Ahmad Khoshnevis

Opportunistic spectral usage: bounds and a multi-band CSMA/CA protocol

In this paper, we study the gains from opportunistic spectrum usage when neither sender or receiver are aware of the current channel conditions in different frequency bands. Hence to select the best band for sending data, nodes first need to measure the channel in different bands which takes time away from sending actual data. We analyze the gains from opportunistic band selection by deriving an optimal skipping rule, which balances the throughput gain from finding a good quality band with the overhead of measuring multiple bands. We show that opportunistic band skipping is most beneficial in low signal to noise scenarios, which are typically the cases when the node throughput in single-band (no opportunism) system is the minimum. To study the impact of opportunism on network throughput, we devise a CSMA/CA protocol, Multi-band Opportunistic Auto Rate (MOAR), which implements the proposed skipping rule on a per node pair basis. The proposed protocol exploits both time and frequency diversity, and is shown to result in typical throughput gains of 20% or more over a protocol which only exploits time diversity, Opportunistic Auto Rate (OAR).

On the asymptotic performance of multiple antenna channels with quantized feedback

In this paper, we analyze the asymptotic performance of multiple antenna channels where the transmitter has finite bit channel state information. Using the diversity multiplexing tradeoff to characterize the system performance, we demonstrate that channel feedback can fundamentally change the system behavior. Even one-bit of information can increase the diversity order of the system compared to the system with no transmitter information. In addition, as the amount of channel information at the transmitter increases, the diversity order for each multiplexing gain increases and goes to infinity for perfect transmitter information. The major reason for diversity order gain is in temporal power control, which adapts the power control strategy based on the average channel conditions of the channel.

Performance of quantized power control in multiple antenna systems

In this paper, we analyze the outage probability of a single user system with multiple antennas at the transmitter, single antenna at the receiver, and finite rate feedback power control. The optimum power control is complex and the analysis is not tractable. Hence we propose a sub-optimal power allocation scheme, with very low computational complexity, which is asymptotically optimum. Analyzing the proposed algorithm we show that the diversity order can potentially be increased unboundedly by increasing the feedback rate and without increasing number of transmit or receive antennas. We find a closed form approximation to this diversity-like gain at large SNRs, as a function of number of transmit antennas, number of quantization levels, and average available SNR. Simulation results confirm the validity of the analysis.

The Case for Transmitter Training

Transmitter side information enables techniques such as beamforming, power control, and rate control in fading channels. It is commonly accepted in the literature that the addition of transmitter information (CSIT) to receiver information (CSIR) provides better performance than receiver information alone. In this work, we examine the performance of a symmetric, single-input, multiple-output (SIMO) channel in which CSIT is acquired through the use of training symbols, and we have a genie-aided receiver. We give a closed form expression for outage probability at high SNR while accounting for the resources consumed by training. We also analyze the diversity-multiplexing tradeoff and find that, though the diversity falls far below that of systems with perfect CSIT, it is still sufficiently superior to that achieved by CSIR-only systems to justify the cost of training. We show that, at zero multiplexing, transmitter training doubles the diversity order of a CSIR-only system and offers nonzero diversity at all achievable multiplexing gains

Achievable diversity and multiplexing in multiple antenna systems with quantized power control

We consider a multiple antenna system with finite rate feedback, in which the quantized channel state information at the transmitter is used solely for temporal power control. We show that similar to systems without feedback, the tradeoff between diversity order and multiplexing gain exists. However, unlike the systems with feedback that apply both rate and power control, systems with only power control are unable of achieving non-zero diversity order at the maximum multiplexing gain. The analysis is based on asymptotic behavior of the distribution of order statistics of the eigenvalues of channel matrix, which is a key step in evaluating the diversity order.

An Achievable Rate Region for a Multiuser Half-Duplex Two-Way Channel

Feedback is known to enlarge the capacity region of a multiuser channel. However, none of the information theoretic analysis account for resource usage of the feedback link. In this paper, we adopt a novel two-way formulation for multiuser systems, which jointly designs the uplink and downlink communication. By assuming that nodes are half-duplex (in time) and feedback shares resources with data, all resource usage is accurately accounted in the system. Our achievable rate region shows that feedback is beneficial only if the the channel is two- way, i.e, there is data to be sent in both directions.