Packet service in UMTS: delay- throughput performance of the downlink shared channel Flaminio Borgonovo, Antonio Capone, Matteo Cesana, Luigi Fratta

Packet service in UMTS: delay-throughput performance of the

downlink shared channel

Flaminio Borgonovo, Antonio Capone, Matteo Cesana, Luigi Fratta

2

1. Introduction

In the last ten years: IP applications has pushed the data traffic to grow

quickly 2G cellular systems have heavily changed the

way in which users access the network. The challenge of third generation mobile

communication systems is to provide access for a wide range of multimedia applications and services.

3

1. Introduction

UMTS: 3G mobile communication system developed by ETSI, extend the present GSM service to include multimedia.

UMTS provides great flexibility and a variety of different physical and logical channel types. Several user rates and protections are possible

by choosing suitable parameters. More implementation complexity.

4

2. UTRA basics (1/3) Two access scheme for the radio interface:

W-CDMA scheme 60MHz for downlink and 60MHz for uplink 3,84 Mchips/s, 5MHz for each channel, QPSK modulation

TD-CDMA scheme 35MHz for downlink and uplink

Physical channels are defined by the associated spreading and scrambling codes. Spreading sequence :

input data XORXOR spreading code XOR XOR scrambling code Spreading code of CDMA: channelization code Scrambling code of CDMA: PN code

5

2. UTRA basics (2/3)

Transport channels: Dedicated channels (DCH)

Devoted to the connection between a single mobile station and the UTRA Network.

Mapped into two physical channel :DPDCH, DPCCH Common transport channel

Broadcast channel (BCH), paging channel (PCH) Random access channel (RACH), forward access

channel (FACH): control information or packet Common packet channel (CPCH), downlink shared

channel (DSCH) : packet only

6

2. UTRA basics (3/3)

Power control DCH: TPC (Transmit power control) symbols in

each slot carry a command for increasing or decreasing

DSCH: computed on the basis of the power of the DCH

7

3. Downlink packet data services (1/3)

Three channels for downlink direction DCH:

assigned to single users through set-up and tear down procedures, subject to closed loop power control and service such as voice.

DSCH No set-up, tear down procedure Doesn’t carry power control signaling, but must have an

associated active DCH FACH

Shared by many users to transmit short bursts of data No DCH must be activated

8


For real-time circuit traffic—DCH Well known results show that CDMA with closed-l

oop power control is very effective.[14] Efficiency can be further enhanced by using powe

rful FEC codes. [15] For packet service—DSCH

Due to the burstiness, the number of interfering channels, its power level, errors can be more efficiently obviated by ARQ than FEC[16][17]

9


it is interesting to investigate whether UMTS achieves the highest data throughput with circuit or packet switching technology For circuit switching, the additional of any further channel

beyond the capacity cannot be accepted since the BER will increase

For packet switching, occasional increases in BER over its target value can be to tolerated Because of the use of ARQ techniques

This paper provide a quantitative evaluation of the performance of the different alternatives.

10

4. Simulator description (1/4)

Propagation model The received power Pr is given by

The path loss L is expressed as

Each cell is assigned a signal tree of orthogonal variable spreading factors, so that channels in the same cell are always orthogonal

2 1010r tP P L

10log (128.1 37.6 log )( )L r dB

11


Traffic model The performance of the UMTS downlink heavily

depends on the input traffic characteristics. Users become active according to a Poisson point

process of intensity λ

12


Receiver model Carrier to interference ratio:

Block error rate: Assumes an ideal ARQ procedure

When system operates far from capacity: Increase powerIncrease power on the same channel to maintain SIR RetransmittingRetransmitting the packets.

maximum interference tolerable is attained with a channel traffic G less than 1, and a further increase in retransmission would causes a strong decrease in throughput

int int

r

ra er N

PC

I I I P

1 (1 )lBLER BER

13


Power control model Closed loop power control mechanism

Inner loop: controls the transmitted power to maintain the SIR at target value

Outer loop: controls the SIR to provide a target BLER→provide different qualities to different services.

Each channel cannot exceed a transmitted power of 30 dBm, whereas the overall power transmitted by a BS is limited to 43 dBm. SIR Proportionally reduced

14

5. Simulation results (1/4)

Effect of codes-1 Channel codes : Convolutional codes

The encoded bits depend not only on the current k input data bits but also on past input bits

Coding rate:

Decoding strategy for convolutional codes: based on Viterbi algorithm

input information bits

output encoded bitsR

15


Effect of codes-1

Light codes

Heavier code

16


Effect of codes-2

17


Effect of codes-3

Wrongly designed:

Require very low interference

Reach 1, but throughput is limited

G: 0.955~0.97

18


Effect of user traffic on downlink shared channel If the amount of information is lower than the space av

ailable, the efficiency is reduced due to the unused space.

Consider sources that generate an average number of packets Np in the range from 1 to 25…

19


Effect of user traffic on downlink shared channel

20


Effect of user traffic on downlink shared channel

21


Effect of control channels Too many users would reduce the system through

put. Limit the number of DCHs. (User arrived system is qu

eued and wait for DCH availability) Power control commands indicate changes in the

transmitted power level Both the power PDCH and PDSCH must change in the sa

me way. The SIR achieved after despreading on the two chann

els are related as:

, ,DSCH DSCH DCH DSCH

DCH DCH DSCH DCH

SIR SF I P CR R SIR SF

SIR SF I P I

22


Effect of control channels Interference generated by the related DSCH:

(loss-of-orthogonality factor = 0.4 [20])

Assume

0.4DSCHP

DSCH BI I;

( ) 0.4 0.4DSCHDCH DSCH DSCH DSCH

DSCH

SIRI P I

SF

0.4 1 0.4DSCH DSCHDCH B DSCH B DSCH DSCH

DSCH DSCH

SIR SIRI I I I I I

SF SF

;

0.4DSCH DSCH DSCH

DCH DCH

SIR SF SIRR

SIR SF

23


24


Effect of control channels Trade off: interference and multiplexing effect SF, coding rate, =>maximum user number

minimum delay

25


Effect of power control The closed-loop power control: introduced to incre

ase the system capacity The burstiness of data transmission may jeopardize t

he gain achieved. DCH introduce additional interference Thus DSCH is compared with FACH.

26


Effect of power control

27

6. Conclusions (1/2)

Present some preliminary results obtained by simulation on the delay-throughput curves Focus on the system parameters and channel

configurations Closed-loop power control mechanism

SIR increases as the speed of the physical channel increases

It has been verified the mechanism is very efficient even with low interference protection

If multiple physical channels is allowed Intra-cell interference is improved by the closed-loop power

control mechanism

28


When the system operates with the highest bearable interference A backoff mechanism is crucial to let the system operate cl

ose to capacity. The use of DCH for power control with DSCH may lead to i

nstability if the number of DCH is not limited.

If low speed users are served A great reduction of capacity may be observed because of

the minimum unit that a single user may use The reduced frame filling degree does not reduce the interf

erence, but yielding a net decrease in throughput

29


In spite of the several limitations of packet switching, due to its intrinsic flexibility, better adapts to interference limited systems than circuit switching.

30

Reference

[14]. Erlang Capacity of a power controlled CDMA system (1993)

[15]. Throughput analysis for Code Division Multiple Access of the Spread Spectrum Channel (1984)

[16]. Channels with block interference

[17]. Retransmissions versus FEC plus interleaving for real-time applications: a comparison between CDMA and MCD-TDMA cellular systems (1999)

[20].3rd Generation Partnership Project, RF system scenarios, 3G TR 25.942, Dic. 1999

31

Normalized energy per information:0

1

2bE C

SFN R I

Documents

Packet service in UMTS: delay- throughput performance of the downlink shared channel Flaminio Borgonovo, Antonio Capone, Matteo Cesana, Luigi Fratta