以OFDM為基礎的無線區域網路分集接收器之性能研究

Abstract

在接收無線電波上,用分集法是大家所知對抗通道衰減效應很有效的一種技術.這篇論文是考慮以OFDM為基礎的無線區域網路分集接收器之性能評估的研究. 首先我們設計一個IEEE802.11a的無線區域網路系統的基頻接收機.我們特別考慮了訊號的同步架構,頻率偏移的估測和通道的估測.利用一具有空間性之通道模型,我們用設計的基頻接收機混合陣列天線來作模擬.結果證明,分集接收可以有效的提升系統的效能.然而,高頻模組和數位處理模組會因陣列天線裡的天線個數而增加.如此,分集接收會大大的增加成本.於是我們提出一個架構來減輕這問題.眾所周知,天線的成本遠低於高頻模組和數位處理模組.因此,我們可以藉著增加天線的數目,以減少高頻模組和數位處理模組.這架構在第一階使用一天線選擇器,接下來在後的是其他的分集技術,例如 最大比組合,頻段選擇或等比組合.廣泛的模擬結果證明,在很多的實例上,我們所提出的架構可以接近傳統分集架構的效能,而且可以降低實現的成本.

Diversity reception is known to be an effective technique combating the fading channel effect. This thesis considers performance evaluation for OFDM-based WLAN diversity receivers. We first are devoted to the baseband receiver design for an IEEE802.11a WLAN system. Specifically, we consider frame synchronization, frequency offset estimation, and channel estimation. Using a spatial channel model, the designed baseband receiver incorporated with an antenna array is then used for simulations. The results show that the system performance can be effectively enhanced by diversity reception. However, the RF as well as FFT modules will be increased by L times where L is the element number in the antenna array. Thus, implementation cost can be greatly increased. We then propose a structure to alleviate this problem. We take the advantage that the cost of antennas is much lower than that of RF and FFT modules. Thus, we can increase the number of antenna and at the same time reduce the number of RF and FFT modules. The proposed structure has antenna selection as the first stage followed by other diversity techniques such as maximal ration combining, tone selection, and equal gain combining. Extensive simulation results show that in many cases the proposed structure can approach the performance of conventional diversity schemes while reduce the implementation cost.