Cdma Cellular System

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Reverse Link Capacity and Coverage Improvement for CDMA Cellular Systems

Using Polarization and Spatial Diversity

Levent Aydin, Eduardo Esteves and Roberto Padovani

Qualcomm Incorporated

5775 Morehouse Dr.

San Diego, CA 92121-1714

Abstract- This paper explores the benefits of increased

diversity order using spatially separated, polarized antennas at

the base station receiver of a CDMA cellular system. Field tests

are conducted in real mobile wireless environments. A

conventional dual diversity receiver is compared with one that

implements four-way diversity combining using spatially

separated cross-polarized antennas. Several measured quantities

are logged at the base station receiver and the mobile

transmitter during a packet data connection at fixed data rate.

Existing published results that explore polarization diversity

focus on correlation measurements between received signal

polarization components. The emphasis of the present study is to

quantify reverse link capacity and coverage improvements for a

CDMA cellular network as a result of exploiting polarization as

an additional source of diversity at the base station receiver.

I. INTRODUCTION

Mobile wireless channels often exhibit multiple reflections

and scattering of the radio signals along with random signal

strength variations due to fading. Several diversity

techniques have been studied and found practical use in many

communication systems in order to improve receiver

performance in fading channel environments. Among these,

spatial diversity has been commonly used at cellular base

station receivers. At a base station site the antenna elements

need to be well separated in order for their respective channel

fading processes to be uncorrelated. The distance required to

achieve this is a function of antenna height, carrier frequency

used and angle of arrival spread. It has been determined

through measurements that horizontally spaced antennas need

to be separated by 10 to 30 times the wavelength in order for

the correlation between antenna observations to be less than

0.7 [1]. For North American cellular band (825-850 MHz)

this distance corresponds to a range of 4 to 11 meters.

Therefore higher order spatial diversity at base station sites

has not been popular because of the additional cost associated

with strict zoning requirements.

Polarization, as a source of diversity, has been studied as

early as 1972 but has not become popular until recently [2]-

[6]. Multiple reflections and scattering caused by the radio

environment between a mobile transmitter and the base

station antennas form a mechanism of decorrelation. In

general, the reflection properties that apply to each

polarization component is different. This gives rise to

different random phase changes for each component. Even if

the transmitted polarization is truly vertical, after a random

number of reflections it is conceivable that the received

polarization along with the random phase of each

observation will be uncorrelated.

Considering the variety of radio propagation environments

along with mobility it is difficult to establish a theoretical

framework to study the diversity provided by polarization. As

a result, studies published so far consist of channel sounding

using a set of polarized antennas and measurement of

correlation between the different components of polarization.

Any two orthogonal base polarizations should be sufficient to

resolve the received wavefront on a plane of base station

antenna apertures. While it seems natural to use vertical and

horizontal directions as basis there is a dependence between

mean branch signal