A. V. Babu

Fairness Analysis of IEEE 802.11 Multirate Wireless LANs

This paper investigates the issue of fairness in IEEE 802.11-based multirate wireless local area networks (WLANs). Distributed coordination function, which is the medium-access control (MAC) protocol used in 802.11 WLANs, provides equal long-term channel access probability to competing stations, irrespective of the time required in sending a frame. When equal-sized frames are used and channel conditions are similar, each station, regardless of its data rate, achieves the same throughput. Furthermore, the aggregate throughput is reduced to a level much closer to what one gets when all stations are of lower rate. This anomaly in the performance is a result of unfair channel time allocation for stations when they use multiple data rates. We consider provisioning of time-based fairness in which each station receives an equal share of the wireless channel occupancy time. We demonstrate that under time-based fairness, the throughput that a tagged node achieves in a multirate WLAN with nodes is identical to what it would achieve in a single-rate WLAN with nodes all at the same data rate as the tagged node. Furthermore, we show that under time-based fairness scheme, the ratio of throughputs per station corresponding to two different bit rates is directly proportional to the ratio of their bit rates. We analyze different mechanisms in achieving time-based fairness by using an analytical model. Using Jains fairness index, optimal MAC parameters required in achieving maximum fairness between slow and fast stations are obtained. The impacts of these mechanisms on throughput of slow and fast stations are explored. We also consider the notion of proportional fairness in a multirate scenario and prove that it is equivalent to fair channel time allocation. Last, our investigation of an alternative fairness criterion also leads us to propose that the IEEE 802.11 MAC protocol should be redesigned with temporal fairness as a design objective in avoiding inefficiencies related to the performance anomaly.

Improving energy efficiency of incremental relay based cooperative communications in wireless body area networks

In this paper, we investigate the energy efficiency of an incremental relay based cooperative communication scheme in wireless body area networks WBANs. We derive analytical expressions for the energy efficiency of direct and cooperative communication schemes taking into account the effect of packet error rate. The following communication scenarios specific to a WBAN are considered: i in-body communication between an implant sensor node and the gateway, and ii on-body communication between a body surface node and the gateway with line-of-sight LOS and non-LOS channels. The results reveal a threshold behavior that separates regions where direct transmission is better from regions where incremental relay cooperation is more useful in terms of energy efficiency. It is observed that, compared with direct communication, incremental relay based cooperative communication schemes improves the energy efficiency significantly. Further, cooperation extends the source-to-destination hop length over, which energy efficient communication can be achieved as compared with direct communication. We also observe that, for both direct as well as cooperative transmission schemes in error prone channels, an optimal packet size exists that result in maximum energy efficiency. Copyright

Ensuring fair access in IEEE 802.11p-based vehicle-to-infrastructure networks

IEEE 802.11p is an approved amendment to the IEEE 802.11 standard to facilitate wireless access in vehicular environments (WAVE). In this article, we present an analytical model to evaluate the impact of vehicle mobility on the saturation throughput of IEEE 802.11p-based vehicle-to-infrastructure (V2I) networks. The throughput model is then used to investigate an unfairness problem that exists in such networks among vehicles with different mobility characteristics. Assuming a saturated network, if all the vehicles in the network use the same MAC parameters, IEEE 802.11p MAC protocol provides equal transmission opportunity for all of them, provided they have equal residence time in the coverage area of a road side unit (RSU). When vehicles have different mobility characteristics (e.g., extremely high and low speeds), they do not have similar chances of channel access. A vehicle moving with higher velocity has less chance to communicate with its RSU, as compared to a slow moving vehicle, due to its short residence time in the coverage area of RSU. Accordingly, the data transfer of a higher velocity vehicle gets degraded significantly, as compared to that of the vehicle with lower velocity, resulting in unfairness among them. In this article, our aim is to address this unfairness problem that exists among vehicles of different velocities in V2I networks. Analytical expressions are derived for optimal minimum CW (CWmin) required to ensure fairness, in the sense of equal chance of communicating with RSU, among competing vehicles of different mean velocities in the network. Analytical results are validated using extensive simulations.

Analytical model for connectivity of vehicular ad hoc networks in the presence of channel randomness

SUMMARY A vehicular ad hoc network (VANET) is a highly mobile wireless ad hoc network formed by vehicles equipped with communication facilities. Developing multihop communication in VANETs is a challenging problem because of rapidly changing network topology and frequent network disconnections. This paper investigates the network connectivity probability of one-dimensional VANET in the presence of channel randomness. Network connectivity is one of the most important issues in VANETs, because the dissemination of time-critical information requires, as a preliminary condition, the network to be fully connected. We present an analytical procedure for the computation of network connectivity probability, taking into account the underlying wireless channel. Three different fading models are considered for the connectivity analysis: Rayleigh, Rician, and Weibull. A distance-dependent power law model is employed for the pathloss in a vehicle-to-vehicle channel. Furthermore, the speed of each vehicle on the highway is assumed to be a Gaussian distributed random variable. The analysis provides a general framework for investigating the dependence of various parameters such as vehicle arrival rate, vehicle density, vehicle speed, highway length, and various physical layer parameters such as transmit power, receive signal-to-noise ratio threshold, path loss exponent, and fading factors (Rician and Weibull) on VANET connectivity. Copyright

Network connectivity of one-dimensional Vehicular Ad hoc Network

In this paper, we study the connectivity of a Vehicular Ad hoc Network (VANET) formed between vehicles that move on a highway. Analytical model to determine the network connectivity of VANET is presented, assuming that the speed of vehicles follow normal distribution. We bring out the exact dependence of vehicle speed statistics on VANET connectivity. We also present the dependence of vehicle speed statistics on both the node isolation probability as well as the critical transmission range required to maintain the desired connectivity probability. The results show that when the vehicle transmission range increases, the network connectivity also gets increased. The results also show that the network connectivity gets degraded when the average vehicle speed increases. Further, as the average speed increases, the critical transmission range required to meet a given connectivity probability criterion increases. On the other hand, when the standard deviation of vehicle speed increases, the network connectivity gets improved. It is also shown that, for a given connectivity probability requirement, the critical transmission range decreases as the standard deviation of the speed increases.

Connectivity Analysis of Vehicular Ad Hoc Networks from a Physical Layer Perspective

This paper investigates the network connectivity properties of a vehicular ad hoc network (VANET) from a physical layer perspective. Specifically, we investigate the minimum transmit power used by all vehicles, sufficient to guarantee network connectivity. As opposed to the conventional graph-theoretic approach, in this paper, the network connectivity problem is analyzed according to a physical layer-based quality of service constraint. Under this approach, a multi-hop path joining a pair of vehicles in a VANET is said to be connected if and only if the average route BER meets a target requirement. We derive closed form analytical expression for the minimum transmit power sufficient to ensure network connectivity. We also derive analytical expression for the maximum number of hops, a packet can traverse satisfying the route BER constraint, for a given transmit power. The validity of our theoretical analysis is verified by extensive simulation studies. The analysis provides a framework for investigating the impact of traffic dependent parameters such as vehicle arrival rate, vehicle density, mean and standard deviation of vehicle speed, highway length and physical layer-based parameters such as path loss exponent, fading factor, Doppler spread, and data rate on VANET connectivity characteristics.

Packet size optimization for energy efficient cooperative Wireless Body Area Networks

In this paper, we investigate packet size optimization to improve energy efficiency of cooperative Wireless Body Area Networks (WBANs). We present models for the packet error rate and the energy efficiency of ARQ scheme for direct as well as cooperative communication scheme. We then consider the packet size optimization for both the transmission schemes. The theoretical analysis and the numerical evaluations reveal that cooperative transmission scheme improves the energy efficiency, increases the optimal payload packet size, and extends the hop length, as compared to direct communication for all scenarios of WBANs that includes both in-body as well as on-body propagation models.

Maximizing the data transmission rate of a cooperative relay system in an underwater acoustic channel

In this paper, we investigate the problem of maximizing the data transmission rate of a cooperative relay system in an underwater acoustic communication channel. With amplify-and-forward relaying and adaptive source transmission, we present optimal transmit signal power adaptation policies that maximize the data transmission rate, considering both frequency and time domains. The analysis takes into account a physical model of acoustic path loss and ambient noise power spectral density. Typical characteristics of underwater channel such as frequency-dependent fading and time variations are also considered. Capacity bounds for channel state information (CSI) only at the receiver and CSI at both transmitter and receiver are presented. To maximize the data rate, we use the notion of an optimal bandwidth which corresponds to efficient allocation of signal power across the transmission bandwidth. Under the constraint of an average transmit power, the optimal transmit power adaptation policy is found to be ‘water-pouring’ in frequency-time domain, while the transmit power adaptation policy with a total power constraint is ‘water-pouring’ in frequency domain. Results show that both frequency domain and frequency-time domain power adaptation schemes provide much greater improvement in average data rate over that of the constant power case. Copyright

Optimal packet size for energy efficient WBAN under m-periodic scheduled access mode

In this paper, we evaluate the optimal packet size for energy efficient communication in IEEE 802.15.6 based wireless body area networks (WBANs) under multi-periodic scheduled access mode. We evaluate the impact of (i) acknowledgement policy, (ii) periodicity of allocation and (iii) number of uploads per super frame on optimal packet size in an error-prone channel. We determine the optimal packet size for various communication scenarios in WBAN and for two distinct acknowledgement policies: immediate and block acknowledgement. Results show that the optimal packet size depends on acknowledgement policy employed and is insensitive to number of uploads per super frame and the periodicity of allocation.

Analysis of Link Life Time in Vehicular Ad Hoc Networks for Free-Flow Traffic State

In this paper, we present analytical models for the probability density function (PDF) of link life time in vehicular ad hoc networks (VANETs), formed on both single lane as well as multi lane highways. Assuming free flow traffic state and Gaussian distributed vehicle speed, we extensively investigate the impact of vehicle mobility, vehicle density and transmission range on the link life time PDF and the mean link life time in VANETs. Our analytical and simulation results suggest that in the free-flow traffic state, exponential distribution with appropriate parametrization is a good approximation for the link life time PDF. We perform the Kolmogorov–Smirnov goodness-of-fit test to ascertain the validity of this claim.