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Performance Analysis of Adaptive Modulation in Cooperative Communication Systems: Ideal Case and Snr-Based Mechanism
To equip the multinode cooperative communication systems with the adaptive modulation isnot straightforward as compared to the conventional point-to-point communication systemswhen the decode-and-forward (DF) scheme is utilized at relays. The trouble comes from therandom decoding behavior of relays. That is, to decide an appropriate modulation level at anytransmission opportunity depends on not only the channel conditions toward the destinationbut also the decoding results at relays. Practically, there is no way for the destination to knowthe decoding results at relays a priori. In order to access the theoretical limitation of theperformance of this system, an ideal case is studied in which we assume that the decodingresults of relays are known to the destination. Based on this assumption, we would like toinvestigate the maximal achievable rate of the overall system and see how the systemefficiency can be improved with the help of AM.One of the popular selection mechanism of constellation size is the SNR-based mechanism.However, the performance of such a selection mechanism in the DF cooperativecommunication systems remains unclear. The difficulty of investigate the performance of sucha mechanism lies in the decoding results of relays, which are not available before themodulation scheme is chosen. This lack of the information of decoding results would degradethe performance of the SNR-based selection mechanism. Hence, in this part of the project, wewould like to first derive the average transmission rate of this mechanism followed by symbolerror rate (SER) performance and outage performance.One of problems of the SNR-based mechanism is that SNR-based mechanism is tooconservative when choosing the modulation scheme for transmission. That is, theconstellation size chosen by the SNR-based mechanism is smaller than the constellationscheme that can be actually supported in multinode cooperative communication systems. Thiswould give the SNR-based mechanism good SER and outage performance at the expense oflow system throughput. To avoid this, the third part of this project is to propose a scheme toselect proper constellation size and hence, the participating relays. The goal of this schemeaims at minimizing the outage rate of the systems while the transmission rate can be made aslarge as possible. The obtained throughput will be compared against that of the ideal case andthat under SNR-based scheme.
在多點合作式通訊系統中使用適應性調變並不是一件顯而易見的事情，特別時當decode-and-forward (DF) 方式使用的時候。主要的困難點來至於參與傳輸訊號之中繼點的解碼結果的不確定性。這解碼的結果是無法在選擇傳輸方式之前獲得的。我們在本計畫中將在假設解碼結果已知的情況之下來了瞭解適應性調變的理想表現。接著我們將會針對較為熱門的傳輸機制選擇的方式來探討適應性調變在在多點合作式通訊系統中的表現。目前較為熱門的傳輸機制選擇的方式為SNR-based 的機制。我們將會針對這樣的機制來探討其調變機制的分布、錯誤率與outage 的表現。從這些分析我們可以了解中繼點的解碼結果的不確定性對於系統效能的影響。最後本計畫將會提出一個新的傳輸機制選擇的機制。其主要是去改善SNR-based 的機制的保守性。在我們初步的模擬中我們發現在多點合作式通訊系統中SNR-based 的機制會選擇較差的傳輸機制來換取較好的錯誤率與outage 的表現。如此系統的傳輸速率就會因此而減少。我們會設計一個選擇機制來獲得較好的傳輸速率同時維持系統的錯誤率與outage 的表現。
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