協力多躍式網路的中繼站選擇機制之研究

Abstract

協力多躍式中繼站網路在近年來有大幅的進步。傳統上，中繼站主要是被用來達到較高的分集增益(Diversity)並改善鏈路可靠性。然而，文獻中很少探討如何選擇中繼站以達到較高的傳送率(Throughput)。 這篇論文的目標在於提出可提升傳送率並且維持鏈路可靠性的中繼站選取方法。我們在這篇文章中提出了兩種方法。第一種方法先計算每個中繼站對應到的傳送率，然後選擇對應傳送率最高的那個中繼站。雖然這個方法可以達到最高的傳送率，但是計算成本相當高。在第二種方法中，我們先比較中繼站到傳送端鏈路的SNR與中繼站到接收端鏈路的SNR，紀錄其中較小的為此中繼站所對應的瓶頸SNR。接著對每個中繼站重複此過程，最後選擇對應到最大瓶頸SNR的中繼站。兩種方法有相似的傳送率表現。然而，第二種方法比第一種有更小的中斷機率。同時，我們所提出的瓶頸SNR方法較傳統以訊號為基礎的同伴選擇機制有較高的傳送率，付出的代價是些微較高的中斷機率(Outage Probability)。此外，我們在多個中繼站的情況下探討我們提出的機制。在我們的模擬架構之下，我們發現多個中繼站的情形卻比單一中繼站有較低的傳送率跟較高的中斷機率。這是因為多個中繼站消耗較多的功率以及有更高的機率選取到不合適的中繼站。 在傳統的功率分配方法中，中繼站到傳送端的傳輸功率相等於傳送端到接收端的傳送功率。當中繼站個數增加的時候，我們建議一套簡單的功率分配演算法，以用來調整中繼站分配到的傳輸功率。每個中繼站的傳輸功率反比於中繼站的數目，所有中繼站的傳輸功率總合應等於傳送端的傳輸功率。模擬結果顯示在一樣的功率消耗中，即使是多個中繼站的情況中斷機率也能獲得改善。我們所建議的功率分配可以消除多個中繼站在第二傳輸傳送階段所造成的不必要功率浪費。 總結來說，我們首先提出了兩個以傳送率為導向的中繼站選擇方法，因此得到傳送率與可靠性之間的抵換關係。其次，我們發展功率分配的演算法，就算在多個中繼站的情況中，仍可以改善中斷機率。最後，我們為多躍式中繼站網路的同伴選取機制設計提供有用的著眼點。

Cooperative multi-hop relay networks progress significantly in recent years. Conventionally, relay stations are used to achieve higher diversity gain and improve link reliability. However, how to choose a relay to obtain higher throughput is rarely discussed in the literature. This thesis is aiming at proposing the relay selection rules to achieve higher throughput, while maintaining link reliability. We propose two partner selection methods in this thesis. The first one is to calculate throughput corresponding to each relay, and then choose the relay achieving the maximal throughput. Though this method can achieve the highest throughput, its computation cost is quite high. In the second method, we first compare the SNR of the relay link from the source with the SNR of the relay link to the destination, and designate the smaller one as the bottleneck SNR associated with that relay. The bottleneck SNR of each relay is recorded and compared. The relay with the largest bottleneck SNR is selected. Both methods have the similar throughput performance. However, the outage probability of the second method is better than that of the first one. Meanwhile, compared to the conventional signal-based partner selection, the proposed bottleneck SNR approach can achieve higher throughput at the cost of small SNR degradation. Furthermore, we examine the performance of the considered relay selection rules in the multiple relay case. We find that at the same consumed power level the outage probability and throughput performance in the multi-relay case is indeed worse than those in the one relay case. This is because the multi-relay case yields more power consumption and higher probability in selecting inappropriate relays. In the traditional method, the transmit power allocated in the relay link from the source is the same as that in the relay link to the destination. Now, we suggest a simple power distribution algorithm to adjust transmit power from the relay as the number of the relays increases. In the suggested power distribution rule, the transmit power of each relay is inversely proportional to the number of relays, and the sum of the total transmit power from the relay is equal to the transmit power from the source. Our results show that at the same consumed power level the proposed power distribution can improve outage probability, while maintaining throughput even in the multi-relay case. The proposed power allocation can eliminate unnecessary power in the second transmission phase with multiple relays. To summarize, we first propose two throughput-oriented relay selection rules and provide the trade-off analysis between throughput and reliability. Second, we develop a power distribution algorithm and improve the outage probability even in the multi-relay case. Last but not least, we obtain insights of relay selection rule for the design in a multi-hop relay network.