Vinay R. Majjigi

Low complexity resource allocation with opportunistic feedback over downlink OFDMA networks

Optimal tone allocation in downlink OFDMA networks is a non-convex NP-hard problem that requires extensive feedback for channel information. In this paper, two constant complexity limited-feedback algorithms are proposed to achieve near-optimal performance. First, using opportunistic feedback, the proposed schemes are shown to reduce feedback overhead by requiring only users likely to be allocated resources to feedback. There are differences between the two proposed schemes for implementation of the feedback protocol. One scheme requires less feedback but is contention-based, while the other scheme is sequential and thus avoids possible collisions leading to slightly higher performance, but needs more feedback. Second, complexity is reduced for resource allocation by solving the optimization problem in a distributed manner, rather than centrally at the base station. As shown both analytically and through numerical results, these distributed algorithms reduce the required feedback overhead significantly, and achieve constant computational complexity with little performance loss compared to the optimal solution.

Buffer State Information: Two-Level Water-Filling for Fixed Rate Applications

When transmitting at a fixed-rate to a wireless mobile terminal with strict latency requirements channel inversion is often used but at the cost of a significant loss in efficiency. Further, many such systems utilize limited packet buffering at the mobile terminal. In this paper we propose a method to recapture the lost spectral efficiency by exploiting the local buffer. Specifically, in severe channel conditions, latency and buffer state information (BSI) affords the transmitter to wait for good channel conditions rather than attempt to waste power through channel inversion. We propose a scheme that has nearly the spectral efficiency of optimal water-filling in time, yet is able to guarantee Quality of Service by ensuring a probabilistic buffer stability criterion. The solution is a cross-layer approach that utilizes event triggered BSI.

Stochastic Optimization for Variable Rate Applications with Time-Varying Statistics

We present a scheme to ensure Quality of Service for buffered variable rate applications. The scheme does not require knowledge of the channel distributions, indeed will track changing statistics, and is effectively spectrum optimal. The scheme employs stochastic approximation where the optimal solution is learned through updates of the Lagrange multiplier. The convergence speed and nearness to optimality are found, and the buffer stability probabilities are met. Analysis and simulations are provided to validate the schemes performance.

User Profiling: A Method for Limited Feedback in OFDMA Systems

In the OFDMA downlink, obtaining Channel State Information (CSI) from users is necessary for the Base Station (BS) to optimize network performance by intelligently allocating resources and scheduling mobile stations (MS). However, the overhead of obtaining CSI is a large burden and therefore, schemes to reduce CSI are necessary for a realizable system. By considering a MSs coherence time Δtcoh and coherence bandwidth Bcoh, and exploiting this redundancy in the MSs CSI, the amount of feedback can be tailored to the specific users profile and greatly decreased. Typically, reducing feedback results in more uncertainty in CSIT and performance degradation. However, in the proposed scheme, the CSITs deviation from the true channel state is bounded, and thus can provide robust scheduling. Specifically, two BS schemes are suggested that either fix the average BER or the average outage probability regardless of user mobility or delay spread.

Mobility dependent feedback scheme for point-to-point MIMO systems

A MIMO diversity scheme that utilizes varying amounts of channel state information (CSI) for mobile users is presented. CSI at the transmitter is obtained through a time-duplexed feedback channel, and thus by varying the periodicity of feedback intervals, an optimal balance is struck between obtaining accurate CSI and minimizing the overhead of uplink transmission. The optimal feedback amount is shown to have a strong dependence on the users average SNR and Doppler spread. The proposed solution optimally switches between beamforming and orthogonal space-time block coding and has negligible loss of performance with respect to more complex optimal schemes.

Buffer Stability in Multi-User Streaming Systems Applying Water-Filling with Adaptive Levels

For multi-user systems with receiver buffers, a transmission strategy for streaming applications guaranteeing buffer stability is proposed. Therefore a power-allocation scheme based on water-filling with adaptive levels is developed. The scheme exploits the multi-user diversity and is shown to operate close to the optimum strategy for transmitting non-delay-critical data.