Data Offloading in Load Coupled Networks: A Utility Maximization Framework

Abstract-We provide a general framework for the problem of data offloading in a heterogeneous wireless network, where some demand of cellular users is served by a complementary network. The complementary network is either a small-cell network that shares the same resources as the cellular network, or a WiFi network that uses orthogonal resources. For a given demand served in a cellular network, the load, or the level of resource usage, of each cell depends in a non-linear manner on the load of other cells due to the mutual coupling of interference seen by one another. With load coupling, we optimize the demand to be served in the cellular or the complementary networks, so as to maximize a utility function. We consider three representative utility functions that balance, to varying degrees, the revenue from serving the users vs the user fairness. We establish conditions for which the optimization problem has a feasible solution and is convex, and hence tractable to numerical computations. Finally, we propose a strategy with theoretical justification to constrain the load to some maximum value, as required for practical implementation. Numerical studies are conducted for both under-loaded and over-loaded networks

ARQ by subcarrier assignment for OFDM-based systems

Abstract-We consider two automatic repeat request (ARQ) schemes based on subcarrier assignment in orthogonal frequency-division multiplexing (OFDM)-based systems: single ARQ subcarrier assignment (single ARQ-SA) and multiple ARQ-SA. In single ARQ-SA, data transmitted on a subcarrier in a failed transmission are repeated on a single assigned subcarrier in the ARQ transmission. In multiple ARQ-SA, the data are repeated on multiple assigned subcarriers in the ARQ transmission. At the receiver, maximum ratio combining is performed on subcarriers that carry the same data. Our goal is to optimize certain system utility functions (such as to minimize bit error rates or to maximize sum capacity) through the choice of the subcarrier assignment. We show that a large class of reasonable system utility functions that we wish to maximize are characterized as Schur-concave. For this class of utility functions, we obtain the optimum subcarrier assignment for single ARQ-SA and propose a suboptimum (heuristic) subcarrier assignment scheme for multiple ARQ-SA. Further, to lower the overhead of signaling the subcarrier assignment information, we consider subcarrier grouping methods. Numerical results indicate that substantial throughput improvement can be achieved by appropriate assignments, especially with the use of incremental redundancy at high signal-to-noise ratios. Index Terms-Automatic repeat request (ARQ), majorization, orthogonal frequency-division multiplexing (OFDM), subcarrier assignment