IEEE 802.16a 分時雙工正交分頻多重進接之下行通道估測之研究與技術

Abstract

正交分頻技術近來因為能在行動環境中穩定高速傳輸而廣受注目，IEEE 802.16a 即是一個基於正交分頻多重進接技術用於無線區域網路和大都會網路的標準。 本論文主要在討論IEEE 802.16a下行通道估測中的兩個重點:內插的技巧以及使用時域的資料來做改善的方法。 我們使用最小平方差的估測器來估計在導訊上的通道頻率響應而不用線性平均最小平方差的估測器，除了因為硬體的計算方便，也因為我們不知道通道的統計特性而不能使用線性平均最小平方差的估測器。而內插的方法我們則研究了線性內插，二次式內插以其三次樣條函數內插（cubic spline）。而在用時域的資料改善的方法有下列四種：移動平均 (moving average)，指數平均 (exponential average)，最小平均平方差適應 (LMS adaptation）以其二維內插法。 我們的在靜態以及瑞雷通道上模擬，在靜態通道上時，當雜訊比高於24分貝時，二維的內插法有最好的表現。而當雜訊比小於24分貝時，最小平均平方差適應的方法最好。 因為在瑞雷通道時通道的變化，我們所提出的方法需要做一些修改。而在瑞雷通道上的模擬則也是二維的內插法有最好的表現。當接收者時速27公里時，二維內插法加上使用四組可變導訊的效果最佳，且在二維內插時若再頻率域使用二次內插也會有較佳的結果。而這個結果與理想的結果相差約十分貝左右。

OFDM (orthogonal frequency division multiplexing) technique has drawn much interest recently for its robustness in the mobile transmission environment and its high transmission data rate. IEEE 802.16a is a wireless local and metropolitan area networks standard which is based on OFDMA (orthogonal frequency division multiple access) technique. This work considers two main subjects of the downlink channel estimation under the specifications of IEEE 802.16a, the interpolation schemes and the time-domain improvement techniques. We use LS instead of LMMS estimator for estimations of pilot carriers, not only because we do not know the statistical properties of channels but also for its low computational complexity. We study the linear, the second-order and the cubic spline interpolations. And the 4 kinds of time-domain improvement skills are the moving average, exponential average, the LMS adaptation, and the two-D interpolation. We did the simulation on both static and Rayleigh fading channels. In the static channel, the two-D interpolation performs the best when Eb/N0 > 24dB; meanwhile, the LMS adaptation technique has the best performance when Eb/N0 < 24dB. In the Rayleigh fading channel, due to the change of the channel impulse response, the time-domain improvement skills mentioned above need to be modified. When the speed of the vehicle is 27 km/h, the two-dimensional interpolation with four sets of variable location pilots and with second-order interpolation on the frequency domain can estimate the channel best and is about 10 dB worse than the ideal one.