Interference Cancellation for DS/CDMA Systems in Flat Fading Channels
Licentiate thesis, 1996
This thesis investigates interference cancellation in direct-sequence code
division multiple access (DS/CDMA) systems that support multiple data rates.
Two approaches for implementing multiple data rates are considered. One
is the use of mixed modulation. The second is the use of parallel channels.
With mixed modulation we denote that each user chooses a modulation format,
e.g., BPSK, QPSK (4-QAM) or 16-QAM, according to the desired data rate.
Parallel channels are realized by assigning several spreading codes to
each user. This enables the transmission of several parallel data streams
that are mutually synchronous.
We introduce and analyse a novel approach that combines the above described
multiple data rate systems with interference cancellation (IC). The cancellation
in the receiver is performed successively on each user, starting with the
user received with the highest power. This procedure can in turn be iterated,
forming a multistage scheme, with the number of iterations set as a design
parameter. We will evaluate the performance of these two multiple data
rate schemes when used in combination with IC and compare them to the performance
of systems employing conventional detectors.
We first analyse IC for M-ary rectangular QAM systems and then apply
the results to mixed modulation systems and systems employing parallel
channels. Our analysis employs a Gaussian approximation for the distribution
of the interference, and it includes both the AWGN and the flat Rayleigh
fading channel. The systems ate also evaluated via computer simulations.
Our analysis and simulations indicate that the IC schemes used in mixed
modulation or parallel channel systems yield a performance close to the
single BPSK user bound and, consequently, give a prospect of a considerable
improvement in performance compared to systems employing a conventional
direct-sequence code division multiple access (DS/CDMA)
multiple data rates