衛星-地面無線網路之合作式通訊效能研究

Researches on Cooperative Communication Performance over Satellite-Terrestrial Wireless Networks

姚永正(Yung-Cheng Yao)

指導教授 ： 邱茂清 ； 溫志宏

國立中正大學/工學院/電機工程學系/博士(2013年)

摘要

多媒體廣播及群播服務(MBMS)將在未來的行動系統中扮演一個舉足輕重的角色，而衛星系統因具有大覆蓋範圍及群播能力，再與地面網路搭配，所形成的衛星-地面無線網路(Satellite-terrestrial wireless network)將是一種提供上述服務的有效途徑。合作式通訊藉由中繼站的協助，可以使用較低複雜度的傳送器與接收器而達到類似多輸入多輸出技術的分集增益的效果。
在本論文中，主要是研究合作式衛星-地面無線通訊網路的效能。首先探討選擇性解碼後傳送的合作式通訊系統效能，系統架構包含一個來源端、多個中繼站與一個目地端，在無線通道模型選擇上則分為三種，衛星到地面及地面之間皆為瑞雷衰減通道、衛星到地面為萊斯衰減通道且地面之間為瑞雷衰減通道及衛星到地面為(瑞雷+Nakagami)衰減通道且地面之間為瑞雷衰減通道。根據上述系統架構設定，推導接收訊號的訊號功率與雜訊功率比(signal-to-noise ratio, SNR)之機率密度函數(probability density function, PDF)。再根據PDF計算差動生成函數(moment generating function, MGF)，進而獲得MPSK調變的平均符元錯誤機率(average symbol error probability)。在本議題中，我們亦計算使用中繼站功率分配策略下的系統效能。
從上述系統的數值結果可知，當中繼站個數增加時，通訊系統的符元錯誤機率會隨之下降，但是所需的系統資源亦隨之上升。系統資源在分頻多功接取系統中是指時槽(time-slot)，在分頻多工接取系統則是指頻段(frequency band)。完成一次符元傳輸所需的時槽增加即代表傳輸延遲(transmission delay)增大，而若要同時接收多個頻段的信號，則代表目地端所需的接收機數量必須增加。所以，我們接著探討使用最佳中繼站選擇策略的系統效能。
除此之外，我們亦分析使用放大後傳送中繼策略時的系統效能。並且使用兩種通道模型，衛星到地面及地面之間皆為瑞雷衰減通道、衛星到地面為萊斯衰減通道且地面之間為瑞雷衰減通道。由於分析使用放大後傳送得系統效能頗為複雜，因此我們針對第二種通道模型分析時，是推導出該系統的平均符元錯誤機率的下限。
最後，我們探討衛星-地面無線網路中的中繼站位置選擇議題。本系統所選用的中繼站策略為放大後傳送，並且假設目地端無法直接收到衛星的訊號。借助於推導出得最大系統容量之結果，我們可以得知中繼站位置、衛星高度與通道衰減指數之間的關係。

關鍵字

選擇性解碼後傳送策略 ； 放大後傳送策略 ； 衛星-地面無線網路 ； 合作式通訊 ； 平均符元錯誤機率

並列摘要

Multimedia Broadcast Multicast Service (MBMS) will be an important service in future mobile communication systems. Since satellite system has large coverage area and capability for broadcasting, satellite-terrestrial wireless network, satellite system combined with terrestrial network, will be an efficient way to support MBMS. With the aid of simpler transmitter and receiver, cooperative communication assisted by relay node can achieve diversity gain similar to multiple input multiple output (MIMO) technique.
This dissertation focused on system performance of cooperative communication over satellite-terrestrial wireless network. First, it discussed system performance of cooperative communications using selective decode-and-forward protocol. The system model includes one source node, multiple relay nodes, and one destination node. Here three channel models were considered: both satellite-terrestrial link and relay-destination link modeled as Rayleigh fading channels, satellite-terrestrial link modeled as Rician fading channel and relay-destination link modeled as Rayleigh fading channel, and satellite-terrestrial link modeled as (Rayleigh+Nakagami) fading channel and relay-destination link modeled as Rayleigh fading channel. The probability density function (PDF) of the maximum ratio combining signal-to-noise ratio (SNR) in destination node was carried out for these three channel models, and then moment generating function (MGF) was derived. According to the MGF, the average symbol error probability of MPSK signals was acquired. Besides, the system performance with power sharing issue among relay nodes was further investigated.
Second, based on the numerical results of the previous chapter, it was found that the average symbol error probability decreases as the number of relay node increases; however, more system resource was needed to complete a symbol transmission. The system resource was referred to the transmission time-slot in time-division multiple access systems and transmission frequency band in frequency-division multiple access systems. Increase of the transmission time-slots led to more transmission delay, while more transceivers in the destination would be required if the signals were to be received from several transmission frequency bands.. To save system resource, the performance analyses under best-relay selection was carried out using order statistic method.
Third, system performance of cooperative communications using amplify-and-forward relaying protocol was analyzed. In this chapter, two channel models were considered: both satellite-terrestrial link and relay-destination link modeled as Rayleigh fading channels as well as satellite-terrestrial link modeled as Rician fading channel and relay-destination link modeled as Rayleigh fading channel. It was rather complicated to analyze the system performance with the latter channel model. Instead of deriving the exact expression of average symbol error probability, this research carried out the lower bound of average symbol error probability.
Finally, this research discussed relay location selection in satellite-terrestrial wireless networks. The relaying protocol adopted was amplify-and-forward. It is assumed that the destination was unable to receive the signal from the satellite directly. By figuring out maximum capacity of this system, this research figured out the relationship between the relay location, the altitude of satellite, and the path-loss exponents.

並列關鍵字

average symbol error probability ； selective decode-and-forward ； amplify-and-forward ； satellite-terrestrial wireless network ； cooperative communication

參考文獻

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