[论文解读] Interference Channels with Coordinated Multi-Point Transmission: Degrees of Freedom, Message Assignment, and Fractional Reuse

Aly El Gamal, V. Sreekanth Annapureddy+1 more|arXiv (Cornell University)|Nov 13, 2012

Advanced MIMO Systems Optimization参考文献 20被引用 4

一句话总结

本文研究了在协调多点（CoMP）传输下，K用户干扰信道中的自由度（DoF）。每个消息最多由M个发送端共享。结果表明，在完全连接的信道中，无论M ≥ 2，其渐近用户自由度仍保持为1/2；但在每个发送端仅连接至自身及L个相邻接收端的局部连接信道中，自由度可提升至至少max{1/2, 2M/(2M+L)}，在L=1的Wyner模型中，通过非对称消息分配和部分频谱复用，可达到2M/(2M+1)。

ABSTRACT

Coordinated Multi-Point (CoMP) transmission is an infrastructural enhancement under consideration for next generation wireless networks. In this work, the capacity gain achieved through CoMP transmission is studied in various models of wireless networks that have practical significance. The capacity gain is analyzed through the degrees of freedom (DoF) criterion. The DoF available for communication provides an analytically tractable way to characterize the capacity of interference channels. The considered channel model has K transmitter/receiver pairs, and each receiver is interested in one unique message from a set of K independent messages. Each message can be available at more than one transmitter. The maximum number of transmitters at which each message can be available, is defined as the cooperation order M. For fully connected interference channels, it is shown that the asymptotic per user DoF, as K goes to infinity, remains at 1/2 as M is increased from 1 to 2. Furthermore, the same negative result is shown to hold for all M > 1 for any message assignment that satisfies a local cooperation constraint. On the other hand, when the assumption of full connectivity is relaxed to local connectivity, and each transmitter is connected only to its own receiver as well as L neighboring receivers, it is shown that local cooperation is optimal. The asymptotic per user DoF is shown to be at least max {1/2,2M/(2M+L)} for locally connected channels, and is shown to be 2M/(2M+1) for the special case of Wyner's asymmetric model where L=1. An interesting feature of the proposed achievability scheme is that it relies on simple zero-forcing transmit beams and does not require symbol extensions. Also, to achieve the optimal per user DoF for Wyner's model, messages are assigned to transmitters in an asymmetric fashion unlike traditional assignments where message i has to be available at transmitter i.

研究动机与目标

• 分析通过协调多点（CoMP）传输增强的K用户干扰信道中的自由度（DoF）。
• 研究在完全连接的干扰信道中，消息分配与协作阶数M是否能实现超过1/2的渐近用户自由度。
• 探讨局部连通性——即每个发送端仅连接至自身及L个相邻接收端——对自由度增益的影响。
• 确定最优的消息分配策略，并验证实现所有用户相等自由度是否需要部分频谱复用。
• 建立外 bound 并通过零迫波束成形证明自由度增益的可实现性，且无需符号扩展。

提出的方法

• 采用自由度（DoF）准则来表征CoMP传输下干扰信道的高信噪比总容量。
• 引入协作阶数M，限制每条消息最多由M个发送端共享，以模拟有限回传容量。
• 分析完全连接与局部连接的干扰信道模型，其中局部连通性定义为每个发送端仅连接至自身及L个相邻接收端。
• 提出一种无需符号扩展的零迫波束成形方案，可在非期望接收端实现干扰消除。
• 采用非对称消息分配策略，即消息i不一定要分配给发送端i，尤其在L=1的Wyner模型中更为显著。
• 引入部分频谱复用，以确保所有用户均能达到相等的自由度，因方案中部分接收端必须保持不活跃。

实验结果

研究问题

• RQ1 在完全连接的K用户干扰信道中，CoMP传输配合协作阶数M ≥ 2是否能实现超过1/2的渐近用户自由度？
• RQ2 当每个发送端仅连接至自身及L个相邻接收端时，局部连通性是否能带来高于完全连通性的自由度？
• RQ3 在协作阶数M受限的局部连接干扰信道中，何种消息分配策略能最大化自由度？
• RQ4 在所提方案中，为实现所有用户相等的自由度，是否必须采用部分频谱复用？
• RQ5 是否仅通过零迫波束成形即可实现自由度增益，而无需符号扩展？

主要发现

• 在完全连接的干扰信道中，无论M ≥ 1（包括M ≥ 2），在满足局部协作约束的任意消息分配下，渐近用户自由度仍保持为1/2。
• 在每个发送端连接至自身及L个相邻接收端的局部连接信道中，渐近用户自由度至少为max{1/2, 2M/(2M+L)}。
• 在L=1的Wyner非对称模型中，渐近用户自由度恰好为2M/(2M+1)，对所有M ≥ 1均超过1/2。
• 通过非对称消息分配可实现最优自由度增益，即消息i不一定要分配给发送端i，尤其在L=1情况下尤为关键。
• 为确保所有用户达到相等的自由度，必须采用部分频谱复用，因为方案中部分接收端必须保持不活跃。
• 所提方案仅通过零迫波束成形即可实现自由度增益，无需符号扩展或复杂信号处理。