摘要
协作通信作为一种新型的传输技术，是利用无线通信系统中随机分布的用户天线，构成虚拟MIMO系统。在不增加硬件复杂度的前提下可获得空间分集增益、提高信道容量和无线链路的传输可靠性，能有效扩大无线传输的覆盖范围，备受学术界和工业界的广泛关注。因此，本文对协作通信中物理层的若干关键技术进行了研究。
根据协作节点的信号处理方式，协作通信协议可分为三种：放大转发、解码转发、编码协作。将这三种基本协作机制应用于不同的中继选择策略中，又形成了自适应(选择性)协作和机会协作机制。本文对这些协作机制的基本原理和容量性能进行了研究。在此基础上，研究了OFDM/OFDMA与协作通信技术的中断概率，分析了二者所获得的分集增益与协作节点的个数之间的关系。
在无线通信系统中，如何高效、合理地使用有限的功率、频率等资源是一个非常重要的问题。根据不同的优化目标，本文对无线通信系统的信道和功率分配技术的研究现状和研究热点进行详细阐述，对OFDM系统的无线资源优化目标进行了规划，并分析了它们之间的相互转化关系。以此为基础，对OFDM/OFDMA协作通信系统中的子信道和功率分配技术进行了深入研究。通过引入等效信道增益的概念，以maxmin准则作为优化准则，提出了一种兼顾用户的速率公平性和系统容量的子信道和功率联合分配算法。理论研究表明，该联合算法使得所有用户获得相同的传输速率的同时，获得较高的系统容量。为了降低计算复杂度，本文还提出了一种次优的子信道和功率分配算法。该算法将子信道和功率分配问题分成两个子问题，首先假设总功率在所有的子载波上平均分配，以此为前提在每个分配周期内将最好的子信道分配给速率最小的用户。在确定子信道分配之后，利用多水平注水算法将总功率在所有的等效中继信道中进行最优分配。计算机仿真表明，该次优算法的性能接近最优算法的性能，而计算复杂度大大降低。
无线协作通信系统中的协作节点是随机分布的，如何从众多的备选协作节点中选择一个或多个合适的节点作为协作伙伴，是用户之间建立协作关系的前提。本文对协作通信系统的中继节点选择算法进行了深入研究。根据不同的通信目标和优化准则，中继节点选择算法可分为基于信道状态信息的中继选择算法、基于效用函数的中继选择算法、基于协作区域的中继选择算法等。与无线资源分配相似，选择协作节点的目标是提高系统容量、降低差错概率以及降低发射功率。因此，本文将功率分配、子信道分配和中继节点选择进行综合考虑，从最优化的角度对OFDMA协作中继系统中的中继节点选择问题进行了研究。以用户之间占用的子信道数为约束条件、以最大化系统的总吞吐量为目标，本文提出了一种基于即时信道增益的机会中继选择算法。仿真结果表明，该算法所获得的吞吐量接近无子载波限制时的系统吞吐量，同时用户占用子信道的公平性最大。因此，该算法对吞吐量和子载波公平性进行了很好的折衷。
在多小区OFDMA系统中，小区内的子载波是相互正交的，因此小区内的干扰较小，影响系统性能、尤其是小区边缘用户性能的关键因素是小区间干扰。为了降低小区间干扰，提高小区边缘用户的传输速率，3GPP LTE中提出了很多降低小区间干扰的技术，如部分频率复用、软频率复用、干扰协调、干扰随机化、干扰删除等。另一个备受关注的研究热点是多点协作技术。本文以部分频率复用为基础，研究了OFDMA系统中的多小区协作传输技术。该算法将每个小区分成内部和外部两个区域，同时将外部区域分成三个扇区。内部区域的频率复用系数为1，外部区域的频率复用系数为3。将相邻三个小区的三个扇区组成一个虚拟小区，以每个用户占用的子信道数量为约束条件，由基站控制器对三个扇区天线进行协调调度，共同对虚拟小区内的用户传输数据。理论研究表明，该算法通过增加每个扇区天线的有效用户数量，提高了每个天线的多用户分集增益。仿真结果表明，该算法有效地提高了小区边缘用户的吞吐量和占用子信道的公平性。
Cooperative communication is a promising diversity technique that has been attracting significant research interests in academic and industry communities, where the antennas of randomly distributed users are shared to form a virtual MIMO system and reap the benefits of spatial diversity without any requirement of additional antennas at each single-antenna users. As a potential technique for 4G, Cooperative communication can improve the channel capacity, enhance the transmission reliability of wireless channel, and enlarge the coverage. In this dissertation, some key techniques in Physical Layer are investigated.
According to the signal processing method, the basic cooperative diversity protocols can be cataloged into three classes:Amplify-and-Forward (AF), Decode-and-Forward (DF), and Coded Cooperation (CC). Combine these cooperation schemes with different relay se-lection strategies, adaptive (selection) cooperation and opportunistic relaying protocols are developed. The dissertation investigates the principle and capacity performance of these cooperation schemes. Based on these results, the outage performance of OFDM/OFDMA cooperative relaying protocols are also studied. Subsequently, the relationship between the cooperative diversity and the number of relays is analyzed.
In wireless communication systems, it is an important research issue that how to utilize power and frequency efficiently and appropriately. There are diverse channel and power al-location methods according to different optimization objectives. All the channel and power allocation methods are expounded in details and are cataloged into MA and RA optimization problem. The transformation between MA and RA are also analyzed. Based on these analy-sis, the subchannel and power allocation techniques in OFDM/OFDMA cooperative relaying networks are investigated thoroughly. A joint subchannel and power allocation algorithm is proposed, in which equal channel power gain is introduced and maxmin criterion is adopted as an optimization metric. The proposed algorithm properly deals with the rate fairness and system capacity. Theoretical investigation and simulation indicate that the proposed opti-mal algorithm can obtain higher system capacity while result in equal transmission rate of each user. In order to reduce the computational complexity, a suboptimal subchannel and power allocation algorithm is also proposed, in which the original optimization problem is divided into two subproblem, namely, subchannel assignment and power distribution. In the suboptimal algorithm, the available subchannels are assigned to each user under the assumption that the total transmission power is distributed equally among all subchannels. After subchannel assignment, the total transmission power is distributed among all deter-mined subchannel pairs by using multi-level water-filling algorithm.
Since the cooperation partners are located randomly in wireless cooperative communi-cation networks, it is the foundation of cooperation among multiple users that how to appro-priately select one or multiple cooperation partners from multiple potential relay nodes. The dissertation studies the relay node selection algorithms in cooperative communication sys-tems. According to different achievable targets and optimization objective, the commonly used relay nodes selection strategies can be classified into the followings which are relay nodes selection strategy based on channel state information, relay nodes selection strategy based on utility function, relay nodes selection strategy based on cooperation regions, and joint relay nodes selection and power allocation, et al.. Compared with radio resource allo-cation, the objectives of relay nodes selection are improve system capacity, reduce the error probability, and save transmit power. Thus, as one of the popular issue on relay selection, relay selection can be considered jointly with subchannel and power allocation. From the point of view on optimization problem, the relay nodes selection problem in OFDMA cooper-ative relaying networks are investigate, in which power distribution, subchannel allocation, and relay nodes selection are considered jointly. An opportunistic relay nodes selection strat-egy base on instantaneous channel state information is propose in the dissertation, of which the objective is to maximize the total throughput of all users under the constraints of fair-ness on subchannel occupation of each user. Simulation results show that the capacity of the proposed algorithm approaches asymptotically to that of the optimal one without fairness constraint and the fairness on subchannel occupation by each is most. Therefore, it can be said that the proposed algorithm can do the best tradeoff between throughput and fairness.
In multi-cell OFDMA networks, the subchannels used within the cell are orthogonal to each other and then intra-cell interference is little. The key factor which degrade the sys-tem performance, especially the performance of cell-edge users, is inter-cell interference (ICI). In order to depress ICI which can further improve the transmission rate of cell-edge users, many ICI reduction methods, such as Fractional Frequency Reuse (FFR), Soft Fre-quency Reuse, ICI Coordination, ICI Randomization, ICI Cancellation, have been proposed in 3GPP LTE. Another hot research topic in 3GPP LTE is Coordinated Multi-Point (CoMP) transmission technique. In the dissertation, a multi-cell cooperative transmission technique is proposed which is based on fractional frequency reuse technique. In this algorithm, each cell is partitioned into interior and exterior region, and subsequently the exterior region is divided into three sector. The frequency reuse factor is 1 in the interior region while 3 in the exterior region. Three adjacent sectors belonging to three neighboring cells is constituted to a virtual-cell. The base station controller coordinately schedules three sector antennas and served all users in in a virtual cell. Theoretical investigation show that the proposed algo-rithm improve the multiuser gain by increasing the effective users of each sector antenna. Simulation results show that the proposed algorithm improve efficiently the throughput of cell-edge users and fairness of channel occupation among all users.
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