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

• Main Authors: Tseng, Chun-Kai, 曾俊凱
• Other Authors: Wu, Sau-Hsuan
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/f6c3y6

博士 === 國立交通大學 === 電信工程研究所 === 106 === 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.