多輸入多輸出正交分頻多工系統於移動環境下的干擾消除與效能提升

ICI Cancellation and Capacity Enhancement for Mobile MIMO-OFDM Systems

林坤毅(Kun-Yi Lin)

指導教授 ： 林信標

國立臺北科技大學/機電學院/電腦與通訊研究所/博士(2013年)

摘要

本論文主要針對多輸入多輸出正交分頻多工系統於高速移動通道下以及高鐵的直視波通道環境，提出干擾消除與傳輸效能改善演算法。在高速移動過程中，都卜勒效應會破壞正交分頻多工系統子載波之間的正交性，引起子載波間干擾問題，造成資料位元錯誤率上升，因此，本論文第一部分主要針對都卜勒擴散造成之子載波間干擾，提出兩種子載波間干擾消除方法；而在高鐵直視波較強的通道環境下，都卜勒效應主要由單一的都卜勒頻率偏移構成，可由一般頻率同步演算法進行補償更正，所以，第二部分主要在探討直視波通道因為空間通道關聯性，對於多輸入多輸出系統的資料傳輸效能影響，提出適用於高鐵直視波通道環境的適應性傳輸機制與通道容量效能提升方案。
在子載波間干擾消除上，首先針對時變通道，提出一等效平緩時變通道之時域上子載波間干擾自我消除演算法，由線性時變通道模型的假設下，設計出一組可用以平緩相對信號區間內等效通道變化之視窗係數，藉此降低由時變通道變化引起之子載波間干擾，模擬結果顯示本方法可改善子載波間干擾功率大小與位元錯誤率效能，並證明與所推導之理論值結果相符，與4階視窗函數、張氏等效視窗法相比，使用所提出之方法，可在8%的最大正規化都卜勒偏移和0.25的可用CP情境下，提供5 dB的子載波間干擾功率改善。擴展至多輸入多輸出的架構下，都卜勒效應除了會造成子載波間干擾外，還會引起天線間干擾，本論文進一步提出基於混合式自動重傳機制之子載波間干擾消除方法，此設計藉由封包信號在重傳時進行編碼，透過展頻的概念，將信號在時域、頻域與天線間進行編碼，讓信號可由干擾中分離出來，達到提升信號訊雜比以及子載波間與天線間干擾消除的效果，與傳統追蹤結合法之混合式自動重傳機制相比，本方法不會特別增加系統複雜度，理論分析與數值模擬結果顯示，所提出之方法在時變通道下，可提供比傳統追蹤結合法更佳之干擾改善與資料位元錯誤率效能。
其次，本論文探討高鐵列車通訊效能。相較於一般都市內的無線電波傳播，高鐵的無線通道環境，容易出現相對較強的直視波路徑，在傳統單一天線收發的適應性傳輸機制中，較強的直視波通道具有較佳的通道品質，可使用較高的調變位階與錯誤更正碼碼率來提升資料傳輸吞吐量，但是，在多輸入多輸出天線架構中，較強的直視波通道可能隱含較大的天線間空間通道關聯性，造成多輸入多輸出系統的空間多工增益下降的問題。本論文首先透過模擬方式建立一多輸入多輸出適應性傳輸切換表，由通道狀態的萊斯通道K參數與信號訊雜比，選擇適合之傳輸參數，此外，由於高速通道狀態的變化，容易造成傳輸參數選擇過時的問題，本論文進一步由高鐵實測的通道資料，建立一隱藏式馬可夫通道模型，透過預測通道的K參數變化，藉以改善因通道回傳延遲造成傳輸參數選擇失準問題，模擬結果顯示加入K參數預測功能，可提升系統平均約5 Mbps之資料傳輸吞吐量。
最後，針對高鐵列車上的直視波通道，提出一分散式多天線架構，藉由適當地設計天線擺放位置，以滿足最佳空間通道矩陣，獲得最大空間通道容量效能，改善一般多輸入多輸出架構在直視波通道環境下，因空間通道間關聯性，形成低秩空間通道矩陣，造成空間多工增益效能下降之問題，模擬結果顯示，所提出之分散式多天線佈建方式，在信號訊雜比及萊斯通道K參數為20 dB的2傳2收多輸入多輸出架構下，可將平均通道容量由8 Mbps提升至13 Mbps。

關鍵字

子載波間干擾消除 ； 都卜勒效應 ； 直視波通道 ； 適應性傳輸機制 ； 混合式自動重傳機制 ； 多輸入多輸出通道容量

並列摘要

This dissertation aims to improve the BER and data throughput performance of MIMO-OFDM systems over high-mobility environments and the LoS channel of high-speed rail. Some algorithms for interference cancellation and data throughput improvement are proposed in this dissertation. During high speed mobile reception, the Doppler effect destroys the orthogonality of OFDM systems among subcarriers which leads the ICI and degrades the error rate performance. In the first part of this dissertation, two ICI cancellation schemes are proposed to reduce ICI caused by the Doppler spread. For the high-speed rail LoS channel, the Doppler effect can be simplified to a Doppler shift which is usually compensated by performing frequency synchronization. Thus, the second part of this dissertation is focus on investigating the MIMO data throughput performance and the channel capacity over the LoS channel. Considering the high-speed rail LoS channel, a link adaptation scheme and a capacity enhancement of MIMO systems are proposed to improve the MIMO data throughput and channel capacity.
For the ICI cancellation, this dissertation firstly proposed a time domain low-complex ICI self-cancellation method from the view of equivalent channel time variation mitigation. A set of window coefficients is derived to equivalently mitigate the channel time-variation of the corresponding information symbol interval under the assumption of linear time-varying multipath channel. It outperforms the 4-step window shape and Chang’s ICI self-cancellation method. The improvement of ICI power cancellation as well as noise power is analyzed. A 5 dB improvement of ICI power can be provided for the scenario of a useful CP ratio of 0.25 with a maximum normalized Doppler frequency of 8%. A nearly ICI-free BER performance can be further obtained for the case of useful CP ratio of 1 on the moving speed of 120 km/hr. Secondly, this dissertation discusses the cross layer performance of an MIMO-OFDM system with the adoption of HARQ over a time-varying channel. In the MIMO-OFDM systems, the Doppler effect not only induces ICI but also IAI. This dissertation proposed a HARQ based interference cancellation coding scheme by encoding the retransmitted signals to reduce ICI and IAI. The interference is mitigated based on the principle of spread spectrum for MIMO-OFDM systems to spread the transmitting signals in time, frequency, and spatial domain to separate the desired signals from interferences. The proposed enhanced chase combining HARQ provides a better BER performance over time-varying channels with the advantages of no spectral efficiency scarification and system complexity increment compared to the conventional HARQ.
The second part of this dissertation discusses the MIMO data throughput performance of high-speed rail communications. Compared to the mobile communications in the urban city, it is more possible to experience a strong LoS channel for high-speed rail communications. The strong LoS channel implies a better channel quality and the higher modulation order as well as error correction coding rate can be applied to increase data throughput in the SISO transceiver. However, the strong LoS may also imply a high MIMO channel correlation which degrades the spatial multiplexing gain of MIMO systems. This dissertation proposes a link adaptation scheme of MIMO-OFDM systems based on the Ricean channel K-factor to save the additional channel feedback information overheads like channel correlations for MIMO transmission modes selection. Besides, a hidden Markov model of Ricean K-factor based on the real channel data measured on the high-speed rail train is developed for the prediction of Ricena K-factor to improve the inaccurate MCS and MIMO modes selection caused by the feedback delay. Simulation result shows that an average throughput of 5 Mbps can be achieved.
The last of this dissertation proposes a distributed MIMO system for high-speed rail communications to improve MIMO channel capacity over the LoS channel. By specially deploying the antenna elements, a full rank channel matrix can be achieved to maximize spatial channel multiplexing gain for the condition of the lack of scatterings which usually leads a low rank channel matrix. Simulation result shows that the average channel capacity can be improved form 8 Mbps to 13 Mbps for the 2x2 MIMO systems with a Ricean K-factor value of 20 dB and the SNR of 20 dB.

並列關鍵字

ICI cancellation ； Doppler effect ； LoS channel ； Link adaptation ； hybrid ARQ ； MIMO capacity

參考文獻

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