1. BER Vs Number of Users in MultiCarrier
CDMA simulated in AWGN Channel
Aditi, Santosh Nagaraj
Electrical and Computer Engineering , San Diego State University
Introduction
• In cellular mobile systems, the problem of multiple users
accessing the same resources can be solved by FDMA, TDMA,
CDMA or SSMA. The basic important parameter while selecting
a Multiple Access scheme is the number of users that can be
accommodated in the system with the available total bandwidth
and given channel conditions while maintaining the signal
quality for all users.
• MC-CDMA is an inflection method which uses multi-user
broadcast of Direct Sequence CDMA signals. MC-CDMA is
grouping of OFDM, a multicarrier modulation, which in this
project we are taking to be Binary Phase Shift Keying (BPSK),
and CDMA, an increase spectrum technique.
Project approach
• The project aims towards simulation and analysis of Multi-
Carrier CDMA signals in an AWGN channel.
• The project will involve generation of Walsh Codes from
Hadamard matrix. These codes are used for spreading the signal
and coding it for Code Division Multiple Access. These codes will
then be modulated using multicarrier OFDM system. These
signals will be passed through a channel affected by AWGN
noise.
• At the receiver, the BER of these signals would be computed
and compared with variation in number of users in the
system at a time.
Results
• BER for each user is calculated with increasing number of
Users in the system.
• MC-CDMA is a fairly good technique of communication as:
Reduces Intersymbol Interference (due to CDMA)
Mitigates frequency select fading (due to OFDM)
• BER is much lower for similar SNR values when the number of
users in the system is lower and also with more number of
users its of small value for low values of SNR.
• The benefit of MC-CDMA modulation scheme for mobile
communications is that for this scheme, the spreading code
chip rate is same as the data rate. In earlier used techniques,
i.e., DS-CDMA, the code chip rate was much higher than the
original data rate. This generated the necessity to synchronize
and track the spreading code at the receiver.
Figure 2 : BER performance 2, 8 and 16 Users
Discussions
• When the MATLAB code was implemented for the project, it
was observed that codes used for encoding the data are
required to be perfectly orthogonal. Hence, Walsh Codes are
used and are BPSK modulated.
• CDMA is used in the project since its not Dimension Limited and
hence more number of users can be easily added to the system.
• OFDM is more immune to Frequency Selective Fading and also
gives higher Bitrate for transmission for same available
Bandwidth
• At the receiver, the data is decoded and BER is computed by
increasing number of Users in the system and the results are
computed.
Concluding Remarks
• This project has presented and shown that The BER of MC-
CDMA with BPSK modulation on AWGN channel varies
directly with number of users .
• More users will require higher values of SNR to ensure a
system with minimum errors, the BER is relatively constant.
Reference
• Rohit Panwar, Sarvda Chauhan --BER Performance of MC-
CDMA Using with MSK Modulation on AWGN and Rayleigh
Channel
• Dinan, E.H.; Jabbari, B., "Spreading Codes for Direct
Sequence CDMA and Wideband CDMA Cellular Networks,"
IEEE Communications Magazine, Vol. 36 Issue 9, pp.5-54,
September 1998.
Figure 1 : MC-CDMA System (Transmitter and Receiver)