Fractional pilot reuse in massive MIMO systems

Abstract

Abstract-Pilot contamination is known to be one of the main impairments for massive MIMO multi-cell communications. Inspired by the concept of fractional frequency reuse and by recent contributions on pilot reutilization among non-adjacent cells, we propose a new pilot allocation scheme to mitigate this effect. The key idea is to allow users in neighboring cells that are closest to their base stations to reuse the same pilot sequences. Focusing on the uplink, we obtain expressions for the overall spectral efficiency per cell for different linear combining techniques at the base station and use them to obtain both the optimal pilot reuse parameters and the optimal number of scheduled users. Numerical results show a remarkable improvement in terms of spectral efficiency with respect to the existing techniques.

A Review on Importance of Improving Spectral Efficiency in Massive MIMO Communications

“…There are many reasons why multi-user MIMO is the most scalable and attractive solution [17]. Firstly, the wavelength is 5-30 cm in the frequency range of cellular communication (1)(2)(3)(4)(5)(6). This limits the number of antennas that can be deployed in a compact user terminal for point-to-point MIMO, while one can have almost any number of spatially separated single-antenna terminals in multi-user MIMO.…”

“…To keep up with the rapid traffic growth, a key goal of the 5G technologies is to improve the area throughput by orders of magnitude; 100× and even 1000× higher throughput are regularly mentioned as 5G design goals. The area throughput of a wireless network is measured in bit/s/km 2 and can be modeled as : Area throughput (bit/s/km 2 ) = Bandwidth (Hz) ×Cell density (cells/km 2 )×Spectral efficiency(bit/s/Hz/cell) This simple formula reveals that there are three main components that can be improved to yield higher area throughput: (1) more bandwidth can be allocated for 5G services; (2) the network can be densified by adding more cells with independently operating access points; and (3) the efficiency of the data transmissions (per cell and for a given amount of bandwidth) can be improved. The improvements in area throughput in previous network generations have greatly resulted from cell densification and allocation of more bandwidth.…”

A Multi-Cell MMSE Precoder for Massive MIMO Systems and New Large System Analysis

“…To verify the accuracy of the large-scale approximations, 10000 independent Monte-Carlo channel realizations for small scale fading are generated to numerically calculate the downlink achievable SE in (14). The numerical result and its approximation from Theorem 3 are shown in Fig.…”

“…As seen from the figure, β f = 0 is preferred when K is small, which means that pilot reuse should also be applied for everyone. However, β f > 0 is beneficial for larger K values since there will likely be cell center users that are not sensitive to pilot contamination, which can be exploited as in [14] to reduce the pilot overhead. However, if we look at the highest SE for each β f among the considered K and β, the increase of β f is not as obvious as it is for M-ZF shown in [14], thanks to the strong ability of our M-MMSE to actively suppress inter-cell interference.…”

“…Finally we consider the refined pilot reuse scheme proposed in [14] where pilot reuse is only applied at the cell edges. This reuse policy selects B = K(β f + (1 − β f )β) pilot sequences, and assign the same β f K pilot sequences to the β f K users that are closest to their serving BSs.…”