Adaptive Modulation & Coding-Based Packet Scheduling with
Inter-Base Station Coordination in Fixed Cellular Broadband Wireless Networks
Abstract
This
thesis is about packet scheduling in wireless networks. We considered a
broadband cellular fixed wireless access network with an aggressive channel
reuse of 1 (more specifically, the reuse is 1/6 as each cell is composed of 6
sectors). Certain interference management/avoidance techniques have to be used
when such an aggressive channel reuse scheme is employed. Towards that end, a
number of packet scheduling schemes based on inter-base station (and thus,
inter-sector) coordination have been proposed in the literature in recent
years. One recent study [Intra and inter-sector
scheduling, ISISS, by M.H. Ahmed et al.] proposes a dynamic time slot
allocation scheme which avoids concurrent transmissions in “interference
groups” (the interference group for a sector is composed of those sectors
which cause the most interference to the sector at hand).
In
this thesis, we have first refined the ISISS in various ways, most notably by
incorporating adaptive modulation and coding (AMC) to improve the throughput;
we refer to this case as the IIS-AMC-ST (intra and inter-sector scheduling with
adaptive modulation and coding: single transmission) scheme. We compared the
performance of the IIS-AMC-ST scheduling discipline with that of the reference
IIS-FM-ST (intra and inter-sector scheduling with fixed modulation: single
transmission) discipline, and we showed that significant enhancements are
achievable in net throughput, area spectral efficiency, rate of the dropped
packets in the scheduler queues, and end-to-end delay. The IIS-AMC-ST
scheduling discipline is the first major contribution in this thesis.
Delay
is an essential factor in delay-sensitive applications as such a packet in the
scheduler queue gets dropped if the delay in the queue exceeds a threshold
value. Although the delay performance of the IIS-AMC-ST scheme is better than
that of the reference IIS-FM-ST scheme, the orthogonal transmissions in an
interference group (composed of a sector and two other sectors which cause the
most interference to this sector) nevertheless cause some packets wait too long
in the queues; such packets are eventually dropped. This observation is
actually valid for all orthogonal scheduling disciplines proposed in the
literature.
The
other (and perhaps more significant) major contribution of this thesis is the
development of a novel scheduling discipline called IIS-AMC-MT (intra and
inter-sector scheduling with adaptive modulation and coding: multiple
transmissions) which allows multiple transmissions in an interference group in
a controlled manner. Basically, if the aggregate spectral efficiency is
predicted to be higher when two or three sectors in the interference group
transmit concurrently, then this multi-transmission mode is chosen. By this way
the aggregate throughput is further increased, the end-to-end delay is further
reduced, and the packet dropping rate in the scheduler queues is also further
decreased in comparison to the IIS-AMC-ST scheme.
The performances
of all three scheduling disciplines (IIS-FM-ST, IIS-AMC-ST, and IIS-AMC-MT) are
analyzed with a realistic packet-level simulation. The effects of some system
parameters, such as the user terminal directional antenna beamwidth,
on the interference management are studied as well.
Since
the IIS-AMC-ST discipline, and especially the IIS-AMC-MT discipline, both
require some link gain information exchange across sectors (as well as sector
traffic information), they might be more suitable for fixed cellular wireless
applications where the link gains change very slowly. However, the concept of
IIS-AMC-MT scheduling discipline is general and can be applied in mobile
wireless networks as well if the required inter-BS signaling happens to be
feasible.
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