Radhakrishna Ganti

Recent work has shown that adaptive CSMA algorithms can achieve throughput optimality. However, these adaptive CSMA algorithms assume a rather simplistic model for the wireless medium. Specifically, the interference is typically modelled by a conflict graph, and the channels are assumed to be static. In this work, we propose a distributed and adaptive CSMA algorithm under a more realistic signal-to-interference ratio (SIR) based interference model, with time-varying channels. We prove that our algorithm is throughput optimal under this generalized model. Further, we augment our proposed algorithm by using a parallel update technique. Numerical results show that our algorithm outperforms the conflict graph based algorithms, in terms of supportable throughput and the rate of convergence to steady-state.

Mobility modeling and analysis is a key issue in wireless networks. In this paper we propose a new and quite general random waypoint (RWP) mobility model which is valid over the entire plane. We derive key properties of the proposed mobility model including transition length, transition time and spatial node distribution. Then the RWP mobility model is applied to study the handover rate in cellular networks under both deterministic (hexagonal) and random (Poisson) base station (BS) models. Closed form expressions for handover rate can be obtained. These results show the expected property that the handover rate is proportional to the square root of base station density. Also, we find that Poisson-Voronoi model for BS coverage areas is about as accurate in terms of mobility (particularly handover) evaluation as the ubiquitous hexagonal model. © 2012 IEEE.

In this paper, we analyze the downlink of a multi-antenna heterogeneous cellular network with distance-dependent interference. In particular, we consider open-loop spatial multiplexing (SM) with minimum mean-square estimation (MMSE) receiver for suppressing self-interference as well as other cell interference. Coverage probability and rate distribution are obtained with biased user association rule and inter-cell interference. The results show that using full rate SM at the macro base stations (BSs) degrades the data rate for a large percentage of cell-edge users compared to a low rate SM transmission. Therefore, the cell edge data rate can be improved substantially by reducing the SM rate at the macro BSs while maintaining full SM rate at the pico BSs. The loss in mean spectrum efficiency (SE) caused by the SM rate reduction is shown to be acceptable for the cases of interest.

It has recently been observed that the SIR distributions of a variety of cellular network models and transmission techniques look very similar in shape. As a result, they are well approximated by a simple horizontal shift of the distribution of the most tractable model, the Poisson point process. This paper makes a first step towards explaining this remarkable property by showing that the asymptotics of the SIR distribution near 0 and near infinity can only differ by a constant.

Device-to-device (D2D) communication is expected to increase the coverage probability by coexisting along with the conventional cellular systems in 5G technology. As the user equipments are often held closer to the human body, D2D links experience shadowing modelled by the amp;#954;-amp;#956; shadowed fading model. Though the cellular links do not experience amp;#954;-amp;#956; shadowed fading, the same is assumed here as it generalizes many popular fading distributions such as Rayleigh, Rician, Nakagami, amp;#954;-amp;#956;, amp;#951;-amp;#956;, Hoyt etc. Thus the above assumption makes the results obtained here generic. We show that by neglecting shadowing in the D2D link, coverage probability of D2D user is over-estimated at low SIR thresholds and under-estimated at high SIR thresholds. Though cellular links are interfered with by the D2D links, we observe that there is negligible impact on cellular coverage probability due to any change of fading parameters in the D2D links. Also, when all the links are Rayleigh faded, the D2D distance below which D2D mode is preferred to cellular mode of communication is obtained analytically.

Random spatial models are attractive for modeling heterogeneous cellular networks (HCNs) due to their realism, tractability, and scalability. A major limitation of such models to date in the context of HCNs is the neglect of network traffic and load: all base stations (BSs) have typically been assumed to always be transmitting. Small cells in particular will have a lighter load than macrocells, and so their contribution to the network interference may be significantly overstated in a fully loaded model. This paper incorporates a flexible notion of BS load by introducing a new idea of conditionally thinning the interference field. For a K-tier HCN where BSs across tiers differ in terms of transmit power, supported data rate, deployment density, and now load, we derive the coverage probability for a typical mobile, which connects to the strongest BS signal. Conditioned on this connection, the interfering BSs of the i^{th} tier are assumed to transmit independently with probability p-i, which models the load. Assuming-reasonably-that smaller cells are more lightly loaded than macrocells, the analysis shows that adding such access points to the network always increases the coverage probability. We also observe that fully loaded models are quite pessimistic in terms of coverage. © 2002-2012 IEEE.

The orders of magnitude increase in projected demand for wireless cellular data require drastic increases in spatial reuse, with picocells with diameters of the order of 100-200 m supplementing existing macrocells whose diameters are of the order of kilometers. In this paper, we observe that the nature of interference changes fundamentally as we shrink cell size, with near line of sight interference from neighboring picocells seeing significantly smaller path loss exponents than interference in macrocellular environments. Using a propagation model proposed by Franceschetti, which compactly models increased interference in small cells, we show that the network capacity does not scale linearly with the reduction in cell size with standard frequency reuse strategies. Rather, more sophisticated resource sharing strategies based on beamforming and base station cooperation are required to realize the potential of small cells in providing high spectral efficiencies and quasi-deterministic guarantees on availability. Numerical results justifying these conclusions include Chernoff bounds on outage probability for random base station deployment (according to a spatial Poisson process), and simulations for deployment in a regular grid. © 2013 IEEE.

A recent way to model and analyze downlink cellular networks is by using random spatial models. Assuming user equipment (UE) distribution to be uniform, the analysis is performed at a typical UE located at the origin. At least one shortcoming of this approach is its inability to model non-uniform UE distributions, especially when there is dependence in the UE and the base station (BS) locations. To facilitate analysis in such cases, we propose a new tractable method of sampling UEs by conditionally thinning the BS point process and show that the resulting framework can be used as a tractable generative model to study current capacity-centric deployments, where the UEs are more likely to lie closer to the BSs. © 2012 IEEE.

The concept of electrical balance is used to achieve wideband isolation in an asymmetric 2.4 GHz directional coupler. An intuitive approach using impedance transformation property of transmission lines is used to develop the theory for designing arbitrary-coupling-ratio directional couplers. A coupler based on this theory is designed and fabricated on a two-layered RO4003C PCB. The coupler achieves a return loss of better than 10 dB over a 1.7-2.7 GHz range. The transmission coefficients of the strongly and weakly coupled ports are around-3.5 dB and-5 dB respectively, at 2.4 GHz. The isolation is better than 25 dB over a 1.4-3 GHz range.

In most developing countries across the world, cellular rural users are predominantly pedestrian and indoor users within village homes, and much less from vehicles, let alone fast-moving ones. Specifically, in India, 85% of the villages in the plains are spaced 2–3 km apart uniformly in every direction. The base station required for providing coverage in rural areas is starkly different from the base station along highways to provide coverage for fast-moving vehicles. Till IMT-2020, ITU did not have a rural use case suited to rural needs of developing countries. Hence, the IMT advanced technology development (such as 4G LTE) did not meet the rural requirements in countries like India. To avoid such a situation for 5G, a new test case requirement, named low-mobility large cell (LMLC) was included as a mandatory use case with performance requirements that must be met by every specification that is approved as an IMT-2020 compliant standard. In this paper, we describe the reasons why such a test case requirement is crucial by taking a case study from the Indian rural setting. In addition, we also describe various technical solutions that can be considered to satisfy such stringent requirements. In particular, we look at a new waveform with low peak-to-average power ratio that has been introduced in 5G for coverage enhancement.