多輸入多輸出正交分頻多工系統之適應性混合式的自動重傳機制

An adaptive HARQ scheme for MIMO-OFDM systems

蘇建忠(Chien-Chung Su)

指導教授 ： 林 信 標

國立臺北科技大學/電資學院/電腦與通訊研究所/碩士(2008年)

https://doi.org/10.6841/NTUT.2008.00154

摘要

多輸入多輸出系統是利用在傳送端與接收端使用多根天線的一種非常吸引人的技術，其系統可以得到較高的頻譜效率或改善系統的效能，而正交分頻多工利用數個子載波來傳送資料，把一個較高傳輸率的資料串用數個較低傳輸率的資料並行傳送，使頻率選擇性衰弱通道可以簡化成平坦衰弱通道，是一種強而有力的傳送方式。混合式的自動重傳機制是一種重傳方法的機制，在接收端利用循環冗餘檢查碼判斷資料的正確與否，若判斷為正確，則回傳一個正確訊息給傳送端，若判斷為錯誤，則回傳一個錯誤訊息給傳送端，傳送端再重傳相同的資料給接收端，利用此種混合式的自動重傳機制可以大大改善系統的效能以及吞吐量。
在本篇論文中，提出一種適應性傳輸之混合式的自動重傳機制，藉此獲得更多的空間性分集增益，此種適應性混合式的自動重傳機制是根據前一次階層依序式干擾消除法的基底來決定重傳的符元。正確的符元將不再重傳，以獲得額外的空間增益，並在接收端結合連續符號串接方法與QR分解法來降低系統之複雜度，經模擬可知，假定多輸入多輸出系統傳送端與接收端各為四根天線，使用本論文之適應性混合式的自動重傳機制來傳輸，與傳統混合式的自動重傳機制相比，在各種不同的子載波數量上，皆可以改善系統的效能，但會隨著子載波數量的增加，其改善效能會漸低。當子載波個數為64時，在重傳第三次時，可以改善將近3.7dB的效能，且重傳第二次之效能會優於使用傳統混合式的自動重傳機制在重傳第三次之效能，意即使用本論文之適應性傳輸的方式可以節省重傳一次的時間，隨著子載波個數變多，其適應性混合式的自動重傳機制的系統最大吞吐量的與傳統混合式的自動重傳機制差距也漸漸降低，當子載波個數增至2048時，其最大吞吐量差異極小，且在較低的訊號雜訊比時其吞吐量可以獲得一定程度的改善，這對於現今的通訊技術是非常有益的。

關鍵字

多輸入多輸出 ； 正交分頻多工 ； 混合式的自動重傳機制 ； 追蹤結合法

並列摘要

Multiple input multiple output (MIMO) system is a very attractive technology which utilizes multiple antennas at the transmitter and receiver terminal, it can achieve high spectral efficiency or improve system performance. Orthogonal frequency division multiplexing (OFDM) is a powerful transmission scheme for improving bandwidth efficiency by dividing a serial high-rate data stream into several parallel low-rate data streams, thus frequency selective fading channel can simplify to flat fading channel. Hybrid automatic repeat request (HARQ) is a retransmission method which can greatly improve system performance and throughput. It uses an error-detecting code, such as a cyclic redundancy check (CRC) code, to check for errors at the receiver. If there is no error then a positive acknowledgement (ACK) feeds back to the transmitter, otherwise the receiver feeds back a negative acknowledgement (NACK) to the transmitter. And then, the transmitter will retransmit the same symbol to the receiver.
In this paper, we propose an adaptive HARQ scheme which can obtain more spatial diversity gain. Those retransmitted symbols will based on previous transmitted OSIC order index. To be mentioned, correct symbol will not to be retransmitted that earn the extra gain for spatial diversity. Utilizing the concatenation assisted symbol level combining (CASLC) with the QR decomposition (QRD). Assume the number of transmit antennas and the number of receiver antennas are both set to four. The simulation results show that the propose adaptive HARQ scheme can improve the system performance in different numbers of sub-carriers. When the number of sub-carriers was raised, the improve performance decayed. The propose adaptive HARQ scheme can offer 3.7dB gain than the convention HARQ scheme with 64 sub-carriers in the third retransmit. The system performance of the propose adaptive HARQ scheme in the second retransmit is better than the convention HARQ method in the third retransmit. To expect, using the propose adaptive HARQ scheme can save one retransmit time. When the number of sub-carriers was raised, the maximum throughput differential of the adaptive HARQ scheme and the convention HARQ scheme decayed. The maximum throughput of the adaptive HARQ scheme is similar to the convention HARQ scheme with 2048 sub-carriers, and the throughput can better than the convention HARQ scheme in lower signal to noise ratio (SNR). It’s very helpful to the present communication technology.

並列關鍵字

MIMO ； OFDM ； HARQ ； Chase Combining

參考文獻

[2] Bingham J.A.C., “Multicarrier modulation for data transmission :an idea whose time has come,” IEEE Communication Magazine, vol. 28, no. 5, pp. 5-14, May 1990.

[3] J. Miller and J. K. Wolf, “High code rate error correction code design for partial response systems,” IEEE Transactions Magnetics, vol. 37, no. 2, pp. 704-707, March 2001.

[6] S. M. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE Journal on select areas in communications, vol. 16, no. 8, pp. 1451-1458, Oct 1998.

[8] V. Tarokh, H. Jafarkhani, and A. R. Calderbank, “Space-time block coding for wireless communications: performance results,” IEEE Journal on select areas in communications, vol. 17, no. 3, pp. 451-460, March 1999.

[9] O. Tirkkonen and A. Hottinen, “Complex space-time block codes for four Tx antennas,” IEEE Global Telecommunications Conference(GLOBECOM), pp.1005-1009, Dec. 2000.