Abstract
We propose adjustable phase shift pilots (APSPs) for channel acquisition in wideband massive multiple-input multiple-output (MIMO) systems employing orthogonal frequency division multiplexing (OFDM) to reduce the pilot overhead. Based on a physically motivated channel model, we first establish a relationship between channel space-frequency correlations and the channel power angle-delay spectrum in the massive antenna array regime, which reveals the channel sparsity in massive MIMO-OFDM. With this channel model, we then investigate channel acquisition, including channel estimation and channel prediction, for massive MIMO-OFDM with APSPs. We show that channel acquisition performance in terms of sum mean square error can be minimized if the user terminals' channel power distributions in the angle-delay domain can be made non-overlapping with proper phase shift scheduling. A simplified pilot phase shift scheduling algorithm is developed based on this optimal channel acquisition condition. The performance of APSPs is investigated for both one symbol and multiple symbol data models. Simulations demonstrate that the proposed APSP approach can provide substantial performance gains in terms of achievable spectral efficiency over the conventional phase shift orthogonal pilot approach in typical mobility scenarios.
Highlights
F ORTHCOMING 5G cellular wireless systems are expected to support 1000 times faster data rates than the currently deployed 4G long-term evolution (LTE) system
We show that the sum mean square error (MSE) of channel prediction (MSE-CP) can be minimized if the user terminals (UTs)’ channel power distributions in the angle-delay domain can be made non-overlapping with proper pilot phase shift scheduling, which coincides with the optimal channel estimation condition
We investigate in the following proposition a property of the large dimensional space-frequency domain channel covariance matrix (SFCCM), and present a relationship between the SFCCM and the power angle-delay spectrum for massive multipleinput multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) channels
Summary
F ORTHCOMING 5G cellular wireless systems are expected to support 1000 times faster data rates than the currently deployed 4G long-term evolution (LTE) system. The most common approach is to estimate the channel response in the delay domain, and optimal pilots sent from different transmit antennas are typically assumed to satisfy the phase shift orthogonality condition in both the single-user case [9]–[11] and the multi-user case [12]. When channel spatial correlations are taken into account, optimal pilot design has been investigated for both the single-user case [14] and multi-user case [15] These orthogonal pilot approaches can eliminate pilot interference in the same cell, they do not take into account the pilot overhead issue, which is thought to be one of the limiting factors for throughput in massive MIMO-OFDM [4]. Portions of this work previously appeared in the conference paper [32]
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