07-05-2014, 11:04 AM
Joint Routing and Spectrum Allocation for Multi-Hop Cognitive Radio Networks with Route Robustness Consideration
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Abstract
In this paper, we introduce the concept of “route
robustness” for path selection in multi-hop cognitive radio
networks. We demonstrate that the aggregate throughput and the
robustness of routes determined by the proposed route selection
strategy are superior to existing rate-based selection strategies.
The rationale behind our approach is to guarantee a basic level
of robustness for a set of routes (referred to as skeletons in this
paper). Then, we select some routes from this robust route set
and determine the spectrum to be allocated on each link along
these routes such that the system throughput is maximized. We
also design a polynomial time algorithm for this problem, and
evaluate our proposed mechanism via simulations. The results
show that our proposed algorithm indeed achieves a near optimal
solution of this problem for multi-hop overlay CR networks.
INTRODUCTION
WIRELESS spectrum is a scarce resource. However,
under the fixed spectrum assignment policy widely
used today, most of the spectrum may be underutilized [1].
To solve this problem, Cognitive Radio (CR) [2], [3] was
introduced. CR is a technique which allows secondary users
(SUs) to access the licensed spectrum when no primary users
(PUs) appear on the frequency band (e.g., TV broadcast bands
and some cellular bands) or under the condition that the
normal operation of PUs will not be interrupted ( [4], [5]).
With the support of software defined radio (SDR) technology,
CR devices can observe and sense the environment, identify
spectrum holes, and dynamically adjust its transmission pa-
rameters to better utilize the resource,
N ETWORK M ODEL AND P ROBLEM D ESCRIPTION
Network Model and Assumptions
In this paper, we consider an overlay CR model. Nodes ex-
change information either through a common control channel,
as in [13], or in a distributed manner, as in [20]. In our model,
the network consists of N CR nodes, L links and C orthogonal
frequency bands (channels). F S-D pairs of flows are injected
into the network from secondary users (SUs). Each flow is
always backlogged, which means that each link, once assigned
to a flow, is always busy. We assume that all SU nodes have a
common transmission range and a common interference range,
as assumed in [13]. Each node is associated with a channel
pool which contains the set of channels available for this node.
This channel pool may be obtained by sensing the activities
of PUs via such methods as feature detection, ambient power
sensing, and beacon-based methods [4], and here we assume
a perfect sensing result. The condition of each channel may
vary with time, location, and radio spectrum, depending on
the presence behavior of PUs. As a result, the channel pool
associated with adjacent nodes may not be identical. Formally,
let X(i) denote the list of available channels for node i. Since
the presence behavior of PUs may vary with frequency bands
and locations, for nodes i and j, i ∕= j, X(i) and X(j)
may be different. For each link (i, j), X(i) ∩ X(j) is called
the available channels on link (i, j). Each link together with
its associated available channels forms a set of link-channel
pairs. For each link, the capacity on different channels may
be different.
ROUTE ROBUSTNESS S ELECTION A LGORITHM
In this section, we propose a joint route selection and
spectrum allocation algorithm for multi-hop CR networks. We
consider the multi-path scenario for each flow. In our algo-
rithm, routes are determined in two stages. First, the possible
skeleton set for each S-D pair in the network is selected
so that the robustness constraint can be satisfied (Skeleton
Formation). Then, the skeleton to use (which is chosen from
the possible skeleton set determined in the first stage) and
the channel to allocate on each link along the route skeleton
are determined such that the system throughput is maximized
(Joint Routing and Spectrum Allocation). Together, the route
to go through and the set of channels to use on the links along
the route for each flow in the network are determined.
CONCLUSION
In this paper, we study the joint routing and spectrum
allocation problem in multi-hop cognitive radio networks.
We take into account the channel heterogeneity property and
the channel dynamics of CR networks. We demonstrate that
route robustness greatly impacts system performance. We then
propose an optimal solution to jointly determining which
routes to use and how to allocate spectrum on each link along
the routes such that the system throughput is maximized. The
rationale behind our solution is to guarantee a basic level of
robustness for a set of routes, based on which routes are
selected and the channel on each link along the routes is
allocated.