Wireless networks - Lecture 23: WCDMA (Part 3)

Wireless networks - Lecture 23: WCDMA (Part 3). The main topics covered in this chapter include: compressed mode measurements; handover measurements; WCDMA packet data access; transport channels for packet data; packet scheduling algorithms; transmission gap lengths (TGL);...

Wireless Networks

Lecture 23 WCDMA (Part III)

Dr Ghalib A Shah

 WCDMA packet data access

 Transport channels for packet data

► Common, dedicated, shared

 Packet scheduling algorithms

► Time division scheduling

► Code division scheduling

Last Lecture Review

 Spreading and Scrambling

Compressed Mode Measurements

 The compressed mode is needed when making

measurement from another frequency without full dual receiver terminal.

 The intention is not to loose data but to

compress

 The transmission and reception are halted for a

short time to perform measurements on the

other frequencies.

Compressed Mode Measurements

 Three methods for compressed mode:

► Lowering the data rate from higher layers because they have

knowledge of compressed mode schedule

► Increasing the data rate by changing the spreading factor

► Reducing the symbol rate by puncturing at the physical layer

multiplexing chain but limited to rather short Transmission Gap Lengths (TGL)

 More power is needed during compressed mode.

 No power control during compressed mode Large step

size is used after a compressed frame to allow the

power level to converge more quickly to the correct

value after the break.

Transmission Gap length (TGL)

 The specified TGL are 3,4,7,10 and 14 slots

► TGL lengths 3, 4 and 7 can be obtained with both single and

double frame methods

► TGL 10 or 14 only obtained with double frame method allowing

minimizing the impact during a single frame

► Very short values of TGL (1 or 2) is excluded the hardware

requires some time to switch to different frequency and not as much time for measurements

► Link performance does not degrade much if the terminal is not

at the cell edge since there is room to compensate with fast power control

TGL

Intra-Mode Handover

CPICH are

► Received Signal Code Power (RSCP): received power on one code

after de-spreading

► RSSI: wideband received power within channel b/w

► Ec/No, representing RSCP/RSSI

timing between the cells to allow coherent combining in the RAKE receiver, otherwise would be difficult to combine

► If cells are within 10ms window, the relative timing can be found from

primary scrambling code phase

► Otherwise terminals need to decode System Frame Number from

primary CCPCH that takes time and may suffer errors.

► The 10 ms window has no relevance when timing information provided

in neighboring cells list.

frequency measurements can be done with aid of compressed

Inter-Mode Handover

 Dual mode FDD-TDD terminals operating in

FDD measure power level from TDD cells

available

 The TDD CCPCH bursts sent twice during

10ms frame can be used for measurement.

 Since TDD cells are synchronized, finding one

slot means that other TDD cells have roughly same timing for their burst.

Inter-System handover

 Terminal receives GSM synch channel during

compressed frames in UTRA FDD.

 GSM 1800 set special requirements for

compressed mode

Packet Data Access

 Four basic types of traffic classes

► Conversational class -> real-time connection,

performed between human users, really low delay, nearly symmetric, e.g., speech

► Streaming class -> real-time connection, transferring

data as a steady and continuous, low delay, asymmetric, e.g., video

► Interactive class -> non-real-time packet data,

response requested from other end-user, reasonable round-trip delay, e.g., Web browsing

► Background class -> non-real-time packet data, no

immediate action expected, less sensitive to delivery

Types of Data Packet Traffic

 Packet data traffic is a non-real-time packet services

including Interactive and Background traffic classes

Their properties are

► Packet data is bursty Sometimes a large amount of data is

transferred At the other times no data is sent Thus, the required bit rate can change rapidly

► Packet data tolerates longer delay than real-time services It is

controllable traffic from the RNC; thus, RNC can decide when and how to send the data

► Packets can be transmitted by the radio link control layer which

provides retransmission and error correction services

Therefore, it allows high frame error rate with low transmission power

 One example of packet data traffic is ETSI packet data

model for web browsing.

 Characteristics of packet service session

► Session arrival process, number of packet calls per session, reading

time, number of packets within a call, inter-arrival time in a call, packet size

Packet service session

Reading time

Packet call

Time

WCDMA packet Access

 In WCDMA packet allocations, e.g., time and bit rate,

are controlled by the packet scheduler (PS) located in RNC PS functions include:

► Properly allocate the available resources (time, code or power)

between the packet data users

► Decide the allocated bit rates and the length of the allocation

► Decide to use the transport channel

► Monitor the packet allocations and the systems loads

 PS can allocate common, dedicated or shared

channels to packet data users It can also change the bit rate during active connection.

 PS can increase or decrease the network load by

increasing or decreasing the bit rates of the packet

bearers respectively.

Transport Channels for Data Packet Access

 Common channels - RACH in the uplink and

FACH in the downlink

► One or few RACH or FACH per sector

► Low setup time

► No feedback channel -> no fast closed loop power

control, no soft handover, use fixed power

► Poor link-level radio performance and generated

more interference

► Suitable for small data amounts

 Dedicated Channel - DCH in the uplink and

downlink

► Use fast power control and soft handover

► Longer setup time

► Up to 2 Mbps

► Suitable for large data amounts

► Not suitable for bursty data

► In case of changing bit rate in the downlink, the

downlink orthogonal code is reserved according to maximum bit rate.

 Shared Channel - uplink and downlink

► A single orthogonal code is shared with many packet

user with established DCH in time division manner - code efficient

► Fast allocation and rate modification

(frame-by-frame basis)

► Suitable for large data amounts and bursty data

► Use fast power control, but no soft handover

 In WCDMA packet scheduling algorithms can be done

in two ways, in a time or code division manner.

 Time division scheduling

► One user is allocated a channel at a time (10 ms frame)

► All available capacity can be allocated to that user

► High data rate for a short period of time

 Code division scheduling

► Many users are allocated the channels simultaneously

► Low data rate for a long period of time

► Increase more users, each user’s bit rate is decreased

Time Division Scheduling

Code Division Scheduling

Trans mis s ion Power­bas ed Scheduling

 Users close to the BS requires less

transmission power and can get a higher bit rate, whereas users at the cell edge could get lower bit rate

► Accurate power estimation

► Unfair resource allocation

 WCDMA packet data access

 Transport channels for packet data

► Common, dedicated, shared

 Packet scheduling algorithms

► Time division scheduling

► Code division scheduling

► Transmission Power-based scheduling

 Next Lecture

► cdma2000