Pilot contamination is a limiting factor in multicell massive multiple-input multiple-output (MIMO) systems because it can severely impair channel estimation. Prior works have suggested coordinating pilot design across cells in order to reduce the channel estimation error caused by pilot contamination. In this paper, we propose a method for coordinated pilot design using fractional programming to minimize the weighted mean squared-error (MSE) in channel estimation. In particular, we apply the recently proposed quadratic transform to the MSE expression which allows the effect of pilot contamination to be decoupled. The resulting problem reformulation enables the pilots to be optimized in closed form if they can be designed arbitrarily. When the pilots are restricted to a given set of orthogonal sequences, pilot optimization reduces to an assignment problem which can be solved by weighted bipartite matching. Furthermore, we consider the max-min fairness of data rates with orthogonal pilots and obtain an extension of the proposed method to correlated Rayleigh fading. Finally, simulations demonstrate the advantage of the proposed (orthogonal and nonorthogonal) pilot designs as compared with state-of-the-art methods in combating pilot contamination.

中文翻译：

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通过协调导频设计增强大规模 MIMO 中的信道估计


导频污染是多小区大规模多输入多输出 (MIMO) 系统的限制因素，因为它会严重损害信道估计。先前的工作建议跨小区协调导频设计，以减少导频污染引起的信道估计误差。在本文中，我们提出了一种使用分数规划来协调导频设计的方法，以最小化信道估计中的加权均方误差（MSE）。特别是，我们将最近提出的二次变换应用于 MSE 表达式，从而可以解耦导频污染的影响。如果可以任意设计，由此产生的问题重新表述使得飞行员能够以封闭形式进行优化。当导频被限制为给定的一组正交序列时，导频优化简化为可以通过加权二分匹配来解决的分配问题。此外，我们考虑了正交导频数据速率的最大-最小公平性，并获得了所提出的方法对相关瑞利衰落的扩展。最后，仿真证明了所提出的（正交和非正交）导频设计与对抗导频污染的最先进方法相比的优势。