Yann Y. L. Lebrun

Maximum SINR-Based Beamforming for the MISO OFDM Interference Channel

We address the problem of co-channel interference (CCI) in wireless mesh networks (WMNs). In such networks, multiple nodes communicate concurrently using the same time/frequency resources. This is recognized as the interference channel (IFC) because the CCI is a major impairment in such a scenario. To mitigate the CCI, non-cooperative beamforming techniques can be employed. Non-cooperative beamformers are simpler to implement than their cooperative counter-parts because they require neither synchronization nor the sharing of information data between the transmit nodes. In this paper, we then propose a non-cooperative beamforming scheme to improve the performance of WMNs with frequency-selective channels. We present an iterative algorithm that maximizes the signal-to-interference-and-noise ratio criterion over all subcarriers of the orthogonal frequency division multiplexing system. The performance of this algorithm is evaluated through simulations.

Performance analysis of distributed ZF beamforming in the presence of CFO

We study the effects of residual carrier frequency offset (CFO) on the performance of the distributed zero-forcing (ZF) beamformer. Coordinated transmissions, where multiple cells cooperate to simultaneously transmit toward one or multiple receivers, have gained much attention as a means to provide the spectral efficiency and data rate targeted by emerging standards. Such schemes exploit multiple transmitters to create a virtual array of antennas to mitigate the co-channel interference and provide the gains of multi-antenna systems. Here, we focus on a distributed scenario where the transmit nodes share the same data but have only the local knowledge of the channels. Considering the distributed nature of such schemes, time/frequency synchronization among the cooperating transmitters is required to guarantee good performance. However, due to the Doppler effect and the non-idealities inherent to the local oscillator embedded in each wireless transceiver, the carrier frequency at each transmitter deviates from the desired one. Even when the transmitters perform frequency synchronization before transmission, a residual CFO is to be expected that degrades the performance of the system due to the in-phase misalignment of the incoming streams. This paper presents the losses of the signal-to-noise ratio gain analytically and the diversity order semi-numerically of the distributed ZF beamformer for the ideal case and in the presence of a residual CFO. We illustrate our results and their accuracy through simulations.

Feasibility Study of the Mesh Extension for the IEEE 802.16e Communication System

The demand for wireless broadband access is rapidly growing. Consequently, the need for wireless systems offering both high speed data rates and high mobility access will become more and more important for present and future communication systems. An interesting candidate to fulfil those requirements is the OFDMA-based IEEE 802.16e wireless metropolitan area network system. In this paper, we study the possible implementation of the Mesh topology for the IEEE 802.16e standard. The reuse of the TDD frame structure considered in the point-to-multipoint topology will be discussed. The performance of the OFDMA air interface will be analyzed, for a varying spectral reuse factor and for different carrier allocations amongst the users

Beamforming for interference mitigation and its implementation on an SDR baseband processor

We present the first implementation of a distributed beam-forming algorithm for interference mitigation on an SDR baseband processor. Co-channel interference (CCI) is becoming a major source of impairments in wireless communications and distributed beamforming is a promising technique to mitigate its negative impact. However, such schemes are challenging to implement in practical scenarios due to their complexity and synchronization requirements. In this paper, we report on implementation of a suboptimal, yet efficient, beamforming scheme for CCI mitigation and present the complexity modeling and algorithm transformations for achieving numerically stability. We also present the fixed-point quantization and the proper mapping on a parallel programmable baseband architecture aimed for software-defined radio (SDR). We optimize this algorithm for a coarse grained reconfigurable array (CGRA) processor and evaluate it in the context of the LTE standard.

Beamforming techniques for enabling spatial-reuse in MCCA 802.11s networks

We address the problem of co-channel interference (CCI) in wireless mesh networks based on the IEEE802.11s extension. The carrier sensing mechanism deployed in those networks insufficiently addresses the CCI problem, causing the hidden and exposed node problems; consequently degrading the throughput and latency. In this paper, we show how beamforming techniques can be implemented on top of the IEEE802.11s medium access control protocol and, using the information readily available, cancel the interference to mitigate this inefficiency of carrier sense and improve the spatial-reuse gain. In addition, we propose the signal-to-jamming-noise ratio (SJNR) beamformer and show that it significantly improves the spatial-reuse gain compared to the simple zero-forcing (ZF) beamformer and the basic IEEE802.11s access scheme. We derive the ergodic capacity of the ZF beamformer and the basic IEEE802.11s access scheme and simulate the performance of the various schemes. We show that improvements of up to 85% are achieved as function of the scenario simulated and the beamforming technique used and that the SJNR scheme outperforms the standard ZF beamformer.

Diversity Order of Spatial Multiplexing with Transmit Antenna Correlation Based Precoding

Spatial Multiplexing (SM) is an effective means for enhancing the transmission data rate in Multiple-Input Multiple-Output (MIMO) systems, particularly when used in combination with precoding techniques. However, it is not always obvious to connect the performance of such a system to its number of data streams and antennas. In this paper, the diversity order of a SM MIMO system using a Minimum Mean Square Error (MMSE) receiver is analytically calculated when a precoder based on transmit antenna correlation is included at the transmitter. It is shown that it is given by d = N r ? N s + 1 where N r is the number of receive antennas and N s is the number of data streams. This result is confirmed by simulations. Although enabling a Signal-to-Noise Ratio (SNR) gain, such a precoder is thus not able to improve the diversity order with respect to a non-precoded SM MIMO system.