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Power Control and Rate Adaptation in WCDMA

By Olufunmilola Awoniyi

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Contents

Overview of WCDMAPaper summary - GoalSystem Model and Assumptions Approach Simulation Results Comments

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WCDMA

Third generation wireless systems designed to fulfill the “communication to anybody, anywhere, anytime” vision.Support voice, streaming video, high speed data. Spread spectrum systems with spread bandwidth of >=5MHzSupport multirate services by using spreading codesDifferent versions of WCDMA – check for names of standards

- Europe - UMTS (asynch). - Japan - Core-A (asynch) - Korea - TTA (I & II) (TTA I – synch, TTA II – asynch) - US - CDMA2000 (synch) - ITU - IMT-2000

*ARIB – Association of Radio Industries and Businesses *ETSI – European Telecommunications Standardization Institute *IMT- 2000 – International Mobile Telecommunications 2000 *ITU - International Telecommunication union *TIA – Telecommunication Industry Association*TTA – Telecommunication Technology Association *UMTS - Universal Mobile Telecommunications System

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WCDMA Standards

IMT-2000 proposal

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Features of the WCDMA Bandwidth 5, 10, 20 MHz

Spreading codes Orthogonal variable spreading factor (OVSF) SF: 4-256

Scrambling codesDL- Gold sequences. (len-18)UL- Gold/Kasami sequences (len-41)

Data ModulationDL - QPSKUL - BPSK

Data rates 144 kbps, 384 kbps, 2 Mbps

Duplexing FDD

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UL and DL Spreading

Downlink Transmitter Design

Uplink Transmitter Design

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Paper Summary

“Power and rate allocation in multirate wideband CDMA system” by J.W Mark and S. Zhut ( University of Waterloo)

Goal – Develop a power distribution law the IMT-2000 WCDMA system so that the QOS requirements are met and transmit power is minimized.

Conclusion – - Power adaptation is a function of spread bandwidth, data

rates and QOS requirements. - The closer the demand for resource is to the available

resource, the higher the required transmit power.

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System Model

Uplink transmissions in a single cell – bottle-neck for capacity M users in the cellNumber of channels for user j is Kj where Kj L

Channel – AWGN, denoted by nj for the jth user

Total Interference (Itj) = Thermal noise + MAI – Gaussian QOS elements have factored in fading and shadowing effects – specified in terms of SIR (BER), j,, such that

with data rates Rbj, where

Total transmit power required (to transmit over Kj channels) for user j is Sj

Each user have a traffic demand, j, and a normalized traffic demand, j.

* MAI – Multiple access interference

jjK,,

j2,j1j α

jbjK

,,bj2

,bj1bj

RRR R

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System Model - Equations can be written in SIR terms as,

such that the required transmit power is

Therefore, Sj can be define as

with a normalized traffic demand defined as

Total interference is

* W – Spread bandwidth

Rbj1, j1

Rbj2, j2

. . . RbjKj, jKj

OVSF code 2

OVSF code Kj

W

OVSF code 1

WtI

bRS α

WtI

bR

S

0Ib

E

tjI'jbjW

1j

S αR

jW1

jbjW1

jΓ αR

jn

M

ji1,i iS

tjI

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Approach (1)

If S = [S1, S2,…,SM ]’, with some manipulation, such that

Perron-Frobenius Theorem – p has positive eigenvalue, equal to the spectral radius and if < 1, the solution is non- negative. Example - M = 2

- By solving the characteristic polynomial, det[p- IM] = 0 - 1= 2 = , n1 = n2 = n (uniform traffic demands and noise)

Observations - - For any power distribution, traffic demand is upper bounded

by spread bandwidth. - The higher the noise or the closer the traffic demands are

to W, the higher the required transmit power.

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Γ1

Γλ

,

Wn

1,2S

,nΓSΓDs

W

nΓ1

PΓ

MIS

D

W21

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Approach (2)Limiting case – Ignore n for each user and minimize transmit power

- By solving for a non-trivial solution, for uniform traffic demands,

therefore, – (necessary condition for convergence - 1) and

Observation - All users transmit the same power and raise the transmit power

until interference can be ignored

1M1Γ

1MW

0SΓs

M

1i iS

M1

jS

1MtjI

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Approach (3)General case - If Sj is such that

Therefore,

Consequently,

– (necessary condition for convergence - 2)

η1η

jn

M

ji1,i iS

jS

jΓ

M

1i iSη

jS

M

η1ηMi

1ii

Γ

η1ηWM

1Mi

1ii

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Admission policy The conditions sufficient for convergence will used to accept or reject a request for connection in the admission controller.

1) For all s (for users already connected and those

requesting), calculate E() and Var() such that

2) Admission policy – - Admit -

- Reject -

- Admit light traffic demand - and

ΓEMΓVarη 3

η1ηW M

1Mi

1ii

1M

MW M

1i i

1MMW M

1i i

η1ηW M

1Mi

1ii

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Simulation Results The higher the variation in the normalized traffic demand, the looser the bound and the higher the capacity. Uniform traffic achieves the minimum capacity.At M , the variation in traffic becomes less significant and the distribution of the traffic demand looks uniform. Admission of a new call can lead to other users having to change their transmit power to achieve their desired SIR values.

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CommentsWorst case scenario - When most users increase their transmit power to meet QOS constraints, the system blows up.

- Total traffic demand < 0.8W. - Better to have power constraints (average or total

power).

Multicell system - “Link Quality in SIR Based Power Control for UMTS CDMA system” by Oppermann et al.

Fading / ISI channel - “Adaptive Multicode CDMA for the uplink Throughput Maximization” by S.A Jafar and A. Goldsmith