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.. |
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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. |