08-05-2012, 01:36 PM
Ad Hoc Resource Allocation in Cellular Systems
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INTRODUCTION
Ideal two-dimensional cellular systems place radio base
stations (BS) according to regular hexagonal grids [10,
11, 12]. Current cellular and PCS systems, with their
small cell sizes, depart significantly from the ideal hexagonal
layout due to terrain variations, difficulties in site
acquisition, and space variations in mobile station density
[7]. In quickly deployed ad hoc military or emergency
communication systems, little or no planning may
be possible. Distributed campus wireless LANs may add
communication elements in a distributed fashion with
little coordination between departments. Such smaller
operators may not have sufficient resources or expertise
for a formal deployment plan. Further, little theory
guides how to evolve cellular networks over time to
meet growing demand. The question we address is how
to allocate network resources in such environments.
AD HOC CELLULAR RESOURCE ALLOCATION:
ACRA consists of three components: channel assignment,
radio transceiver allocation, and new BS placement.
We pause to emphasize that the ACRA algorithm
is unrealistic in the sense that it allows resources to be
instantly allocated or reallocated, but it does provide a
basis for understanding how more realistic ad hoc
deployments behave (See Section VI.).
A. Dynamic Channel Assignment
Many DCA schemes are known [9] and the goal of this
paper is not to find the best of these schemes. The goal
instead is to show representative performance with
DCA.
SIMULATION:
To evaluate the dynamic channel assignment for different
channel and BS layouts, we use a simulated cellular
system. This section describes the details of this simulation.
The simulation proceeds as follows:
1. Choose scenario parameters: the total number of
channels, N; total traffic in Erlangs, E, background
noise, NB; path loss exponent e; shadow
fading standard deviation, s; required C/I threshold,
T; grade of service, BMax; and traffic spatial
distribution.
2. Call arrivals and departures are generated according
to the total Erlangs and their spatial distribution.
3. For each call arrival: the call is placed according
to the ACRA algorithm.
4. For each call departure, the call is simply
removed.
ACRA IN PRACTICE
The ACRA algorithm is unrealistic for a number of reasons.
First, there are time delays between when a need for
resources is identified and when the resources are deployed.
In practice, operators monitor call block rates at BS and
deploy sufficient radios as the blocking rates become too
large. Similarly, holes in coverage, and cells that require
splitting to increase capacity are identified, and future sites
are mapped out well in advance of the actual need. None-theless,
the ACRA results suggest that no matter how little foresight
is put into this process, as long as resources are
deployed where there is a demonstrated need, the total
resource usage should be close to that of what an omnipotent
operator with full information of traffic demands would
build.
CONCLUSION
This paper presented ad hoc cellular resource allocation
(ACRA), a very simple algorithm for deploying BS and radio
transceiver resources in a wireless cellular network. The
algorithm was able to find good deployments with approximately
the same number of total radio transceivers and 25%
more BS compared to an ideal hexagonal layout with uniform
distribution of users in an interference limited environment.