Burts-by-burst (BbB) adaptive high-speed down-link packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER. Following a basic portrayal of adaptive OFDM, the paper investigates a combined system constituted by a constant-power adaptive modem employing space-time coded diversity techniques in the context of both OFDM and multicarrier code division multiple access (MC-CDMA). The combined system is configured to produce a constant uncoded BER and exhibits virtually error-free performance when a turbo convolutional code is concatenated with a space-time block code. Again, it was found that the advantage of rendering the multiple-input multiple-output (MIMO)-aided modem BbB-adaptive erodes, when the affordable system complexity facilitates the implementation of multiple transmitters and receivers, if it can be ensured that no shadowing and no multiuser interference fluctuation is experienced. By contrast, in the presence of these impairments, only BbB-adaptive transceivers can provide a near-constant BER.
['Lajos Hanzo', 'Byoung-Jo Choi']
Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems