INVITED PAPER

IETE Technical Review
Vol 23, No 4, July-August 2006, pp 247-252

The Road to Multi-Carrier CDMA

S B LANDE AND NIKHILESH GOEL
Kavikulguru Institute of technology and science, ramtek 441 106, India.
email : landeeed@yahoo.co.in ; nikhilesh_goel@yahoo.co.in

This paper presents an overview of the evolution of a new multiple access technique called the multicarrier Code Division multiple Access (MC-CDMA). We take a look at the basic techniques of Frequency division multiple Access (FDMA) and time Division multiple Access (TDMA) and explain the two schemes orthogonal Frequency Division multiplexing (oFDm) and Direct sequence CDMA (DS-
CDMA), the merger of which has led to MC-CDMA. Finally, a comparison is presented between the three proposed MC-CDMA techniques and their predecessor DS-CDMA and the superiority of MC-CDMA is exhibited through the simulation graphs of bit error rate (ber) Vs signal to Noise ratio (SNR) for MCCDMA and DS-CDMA when serving multiple users.


INTRODUCTION

Multiple access schemes are used to allow many simultaneous users to use the same fixed bandwidth radio spectrum. In any radio system, the bandwidth that is allocated to it is always limited. Sharing of the spectrum is required in order to increase
the user capacity of any wireless network. Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) were the three prevalent methods of sharing the available bandwidth to multiple users in wireless system. Active research for schemes that can accommodate a large number of users has given rise
to many extensions and hybrid techniques based on these methods. These include Orthogonal Frequency Division Multiplexing (OFDM), hybrid TDMA and FDMA systems, and Multi-Carrier CDMA systems. This paper presents an overview of the evolution of the recently proposed multicarrier modulation technique, developed through the merger of OFDM and CDMA, called the Multicarrier Code Division Multiple Access (MC-CDMA) system.


This paper is organised as follows: Section 1 briefly discusses the basic multiplexing techniques namely FDMA, TDMA and Direct Sequence (DS-) CDMA. Section 2 explains the working of the OFDM system while section 3 takes a look at Multicarrier CDMA and the three techniques for its implementation. The comparison between MC-CDMA and DS-CDMA is illustrated in section 4 and the conclusion is presented in section 5.

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Paper No 36-C; Copyright © 2006 by the IETE..

 

1. BASIC MULTIPLE ACCESS TECHNIQUES

1.1. Frequency Division Multiple Access (FDMA)

In Frequency Division Multiple Access (FDMA), the available bandwidth is subdivided into a number of narrower band channels. Each user is allocated a unique frequency band in which to transmit and receive on. The transmitted signal on each of the channels is continuous allowing analog transmissions. FDMA is used as the primary breakup of large allocated frequency bands and is used as part of most multichannel systems (Fig 1).

1.2. time Division Multiple Access (TDMA)

Fig 1 Division of available bandwidth into narrower band channels in FDMA

TDMA divides the available spectrum into multiple time slots, by giving each user a time slot in which they can transmit or receive. Figure 2 shows the division of time slots to allot a time slot to users in a round robin fashion, with each user being allotted one time slot per frame.

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