Traffic Driven Resource Allocation in Heterogenous Wireless Networks

Most work on wireless network resource allocation use physical layer performance such as sum rate and outage probability as the figure of merit. These metrics may not reflect the true user QoS in future heterogenous networks (HetNets) with many small cells, due to large traffic variations in overlapping cells with complicated interference conditions. This paper studies the spectrum allocation problem in HetNets using the average packet sojourn time as the performance metric. To be specific, in a HetNet with $K$ base terminal stations (BTS's), we determine the optimal partition of the spectrum into $2^K$ possible spectrum sharing combinations. We use an interactive queueing model to characterize the flow level performance, where the service rates are decided by the spectrum partition. The spectrum allocation problem is formulated using a conservative approximation, which makes the optimization problem convex. We prove that in the optimal solution the spectrum is divided into at most $K$ pieces. A numerical algorithm is provided to solve the spectrum allocation problem on a slow timescale with aggregate traffic and service information. Simulation results show that the proposed solution achieves significant gains compared to both orthogonal and full spectrum reuse allocations with moderate to heavy traffic.Comment: 6 pages, 5 figures IEEE GLOBECOM 2014 (accepted for publication

Traffic-Driven Spectrum Allocation in Heterogeneous Networks

Next generation cellular networks will be heterogeneous with dense deployment of small cells in order to deliver high data rate per unit area. Traffic variations are more pronounced in a small cell, which in turn lead to more dynamic interference to other cells. It is crucial to adapt radio resource management to traffic conditions in such a heterogeneous network (HetNet). This paper studies the optimization of spectrum allocation in HetNets on a relatively slow timescale based on average traffic and channel conditions (typically over seconds or minutes). Specifically, in a cluster with $n$ base transceiver stations (BTSs), the optimal partition of the spectrum into $2^n$ segments is determined, corresponding to all possible spectrum reuse patterns in the downlink. Each BTS's traffic is modeled using a queue with Poisson arrivals, the service rate of which is a linear function of the combined bandwidth of all assigned spectrum segments. With the system average packet sojourn time as the objective, a convex optimization problem is first formulated, where it is shown that the optimal allocation divides the spectrum into at most $n$ segments. A second, refined model is then proposed to address queue interactions due to interference, where the corresponding optimal allocation problem admits an efficient suboptimal solution. Both allocation schemes attain the entire throughput region of a given network. Simulation results show the two schemes perform similarly in the heavy-traffic regime, in which case they significantly outperform both the orthogonal allocation and the full-frequency-reuse allocation. The refined allocation shows the best performance under all traffic conditions.Comment: 13 pages, 11 figures, accepted for publication by JSAC-HC

'Slow'- and 'fast'-light in a single ring-resonator circuit: theory, experimental observations, and sensing applications

Transfer matrix method (TMM) was used to study the phenomena of ‘slow’- and ‘fast’-light in a single two-port ring-resonator (TPRR) circuit theoretically. Their classifications into ‘slow’- and ‘fast’-light with negative and positive group velocity (v_g), where ‘slow’ means |v_g|<c and ‘fast’ means |v_g|>c, will be introduced. The role of such phenomena in controlling light-matter interaction and pulse delay/’advancement’ will be discussed. Direct experimental observations on pulse temporal behaviors in the regimes of ‘slow’- and ‘fast’-light with negative and positive v_g will be demonstrated, showing large and small pulse ‘advancement’ and delay, respectively. Pulse splitting phenomenon as a transition from a highly delayed to a highly ‘advanced’ pulse and vice versa, will also be experimentally demonstrated. Theoretical simulations on the pulse delay and ‘advancement’ based on the TMM and Fourier transform, which show a good qualitative agreement to the experimental results, will also be presented. The exploitation of ‘slow’-light, either with positive or negative v_g for enhancing light-matter interaction will be discussed through evaluating their effects to the performance of integrated-optical refractometric sensor. It will be shown that when the light is ‘slow’, either with negative or positive v_g, there is enhancement of the sensor sensitivity. An integrated-optical sensor which exploits such properties and exhibits sensitivity of one order better than the present day state-of-the-art commercial Mach-Zehnder interferometer refractometric sensor, will be presented

Direct experimental observation of pulse temporal behavior in integrated-optical ring-resonator with negative group velocity

We report a direct experimental observation of pulse temporal behavior in an integrated optical two-port ring-resonator circuit as a function of coupling strength, including the transition across the critical coupling point. We demonstrate the observation of pulse ‘advancement’ in the negative v_g regime and pulse delay in the positive v_g regime. We also observed a smooth transition of the pulse shape from highly negative to highly positive v_g (or vice versa) through a pulse splitting phenomenon. The observed phenomena agree well to theoretical simulations

Observing 'back to the future' phenomenon with photonic chip

The possibility to engineer the group velocity $(v_g)$ of light has attracted much attention in the last couple of years. One of the most exotic phenomena in this research field is the negative $v_g$phenomenon. Negative $v_g$ implies that if we send light pulse into the optical medium, the peak of the output will leave the output before the peak of the input pulse entering the medium, i.e. a pulse 'advancement' or negative delay. This paper will discuss such counter-intuitive 'back-to-the-future' phenomenon and its direct time-domain experimental observations on a real photonic chip using measurement equipments available in a typical optical telecommunication laboratory. Comments on the consistency of the phenomenon with the causality principles as well as possible application will also be briefly discussed