使用展頻碼在多輸入多輸出正交分頻多工系統之FPGA實現

正交分頻多工(Orthogonal frequency division multiplexing, OFDM)可以結合在傳送端與接收端的多根天線，組成多輸入多輸出(MIMO)天線型態，以增加分集增益(Diversity gain)且/或提升系統容量與對抗時變和多路徑衰減(Multi-path fading)，此外整合展頻碼技術，能更進一步的抵抗多路徑干擾，使得效能提升。本論文以傳送分集(Transmit diversity)著名的Alamouti方法與展頻碼，利用可程式邏輯閘陣列(Field Programmable Gate Array, FPGA)實現了簡易架構的展頻碼在多輸入多輸出正交分頻多工系統，並比較不同傅立葉點數與系統複雜度的關係，在此系統中使用QPSK (Quadrature Phase Shift Keying)做調變，而在快速傅立葉轉換(Fast Fourier Transform, FFT)電路和快速哈達碼轉換(Fast Hadamard Transform, FHT)方面，使用了Radix-2 演算法以及Single-path Delay Feedback (SDF)，藉由Matlab上的定點數模擬，在電路的面積以及資料的精確度上取得平衡。我們運用Matlab來檢驗系統的錯誤率並在FPGA上實現，結果顯示范德蒙展開的MIMO-OFDM都比哈達碼展開約增加35%的硬體複雜度。

關鍵字

傳送分集；正交分頻多工；展頻碼展開；可程式邏輯閘陣列

並列摘要

Orthogonal frequency division multiplexing (OFDM) system could link up with multiple antennas at transmitter and receiver resulting in a multiple in multiple output (MIMO) configuration. MIMO can increase the diversity gain and/or to enhance the system capacity on time variant and multipath fading channel. Moreover, MIMO OFDM integrated with code spreading would be more robust in multipath interference. This thesis adopts the well known Alamouti’s transmit diversity scheme and spread spectrum code for MIMO OFDM system. We implement the system with field programmable gate array (FPGA), and compare the complexity of various numbers of points in Fast Fourier Transform and Fast Hadamard Transform. The Radix-2 algorithm and the single-path delay feedback structure are used for the realization of the FFT and FHT circuits. We verify the BER performances by using Matlab simulation and FPGA implementation. The experimental results show that the hardware complexity for the Vandermonde spreading MIMO-OFDM is about 35% higher than Hadamard spreading MIMO-OFDM.

並列關鍵字

Transmit Diversity ； OFDM ； Code Spread ； FPGA.

參考文獻

[1] G. L. Stüber, et al., “Broadband MIMO-OFDM Wireless Communications,” Proc. IEEE, vol. 92, no. 2, Feb. 2004, pp. 271–94.

Google Scholar

[2] S. M. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Select. Areas Commun., vol. 16, pp. 1451–1458, Oct. 1998.