Adaptive Multicarrier CDMA Space-Time Receivers - Adaptive Signal Processing in Wireless Communications - page 267

10.1 Motivation of the Chapter

One important challenge for future wireless networks is the design of appropriate trans-
ceivers that can reliably transmit high data rates at a high bandwidth efficiency. Multi-
carrier code division multiple access (MC-CDMA) systems in particular have received
considerable attention, because they have the attractive feature of high spectral efficiency
and because they can be easily implemented using fast Fourier transform (FFT) without
significantly increasing the transmitter and receiver complexities [1, 2].
The multicarrier systems include different combinations of multicarrier modula-
tion (accomplished by orthogonal frequency division multiplex [OFDM]) and direct-
sequence code division multiple access (DS-CDMA). This combination provides both
high-data-rate transmission and multiple access capabilities. An excellent overview of
the different multicarrier CDMA systems is found in [2] and [3]. They can be divided
into two categories of multicarrier CDMA: one combines multicarrier modulation with
frequency-domain spreading, and the other transmits several DS-CDMA waveforms
in parallel with the spreading operation performed in time. The transmitter proposed
here belongs to the second group, and it can be divided into MC-DS-CDMA and multi-
tone (MT) CDMA, the difference between the two being the subcarrier frequency
separation.
However promising, challenges remain before multicarrier CDMA can achieve its
full potential. One open area is the design of transceivers that will enable the future
upgrade of current wireless networks beyond the third generation (3G). Transceivers
selected for early implementation need to achieve high spectrum efficiency in realistic
propagation channels while being robust to imperfections such as time and frequency
mismatch. Multicarrier CDMA, similar to other multicarrier schemes, is sensitive to the
signal distortion generated by the imperfect frequency downconversion at the receiver
due to local oscillator frequency offset. It has been found that carrier frequency offset
(CFO) gives rise to a reduction of the useful signal power and to the intercarrier interfer-
ence (ICI) [5]. Furthermore, one of the major obstacles in detecting multicarrier CDMA
signals is interference. The multiple access interference (MAI) and the intersymbol
interference (ISI), which are inherited from conventional DS-CDMA, affect likewise the
performance of multicarrier CDMA systems.
The subject of this work is the design of a new adaptive multicarrier CDMA space-
time receiver that provides solutions to these problems, with particular emphasis on the
comparison of the MC-DS-CDMA- and MT-CDMA-air interface configurations. The
proposed receiver, named MC-ISR* (multicarrier interference subspace rejection), will
hence (1) perform blind channel identification and equalization as well as fast and accu-
rate joint synchronization in time and frequency, and (2) mitigate the full interference
effect. In addition, the assessment of this new receiver is oriented toward an implemen-
tation in a future, real-world wireless system.

* This work was presented in part at the IEEE SPAWC 2005 and IEEE ISSPA 2005 conferences and
accepted for publication in IEEE Transactions on Vehicular Technology [6].