Samer Bazzi

Design aspects for 5G V2X physical layer

Next generation 5G communication systems will need to employ a range of diverse technologies in order to support vehicle-to-everything (V2X) use cases, ultimately leading to accident-free, cooperative autonomous vehicles that use the available roadway efficiently. V2X is considered as one of the most challenging applications of 5G, as it requires ultra-reliable and low-latency communication for safety-critical use cases and has to provide high data rates in many scenarios. This paper discusses design aspects for the radio access in 5G V2X. Selected key technologies and their integration towards future 5G V2X physical layer are addressed. We discuss channel modeling in the context of 5G V2X use cases and we present first results for frame structure and numerology design, coexistence with earlier systems, multi-link synchronization, and multi-antenna transmission techniques.

Interference Coordination for 5G New Radio

The arrival of the 5G NR provides a unique opportunity for introducing new inter-cell interference coordination (ICIC) mechanisms. The objective is twofold: to better exploit the benefits of ICIC in coherence with the rest of radio resource management (RRM) principles in 5G, and to support new services and deployment scenarios. We propose several enhanced techniques. In the uplink, inter-cell coordination of the pilot sequence configuration mitigates the inter-cell interference problem of such pilots, which is especially severe for cell-edge users. In the downlink, coordinated small cell DTX aims at network interference control and energy consumption reduction, whereas intercell rank coordination can unleash the potential of advanced receivers with minimal control overhead. Besides, on-demand power boosting and coordinated muting can be tailored to meet URLLC requirements. The simulation results quantify the performance benefits of the different techniques under heterogeneous key performance indicators (KPIs). We also discuss the standardization effort required for having each of these techniques included in the 5G NR specifications.

Mitigating Inter-Cell Pilot Interference via Network-Based Greedy Sequence Selection and Exchange

We revisit the pilot contamination problem using the LTE pilot frame structure, and perform channel estimation in the time domain using Zadoff-Chu pilot sequences. Sequences in each cell occupy the same time slot, multiple subcarrier frequencies, and are orthogonal in time via properly chosen cyclic shifts. Sequences in multiple cells are chosen from a pool of non-orthogonal yet distinguishable sequences through their root indices. Exchanging root indices among cells allows a given cell to reconstruct sequences used in neighboring cells and to estimate interfering channels as the number of channel taps is usually limited, thus mitigating pilot contamination. Furthermore, we propose a greedy selection algorithm to find combinations of sequences that further reduce the channel estimation mean-square-error. The greedy algorithm exhibits increasing gains with increasing signal-to-noise-ratios and sequence lengths.

Robust Bayesian Precoding for Mitigation of TDD Hardware Calibration Errors

The predominantly used channel reciprocity condition in time-division-duplex mode only holds in practical systems if the transmit and receive hardware chains at the base station (BS) and at the user equipments are perfectly calibrated. Otherwise, there is a mismatch between the uplink (UL) and downlink channel state information (CSI) that leads to performance degradations. In this paper, we consider a multi-user MIMO system with hardware calibration errors (CaEs) at the BS and derive robust Bayesian precoders that minimize inter-user interference using the UL CSI and CaEs statistics as side information. The proposed robust precoding outperforms conventional non-robust precoding, and its gains compared to non-robust precoding increase with increasing numbers of users and CaEs. Furthermore, robust precoding can be used to relax calibration requirements, which is a result of practical interest. Our approach is applicable to any number of BS antennas but is especially important for massive MIMO systems where larger CaEs might be expected.

A simple over-the-air hardware calibration procedure in TDD systems

Accurate hardware calibration is necessary to achieve channel reciprocity and enable closed-loop MIMO operation in time-division-duplex mode. In this paper, a simple over-the-air procedure that efficiently estimates the base station (BS) hardware chains is presented. The procedure is to be performed between the BS and a dedicated user equipment (UE) experiencing a high signal-to-noise-ratio, and is based on a downlink (DL) and a following uplink (UL) signaling procedure. The UE first estimates the DL channel of each BS antenna and then sends, in the UL, normalized inverses of the estimated DL channels. In addition, the UE feeds back a set of normalization parameters to the BS. The obtained UL signals in addition to the normalization parameters allow for the estimation of the BS hardware chains and ensure channel reciprocity is achieved. Numerical results illustrate the performance of the proposed procedure when a zero-forcing precoder is used at the BS.