In the radio environment, transmitted signals arrive
at the receiver via a direct, unobstructed
path or via multiple paths from the reflection, diffraction
and scattering of surrounding
objects such as buildings and trees. This multipath
propagation causes the signal at the
receiver to distort and fade significantly, leading
to inter-symbol interference (ISI).
Spread spectrum mobile communication systems use RAKE
receivers to minimize these
communication errors resulting from multipath effects.
Ideally, the number of correlators in
the RAKE receiver should match the number of multipath
signals. In general, however the
number of multipath signals is unknown and consequently
RAKE receivers either contain an
excessive number of correlators or the receiver performs
sub optimally.
The aim of this thesis is to incorporate a new signal
detection technique within the RAKE
receiver of the Code Division Multiple Access (CDMA)
system, which is able to identify
every ‘important’ multipath in the unknown channel
using the Least Mean Square (LMS)
algorithm. This detection technique involves an activity
measure and an activity threshold. A
multipath signal whose activity measure exceeds the
threshold is deemed as an ‘important’
tap. The threshold has a theoretical basis, but may
be increased to reduce the number of
detected active multipath components resulting in
a lower computational cost.
Simulations demonstrate that the CDMA Detection Guided
RAKE Receiver has a significant
improvement in the signal-to-noise ratio (SNR), subsequently
leading to a lower bit error rate
than the current RAKE receiver with three correlators.
Index Terms – Activity measure, activity threshold,
asymptotic error, CDMA system, least
mean square (LMS) algorithm, multipath effects, RAKE
receiver.
Introduction
With the technology advancement in today’s society,
the ability to communicate with pn the move has evolved
remarkably. However, the transmission quality of the
signeteriorated due to the modernization of the urban
cities with skyscrapers and other manbstacles. This
results in the transmitted signal having to take multiple
paths before rea
he intended receiver. Through the multipath transmissions,
the signal is severely disnd attenuated. Methods have
to be developed to improve on the signal quality.
.1 Background
Code Division Multiple Access (CDMA) systems use the
spread spectrum technology a
RAKE receiver concept to minimize communication errors
resulting from multipath en general, the number of
multipath signals in the wireless channel is unknown
and di
o predict. The spread spectrum technology aims to
spread the information signal o
wider bandwidth to make jamming and interception more
difficult. A RAKE receiver aach arriving multipath
signal to be individually demodulated and then combined
to pr stronger and more accurate signal. he RAKE receiver
in the IS-95A CDMA system uses three correlators and
a searcher,
he TIA/EIA-95B CDMA system limits the number of correlators
in the RAKE receiix . The searcher receives pilot
signals for synchronizing the spreading code. Both
ofystems have a fix number of correlators and leads
to the RAKE receivers either contn excessive number
of correlators or that the receiver performs sub optimally.
.2 Motivation and Objectives of this thesis he ideal
approach is to match the number of multipath signals
with the numborrelators, but this would be a waste
of resources and add unnecessary expense
manufacture of the phone. This thesis aims to incorporate
a new signal detection tech
within the RAKE receiver, where the detection technique
is used to determine the numorrelators required for
demodulating the ‘important’ multipath signals. This
techninlike the method use in the current CDMA system,
which has a fix number of correespite the number of
multipath signals in the channel.
7
Qualcomm home page
Qualcomm, Inc. developed the CDMA technology and is
one of the leading providers of CDMA products. Its web
site provides corporate information as well as data
about CDMA and CDMA products.
The CDMA Development Group
Home page of the CDG (CDMA Development Group), an industry
consortium of companies developing CDMA products and
services. Here you'll find background information about
CDMA as well as updated news stories.
The CDMA Revolution
Technical overview of CDMA and how it differs from TDMA-based
systems.
Wireless Design Online
Excellent resource for those involved in the design
and development of wireless communications systems and
equipment. Includes news items, a Web resource center,
discussion forums, and buyer's guides.
