3G Air Interface

Feb 1999 IMT-2000 Air Interface 1

UMTS’s W-CDMA air interface for IMT-2000

• Introduction

• Physical Channels and structures

• Spreading and Modulation

• Channel Coding, Multiplexing and Rate Matching

• Multiple Access

• Asynchronous Base-station Operation

• RLC and MAC Layers

• Mode of Operation

• Reference: Sept 1998, IEEE Commun. Magazine


Introduction

• ETSI’s Special Mobile Group is responsible for UMTS standardization. It has selected the basic technology for UTRA (UMTS Terrestrial Radio Access)– 1920 - 1980 MHz, 2110 - 2170 MHz paired band will use WCDMA in

FDD operation– 35 MHz unpaired band will use TD-CDMA in TDD operation

• ARIB (Association of Radio Industry and Business) and ETSI are working towards a common proposal

• Requirement of IMT-2000 is 384 kb/s for full area and 2 Mb/s for local area access

• GSM evolution to IMT-2000– EDGE use high level modulation for a 200 kHz TDMA migration from

existing bands in small spectrum chunks– UMTS is based on 5 MHz WCDMA– Reuse GSM’s mobility support, authentication, circuit switching


Physical layer (Basic Radio Parameters)

• UTRA/FDD is based on 4.096 Mchips/s in 5 MHz band. 8.192 and 16.384 Mchips/s are also specified for future evolution to >2 Mb/s data rate.

• 10 ms radio frame for low delay speech and fast control message

• Carriers are on a 200 kHz carrier grid with spacing 4.2 - 5 MHz. Larger spacing is needed in different cell layers


Physical Layer (Transport Channels)

• Transport channels are PHY services offered to higher layers. • Always unidirectional, either common or dedicated• Broadcast control channel (BCCH) -- downlink channel for

broadcasting system and cell specific control information• Forward Access Channel (FACH) -- downlink common channel to

entire cell or part of cell using adaptive antenna array• Paging Channel (PCH) -- downlink common channel to mobiles whose

locations are not known• Random Access Channel (RACH) -- uplink common• Dedicated Channel (DCH) -- down or up link dedicated channel to and

from a mobile station• Data are organized into transport blocks. Different channels have

different transmission time interval ranging from 10 to 80 ms


Table 1. Transport-channel formats for some typical service cases

• Service //Transmission-time interval// blocks per tran. Interval//block size

• Variable-rate speech 10 or 20 ms Fixed (=1) Variable

• Packet data 1080 ms Variable Fixed (~300 bits)

• Circuit-switched data 1080 ms Fixed (>=1) Fixed


Physical channel structuredownlink dedicated channel frame structure

•


Uplink dedicated channel frame structure


Physical Channel Structure

• 10 ms split into 16 slots of size 0.625 ms (one power control period)• downlink dedicated channel

– layer 2 data is time multiplexed with layer 1 control information within one slot

– pilot bits are for downlink channel estimation – TPC (Transmit power control) bits are for power control

commands for uplink close-loop power control– TFI (Transport Format Indicator) gives the rate information, block

size and number of blocks of each transport channel multiplexed on the physical channel

– data bit per slot ranges from 20 - 1280 corresponding to 32 - 2048 kb/s

– multiple downlink channels can be transmitted in parallel for still higher rates


Physical Channel Structure

• Uplink data and control channel

– 10 - 640 bits per slot corresponding to 16 - 1024 kb/s

– use multiple channels in parallel to achieve higher rates

– layer 2 data and layer 1 control are transmitted in parallel on different physical channels


Downlink Spreading and Modulation


Spreading and Modulation

• Downlink dedicated channel (fig 6)

– QPSK

– use different channelization codes called OVSF (Orthogonal Variable Spreading Factor) codes. Can support different channelbit rates

– Scrambling code -- 512 codes divided into 32 groups of 16 codes

• Uplink dedicated channel (fig 7) -- use dual channel QPSK

• Downlink common channel -- same as dedicated channel except that TIF and power control are not present


Uplink spreading and modulation


Transport channel Coding and multiplexing


Channel coding and Rate Matching

• Channel Coding– 1/3 con. coding for low delay and BER < 10E-3 service

– 1/3 con. Coding and outer RS coding plus interleaving for high quality service BER <10 E-6

– Turbo codes are being considered for use

• Rate Matching– for matching the bit rate of the CC-TrCh (Coded Composite Transport

Channel) to one of the bit rates supported on the up or downlink physical channel.

– Static rate matching and dynamic rate matching


Random Access

• Slotted Aloha with access slot width 1.25 ms

• preamble part consists of a length-16 complex symbol sequence, the random access signature, spread by a cell-specific preamble code of 256 chips

• message part is split into data and control parts(pilot + TFI). With TFI variable rate transmission is supported.

• idle part of 0.25 ms is for preamble detection and preparation for online processing of the subsequent message part.

• Random Access protocol -- MS selects spreading codes for preamble and message parts and also spreading factor. Randomly select the signature and access slot. Estimate the downlink path loss and and set the required uplink power for the RA burst. Send the burst. Wait for acknowledgment on FACH. Retransmit if necessary.


Structure of the Random Access Burst


Asynchronous Base-station Operation

• For easy deployment, tight inter BS synchronization is not required

• affect cell search and soft handoff synchronization

• For cell search by the Mobiles, each base-station transmits 2 syn signals per slot. Primary syn code is for the mobile to detect slot times, while the secondary syn codes are for frame timing and long-code group (scrambling code) acquisition.


RLC and MAC Layers

• The Network-PDUs are first segmented into Link Access Control LAC-PDU. The LAC overhead consists of a service access point identifier, sequence number for high level ARQ and others.

• LAC-PDU segment into RLC-PDU which forms the transport blocks. Each header consists of seq number for low level ARQ and CRC.

• Each RLC-PDU has the the same size. But the transmission rate may change every 10 ms leading to variable number of RLC-PDU per frame.

• Three services :– Packet data service

– real-time service

– mixed service


Segmentation and transformation of network layer protocol data units


Mode of Operation

• Packet Date Service– For short and infrequent packets and when the network traffic is light, the

MS RRC (Radio Resource Control) may transmit layer 3 packets on RACH

– The second method is to make reservation on RACH. The BS then sends a resource allocation message on the FACH. It indicates the transport format and the transmission time on DCH for the MS to send its packet.

– The third method is for MS with a dedicated channel (DCH) available. Link maintenance is performed between packet transmissions. Large transmission may be preceded by a capacity request message.


Mode of Operation (con.)

• Real-Time Service– MS issues a resource request message on the RACH, receive a

resource allocation message, immediate transmission on the assigned DCH

– direct txn on DCH if one is already available

• Mixed Service– The PHY layer is capable of multiplexing bit streams from

different services. MAC controls the multiplexing.

– MAC allocates multiple transport formats. MS use these formats subject to output power/rate threshold constraints.

3G Air Interface

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