Vinay Majjigi, Daniel O’Neill, Carolin Huppert and John Cioff. Globecom 2010.
Abstract - Multi-user buffer control is a closed-loop transmission strategy to ensure users buffers do not underflow or overflow. The dynamic scheme requires buffer state information and cooperation among transmitters. By altering the average arrival rate to users, the transmitters cause users’ buffer levels to drift away from underflow and overflow conditions. The paper presents three schemes that suggest coordination among transmitters significantly reduces the resource requirement to ensure multi-user buffer stability.
Vinay Majjigi, Daniel O’Neill and John Cioffi. ICC 2010.
Abstract - 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 scheme’s performance.
Vinay Majjigi, Daniel O’Neill and John Cioffi. Globecom 2009.
Abstract - 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.
Vinay Majjigi, Rajiv Agarwal, John Cioffi. ICC 2009.
Abstract - 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 MS’s coherence time and coherence bandwidth, and exploiting this redundancy in the MS’s CSI, the amount of feedback can be tailored to the specific user’s profile and greatly decreased. Typically, reducing feedback results in more uncertainty in CSIT and performance degradation. However, in the proposed scheme, the CSIT’s 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.
Gonzalo Vazquez-Vilar, Vinay Majjigi, Aydin Sezgin and Arogyaswami Paulraj. Asilomar 2008.
Abstract—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 user’s 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.
Vinay Majjigi, Rajiv Agarwal, Rath Vannithamby and John Cioffi. Asilomar 2007.
Abstract—We consider optimal allocation of resources to users in a downlink OFDMA system to support heterogeneous applications consisting of both deadline-sensitive (DS) and best-effort (BE) users. Given the user rate-requirements and long-term statistics, the proposed persistent scheduling algorithm allocates the minimum resources to ‘just’ meet the deadlines for DS packets. The remaining resources are optimally split to maximize a rate objective for BE users. We resort to a greedy tone allocation algorithm to solve the optimization problem. By considering persistent scheduling instead of instantaneous scheduling, the proposed scheme’s complexity and required feedback overhead is significantly reduced, hence a practical algorithm for a real-system.
Rajiv Agarwal, Vinay Majjigi, Rath Vannithamby, and John M. Cioffi. Globecom 2007.
Abstract—We consider optimal allocation of resources to users in a downlink OFDMA system to support heterogeneous applications consisting of both deadline-sensitive (DS) and best-effort (BE) data in a cross-layer manner. Given the user queue-states and long-term channel statistics, the proposed persistent scheduling algorithm allocates the minimum resources to ‘just’ meet the deadlines for DS packets (QoS guarantee). The remaining resources are optimally split between the users for their BE data to maximize sum-rate, minimize buffer holding cost or any general utility function. We formulate the resource allocation problem as a single geometric program (GP) that can be solved using standard convex optimization software tools. Simulation results are presented to compare the performance of different objectives for the BE applications in presence of DS traffic.
Rajiv Agarwal, Vinay Majjigi, Rath Vannithamby and John Cioffi. VTC Fall 2007.
Abstract-This paper studies optimal allocation of resources to users in a fading broadcast channel (BC) to support heterogeneous traffic. Resource allocation is done in a cross-layer manner for both deadline-sensitive (DS) and best-effort (BE) data to provide users guaranteed Quality of Service (QoS). Given the user queue-states, the proposed algorithm allocates the minimum resource to `just' meet the deadlines for DS packets a fixed % of time. The remaining resources are then optimally split between the users for their BE data to maximize sum-rate, minimize buffer holding cost or any general utility function. The resource allocation problem is formulated as a linear program (LP) followed by a geometric program (GP) which can be solved very efficiently. Simulation results are presented for Poisson packet arrivals and exponentially distributed packet lengths on a Rayleigh fading channel for both the DS and BE traffic.
Rajiv Agarwal, Vinay R. Majjigi, Zhu Han, Rath Vannithamby and John M. Cioffi. IEEE JSAC Special Issue on Limited Feedback. Oct 2008.
Abstract—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 feed back. 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.