Adaptive Multicarrier CDMA Space-Time Receivers - Adaptive Signal Processing in Wireless Communications - page 268

10.2 Overview of Multicarrier CDMA

for Wireless Communications

In this section, we review concisely the class of multicarrier CDMA schemes, which have
been discussed in the literature. Specifically, we discuss their parameters, spectral char-
acteristics, advantages, and disadvantages in terms of design and structure. For more
detailed information on the different multicarrier CDMA schemes, the reader is referred
to the excellent monographs by Hanzo et al. [3].
The multicarrier CDMA schemes are categorized mainly in two groups. One spreads
the original data stream using a given spreading code, and then modulates a different
subcarrier with each chip (i.e., the spreading is in the frequency domain), and the other
spreads the serial-to-parallel (S/P) converted data streams using a given spreading code,
and then modulates a different subcarrier with each of the data streams (spreading is in
the time domain).

10.2.1 Frequency-Domain Spreading Multicarrier CDMA:

FD-MC-CDMA

Figure 10.1a shows the spectrum of the multicarrier CDMA scheme associated with
frequency-domain spreading [7-10]. We refer to this scheme as FD-MC-CDMA. The
FD-MC-CDMA transmitter spreads the original data stream over N c subcarriers using
a given spreading code in the frequency domain. This scheme does not include serial-to-
parallel data conversion, and there exists no spreading modulation on each subcarrier.
Therefore, the data rate on each of the N c subcarriers is the same as the input data rate.
However, by spreading each data bit across all of the N c subcarriers, the fading effects
of multipath channels are mitigated. In this FD-MC-CDMA system, the subcarrier
frequencies are chosen to be orthogonal to each other, i.e., the subcarrier frequencies
satisfy the following condition:

−= − ,

ni j

f f

i

T

where n ∈  and T is the symbol duration. Therefore, the minimum spacing ∆ between
two adjacent subcarriers satisfies 1/ T , which is a widely used assumption [7-10] and is
also the case employed in Figure 10.1a. If no overlap is assumed, then the minimum
spacing ∆ between two adjacent subcarriers is 2/ T .
Yee et al. [7] have considered an FD-MC-CDMA system, in which the subcarriers'
frequency separation is higher than the coherence bandwidth of the channel, and there-
fore the individual subcarriers experience independent fading. As a result, the frequency
diversity is maximized. This is the main advantage of the FD-MC-CDMA scheme over
other multicarrier CDMA schemes [2]. However, this system may require a considerable
transmission bandwidth. Besides that, a large delay spread per subcarrier would lessen
this bandwidth requirement. But in a frequency-selective fading channel, different