Abstract
Massive multiple-input multiple-output (MIMO) system uses a large number of antennas in the base station (BS) serving a set of users in order to increase the spectral efficiency. The pilot-contamination is considered the most remarkable impairment that limits the performance of massive MIMO system. In this paper, two different pilot assignment approaches are jointly proposed to reduce pilot contamination effect. A time-shifted pilot assignment (TSPA) approach is used, where a cellular network is divided into exclusive groups, wherein users in the same group send their uplink-pilots simultaneously, while other users receive their downlink-data. Through the uplink-pilot phase, a heuristic weighted graph coloring-based pilot assignment (WGC-PA) approach is used to reduce intra-group interference caused by pilot contamination. Different uplink-pilots are allocated to the users in the same group having the largest pilot contamination severity (PCS), whereas other users with the smallest PCS share the same uplink-pilots. To divide the cellular network into exclusive groups, we propose a cells grouping technique based on the adjacent distance between cells. This proposed technique uses a backtracking based graph coloring algorithm. It ensures that no two adjacent cells are in the same group in the time-shifted approach. The simulation results show that the proposed joint WGC/TS-PA strategy not only reduces the pilot contamination effect but also reduces computational complexity than the weighted graph coloring approach.
Highlights
Quite recently, considerable attention has been paid to massive multiple-input multiple-output (MIMO) system as a promising technology to meet the requirements of the fifth generation (5G) cellular network
The estimated uplink channel is contaminated by the user equipment (UE) in the other cells that use the same pilot sequence which leads to the so-called pilot contamination effect
The cellular system considered in this paper is composed of L hexagonal cells; each cell has a central base station (BS) with M antennas and K single-antenna UEs
Summary
Considerable attention has been paid to massive multiple-input multiple-output (MIMO) system as a promising technology to meet the requirements of the fifth generation (5G) cellular network. The time-shifted pilot assignment (TSPA) approach is considered through this paper, whereby the whole cellular network (denoted as G) is partitioned into groups, i.e., G1, G2, . B. PROPOSED CELLS GROUPING USING GRAPH COLORING ALGORITHM the TSPA approach in previous researches is an effective way to reduce interference between groups [12]–[14], there is no explicit suggestion of how cells are grouped. The grouping of cells in the TSPA approach based upon the adjacent distance between BSs can be considered as a graph coloring problem. The steps of the proposed cells grouping scheme in TSPA using backtracking-graph coloring algorithm are: 1) Step 1: Input the maximum adjacency distance between two cells denoted as (dadj). This is appropriate in our situation as the cells grouping is done only once during the system planning and design
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