Value of adaptive transmission and effect of imperfect decision feedback in practical wireless systems.

摘要

High-performance wireless communication system design is often aided by information-theoretical results that shed useful insights into the design strategy of real systems. In some cases, however, the gap between the practical system and the theoretical model requires careful interpretation of such intuitions. The main theme of this dissertation is to demonstrate two such examples in a single-user communication link; the value of adaptive transmission in a frequency selective fading channel and the difficulty of decision-feedback receiver processing for a multi-antenna fading channel.;The first part of the thesis exemplifies the importance of channel feedback to the transmitter in a frequency-selective fading channel where adaptive modulation and coding is proposed as a method for bit-interleaved coded OFDM packet transmission. The total data rate is maximized by optimally selecting the code and efficiently allocating rate and power over the frequency band. In the second part of the thesis, the impact of imperfect decision feedback in a coded multi-antenna system is investigated. For a practical low-complexity coded system the error performance of vertical Bell Laboratories Space-Time Architecture (V-BLAST), a well-known decision-feedback-based receiver method, is shown to be even worse than simple linear receivers. Through a simplified error-propagation model, it is shown that the coding scheme that is matched to the perfect cancellation cannot overcome the probability of incorrect decision of V-BLAST. Therefore, decision feedback is beneficial only if decisions are "correct" at the error-probability level of the coding used. Finally, the minimum mean square error design of decision feedback processing in the presence of decision errors is proposed, and its asymptotic convergence to linear and V-BLAST receivers is discussed.