Title: 基於快速傅立葉轉換之寬頻分碼多重進接系統犁耙式接收機架構設計與模擬
On Design and Simulation of FFT-based RAKE Receiver Architectures for Wideband CDMA Systems
Authors: 王信淵
Shin-Yuan Wang
Chia-Chi Huang
Keywords: 犁耙式接收機;多用戶偵測器;智慧型天線;RAKE Receiver;Multiuser Detector;Smart antenna
Issue Date: 2000
Abstract: 全世界第三代無線蜂巢通訊幾乎都朝向使用寬頻分碼多重進接系統。 而分碼多重進接系統犁耙式接收機在對抗多重通道衰減及提升系統效方面能扮演重要的角色。在這篇論文中,我們設計了三種基於快速傅立葉轉換之寬頻分碼多重進接系統犁耙式接收機架構,並以電腦模擬的方法來驗證其系統效能。 我們提出一個用於下鏈傳輸之基於快速傅立葉轉換之犁耙式接收機。這一個接收機在頻域偵測資訊,其展頻碼匹配濾波器、通道匹配濾波器及通道探測器皆是以基於快速傅立葉轉換之匹配濾波器來製作並整合在同一個系統中。再者此接收機可估計下鏈領航信號然後將其干擾移除,所以使用較強的下鏈領航信號不會降低系統效能。此基於快速傅立葉轉換之匹配濾波器的信號處理採取一個符元接著一個符元的方式,因此這一個犁耙式接收機可以有效地反應快速變化的通道。我們的模擬結果顯示此基於快速傅立葉轉換之犁耙式接收機在靜止通道中其錯誤率可以達到接近二元相位鍵同步解調的理論值。在雙路徑衰減通動中則其錯誤率會隨其都卜勒頻率增加而增加。 進階技術如多用戶偵測器及智慧型天線已被建議用來增加分碼多重進接系統的容量。在這論文中,我們也提出適用於寬頻分碼多重進接系統之多用戶接收機及二維犁耙式接收機,其中我們也使用基於快速傅立葉轉換之匹配濾波器來減少接收機的計算複雜度。 在同步分碼多重進接系統中,我們提出一個基於快速傅立葉轉換之多用戶接收機。我們使用次佳之平行干擾消除多用戶偵測技術,並將平行干擾消除結合在基於快速傅立葉轉換之犁耙式接收機中。在多用戶接收機的每一級,我們以快速傅立葉轉換之犁耙式接收機偵測資訊,然後在頻域重建此信號。而來自其他用戶的干擾(相當於此重建信號)會在下一級資訊偵測前被移除,如此可以減少來自其他用戶的干擾使得資訊偵測越來越正確。 這裡我們也會依據資訊偵測的可靠度來進行軟決策。為了實際應用,我們可以改變傳輸信號並加入一循環延展區間,如此可以放寬用戶間完全同步的要求。透過電腦模擬,我們發現此基於快速傅立葉轉換之多用戶接收機可顯著地提升系統容量。 基於快速傅立葉轉換之二維犁耙式接收機將陣列天線及犁耙式接收機結合在一起成為一個在頻域之空時濾波器。我們先以快速傅立葉轉換來計算陣列天線之接收信號的空間頻譜而形成天線波束。接著以一組基於快速傅立葉轉換之匹配濾波器來匹配展頻碼。然後頻域之空時通道匹配濾波器以最大比相加將展頻碼匹配後的信號能量相加。這一個二維犁耙式接收機也包含一個二維通道探測器。此通道探測器可估計通道空時參數例如多重路徑之延遲時間、方向及複數增益。電腦模擬顯示此基於快速傅立葉轉換之二維犁耙式接收機可在靜止及行動的二維通道中良好地運作。
Wireless cellular communications around the world are moving toward wideband code division multiple access (WCDMA) systems in almost all third generation approaches. A CDMA RAKE receiver plays an import role to combat the effect of multipath fading and improve the system performance. In this dissertation, we design three FFT-based RAKE receiver architectures for WCDMA systems and evaluate the system performance by computer simulations. We first proposed a downlink FFT-based RAKE receiver, which computes a data detection in the frequency domain. A spreading code matched filter, a channel matched filter, and a sounding receiver are all implemented by FFT-based matched filters and integrated in a unified architecture. Furthermore, the strong downlink pilot signal does not degrade the receiver BER performance because the receiver estimates the pilot signal and subtracts the estimated pilot signal from the received signal before data detection. Since the signal processing of the FFT-based matched filter is done on symbol-by-symbol basis, the RAKE receiver is robust to the changing of the channel. Our simulation results show that average bit error probability of the system is close to the theoretic optimal performance in a static channel. In a two-path fading channel, we found the system performance degrades as the maximum Doppler frequency increases. Advance technologies, such as multiuser detectors and smart antennas, have been suggested to increase the system capacity of CDMA systems. In this dissertation, we propose a multiuser receiver and a 2 dimensional (2D) RAKE receiver for WCDMA systems, in which FFT-based matched filters are used to reduce the computation complexity. An FFT-based multiuser receiver adopts parallel interference cancellation (PIC), a suboptimal multiuser detection, with the FFT-based RAKE receiver architecture for synchronous CDMA systems. At each stage of the multiuser receiver, we make data detections using FFT-base RAKE receivers and reconstruct the interference signals in the frequency domain. Then all the multiple access interferences (i.e. the replicas of the signals) are removed from the received signal in parallel before next stage data detection. Because the interference can be more and more reduced in this stage-by-stage PIC processes, the data decision will become much more accurate. Here, a soft decision is used in the receiver to account for the reliability of data dectection. For practical implementation, we modify a transmitted symbol by adding a cyclicly extended guard interval to loose the requirement for perfect multiuser synchronization. Through computer simulations, we found the FFT-based multiuser receiver significantly increases the system capacity. An FFT-based 2D RAKE receiver, which combine an antenna array with RAKE receivers, is implemented by a spatial-temporal matched filter in the frequency domain. To form a beam pattern, we calculate the spatial frequency spectra of received signals on the antenna array using fast Fourier transform (FFT). After FFT-beamforming, a bank of FFT-based matched filters is first used to perform code matching. Then, the code matched signals are combined with maximal-ratio combining technique by a spatial-temporal channel matched filter implemented in the frequency domain. This 2D RAKE receiver also includes a channel sounder, which estimates the spatial and temporal channel impulse response parameters such as delays, directions of arrivals, and complex gains of multipath components. Computer simulations show that the 2D RAKE receiver performs well in both a static 2D channel and a mobile radio channel environments.
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