高速多碼展頻CDMA系統下高效率接收機之設計

Highly Efficient Receivers for High-Rate Multi-Code Spread Spectrum CDMA Systems

邱育倫(Yu-Lun Chiu)

指導教授 ： 黃正光

元智大學/電機通訊學院/通訊工程學系/博士(2008年)

摘要

展頻系統由於其固有的抗惡意干擾能力，常被採用於無授權頻帶中無線通訊系統的設計，此外近年來更有部分學術研究及工業規格，進一步思考利用多展頻碼方式來提升資料傳輸率。因此在本論文中，吾人主要探討在下列兩大類的高速多碼展頻系統下，相對應的高效率接收機設計：一是在時域進行展頻的直接序列展頻系統，另一則為在頻率展頻且包含循環字首的多載波展頻系統。
在時域的直接序列展頻系統方面，由於高速展頻系統在室內無線通道環境中，其多路徑效應不但密集且常擴展至許多個符碼長度，因此吾人結合決策迴授機制，提出數種接收機架構設計。首先為結合決策迴授的犁耙接收機，以及最小均方誤差決策迴授接收機兩種接收機架構，並利用高斯近似的方式，分析在沒有錯誤傳遞下的位元錯誤率。由結果可見，決策迴授犁耙接收機將因無法消除的前標記符際間干擾，而在位元錯誤率上有明顯的錯誤平層的現象，最小均方誤差決策迴授接收機雖可抑制前標記符際間干擾的影響，卻需高運算量的反矩陣運算。因此吾人在此提出一創新且具高效率的雙向決策迴授犁耙接收機，可有效率的利用操作在符碼速率下的雙向決策迴授機制，消除前後標記符際間干擾，並在性能上能夠貼近理想的匹配濾波器性能界限，且僅需較決策迴授犁耙接收機稍高的運算複雜度。
在學術及應用的領域中，M元雙正交移鍵一直被認為是一種相當適合的多碼直接序列展頻系統調變方式，因此吾人接著提出將上述之雙向決策迴授犁耙接收機，延伸至直接序列M元雙正交移鍵多碼展頻系統。由於是利用多組碼的展頻系統，除了於前述單碼系統中所考慮的前後標記符際間干擾外，此接收機同時亦需考慮多碼間干擾的消除，且同時提出應用於在實現時，可快速計算迴授參數的演算法。在室內多路徑環境中的模擬顯示，此一應用於M元雙正交移鍵多碼展頻系統的雙向決策迴授犁耙接收機亦可達到接近理想匹配濾波器性能界限並保持低預算複雜度的特性。
進一步吾人考慮到，在直接序列M元雙正交移鍵分碼多工多重進入系統中，接收機的設計問題，除了上述所提的符際間、多碼間的干擾外，還有基地台同時接收許多不同使用者上鏈路訊號時的多重進入干擾。此外依據不同的系統使用者負載程度，吾人亦提出不同複雜度的多使用者偵測器。首先在中等程度的使用者負載中，提出一利用平行干擾消除的概念，且運作在符碼速率下的雙向決策迴授犁耙多用戶偵測器，當使用者負載變重時，可進一步串接一多階層的依序干擾消除架構以提升性能。
對於頻域率的展頻系統，吾人在本論文中提出一種用於多碼多載波展頻系統，且同時具備頻寬及功率效益的循環位移正交鍵調變方法，配合多相位的Chu序列，此一新式的調變方式可輕易的組成彼此互相正交的展頻碼組，且擁有極低的功率峰均比。此外吾人在本論文中提出兩種用以提高傳輸率的操作模式，並推導相對應的低複雜度最大似然法則接收機。模擬結果中顯示，此循環位移正交鍵多載波展頻系統不論在性能、頻寬效益、功率峰均比、抗惡意干擾能力上，都顯著優於傳統華許碼多載波展頻系統。

關鍵字

展頻系統 ； 犁耙接收機 ； 決策迴授 ； 干擾消除 ； 符際間干擾 ； M元雙正交移鍵 ； 多用戶偵測器 ； 多載波展頻系統 ； 功率峰均比

並列摘要

Spread spectrum (SS) is known as a key technology of wireless transmission in unlicensed-band due to its inherent anti-jamming capability. In this dissertation, we investigate multi-code SS transmission scheme for further improving the throughput. Efficient receivers are designed for two prime multi-code SS systems: (1) direct-sequence spread spectrum (DSSS) system with time-domain spreading, and (2) multicarrier spread spectrum (MCSS) system with frequency-domain spreading and cyclic prefix.
In time-domain DSSS system, first we investigate three decision-feedback receivers for high-rate single-code QPSK DSSS system under indoor wireless channel. Based on a compact signal model for multipath channel dispersion over many consecutive symbols, we consider the design of a RAKE with decision feedback (RAKE-DF) and MMSE-DF receiver, and derive its analytic performance under no error propagation using the Gaussian approximation technique. It is shown that the RAKE-DF receiver suffers from a significant error floor due to the uncancelled pre-cursor intersymbol interferences (ISI). On the other hand, the MMSE-DF receiver can achieve a better performance by suppressing the pre-cursor ISI (or pre-ISI abbreviated), but require costly matrix inversion. Hence, a novel and highly efficient receiver, called the RAKE with bi-directional decision feedback (RAKE-BDDF) receiver is then proposed for efficiently canceling both the post-ISI and pre-ISI at symbol rate. It can not only attain the matched filter bound (MFB) approximately, but also maintain a similar complexity as that of the RAKE-DF receiver.
Next, we extend the RAKE-BDDF receiver to direct sequence M-ary Bi-orthogonal Keying (DS-MBOK) SS system which has been regarded as one promising candidate for high-rate multi-code DSSS transmission. The proposed receiver is not only required to cancel all the post-/pre-ISI but also the multi-code interference (MCI) at symbol rate. For practical implementation, a fast computation algorithm for feedback coefficients is also derived. Under the indoor multipath channels, simulation results show the receiver can also attain a near-optimum performance in terms of the MFB, with only a moderate increase in the receiver complexity compared with the baseline RAKE receiver.
Then a true DS-MBOK code-division multiple access (CDMA) system is considered, where the base-station should receive multiple uplink users simultaneously, leading to multiple access interference (MAI) issue. Efficient and high performance multiuser detector (MUD) receivers are developed under different system loading situations and in the presence of severe indoor channel dispersion. Under low to moderate loading, a RAKE-BDDF MUD receiver based on the parallel interference cancellation (PIC) concept and the symbol-rate BDDF structure is presented for ISI/MCI/MAI cancellation. When the system loading becomes heavier, the RAKE-BDDF MUD can be further enhanced by a cascaded multi-stage successive interference cancellation (SIC) structure.
For system with frequency-domain spreading, a bandwidth and power efficient multi-code multicarrier spread spectrum (MCSS) system is proposed in terms of a new cyclic shift orthogonal keying (CSOK) scheme. The scheme exploits poly-phase Chu sequence for easy orthogonal code set generation and achieving constant-modulus signal. The proposed system is versatile, which can operate in either the Hybrid CSOK (HCSOK) mode that combines PSK/QAM with CSOK, or the Quadrature CSOK (QCSOK) mode with higher data rate. For both modes, low-complexity maximum likelihood (ML) receivers are derived, along with a bit error rate (BER) analysis of the important QPSK-HCSOK case. It is shown that the CSOK MCSS system can considerably outperform the traditional Walsh-coded MCSS system in terms of bandwidth efficiency, PAPR, bit error rate, and anti-jamming capability. Furthermore, under indoor multipath fading channel, the QCSOK system is only slightly inferior to the QPSK-HCSOK system in BER, while achieving almost twice the data rate of the later.

並列關鍵字

Spread spectrum system ； rake receiver ； decision feedback ； interferences cancellation ； intersymbol interferences ； MBOK ； multiuser detector ； multicarrier spread spectrum ； PAPR

參考文獻

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