wcdma air interface

WCDMA Air Interface

Layered three-layer protocol model

Layer 1 Lowest layer in this interface is

physical layer,(PHY).

The physical layer has interface to both MAC and RRC sub-layer.

Layer 2Layer 2 Consists of

Medium Access Control (MAC) Radio Link Control (RLC) The Broadcast Multicast Control (BMC) Packet Data Convergence Protocol

(PDCP)

Layer 3

Layer 3 consists of: RRC Mobility Management (MM) GPRS Mobility Management (GMM) Call control (CC) Supplementary services (SS) Short Message Service (SMS) Session Management (SM) GPRS Short Message Service

support(GSMS)

WCDMA Air Interface : Physical Layer

Physical layer interfaces

Functions of physical layer FEC encoding/decoding of transport

channels Radio measurements and indications

to higher layers Macro diversity

distribution/combining and SHO execution

Error detection on transport channels Multiplexing of transport channels

and de multiplexing of coded composite transport channels (CCTrCHs)

Functions of physical layer …. Rate matching Mapping of CCTrCHs on physical

channels Modulation, spreading/demodulation,

and de-spreading of physical channels

Frequency and time synchronization Closed-loop power control Power weighting and combining of

physical channels RF processing

FEC encoding/decoding of transport channels FEC scheme aims to reduce

transmission error UTRAN employs two FEC schemes

(i) Convolution codes(ii) turbo codes

Radio measurements and indications to higher layers Radio measurements is to be carried

by physical layer and result is to be reported to higher layers.

These measurements can be specific to either UE or Node B

Radio measurements and indications to higher layers….

Possible measurement types of UE : Received Signal Code Power (RSCP) Received signal strength indicator

(RSSI) Block error rate (BLER) UE transmitted power CFN-SFN observed time difference UE Rx-Tx time difference Observed time difference to GSM cell

Radio measurements and indications to higher layers….

Possible measurement types for UTRAN Received total wide band power Signal to interference ratio (SIR) SIR error Transmitted carrier power Transmitted code power Bit error rate (BER) Round trip time SFN-SFN observed time difference

Macro diversity Distribution/Combining and SHO Execution

Macro diversity (SHO) is a situation in which a receiver receives the same signal from different sources.

This macro diversity should be combined, Without combining interference level would increase and capacity decreased by considerable amount.

This combining is done at physical layer using RAKE receiver

Macro diversity Distribution/Combining and Soft Handover Execution

RAKE receiver

Error Detection on Transport Channels The purpose of error detection is to

find out whether a received block of data was recovered correctly.

CRC is used for this. There are five CRC polynomial

lengths in use (0, 8, 12, 16, and 24 bits)

Multiplexing of Transport Channels and De multiplexing of CCTrCHs Each UE can have several transport

channels in use simultaneously. Every 10 ms, one radio frame from

each transport channel is multiplexed into a (CCTrCH), serially.

For uplink, FDD mode has 1 CCTrCH TDD mode has multiple

CCTrCHs For downlink, both FDD and TDD

modes have multiple CCTrCHs

Rate Matching

The number of bits on a transport channel can vary with every transmission time interval. However, the physical channel radio frames must be completely filled.

To match bit rate after transport channel with total physical channel bit rate either repeating or puncturing bits techniques is used.

Mapping of CCTrCHs on Physical Channels If there is more than one physical

channel in use, then the bits in the CCTrCH must be divided among them. This is done by segmenting the input bits evenly for each physical channel.

Since rate matching is already done in an earlier phase, so the bits should fit nicely into physical channels.

Modulation, Spreading/Demodulation, and De spreading of Physical Channels The spreading process in UTRAN

consists of two steps: channelization & scrambling.

Channelization transforms each data symbol into several chips.

Data symbols on I- and Q-branches are multiplied with a channelization code.

Channelization codes are orthogonal codes.

Modulation, Spreading/Demodulation, and De spreading of Physical Channels… In the scrambling procedure, the I-

and Q-phases are further (after channelization) multiplied by a scrambling code.

These scrambling codes have good autocorrelation properties.

The modulation scheme in the UTRAN is quadrature phase shift keying(QPSK).

Modulation rate = 3.84 Mcps

Frequency and Time Synchronization This procedure takes place when the

power is turned on in the UE, starts with downlink SCH synchronization.

The UE knows the SCH primary synchronization code, which is common to all cells.

P-SCH and S-SCH both are sent over first 256 chips of each slot (of 2560 chips).

There are 15 such slots in each radio frame.

Frequency and Time Synchronization…

Structure of synchronization channel

Inner-Loop Power Control In UTRAN two forms of power

control : open loop & closed loop. Closed loop power control is further

divided into : inner loop & outer loop power control.

Outer loop power control sets the Signal to Interference Ratio (SIRtarget) and Inner loop power control adjusts the peer entity transmit power so that measured SIR fulfills the SIRtarget requirement.

Power Weighting and Combining of Physical Channels

In Uplink

Power Weighting and Combining of Physical Channels…

In the uplink, one UE can transmit simultaneously one DPCCH and up to six DPDCHs.

The control channel (DPCCH) will be sent in the Q-plane, and the data channels (DPDCH) in both planes.

The channelization codes are orthogonal codes, and the scrambling code is a pseudo-random sequence.

Power Weighting and Combining of Physical Channels…In downlink

Power Weighting and Combining of Physical Channels…

All channels have their own power weight factor G.

All physical channels (except the SCH) are processed in the same way as a DPDCH.

All channels (except the SCH) are scrambled with the same scrambling code.

RF Processing

Four UE Power Classes

RF Processing…

Frequency bands

RF Processing…

RF specification

Channels

There are three separate channel concepts in the UTRAN: logical, transport and physical channels.

Logical Channel

Logical channels define what type of data is transferred.

It can be divided into control channel and traffic channel.

Control channels transfer Control plane (C-plane) information and traffic channels User plane (U-plane) information.

A control channel can either be common or dedicated.

Logical Channel… A common channel is a point-to-multipoint

channel; i.e. common to all users in a cell. A dedicated channel is a point-to-point

channel; i.e. used by only one user. The defined logical control channels are:

Broadcast control channel (BCCH)Paging control channel (PCCH)Dedicated control channel (DCCH)Common control channel (CCCH)Shared channel control channel (SHCCH)

The used logical traffic channels are:Dedicated traffic channel (DTCH)Common traffic channel (CTCH)

Transport Channel

The transport channels define how and with which type of characteristics the data is transferred by the physical layer.

Transport channels are divided into common channels and dedicated channels.

All these channels are unidirectional.

Transport Channel…

Common transport channels include:Broadcast channel (BCH)Paging channel (PCH)Random access channel (RACH)Common packet channel (CPCH)Forward access channel (FACH)Downlink shared channel (DSCH)High-speed downlink shared channel (HS-DSCH)Uplink shared channel (USCH)

The only dedicated transport channel type is:

Dedicated channel (DCH)

Physical Channel

Physical channels define the exact physical characteristics of the radio channels.

In frequency-division duplex (FDD) mode, both the uplink and downlink bands have their own frequency channels.

In time-division duplex (TDD) mode, there is only one frequency channel, which is then dynamically time-divided for both uplink and downlink slots.

FDD Physical Channels Downlink

Synchronization channel (SCH)Common pilot channel (CPICH)Primary common control physical channel (P-CCPCH)Secondary common control physical channel (S-CCPCH)Physical downlink shared channel (PDSCH)Paging indicator channel (PICH)Acquisition indicator channel (AICH)

Downlink and UplinkDedicated physical data channel (DPDCH)Dedicated physical control channel (DPCCH)

UplinkPhysical random access channel (PRACH)Physical common packet channel (PCPCH)Uplink dedicated control channel for HS-DSCH (HS-DPCCH)

TDD Physical Channels Downlink

Primary common control physical channel (P-CCPCH)Secondary common control physical channel (S-CCPCH)Synchronization channel (SCH)Paging indicator channel (PICH)Physical downlink shared channel (PDSCH)Physical Node B synchronization channel (PNBSCH)High speed physical downlink shared channel (HS-PDSCH)Shared control channel for HS-DSCH (HS-SCCH)

Downlink and UplinkDedicated physical channel (DPCH)

UplinkPhysical random access channel (PRACH)Physical uplink shared channel (PUSCH)Shared information channel for HS-DSCH (HS-SICH)

Spreading and Scrambling The spreading process actually

consists of two phases, spreading and scrambling, and both of them use different types of codes with different characteristics.

Spreading and Scrambling… Spreading increases the bandwidth

of the signal. Spreading codes used are orthogonal Scrambling shuffles the bit using a

code called scrambling code. Scrambling is done after the

spreading in the transmitter Scrambling codes are pseudo-

random sequence of bits.

Diversity A signal can be subjected to time

diversity, multipath diversity, and antenna diversity.

Time Diversity

Time diversity means that the signal is spread in the time domain.

Time diversity spreads the faulty bits over a longer period of time, and thus makes it easier to reconstruct the original data.

Multipath Diversity

Multipath diversity is a phenomenon that happens when a signal arrives at the receiver via different paths.

Multipath Diversity…

In a WCDMA system the receiver is typically able to track and receive several multipath components and combine them into a composite signal.

The receiver is of the RAKE variety.

RAKE receiver

Macro diversity

In a CDMA system the same signal can be transmitted over the air interface, on the same frequency, from several base stations separated by considerable distances. This scheme is called the soft handover (SHO).

In a SHO all the participating base stations use the same frequency, and the result is a macro diversity situation.

Macro diversity…

A typical SHO situation

WCDMA Air Interface : Protocol Stack

Control Plane

Radio interface protocols can be divided in two ways: horizontal layers and vertical planes.

Medium Access Control (MAC) The UTRAN MAC can even contain

different functionalities depending on whether it supports FDD, TDD, or both modes.

The MAC is not a symmetric protocol; the entities in the UE and in the UTRAN are different.

MAC Services

The services MAC provides to the upper layers include the following: Data transfer; Reallocation of radio resources and

MAC parameters; Reporting of measurements to RRC.

MAC Functions

Mapping between logical channels and transport channels;

Priority handling between data flows of one UE;

Priority handling between UEs by means of dynamic scheduling;

Identification of UEs on common transport channels;

Traffic-volume monitoring;

MAC Functions…

Transport-channel type switching; Ciphering for transparent RLC; Selection of the appropriate

transport format for each transport channel depending on the instantaneous source rate;

Radio Link Control (RLC)

RLC layer is in charge of the actual data packet transmission over the air interface.

The RLC task maintains a retransmission buffer, performs ciphering, and routes the incoming data packets to the right destination task (RRC, BMC, PDCP, or voice codec).

Radio Link Control (RLC)

RLC Services

Transparent Data Transfer ServiceSegmentation and reassembly;Transfer of user data; SDU discard.

Unacknowledged Data Transfer ServiceSegmentation and reassembly;Concatenation;Padding;Transfer of user data;Ciphering;Sequence number check;SDU discard.

RLC Services…

Acknowledged Data Transfer ServiceConcatenation;Padding;Transfer of user data;Error correction;In-sequence delivery of higher-layer PDUs;Duplicate detection;Flow control;Protocol error detection and recovery;Ciphering;SDU discard.

RLC Functions

Segmentation and reassembly of higher-layer PDUs into/from smaller RLC payload units;

Concatenation (RLC SDUs may be concatenated so that they will fill

the RLC PUs); Padding; Transfer of user data; Error correction;

RLC Functions…

In-sequence delivery of higher-layer PDUs;

Duplicate detection; Flow control; Sequence number check (in

unacknowledged data transfer mode);

Protocol error-detection and recovery;

Ciphering (in UM and AM modes); Suspend/resume function.

Radio Resource Control (RRC) The RRC controls the configuration of

the lower layers in the protocol stack, and it has control interfaces to each of the lower layers (PDCP, BMC, RLC, MAC, and layer 1).

RRC Services

General control Notification Dedicated control

RRC Functions

Initial cell selection and cell reselection

Broadcast of information reception of paging messages;

Establishment, maintenance, and release of RRC connection;

Establishment, reconfiguration, and release of radio bearers;

RRC Functions…

Handovers (HOs); Measurement control; Outer-loop power control; Security mode control; Control of requested QoS; Contention resolution in the TDD

mode; Timing advance in the TDD mode;

Core Network Protocols : Circuit-Switched Core Network Mobility Management (MM)

Main function of the MM is location management.MM procedures can be divided into three groups: MM common procedures; MM specific procedures; MM connection-management

procedures.

Core Network Protocols : Circuit-Switched Core Network GPRS Mobility Management (GMM)

The GPRS mobility management (GMM) sub layer provides services to the session management (SM) entity and to the SMS support entity for message transfer.GMM procedures can be of two types:▪ GMM common procedures;▪ GMM-specific procedures.

Core Network Protocols : Circuit-Switched Core Network Call Control (CC)

A CC entity supports the following elementary procedures:▪ Call-establishment procedures;▪ Call-clearing procedures;▪ Call-information-phase procedures;▪ Miscellaneous procedures.

Core Network Protocols : Circuit-Switched Core Network Supplementary Services (SS)

Supplementary services (SS) are value-added services that may or may not be provided by the network operator.

Short Message Service (SMS)The purpose of the SMS is to provide a means to transfer short text messages between a UE and a short message service center (SMSC). These messages are sent using the control signaling resources, and their maximum length can be only 160 characters.

Core Network Protocols : Packet-Switched Core Network Session Management (SM)

The main function of the SM protocol is to support packet data protocol (PDP) context handling of the user terminal.there is no “connection” concept in a (IP) packet-switched system.

GPRS Short Message Service Support (GSMS)

The GPRS Short Message Service (GSMS) protocol task handles the SMS service while the UE is attached to the GPRS system. This protocol is an extension of the circuit-switched SMS protocol.

User Plane

MAC and RLC are in both the planes. PDCP and BMC exist only in user

plane. The U-plane is responsible for the

transfer of user data, such as voice or application data, whereas the C-plane handles the control signaling and the overall resource management

Packet Data Convergence Protocol (PDCP) The functions the PDCP perform

following: Header compression and

decompression of IP data streams; Transfer of user data; Maintenance of PDCP sequence

numbering; It uses the service provided by a

lower layer called Radio Link Control (RLC).

Broadcast/Multicast Control (BMC) Broadcast/multicast control is a layer 2

sub-layer that exists only in the U-plane. This layer handles only downlink

broadcast/multicast transmission. The functions of BMC are:

▪ Storage of cell broadcast messages;▪ Traffic-volume-monitoring and radio resource

requests for CBS;▪ Scheduling of BMC messages;▪ Transmission of BMC messages to UEs;▪ Delivery of cell broadcast messages to the upper

layer

Queries