Abstract

Current and future wireless applications demand high data rates, which can be achieved through wider bandwidths. However, the available bandwidth is inherently limited and cannot be expanded beyond physical constraints. Therefore, improving the efficiency of the available spectrum—referred to as spectral efficiency (SE)—is essential. Massive multiple input multiple output (MIMO) systems operating with time division duplexing (TDD) are considered a promising solution for enhancing SE. A critical factor influencing SE is the accuracy of channel state information (CSI), which can be obtained through pilot-based channel estimation methods. While assigning orthogonal pilots to each mobile station (MS) is theoretically ideal for improving CSI accuracy and SE, it is not practically feasible due to limited resources. To reduce pilot overhead in transmission frames, pilot reuse is employed, where multiple MSs share the same pilot. However, both extremely low and high pilot reuse factors (PRFs) fail to optimize SE, making it essential to determine the optimal PRF under given system constraints. The receive combining method also significantly impacts SE and is closely tied to the channel estimation approach. The combining vector is used to extract relevant information from the received signal, and the optimal PRF may vary depending on the chosen combining method. Rician fading is adopted in this study as a realistic channel fading model for evaluating SE performance and results are also compared with the Rayleigh fading model for benchmarking. Minimum mean square error (MMSE) is selected for channel estimation. The PRFs considered in the analysis are 1, 2, 4, 8, and 16, while the receive combining techniques analyzed include multi-cell MMSE (M-MMSE), single-cell MMSE (S-MMSE), regularized zero-forcing (RZF), zero-forcing (ZF), and maximum ratio (MR). The resulting SEs are 103, 71.7, 58.2, 58.2, and 18.4 bits/sec/Hz/cell, respectively, using M-MMSE (with PRF = 4), S-MMSE (PRF = 2), RZF (PRF = 2), ZF (PRF = 2), and MR (PRF = 1). These results demonstrate that selecting the optimal PRF for a given receive combining method yields significantly better SE compared to arbitrary PRF values.

Keywords

Spectral efficiency, Massive MIMO, Pilot reuse factor (PRF), Rician fading, Channel state information (CSI), Minimum mean square error (MMSE).

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