Power Control in CDMA Cellular System

By,P. D. Aditya , Rishabh Agarwal

BE IV/IV ECEUniversity College Of Engineering,

Osmania University

Contents

Introduction

Power in Wireless Networks

Main disadvantage in CDMA

Capacity of CDMA System

Power Control Schemes

Introduction Code Division Multiple Access

Narrowband message signal multiplied with spreading signal

Unique Pseudo Random Noise code given to each user- orthogonal to other users

Correlation at receiver- Detection of specific code word

Narrowband Data Signal (Voice)

Pseudo Random code

Wideband Transmitted Signal

Spread Spectrum:

Hi

Hello

Orthogonality:

Transmission through same channel

PN Sequence of user X

PN Sequence of user Y

Correlation at receiver:

Hi

Hello

PN Sequence of user X

PN Sequence of user Y

No Interference ideally

Users in CDMA share same frequency band

CDMA is Interference limited

Increase in interference → Decrease in capacity of system

Power control → Interference reduced → Capacity increased with sufficient Quality of Service(QoS)

Facts:

Power in Wireless Networks

Path losses occur when signal propagates from transmitter to receiver

According to Free Space Propagation Loss(FSPL) theory,

S - power per unit area at distance dPt - total power transmitted

For an isotropic antenna

Pr-received power

FSPL = Pt/Pr

S = Pt( 1/4πd2)

Pr= S (λ2 /4π)

Received signal strength - Raleigh fading:

Disadvantage in CDMA

Users transmitting signals with same power strong signal from near end masks weak signal from far end.

Multipath

fading an

d shad

owing

Far mobile unit

Near mobile unitNear Far Effect

Forward Link:

Single Cell Multiple Cell

Reverse Link:

Single Cell Multiple Cell

Capacity in CDMA System

Forward link coherent demodulation by pilot carrier

Performance superior to reverse link

Reverse link non coherent reception, non coordinated, independent fading of all users.

Hence emphasis is made on reverse link capacity.

Capacity dependency on power:

RMS powerVar. (%)

RMS powerVar. dB

Reduction inUsers (%)

0 0 0

10 -0.4,+0.4 2

20 -1,+0.4 10

30 -1.5,+0.8 25

40 -2.2,+1.5 50

Sectorization

Uses Directional antennas with 1200 beam width

Figure shows the radiation pattern of a sector with 1200 beam width

Each site uses 3 such antennas

On an average, number of users can be increased by a factor of 5 to 6

Power Control Schemes

Open Loop Power Control

Closed Loop Power Control

Types

Open Loop Power Control System

Assume Mobile unit is nearer to Antenna

Hence, good signal will be received at base station

Antenna sends the signal with

reduced amplitude

Open Loop Power Control System

Assume Mobile unit is far from Antenna

Signal at antenna is weakAntenna sends the signal with increased amplitude

Closed Loop Power Control System

Antenna receives some power

Sends command signal based upon the received power

Adjusts the power based upon the command signal

About 1000 times per sec

When mobile is nearer to antenna:

Loses are less

Commands to Reduce the power

Continued until power is controlled at both the ends

When mobile is far from antenna:

Losses are more

Commands to increase the power

Conclusion

Users in CDMA use same frequency band Practically combination of open and closed loop

power control schemes is desired Power control Interference minimized

Capacity increased

References 1. Akinniyi, A.R., Lehnert, J.S. “Characterization of Noncoherent spread spectrum multiple access communications” IEEE transaction on communication. Vol. 42, No. 1, January, 1994.  2. Felhaure, T. Klein, A. and Baier, P.W. “A low cost method for CDMA and other application to separate non orthogonal signals” IEEE transaction on communication.  3. Grandhi.S “Centralized power control in cellular radio systems” Trans.On Vehicular tech. vol.42 No. 4, November, 1993.  4. www.cdmaonline.com   5. www.itr.unisa.edu.au/research/publications/thesis  6. www.cdmaonline.com  7. http://ieeexplore.ieee.org

Thank you

Any Queries