Mehdad N. Shirazi

Message Dissemination in Inter-Vehicle CDMA Networks for Safety Driving Support

Although the near-far effect has been considered to be the major issue preventing CDMA from being used in ad-hoc networks, in this paper, we show that the near-far effect is not a severe issue in inter-vehicle networks for safety driving support, where packet transmissions are performed in the broadcast manner. Indeed, the near-far effect provides extremely reliable transmissions between near nodes, regardless of node density, which can not be achieved by CSMA/CA. However, CDMA can not be directly applied in realistic traffic accident scenarios, where highly reliable transmissions are required between far nodes as well. This paper proposes to apply packet forwarding and transmission scheduling methods that try to expand the area, where reliable transmissions are achievable. Simulation results show that the proposed scheme achieves approximately 100% of delivery ratio and 4 milliseconds of delay in a realistic traffic accident scenario, where CSMA/CA achieves approximately 60% of delivery ratio and 80 milliseconds of delay.

A reliable and energy-efficient MAC protocol for cluster-tree wireless sensor networks

This paper presents a reservation based enhancement of ZigBee beacon-only period approach, called R-BOP protocol, which addresses energy waste caused by not only idle listening but also overhearing and packet collision over cluster-tree wireless sensor networks. The protocol is based on traffic load estimation and a distributed medium reservation scheme to achieve low power consumption while ensuring highly reliable delivery of sensor data. The proposed protocol is implemented on QualNet network simulator and its performance is evaluated in comparison to an adaptive duty-cycle version of ZigBees BOP protocol. As confirmed by simulation results, the protocol outperforms BOP protocol in terms of packet delivery ratio and energy consumption.

Exploiting network coding for pseudo bidirectional relay in wireless LAN

Network coding is an effective method to improve forwarding efficiency in multi-hop wireless networks. Previous network coding schemes proposed for unicast transmissions focus on scheduling where paths are already established, and assume a fixed rate for all links, neglecting link rate heterogeneity. In this paper we study a network coding based pseudo bidirectional relay scheme for wireless LAN, taking link quality into account. We first show when network coding based relay improves channel efficiency and how to select such a relay in a distributed way. Then we enhance network coding based transmissions from three aspects: (i) Coordinated rate adaptation is adopted to reduce the probability with which a priori packet is missing in times of network decoding. (ii) The retransmission scheduling completely solves the lack-of-a-priori problem. (iii) Piggybacking receiving status in ACK/NAK further enables opportunistic reception and avoids unnecessary transmissions. Simulation results confirm that compared with direct transmissions, the proposed relay scheme can reduce per-packet transmission time by up to 39% in Rayleigh fading environment.

Layer-2 retransmission and combining for network coding-based forwarding in wireless networks

Network coding is an effective method to improve channel efficiency by reducing the number of transmissions. However, most existing network coding schemes are based on network layer broadcast. As a result packets become vulnerable to channel errors due to lack of retransmissions. In this paper we present a cross-layer signal processing technique and show that network coded signal mixed from different flows can be separated in the physical layer by exploiting a priori knowledge. Then we propose the soft network coding framework for multi-hop forwarding. This framework enables layer-2 retransmission and further reduces packet errors by combining retransmitted copies. It supports both conventional unicast and the proposed network coding based bi-cast. Simulation results show that compared with plain Decode-and-Forward (DF), the proposed scheme can reduce air-time by about 30% for Poisson arrival traffic while retaining almost the same packet error rate as DF with frame combining.

Reliable Cut-Through Forwarding for Inter-Vehicle Networks

In inter-vehicle networks, it is required that collision warning messages be distributed over multiple hops with significantly short delay. Cut-through packet forwarding is known to be an effective method to reduce forwarding latency. However, it suffers from the reliability issue, which is especially severe in highly mobile scenario, due to multi-path fading. This paper tries to establish a reliable cut-through forwarding scheme by applying diversity combining method to collision warning messages. By exploiting the nature of the multi-hop forwarding of a broadcast packet in CDMA wireless networks, the diversity combining scheme does not incur any additional communication overhead or delay. The simulation results show that diversity combining schemes, especially Maximal Ratio Combining (MRC), can effectively reduce BER by orders of magnitude.

Opportunistic cooperation and selective forwarding, a virtual MIMO scheme for wireless networks

Multipath fading greatly degrades system performance of wireless networks. Conventionally diversity is introduced to mitigate fading. In this paper we study space diversity in multi-hop wireless networks under power constraints. Point-to-point links are extended to group-to-group virtual MIMO links. Then we propose a cross-layer design of MAC and routing that exploits opportunistic cooperation and selective forwarding (OCSF). Packets are forwarded around the pre-calculated anchor route and the proposed OCSF scheme enables local post selection of both transmitter and forwarder according to channel state information. Simulation results confirm that OCSF outperforms existing schemes that utilize distributed space-time coding or selection diversity.

Delay-Based Priority Control over Multi-Hop Wireless Mesh Networks

Wireless mesh networks are attracting more and more attention as a promising technology for the next generation access infrastructure. QoS support is a unavoidable task given the rising popularity of multimedia applications, and also a challenging task for multi-hop wireless mesh networks. Among the numerous QoS factors, end-to-end delay is one of the most critical and important issues, especially for the real time applications. Over multi-hop wireless mesh networks, end-to-end delay of a flow is highly dependant on the number of hops as well as congestion condition of the hop nodes that the flow traverses through. In this paper, we propose QoS priority control schemes based on the end-to-end QoS delay metrics in order to increase traffic accommodation, i.e., the numbers of real-time flows which satisfy the requirements of end-to-end delay and packet delivery ratio over multi-hop wireless mesh networks. The first scheme enables source and forwarding nodes to perform priority control based on the number of hops of routes. The second scheme enables nodes to perform priority control based on the congestion condition of the hop nodes, where the flow traverses through. The effectiveness of the proposed schemes is investigated with NS-2 network simulator for voice and video traffics over multi-hop wireless mesh networks. Simulation results show that the scheme greatly improves the traffic accommodation for voice and video applications in multi-hop wireless mesh networks.

A Reservation-Based Enhancement of IEEE 802.11 DCF Protocol

IEEE 802.11 DCF is an asynchronous and distributed MAC protocol which does not require the existence of a central controller for medium access coordination. This flexibility, which is due to DCFs contention-based nature, comes at the expense of the overhead associated with contention resolution. The overhead consists of frame collision time and channel idle time, which is particularly severe when channel is saturated. In this paper, we present an enhancement of DCF which aims at reducing its contention resolution overhead by equipping it with a distributed reservation mechanism. The proposed reservation mechanism enhances collision avoidance mechanism of DCF by enforcing a partially ordered medium access through an implicit agreement between neighboring nodes. Simulation results, using ns-2 network simulator, show that the added reservation scheme 1) effectively reduces DCFs overhead and improves channel utilization particularly when node density and traffic load is high, 2) significantly enhance DCFs fairness.

A concurrent power supply and data transmission protocol for 2D communication sensor system

The two-dimensional communication system (2DCS) is a novel technology that allows the surface physical as a communication medium for providing simultaneous power supply and data transmission to any device. However, how to carry out simultaneous power supply and data transmission in an efficient manner is still an open issue for 2DCS. In this paper, we propose a concurrent power supply and data transmission protocol based on the modifications of IEEE 802.15.4 MAC protocol standard, and then propose a generic rechargeable battery model by considering the features of 2DCS. Simulation results reveal that our proposed scheme highly improves the energy charging efficiency of 2DCS.

An Opportunistic Forwarding Scheme Exploiting both Long Progress and Adaptive Rate in Wireless Networks

In Mobile Ad hoc Networks (MANET) geographic routing is characterized by local forwarding decision. Links with a long progress are preferred under the greedy forwarding rule. However in a real system long links tend to have a high packet loss rate due to multipath fading. A sub-optimal solution may separately exploit path diversity or rate adaptation. In this paper we study channel efficiency of multi-hop forwarding and try to jointly optimize rate adaptation and forwarder selection in geographic routing by the tradeoff between progress and instantaneous rate. We define a new metric-Bit Transfer Speed (BTS)-as the ratio of the progress made towards the destination to the equivalent time taken to transfer a payload bit. This metric takes overhead, rate and progress into account. Then we propose a packet forwarding scheme that Opportunistically exploits both long Progress and Adaptive Rate (OPAR) by a cross-layer design of routing and MAC. In OPAR each node selects for a packet the forwarder with the highest BTS. The forwarder changes as local topology (progress), packet size (overhead ratio) or channel state (data rate) varies. Simulation results show that compared with the normalized advance (NADV) [7] scheme and contention-based forwarding (CBF) [17] scheme, OPAR has lower packet loss and can effectively reduce channel occupation time by over 30% in the scenario with moderate mobility speeds.