Location-Aided Pilot Contamination Avoidance for Massive MIMO Systems

Abstract

ilot contamination, defined as the interference during the channel estimation process due to reusing the same pilot sequences in neighboring cells, can severely degrade the performance of massive multiple-input multiple-output systems.ilot contamination, defined as the interference during the channel estimation process due to reusing the same pilot sequences in neighboring cells, can severely degrade the performance of massive multiple-input multiple-output systems.P In this paper, we propose a location-based approach to mitigating the pilot contamination problem for uplink multiple-input multiple-output systems. Our approach makes use of the approximate locations of mobile devices to provide good estimates of the channel statistics between the mobile devices and their corresponding base stations. Specifically, we aim at avoiding pilot contamination even when the number of base station antennas is not very large, and when multiple users from different cells, or even in the same cell, are assigned the same pilot sequence. First, we characterize a desired angular region of the target user at the serving base station based on the number of base station antennas and the location of the target user, and make the observation that in this region the interference is close to zero due to the spatial separability. Second, based on this observation, we propose pilot coordination methods for multi-user multi-cell scenarios to avoid pilot contamination. The numerical results indicate that the proposed pilot contamination avoidance schemes enhance the quality of the channel estimation and thereby improve the percell sum rate offered by target base stations.

Channel Covariance Estimation in Multiuser Massive Mimo Systems with an Approach Based on Infinite Dimensional Hilbert Spaces

“…From a practical perspective, the inner product ρj, T * T (ρ l ) H 3 in (10) and (11) explicitly reveals the individual influences of the signals impinging on the array (the angular power spectra ρj and ρ l ) and the array response (the self-adjoint operator T * T ) on the interference pattern. This explicit separation of the contributions of impinging signals and the array response can be used to generalize in a unified way previous results such as the schedulers in [10][11][12]. To be concrete, suppose that we know ρ l ∈ K, and let Z ⊂ Ω be angles for which the function T * T (ρ l ) ∈ K takes values close to zero.…”

“…For example, the operator T * T is typically a smoothing operator, so it is expected to increase the support of functions in K. See, in particular, the integral form of T * T with the kernel κ in (9), which is the standard form of a smoothing operator. As a result, (10) shows that interference can be strong even if the angular power spectra of interfering users have disjoint support. The effects of T * T on the interference pattern cannot be ignored.…”

An Efficient Pilot Assignment Scheme for Addressing Pilot Contamination in Multicell Massive MIMO Systems

“…The authors in [14,15] developed the location-based pilot assignment approaches for pilot decontamination. A new expression for line of sight (LOS) interference is derived in [14] which is considered as the criteria for pilot allocation.…”

“…Although, there was an improvement in the sum spectral efficiency (SE), but the pilot assignment process takes a longer time to be implemented, especially in large networks. The work in [15] characterizes the angular region of the targeted user, and the pilot assignment process was implemented with the aim of making this region interference-free. This angular region is characterized by both the number of BS antennas and the location of the targeted user.…”

“…Differing from the aforementioned works [11,20], we consider the source of inter-cell interference throughout pilot assignment, which is essentially the cause of the pilot contamination. In some other works [12][13][14][15], the availability of some factors (e.g., user location, AoA, or LOS interference) are needed for pilot assignment which are not always easy to estimate, while our approach requires only large-scale fading coefficients, which can be tracked easily as they do not frequently change during coherence interval. Besides, comparing to previous works [17][18][19]21], our algorithm is not computationally intensive, and therefore it can be applied for large-scale networks.…”