The introduction of massive MIMO (Multiple Input Multiple Output) communication systems enables base stations (BS) to perform beamforming for enhancing communication reliability. A typical key assumption, however, is the availability of accurate downlink channel state information (CSI). In practice, CSI estimation and reporting delays coupled with the process of channel aging result in the BS receiving outdated CSI information, which in turn impacts the system's spectral efficiency. To combat this latency, this paper develops efficient methods of CSI prediction that preemptively predict future downlink CSI based on historical data. We leverage the spatial and temporal correlation properties of the channel and use explicit feature extraction frameworks for both dimensions to accurately predict future CSI. We analyze combinations of spatial and temporal feature extractors in terms of a tradeoff between performance and latency. We evaluate the performance of the proposed prediction model in terms of proximity to the ground truth, prediction latency, and model footprint. Our experiments show that our method outperforms classical statistical methods as well as existing CSI prediction baselines.

The new applications enabled by the 5G new radio (NR) use cases require novel solutions capable of supporting high end-user data rates at low latency. To meet these needs, it is critical to have accurate channel state information at the receiver. This requires sending a large number of reference signals which scales up with the number of users and layers, thereby introducing significant overhead that eventually impacts the system's spectral efficiency. This paper introduces a new downlink data structure that is free from demodulation reference signals, and does not require channel estimation at the receiver. The proposed structure involves a repetition step of part of the user data across the time-frequency grid. Exploiting the repetition structure at the receiver, it is shown that reliable recovery is possible via canonical correlation analysis (CCA). This paper also proposes two effective mechanisms for optimizing the CCA performance in OFDM systems, one for repetition pattern selection and another to deal with the severe frequency selectivity issues. The proposed approach provides rigorous recovery guarantees, exhibits favorable complexity-performance tradeoff, and is shown to be robust to high interference scenarios, thus rendering it appealing for practical implementation. The paper provides simulation results, using a 3GPP link-level testbench, that demonstrate the feasibility of the proposed approach.

Hybrid beamforming has evolved as a promising technology that offers the balance between system performance and design complexity in mmWave MIMO systems. Existing hybrid beamforming methods either impose unit-modulus constraints or a codebook constraint on the analog precoders/combiners, which in turn results in a performance-overhead tradeoff. This paper puts forth a tensor framework to handle the wideband hybrid beamforming problem, with Vandermonde constraints on the analog precoders/combiners. The proposed method strikes the balance between performance, overhead and complexity. Numerical results on a 3GPP link-level test bench reveal the efficacy of the proposed approach relative to the codebook-based method while attaining the same feedback overhead. Moreover, the proposed method is shown to achieve comparable performance to the unit-modulus approaches, with substantial reductions in overhead.

The ever-growing demands on wireless connectivity, especially with the emergence of various data-intensive low-latency applications, require novel multiplexing solutions capable of reliably supporting high rates at low latency. Non-orthogonal time division duplex (TDD) coupled with multiuser detection can meet these emerging needs, provided that accurate channel state information is available. This paper proposes a new pilot-free TDD frame structure that allows designing highly effective multiuser decoders and precoders for uplink and downlink multicell systems, in an unsupervised manner. The key idea is that each user repeats and permutes its uplink data using a pre-assigned permutation code. Invoking canonical correlation analysis (CCA) at the serving BS on the two deinterleaved uplink blocks yields high quality CCA-based beamformers capable of both recovering the uplink and precoding the downlink user signals in a way that effectively mitigates interference. The paper includes a pilotless synchronization framework that leverages CCA to recover the timing and frequency offsets in an asynchronous multiuser MIMO setup, without using pilots. Simulations are used to study the performance of the proposed approach on a large-scale network with multiple users and cells, while laboratory experiments with a small-scale network of software radios are used to demonstrate that the approach works well in practice under common hardware imperfections.

The unprecedented growth in wireless Internet-of-Things and WiFi devices has renewed interest in mechanisms for efficient spectrum reuse. Existing schemes require some level of primary-secondary coordination, cross-channel state estimation and tracking, or activity detection– which complicate implementation. For low-power short-range secondary communication, the main impediment is strong and time-varying (e.g., intermittent) interference from the primary system. This paper proposes a practical underlay scheme that permits reliable secondary communication in this regime. The secondary transmitter merely has to send its signal twice, at very low power - a few dBs above the noise floor, but far below the primary’s interference. Exploiting the repetition structure, reliable and computationally efficient recovery of the secondary signal is possible via canonical correlation analysis (CCA). Experiments using a software radio testbed reveal that, for a secondary user with only two receive antennas, reliable detection of the secondary signal is possible for signal to interference plus noise ratio (SINR) in the range of −20 to −40 dB. The approach works with unknown time-varying channels, digital or analog modulation, it is immune to carrier frequency offset, and, as a side-benefit, it provides means for accurate synchronization of the secondary user even at very low SINR.

The spectrum underlay concept promises enhanced spectrum utilization without disturbing legacy / licensed or scientific primary users, so long as their interference constraints can be met. Existing underlay schemes assume that both the primary signal to secondary interference plus noise ratio, and the secondary signal to primary interference plus noise ratio can be high enough at the primary and secondary receiver, respectively. Even if the cross-network channel state information is available at the secondary users, these two conflicting requirements are hard to achieve simultaneously in practice. This work proposes a practical data-driven approach that allows a pair of secondary users to reliably communicate in underlay mode while keeping the interference at the primary receiver close to its noise floor. The secondary transmitter merely has to transmit its signal twice, at very low power - above the noise floor, but well below the primary’s interference. It is shown here that reliable detection of the secondary signal is possible via canonical correlation analysis (CCA). Theoretical and experimental results reveal the remarkable detection performance of the proposed CCA-based approach, which does not require any cross-network coordination, or even channel state information.

Providing reliable service to users close to the edge between cells remains a challenge in cellular systems, even as 5G deployment is around the corner. These users are subject to significant signal attenuation, which also degrades their uplink channel estimates. Even joint detection using base station (BS) cooperation often fails to reliably detect such users, due to near-far power imbalance, and channel estimation errors. Is it possible to bypass the channel estimation stage and design a detector that can reliably detect cell-edge user signals under significant near-far imbalance? This paper shows, perhaps surprisingly, that the answer is affirmative - albeit not via traditional multiuser detection. Exploiting that cell-edge user signals are weak but common to different base stations, while cell-center users are unique to their serving BS, this paper establishes an elegant connection between cell-edge user detection and canonical correlation analysis (CCA) of the associated space-time baseband-equivalent matrices. It proves that CCA identifies the common subspace of these matrices, even under significant intra-and inter-cell interference. The resulting mixture of cell-edge user signals can subsequently be unraveled using a well-known algebraic signal processing technique. Interestingly, the proposed approach does not even require that the signals from the different base stations are synchronized - the right synchronization can be automatically determined as well. Experimental results demonstrate that the proposed approach achieves order of magnitude BER improvements compared to `oracle' multiuser detection that assumes perfect knowledge of the cell-center user channels.

This paper investigates the weighted sum degrees of freedom (DoF) of the MIMO Y channel that consists of three users, where the j-th user is equipped with Mjantennas, and a relay equipped with N antennas. In this network, each user conveys two private messages to the other two users in addition to a common message directed to both of them. As there is no direct link between the users, communication occurs through the relay. We define a weighted sum DoF metric that integrates all the network messages and weights the common message by a factor of α. Then, we study the weighted sum DoF maximization problem for α ≥ 0. Specifically, we show that the weighted sum DoF of the network, with M1≥ M2≥ M3, is given by min{2N, 2M2+ 2M3, M1+ M2+ M3}, for 0 ≤ α ≤ 4/3. While, for 4/33) + 3α/2 M3, 2(2N - M2- M3) + 3α/2 (M2+ M3- N), (6N - 2(M1+ M2+M3))+ 3α/2 (M1+ M2+ M3-2N), 2M2+ 2M3, M1+ M2+ M3}, and finally, for α > 2, the weighted sum DoF is equal to min{3α/2 N, α/2 (2N + M3), 2(N - 3/2M3) + 2αM3, α/2 (N + M2+ M3), N + M2- 3M3+ 2αM3, α/2 (M1+ M2+ M3), M1+ M2- 3M3+ 2αM3, 2(M2- M3) + 2αM3}. Achievability of the sum DoF is shown by using signal space alignment for network coding in the uplink phase, and zero-forcing precoding in the downlink phase.

In this paper, we study the general multiple input multiple output (MIMO) multi-way relay channel, i.e., MIMO Y channel, with common and private messages. In this channel, K users exchange messages through a common relay. Each user transmits a private message to each user in addition to a common message to all the other users. The ith user and the relay are equipped with Miand N antennas, respectively. First, we derive the degrees of freedom (DoF) region of the symmetric three-user MIMO Y channel, where Mi= M and i ∈ (1,2,31. Due to the symmetry of the network, we focus on the case where the DoF of all private messages are equal and the DoF of all common messages are equal. In this case, the DoF region has two dimensions: the DoF of private messages and the DoF of common messages. We develop an outer bound on the DoF region based by using cut-set and one-sided genie bounds. We prove the achievability of the outer bound on the DoF region by using linear beamforming and signal space alignment (SSA) schemes. Second, based on our study of the DoF region of the symmetric channel, we define a weighted sum DoF metric that integrates all the network messages and weights the common messages by a factor of α. We study the weighted sum DoF maximization problem and show that sending common messages only is optimal when α exceeds 43. Next, we focus on the weighted sum DoF with α = 2 that represents the total number of received interference-free streams at the users. First, we show that the weighted sum DoF, with α = 2, of the MIMO Y channel with an arbitrary number of antennas is given by min(3N, 2N + M3, N + M2+ M3, 2M2+ 2M3, M1+ M2+ M3). Second, we study the weighted sum DoF, with α = 2, of the K-user case in the symmetric setting. We derive an outer bound on the weighted sum DoF using cut-set bounds, and show that the network has K min(N, M) weighted DoF. The achievability results are obtained by using SSA for network coding in the multiple access phase, and zero-forcing precoding in the broadcast phase.