Downlink Rate Analysis for Virtual-Cell Based Large-Scale Distributed Antenna Systems
Downlink Rate Analysis for Virtual-Cell Based Large-Scale Distributed Antenna Systems
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DOI：
10.1109/twc.2015.2497678
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发表时间：
2015-06
影响因子：
10.4
通讯作者：
Junyuan Wang;L. Dai
中科院分区：
文献类型：
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作者：
Junyuan Wang;L. Dai
尽管通过大量地理上分布的天线的联合传输实现了显著的速率提高，但由此产生的计算成本和信道测量开销对于大规模分布式天线系统（DAS）来说可能是负担不起的。因此，一个可扩展的信号处理框架是非常需要的，它可以基于虚拟单元的概念建立。在基于虚拟小区的DAS中，每个用户选择几个相邻的基站（BS）天线组成其虚拟小区，即其自己的服务BS天线集。本文以具有大量用户且BS天线均匀分布在一定区域的下行DAS为研究对象，研究虚拟小区大小对平均用户速率的影响。具体而言，假设每个用户的虚拟小区采用最大比传输（MRT），推导出每个用户可实现的遍历率作为所有用户到其虚拟小区的大规模衰落系数的显式函数，并建立了平均用户速率的上界，在此基础上建立了确定最佳虚拟小区大小以最大化平均用户速率的经验法则。进一步将分析扩展到考虑多个用户组在一起并使用零强迫波束形成（ZFBF）由虚拟小区共同服务。与不分组的情况（首选小的虚拟单元大小）相比，研究表明，通过将具有重叠虚拟单元的用户分组，可以通过增加虚拟单元大小来显着提高平均用户速率，尽管代价是更高的信号处理复杂性。
Despite substantial rate gains achieved by joint transmission from a massive amount of geographically distributed antennas, the resulting computational cost and channel measurement overhead could be unaffordable for a large-scale distributed antenna system (DAS). A scalable signal processing framework is therefore highly desirable, which could be established based on the concept of virtual cell. In a virtual-cell based DAS, each user chooses a few neighboring base-station (BS) antennas to form its virtual cell, i.e, its own serving BS antenna set. In this paper, we focus on a downlink DAS with a large number of users and BS antennas uniformly distributed in a certain area, and aim to study the effect of the virtual cell size on the average user rate. Specifically, by assuming that maximum ratio transmission (MRT) is adopted in each user's virtual cell, the achievable ergodic rate of each user is derived as an explicit function of the large-scale fading coefficients from all the users to their virtual cells, and an upper-bound of the average user rate is established, based on which a rule of thumb is developed for determining the optimal virtual cell size to maximize the average user rate. The analysis is further extended to consider multiple users grouped together and jointly served by their virtual cells using zero-forcing beamforming (ZFBF). In contrast to the no-grouping case where a small virtual cell size is preferred, it is shown that by grouping users with overlapped virtual cells, the average user rate can be significantly improved by increasing the virtual cell size, though at the cost of a higher signal processing complexity.