IEEE 802.16a分時雙工正交分頻多重進接用於固定及行動無線通訊時之傳輸濾波與同步技術研究

Abstract

正交分頻多工 (OFDM) 技術可有效地解決通訊系統中的許多問題，如多重路徑衰落、窄頻干擾等，多用戶正交分頻多工 (multiuser OFDM) 系統能依據使用者之需求將頻寬作更有效之分配。此篇論文中我們依據IEEE 802.16a之規格對正交分頻多重進階 (OFDMA) 系統作探討，並提出了適用於OFDMA系統之傳輸濾波器與同步架構。 為了模擬多重路徑非整數點延遲的影響，我們在傳送端對訊號作四倍頻取樣，而後依照四倍頻取樣時間加入通道模擬。由於升餘弦平方根 (SRRC) 濾波器可使接收端經過濾波後之訊號不受到符元間干擾(inter-symbol interference)，因此我們採用升餘弦平方根 (SRRC) 濾波器來濾除漏在使用頻帶之外的能量，使其符合802.16a之規定。 再者，我們針對上行與下行傳輸分別提出不同的同步架構。不論上行或下行傳輸都需要作時間同步以偵測信號到達的時間，如果估測錯誤會降低間格區間 (guard interval) 用來防止多重路徑延遲造成符元間 (ISI) 干擾的能力。此外，OFDM系統對載波頻率偏移非常敏感，些許偏移即可能造成次載波之間的正交性喪失，因而在下行傳輸中需對頻率偏移作同步處理。 我們將下行同步分為四級，第一、二級利用OFDM系統特有之間格區間(guard interval) 估測OFDM 符元(symbol) 開始時間與分數頻率偏移，此乃由於間格區間使單一符元內具有高度的自相關。第三級利用802.16a規範之保護頻帶 (guard bands) 及部分的嚮導載波 (pilot carrier) 判斷整數頻率偏移量。最後一級藉由下行傳輸前置資訊 (preamble) 判斷框位開始的時間。上行部分嘗試用兩種方式作時間同步，分別為在時間域及頻率域對收到的訊號與上行傳輸前置資訊 (preamble) 作相關性 (correlation) 分析，找到具有最大相關性的時間。我們針對加法性白色高斯雜訊 (AWGN) 及不同的都卜勒偏移衰減通道來模擬以分析同步架構之效能。

OFDM is an effective transmission scheme to cope with many transmission impairments, such as multipath fading and narrowband interference. Multiuser OFDM can provide highly flexible to allocate the bandwidth according to the needs of users. In this thesis, we study the OFDMA system according to IEEE 802.16a. In order to simulate multipath delay at non-integer sample times, 4-times oversampling is applied to the signal at the transmitter so that we can add the channel effect according to 4-times oversampling time. For SRRC (square-root raised cosine) filter, there is no inter-symbol interference (ISI) introduced in the receiver. So we adopt a SRRC filter to suppress the out-of-band power to meet the power mask specified in 802.16a. The DL (downlink) and UL (uplink) synchronization schemes are presented respectively. Time synchronization is performed to detect the start time of symbols for both DL and UL . Time synchronization errors would decrease the ability of guard interval to avoid ISI introduced by multipath channel. Moreover, OFDM system is very sensitive to carrier frequency offset that would possibly destroy the orthogonality between subcarriers. So we have to do frequency synchronization in the DL. There are four stages in the DL synchronization. The first and second stages use the guard interval to estimate the OFDM symbol start time and fractional frequency offset. The reason of using the guard interval is that it provides strong autocorrelation within an OFDM symbol. The third stage uses the guard bands and partial pilot carriers (specified in 802.16a) to detect the integer frequency offset. The final stage determine when a new frame starts from the information of DL preamble. As for UL, we present two schemes to do time synchronization. One is using the correlation of received signal with UL preamble in the time domain and the other is in the frequency domain. The symbol start time is determined as the location with maximum correlation value. Simulation are taken in the AWGN (additive white Gaussian noise) channel and multipath fading channel under different Doppler shifts.