Adaptive Multicarrier CDMA Space-Time Receivers - Adaptive Signal Processing in Wireless Communications - page 270

Amplitude

Time-domain spreading
Data bits +1+1-1
Spreading sequence -1+1-1+1-1+1-1+1

Frequency

T C

T MC

Time

MT-CDMA system
(c)

FIgure 10.1 (cont inued).

this characteristic is effective for establishing a quasi-synchronous channel. In [12],
Kondo and Milstein proposed a similar transmitter, except that band-limited subcar-
rier signals and larger subcarrier separation are employed. This scheme yields both fre-
quency diversity improvement and narrowband interference suppression. The spectrum
of the MC-DS-CDMA signal with 50% overlap having three subcarriers is shown in
Figure 10.1b . In the MC-DS-CDMA system, the subcarrier frequencies are usually cho-
sen to be orthogonal to each other after spreading, which can be formulated as

−= − ,

ni j

f f

i j

T

c

where n ∈  and T c is the chip duration. Therefore, the minimum spacing Δ between two
adjacent subcarriers satisfies 1/ T c .
Frequency diversity in MC-DS-CDMA systems can be achieved by repeating
the transmitted signal in the frequency (F) domain with the aid of several subcarri-
ers [12-14]. Alternatively, in MC-DS-CDMA systems the F-domain repetition can be
replaced by F-domain spreading [15] using a spreading code. One of the advantages of
using F-domain spreading instead of F-domain repetition in MC-DS-CDMA systems
is that frequency diversity can be achieved without reducing the maximum number of
users supported by the system [15, 16]. The MC-DS-CDMA scheme can provide the fol-
lowing advantages [13]. First, the spreading processing gain is increased compared to
the corresponding single-carrier DS-CDMA scheme. Second, the effect of multipath
interference is mitigated because of DS spreading. Third, frequency/time diversity can
be achieved. Finally, a longer chip duration may lead to more relaxed synchronization
schemes.