Downlink Performance of Pilot-Reused HetNet With Large-Scale Antenna Arrays

Abstract

Considering a heterogeneous network (HetNet) where both macro base station (BS) and small cell (SC) nodes are equipped with massive antennas, this paper studies the performance for multiple-input multiple-output (MIMO) downlinks when the macro and small cells share the same spectrum and hence interfere with each other. Suppose that the largescale antenna arrays at both macro BS and SC nodes employ maximum-ratio transmission (MRT) or zero-forcing transmission (ZFT) precoding, and transmit data streams to the served users simultaneously. A new pilot reuse pattern among SCs is proposed for channel estimation. Taking into account imperfect channel state information (CSI), capacity lower bounds for MRT and ZFT are derived, respectively, in closed-form expressions involving only statistical CSI. Then asymptotic analyses for massive arrays are presented under specific power scaling laws. Subsequently, two user scheduling algorithms, greedy scheduling algorithm and asymptotical scheduling algorithm (ASA), are proposed based on derived capacity lower bounds and asymptotic analyses, respectively. ASA is demonstrated to be a near optimal in the asymptotic regime and has low complexity. Finally, the derived closed-form expressions are verified to be accurate predictors of the system performance by Monte-Carlo simulations. Numerical results demonstrate the effectiveness of asymptotic analysis and proposed user scheduling schemes. Index Terms-Heterogeneous network, MIMO, large-scale antenna arrays, pilot reuse, user scheduling. Y. Dai and X. Dong are with the

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

“…Pilot reuse seriously affects the channel estimation accuracy at BSs, and this influence is known as the pilot contamination [6]. Pilot contamination is one major factor that restricts the performance of TDD massive MIMO systems, so many methods are proposed to reduce pilot contamination, such as time-shifted pilot allocation [7], spatial domain-based pilot allocation [8], design and allocation of pilot sequences [9,10], and pilot reuse [11][12][13].…”

“…For massive MIMO multi-tier heterogeneous networks (HetNets), literature [12] uses a fixed pilot allocation to reduce system interference. Paper [13] considers the case of one macro cell and many micro cells, adopts pilot allocation method that distinguish the users of micro cells, and uses iterative algorithms to find the optimal pilot allocation, so its complexity is very high. The methods of literatures [12,13] assume that the number of users in each macro cell is the same, and the number of users in each micro cell is the same.…”

“…Reviewing these studies [7][8][9][10][11][12][13][14][15], reasonable pilot allocation can effectively reduce pilot contamination and improve system performance. However, in order to facilitate theoretical analysis, most of these pilot allocation methods, whether for one-tier or multi-tier networks, assume that cells have regular shape (most are hexagons) or the number of users per cell is fixed.…”

“…Moreover, mobile users are randomly distributed in reality, so it is impossible that the number of users in each cell is equal or fixed. In addition, in order to avoid the use of the same pilot in adjacent cells, these traditional pilot allocation methods [7][8][9][10][11][12][13][14][15] always reserve a certain number of pilots for each cell. Therefore, these existing pilot allocation methods [7][8][9][10][11][12][13][14][15] are inflexible and not applicable to multi-tier HetNets with irregular shaped cells and random distributed users.…”

“…For example, if the number of users is large in a cell, using these methods [7][8][9][10][11][12][13][14][15], pilots are insufficient, and pilot reuse will lead to serious pilot contamination. On the other hand, if the number of users is small in a cell, using these methods [7][8][9][10][11][12][13][14][15], the pilot resource cannot be fully utilised, and this will lead to the waste of pilot resources, thus affecting system performance. That is to say, these methods [7][8][9][10][11][12][13][14][15] lack flexibility; even if they are modified or generalised to massive MIMO multi-tier HetNets, they cannot guarantee system performance.…”