| Wireless LAN and many other wireless communication systems have adopted OFDM techniques to mitigate the multi-path problem. In this dissertation, we study two of the major issues involved in the applications of OFDM: (i) improving channel access efficiency with OFDM, and (ii) develop blind equalization techniques for OFDM with even insufficient cyclic prefix or ill-conditioned channels.; Firstly, we show that OFDM techniques can be exploited to assist contention resolution in random access wireless networks to greatly improve random access efficiency. A cross-layer design methodology is proposed, where both the OFDM-based physical-layer (PHY) signal processing and the MAC-layer packet scheduling are jointly designed and optimized. This way, contentions can be completely resolved, which enhances network throughput. More stringent QoS requirements may also be supported. In our design, the PHY-layer signal detection is assisted by the MAC-layer smart scheduling, whereas the latter is also enhanced by the former.; Secondly, a new blind equalization method is proposed for OFDM transmissions. Besides working in classical OFDM transmissions with sufficient cyclic prefix, it works even in OFDM transmissions when the cyclic prefix becomes insufficient. Further more, it works not only for good channels, but also for ill-conditioned channels. The new method uses constrained minimum-output-energy (MOE) optimization to derive a bank of linear equalizers. When the cyclic prefix is sufficient, the complexity of the method is comparable to the traditional FFT-based OFDM receivers. When the cyclic prefix is insufficient, the complexity of the new method is still linear, while the performance is exceptionally better than many other methods. Another important advantage of the proposed method is its robustness to ill-conditioned channels. While traditional FFT-based OFDM receivers suffer from performance loss or even complete failure on ill-conditioned channels, the proposed method still works. As a result, the proposed method provides an effective way to improve the bandwidth efficiency and the performance of OFDM transmissions. |
