07-05-2014, 01:02 PM
CRP: A Routing Protocol for Cognitive Radio Ad Hoc Networks
Routing Protocol for Cognitive .pdf (Size: 534.47 KB / Downloads: 13)
Abstract
Cognitive radio (CR) technology enables the op-
portunistic use of the vacant licensed frequency bands, thereby
improving the spectrum utilization. However, the CR operation
must not interfere with the transmissions of the licensed or
primary users (PUs), and this is generally achieved by incur-
ring a trade-off in the CR network performance. In order to
evaluate this trade-off, a distributed CR routing protocol for
ad hoc networks (CRP) is proposed that makes the following
contributions: (i) explicit protection for PU receivers that are
generally not detected during spectrum sensing, (ii) allowing
multiple classes of routes based on service differentiation in CR
networks, and (iii) scalable, joint route-spectrum selection. A key
novelty of CRP is the mapping of spectrum selection metrics,
and local PU interference observations to a packet forwarding
delay over the control channel. This allows the route formation
undertaken over a control channel to capture the environmental
and spectrum information for all the intermediate nodes, thereby
reducing the computational overhead at the destination. Results
reveal the importance of formulating the routing problem from
the viewpoint of safeguarding the PU communication, which is
a unique feature in CR networks.
INTRODUCTION
COGNITIVE radio (CR) technology aims to enhance the
spectrum utilization in the licensed frequencies, and also
alleviate the congestion in the 2.4 GHz ISM band. Recent
research in this area has mainly focused on spectrum sensing
and sharing issues in infrastructure-based networks that relies
on the presence of a centralized entity for collecting the
spectrum information, deciding the best possible spectrum for
use, and allocating transmission schedules to the CR users
served by it. Moreover, such architectures are generally single-
hop, with each CR directly communicating with the central
entity as the end destination. Thus, the application of CR
technology in distributed scenarios is still in a nascent stage,
and several open research challenges are outlined in [1]. This
paper proposes a CR routing protocol for ad hoc networks
(CRP) that specifically addresses the concerns of end-to-end
CR performance over multiple hops, and the problem of
protecting the PU transmissions from interference with limited
knowledge of the environment.
Explicit protection for PU receivers:
CR users periodically sense the spectrum and decide on
the spectrum availability. Typically, the CR users that are
located in regions with fewer cases of positive PU transmis-
sion detections may be preferred for routing. However, this
approach only guarantees protection to the PU transmitters
that are within the range of the CR devices. For certain PU
applications such as television broadcast, the transmission is
uni-directional, and the PU transmitters do not suffer from
CR network interference. Rather, transmission by neighboring
CR users may affect the PU receivers that cannot be detected
easily (no transmission, low leakage power from the reception
circuitry). The CR routing protocol must provide protection
to these PU receivers by avoiding entire regions where such
devices may possibly be present, and this has yet not been
addressed in the existing literature.
Allowing CR routing classes:
The protection provided to the PUs results in a performance
tradeoff for the CR network operation. As an example, the
CR network may proactively avoid regions of PU activity,
thereby incurring longer paths with an aim of reducing the
possibility of interference to the PUs. Depending upon the
desired level of protection for the PU activity, and the CR
user’s end-to-end latency demands, multiple classes of routes
are possible. CRP considers two routing classes, with class
I assigning higher significance to end-to-end latency while
meeting minimum PU interference avoidance.
Networks with specific architectural assumptions
Several existing works assume knowledge of the entire
topology graph, with known edge weights between any given
node pair. Such an approach is seen in [7], wherein the edge
weights represent the wireless capacity, and are calculated
probabilistically based on interference from the PUs, the re-
ceived signal strength, among others. A path-centric spectrum
assignment framework (CogNet) is proposed in [16] that
constructs a multi-layered graph of the network at each node
such that the edge weights of the graph represent the spectrum
availability between the nodes. In either case, a Dijsktra or
Bellman Ford-like algorithm is run over the topology graph
to find the optimal path. The dissemination of the network-
wide edge weights to each node incurs a prohibitive overhead,
as is hence not suited for ad hoc network routing. Other works
have also been proposed for mesh networks arranged in a tree
hierarchy [6][10].
CRP ROUTING P ROTOCOL OVERVIEW
The route-setup in the CRP protocol is composed of two
stages - (i) the spectrum selection stage, and the (ii) next
hop selection stage. The source node broadcasts the RREQ
over the control channel, and this packet is propagated to the
destination. Each intermediate forwarder identifies the best
possible spectrum band, and the preferred channels within
that band during spectrum selection. To enable this, we
have proposed several unique CR metrics that are weighted
appropriately in an optimization framework for choosing the
spectrum. Moreover the function is cast differently for each
Class of CR route. As an example, for the class I route,
the CR network end-to-end latency is the key consideration.
Here, the spectrum chosen by a given candidate forwarding
node must (i) support the highest propagation distance, with
the (ii) longest allowed duration for transmission given the
sensing schedules of the neighboring nodes. Consequently, the
optimization function for class I route attempts to maximize
these two factors during the spectrum selection stage.
CONCLUSION
In this paper, we proposed the CRP routing protocol that
specifically addresses the problems the PU receiver protection,
service differentiation in CR routes, and joint spectrum-route
selection. Our protocol allows for two classes of routes - class
I routes that provide better CR network performance, while
class II routes aim to achieve a higher measure of protection
for the PUs. Our performance evaluation shows the tradeoff
that incurs in the CR performance to reduce the interference
to PU receivers, thereby motivating separate routing classes
based on the specified operational limits of the CR network.
Our future work will include the following: We assume the
“on-off” PU activity model in our work, which we propose to
update with the recent experimental findings of an exponential
distribution of the on times. In addition, spectrum
bands to extract accurate values for Bc
sensing has a finite probability of detection error, which we
would like to incorporate for a realistic performance analysis.
Finally, our work considers two classes depending upon the
level of protection to the PU receivers. We will extend this
concept further to incorporate traffic class requirements for
the CR users.