循環延遲多樣性正交分頻多工系統中通道估測之領航訊號最佳化設計

Optimal Design of Pilot Symbols for Channel Estimation in CDD-OFDM Systems

黃偉傑(Wei-Chieh Huang)

指導教授 ： 李學智

共同指導教授 ： 李志鵬

國立臺灣大學/電機資訊學院/電信工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.00639

摘要

多重輸入與多重輸出 (multiple-input multiple-output, MIMO)技術可以有效增進正交分頻多工 (orthogonal frequency division multiplexing, OFDM)系統的效能。近幾年來，有文獻提出循環延遲多樣性(cyclic delay diversity, CDD)技術以利用多根傳送天線所提供的多樣性。和傳統的傳送多樣性技術相比，例如塊狀時空編碼(space-time block coding , STBC)，循環延遲多樣性技術的主要優點為對通道響應的限制條件較寬鬆以及傳輸端和接收端都有較低的複雜度。
一般而言，通道資訊(channel state information, CSI)在同調解調中是不可或缺的。而在多路徑通道的環境當中，通道估測多半利用領航訊號輔助調變法(pilot symbol assisted modulation, PSAM)而達成。領航訊號的架構更對通道估測的準確度有決定性的影響。然而，針對傳統多重輸入與多重輸出正交分頻多工系統所提出最小化通道估測均方誤差(mean square error, MSE)的最佳化領航訊號序列，並不適用於循環延遲多樣性正交分頻多工系統。在本篇論文當中，我們將推導出最佳化領航訊號的架構，能最小化循環延遲多樣性正交分頻多工系統中通道估測的均方誤差。理論推導結果顯示，妥善設計領航訊號以及循環延遲可以最小化通道估測的均方誤差。我們並利用模擬結果驗證所推導出的領航訊號序列的確能達到通道估測均方誤差的下限。
由於領航訊號輔助調變法中領航訊號子載波的使用會降低頻寬使用效率，本論文將更進一步探討在循環延遲多樣性正交分頻多工系統中利用疊加序列法(superimposed training, ST)進行通道估測的架構。在疊加序列法當中，領航訊號直接和資料序列相加後送出。理論分析結果顯示，針對領航訊號輔助調變法所推導的最佳化領航訊號序列，也能夠最小化疊加序列通道估測法的均方誤差。此外，當假設領航序列的能量固定時，我們發現通道估測的效能和領航序列的長度無關。
本論文最後分析在領航訊號輔助調變法和疊加序列法這兩種架構中循環延遲多樣性正交分頻多工系統的平均通道容量(average channel capacity)。我們推導出領航訊號的最佳傳送能量比值，以最大化平均通道容量的下限。研究結果顯示，當通道假設在連續多個正交分頻多工符元中有相同的通道響應，疊加序列法可以達到比領航訊號輔助調變法更高的系統容量(system capacity)。

關鍵字

正交分頻多工系統 ； 多重輸入與多重輸出 ； 循環延遲多樣性 ； 通道估測 ； 領航訊號輔助調變 ； 疊加序列 ； 系統容量

並列摘要

Multiple-input multiple-output (MIMO) technique is an attractive candidate for performance improvement in orthogonal frequency division multiplexing (OFDM) systems. Recently, cyclic delay diversity (CDD) scheme is developed in order to exploit the transmit diversity offered by multiple transmit antennas. The main advantages of CDD over traditional transmit diversity technique, e.g., space-time block coding (STBC), are the easier requirement for channel response and the fact that transmitter and receiver both have relatively lower complexity.
Channel state information (CSI) is generally required for coherent demodulation technique. In multi-path environments, channel estimation is typically performed by using pilot symbol assisted modulation (PSAM) scheme. The accuracy of channel estimation is critically dependent on the structure of pilot sequences. However, the optimal pilot sequences, being derived to minimize the mean square error (MSE) of channel estimation in traditional MIMO-OFDM systems, are not suitable for CDD-OFDM systems. In this dissertation, we present a general expression of the optimal pilot sequences for channel estimation in CDD-OFDM systems. The derived results illustrate that the MSE of channel estimation can be minimized if pilot sequences and cyclic-delays are appropriately designed. Through simulation experiments, we confirm that the MSE of channel estimation using the proposed pilot sequences achieves the lower bound in CDD-OFDM systems.
Since PSAM schemes require the use of dedicated pilot sub-carriers, the bandwidth utilization is substantially reduced. Therefore, the dissertation further investigates a channel estimation approach for CDD-OFDM systems using a superimposed training (ST) scheme, in which pilot symbols are superimposed onto data streams prior to transmission. It is revealed that the developed pilot sequences are also optimal for ST-based channel estimation in CDD-OFDM systems. Additionally, under the assumption that the total power of pilot sequence is a constant, we show that the performance of channel estimation is independent of the length of pilot sequence.
Under both PSAM and ST schemes, the average channel capacity of CDD-OFDM systems is investigated. Moreover, the optimal ratios of pilot symbol power to total transmission power are analyzed in order to maximize the lower bound of average channel capacity. The current results show that, when channel is assumed to be stationary over a number of OFDM symbols, the ST-based channel estimation schemes yield a higher system capacity than PSAM-based channel estimation schemes.

並列關鍵字

Orthogonal frequency division multiplexing (OFDM) ； multiple input multiple output (MIMO) ； cyclic delay diversity (CDD) ； channel estimation ； pilot symbol assisted modulation (PSAM) ； superimposed training (ST) ； system capacity

參考文獻

[3] H.-C. Wu, “Analysis and characterization of intercarrier and interblock interferences for wireless mobile OFDM systems,” IEEE Trans. Broadcasting, vol. 52, no. 2, pp. 203-210, Jun. 2006.

[4] L. Goldfeld, V. Lyandres, and D. Wulich, "Minimum BER power loading for OFDM in fading channel," IEEE Trans. Commun., vol. 50, pp. 1729- 1733, Nov. 2002.

[5] P. F. Xia, S. L. Zhou, and G. B. Giannakis, "Adaptive MIMO-OFDM based on partial channel state information," IEEE Trans. Signal Processing , vol. 52, no. 1, Jan. 2004

[6] A. M. Wyglinski, F. Labeaua, and P. Kabal, "Bit loading with BER-constraint for multicarrier systems," IEEE Trans. Wireless Commun., vol. 4, pp. 1383-1387, July 2005.

[7] A. Marques, F. Digham, and G. Giannakis, "Optimizing power efficiency of OFDM using quantized channel state information," IEEE J. Select. Areas Commun., pp. 1582-1592, Aug. 2006