A New Joint TSPA/WGC Pilot Contamination Reduction Strategy Based on Exact Graph Coloring Grouping Algorithm

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.INDEX TERMS Cells grouping, graph coloring, massive MIMO, pilot assignment, pilot contamination, time-shifted pilot.

Downlink Massive MIMO Systems: Reduction of Pilot Contamination for Channel Estimation with Perfect Knowledge of Large-Scale Fading

“…To improve SINR, active user evaluation aligns path loss with the required pilot signal. The number of PRS is determined to optimize achievable DR. High SINR is essential for accurate channel estimation, which is obtained using a correlated received pilot matrix with low complexity.𝑔𝑔 � 𝑙𝑙𝑙𝑙𝑙𝑙 is convenient for a large number of 𝑀𝑀 [5], [23], [26] (4)…”

“…The PS, denoted by 𝛶𝛶 𝑝𝑝 , is allocated within each coherence interval and can be used for both the UL and DL transmission. For the EC, the properties of MMSE 𝑔𝑔 𝑙𝑙𝑙𝑙𝑙𝑙 ~= 𝑔𝑔 � 𝑙𝑙𝑙𝑙𝑙𝑙 − 𝑔𝑔 𝑙𝑙𝑙𝑙𝑙𝑙 the EC between users 𝐾𝐾 and BS cells 𝐿𝐿 was originate, where 𝑔𝑔 𝑙𝑙𝑙𝑙𝑙𝑙 ~𝒞𝒞𝒞𝒞�0, 𝜑𝜑 𝑙𝑙𝑙𝑙 � is the uncorrelated estimation independent of 𝑔𝑔 𝑙𝑙𝑙𝑙𝑙𝑙 ~~𝒞𝒞𝒞𝒞�0, �𝜑𝜑 𝑙𝑙𝑙𝑙 − Ѻ 𝑙𝑙𝑙𝑙𝑙𝑙 �𝐼𝐼 𝑀𝑀 �, and Ѻ 𝑙𝑙𝑙𝑙𝑙𝑙 = Ω 𝑙𝑙𝑙𝑙𝑙𝑙 𝜑𝜑 𝑙𝑙𝑙𝑙 Ω 𝑙𝑙𝑙𝑙𝑙𝑙 , which is also independent of antenna 𝑀𝑀 for LSF [6][7][8][9][10], [22][23][24][25][26][27][28]. The MMSE of the 𝐾𝐾𝐾𝐾ℎ UEs can be expressed as…”

“…This optimization, in turn, enables more accurate channel estimation, enhanced pre-coding performance, and overall improved system efficiency. By mitigating the effects of pilot contamination, wireless communication systems can effectively utilize available resources, maximize capacity, and provide users with enhanced connectivity, throughput, and quality of service [23] In this research, we address the challenge of PC for edge users by proposing the adoption of pilot reuse sequences (PRS). By utilizing PRS, we aim to mitigate the interference caused by PC, leading to improved system performance.…”

Pilot Allocation Technique for Massive MIMO Systems Based on Clustering

“…Two distinct pilot allocation procedures are proposed in [16] to lessen the pilot contamination issue. A time-shifted pilot assignment (TSPA) method is utilized, in which a cellular network is separated into exclusive groups, with users in the same group sending uplink-pilots at the same time as users in other groups receive downlink-data.…”

Power Allocation in Massive MIMO‐HWSN Based on the Water‐Filling Algorithm

“…In actual wireless communication systems, the number of orthogonal pilots is often less than active users, so it is impossible for each user to use the orthogonal pilots because of the limited length of the coherence time. When several users in the system use the same pilots, pilot contamination will result in inaccurate channel estimation and the BS cannot obtain accurate channel state information (CSI) [23][24][25][26][27][28][29]. In the time-division duplex (TDD) massive MIMO-HWSN system which employs pilots to conduct uplink channel estimation, the pilot contamination seriously hinders the increasing trend of wireless communication system capacity and delays the process of standardization and industrialization [30,31].…”