Adaptive digital transceiver for rayleigh fading channels

by Hughes, Jeffery R.

This thesis presents an adaptive modulation scheme using a Walsh-code

modulator. The Walsh-code modulator consists of a dynamic demultiplexer and

predetermined sets of orthogonal Walsh-Hadamard codes. The demultiplexer can demultiplex

the input bit stream into a maximum of thirty-two sub-streams. The Walsh-

Hadamard codes are used to spread the spectrum of the demultiplexed sub-streams.

The Walsh-code modulator produces a multi-level pulse amplitude modulated (PAM)

signal by summing the spread sub-streams. The number of levels contained in the

constellation of the PAM signal varies, based on the number of demultiplexed substreams.

The error rate performance of different constellations produced by the

Walsh-code modulator, differ when transmitted through a Rayleigh fading plus

additive white Gaussian noise (AWGN) channel. As the level of fading experienced

in the channel fluctuates, a desired level of error rate performance can be maintained

by switching constellations, while trying to use the maximum number of sub-streams,

or links, to transmit the data. Simulation results show the performance of several constellations when transmitted through an AWGN channel and a Rayleigh fading

plus AWGN channel. Also, mapping of the PAM signal onto a quadrature amplitude

modulation (QAM) constellation, in order to improve the performance of the system is

discussed. It is found that for a Rayleigh fading plus AWGN channel, at a given error

performance level, the amount of gain in signal-to-noise ratio (SNR), from the largest

constellation to the smallest constellation, is 8dB for the symbol error rate (SER) and

14dB for the bit error rate (BER).