時空方塊碼-正交分頻多工系統之通道估測技術

Abstract

在無線通訊裡，頻寛是一項非常重要的資源；要如何在不增加頻寛的情形下增加系統的可靠度一直是大家所研究的方向。在1998年，時空方塊碼 (space-time block code, STBC) 被提出來解決這一個問題，時空方塊碼的基本概念是在傳送端或接收端增加天線的數目以逹到系統可靠度的提升；而為了使系統簡單化，在這篇論文中，我們只討論系統裝一根接收天線與二根傳收天線的效能。 時空方塊碼在頻率選擇性衰減通道 (frequency-selective fading channel)中的效能並不好，如果我們將時空方塊碼應用在正交分頻多工 (orthogonal frequency division multiplexing, OFDM) 系統上，形成了時空方塊碼-正交分頻多工 (STBC-OFDM) 系統；正交分頻多工可將頻率選擇性衰減通道轉換成多個頻率無選擇性衰減子通道 (frequency-nonselective fading subchannel)，則時空方塊碼則可藉著正交分頻多工的幫助，能在頻率選擇性衰減通道的環境下作用。 多通道的估測是時空方塊碼-正交分頻多工系統中一個相當要的部分，在這篇論文中，我們提出兩種多通道估測的方法，其主要是應用導引符號 (pilot symbol) 來幫助多通道估測，其複雜度較一般多通道估測的方法為低；第一種方法是梳型導引次載波排列 (comb-type pilot subcarrier arrangement)，而第二種方法是方塊型導引次載波排列 (block-type pilot subcarrier arrangement)，我們並用電腦模擬來比較這兩種方法的位元錯誤率 (bit error rate, BER)。最後我們考慮IEEE標準802.16加上時空方塊碼的系統，但是我們不加上通道編碼的技術；我們將之前所提出的二種多通道估測方法用在此系統上，電腦摸擬的結果顯示它在此系統環境下只有2~3 dB的損失。

Bandwidth is a very important resource in the wireless communication system. Improving the reliability of the wireless communication system without bandwidth expansion is always an interesting research topic. In 1998, space-time block code (STBC) is proposed to solve this problem. The basic concept of STBC is equipping multiple transmit antennas and/or receive antennas in the system to improve the performance. For simplicity, we only consider the system equipped with one receive antenna and two transmit antennas in this thesis. The performance of STBC employed in conventional single-carrier system is usually poor in the frequency-selective fading channel. Therefore, we apply STBC to the orthogonal frequency division multiplexing (OFDM) system to form the space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) system. OFDM can transform frequency-selective fading channel to multiple flat fading subchannels. Aided by OFDM, STBC-OFDM system can work quite well over the frequency-selective fading channel. Multi-channel estimation plays an important role in the STBC-OFDM system. In this thesis, we present two simple channel estimation methods by using pilot signals. The first method (method Ⅰ) is the comb-type pilot subcarrier arrangement, and the second method (method Ⅱ) is the block-type pilot subcarrier arrangement. We also show the simulation results of comparison between method Ⅰ and method Ⅱ. Finally, we consider the IEEE standard 802.16 WirelessMAN with STBC. Equipping one receive antenna and two transmit antennas to the IEEE 802.16 system without channel coding, we apply our two channel estimation methods to improve the performance. Simulations show that the performance of our channel estimation methods is 2~3 dB loss compared with the ideal channel estimation in the IEEE 802.16 environment.