Flexible pilot allocation scheme for massive MIMO two-tier heterogeneous networks

Existing research demonstrates that pilot contamination is one major factor that restricts the system performance of time division duplex massive multiple-input multiple-output (MIMO) systems. Reasonable pilot allocation is an effective method to reduce pilot contamination. However, most existing pilot allocation methods (whether for one-tier or multi-tier networks) focus on cells with the same size, or regular shape, or fixed number of users per cell, so they are inflexible and not applicable to massive MIMO multi-tier heterogeneous networks (HetNets) with irregular shaped cells and random distributed users. To solve this problem, this study introduces the idea of coalition game into pilot allocation and proposes a pilot allocation scheme based on coalition game. The utility function of each user for coalition game is analysed; the adjustment principle of the coalition structure and the coalition formation algorithm are given. Simulations results show that the proposed scheme can be flexibly used in many massive MIMO scene that base stations and users are randomly distributed (such as one-tier networks, multi-tier HetNets, ultra-dense HetNets, etc.), and it can effectively reduce pilot contamination and improve the received signal to interference plus noise ratio (SINR) and spectral efficiency (SE) of the uplink.