Guido Carlo Ferrante

Group-blind detection with very large antenna arrays in the presence of pilot contamination

Massive MIMO is, in general, severely affected by pilot contamination. As opposed to traditional detectors, we propose a group-blind detector that takes into account the presence of pilot contamination. While sticking to the traditional structure of the training phase, where orthogonal pilot sequences are reused, we use the excess antennas at each base station to partially remove interference during the uplink data transmission phase. We analytically derive the asymptotic SINR achievable with group-blind detection, and confirm our findings by simulations. We show, in particular, that in an interference-limited scenario with one dominant interfering cell, the SINR can be doubled compared to non-group-blind detection.

Group-Blind Detection for Uplink of Massive MIMO Systems

When paired with traditional channel estimation and detection, massive MIMO is severely affected by pilot contamination. While sticking to the traditional structure of the training phase where orthogonal pilot sequences are reused in different cells, we propose a group-blind detector that takes into account the presence of pilot contamination. Our detector uses the excess antennas to partially remove interference during the data transmission phase. We derive asymptotic expressions for the SINR gain and the achievable rate in the massive regime, i.e., when the number of antennas tends to infinity while keeping the number of users per cell fixed. Implementing the group-blind detector requires an estimate of the aggregate out-of-cell channel covariance. We propose a simple scheme, referred to as method of silences, to obtain such estimate. Numerical results confirm our analysis in practical scenarios, and show cases where the method of silences achieves a large fraction of the promised SINR gain over conventional detectors.

An achievable rate region for superposed timing channels

A multiple-access channel where point processes are randomly transformed by timing channels and then superposed is considered. An achievable rate region for the K-user channel is established. A single-user achievable rate in the presence of “many” interfering users is proposed. Results are applied to exponential server timing channels.

Robustness of Time Reversal versus All-Rake Transceivers in Multiple Access Channels

Time reversal (TR) is an effective solution in both single user and multiuser communications for moving complexity from the receiver to the transmitter, in comparison to traditional postfiltering based on Rake receivers. Imperfect channel estimation may, however, affect pre- versus postfiltering schemes in a different way; this paper analyzes the robustness of time reversal versus All-Rake (AR) transceivers, in multiple access communications, with respect to channel estimation errors. Two performance indicators are adopted in the analysis: symbol error probability and spectral efficiency. Analytic expressions for both indicators are derived and used as the basis for simulation-based performance evaluation. Results show that while TR leads to slight performance advantage over AR when channel estimation is accurate, its performance is severely degraded by large channel estimation errors, indicating a clear advantage for AR receivers in this case, in particular when extremely short impulsive waveforms are adopted. Results however also show a stronger non-Gaussianity of interference in the TR case suggesting that the adoption of a receiver structure adapted to non-Gaussian interference might tilt the balance towards TR.

On information-theoretic limits of code-domain NOMA for 5G

Motivated by recent theoretical challenges for 5G, this study aims to position relevant results in the literature on code-domain non-orthogonal multiple access (NOMA) from an information-theoretic perspective, given that most of the recent intuition of NOMA relies on another domain, that is, the power domain. Theoretical derivations for several code-domain NOMA schemes are reported and interpreted, adopting a unified framework that focuses on the analysis of the NOMA spreading matrix, in terms of load, sparsity, and regularity features. The comparative analysis shows that it is beneficial to adopt extreme low-dense code-domain NOMA in the large system limit, where the number of resource elements and number of users grow unboundedly while their ratio, called load, is kept constant. Particularly, when optimum receivers are used, the adoption of a regular low-dense spreading matrix is beneficial to the system achievable rates, which are higher than those obtained with either irregular low-dense or dense formats, for any value of load. For linear receivers, which are more favourable in practice due to lower complexity, the regular low-dense NOMA still has better performance in the underloaded regime (load < 1).

A Group-Blind Detection Scheme for Uplink Multi-Cell Massive MIMO

With the reuse of identical training sequences by users in different cells, massive MIMO is severely affected by pilot contamination due to residual error in channel estimation. In this paper, we consider the traditional structure of the training phase, where orthogonal pilot sequences are reused, and analyze a recently proposed group- blind detector in the uplink of an interference- limited network. We derive the asymptotic SINR gain achievable by the group-blind detector compared to conventional schemes, and find that it depends on the number of cells and channel gains only. We show that group-blind detection can significantly improve the asymptotic achievable rate. We propose a simple scheme, that we call method of silences, to estimate the aggregate instantaneous out-of-cell channel covariance that is required to implement the group-blind detector. Numerical results confirm our analysis in scenarios of practical interest, and show cases where a scheme as simple as the method of silences allows to achieve a large fraction of the promised SINR gain.

Timing capacity of queues with random arrival and modified service times

A queue timing channel with random arrival and service times is investigated. The message is encoded into a sequence of additional delays that packets are subject to before they depart from the queue. We derive upper and lower bounds of the channel capacity for general arrival and service processes, and general load. We establish the channel capacity for the queue with exponential server and no load constraint by keeping the queue stable. We discuss the consequences of this result and describe a possible application where the timing channel is used to send covert information.

Revisiting the capacity of noncoherent fading channels in mmWave system

Millimeter wave (mmWave) communications use large transmission bandwidth and experience severe propagation conditions. This forces the communication system to operate in the so-called wideband regime, where signals must be increasingly “peaky” in order to attain a large fraction of the peak unconstrained wideband capacity. This paper investigates the capacity of the noncoherent channels as a function of bandwidth for signals with average and peak power constraints operating in the millimeter spectrum. Upper and lower bounds on capacity are provided, and the impact of features peculiar to mmWave channels is investigated. Numerical results for a scenario based on recent experimental campaigns are provided. It is shown that the rate achievable by a typical user in a mmWave cell with “non-peaky” signaling can be bounded away from the peak unconstrained wideband capacity. This suggests to reconsider the role of signaling “peakedness” in future mmWave communications.