IEEE802.16a分時雙工正交分頻多重進接無線通訊標準之正交分頻多工信號描述與通道編碼研究

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，IEEE802.16a 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。 本論文介紹了IEE802.16a的實體層，並研究其中的兩個部份:正交分頻信號的描述與通道編碼。通道編碼是本論文的重點。 在正交分頻信號的描述方面，本論文推導了常用之類比正交分頻信號描述方法與數位描述方法兩者間的關係。IEEE802.16a使用了前者。 IEEE802.16a的通道編碼採用串接碼(concatenated code)，外層是經過縮短(shorten)和穿孔(puncture)的里德-索羅門碼(Reed-Solomon code)，內層是咬尾(tail-biting)和穿孔的迴旋碼(convolutional code)。此外，在串接碼後接了一個位元交錯器(bit interleaver)和M位元正交振幅調變器(M-ary QAM)。在本論文中，我們為整個通道編碼設計了演算法。我們在加成性白色高斯通道下和淡化的通道下分別模擬了里德-索羅門碼，迴旋碼，和串接碼，並把模擬之結果與一些分析的結果做比較，包含在加成性白色高斯通道下的IEE802.16a的編碼增益需求值，用Shannon極限求出的編碼極限值，及用最短碼字間的距離求出的增益值。在加成性白色高斯通道下，里德-索羅門碼和迴旋碼的編碼增益值幾乎達到理論值，但整體的編碼增益值離理論值或IEE802.16a的編碼增益需求值卻有很大的差距。我們預測在高信號雜訊比下，編碼增益值會比較趨近以上的編碼增益值。

OFDM (orthogonal frequency division multiplexing) technique draws great interesting recently for its advantage of robust in mobile transmission environment with high data rate, and IEEE802.16a is a wireless local and metropolitan area networks standard based on OFDMA (orthogonal frequency division multiple access) technique. This work introduces the PHY layer of IEEE802.16a and studies two parts of it: one is the OFDM signal description and the other is the channel coding scheme. The focus is on the channel coding. For the OFDM signal description, this work derives the relation between the commonly used analog and digital descriptions of OFDM symbol waveforms. The former is used in IEEE802.16a. The channel coding scheme in IEEE802.16a employs concatenated coding with shortened punctured Reed-Solomon code as outer code and tail-biting punctured convolutional code as inner code. In additional, bit interleaver and M-ary QAM modulation are used after the concatenated code. In this work, we design algorithms for the overall coding scheme. We simulate Reed-Solomon code, convolutional code, and concatenated code respectively in AWGN channel and fading channels, and compare the simulation results with some analytical results, including, in AWGN channel, the coding gain requirements in IEEE802.16a, Shannon bounds on coding gain, and theoretic coding gains based on minimum codeword distance. In AWGN channel, both Reed-Solomon and convolutional coding gains almost achieve theoretic values, but the overall coding gains are far away from the theoretic values or the requirements in IEEE802.16a, and we conjecture that the performance will more achieve above values with high signal to noise ratio.