運用傳送功率分配於欠定多輸入多輸出系統達到公平傳輸速率

Abstract

在無線通訊系統中，多輸入多輸出技術不需額外頻寬及傳輸功率即能提高傳輸速率及改善傳輸品質。然而，在多用戶多輸入多輸出上鏈系統的環境中，由於部分用戶可能距離基地台較遠或被障礙物所阻擋，以致通道增益較小，導致傳輸速率變差。在本篇論文中，吾人提出一個傳送功率分配法達到公平的傳輸速率。吾人將一非線性最佳化問題轉換成適當形式，將之套用到現有的演算法中。此公平化的機制可使得等效子通道增益較為均勻，因此等效通道矩陣會傾向於良置（well-conditioned）。因為在欠定多輸入多輸出系統中，有效率的解碼器均基於球體解碼器，因此通道矩陣有較小的條件數（condition number），可使得解碼的複雜度降低。最後，吾人亦提出另一功用函數用於改善條件數，進一步降低運算複雜度。模擬結果顯示吾人提出方法的有效性。

In wireless communication systems, multiple-input multiple-output (MIMO) technology offers significant increases in data rate and link range without additional bandwidth and transmit power. However, in uplink multi-user MIMO systems, some users will suffer from small channel gains due to being far away from the base station or blocked by obstacles in practical environments. This result in poor data rates for those users. In this thesis, we propose a transmit power allocation scheme with fair rate allocation for all users. We reformulate a nonlinear optimization problem to a modified form which can be applied to the existing algorithms. The proposed fairness scheme also leads to uniform sub-channel gains. Thus the equivalent channel matrix will tend to be well-conditioned. Since efficient decoders of underdetermined MIMO systems are based on sphere decoders, the decoding complexity can be reduced with a smaller channel condition number. Finally, we also propose an alternative utility function to improve the condition number, to further reduce the decoding complexity. Simulation results confirm the effectiveness of the proposed methods.