Venkatesh Ramaiyan

Fixed point analysis of single cell IEEE 802.11e WLANs: uniqueness and multistability

We consider the vector fixed point equations arising out of the analysis of the saturation throughput of a single cell IEEE 802.11e (EDCA) wireless local area network with nodes that have different backoff parameters, including different arbitration interframe space (AIFS) values. We consider balanced and unbalanced solutions of the fixed point equations arising in homogeneous (i.e., one with the same backoff parameters) and nonhomogeneous networks. By a balanced fixed point, we mean one where all coordinates are equal. We are concerned, in particular, with: 1) whether the fixed point is balanced within a class, and 2) whether the fixed point is unique. Our simulations show that when multiple unbalanced fixed points exist in a homogeneous system then the time behavior of the system demonstrates severe short term unfairness (or multistability). We provide a condition for the fixed point solution to be balanced, and also a condition for uniqueness. We then extend our general fixed point analysis to capture AIFS based differentiation and the concept of virtual collision when there are multiple queues per station; again a condition for uniqueness is established. For the case of multiple queues per node, we find that a model with as many nodes as there are queues, with one queue per node, provides an excellent approximation. Implications for the use of the fixed point formulation for performance analysis are also discussed.

Fixed point analysis of single cell IEEE 802.11e WLANs: uniqueness, multistability and throughput differentiation

We consider the vector fixed point equations arising out of the analysis of the saturation throughput of a single cell IEEE 802.11e wireless local area network with nodes that have different back-off parameters, including different Arbitration InterFrame Space (AIFS) values. We consider balanced and unbalanced solutions of the fixed point equations arising in homogeneous and nonhomogeneous networks. We are concerned, in particular, with (i) whether the fixed point is balanced within a class, and (ii) whether the fixed point is unique. Our simulations show that when multiple unbalanced fixed points exist in a homogeneous system then the time behaviour of the system demonstrates severe short term unfairness (or multistability). Implications for the use of the fixed point formulation for performance analysis are also discussed. We provide a condition for the fixed point solution to be balanced within a class, and also a condition for uniqueness. We then provide an extension of our general fixed point analysis to capture AIFS based differentiation; again a condition for uniqueness is established. An asymptotic analysis of the fixed point is provided for the case in which packets are never abandoned, and the number of nodes goes to ∞. Finally the fixed point equations are used to obtain insights into the throughput differentiation provided by different initial back-offs, persistence factors, and AIFS, for finite number of nodes, and for differentiation parameter values similar to those in the standard.

WiMAX relay networks: opportunistic scheduling to exploit multiuser diversity and frequency selectivity

We study the problem of scheduling in OFDMA-based relay networks with emphasis on IEEE 802.16j based WiMAX relay networks. In such networks, in addition to a base station, multiple relay stations are used for enhancing the throughput, and/or improving the range of the base station. We solve the problem of MAC scheduling in such networks so as to serve the mobiles in a fair manner while exploiting the multiuser diversity, as well as the frequency selectivity of the wireless channel. The scheduling resources consist of tiles in a two-dimensional scheduling frame with time slots along one axis, and frequency bands or sub-channels along the other axis. The resource allocation problem has to be solved once every scheduling frame which is about 5 - 10 ms long. While the original scheduling problem is computationally complex, we provide an easy-to-compute upper bound on the optimum. We also propose three fast heuristic algorithms that perform close to the optimum (within 99.5%), and outperform other algorithms such as OFDM2A proposed in the past. Through extensive simulation results, we demonstrate the benefits of relaying in throughput enhancement (an improvement in the median throughput of about 25%), and feasibility of range extension (for e.g., 7 relays can be used to extend the cell-radius by 60% but mean throughput reduces by 36%). Our algorithms are easy to implement, and have an average running time of less than 0.05 ms making them appropriate for WiMAX relay networks.

Delay Optimal Scheduling in a Two-Hop Vehicular Relay Network

We study a scheduling problem in a wireless network where vehicles are used as store-and-forward relays, a situation that might arise, for example, in practical rural communication networks. A fixed source node wants to transfer a file to a fixed destination node, located beyond its communication range. In the absence of any infrastructure connecting the two nodes, we consider the possibility of communication using vehicles passing by. Vehicles arrive at the source node at renewal instants and are known to travel towards the destination node with average speed v sampled from a given probability distribution. The source node communicates data packets (or fragments) of the file to the destination node using these vehicles as relays. We assume that the vehicles communicate with the source node and the destination node only, and hence, every packet communication involves two hops. In this setup, we study the source node’s sequential decision problem of transferring packets of the file to vehicles as they pass by, with the objective of minimizing delay in the network. We study both the finite file size case and the infinite file size case. In the finite file size case, we aim to minimize the expected file transfer delay, i.e., expected value of the maximum of the packet sojourn times. In the infinite file size case, we study the average packet delay minimization problem as well as the optimal tradeoff achievable between the average queueing delay at the source node buffer and the average transit delay in the relay vehicle.

Optimal Hop Distance and Power Control for a Single Cell, Dense, Ad Hoc Wireless Network

We consider a dense, ad hoc wireless network, confined to a small region. The wireless network is operated as a single cell, i.e., only one successful transmission is supported at a time. Data packets are sent between source-destination pairs by multihop relaying. We assume that nodes self-organize into a multihop network such that all hops are of length d meters, where d is a design parameter. There is a contention-based multiaccess scheme, and it is assumed that every node always has data to send, either originated from it or a transit packet (saturation assumption). In this scenario, we seek to maximize a measure of the transport capacity of the network (measured in bit-meters per second) over power controls (in a fading environment) and over the hop distance d, subject to an average power constraint. We first motivate that for a dense collection of nodes confined to a small region, single cell operation is efficient for single user decoding transceivers. Then, operating the dense ad hoc wireless network (described above) as a single cell, we study the hop length and power control that maximizes the transport capacity for a given network power constraint. More specifically, for a fading channel and for a fixed transmission time strategy (akin to the IEEE 802.11 TXOP), we find that there exists an intrinsic aggregate bit rate (\Theta_{opt} bits per second, depending on the contention mechanism and the channel fading characteristics) carried by the network, when operating at the optimal hop length and power control. The optimal transport capacity is of the form d_{opt}(\bar{P_t}) \times \Theta_{opt} with d_{opt} scaling as \bar{P_t}^{{1\over \eta}}, where \bar{P_t} is the available time average transmit power and \eta is the path loss exponent. Under certain conditions on the fading distribution, we then provide a simple characterization of the optimal operating point. Simulation results are provided comparing the performance of the optimal strategy derived here with some simple strategies for operating the network.

Jointly Optimal Power Control and Routing for a Single Cell, Dense, Ad Hoc Wireless Network

We consider a dense, ad hoc wireless network confined to a small region, such that direct communication is possible between any pair of nodes. The physical communication model is that a receiver decodes the signal from a single transmitter, while treating all other signals as interference. Data packets are sent between source-destination pairs by multihop relaying. We assume that nodes self-organise into a multihop network such that all hops are of length d meters, where d is a design parameter. There is a contention based multiaccess scheme, and it is assumed that every node always has data to send, either originated from it or a transit packet (saturation assumption). In this scenario, we seek to maximize a measure of the transport capacity of the network (measured in bit-meters per second) over power controls (in a fading environment) and over the hop distance d, subject to an average power constraint. We first argue that for a dense collection of nodes confined to a small region, single cell operation is efficient for single user decoding transceivers. Then, operating the dense ad hoc network (described above) as a single cell, we study the optimal hop length and power control that maximizes the transport capacity for a given network power constraint. More specifically, for a fading channel and for a fixed transmission time strategy (akin to the IEEE 802.11 TXOP), we find that there exists an intrinsic aggregate bit rate (Thetaopt bits per second, depending on the contention mechanism and the channel fading characteristics) carried by the network, when operating at the optimal hop length and power control. The optimal transport capacity is of the form dopt(Pmacrt) x Thetaopt with dopt scaling as Pmacrt 1 /eta, where Pmacrt is the available time average transmit power and eta is the path loss exponent. Under certain conditions on the fading distribution, we then provide a simple characterisation of the optimal operating point.

Performance analysis of an IEEE 802.11ac WLAN with dynamic bandwidth channel access

We consider a collocated WLAN with Nac 802.11ac users using dynamic 20/40 MHz channel access and Nl 802.11a/n legacy users operating in the secondary 20 MHz channel. Under ideal channel conditions, we seek to characterise the saturation throughput performance of the 802.11ac users and the legacy users. We propose an analytical model based on a decoupling approximation for saturation throughput analysis. Dynamic bandwidth channel access leads to correlation in the channel access of 802.11ac and legacy users and hence, we consider a DTMC to model the transmission opportunities in the primary and the secondary channel. Using simulations, we show that the proposed model accurately captures both high level performance measures such as long term average throughput and low level performance measures such as collision probability very well. Finally, we characterise the effect of Nac, Nl and packet size on the network performance.

Reliable Multihop Broadcast Protocols for Inter-Vehicular Communication in a Fading Channel

We consider a inter-vehicular communication scenario with vehicles moving along a highway. The vehicles are equipped with radio devices and can communicate among themselves. We study the problem of reliable multihop broadcasting in such a scenario. We propose two broadcasting algorithms -grid based multihop broadcast (GBMB) and controlled flooding (CF). Controlled flooding is derived from classical flooding. Grid based multihop broadcasting is a simple grid based multihop scheme that adds reliability as well as increases throughput by reducing collisions. We have shown through simulations that our proposed protocols have high reliability and yield efficient channel utilization when compared to Flooding, Probabilistic Flooding and Area Based Protocols in scenarios where the channel is lossy.

On the Limits of Spatial Reuse and Cooperative Communication for Dense Wireless Networks

We consider a dense ad hoc wireless network comprising n nodes confined to a given two dimensional region of fixed area. For the Gupta-Kumar random traffic model and a realistic interference and path loss model (i.e., the channel power gains are bounded above, and are bounded below by a strictly positive number), we study the scaling of the aggregate end-to-end throughput with respect to the network average power constraint, P macr, and the number of nodes, n. The network power constraint P macr is related to the per node power constraint, P macr, as P macr = np. For large P, we show that the throughput saturates as Theta(log(P macr)), irrespective of the number of nodes in the network. For moderate P, which can accommodate spatial reuse to improve end-to-end throughput, we observe that the amount of spatial reuse feasible in the network is limited by the diameter of the network. In fact, we observe that the end-to-end path loss in the network and the amount of spatial reuse feasible in the network are inversely proportional. This puts a restriction on the gains achievable using the cooperative communication techniques studied in and, as these rely on direct long distance communication over the network.

A completely uncoupled learning algorithm for general utility maximization

In this paper, we study completely uncoupled learning algorithms for general utility maximization. We illustrate the algorithm with a wireless network application viz distributed user association. Our main contribution is expansion of achievable rate region by allowing time sharing of resources, which the previous works based on completely uncoupled strategies have ignored. First, we present a distributed user association algorithm based on a state space expansion that can achieve any desired throughput vector in the rate region of the wireless network. Then, for concave utility functions, we present a stochastic gradient algorithm with fewer synchronization requirements than known references.