Petar Djukic

Soft-TDMAC: A Software TDMA-Based MAC over Commodity 802.11 Hardware

We design and implement Soft-TDMAC, a software Time Division Multiple Access (TDMA) based MAC protocol, running over commodity 802.11 hardware. Soft-TDMAC has a synchronization mechanism, which synchronizes all pairs of network clocks to within microseconds of each other. Building on pairwise synchronization, Soft-TDMAC achieves network wide synchronization. With, out-of-band, network wide synchronization Soft-TDMAC can schedule arbitrary TDMA transmission patterns. We summarize hundreds of hours of testing Soft-TDMAC on a multi-hop testbed. Our experimental results show that Soft-TDMAC synchronizes multi-hop networks to within a few microsecond sized TDMA slots. Soft-TDMAC can schedule transmissions to take end-to-end demands into account and in a way that decreases end-to-end delay. With no collisions, under good channel conditions, TCP achieves almost the full wireless channel bandwidth.

Zoning for hierarchical network optimization in software defined networks

Software defined networking (SDN) decouples control plane functionality from the data plane and features the presence of programmable dumb network devices, which have no or little intelligence and take control commands from a central controller at the control plane. The central controller is responsible for controlling data plane hardware and optimizing network operation. Centralized network optimization and control is impractical or infeasible when the network becomes too large in size or loading. Distributed network optimization comes into play under this circumstance. Fully distributed network optimization requires local intelligence at individual network elements, against the basic concept of SDN. In this paper we consider SDN-friendly zone-based distributed network optimization and studies the integral network zoning problem, that is, how to group network elements into zones such as to minimize the overhead of distributed network optimization. We give a mathematical formulation of the problem and show that it is NP complete. We then present three heuristic solutions and evaluate their performance through simulation.

Optimized Nonuniform Constellation Rearrangement for Cooperative Relaying

The performance of cooperative relaying networks can be significantly enhanced by using constellation rearrangement (CoRe). In CoRe, the base-station and the relay-station use different constellations, each having the same number of signal points, to communicate with the user terminal. A number of CoRe schemes have been proposed in the literature based on uniform quadrature-amplitude modulation (QAM) constellations. However, it is still unclear whether nonuniform QAM constellations can further enhance the performance of CoRe. Toward this end, we investigate the problem of designing the optimum nonuniform QAM constellations for CoRe. Our motivation is that nonuniform constellations have the potential to outperform their uniform counterparts because the set of nonuniform constellations is a superset of uniform constellations. Nonuniform QAM constellations can be categorized as either decomposable or nondecomposable . Unlike nondecomposable QAM constellations, decomposable QAM constellations are generated from the Cartesian product of two pulse-amplitude modulation (PAM) constellations. We formulate an optimization problem to find the nonuniform constellations that have the minimum union bound on the uncoded symbol error rate (SER). Using convex analysis, we devise a search method to find globally optimum nonuniform decomposable constellations. We also devise a simple heuristic to find good locally optimum nonuniform nondecomposable constellations, which perform better than their decomposable counterparts.

Max-Min Fair Resource Allocation for Multiuser Amplify-and-Forward Relay Networks

We investigate the problem of multi-user radio resource allocation for orthogonal frequency division multiple access (OFDMA) amplify-and-forward (AF) relays. In the single-user case, the problem reduces to the well know assignment problem, which maximizes the user rate. For the multi-user case we devise a resource allocation algorithm to achieve max-min fairness. We find max-min fairness since it can provide almost flat ubiquitous coverage. We start by formulating a convex optimization, which takes a parameter that asymptotically makes the optimization produce max-min fair rates. Since the optimization is a convex problem, we are able to devise a sub-optimal gradient-based algorithm to solve it quickly. Simulations show that the algorithm achieves results very close to the optimum solutions due to its gradient origins.

Generalized Proportionally Fair Scheduling for Multi-User Amplify-and-Forward Relay Networks

Providing ubiquitous very high data rate coverage in next generation wireless networks is a formidable goal, requiring cost-effective radio access network (RAN) devices, such as multi-user enabled amplify- and-forward (AF) relays, and fair radio resource management (RRM). To further this goal, we investigate multi-user enabled AF relays which multiplex users data in orthogonal frequency division multiple access (OFDMA). These relays are cost-effective, simpler to implement, and introduce less delay in comparison to other relay based routers. We devise a generalized proportionally fair (GPF) RRM framework for multi-user enabled AF relays. In GPF scheduling a single parameter is used to gradually change schedules from throughput optimal to proportionally fair. We formulate the GPF scheduling problem and due to its complexity devise a low complexity heuristic to solve it. We evaluate the performance of the heuristic with extensive simulations and show that the heuristic performs well.

Generalized Constellation Rearrangement in Cooperative Relaying

In constellation rearrangement (CoRe), the base-station and the relay use different constellations, with the same number of signal points, to communicate with the user terminal. In contrast to the existing CoRe techniques, which restrict the possible constellations, we propose generalized quadrature amplitude modulation (QAM) constellations. Since generalized CoRe schemes do not restrict constellations, they have the potential of outperforming all other CoRe schemes, with a complexity penalty in decoding. We pose an optimization to find the generalized QAM constellations, which minimize an upper bound on the uncoded symbol error rate (SER). However, since the optimization is not convex, it is not possible to find constellations with globally minimum SER in a reasonable time. Nevertheless, we use a convex solver to find constellations, which are local minima to the optimization. We input the best known restricted CoRe constellations as starting points to the solver to find constellations, which are guaranteed to improve SER. We demonstrate the significant gains achieved by the proposed CoRe scheme with simulations.

Near-optimal non-uniform constellation rearrangement for cooperative relaying

In constellation rearrangement (CoRe), the basestation and the relay use different constellations, with the same modulation level, to communicate with the user terminal. Unlike the existing CoRe techniques, which are restricted to uniform constellations, we propose to use non-uniform quadrature amplitude modulation (QAM) constellations for CoRe. We devise an optimization to find the optimal QAM constellation, which minimizes an upper bound on the symbol error rate (SER). We show how to decompose the optimization problem, which is not convex, into a set of convex optimization problems to solve it. Since the set of non-uniform constellations is a superset of uniform constellations (used in literature), the proposed non-uniform CoRe outperforms all the existing uniform CoRe schemes, with respect to the SER bound. Simulation results show that the proposed non-uniform CoRe scheme yields large gains, with no additional complexity penalty.

Advanced Radio Access Networks for LTE and Beyond

Standardization efforts towards achieving 4G objectives have resulted in the development of Orthogonal Frequency Division Multiple Access (OFDMA) technology. The flexible frequency assignment enabled by OFDMA allows for the introduction of new network elements such as distributed antenna ports, femto base-stations, and relays, and new coordinated multipoint (CoMP) transmission and reception techniques. New network elements require new radio access network (RAN) architectures and make radio resource management (RRM) a very challenging problem, requiring a new set of optimization techniques. First, we overview the evolution of 4G RANs, with special emphasis on the 3 Generation Partnership Project (3GPP) Long Term Evolution (LTE) and LTE-Advanced RAN architectures, and RRM optimization algorithms for OFDMA-based 4G RANs, such as LTE and LTE-Advanced. Second, P. Djukic, M. Rahman and H. Yanikomeroglu are with the Department of Systems and Computer Engineering, Carleton University, Canada, e-mail: {djukic,mmrahman,halim}@sce.carleton.ca, J. Zhang is with the Huawei Wireless Research, China, e-mail: jtzhang@huawei.com

Mixed Time-Scale Generalized Fair Scheduling for Amplify-and-Forward Relay Networks

We devise an optimization framework for generalized proportional fairness (GPF) under different time scales for amplify-and-forward (AF) relay networks. In GPF scheduling, a single input parameter is used to change the fairness from throughput optimal, to proportionally fair and asymptotically to max-min fair. We extend the GPF scheduling to include a new input parameter, which determines the time-scale of fairness from short-term GPF to long-term GPF. We devise a low-complexity near-optimal algorithm to find schedules satisfying the given fairness criteria in a given time-scale. Simulations show that the proposed algorithm indeed allows the flexibility to change the fairness and its time- scale. To the best of our knowledge, this paper is the first to provide a multi-user scheduling framework for AF relays with both flexible fairness and flexible time-scales.