結合動態通道調配機制和天線波束形成技術以支援包含非對稱性傳輸的分時雙工分碼多工存取系統

Abstract

本論文主要目的在於希望能夠藉由適當地運用時間與空間二個不同維度上的無線資源，以期在分時雙工（Time Division Duplex, TDD）分碼多工存取（Code Division Multiple Access, CDMA）系統下充分地支援非對稱性的資料傳輸，並同時提高整體系統效能。隨著非對稱性傳輸需求的日益增加，分時雙工分碼多工存取系統在未來的無線網路中將扮演著一個重要的角色。然而，由於分時雙工分碼多工存取系統中的細胞都使用相同的頻帶來上傳和下載資料，因此非對稱性的資料傳輸將會導致兩個彼此相鄰但是傳輸方向相反的基地台間產生極大的交錯時槽干擾（Cross-Slot Interference）。許多的研究顯示，交錯時槽干擾不但嚴重地影響系統效能，並造成龐大無線資源的浪費。 為了解決交錯時槽干擾的問題並增進整體系統性能，我們致力於研究關於各種動態通道調配（Dynamic Channel Assignment, DCA）機制以及天線波束形成技術（Antenna Beamforming）的特點。在前半部中，我們提出一個鏈接相對性動態通道調配（Link-Proportional DCA）機制搭配三區域指向性天線的優點，以減緩交錯時槽干擾對分時雙工分碼多工存取系統的影響。在指向性天線的幫助下，三區域的細胞系統將會由三個不同基地台的相鄰區域來形成一個虛擬細胞（virtual cell）。我們發現在這種細胞架構下，交錯時槽干擾將會被限制在一個虛擬細胞中，因此，鏈接相對性動態通道調配機制能夠專注在虛擬細胞中藉由使用者的無線鏈結品質來做時槽的配置，以達到充分降低交錯時槽干擾的需求。許多結果都顯示鏈接相對性動態通道調配機制能夠顯著的勝過其他動態通道調配機制，並能提供分時雙工分碼多工存取系統一個更有效率的資源配置法。 然而，我們發現大部分的動態通道調配機制，包括鏈接相對性動態通道調配，都無法有效的解決上傳時的交錯時槽干擾。為了更進一步解決這種交錯時槽干擾，我們更進一步提出了一個結合智慧型天線波束形成技術的交錯時槽干擾為主的動態通道調配機制（Cross-Slot Interference-Based DCA）。我們所提出的交錯時槽干擾為主的動態通道調配機制，主要希望能夠降低下載時的交錯時槽干擾，並利用細胞各自分散的方式調整上傳和下載的時槽數目，以其在個別細胞中充分地支援非對稱性的資料傳輸。智慧型天線的波束形成技術，在這邊將被用來對付上傳時嚴重的交錯時槽干擾。我們實驗的結果顯示所提出的交錯時槽干擾為主的動態通道調配機制能夠充分地壓制交錯時槽干擾的影響，進而使得分時雙工分碼多工存取系統能夠充分地滿足不同細胞間對於非對稱資料傳輸的個別需求，並同時能達到更高更好的系統效能。

The key idea of this thesis is to efficiently utilize the two dimensions of radio resource - time and space diversity to support the traffic asymmetry and enhance the system performance in the TDD-CDMA systems. To support the high speed multi-media services is one of the main objective of the future wireless communication systems. An important feature of these services which are different from the traditional voice services is the asymmetric traffic requirement. By exploiting the extra dimension in time slots, time division duplex (TDD) code division multiple access (CDMA) systems with unpaired frequency bandwidth is expected to support asymmetric traffic services and flexible radio resource allocation.

However, in TDD code division multiple access (CDMA) systems, because uplink and downlink transmissions share the same frequency band in every cell, the transmissions of asymmetric traffic from adjacent cells may cause heavy \emph{cross-slot} interference, which will seriously degrade the system performance. Therefore, it is usually suggested that the same time slot be used for the same transmission direction for two neighboring cells unless a base station is willing to leave the time slot unused. Apparently, this approach may waste resources and lose the key advantages of the TDD systems in supporting asymmetric traffic services.

To alleviate the impact of cross-slot interference and furthermore enhance the system performance, we investigate the advantages of the dynamic channel assignment (DCA) and multiple antenna techniques. So far, the dynamic channel assignment (DCA) algorithm has been considered as an important mechanism to enhance the performance of the TDD systems. Even so the capacity of TDD-CDMA system is still strong limited by the increasing co-channel interference. Recently, the advanced antenna techniques are charming in their high capacity and signal quality improvement. Because there are still some constraints in the TDD-CDMA systems, the antenna techniques cannot be well applied. We are highly encouraged to develop an efficient DCA algorithm with the advanced antenna techniques to enhance the system capacity and support the diverse traffic asymmetry.

Since the space diversity can provide an additional degree of freedom for allocating radio resource, we proposed two kinds of efficient algorithms, called the link-proportional dynamic channel assignment (LP-DCA) and smart dynamic channel assignment(SDCA) algorithms, respectively. The key idea of LP-DCA scheme is to classify the cross-slot interference and allocate the radio resource according to the users' received signal quality. With employment of sectorized antennas in the LP-DCA, the co-channel interference can be limited in the smaller area and alleviated easily. The SDCA employs time/space channel assignment to improve the received signal quality and minimize the co-channel interference. In the SDCA algorithm, we also take the advantages of antenna array to avoid the strong co-channel interfering sources. Though the numerical results we can demonstrate that these proposed algorithms not only offer flexibility in processing the un-uniform traffic environments but also achieve better link quality and call blocking performance.