbps) with NGMN assumptions.

CDF—Cumulative distribution functionHSDPA—High speed downlink packet accessLTE—Long term evolutionNA—Not applicableNGMN—Next-generation mobile networkUE—User equipment


3GPP performance verification framework [5], which

is based on TS 25.814 [4]. Contrary to this frame-

work, we scaled the average number of users per sec-

tor according to the bandwidth in order to have a fair

comparison across the different systems. For HSDPA

mobile terminals we used enhanced receiver type 1,

i.e., a rake receiver with two antenna receive diver-

sity. For LTE, we used a maximum ratio combining

receiver in the case of single stream transmission and

a linear minimum mean square error (LMMSE)

receiver in the case of dual stream transmission.

Switching between single and dual stream in the case

of 2�2 and 4�2 transmission was performed accord-

ing to the precoding matrices presented in 3GPP TS

36.211 [7]. For all DL cases, we used the spatial channel

model WiM C2 “macro urban” from the Information

Society Technologies Wireless World Initiative New

Radio (IST WINNER) project. We performed the

simulations for case 1 with an inter site distance of

500 meters (m) and for case 3 with an inter site dis-

tance of 1732 m; both cases use a penetration loss

of 20 decibels (dB) and are at a carrier frequency of

2 GHz. The 3GPP performance verification framework

requests an improvement factor between 3 and 4 for

DL spectral efficiency and between 2 and 3 for DL cell

border throughput, which is defined as the fifth per-

centile of the mobile terminal’s cumulative through-

put distribution function. However, for the 3GPP

performance verification framework, HSDPA with

5 MHz bandwidth is compared to LTE with 10 MHz

bandwidth, and with 10 users in an average per sec-

tor for both systems, which penalizes the average

mobile device and cell border throughput of HSPA by

a factor of two. Consequently, in our comparison, as

we scale the number of users with the bandwidth,

the required improvement factor for the cell border

throughput of 3GPP has to be divided by a factor of 2

and, hence, shall be between 1 and 1.5. The required

improvement factor for spectral efficiency remains the

same. For all simulations, a standard proportional fair

scheduler has been used which, in the long term,

assigns approximately the same number of resources

to the mobile devices. For HSDPA, the proportional

fair algorithm has been performed in time, for LTE DL

in time and frequency. It is interesting that for case 3,

i.e., for an inter-site distance of 1732 m, the most

comparable case, namely 1�2 with 5MHz bandwidth,

the LTE cell border throughput is 18 percent smaller

but the spectral efficiency is 180 percent higher

compared to HSDPA. However, the LTE DL propor-

tional fair scheduler could be easily tuned towards

higher cell border throughput on the cost of spectral

efficiency. For case 1, a 500 meter inter-site distance,

cell border throughput is superior to HSDPA Release

6 while we still obtain a 180 percent gain in spectral

efficiency. Due to a higher channel diversity, spectral

efficiency and cell border throughput increases with

increasing bandwidth. With increasing numbers of

transmit antennas, we also obtain a gain in both spec-

tral efficiency and cell border throughput due to better

exploitation of channel diversity. As a consequence, if

we combine both effects, the spectral efficiency gains

for increasing bandwidth decreases with increasing

numbers of transmit antennas due to the fact that

both exploit channel diversity.

Further improvements are possible for HSDPA and

LTE. HSDPA, according to Release 7, introduces 64

QAM and MIMO in the DL as well as new enhanced

receiver types with equalizers and with intra-cell inter-

ference cancellation. 64 QAM will lead to an improve-

ment of spectral efficiency. Dual stream MIMO will

not lead to an improvement of cell border through-

put. However, transmit diversity, the fallback mode

for 2�2, may enhance cell border throughput as well

as the new enhanced receiver types. All these features

will reduce the gap between HSDPA and LTE DL per-

formance. However, all results presented for LTE are

without interference rejection combining, which will

reduce neighbor cell interference and, consequently,

will increase LTE DL cell border throughput.

Key Features in the Uplink of LTE and HSPA�

The uplink is most often the limiting link of

mobile broadband technologies, due in large part to

the limited transmit power available at the terminal as

well as the complexity and battery life constraints

imposed by practical handheld and portable devices.

In this section, we describe the key features of the

LTE uplink followed by the uplink enhancements in

HSPA�.


LTE Uplink Key FeaturesThe key features of the LTE uplink include a sin-

gle carrier multiple access technique which provides

in-cell orthogonality, the ability to obtain scheduling

gains based on both the time and frequency varia-

tions of the radio channel, and the ability to provide

the always-on connectivity experience for the

end user.

Multiple Access TechniqueUnlike OFDM used for the LTE downlink, which

is a multi-carrier OFDM transmission technique,

single carrier frequency division multiple access

(SC-FDMA) was chosen for the LTE uplink due to its

more favorable peak to average power ratio (PAPR)

characteristics. Consideration of PAPR is crucial in the

uplink where power efficient amplifiers are required

in the mobile device. In 2005, there was significant

discussion in 3GPP on whether single carrier fre-

quency division multiple access (SC-FDMA), orthogo-

nal frequency division multiple access (OFDMA),

and even multi-carrier CDMA (MC-CDMA) should

be chosen as the uplink multiple access method. In

the end, a majority of companies took the position

that SC-FDMA was the best uplink multiple access

method for LTE [1]. While SC-FDMA does have bet-

ter PAPR than OFDMA, unlike OFDMA the

SC-FDMA method suffers from inter-symbol inter-

ference when the assigned bandwidth is comparable

to or larger than the coherence bandwidth of the

channel (i.e., when the channel is frequency selec-

tive within the assigned bandwidth); OFDMA does

not have this drawback. Therefore, equalization is

required in the SC-FDMA receiver. To facilitate a sim-

ple one-tap frequency domain equalizer, the chosen

SC-FDMA technique uses a cyclic prefix as done in

the OFDMA downlink. Further, the basic numerol-

ogy in the uplink and downlink is the same; they

share the same resource block size of 180kHz (12 sub-

carriers), the maximum bandwidth utilization is the

same, and the SC-FDMA symbol time is the same as

the OFDMA symbol time for the downlink. In fact,

one way to implement the SC-FDMA transmitter is to

simply first take a DFT of the modulation symbols

prior to mapping the IFFT in a conventional OFDMA

transmitter. The only restriction is that the subcarriers

which are utilized for a particular user must be

contiguous in order to maintain the single carrier

property. While the single carrier property can also

be obtained using a distributed allocation with uni-

formly spaced subcarriers and inserting zeros in

between, this option was rejected by 3GPP due to

poor channel estimation performance and the

increased susceptibility to small frequency offsets from

the individual mobile devices.

In-Cell OrthogonalityOne important characteristic of the LTE uplink is

that the users remain orthogonal even in the pres-

ence of multipath, which is very different from the

HSUPA uplink based on asynchronous CDMA.

Orthogonality in the LTE uplink is maintained in two

ways: first, by time synchronizing the users to within

a small fraction of the CP through the use of timing

advance (TA) signaling; and second by the base station

scheduler ensuring that different users are assigned

different subcarriers. Loss of orthogonality between

users only occurs due to non-idealities in the system

such as a residual frequency offset between users or in

the case of very high Doppler where there is appre-

ciable variability in the channel during the SC-FDMA

symbol time. The fact that the LTE uplink is orthogo-

nal means that, ideally, users do not see intra-cell (i.e.,

same cell) interference as in the case of CDMA, and

hence the rise over thermal (RoT) is no longer the

factor which determines performance, rather it is

the interference over thermal (IoT), which is defined

as the total interference power (other cell interfer-

ence plus thermal noise) divided by the thermal noise.

Techniques to manage and control the IoT are still

being discussed in 3GPP, and require communication

between eNBs, which is made possible through the

X2 interface; this is sometimes also referred to as inter-

cell power control. As intra-cell interference is signifi-

cantly reduced in the LTE uplink, the use of fast

intra-cell power control is no longer necessary as in

the case of a CDMA uplink; rather, slow power con-

trol to compensate the path loss and shadowing is the

baseline power control technique for the LTE uplink.

Practically, such power control is needed to ensure that

the received power level from different mobile terminals

stays within a prescribed range due to the non-idealities


which lead to some degree of intra-cell interference,

as well as dynamic range and bit-width considera-

tions in the base station receiver.

Frequency Selective SchedulingAnother important feature of the LTE uplink

which differentiates it from HSUPA is the availability

of a channel sounding reference signal (SRS). The

SRS is a known sequence which is transmitted by the

mobile device, possibly over a wide bandwidth, in

order to allow the base station scheduler to obtain

channel state information. In this way, channel sen-

sitive scheduling in both time and frequency, referred

to generally as frequency selective scheduling,

becomes possible. This enhances the spectral effi-

ciency of the LTE uplink compared to HSUPA, espe-

cially for latency insensitive traffic, i.e., best-effort

traffic such as File Transfer Protocol (FTP) uploads or

e-mail attachments, in low Doppler conditions.

In order to capitalize on the improved SINR

offered by the orthogonal uplink and frequency

selective scheduling, the LTE uplink allows not only

QPSK modulation as in HSUPA Release 6, but also for

16 QAM and optionally 64 QAM.

Always-On ConnectivityFinally, a key feature of next-generation mobile

broadband technologies is to provide the end user

with the feeling of so called “always-on” data con-

nectivity; that is, the end user of a mobile broadband

device should experience connectivity similar to the

always-on wired broadband connections used in the

home or office. The simple solution is to keep users

fully connected to the wireless network as long as

the device is powered on; however this will pose

significant problems not only with device battery

lifetime, but also may result in inefficiency in terms

of air interface and base station channel card

resources.

In UMTS/HSPA, several levels of connectivity are

defined through the radio resource control (RRC)

states illustrated in Figure 10, which allow efficient

Idle mode

RRC connected mode

Establish RRCconnection

Release RRCconnection

Triggered by data activity

DCH—Dedicated channel FACH—Forward access channelPCH—Paging channelRRC—Radio resource control

Triggered by data inactivityURA_PCH or

CELL_PCH

CELL_FACH CELL_DCH

UMTS—Universal Mobile Telecommunications SystemURA—UTRAN registration areaUTRAN—UMTS terrestrial radio network

Figure 10.Radio resource control states for UMTS.


management of terminal battery lifetime as well as

base station channel card resources. The RRC con-

nected states are characterized as follows:

• URA_PCH/CELL_PCH allows the terminal to be

kept in a dormant (or standby) mode while

retaining an RRC connection with the RAN, as

well as a signaling and bearer plane connection to

the core network (i.e., the terminal retains its IP

address). This is very efficient in terms of terminal

battery life, as the device transceiver powers-on

periodically only to listen for paging messages, or

when it needs to send signaling messages.

• CELL_FACH allows for connectionless packet data

transfer that involves the use of the shared for-

ward access channel (FACH) in the downlink, and

the contention-based random access channel

(RACH) in the uplink. This state is used when only

small amounts of data need to be exchanged, i.e.,

via short message service (SMS), or as a transi-

tional state in which signaling messages are

exchanged between the terminal and the network,

i.e., to move from URA/CELL_PCH to CELL_DCH.

• CELL_DCH is a fully connected state in which

large amounts of data can be exchanged efficiently

between the terminal and the network; note that

both HSDPA and HSUPA are only applicable to

the CELL_DCH state in UMTS Release 6. In

Release 6, this state is characterized in the uplink

by the use of an always-on dedicated physical

control channel (DPCCH) which continuously

transmits a pilot, even in the absence of data, so

that power control can track the fading channel.

While the number of RRC connected sub-states

offers flexibility and efficiency in terms of terminal

battery life, air interface usage, and base station chan-

nel card resource utilization, it also results in relatively

long latencies when the terminal needs to transition

between dormant states (i.e., URA_PCH/CELL_PCH)

and fully active states (i.e., CELL_DCH), which makes

it difficult to give the end user the always-on data con-

nectivity experience.

In LTE, the states and state transitions are simpli-

fied significantly, as illustrated in Figure 11. Only an

RRC_IDLE and RRC_CONNECTED state are defined,

and in the RRC_CONNECTED state data can be quickly

exchanged between the terminal and the network

while terminal battery life can be managed via highly

flexible discontinuous receive (DRX) periods which

are under the control of the eNB. As the EPS archi-

tecture contains only a single network element in the

RAN (the eNB), the terminal can quickly move from

being dormant to being active as signaling only needs

to be exchanged between the terminal and the eNB.

In addition, there is no continuously transmitted pilot

channel in the LTE uplink as there is in HSUPA

Release 6; a data demodulation reference signal

(DM-RS) is transmitted by the terminal only when

there is data transmitted on the uplink, which is more

RRC_IDLE

Establish RRCconnection

Release RRCconnection

RRC_CONNECTED

LTE—Long term evolutionRRC—Radio resource control

Figure 11.Radio resource control states for LTE.


efficient from both an air interface point of view and

a terminal battery lifetime point of view.

HSPA� Uplink EnhancementsThe main enhancements for the HSPA� uplink

include the addition of 16 QAM in order to improve

peak user data rates as well as the continuous packet

connectivity feature which allows for an improved

always-on connectivity experience. Neither of these

features results in significant improvements in uplink

spectral efficiency, particularly in the typical macro-

cellular deployment conditions; hence HSPA� uplink

data capacity still falls short of LTE.

16 QAM ModulationHSPA� extends the uplink peak rate of HSUPA

from 5.76Mbps to 11.52Mbps through the addition of

16 QAM modulation. HSUPA Release 6 only allowed

QPSK modulation, more specifically, multi-code

binary phase shift keying (BPSK). 16 QAM is only

available with the 2 ms transport time interval (TTI)

length. As significantly higher SINRs are required to

support the extended rates offered by 16 QAM, an

advanced receiver at the node B, such as an LMMSE

sub-chip equalizer, is needed in order to prevent

SINR saturation due to self-noise in highly dispersive

channels. In addition, 16 QAM requires a stronger

phase reference than QPSK, so 3GPP has introduced

the concept of boosting the enhanced dedicated physi-

cal control channel (E-DPCCH) power level when 16

QAM modulation is used, with the intent that the

E-DPCCH be used as additional pilot in the demodu-

lation. Due to the range of SINRs experienced in typi-

cal macro-cellular deployments, 16 QAM offers little

gain in terms of average spectral efficiency; however

environments which offer a higher degree of cell

isolation (e.g., femtocells) may benefit more from

16 QAM.

It should be pointed out that if we consider an

LTE system in 5 MHz and consider that the minimum

amount of bandwidth is reserved for the physical

uplink control channel (PUCCH) is two resource blocks,

and also account for standards-based restrictions on the

number of subcarriers that can be allocated to a single

user, we find with 16 QAM (the highest mandatory

modulation in LTE) the peak user data rate in 5 MHz

of exactly 11.52 Mbps, which is precisely the uplink

peak rate offered by HSPA�.

Continuous Packet ConnectivityThe importance of the always-on connectivity

experience has already been discussed. It was recog-

nized in 2005 that HSPA would require enhance-

ments to efficiently support this feature, and a work

item called continuous packet connectivity was intro-

duced in 3GPP. Recall from Figure 10 that a user

needs to be in the CELL_DCH state in order to effi-

ciently exchange large amounts of data between the

user and the network; however, in the CELL_DCH

state the terminal is continuously transmitting the

DPCCH (consisting mainly of pilot and power control

bits). The DPCCH always transmits in the CELL_DCH

state even when the terminal has no data to transmit

in the uplink. In HSPA,it is common to move the user

into a CELL_PCH or URA_PCH state when there has

been a sufficiently long period of data inactivity from

the terminal; doing so not only reduces the uplink

interference level, but also extends terminal battery

life and saves base station channel card resources.

When data arrives in the terminal’s buffer, signaling

messages must be exchanged between the terminal

and the RNC in the CELL_FACH state in order to

move the terminal back into the CELL_DCH state.

This process involves a random access procedure and

establishment of radio bearers, which can take 700 ms

to 1 second depending on radio conditions. The latency

involved in moving a user between CELL_DCH and

CELL/URA_PCH repeatedly during a packet session

immediately detracts from the feeling of being always

connected. Hence, HSPA� introduced the CPC fea-

ture in order to make it more efficient to keep a user

in the CELL_DCH state. As illustrated in Figure 12,

the DPCCH is gated off when there is no data to trans-

mit on the uplink, which acts as a very quick way to

get the benefits offered by the CELL_PCH and

URA_PCH states; i.e., it reduces the level of interfer-

ence generated in the uplink as well as preserves the

terminal battery life. A DPCCH transmission cycle is

defined so that the DPCCH is transmitted periodically

even during data inactivity, in order to loosely main-

tain the power control state and synchronization with

the network. This allows the terminal to remain in


an efficient semi-dormant sub-state, and the transi-

tion back to a fully active sub-state can occur in less

than 50 ms; essentially this is the time required to

fully regain the power control state.

Use of HSDPA and HSUPA in CELL_FACHThe use of CPC in CELL_DCH state does not

eliminate the need for the URA_PCH or CELL_PCH

states, as it is still more efficient from a terminal bat-

tery lifetime point of view to be in the URA_PCH or

CELL_PCH states. In addition, when in CELL_DCH

state, the user mobility between cells is controlled

by the network, which incurs a higher signaling

overhead compared to the user-controlled mobility

through cell reselection in the URA_PCH and

CELL_PCH states. CPC allows for the expiration

timer to be set at much longer intervals before the

network decides to move the terminal into the

URA_PCH or CELL_PCH state, so that the user can

experience the feeling of always-on connectivity

during an extended data session. To further improve

the always-on experience, HSPA� introduced

enhancements known as enhanced CELL_FACH and

enhanced uplink in CELL_FACH which allows the

use of HSDPA and HSUPA in the CELL_FACH state,

respectively. HSDPA is used in lieu of the secondary

common control physical channel (S-CCPCH) to

carry both FACH and paging information and HSUPA

is used in lieu of sending RACH messages. In addi-

tion, it has been agreed that HSUPA can be used in

lieu of RACH messages even in the transition from

idle mode, i.e., for transmission of the common con-

trol channel. The intent here is to allow for faster

exchange of signaling messages to move the user

more quickly from idle, URA_PCH, or CELL_PCH

states to the CELL_DCH state. CPC, combined with

the use of HSDPA and HSUPA in the CELL_FACH

state, is the HSPA� solution to provide the user

experience of always-on data connectivity. Note that

the CPC feature does not directly translate into sig-

nificant improvement in spectral efficiency for best

effort data applications.

Uplink Performance ComparisonGiven that the HSPA� uplink features do not

result in significant spectral efficiency improvement

for best effort data applications, we focus here on

comparing the performance of LTE with HSUPA

Release 6 to check if the desired performance require-

ments of LTE were met. As in the case of the downlink,

the simulation assumptions from the next-generation

mobile networks (NGMNs) forum in [5] have been

Without CPC(Release 6)

With CPC(Release 7)

CPC—Continuous packet connectivityDPCCH—Dedicated physical control channel

DPCCH Data burst

Figure 12.Comparison of DPCCH transmission with and without CPC.


used; detailed assumptions can be found in [6]. We

have provided simulation results for the average cell

spectral efficiency in Table III for HSUPA and LTE

using the NGMN simulation assumptions. Results for

LTE have been given for a 5 MHz carrier for direct

comparison with HSUPA, as well as with a 10 MHz

carrier which is a more likely deployment scenario

for LTE. Unlike the NGMN simulation assumptions,

which simulate ten UEs per cell for both HSUPA in

5 MHz and LTE in 10 MHz, we have taken a more fair

approach and assumed five UEs per cell for HSUPA

as well as LTE in 5 MHz, and assumed ten UEs per cell

for LTE in 10 MHz. This ensures that the number of

UEs scales proportionally with the bandwidth, allow-

ing for a fair comparison of user throughputs. TableIV provides the cell edge user data rates for the

NGMN simulation assumptions. Note from Table III

and Table IV that LTE is able to provide the targeted

two to three times improvement in spectral efficiency

and cell edge user data rate compared to HSUPA in

Release 6.

While the NGMN set of simulation assumptions

described in [5] do provide a framework in which

results can be compared across different companies,

we feel that it is not necessarily a very realistic set of

assumptions even for the purposes of evaluating rela-

tive performance gains. One point in particular we

would like to highlight is that the NGMN assumption

set uses a typical urban (TU) channel model with a 3

kilometer/hour (km/hr) velocity. This channel type

has significant frequency selectivity (i.e., significant

multi-path delay spread) which hinders the performance

HSUPA Release 6 LTE LTE

(5 UEs/cell in (5 UEs/cell in (10 UEs/cell in5 MHz) 5 MHz) 10 MHz) Improvement

Case 1 (1 � 2) 0.26 0.72 0.75 2.8–2.9x

Case 1 (1 � 4) 0.36 0.96 0.97 2.6–2.7x

Case 3 (1 � 2) 0.27 0.61 0.67 2.3–2.5x

Case 3 (1 � 4) 0.33 0.83 0.93 2.5–2.8x

Table III. Average uplink spectral efficiency (bps/Hz/cell) with NGMN assumptions.

HSUPA—High speed uplink packet accessLTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipment



Case 1 (1 � 2) 125 245 306 2.0–2.4x

Case 1 (1 � 4) 168 368 430 2.2–2.6x

Case 3 (1 � 2) 35 50 60 1.4–1.7x

Case 3 (1 � 4) 42 85 125 2.0–3.0x

Table IV. Five percent CDF uplink user throughput (kbps) with NGMN assumptions.

CDF—Cumulative distribution functionHSUPA—High speed uplink packet accessLTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipment


of the HSUPA Release 6 system using a rake receiver

at the base station, while providing an advantage to

the LTE system, which capitalizes on the frequency

selectivity and low Doppler with its frequency selec-

tive scheduling feature. For additional insight on the

comparison between HSUPA Release 6 and LTE per-

formance, Table V and Table VI provide the per-

formance figures using all the NGMN assumptions

with the exception that the channel model is changed

to a mixture of ITU channel types given by the fol-

lowing: 30 percent Pedestrian A 3 km/hr, 30 percent

Pedestrian B 10 km/hr, 20 percent Vehicular A

30 km/hr, 10 percent Pedestrian A 120 km/hr, and 10

percent Ricean with K factor of 10 dB. This channel

mix uses channel types with both low and high fre-

quency selectivity as well as low and high Doppler.

We see from Tables V and VI that with this channel

mixture, LTE now improves performance only by a

factor of one to two over HSUPA Release 6. An

overview of LTE performance, with a more compre-

hensive set of realistic assumptions compared to those

used by NGMN, is given in [11].

Voice Over IP TransmissionWhile there is an increasing demand for high speed

data transmission over cellular networks, voice

still remains the dominant application today. Compared

to the traditional circuit-switched transmission, Voice

over IP offers a great deal of flexibility in providing

value-added services to the consumer, such as

enhanced caller identification (ID), call waiting and

voice mail features. In addition, VoIP is much easier to

integrate into multimedia applications which make use

of voice features (i.e., video calling, push-to-talk, and



Case 1 (1 � 2) 0.42 0.74 0.75 1.8x

Case 1 (1 � 4) 0.64 1.01 1.03 1.6x

Case 3 (1 � 2) 0.38 0.58 0.63 1.5–1.7x

Case 3 (1 � 4) 0.58 0.83 0.94 1.4–1.6x

Table V. Average uplink spectral efficiency (bps/Hz/cell) using channel mixture.

HSUPA—High speed uplink packet accessLTE—Long term evolutionUE—User equipment



Case 1 (1 � 2) 200 216 238 1.1–1.2x

Case 1 (1 � 4) 308 380 435 1.2–1.4x

Case 3 (1 � 2) 42 42 45 1.1x

Case 3 (1 � 4) 50 65 100 1.3–2.0x

Table VI. Five percent CDF uplink user throughput (kbps) using channel mixture.

CDF—Cumulative distribution functionHSUPA—High speed uplink packet accessLTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipment


interactive gaming). Another inherent advantage of

VoIP, especially in an end-to-end VoIP call, is that the

speech packets no longer need to traverse the public

switched telephone network (PSTN), which has a lim-

ited bandwidth and hence only supports an 8 KHz

(narrowband) sampling frequency. Significant

improvements in voice quality are possible by moving

to wideband vocoders which utilize a 16kHz sampling

rate, and this sampling rate can be maintained as the

packets traverse an IP network as opposed to

the PSTN.

In a VoIP system, the speech signal, after being dig-

itized and compressed by a vocoder, is packetized into

voice frames of a fixed duration in the application layer

called the Real Time Transport Protocol (RTP). A voice

frame duration of 20 ms is used by adaptive multi rate

(AMR) vocoders that are used in LTE and HSPA net-

works. The output bit-rate of AMR vocoders can be

adjusted between 12.2 kbps (best quality, highest bit-

rate) to 7.95 kbps to 5.9 kbps (lowest bit-rate, at the

expense of some voice quality). The RTP packets are

then transported in the network using a transport pro-

tocol such as User Datagram Protocol (UDP), and

routed using the Internet Protocol. Since a large over-

head of 40 bytes (more than 100 percent overhead) is

added by the RTP/UDP/IP version 4 (IPv4) layers, a

technique to compress the header information, called

robust header compression (RoHC), can be used to sig-

nificantly reduce the overhead. For example, in the

absence of RoHC, the 244 source bits that comprise a

20 ms AMR 12.2 kbps speech packet increases to 576

bits with the addition of headers and other overhead

before it reaches the LTE packet core. RoHC compresses

the 40 byte RTP/UDP/IPv4 header down to just 4 bytes.

Then we have two bytes of radio link control (RLC)

and MAC header that get added in the RAN.

VoIP Transmission Over LTEBoth LTE and HSPA� have incorporated air inter-

face enhancements in the MAC and physical layers

to enable efficient transmission of VoIP packets. In

this section we describe these features and provide

results of performance analysis for LTE and HSPA�.

SchedulingIn LTE, there is a choice of semi-persistent or

dynamic scheduling for VoIP packets. Semi-persistent

scheduling refers to the mode of scheduler operation

where a set of dedicated resources in time and fre-

quency are pre-allocated for the initial HARQ trans-

mission of every MAC packet. This means that the

network can allocate to a user a set of resource units

at specific intervals of time (e.g., once every 20 ms) on

the downlink and/or uplink, which will be used for the

transmission of initial HARQ transmissions of VoIP

packets. Retransmissions for these packets will have to

be scheduled dynamically, which on the uplink is

dynamic only in the frequency domain due to syn-

chronous HARQ operation. The benefit of semi-

persistent scheduling is in the reduction of MAC

control signaling that results from not having to trans-

mit dynamic scheduling grants on the uplink and data

associated signaling on the downlink for initial HARQ

transmissions.

A VoIP user may also be scheduled in a purely

dynamic scheduling mode, similar to any other data

user. Dynamic scheduling provides the flexibility

of scheduling the user’s transmissions at any time

and frequency, at the expense of higher control

signaling.

Frequency HoppingIn many instances, it may not be possible or effi-

cient to implement channel-selective scheduling for

VoIP users in LTE. Channel sensitive scheduling

cannot be used for persistently scheduled uplink

transmissions as the resources are pre-allocated. For

uplink transmissions that are dynamically scheduled,

channel-sensitive scheduling requires the uplink

channel sounding reference signal to be transmitted

over multiple resource blocks, which consumes

uplink bandwidth, and becomes infeasible when the

number of users is large. On the downlink, channel-

sensitive scheduling requires the mobile terminal to

provide CQI feedback for different frequency

resource blocks, which again becomes infeasible

when the number of voice users in the cell is large.

In situations where channel quality information can-

not be used for scheduling in the frequency domain,

frequency hopping can provide a significant diversity

gain by ensuring that a slow-moving user is not

“stuck” with a bad channel for a long time. Physical

layer frequency hopping is allowed in LTE.


Uplink Power ControlThe light load and low delay tolerance of VoIP

traffic imply that we cannot rely on a large number of

HARQ transmissions over a long period of time to

average out channel variations. Some form of power

control can hence be very useful to ensure that the

required signal-to-interference-plus-noise ratio is

consistently maintained and no more interference

than necessary is generated. In LTE, uplink power

control is applied on the transmit power spectral den-

sity (PSD) at the mobile device, and defined as the

transmitted power per physical resource block of

180 kHz. The PSD is computed using an open-loop

portion that depends on the long-term path loss

experienced at the mobile device, the average inter-

ference level seen at the base station receiver, and a

closed-loop portion that adjusts the open-loop set-

point using power control commands that may be

issued by the base station.

Coverage Enhancement TechniquesAs discussed in the next subsection on capacity

analysis, semi-persistently scheduled VoIP users on

LTE uplink face a coverage limitation in macro-cell

deployments with large building penetration losses.

Two coverage enhancement techniques are currently

being discussed in 3GPP to overcome this problem:

one is to let a single HARQ transmission span several

subframes before receiving an acknowledgement/

negative acknowledgement (ACK/NACK); the other is

to segment voice frames within the MAC layer before

transmission and use multiple HARQ processes to trans-

mit the different segments. Both techniques aim to

increase the amount of energy received from a cell-

edge mobile device per unit of time, thereby enhanc-

ing the cumulative received SINR for a voice frame.

Capacity AnalysisSimilarly to most other air interfaces, VoIP capac-

ity in LTE turns out to be uplink-limited. Using

semi-persistent scheduling, frequency-hopping and

closed-loop power control, we simulated uplink VoIP

transmissions at the system level under the NGMN

set of assumptions described in [5]. Capacity is defined

as the largest value of the average number of users per

cell for which no more than 5 percent of the users

each experience larger than 2 percent voice frame

outage. A voice frame is declared to be in outage if it

is not received successfully at the receiver, or if it is

received after the maximum tolerable one-way air

interface delay of 50 ms. The results are summarized

in Table VII for case 1 using an AMR 12.2 kbps

vocoder. We see that LTE provides approximately a

200 percent capacity improvement over Release 6

HSPA for case 1, which corresponds to a micro-cell

deployment. For case 3, which corresponds to a large

macro-cell deployment with 20 dB in-building pene-

tration loss, coverage was found to be inadequate for

the LTE uplink. The coverage enhancement tech-

niques outlined in the previous sub-section would

HSUPA Release 6 HSPA� LTE(5 MHz) (5 MHz) (5 MHz) Improvement

Case 1 (1 � 2) 73 100 220 3x HSUPA

Release 6

2.2x HSPA�

Table VII. VoIP capacity for uplink LTE and uplink HSPA for case 1 with NGMNassumptions, AMR 12.2 kbps vocoder.

AMR—Adaptive multi rateHSPA—High speed packet accessHSPA� —HSPA evolutionHSUPA—High speed uplink packet access

LTE—Long term evolutionNGMN—Next-generation mobile networkUE—User equipmentVoIP—Voice over Internet Protocol


have to be used before evaluating capacity for this

case.

VoIP Transmission Over HSPA�

HSPA� inherits all the key features of Release 6

HSPA that are useful for VoIP transmission, namely,

the scheduled nature of transmission, HARQ retrans-

missions, and link adaptation. The following are some

new features in HSPA� that further enhance VoIP

performance.

Uplink CPCAs described above in the section on HSPA�

uplink enhancements, uplink CPC decreases the frac-

tion of time that the mobile device is transmitting the

pilot channel (DPCCH). This is useful not only during

voice inactivity on the uplink, but even during a talk

burst since not all HARQ processes are utilized when

the 2 ms TTI length is chosen for HSUPA. The reduc-

tion in the DPCCH transmission, both during a talk

spurt and also during voice inactivity, results in a

reduction of the total interference level seen at the

base station receiver, which allows a larger number of

VoIP users to be supported for a given target loading.

In Table VII, we have included the HSPA� VoIP

capacity which utilizes the CPC feature, and we see a

37 percent improvement over HSUPA Release 6 VoIP

capacity. However, LTE still offers a 120 percent

improvement in VoIP capacity over HSPA�.

HS-SCCH Less OperationIf a system is loaded with a high number of low bit

rate users, the HS-SCCH would use a significant

amount of spreading codes and power on the HSDPA

downlink. A means to avoid this is the introduction of

the HS-SCCH less operation. A mobile device that

takes part in this operation mode has to blindly decode

all transport blocks sent over one spreading code of

the high speed downlink shared channel (HS-DSCH).

The corresponding transport blocks have only a limited

set of code rates and sizes, which are configurable per

mobile device, and sent with QPSK modulation only.

ConclusionIn this paper, we have described the key features

of the two technology paths in 3GPP for global mobile

broadband: the evolutionary HSPA� approach and

the revolutionary LTE approach. HSPA� offers the

advantage of backwards compatibility with earlier

releases, allowing operators an easier upgrade while

exploiting their current HSPA investment. On the

other hand, LTE offers significant improvements in

performance, especially in larger spectrum allocations,

but does not offer backwards compatibility to HSPA.

To deploy LTE, operators will need to consider new

spectrum which is becoming available, and eventu-

ally swap-out existing GSM/EDGE/UMTS/HSPA spec-

trum as the LTE technology matures.

The comparison provided in this paper illustrates

that there are many feature similarities between

HSPA� and LTE: the flatter network architecture with

clean separation of the user plane and control plane,

the availability of higher order modulations such as 64

QAM on the downlink and 16 QAM on the uplink,

MIMO techniques, and the ability to provide the

always-on data connectivity experience. While these

similarities exist, there are fundamental differences

between LTE and HSPA�; namely, the use of orthogo-

nal multiple access in LTE (OFDMA in the downlink,

SC-FDMA in the uplink) and the ability to exploit the

frequency-selective nature of the channel in both the

downlink and uplink. We have seen that LTE offers a

considerable advantage in spectral efficiency for best

effort data in the downlink and especially the uplink.

We have also seen that while both LTE and HSPA� pro-

vide features that enhance VoIP performance, LTE is

able to offer twice the VoIP capacity compared to

HSPA� for micocell deployments.

AcknowledgementsWe gratefully acknowledge Lutz Schönerstedt for

the LTE DL performance data and Michael Wilhelm

for the HSDPA performance data.

*Trademarks3GPP is a trademark of the European Telecommuni-

cations Standards Institute.CDMA2000 is a trademark of the Telecommunications

Industry Association.GSM and Global System for Mobile Communications are

registered trademarks of the GSM Association.

References[1] 3rd Generation Partnership Project, “LS on

UTRAN LTE Multiple Access Selection,” 3GPPTSG RAN Meeting #30, RP-050758, Nov.2005, �http://www.3gpp.org�.


[2] 3rd Generation Partnership Project,“Requirements for Evolved UTRA (E-UTRA) andEvolved UTRAN (E-UTRAN) (Release 7),” 3GPPTR 25.913, v7.3.0, Mar. 2006, �http://www.3gpp. org/ftp/Specs/html-info/25913.htm�.

[3] 3rd Generation Partnership Project, “Scope ofFuture FDD HSPA Evolution,” 3GPP RANPlenary #31, RP-060217, Mar. 2006, �http://www.3gpp.org�.

[4] 3rd Generation Partnership Project, “PhysicalLayer Aspects for Evolved Universal TerrestrialRadio Access (UTRA) (Release 7),” 3GPP TR25.814, v7.1.0, Sept. 2006, �http://www.3gpp.org/ftp/Specs/html-info/25814.htm�.

[5] 3rd Generation Partnership Project, “LTEPhysical Layer Framework for PerformanceVerification,” 3GPP TSG-RAN1 #48, R1-070674,Orange, China Mobile, KPN, NTT DoCoMo,Sprint, T-Mobile, Vodafone, Telecom Italia, Feb.2007, �http://www.3gpp.org�.

[6] 3rd Generation Partnership Project, “Uplink E-UTRA Performance Checkpoint,” 3GPP TSG-RAN WG1, R1-071989, Alcatel-Lucent, Apr.2007, �http://www.3gpp.org�.

[7] 3rd Generation Partnership Project, “EvolvedUniversal Terrestrial Radio Access (E-UTRA),Physical Channels and Modulation (Release 8),”3GPP TS 36.211, v8.1.0, Nov. 2007, �http://www.3gpp.org/ftp/Specs/html-info/36211.htm�.

[8] 3rd Generation Partnership Project, “High SpeedPacket Access (HSPA) Evolution, FrequencyDivision Duplex (FDD) (Release 7),” 3GPP TR25.999, v7.0.1, Dec. 2007, �http://www.3gpp.org/ftp/Specs/html-info/25999.htm�.

[9] 3rd Generation Partnership Project “High SpeedDownlink Packet Access (HSDPA), OverallDescription, Stage 2 (Release 7),” 3GPP TS25.308, v7.5.0, Dec. 2007, �http://www.3gpp.org/ftp/Specs/html-info/25308.htm�.

[10] 3rd Generation Partnership Project, “EvolvedUniversal Terrestrial Radio Access (E-UTRA),Base Station (BS) Radio Transmission andReception (Release 8),” 3GPP TS 36.104, v8.0.0,Dec. 2007, �http://www.3gpp.org/ftp/Specs/html-info/36104.htm�.

[11] K. Balachandran, Q. Bi, A. Rudrapatna, J. Seymour, R. Soni, and A. Weber, “PerformanceAssessment of Next-Generation Wireless MobileSystems,” Bell Labs Tech. J., 13:4 (2009), 35–58.

[12] Informa Telecoms & Media, WCIS, and 3GAmericas, Global UMTS and HSPA OperatorStatus, Feb. 27, 2008.

(Manuscript approved October 2008)

ANIL M. RAO is a member of technical staff in Alcatel-Lucent’s wireless research anddevelopment (R&D) organization inNaperville, Illinois. He received a B.S. inapplied mathematics from the University ofAlaska, Fairbanks, and M.S. and Ph.D.

degrees in electrical engineering from the Universityof Illinois at Urbana Champaign where he held aNational Science Foundation graduate researchfellowship. Dr. Rao joined Alcatel-Lucent afterassignments with NASA’s Jet Propulsion Laboratoryand TRW. His work at Alcatel-Lucent has involvedvarious aspects of system design, performance analysis,and algorithm development for UMTS, HSPA/HSPA�,and LTE. He has actively contributed to both thestandardization and product realization of thesetechnologies. His interests include intelligentantennas, scheduling and resource allocationalgorithms, and optimizing the end-to-endperformance of mobile broadband wireless systems.

ANDREAS WEBER is team leader of the mobile systemperformance evaluation group in Bell Labs’Radio Access domain in Stuttgart, Germany.He received Dipl.-Ing. and Dr.-Ing. degreesin electrical engineering from the Universityof Stuttgart, Germany. Prior to joining

Alcatel-Lucent, Dr. Weber worked in the field ofsatellite communications as a member of scientific staffat the Institute of Communications Switching and DataTechnics, University of Stuttgart. During his tenure atAlcatel Research & Innovation and later at Bell Labs, heworked on the performance evaluation andoptimization of 2G, 3G, and beyond 3G mobilecommunication systems. Currently, he and his teamwork on LTE Advanced and WiMAX.

SRIDHAR GOLLAMUDI is a member of technical staffwith Alcatel-Lucent’s Wireless research anddevelopment (R&D) organization inWhippany, New Jersey. He received his Ph.D.in electrical engineering from the Universityof Notre Dame, Indiana.

Dr. Gollamudi worked at Motorola Inc. beforebeginning his career at Alcatel-Lucent. His researchinterests include statistical signal processing, resourceallocation in wireless systems, physical and MAC layeralgorithm design, and performance analysis ofcommunications systems.


ROBERT SONI is a technical manager in Alcatel-Lucent’sWireless business group in Whippany, NewJersey. He supervises a group which isinvestigating and developing new advancedantenna, physical layer and MAC layertechnologies for 3G/4G cellular systems. He

received a Ph.D. and MSEE in electrical engineeringfrom the University of Illinois at Urbana-Champaign,and received his BSEE, summa cum laude, from theUniversity of Cincinnati in Ohio. Dr. Soni began hiscareer as a member of technical staff at Alcatel-Lucentten years ago. He also teaches part-time at ColumbiaUniversity in New York City, and the New JerseyInstitute of Technology in Newark, New Jersey. ◆

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