應用於無線干擾環境下結合時空編碼技術及展頻系統之研究

Abstract

在無線傳輸的環境中，傳輸的信號常遭受到惡意的干擾源及通道衰減效應，導致接收訊號產生嚴重的失真。跳頻展頻系統是一般最常用來抑制干擾效應的技術，而具有分集增益及編碼增益的時空編碼技術可有效的降低通道衰減效應。因此，在本篇論文裡吾人便結合了兩者之優點，提出了時空編碼結合跳頻展頻技術以提升傳輸系統在無線干擾環境中之整體效能。 為了能夠專注於分析時空碼的解碼設計，吾人考慮了三種較簡單的跳頻方式。第一種定義為所有傳送天線的訊號都跳至相同的頻帶上，稱此為最差跳頻。第二種情形為所有傳送信號皆設計為避免互相發生碰撞，稱此為最佳跳頻。而最後一種所提出的跳頻方式，便是任何傳輸信號皆隨機的跳至任一頻帶上，稱此為均勻跳頻。其中最差及最佳跳頻方式分別代表為此系統效能分析的上界與下界。而在跳頻頻帶數夠大及傳送天線個數少的情況下，均勻跳頻方式可使系統效能近似於使用最佳跳頻方式。針對上述跳頻方式，吾人推導出在路徑增益已知或未知情況下此系統的最大可能性解碼。然而，由於最大可能性解碼相當的複雜，在實際應用中較不易實現，因此，吾人提出了一些次佳解碼方式來降低解碼複雜度，並模擬其在多重傳送及接收天線架構下之效能。此外，吾人亦針對此系統提出了在無線干擾環境下好的時空碼準則。最後經由模擬結果驗證出，在相同的訊雜比及頻寬效益的考量之下，此系統比傳統單進單出編碼效能來的更佳。

In wireless jamming environments, the transmitted signals usually suffer from hostile jammers and undesired channel impairments, e.g., multipath fading. Conventionally, frequency-hopping spread spectrum (FHSS) systems are most effective anti-jamming techniques, and space-time coding (STC), which introduces temporal and spatial correlation into the transmitted signals to achieve transmitter diversity without sacrificing the bandwidth, has been shown to provide excellent performance against multipath fading. Therefore, in this thesis, we combine STC with the FHSS to construct a powerful high-rate transmission scheme for wireless jamming channels. Three cases of FH are considered here to simplify the design of STC. One is the worst-case frequency hopping which hops the symbols from all transmitter antennas into the same frequency band, another is the perfect frequency hopping which avoids any possible collision of the transmitted symbols, and the other is the uniform frequency hopping which hops the transmitted symbol randomly over the total frequency hopping bands. The actual performance of the combined STC/FHSS system with arbitrary hopping patterns can then be upper and lower bounded by the evaluated performance of the worst case and perfect case, respectively. Moreover, the performance of the uniform case can approach the perfect case as the number of frequency bands is very large and few antennas are used for transmission. The maximum likelihood decoding of space-time codes is derived with respect to different reception conditions. Several suboptimal schemes are proposed for complexity reduction. We also present the design criteria for constructing good space-time codes with respect to the wireless jamming channels. Verified by the simulation results, the proposed system can provide better performance than the conventional schemes in terms of both bandwidth efficiency and signal-to-noise ratio.