Uplink Power Control in LTE Relay Enhanced Cells

Masters Thesis PresentationDepartment of Communications and Networking

Student: Aydin Karaer

Supervisor: Prof. Jyri Hämäläinen / HUTInstructor: Doc. Simone Redana / NSN

Agenda

LTE Advanced Why Relaying? Relay Enhanced Cell (REC) Scenario LTE Uplink Power Control Simulation Parameters Power Control Optimization and System Performance

Evaluation in REC

LTE Advanced (LTE-A)

First set of requirements were addressed in June 2008 Not a revolution but an evolution of LTE Promises to support peak data rates of 1 Gbps in downlink and 500

Mbps in uplink Bandwidth scalability up to 100 MHz Improved user and control plane latencies Improved cell edge performance

Why Relaying? (1)Reasons

Future wireless communication systems are operating carrier frequencies over 2 GHz Heavy pathloss in radio transmission

Aggressive propagation conditions restrict the radio coverage especially in urban areas

Possible solutions: Power increase? => Interference, decreased battery lifetime More base stations? => Deployment and maintenance costs,

possibly not enough subscribers, no cell edge performance enhancement

Relay nodes (RN) provide an attractive solution to satisfy tough throughput and coverage requirements for LTE-A

Less eNBs and smaller OPEX

OFDMA flexible enough to fine tune e.g. resource allocation

Reasoning Result

Due to low TX power, no auxiliary equipment

Relays can save costs

Relaying and LTE

technology fit

Relays will be cheap

Easier to find sites, no backhaul costs, easier installation

Relay OPEX will be very

low

Why Relaying? (2)Benefits

Why Relaying? (3)Drawbacks

Relays introduce extra delay and overhead Resource partitioning and interference management

becomes important Deployment is challenging Additional set of signaling protocols are needed in case

of Layer 2 and Layer 3 relays Increasing number of hops introduce more complexity

and overhead in the system

Relay Enhanced Cell (REC) Scenario

Simple infrastructure multi-hop scenario is considered with decode-and-forward relays

Idea is to deploy the relay nodes in the cell edges in order to improve the low SINR experienced by users and minimize the cell outage

Downlink received signal power in RECTwo-hop relay based deployment

LTE Uplink Power Control (1)Rationale

Full frequency reuse (reuse one) is highly desirable for future communication systems so as to exploit the spectrum efficiently

Intra-cell interference was the limiting factor in WDCMA uplink

LTE uplink transmission scheme SC-FDMA mitigates intra-cell interference near far effect

However, the LTE system is sensitive to inter-cell interference

LTE Uplink Power Control (2)Analysis

Standardized LTE uplink power control formula is simple and robust:

Fractional power control (FPC) utilizes a compensation factor for the pathloss and it is introduced to improve the performance of cell center users by inducing an acceptable inter-cell interference

Open loop power control is considered in this work, thus closed loop corrections are omitted

Used formula is given as:

)}()(log,min{ max ifiLMPPP TF 100 10

)log,min{ max LMPPP 100

Power Control in REC (1)Motivation

REC requires detailed dimensioning and planning New cell edges introduced by RNs will lead to severe

intra-cell and inter-cell interference in particular when high number of relay nodes are deployed in the

cell with reuse one Power control becomes an important means in the uplink

transmission of REC to mitigate the interference and increase the cell edge and system

capacity Approved LTE uplink power control scheme should be re-

investigated in REC to achieve an optimal performance in this work, approved LTE uplink power control formula is applied

in each relay node

Power Control in REC (2)Main Simulation Parameters

PARAMETER (Ref.1) ASSUMPTIONS

System Layout 19 cells & 3 sectors/cell & 1 tier (9 RNs) of RNs/sector

Carrier Frequency 2 GHz

Propagation Scenario Macro 1 (500m ISD)

Frequency Planning Reuse one (each eNB and RN uplink transmission interferes with each other)

System Bandwidth 10 MHz (48 PRBs for data)

Channel Models eNB-UE => (R in km) eNB height/location = 25 m (above rooftop)

eNB-RN => (R in km) RN height/location = 5 m (below rooftop)RN-UE => (R in km) UE height/location = 1.5 m

Antenna Configurations(Pattern & Number of Ant.)

eNB antennas per sector = 2 tx, 2 rx

RN antennas per sector = 2 tx, 2 rx

UE antennas = 1 tx, 2 rx

UE Transmit Power 23 dBm

eNB Transmit Power 46 dBm

RN Transmit Power 30 dBm

Extra Margins 0 dB (No shadow fading, fast fading)

User Drop 48 users per Sector / 200 iterations

UE Scheduling/Traffic Model Round robin, full buffer

Simulation Window 1 TTI

RL 106371128 log.. RL 106375124 log.. RL 107367140 log..

m

dB

AA ,min)(2

3

12

odB 703

dB 25mA

Ref.1: TR 36.814 v0.3.1 (2008-09), Further Advancements for E-UTRA, Physical Layer Aspects, 3GPP TR 25.942, 3GPP R1-084026

Power Control in REC (3)Parameter Configuration in a Macro Cell Scenario

Cell coverage prioritized Cell capacity prioritized

Po & Alpha -83 dBm & 1 -42 dBm & 0.6

Average IoT 5.4 dB 5.1 dB

Cell capacity 9354 kbps 11032 kbps

Cell coverage 3757 kbps 3382 kbps

• Rationale is based on the cell capacity and cell coverage with considering the corresponding average interference over thermal (IoT) level in the system adopted from Ref.2

Ref.2: C. Castellanos, D. L. Villa, C. Rosa, I. Z. Kovacs, F. Frederiksen, and K. I. Pedersen, ‘’Performance of Fractional Power Control in UTRAN LTE Uplink’’, The 2008 IEEE, ICC, Beijing, China, May 2008

• Acceptable IoT level is decided according to the eNB receiver dynamic range (see Appendix A)

Disclaimer: Resulting Po values are not same with the Ref.2 due to that shadowing is not considered.

Power Control in REC (4)Suboptimal Settings for REC

Macro cell scenario parameter configurations are named as full compensation power control (FCPC) and fractional power control (FPC) according to coverage and capacity priorities respectively

The eNB-only deployment with optimal parameter settings for cell capacity prioritized scenario by FPC was assumed as reference case for the performance evaluation in REC scenario. Notations are as following: FPC: optimal parameter setting for fractional power control in eNB-only

deployment FCPC (eNB and RN): optimal parameter setting for FCPC in eNB-only

deployment is applied in relay based deployment both at eNB and RN FPC (eNB and RN): optimal parameter setting for FPC in eNB-only

deployment is applied in relay based deployment both at eNB and RN

Power Control in REC (5)Results of Suboptimal Settings

• Very high throughput at RNs

• FCPC outperforms FPC up to 50% ile

• 80 % of the UEs connected to eNB experience higher throughput compared to FPC

• FPC boosts the performance of UEs served by RNs

• Do we need high capacity at RNs?

• it should be noted that an ideal relay link is assumed (see Appendix B)

• Parameter settings should be re-adjusted to achieve an optimal performance

CDF of Throughput per UE at sector for FCPC (eNB and RN) vs. FPC (eNB and RN) in 1 tier (9 RNs deployed at the cell edges) REC

scenario

Power Control in REC (6)Analysis of Po at eNB and RNs

• Analysis of Po at RNs in REC => Feasible SINR threshold at RNs (-15 dBm), 12 dB lower Po value can be used in FPC case

• Po value can be set as small as possible for the UEs served by RNs in order to improve the performance of UEs served by eNBs

• Analysis of Po at eNB in REC => Optimum cell edge performance can be maintained with suboptimal settings found in eNB-only scenario

Power Control in REC (7)Analysis of No Power Control at RNs

Performing a power control scheme might still be regarded as an extra overhead at RNs No power control by considering fixed maximum allowed transmit power for UEs at RNs Scheme maintains the SINR performance of the cell edge users connected to RNs with a

fixed maximum Tx power leads to higher throughput for the cell center UEs at RNs simpler RN design without penalizing the UEs served by eNB

• 18 dBm illustrates similar performance to FCPC

• 15 dBm results in better performance for the UEs at eNB

Power Control in REC (8)Power control with Maximum Allowed Tx Power Setup

• Po configuration with fixed maximum allowed transmit power can be still re-adjusted to reduce the experienced high throughput for the UEs served by RNs and enhance the UEs served by eNBs

• 5% ile user throughput improved by

• 9 % for FCPC

• 25 % for FPC

• Average user throughput improved by

• 17 % for FCPC

• 40 % for FPC

compared to non-adjusted suboptimal settings

Power Control in REC (9)Results of Optimized Parameter Settings

It is observed that FPC outperforms FCPC after parameter optimization

For the UEs at eNB in the REC FPC provides:

70 % better cell edge user throughput (5 %ile) than eNB-only 55 % better average user throughput than eNB-only

For the same cell edge performance FPC provides: 23 % better average user throughput at eNB than FCPC 13 % better average user throughput at RNs than FCPC 15 % better average user throughput at sector than FCPC

Summary & Conclusions

Relaying is a promising solution for the demands of LTE-A

REC provides performance enhancement in the cell edge throughput and the system capacity compared to Macro cell scenario

Standardized LTE UL Power control scheme is feasible to use in REC scenarios

Parameter optimization and transmit power setup is important to achieve optimal performance

FPC outperforms traditional FCPC with an appropriate parameter configuration and transmit power setup in REC scenarios

Appendix AeNB Receiver Dynamic Range vs. Average IoT

Assuming a maximum allowed receiver dynamic range of 35 dBm, compensation factors lower than 0.6 do not seem suitable to use because of non-acceptable eNB receiver dynamic ranges

Appendix BRelay Link Overhead

This study assumes an ideal relay link (can be maintained via Microwave transmission) However, a possible resource allocation scheme is also studied to see the overhead that is introduced by relay link given as in Fig.1: where half duplex transmission is applied to define the signal reception between direct link and relay link.

End-to-end user throughput is calculated according toa minimum formula given as:

It is observed that excessive user throughput experienced from the access link is limited by the relay link Described resource allocation scheme only improvesthe cell edge users while it does not increase the average user throughput and system capacity compared to an eNB-only scenario

This can be achieved with bandwidth scalability of 100 MHz by LTE-Aand more efficient resource allocation and frequency reuse scheme

f

t

UE - eNB

UE - RNRN - eNB

)RNby served UEsofNumber

RNper ThroughputLink Relay , per UE ThroughputLink (Access 2 minThroughputUEee

Fig.1

THANK YOU!