軟體定義之多倍取樣可調適濾波器及其在無線通訊之應用

Software Definable Oversampling Adaptive Filter and its Applications in Wireless Communications

張元龍(Yuan-Lung Chang)

指導教授 ： 賴國華

元智大學/資訊學院/資訊工程學系/博士(2007年)

摘要

針對下一世代寬頻無線網路(broadband wireless access), 許多研究指出, 未來的行動設備應能夠漫遊於不同型態的網路與無線協定, 諸如3G, 4G, 802.11 WLAN, 802.15 WPAN, 還有 802.16 WMAN. 同時, 多媒體服務的頻寬將可以有2-100 MB/sec的高速傳輸性能. 為了達到這樣的目標, 中頻(intermediate frequency)與射頻(radio frequency)段都需要創新的技術, 其中包括發展經濟有效且能夠支援多種規範的可調適濾波器之傳收器架構(transceiver architecture).

本論文提出了一個嶄新的軟體定義可調適濾波器(software definable adaptive filter)用以支援多種無線網路規範的架構. 關鍵在於, 如何使用多倍取樣理論(oversampling theory)找出一個有效的替代方案用以設計中頻及射頻可使用的可調適濾波器. 實際上來說, 將多倍取樣可調適濾波器以軟體定義的技術實踐後, 調整多倍取樣係數(oversampling ratio)將使濾波器廣泛適用於多種規範. 透過增加多倍取樣係數維持系統效能(performance)時, 演算法可以使用較不精確的元件(less-precision elements), 因此減少計算複雜度(computational complexity)與建置成本(implementation cost). 多倍取樣理論可提供近似的系統效能, 但不會優於傳統的可調適濾波器演算法. 也就是說, 軟體定義的可行性在於犧牲部份的效能, 進而換取較少的計算複雜度, 並改善空間使用率與減少電力消耗.

由於多倍取樣理論原本是使用於類比數位訊號轉換之用, 故本研究必須從建構可調適濾波器的基礎工程開始, 涵蓋多倍取樣可調適濾波器的結構, 優化的權重因子(optimization of weigh factors), 當然還有效能分析的推導(derivation of close-form). 緊接著, 我們提出軟體定義的多倍取樣可調適濾波器的架構. 基於不同的無線通訊特質與頻段, 我們討論最常使用的濾波器, 諸如標準模型(canonical model), 橫斷濾波器(transversal filter), 線性加強器(line enhancer), 可調適天線(adaptive antenna), 還有寬頻智慧天線(wideband smart antenna). 同時驗證多倍取樣可調適濾波器的效能, 並討論其計算複雜度之改善. 針對本研究的實用性, 我們實現多倍取樣可調適陣列天線 (oversampling adaptive array antenna)於 DS-CDMA (Direct Sequence Code Division Multiple Access)技術, OFDMA (Orthogonal Frequency Division Multiple Access)技術, 還有OFCDMA (Orthogonal Frequency Code Division Multiple Access)技術.

不同於傳統的可調適濾波器, 本論文提出的多倍取樣濾波器有下列特點與獨到之處. 1) 近似的效能, 卻可使用較簡單且不精準的原件. 2) 設計複雜度減低, 因為多倍取樣係數可多次設定用以控制接收訊號的品質與計算需求. 3) 一個能支援未來多規範的統一平台也成功建構. 無論是針對無線頻段亦或中頻頻段, 都可以採用多倍取樣可調適濾波器. 4) 系統容量得以提升, 傳輸更加穩定, 並減少多通道之成本. 5) 軟體定義的多倍取樣可調適濾波器將可適用於未來各式各樣的無線系統.

關鍵字

軟體無線電 ； 可調適濾波器 ； 多倍取樣理論 ； 多天線接收機

並列摘要

One consistent theme of many standards and proposals for next-generation broadband wireless access (BWA) is that future mobile devices should be able to roam across multiple networks and air interfaces, such as 3G, 4G, 802.11 WLAN, 802.15 WPAN, and 802.16 WMAN, and be able to access to data at rates of 2-100 MB/sec for multimedia application. This will require several innovations at both radio and intermediate frequency levels, including economical development of the adaptive filters for the multi-standard transceiver architecture.

This dissertation presents a novel software definable architecture for adaptive filters to be used at mobile stations in multi-standard wireless communication systems. The key to which is the development of a valid and effective alternative to the design of adaptive filters at intermediate frequency level with oversampling theory. In practice, an SDR realization of oversampling adaptive filters (OAF) can be operated at a wide range of requirements according to their oversampling ratio (OSR). The required computational complexity and implementation cost can be greatly reduced by restricting the algorithm to adopt much simpler operations with less-precision elements, while maintaining a comparable performance by increasing the oversampling ratio. Consequently, the oversampling approach should perform comparably to, but not better than the traditional adaptive filtering algorithms. That is, the feasibility of SDR depends on its trading some performance for reduced computational complexity, improved area efficiency, and less power consumption.

Since the oversampling data converter differs fundamentally from adaptive filter, this dissertation first has to develop the fundamentals of adaptive filter design with the oversampling theory, including the framework of oversampling adaptive filters, the scheme discussion, the optimization of weigh factors, and the derivation of close-form for performance analysis. Then, we present an SDR architecture of oversampling adaptive filters. While, depending on the radio characteristics and intended use, there are a variety of implementation structures at different frequency levels, we focus only on those that are most commonly used in wireless communication systems, including canonical model, transversal filter, line enhancer, adaptive antenna, and wideband smart antenna. The performance of oversampling adaptive filters is also examined and compared with that of conventional adaptive filters along with their computational complexity. To illustrate the practicality of our work, the proposed architecture is also used to implement an adaptive array antenna (also known as receiver diversity) in real-world wireless communication systems, including DS-CDMA (Direct Sequence Code Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), and OFCDMA (Orthogonal Frequency Code Division Multiple Access).

In contrast to conventional adaptive filters, the oversampling adaptive filters presented in this dissertation have the following characteristics and features. A comparable performance is offered with simpler and less-precision elements. Design complexity is much simpler, because the reconfigurable oversampling ratio effectively controls the quality of received signal and computational requirements. It also renders a single computational framework for future multi-standard systems. Meanwhile, ease of adoption and realization at both radio and intermediate frequency levels. This could greatly improve the system capacity, offer a more reliable link, and reduce the cost of mixed channel access. Consequently, easy migration to future wireless networks and air interfaces.

並列關鍵字

software radio ； adaptive filter ； oversampling theory ； receiver diversity

參考文獻

Chang, Y. L. and Y. H. Chang, 2004. ``A Lower Power Adaptive Space Diversity for Software Definable Base Station in 3G DS-CDMA Systems,' World Wireless Congress, San Francisco, May25-28.

Chen, K. C. and S. T. Wu, 1999. ``A Programmable Architecture for OFDM-CDMA,' IEEE Commun. Mag., Vol. 37, No. 11, pp. 76-82.

Choi, R. L., K. B. Letaief and R. D. Murch, 2001. ``MISO CDMA Transmission with Simplified Receiver for Wireless Communication Handsets,' IEEE Trans. Commun, Vol. 49, No. 5, pp 888-898.

Correal, N. S., R. M. Buehrer, and B. D. Woerner, 1999. ``A DSP-Based DS-CDMA Multiuser Receiver Employing Partial Parallel Interference Cancellation,' IEEE J. Select. Areas Commun., Vol. 17, No.4, pp. 613-630.

Cummings, M. and S. Haryhama, 1999. ``FPGA in the Software Radio,' IEEE Commun. Mag., Vol. 37, No. 2, , pp. 108-112.