具空間分集TDMA行動通訊系統之MMSE-DFE多用戶接收機性能分析

Performance Analysis of MMSE DFE Multiuser Receiver for TDMA Mobile Systems with Spatial Diversity

陳泉旭(Chiuan-Hsiu Chen)

指導教授 ： 黃正光

元智大學/工程學院/電機工程學系/碩士(2000年)

摘要

在本篇論文中，吾人考慮了TDMA系統結合空間分集於單一使用者及多個使用者的情形下，系統的性能分析。首先，以一般傳統的固定頻率、一根天線，一個時槽只給一個使用者使用，即SISO的性能開始，進而推廣至多根天線，即SIMO，再到使用多根天線，一個時槽給兩個甚至多個使用者使用，即MIMO的多用戶性能分析。在將符碼間干擾（ISI）及相同存取頻道干擾（MAI）近似為高斯雜訊的條件下，可以獲得等化器決策輸入端的訊雜比公式，藉此用來評估在已知環境下決定該用何種系統便可達到所要求的性能。而當兩個使用者之間產生遠近效應時，可以藉由調整兩個決策迴授等化器的決策延遲(decision delay)來解決這個問題，而不需要再多加功率控制的機制。
在行動通訊系統中通道並非靜態不變的，隨著使用者的移動便會對訊號頻譜產生都卜勒位移，使系統性能變差，因此亦需瞭解此因素對系統的影響。由於在接收機之中使用了多根天線，在天線實際接收到的訊號並非完全不相關，而天線的相關性卻拉低了它原有的分集增益，所以針對以上兩個問題由模擬得知，都卜勒位移的影響較嚴重，而天線的相關性並沒有造成十分嚴重的後果。

關鍵字

多用戶偵搜 ； 分時多工多重存取 ； 半解析 ； 符碼干擾 ； 多重存取干擾 ； 空間衰減相關性 ； 都卜勒擴散

並列摘要

In this thesis, we consider the design and performance analysis of TDMA MMSE DFE receiver with spatial diversity, especially for the case of TDMA multiuser detection by using smart antennas.
To tackle the problem, we first consider the most fundamental case of a single-user single antenna case, i.e., the SU-SISO case. The SISO-MMSE-DFE receiver and its analytic BER performance are derived by using the Gaussian approximation technique, which treats the ISI terms as a Gaussian random variable. Then the result is further extended to single-user multiple antenna case, i.e., the SU-SIMO case. Next, we focus on the problem of TDMA multiuser detection, which means that the same time slot is allocated to two simultaneous users within the same cell. At the basestation, both the ISI and multiple access interference (MAI) should be suppressed, so we use multiple receiving antennas and a multiple-input multiple output (MIMO) MMSE-DFE receiver to detect the two users at the same time. As above, the receiver''s BER performance is derived in terms of the known channel response and some system parameters. Moreover, it is shown that if the receiving power levels of the two users are quite imbalanced due to lack of power control mechanism in TDMA existing TDMA system, the receiver suffers no near-far effect and can still achieve satisfactory performance by adjust the two decision delays to their optimum values. Interesting yet, the weaker signal can even be beneficial by the stronger signal. To confirm our theoretical works, computer simulations are conducted which show that our analytic BER prediction is quite matched to the simulation results.
It is well known that as the antenna spacing is not large enough, the fading processes for different antennas are then not uncorrelated but correlated. So the second main theme of this thesis is to consider the effects of spatial fading correlation and Doppler spread on the receiver''s performance. We first elaborate on the simulation model and procedure for generating spatially correlated time-varying fading processes. Then from the simulational results, it is noted that the Doppler rate has a much significant adverse effect on the system performance than the spatial correlation does. In fact, as long as the spatial correlation coefficient is kept below 0.5, only slight performance degradation is observed.

並列關鍵字

Multiuser detection ； TDMA ； Quasi-analytical(QA) ； ISI ； MAI ； spatial fading correlation ； Doppler spread

參考文獻

[2] B. Natarajan, Carl R. Nassar, and V. Chandrasekhar, "Generation of Correlated Rayleigh Fading Envelopes for Spread Spectrum Applications," IEEE Comm. letters, Vol. 4, No. 1, pp.9-11, Jan. 2000.

[4] T. Fulghum, K. Molnar, and Alexandra Duel-Hallen, "The Jakes fading model incorporating angular spread," in Processing of CISS, Baltimore, USA, pp.364-369, Mar. 1997.

[6] J. K. Hwang, "Multiuser detection scheme for TDMA Mobile systems using spatial diversity and MMSE decision-feedback receiver", IEEE Region 10 Conf. (TENCON),South Korea, pp. 1247-1251, 1999.

[7] Jei-Kuo Chiang and Jeng-Kuang Hwang, "Multiuser Detection Scheme for TDMA Mobiles System Using T/2-MMSE Decision Feedback Receiver", Master''s Thesis, Institute of Electrical Engineering, Yuan-Ze University, Chungli, Taiwan, R.O.C., 1999.

[9] P. Monsen, "Theoretical and Measured Performance of a DFE Modem on a Fading Multipath Channel," IEEE Trans. on Comm., Vol COM-25, No. 10, pp. 1144-1153, Oct. 1977.