Elsevier

Computer Communications

Volume 32, Issue 18, 15 December 2009, Pages 1983-1997
Computer Communications

Search: A routing protocol for mobile cognitive radio ad-hoc networks

https://doi.org/10.1016/j.comcom.2009.06.011Get rights and content

Abstract

Recent research in the emerging field of cognitive radio (CR) has mainly focussed on spectrum sensing and sharing, that allow an opportunistic use of the vacant portions of the licensed frequency bands by the CR users. Efficiently leveraging this node level channel information in order to provide timely end-to-end delivery over the network is a key concern for CR based routing protocols. In addition, the primary users (PUs) of the licensed band affect the channels to varying extents, depending on the proportion of the transmission power that gets leaked into the adjacent channels. This also affects the geographical region, in which, the channel is rendered unusable for the CR users. In this paper, a geographic forwarding based SpEctrum Aware Routing protocol for Cognitive ad-Hoc networks (SEARCH), is proposed that (i) jointly undertakes path and channel selection to avoid regions of PU activity during route formation, (ii) adapts to the newly discovered and lost spectrum opportunity during route operation, and (iii) considers various cases of node mobility in a distributed environment by predictive Kalman filtering. Specifically, the optimal paths found by geographic forwarding on each channel are combined at the destination with an aim to minimize the hop count. By binding the route to regions found free of PU activity, rather than particular CR users, the effect of the PU activity is mitigated. To the best of our knowledge, SEARCH takes the first steps towards a completely decentralized, CR routing protocol for mobile ad-hoc networks and our approach is thoroughly evaluated through analytical formulations and simulation study.

Introduction

The emerging field of cognitive radio (CR) networks is geared to address the increasing congestion in the unlicensed band by opportunistically using vacant spectrum, such as, frequencies licensed for television broadcast, public service, among others [1]. While there has been considerable research effort in devising efficient spectrum sensing and sharing algorithms at the node level, it is important to seamlessly integrate these designs in the implementations of the end-to-end network protocols. As an example in a CR network, routes constructed at the network layer must not affect the ongoing transmission of the primary users (PUs) of the licensed spectrum and thus, they must have an awareness of the spectrum availability. Moreover, when a PU is detected, the routing protocol must make the key decision of either (i) switching the channel in the affected portion of the route, or (ii) passing through entirely different regions altogether, thus increasing the latency. The frequently changing PU activity and the mobility of the CR users make the problem of maintaining optimal routes in ad-hoc CR networks challenging. In this paper, we propose the SpEctrum Aware Routing for Cognitive ad-Hoc networks (SEARCH) protocol based on geographic routing, that adapts to the dynamic spectrum availability and the node mobility, while trying to maintain end-to-end connectivity.

In this paper, we make the following contributions for routing in CR networks.

Existing routing protocols for ad-hoc networks do not account for the region affected by an active PU and are unaware of the changing spectrum opportunity. In CR networks, routes must be constructed to avoid these regions and must also adapt themselves to subsequent changes induced by new PU arrivals. We discuss this further for geographic routing protocols, as the route formation in SEARCH is based on this principle.

Geographic routing protocols for classical wireless networks rely on a greedy positive advance towards the destination [4]. By knowing the location of the destination and that of the candidate forwarding nodes within its range, a node can choose the next hop that gives the greatest advance. Whenever a void is encountered, the path traverses the perimeter of the void and then resumes the greedy forwarding to the destination. However, in a CR network, as the PUs have precedence in accessing the spectrum resource, the routing protocol must compromise on the greedy advance when it intersects a region of PU activity. Two possible solutions to this are: circumventing the affected region or switching the affected channel. The choice between these options needs to made taking into account the overall end-to-end performance. Classical geographic routing protocols are oblivious to these PU specific considerations and limit their operation to a single channel. SEARCH, on the other hand, provides a hybrid solution. It first uses greedy geographic routing on each channel to reach the destination by identifying and circumventing the PU activity regions. The path information from different channels is then combined at the destination in a series of optimization steps to decide on the optimal end-to-end route in a computationally efficient way. Instead of binding the path to a set of forwarding nodes, it is described by anchor locations, i.e., regions found free of PU activity. Moreover, the route management function in SEARCH modifies an existing route with minimal overhead whenever a new PU is detected.

From practical considerations, the channels are not completely orthogonal and a finite amount of transmitted power leaks into the adjacent channels. This constitutes the spectral leakage power, which is a source of the interference in the affected channels. Based on the proportion of the leakage power, channels used by the CR may be affected to different geographical extents due to a single PU. As an example, consider Fig. 1, in which, a WLAN transmitter is located at the origin O and follows the IEEE 802.11b standard [13]. The x-y axes represent the two dimensional cartesian space and the z axis is the frequency scale. S1 and S2 are two other nodes that sense the transmitted power and are at a distance of 1 and 2, respectively, from the WLAN where 1>2. The nodes S1,S2 and the WLAN lie in the x-y plane. When the WLAN transmits on channel 6, the nodes observe the leakage power by tuning to the channels Ch:4,5,7 and 8, each separated by a difference of 5 MHz. The spectral shape of the WLAN signal is such that the leakage power is high in the frequencies close to that of the transmission channel, and falls as one moves further away on the frequency scale. The received power also falls exponentially with increasing distance on the x-y plane. Consequently, the coverage radius of the transmitter, i.e., the distance up to which the channel power serves as a source of interference, depends upon the (i) frequency difference in the channels used for transmission and power measurement, and the (ii) distance between the transmitter and the sensing node. This radius is shown by R4,,R8 for the WLAN channels Ch:4,,8, respectively. We observe that node S1 is further away from the origin, as compared to S2, and is within the coverage radius of the WLAN on channel 6 only. Node S2, being closer to the WLAN, is under the coverage range of all the channels considered above.

Extending this example for CR networks, a single PU located at the origin may affect the channels used by the CR users, say S1 and S2, differently. As the extent of the coverage is not completely known at a given node, a local channel switching decision may not be optimal in a network-wide context. From Fig. 1, consider a path that passes through the nodes S1 and S2. Node S1 finds itself under the PU coverage region on channel 6 and switches to a free channel, say 5. The subsequent route, however, intersects the PU coverage region in the new channel within a few hops, when node S2 is reached. In addition, the node S1 does not have any information about the extent of the detour that will be required if the route formation is continued on the affected channel 6. SEARCH solves this problem by estimating the route detours on each channel, taking into account their dissimilar coverage ranges. It then computes whether the cost of the detour is less than the added delay in switching the channel, with respect to the end-to-end latency.

The node mobility is one of the chief factors of route outages in ad-hoc networks. Specifically, in a CR environment, we identify a new problem arising out of mobility that needs to be addressed. Consider the case in which the nodes continue to form a connected route but may stray, due to their mobility, in the coverage region of a PU on the current channel. Thus, even though the route is connected, the nodes in the PU region may be unable to transmit. We recall that SEARCH uses anchor locations for routing, that are found to be free of PU activity. It addresses the CR mobility concerns by checking if the next hop node is within a threshold distance of the anchors. If this condition is not satisfied, the forwarding node identifies a new next hop that is closest to the anchor, thus ensuring that PU regions are always avoided.

Geographic routing protocols rely on the knowledge of the location of the destination for route computation. When the destination itself moves with time, the current route needs to be extended by progressively adding nodes at the end. SEARCH predicts the destination location in advance through Kalman filtering approaches, so that the route is suitably extended and packets are reliably delivered to the new location.

The rest of this paper is organized as follows. Section 2 describes the related work in this area, while the assumptions of our approach are given in Section 3. In Section 4, we present the route setup phase of SEARCH, our CR routing protocol in detail. Section 5 describes the route maintenance functions related to the changing spectrum and node mobility. A thorough performance evaluation is conducted in Section 6. Finally, Section 7 concludes our work.

Section snippets

Related work

Routing is a well researched area in classical ad-hoc networks with protocols designed for diverse mobility considerations, optimization constraints and hardware assumptions. SEARCH is designed for CR networks and differs from the general class of protocols for ad-hoc networks in its consideration of (i) regions affected by PU activity, (ii) route optimization undertaken over several channels, and (iii) awareness of the dynamically changing channel environment. In this section, we specifically

Network architecture

In our CR mobile ad-hoc network nodes may move freely in the two dimensional cartesian space. The PUs are assumed to be stationary and they coexist in an overlapping region with the CR users. Each PU operates with an ON–OFF switching cycle that is unknown to the CR network. In this section, we describe the network architecture by considering the node and channel aspects of the network in detail.

Search: a CR routing protocol

SEARCH attempts find the length of the shortest path based on greedy advancement that may be traversed on a combination of channels to the destination. The key functionality in our proposed approach is evaluating when the coverage region of the PU should be circumvented, and when changing the channel is a preferred option. First, the shortest paths to the destination, based on geographic forwarding and consideration of the PU activity, are identified on each channel. The destination then

Route maintenance

This phase of the SEARCH protocol addresses the following concerns:

  • PU awareness: The appearance of a PU may render the region in its vicinity unsuitable for routing. In such cases, the nodes in the affected regions must immediately cease operation in the occupied channel and, if needed, explore alternate routes to the destination.

  • CR user mobility: Node mobility may cause route disconnections. Even if the route stays connected, nodes may stray into PU activity regions and cause undesirable

Performance evaluation

In this section, we evaluate the performance of the SEARCH protocol under different network conditions, traffic loads and mobility factors with the parameters as listed in Table 1. The simulation model is built in the NS-2 simulator with multi-radio multi-channel extensions [15]. We model the primary users’ activities by using the exponential ON–OFF process described in Section 3. Simulations are performed in random multi-hop network topologies, in which, 400 nodes, unless specified otherwise,

Conclusions

In this paper, we presented SEARCH, a distributed routing protocol for mobile CR networks. Our approach jointly optimizes the path and channel decisions so that the end-to-end path latency is minimized. It is sensitive to the PR activity and ensures that the performance of the CR network is minimally affected as well as no interference is caused to the licensed users during their transmission. The route management functionality effectively manages to meet the challenges of a mobile environment

Acknowledgements

The authors would like to thank Prof. Ian Akyildiz for his valuable advice during the course of this work.

References (20)

  • I.F. Akyildiz et al.

    Next generation/dynamic spectrum access/cognitive radio wireless networks: a survey

    Elsevier Comput. Netw. J.

    (2006)
  • S. Krishnamurthy, M. Thoppian, S. Venkatesan, R. Prakash, Spectrum aware on-demand routing in cognitive radio networks,...
  • G. Cheng, W. Liu, Y. Li, W. Cheng, Spectrum aware on-demand routing in cognitive radio networks, in: Proc. of IEEE...
  • B. Karp, H.T. Kung, GPSR: greedy perimeter stateless routing for wireless networks, in: Proc. of ACM MobiCom, August...
  • B. Leong, B. Liskov, R. Morris, Geographic routing without planarization, in: Proc. of Symp. on Network Sys. Design and...
  • C. Xin, B. Xie, C. Shen, A novel layered graph model for topology formation and routing in dynamic spectrum access...
  • Q. Wang, H. Zheng, Route and spectrum selection in dynamic spectrum networks, in: Proc. of IEEE Consumer Comm. and...
  • V. Dumitrescu, J. Guo, Context assisted routing protocols for inter-vehicle wireless communication, in: Proc. of IEEE...
  • K.C. Lee, J. Haerri, U. Lee, M. Gerla, Enhanced perimeter routing for geographic forwarding protocols in urban...
  • C.H. Chou et al.

    Geographic forwarding with dead-end reduction in mobile ad hoc networks

    IEEE Trans. Veh. Technol.

    (2008)
There are more references available in the full text version of this article.

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This work was completed when the author was a visiting researcher at the Broadband Wireless Laboratory, Georgia Institute of Technology, Atlanta, GA 30332, USA.

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