小細胞網路中合作式中繼與多點傳輸之協調機制

Coordination Mechanisms for Cooperative Relaying and Multi-point Transmissions in Small Cell Networks

曾俊凱(Chun-Kai Tseng)

指導教授 ： 伍紹勳

國立交通大學/電機學院/電信工程系所/博士(2018年)

摘要

為了提高下一代無線通訊系統服務質量，小細胞(small cell)技術已經被視為一項嶄新且有效解決方案 。通過在大型基地台(macro base station)的通訊覆蓋範圍下密集佈署小細胞基地台，可以使得提供或輔助數據傳輸服務的無線接取點(access point)盡可能地接近終端使用者，這種方式大大地改善了接收端的連線品質(link quality)。在本論文中，我們探討了兩種類型的小細胞基地台，每種類型都針對不同的傳輸情境和要求。對於第一種類型，我們考慮使用一組由無線迴路網路(wireless backhaul)構成，具有成本效益的小細胞基地台，其旨在幫助大型基地台轉傳訊號至接近蜂巢邊緣(cellular edge)的使用者。簡而言之，這類型的小細胞基地台基本上是大型基地台的延伸。在本論文的後續當中，我們考慮另一種不同類型的小細胞基地台，它們在城市地區內超密集部署，用來顯著提高整體的網絡容量(network capacity)。為了避免層間(inter-tier)干擾，這類型的小細胞基地台通常被配置與大型基地台不同的頻帶上發送訊號。換句話說，與前一類型不同，此小細胞基地台們可以獨立於大型基地台工作。

合作式中繼—可讓第一種類型的小細胞基地台支援數據上的傳輸，針對提高傳輸的可靠度， 提供了一種具有前景且有成本效益的解決方案。合作式中繼不僅可以補償無線電傳播中訊號功率的路徑損耗，而且還可以提供不同空間的通道多樣性來對抗通道的多重路徑衰減。就一般認知而言，將中繼技術整合至自動重傳請求(automatic repeat request)機制將能夠增強系統的通道多樣性和吞吐量。然而，當投機式(opportunistic)放大轉傳(amplified-and-forward)中繼技術應用於合作式自動重傳請求時，其系統設計變得更加複雜。首先，從我們的通道容量失效(capacity outage)分析中顯示，除非中繼訊號的品質高於某門檻值，否則在單一放大轉傳中繼協助下的自動重傳請求不能有效利用重傳機制在時間上的通道多樣性。我們進一步將這種選擇性放大轉傳中繼的概念擴展到具有多個中繼的系統，試圖能夠在自動重傳機制上共同使用時間與空間上的通道多樣性。然後針對這種類型的中繼輔助自動重傳請求，我們開發了兩種類型的選擇性投機式放大轉傳中繼方法。從我們進一步的分析顯示，如果不使用中繼器之間的監聽(overhearing)機制以及適當的通道品質控制機制來預先檢測被監聽的訊號，其合作式重傳機制就不能有效地充分利用時間和空間通道的多樣性。該通道品質控管機制可由一組門檻值組成，其中每個門檻值是用來控管中繼路徑上傳送與監聽者之間的通道品質。相對應的，不同於我們的設計，對於典型的機會式放大轉傳中繼輔助下的自動重傳請求，其機制則遭受到嚴重的通道多樣性損失。

要充分使用所提出的自動重傳請求機制上的通道多樣性，其關鍵在於對中繼訊號適當的通道品質控制機制。雖然已從容量失效機率的角度提出了一些品質控制的設定方法，但在後續的分析中顯示，依照這些法設定下的自動重傳請求，在封包錯誤率(packet error rate)方面，不足以充分的利用其潛在豐富的通道多樣性。對此，我們從最大相似解碼(maximum likelihood decoding)的角度來分析其系統錯誤率的效能。從分析的結果，我們發現到在通道品質控制上的門檻設定要求，與從失效機率理論分析角度上的設計方法，兩者有著本質上的不同。首先，在錯誤率方面，要利用通道多樣性，通道品質控管的門檻值需隨著訊雜比(signal to noise ratio)的大小來增加，其為必要條件。接著我們提出了兩個充足的門檻條件。根據這些門檻條件，進而分析出條件本身，系統架構，通道編碼(channel code)的最小距離與訊雜比之間的關係。接著利用此結果與先前的失效機率分析的結果，發展出一套啟發性但有效的門檻值搜尋方法，來達到在錯誤率方面同時開發通道多樣性增益以及訊雜比增益。透過真實通道編碼的模擬也驗證了我們的分析和所提出的通道品質控制方法的有效性。模擬結果還顯示，所提出的合作式自動重傳機制能夠有效的增強系統的吞吐量，並且與沒有中繼輔助的自動重傳機制相比，可為細胞邊緣用戶提供幾乎兩倍的吞吐量增益。

除了合作式中繼之外，超密集小細胞網絡(ultra-dense small cell network)也被認為是未來移動通訊系統中最有前途的解決方案之一，用來滿足未來十年的爆炸性行動通訊容量需求。儘管超密集式小細胞網路強整體系統吞吐量的潛力，但來自相鄰小細胞的干擾仍可能會惡化接近細胞邊緣使用者的性能，而成為提高服務品質上的瓶頸。為了解決這個問題，將協調式多點傳輸(coordinated multi-point transmission)技術應用在小細胞網路似乎是可行解決方案。但因細胞網路下幾乎隨機部署基地台的特性和有限的小細胞基地台天線資源，使得難以針對小細胞網路的協調多點服務設計出有效的資源分配機制。鑑於此，在一個集中式合作無線接取網路架構下，我們針對小細胞網路中的干擾和天線資源管理提出了簡單但有效的協調多點傳輸方法。該方法的操作基本上遵循兩步機制：第一步將整個服務區劃分為多個區域，第二步調整每個區域的活動使用者的數量。基於這種思想，我們採用了一種實際的區域劃分方法，並利用空間多工(spatial multiplexing)增益，陣列(array)增益和協作分集(cooperative diversity)增益來開發多層次的天線分配演算法，以提高系統邊緣使用者的吞吐量。另外，為了提改善所提出架構的可擴展性，我們進一步採用兩層式的機制來實現基於服務區域分群的多點傳輸方法。模擬的結果顯示，我們提出的服務架構可以在保持相對較高的平均用戶吞吐量的同時，顯著地改善超密集小細胞網絡下細胞邊緣使用者的吞吐量。

關鍵字

合作式通訊 ； 小細胞 ； 合作式自動重傳請求機制 ； 選擇性投機式放大轉傳中繼技術 ； 通道品質控制 ； 集中式合作無線接取網路 ； 協調式多點傳輸服務 ； 超密集式小細胞網路 ； 天線配置

並列摘要

Small cell techniques have been regarded as an emerging and effective solution for enhancing the quality of services (QoS) in the next generation of wireless communication systems. By densely deploying small cell base stations (SBSs) within the coverage of a macro base station (MBS), the data transmission service can be provided or assisted with an access point as close as possible to the end users, which largely improves the link qualities at the receiving ends. In this dissertation, we investigate two types of SBSs, with each targeted at a different transmission scenario and requirement. For the first type, we consider a set of cost-efficient SBSs deployed with wireless backhaul, aiming to help the MBS retransmit signals to users that are close to the cellular edge. Simply speaking, the SBSs are an extension for the MBS. In the sequel, we consider a different type of SBSs, where they are ultra-densely deployed in an urban area in order to significantly enhance the entire network capacity. To avoid inter-tier interference, the SBSs are usually configured to transmit on a dedicated frequency band different from that of the macrocells. In other words, different from the former type, the SBSs can work independently of the underlying MBSs.

Cooperative relaying, enabling the first type of SBSs to support data transmissions, provides a promising and cost-effective approach to increase the transmission reliability, which not only can compensate the path loss of signal power in radio propagation but also can exploit the spatial diversities offered by alternative transmission routes to combat wireless fading. Incorporating relaying techniques into Automatic Repeat reQuest (ARQ) in general will provide diversity and throughput enhancements. However, when opportunistic amplified-and-forward (AF) relaying is applied to cooperative ARQ, the system design becomes much more involved. First, our capacity outage analysis shows that the temporal diversities of ARQs with a single AF relay cannot be exploited unless the channel quality to the relay exceeds a threshold. This notion of selective AF relaying is extended to systems with multiple relays in an attempt to jointly explore the temporal and spatial diversities with ARQ. Two types of selective and opportunistic AF (SOAF) relaying schemes are then developed for such kinds of relay-assisted ARQ. And our analysis further shows that the temporal and spatial diversities cannot be fully exploited without the use of overhearing among relays and a proper link quality control mechanism to pre-screen the overheard signals. This quality control mechanism is implemented with a set of thresholds designed for each hop of the relaying path. In contrast to our designs, the ARQ scheme with the typical opportunistic AF (OAF) relaying method suffers from severe diversity losses.

The key to exploit the diversity potential of SOAF ARQ lies in a proper quality control mechanism for the relayed signals. Although quality control methods have been proposed from the viewpoint of capacity outage probability, the methods are shown in the sequel not enough for SOAF ARQ to exploit the full diversity in packet error rates (PERs). Reinvestigating the quality control problem from the perspective of maximum likelihood decoding (MLD), our analysis shows that the PERs can attain the full spatial diversity offered by relays in every ARQ round only if the thresholds to control the received signal qualities of the forwarding relays increase with the signal-to-noise ratio (SNR). Sufficient conditions on the thresholds settings are proposed, which explicitly characterize the relationship among the threshold requirements, the SNR, and the minimum codeword distance of the employed channel code. An effective threshold searching algorithm is further developed for SOAF ARQ to exploit both the diversity and the SNR gains in PER. Simulations with trellis codes also verify our analysis and the effectiveness of the proposed quality control method. Simulations also show that the proposed ARQ schemes are more effective in throughput enhancement, and can provide cell-edge users almost two times the throughput gain in comparison with ARQ with no relay- assisted forwarding.

Besides cooperative relaying, ultra-dense small cell network (UDN) has also been regarded as one of the most promising solutions for the future mobile communication systems to meet the explosive wireless capacity demand in the next decade. Despite the expected system throughput enhancement, the throughput of cell-edge users remains to be a bottleneck for improving the quality of experience in UDN. Applying coordinated multi-point (CoMP) transmissions among cells appears to be a feasible solution for resolving this issue. However, the nature of the almost randomly deployed locations of small cell base stations and their limited numbers of antennas make it difficult to design an effective resource allocation mechanism for CoMP services in UDN. In view of this, we present herein a simple but effective two-step approach under a centralized cooperative radio access network (C-RAN) architecture to help implement CoMP services in UDN. The first step is to partition the entire service region into multiple areas, and the second step is to adjust the number of active users of each transmission frame interval for each area. Based on this idea, we adopt a practical area partition method, and then develop a multi-layer antenna allocation scheme to enhance the throughput of cell-edge users. To improve the scalability of the proposed idea, we further apply a two-tier service architecture to realize the proposed method based on clustering of the service areas. Simulation results show that our proposed service architecture can provide a significant improvement on the cell-edge throughput of UDN while maintaining a good average user throughput.

並列關鍵字

cooperative communication ； small cell ； cooperative ARQ ； selective and opportunistic AF relaying ； channel quality control ； C-RAN ； CoMP service ； UDN ； antenna allocation

參考文獻

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