Philippe Sartori

Cooperative Algorithms for MIMO Amplify-and-Forward Relay Networks

Interference is a common impairment in wireless communication systems. Multi-hop relay networks use a set of intermediate nodes called relays to facilitate communication between multiple transmitters and multiple receivers through multiple hops. Relay based communication is especially sensitive to interference because the interference impacts both the received signal at the relay, and the received signal at the destination. Interference alignment is a signaling technique that provides high multiplexing gain in the interference channel. In this paper, inspired by an algorithmic approach for interference alignment, three cooperative algorithms are proposed to find suboptimal solutions for end-to-end sum-rate maximization problem in a multiple-antenna amplify-and-forward (AF) relay interference channel. The first algorithm aims at minimizing the sum power of enhanced noise from the relays and interference at the receivers. The second and third algorithms aim at minimizing matrix-weighted sum mean square errors with either equality or inequality power constraints to utilize a connection between mean square error and mutual information. The resulting iterative algorithms are convergent to points that we conjecture to be stationary points of the corresponding problems. Simulations show that the proposed algorithms achieve higher end-to-end sum-rates and multiplexing gains that existing strategies for AF relays, decode-and-forward relays, and direct transmission. The first algorithm outperforms the other algorithms at high signal-to-noise ratio (SNR) but performs worse than them at low SNR. Thanks to power control, the third algorithm outperforms the second algorithm at the cost of additional overhead.

LTE evolution for vehicle-to-everything services

Wireless communication has become a key technology for competitiveness of next generation vehicles. Recently, the 3GPP has initiated standardization activities for LTE-based V2X services composed of vehicle-to-vehicle, vehicle- to-pedestrian, and vehicle-to-infrastructure/network. The goal of these 3GPP activities is to enhance LTE systems to enable vehicles to communicate with other vehicles, pedestrians, and infrastructure in order to exchange messages for aiding in road safety, controlling traffic flow, and providing various traffic notifications. In this article, we provide an overview of the service flow and requirements of the V2X services LTE systems are targeting. This article also discusses the scenarios suitable for operating LTE-based V2X services, and addresses the main challenges of high mobility and densely populated vehicle environments in designing technical solutions to fulfill the requirements of V2X services. Leveraging the spectral-efficient air interface, the cost-effective network deployment, and the versatile nature of supporting different communication types, LTE systems along with proper enhancements can be the key enabler of V2X services.

Initial beamforming for mmWave communications

Cellular systems were designed for carrier frequencies in the microwave band (below 3 GHz) but will soon be operating in frequency bands up to 6 GHz. To meet the ever increasing demands for data, deployments in bands above 6 GHz, and as high as 75 GHz, are envisioned. However, as these systems migrate beyond the microwave band, certain channel characteristics can impact their deployment, especially the coverage range. To increase coverage, beamforming can be used but this role of beamforming is different than in current cellular systems, where its primary role is to improve data throughput. Because cellular procedures enable beamforming after a user establishes access with the system, new procedures are needed to enable beamforming during cell discovery and acquisition. This paper discusses several issues that must be resolved in order to use beamforming for access at millimeter wave (mmWave) frequencies, and presents solutions for initial access. Several approaches are verified by computer simulations, and it is shown that reliable network access and satisfactory coverage can be achieved in mmWave frequencies.

Device-to-device proximity discovery for LTE systems

Device-to-device (D2D) proximity discovery enables spectral reuse via D2D communications as well as a range of innovative proximity services, such as enhanced social networking and location services. To develop a scalable discovery mechanism, signals transmitted by geographically disparate devices should not differ drastically in their received power at each monitoring device. Discovery schemes are proposed to deal with this near-far issue. Analysis and simulation results are provided to characterize the performance of discovery schemes.

Performance Analysis of Layer 1 Relays

Relays are expected to be included in 3GPP Release 10 also known as LTE-Advanced (LTE-A). Relays can be used to improve coverage and throughput and could be an important tool to improve the wireless experience. In this paper, the discussion on relay technologies in 3GPP LTE-A is overviewed firstly. Because both low cost and small delay make L1 relay more attractive than other types of relay, the SINR and information rate performance of L1 relays are analyzed mainly in this paper. With a simple one-dimensional relay model and an approximate self-interference model, the capability of L1 relays to improve cell-edge throughput is verified.

Design of a D2D Overlay for Next Generation LTE

With the ever-growing popularity of smartphones, new services are emerging where the local and positioning aspects becomes more important. Additionally, new types of traffic, such as vehicle-to-vehicle communications are being developed. In order to address these new trends, release-12 of LTE has started to evaluate, and possibly standardize, solutions for device-to-device (D2D) communication. This provides the foundations for the deployment of a D2D overlay. However, this should be viewed as a first step, and future LTE releases should encompass more advanced D2D communication techniques and architectures. In this paper, after reviewing the standardization progress for LTE in release-12, several possible improvements are examined for future releases both D2D discovery and communication. It is shown that the future D2D architecture should exploit the network when possible in order to provide an improved user experience and satisfy the need for more local services.

LTE relay backhaul design for sparsely-populated environments

In the latest release of the Long Term Evolution (LTE) standards, which will be completed in September 2011, it was decided to standardize relay nodes (RN) for the purpose of improving coverage. This makes RNs very suitable tools for increasing the coverage of low population density areas. The RN is a new type of node that was not previously standardized in 3GPP before and, as such, created new challenges. In particular, for RN, the backhaul link (connecting the enhanced Node B (eNB) and the RN) needs to be defined, and both uplink and downlink grants need to be conveyed to the RNs, either using existing channels (e.g., the Packet Dedicated Control Channel (PDCCH)), or a new channel (commonly referred to as the Relay-PDCCH (R-PDCCH)). In this paper, we discuss both channels for the Frequency Division Duplex (FDD) mode and show that using a R-PDCCH is a more efficient channel. In addition, several features are provided to ensure good performance for the R-PDCCH, such as robust use of frequency diversity by transmitting the R-PDCCH using Distributed Virtual Resource Blocks (DVRBs), or use of rate-matching to ensure optimal usage of the allocated resources for R-PDCCH transmission. This robustness is considered necessary to ensuring good reliability for sparsely populated areas.