Ling Fu Xie

A survey of inter-flow network coding in wireless mesh networks with unicast traffic

Wireless network performance is much restricted by the unreliability of the wireless channel and the competition among different flows for the shared network resources such as the bandwidth. Network coding is a technique that exploits the broadcast of the wireless channel and can effectively address these two restrictions to improve network performance. For example, with network coding, an intermediate node of multiple flows can encode packets from these flows into one mixed packet and serve these flows using only one transmission instead of multiple transmissions in the traditional way, thus mitigating the competition among flows. Inter-flow network coding (XNC), as a form of network coding, considers encoding packets from different flows, and it can benefit wireless mesh networks (WMNs) with either reliable or lossy links. In this paper, we present a survey on XNC in WMNs for unicast traffic, with various design factors related to XNC being covered. Specifically, our survey considers two types of WMNs, one with reliable links and the other with lossy links, and we study how XNC can be effectively utilized in both two types of WMNs. In addition to the benefits of XNC, we also present in this survey some drawbacks of applying XNC in WMNs. With this paper, we believe that readers will have a more thorough understanding of XNC on how it effectively mitigates the resource competition among flows and the channel unreliability problem in WMNs.

A Review of Group Mobility Models for Mobile Ad Hoc Networks

The importance of accurate mobility models for mobile ad hoc network (MANET) evaluation is widely recognized in literature. Existing MANET mobility models can be generally classified into two categories: entity and group mobility models. The latter is of particular interest as group motion occurs commonly in scenarios where MANETs are deployed, such as firefighting and battlefield operations. This paper provides a systematic and critical review of recent group mobility models proposed for MANETs. It also discusses the potential of using or adapting these mobility models for other emerging network types.

Leader Based Group Routing in Disconnected Mobile Ad Hoc Networks with Group Mobility

When there is disconnection in mobile ad hoc network under group mobility, it falls into the delay-tolerant network (DTN). However, most existing research in DTN targets entity mobility. In this paper, we consider the routing strategy for DTN with group mobility, and propose the leader based group routing (LBGR) by making full use of group structure in group mobility. Three major mechanisms closely related to the group mobility are proposed in LBGR. First, we treat each group as one individual unit during routing execution to substantially reduce the routing overhead and the resource requirement. Second, we consider the resource allocation in each group and propose the leader-dominating routing in LBGR to reduce the impact of the group dynamics on network performance. Third, to make better use of the rare contact opportunities in DTN, we propose the group based packet exchange, in which the contact of any two nodes from two groups will trigger the packet exchange between the two groups. By extensive simulation we show that LBGR outperforms two traditional routing protocols, epidemic routing and DSR, in various network conditions. Especially, we will find that the impact of the group dynamics on LBGR is very limited.

CEO: Consistency of Encoding and Overhearing in Network Coding-Aware Routing

Network coding-aware routing attempts to find coding opportunities between an incoming flow and existing flows in wireless ad hoc networks. However, we find that a problem of most proposed coding-aware routing schemes is that they may misidentify the coding opportunities. Specifically, the opportunity misidentification may appear when a packet is about to be re-encoded. We call this the re-encoding problem. To solve this problem, we propose a principle called consistency of encoding and overhearing (CEO). By adhering to CEO, the encoding node is able to avoid misidentifying coding opportunities, thus ensuring the successful decoding of all encoded packets. In this letter, we first show the importance of CEO by analysis in a simple topology. Then, we introduce a distributed way to apply CEO to a coding-aware routing scheme, and show the improved network performance with CEO by simulation in different scenarios.

Routing strategy in disconnected mobile ad hoc networks with group mobility

Most of the current proposed routing protocols in delay-tolerant network (DTN) are designed based on the entity mobility. In this article, we consider the routing in DTN with group mobility, which is useful in modeling those cooperative activities. The new proposed routing scheme is called group-epidemic routing (G-ER). G-ER is designed on the basis of one DTN protocol called epidemic routing (ER). In G-ER, two strategies related to the unique characteristics of the group mobility have been proposed to greatly improve ER. The first is to treat each group as a single node and exchange packets between groups instead of individual nodes. Thus, the resource-consuming problem of ER could be much alleviated. In the meantime, exchanging packets between two groups could speed up the packet delivery. The second is the buffer sharing inside a group, which is supported by the cooperative nature in group mobility. Moreover, we specifically propose a group dynamic model for group mobility to realize group splitting and merging. The performance of G-ER is studied by extensive simulations and compared with ER and dynamic source routing (DSR). Results show that G-ER outperforms ER and DSR in different network scenarios even with group dynamics.

A Performance Comparison of Flat and Cluster Based Routings in Mobile Ad Hoc Networks

The objective of this paper is to study the performance comparison between flat and cluster-based routings in MANETs. Previous study shows that an earlier proposed cluster-based routing, Cluster based Routing Protocol (CBRP), has poorer performance due to its inefficient clustering algorithms based on Least Clusterhead Change (LCC) and Lowest ID (LID). Thus, in this paper, we first propose a new cluster-based routing, namely efficient cluster based routing protocol (ECBRP), which is based on a more efficient and stable clustering scheme. In addition, a new information table update mechanism utilizing routing events and data forwarding events is introduced in ECBRP to enhance the clustering and routing performance. The performance comparison in terms of packet delivery ratio and normalized routing overhead between flat and cluster based routings are studied extensively. Two flat routings (Dynamic Source Routing (DSR) and Ad Hoc On-demand Distance Vector (AODV)) and two cluster-based routing (CBRP and ECBRP) under various scenarios such as traffic load, mobility, network size and network area are considered in our study. By evaluating the performance of ECBRP and CBRP, we can study how the performance of a CBRP is affected by its underlying cluster structure. Simulation results show that ECBRP can achieve satisfying routing performance and outperform other routing schemes, DSR, AODV and CBRP, under the same simulated scenarios.

Performance Analysis of Network Coding With Virtual Overhearing in Wireless Networks

Due to the broadcast nature of wireless channels, overhearing plays an important role in interflow network coding in that it serves as a way to acquire the side information for the decoding of network-coded packets. Since overhearing only takes effect in the one-hop neighborhood of nodes, it, in turn, restricts the identification of network coding opportunities. That is, if the side information for packet decoding cannot be overheard, no coding opportunity can be identified. Hence, in this paper, we aim to relax such a restriction to further explore network coding opportunities. Specifically, we propose a virtual overhearing (VOH) technique to enable network coding when the side information cannot be obtained from traditional overhearing previously mentioned. VOH allows a node to obtain a packet sent by another node that can be two or more hops away. As a result, VOH can serve as a supplementary way for side information acquirement, thus still enabling network coding between network flows when traditional overhearing is not possible to obtain the needed side information. The additional coding opportunities brought by VOH can be translated into additional performance gains. We first analyze network coding with VOH in a simple topology and show that with VOH, network capacity could be improved by over 12% (in some situations, network throughput could be improved by 50%). Then, we conduct simulations in NS-2 to show the advantage of VOH.

Virtual overhearing

Overhearing is of great importance to wireless network coding in that it can be exploited to obtain the side information needed for packet decoding. Recently, a new technique called virtual overhearing (VOH) was proposed to allow a node to obtain the packet sent by another node that is multiple hops away for free. This can overcome the limitation of overhearing and be used to discover more coding opportunities. In this paper, we take advantage of VOH and propose two modes of exploiting VOH to increase coding opportunities in wireless ad-hoc networks. First, we make use of VOH to increase the chance of finding a route with coding opportunities for a new incoming flow. Second, and more importantly, we make use of VOH to create coding opportunities between two established flows which are currently unmixable. Note that most previous studies only attempt to find coding opportunities rather than create them. Based on these two modes of VOH usage, we design two routing protocols: distributed coding-aware routing with virtual overhearing (DCAR-VOH), and its enhanced version DCAR-VOH+. DCAR-VOH implements only the first mode of usage, whereas DCAR-VOH+ incorporates both modes of usage. Our extensive simulations indicate that VOH provides an effective way to discover coding opportunities, resulting in improved network performance. The positive effect of the second mode of usage stands out especially.

Mitigating Doppler effects on physical-layer network coding in VANET

This paper considers physical-layer network coding (PNC) in vehicular ad-hoc network (VANET) to solve the problem of short contact time between fast-moving vehicles. PNC enables data exchange between nodes in a relay network within a short airtime, e.g., twice faster than relay networks based on traditional communication, and can be a powerful performance booster in VANET. One of the most important challenges in applying PNC to VANET, however, is the Doppler shift caused by vehicular motions. Doppler shift leads to carrier frequency offset (CFO) that induces inter-carrier interference (ICI) in OFDM systems. The ICI destroys the orthogonality of modulated symbols, causing degradation in PNC signal detection. This paper puts forth a detection method to mitigate the CFO/ICI effect on PNC. The method, referred to as BP-VPNC, makes use of a belief propagation (BP) algorithm to process the outputs of the OFDM correlators. BP extracts useful hidden information embedded in ICI to improve signal detection in VANET PNC. Our study shows that the BER performance of PNC VANET operated with BP-VPNC can be achieved close to that of traditional VANET at various CFO levels. These results suggest that with BP-VPNC, a potential shortcoming of PNC, vulnerability to CFO, can be circumvented, and that PNC can be used to overcome the short vehicular contact time in VANET.

Routing for group mobility in mobile ad hoc networks with delay-tolerant approach

Since network disconnection will happen more frequently in mobile ad hoc network with group mobility model, the routing strategy proposed in delay-tolerant network could be applied for it. However, most of the current delay-tolerant routing protocols are designed based on the entity mobility model, not on the group mobility model. Motivated by this, in this paper we have proposed a new routing scheme called group-epidemic routing (G-ER), on the basis of one proposed delay-tolerant routing protocol called epidemic routing (ER), to make it perform more effectively for group mobility model. Through making use of the uniqueness of the group mobility models, one significant improvement measure has been proposed in which a group of nodes will be behaved as a single node. In addition, we introduce the individual node buffer management to further enhance the network performance especially in the more disconnected scenarios. The simulation results show that the proposed G-ER outperforms ER in both packet delivery ratio and delay under group mobility scenario.