14-08-2012, 04:39 PM
PERFORMANCE EVALUATION OF DS-CDMA SYSTEM USING MATLAB
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ABSTRACT
The author evaluated the performance of synchronous DS-CDMA systems over multipath fading channel and
AWGN Channel. The synchronous DS-CDMA system is well known for eliminating the effects of multiple access
interference (MAI) which limits the capacity and degrades the BER performance of the system. This paper
investigated the bit error rate (BER) performance of a synchronous DS-CDMA system over AWGN and
Rayleigh channel, which is affected by the different number of users, as well as different types spreading codes.
The promising simulation results explore the comparative study of different DS-CDMA system parameter and
showed the possibility of applying this system to the wideband channel. Different MATLAB functions and
MATLAB program segments are explained for the simulation of DS-CDMA system.
INTRODUCTION
Direct-sequence code-division multiple access (DS-CDMA) is currently the subject of much research
as it is a promising multiple access capability for third and fourth generations mobile communication
systems. Code-division multiple access (CDMA) is a technique whereby many users simultaneously
access a communication channel. The users of the system are identified at the base station by their
unique spreading code. The signal that is transmitted by any user consists of the user’s data that
modulates its spreading code, which in turn modulates a carrier. An example of such a modulation
scheme is quadrature phase shift keying (QPSK). In this paper, we introduce the Rayleigh channel and
AWGN Channel, and investigated the bit error rate (BER) performance of a synchronous DS-CDMA
system over these channels. In the DS-CDMA system, the narrowband message signal is multiplied
by a large bandwidth signal, which is called the spreading of a signal. The spreading signal is
generated by convolving a M-sequence & GOLD sequence code with a chip waveform whose
duration is much smaller than the symbol duration. All users in the system use the same carrier
frequency and may transmit simultaneously. The receiver performs a correlation operation to detect
the message addressed to a given user and the signals from other users appear as noise due to decorrelation.
The synchronous DS-CDMA system is presented for eliminating the effects of multiple
access interference (MAI) which limits the capacity and degrades the BER performance of the system.
MAI refers to the interference between different direct sequences users.
CHANNEL MODEL
Rayleigh fading channel Model:
Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal,
such as that used by wireless devices. Rayleigh fading models assume that the magnitude of a signal
that has passed through such a transmission medium will vary randomly, or fade, according to a
Rayleigh distribution the radial component of the sum of two uncorrelated Gaussian random
variables. [C.Trabelsi et.al]. Rayleigh fading is viewed as a reasonable model for tropospheric and
ionospheric signal propagation as well as the effect of heavily built-up urban environments on radio
signals. Rayleigh fading is most applicable when there is no dominant propagation along a line of
sight between the transmitter and receiver Rayleigh fading is a reasonable model when there are many
objects in the environment that scatter the radio signal before it arrives at the receiver, if there is
sufficiently much scatter, the channel impulse response will be well modelled as a Gaussian process
irrespective of the distribution of the individual components. If there is no dominant component to the
scatter, then such a process will have zero mean and phase evenly distributed between 0 and 2
radians. The envelope of the channel response will therefore be Rayleigh distributed [Theodore S.
Rappaport].
AWGN channel Model
Additive White Gaussian Noise channel model as the name indicate Gaussian noise get directly added
with the signal and information signal get converted into the noise in this model scattering and fading
of the information is not considered[Theodore S. Rappaport].
MODULATOR AND DEMODULATOR
A QPSK signal is generated by two BPSK signals. To distinguish the two signals, we use two
orthogonal carrier signals. One is given by cos2fct, and the other is given by sin2fct. A channel in
which cos2fct is used as a carrier signal is generally called an in-phase channel, or Ich, and a channel
in which sin2fct is used as a carrier signal is generally called a quadrature-phase channel, or Qch.
Therefore, dI(t) and dq(t) are the data in Ich and Qch, respectively. Modulation schemes that use Ich
and Qch are called quadrature modulation schemes.