高速下行鏈路封包存取及多載波直接序列分碼多重接取系統之無線電資源管理

Abstract

無線電資原管理在未來的無線電系統中之服務品質控制及系統容量最佳化扮演了很重要的角色。在本篇論文中，我們針對了寬頻分碼多重接取之高速下行鏈路封包存取系統及多載波直接序列分碼多重接取系統中幾個重要的無線電資源管理議題進行深入的研究。 在本論文的第一個部份中，我們著重在寬頻分碼多重接取之高速下行鏈路封包存取系統所使用的平行多通道停止並等待混合自動重傳要求機製中所謂的停滯問題之研究。當停滯的情況發生時，接收端會持續的等待一個不會被重傳的封包並且中斷了將媒體存取控制層所接收的封包送往上層的程序。依據一個新的評量標準，稱為間空處理時間，我們提出了一個評量三種停滯防止機製的分析模型，其中包含了以計時器為基礎、以視窗為基礎以及以指示器為基礎的停滯防止機製。從分析及模擬結果中我們發現，以指示器為基礎之停滯防止機製是三種機製中表現最好的方法。最後，我們更進一步分析以指示器為基礎之停滯防止機製在交錯式排程法中的系能表現。我們所提出的分析方法具有下列幾個優點：（一）有助於決定平行多通道停止並待待自動重傳要求之通道數目以及在限定間空處理時間情況下，系統之允許控制政策可容納的滿載用戶數量；（二）有助於媒體存取控制層與無線電鏈路控制層的跨層級設計，如無線電鏈路控制層之暫停時間及視窗大小之設計。 在本論文的第二部份中，我們則探討了多載波直接序列分碼多重接取系統中之功率控制、碼通道分配以及子載波功率分配之無線電資源管理議題。首先，我們分析了功率控制錯誤及完整多重接取干擾對多速率多載波直接序列分碼多重接取系統中上行鏈路的影響。我們發現（一）功率控制錯誤會加重多重接取干擾的嚴重性，反之亦然；（二）增加頻域及時域之展頻增益會增加系統對功率錯誤反應的敏感度；（三）較大的功率控制錯誤所會造成的多樣性増益下降在頻域展頻比時域展頻更為顯著。 而在於多載波直接序列分碼多重接取系統的下行鏈路中，我們發展了一套以多重接取干擾系數為基礎之干擾防止碼通道分配法。多重接取干擾系數可用來衡量加諸於多載波直接序列分碼多重接取系統中碼通道的干擾量。藉由數據結果的呈現，我們驗證了干擾防止碼通道分配法能有效的降低碼通道所受的干擾量並且維持高水準的碼通道允許率。再者，我更進一步提出了混合子載波功率與碼通道分配法以提升在多重接取干擾下之訊號品質。 總結而言，我們探討了使用了多通道停止並等待自動重傳要求之高速下行鏈路封包存取系統在間空處理時間與系統流量之間的權衡關係。再者，我們分析了多載波直接序列分碼多重接取系統上行鏈路在功率控制誤差及多重接取干擾雙重影響下的性能表現。最後，我們研究多載波直接序列分碼多重接取系統下行鏈路中如何在干擾量較小的條

Radio resource management (RRM) plays a key role in quality-of-service (QoS) control and capacity optimization for future wireless systems. In this dissertation, we investigate several critical RRM issues in the high speed downlink packet access (HSDPA) of wideband code division multiple access (WCDMA) system and the multi-carrier direct-sequence code division multiple access (MC-DS-CDMA) system.

In the first part of this dissertation, we focus on the stall issue of the parallel multi-channel stop-and-wait (SAW) hybrid auto-retransmission request (HARQ) mechanism for the HSDPA of WCDMA system. In the stall situation, the receiver may wait for a packet that will be no longer be sent by the transmitter and stops delivering the medium access control (MAC) layer packets to the upper layer. We present an analytical approach to evaluate three stall avoidance schemes \rule[1mm]{0.4cm}{0.03cm} the timer-based, the window-based, and the indicator-based schemes based on a newly proposed performance metric \rule[1mm]{0.4cm}{0.03cm} gap processing time. We demonstrate that the indicator-based stall avoidance scheme outperforms the timer-based and the window-based schemes. Finally, we further derive the closed-form expression for the gap-processing time of the indicator-based stall avoidance mechanism when applying the interleaving scheduling. The developed analytical approaches have several advantages including: (1) help determine a proper number of processes for the parallel SAW HARQ mechanisms and the number of acceptable fully loaded users for an admission control policy subject to the gap processing time constraint; (2) facilitate the MAC/radio link control (RLC) cross-layer design for the RLC timeout and RLC window size.

In the second part of this dissertation, we focus on the RRM issues in the MC-DS-CDMA systems, including power control, code admission, and subcarrier power allocation. We first analyze the joint effects of the power control errors (PCE) and the complete MAI in the multi-rate uplink MC-DS-CDMA system. We find that (1) the effect of PCE can exacerbate the impact of the complete MAI, or vice versa; (2) increasing frequency or time domain spreading gain results in a higher sensitivity to the PCEs; (3) a larger PCE can possibly make the frequency domain diversity diminish faster than the gain obtained from the time-domain spreading.

For the downlink MC-DS-CDMA system, we develop an MAI-coefficient-based interference avoidance code assignment scheme. The \textit{MAI coefficient} can be applied to quantitatively predict the MAI effect imposed on a particular code in a multi-rate MC-DS-CDMA system. We show that the proposed interference avoidance code assignment method can effectively reduce the MAI in a multi-rate MC-DS-CDMA system, while maintaining a very good call admission rate. Moreover, we further propose a joint subcarrier power allocation and code assignment scheme to optimize the received signal quality in the presence of MAI.

In summary, we investigate the performance tradeoffs between the gap processing time and throughput of the multi-process SAW HARQ mechanism in the HSDPA system. Moreover, we analyze the joint effects of PCEs and MAI in the uplink MC-DS-CDMA system. Finally, we investigate how to assign a code with less MAI and how to allocate the subcarrier power allocation for the downlink MC-DS-CDMA system.