The diversity is probably one of the most well-investigated topics in the wireless communications literature. Using the idea of independence of fading signals, different diversity approaches are briefly discussed next.
2.7.1 Time Diversity
The fast fading channels can also be regarded as ergodic process [2] and hence, there is a natural time diversity that can be exploited to improve the reliability of signal transmission. By simply transmitting multiple copies of the information over L symbol periods, Lthorder diversity can be achieved. When the channel is slowly fading, to fully exploit the time diversity, the information should be transmitted over several channel coherence period or average fade durations. A well known approach to exploits time diversity in such channels is to perform coding and interleaving at the transmitter and perform reverse operation at the receiver. Provided that the interleaving depth is sufficiently long to cover L coherence period, full L-order diversity is achieved.
2.7.2 Frequency Diversity
High speed data transmission over the fading multipath channels are often characterized with mul- tiple impulse response within the signal bandwidth. The frequency selective fading behavior of such channels can be exploited to provide frequency diversity. This is dependent upon the measure of correlatedness of channels over different frequency bands. For example, multiple tap equalizers using techniques such as zero-forcing, MMSE [2] can used to invert the effects of the channels to obtain the diversity. In wideband transmissions such as CDMA, multiple delayed copies of the arriving transmitted data can be resolved and coherently combined at the RAKE receiver to give frequency diversity [5]. The use of RAKE receiver allows to gain some frequency diversity by favorably exploiting the multipath signals for the downlink and uplink CDMA. However, as the number of multipath increases their effect on the basestation receiver that has to simultaneously detect large number of users degrades drastically with each additional multipath [65]. This is be- cause each multipath separated by more than a single chip period potentially becomes an interferer if not resolved.
Figure 2.12: The effect of channel delay spread on wideband transmission signals: frequency domain view
The multicarrier or OFDM based transmission effectively solves this problem by reducing the chip rate and transmitting the slower rate information over many subcarriers simultaneously. As an example the frequency response of OFDM transmission using 64 subcarriers for different ratio of channel delay spread to OFDM symbol duration (Td/Tb) is shown in Figure 2.12. The benefit
of this approach is that multipath effects only a single chip signal rendering the fading to be flat on each subcarrier so that low complexity discrete Fourier transform can be applied to recover the signals from the subcarriers. The technique that combines CDMA spreading and OFDM technique
are broadly called as Multicarrier CDMA (MC-CDMA) [69], [70], [71]. To exploit the frequency diversity of wideband CDMA with multicarrier transmission different subcarrier combining tech- niques such as MRC, EGC in the uplink and also MMSE Combining (MMSEC) in the downlink are investigated in the literature [69], [72]. The performance of uplink MC-CDMA is dependent on different system parameters such as: Receiver combining methods, type of sequences, subcarrier fading correlation, system loading, nearfar condition etc. To address the performance limitations of existing receiver approaches such as MRC, EGC, this thesis also proposes a novel blind adaptive subcarrier combining receiver technique in Section 3.4. The blind approach has shown to signif- icantly improve the frequency diversity gains from MC-CDMA transmission and allows much higher number of users to share the same system bandwidth for the same target performance.
2.7.3 Space Diversity
It is shown earlier in (2.14) that, if transmission of information can be carried out over multiple suf- ficiently separated antenna channels, the information signals experience independent fading. With the use of channel knowledge, the signals can be coherently combined to significantly decrease the probability of deep fade of the resulting combined signal. The spatially separated diversity channels can be obtained either at the transmitter or at the receiver. The use of multiple receive antennas and combining their signals provides the diversity gain as well as so-called array gain as shown in Figure 2.13. Conversely, the transmitter diversity based scheme requires special coding techniques to avoid the received signals to interfere each other. With the use of space-time codes [73], it has been demonstrated that full L-order diversity can be achieved with L transmit antennas without extra time, frequency or power requirements.
Among the diversity schemes described above, the attractive property of the space diversity scheme is that, it offers the diversity gain at no cost of bandwidth expansion. The use of multiple antennas to improve the capacity of multiuser wireless communications has been pioneered by Winters [2] in 1980s. Since then, there has been tremendous research interests recently on ex- ploiting the spatial dimension for both improving the diversity and transmission rates under the research theme of MIMO Spatial Multiplexing [32], [74]. Another stream of research works have been focused at the practical aspects of achieving the spatial diversity gain in multiuser settings by employing technique called ‘User Cooperation Diversity’, initially proposed by Sendonaris et.al. in [7]. The elements of this new technique and recent developments are briefly reviewed in the next subsection. Where, the motivations for our low complexity user cooperation techniques with successive interference cancellation for a practical uplink of CDMA is also discussed.
Figure 2.13: Different approaches for achieving spatial diversity