Veria Havary-Nassab

Distributed Beamforming for Relay Networks Based on Second-Order Statistics of the Channel State Information

In this paper, the problem of distributed beamforming is considered for a wireless network which consists of a transmitter, a receiver, and relay nodes. For such a network, assuming that the second-order statistics of the channel coefficients are available, we study two different beamforming design approaches. As the first approach, we design the beamformer through minimization of the total transmit power subject to the receiver quality of service constraint. We show that this approach yields a closed-form solution. In the second approach, the beamforming weights are obtained through maximizing the receiver signal-to-noise ratio (SNR) subject to two different types of power constraints, namely the total transmit power constraint and individual relay power constraints. We show that the total power constraint leads to a closed-form solution while the individual relay power constraints result in a quadratic programming optimization problem. The later optimization problem does not have a closed-form solution. However, it is shown that using semidefinite relaxation, this problem can be turned into a convex feasibility semidefinite programming (SDP), and therefore, can be efficiently solved using interior point methods. Furthermore, we develop a simplified, thus suboptimal, technique which is computationally more efficient than the SDP approach. In fact, the simplified algorithm provides the beamforming weight vector in a closed form. Our numerical examples show that as the uncertainty in the channel state information is increased, satisfying the quality of service constraint becomes harder, i.e., it takes more power to satisfy these constraints. Also our simulation results show that when compared to the SDP-based method, our simplified technique suffers a 2-dB loss in SNR for low to moderate values of transmit power.

Compressive detection for wide-band spectrum sensing

In this paper we propose a novel wide-band spectrum sensing scheme using compressive sensing. The wide-band signal is fed into a number of wide-band filters and the outputs of the filters are used to reconstruct the vector of channel energies through the ℓ1 norm minimization. An energy detection is then performed by comparing the obtained vector to a vector of energy thresholds to decide about the occupancy of each channel. Performance of the proposed approach is compared to the current wide-band spectrum sensing algorithms as well as the conventional channel-by-channel scanning method.

Joint Receive-Transmit Beamforming for Multi-Antenna Relaying Schemes

In this correspondence, we study the problem of joint receive and transmit beamforming for a wireless network consisting of a transmitter, a receiver, and a relay node. The relay node is equipped with multiple antennas while the transmitter and the receiver each uses only one antenna. Our communication scheme consists of two phases: first the transmitter sends the information symbols to the relay. In the second phase, the relay re-transmits a linearly transformed version of the vector of the signals received at its multiple antennas. We introduce the novel concept of general rank beamforming which can be applied to our communication scheme. In our general rank beamforming approach, the relay multiplies the vector of its received signals by a general-rank complex matrix and re-transmits each entry of the output vector on the corresponding antenna. Through maximizing the signal-to-noise ratio (SNR), we obtain a closed-form solution to the general rank beamforming problem. We also prove that for the case of statistically independent transmitter-relay (TR) and relay-receiver (RR) channels, the general rank beamforming approach results in a rank-one solution for the beamforming matrix regardless of the rank of the channel correlation matrices. Simulation results show that when applied to the case of statistically dependent TR and RR channels, our general rank beamforming technique outperforms the separable receive and transmit beamforming method by a significant margin.

Network beamforming based on second order statistics of the channel state information

The problem of distributed beamforming is considered for a network which consists of a transmitter, a receiver, and r relay nodes. Assuming that the second order statistics of the channel coefficients are available, we design a distributed beamforming technique via maximization of the receiver signal-to-noise ratio (SNR) subject to individual relay power constraints. We show that using semi-definite relaxation, this SNR maximization can be turned into a convex feasibility semi-definite programming problem, and therefore, it can be efficiently solved using interior point methods. We also obtain a performance bound for the semi-definite relaxation and show that the semi-definite relaxation approach provides a c-approximation to the (nonconvex) SNR maximization problem, where c = O((log r)-1) and r is the number of relays.

Optimal network beamforming for bi-directional relay networks

We consider a relay network which consists of two transceivers and r relay nodes. We study a half-duplex two-way relaying scheme. First, the two transceivers transmit their information symbols simultaneously and the relays receive a noisy mixture of the two transceiver signals. Then each relay adjusts the phase and the amplitude of its received signal by multiplying it with a complex beamforming coefficient and transmits the so-obtained signal. Aiming at optimally calculating the beamforming weight vector as well as the transceiver transmit powers, we minimize the total transmit power subject to two constraints on the receive signal-to-noise ratios (SNRs) at the two transceivers. We show that the optimal weight vector can be obtained through a simple iterative algorithm which enjoys a linear computational complexity per iteration.

An SNR balancing approach to two-way relaying

We consider a relay network which consists of two transceivers and r relay nodes. Assuming that the transceivers and the relays are all equipped with single antennas, we devise a two-way amplify-and-phase-adjust relaying scheme. In this scheme, each relay multiplies its received signal by a complex weight and transmits the so-obtained signal thereby participating in a distributed beamforming process. We deploy an SNR balancing technique where the smallest of the two transceiver SNRs is maximized while the total transmit power is kept below a certain power budget. We show that this problem has a unique solution which can be obtained through an iterative procedure with a linear computational complexity per iteration. We also prove that for any channel realization, this approach leads to a power allocation scheme where half of the maximum power budget is allocated to the two transceivers and the remaining half will be shared among all the relay nodes. We further devise a distributed implementation of our proposed scheme which requires a minimal cooperation among the two transceivers and the relays. In fact, we show that our technique can be implemented such that the bandwidth required to obtain the beamforming weights in a distributed manner remains constant as the size of the network grows.

General-Rank Beamforming for Multi-Antenna Relaying Schemes

We consider a wireless network consisting of a single-antenna transmitter, a single-antenna receiver, and a multi-antenna relay node. For such a network, we introduce the novel approach of general rank beamforming. In this approach, the relay multiplies the vector of its received signals by a general-rank complex matrix to obtain a new vector. Each entry of this new vector is then transmitted on one of the antennas available at the relay. We show that maximizing the receiver SNR subject to total relay power constraint yields a closed-form solution for the beamforming matrix. We also prove that if the channel coefficients from the transmitter to the relay antennas and those from the relay antennas to the receiver are statistically independent, the general rank beamforming approach results in a rank-one solution for the beamforming matrix.

Mobility diversity in mobile wireless networks

In this paper, we introduce the novel concept of mobility diversity as the diversity gained by transmitting the information of the nodes of a mobile network over different network topologies. Due to the mobility of the nodes, different network topologies emerge which can benefit the information transmission throughout the network. In traditional diversity schemes, such as frequency, time, or spatial diversity, a signal is transmitted over different diversity dimensions (e.g., different frequency bands, different time intervals, or different spatial paths) to combat the destructive effects of fading in each individual channel. In a mobile wireless network, the nodes can exploit the topology diversity to communicated with their corresponding destinations more reliably as compared to the case when the topology of the network is fixed. In fact, in a fixed topology, the probability of a source having a poor connectivity to its destination is higher than the case when there are multiple topologies over which the communication can occur.

Denial of Service Attacks in Networks with Tiny Buffers

Recently, several papers have studied the possibility of shrinking buffer sizes in Internet core routers to just a few dozen packets under certain constraints. If proven right, these results can open doors to building all-optical routers, since a major bottleneck in building such routers is the lack of large optical memories. However, reducing buffer sizes might pose new security risks: it is much easier to fill up tiny buffers, and thus organizing Denial of Service (DoS) attacks seems easier in a network with tiny buffers. To the best of our knowledge, such risks have not been studied before; all the focus has been on performance issues such as throughput, drop rate, and flow completion times. In this paper, we study DoS attacks in the context of networks with tiny buffers.We show that even though it is easier to fill up tiny buffers, synchronizing flows is more difficult. Therefore to reduce the network throughput, the attacker needs to utilize attacks with high packet injection rates. Since such attacks are easily detected, we conclude that DoS attacks are in fact more difficult in networks with tiny buffers.

Experimental performance evaluation of blind channel estimation for orthogonal space-time block codes

In this paper, we use a 4times4 MIMO testbed to investigate the experimental performance of the blind channel estimation technique presented in (Shahbazpanahi et al., 2005). The operating frequency throughout all experiments was selected to be 2.47 GHz and the transmission bandwidth was 20 MHz. Our experimental results show that the performance of the blind technique can be very close to that of non-blind training based receiver which uses a significant bandwidth overhead as compared to the blind approach developed in (Shahbazpanahi et al., 2005).