22-01-2013, 12:20 PM
Handoff Minimization Through a Relay Station Grouping Algorithm With Efficient Radio-Resource Scheduling Policies for IEEE 802.16j Multihop Relay Networks
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Abstract
The IEEE 802.16j standard has been developed to
provide performance enhancement to the existing IEEE 802.16e
network by incorporating the multihop relay (MR) technology.
However, frequent handoffs and low spectrum-utilization issues
that were not encountered in IEEE 802.16e may be incurred in
IEEE 802.16j. The relay station (RS) grouping is one optional
mechanism in the IEEE 802.16j MR standard to overcome these
problems. The concept of RS grouping is to group neighboring
RSs together to form an RS group, which can be regarded as a
logical RS with larger coverage. In this paper, we investigate RS
grouping performance enhancement in terms of throughput and
handoff frequency. This paper designs an RS grouping algorithm
to minimize handoffs by utilizing a greedy grouping policy: RS
pairs with higher handoff rates will have higher priority for
selection. The simulation results show that the handoff frequency
of the considered MR network can significantly be reduced, and
suitable RS grouping patterns can be derived using our grouping
algorithm. In addition, we propose two centralized scheduling
policies, i.e., the throughput-first (TF) policy to maximize the
system throughput and the delay-first (DF) policy to minimize the
average packet delay. By integrating our RS grouping algorithm
and centralized scheduling algorithms, the simulation results indicate
that, for the case of fixed users, groupings with smaller
group sizes can result in better throughput performance. However,
when user mobility is considered, the throughput value increases
as the group size increases.
INTRODUCTION
INCORPORATING the multihop relay (MR) technology
[25], the IEEE 802.16j MR standard [14] has been developed
to provide throughput improvement, coverage extension,
and capacity enhancement to the existing IEEE 802.16e protocol
[1]. By deploying relay stations (RSs), the end-to-end
communication quality between base stations (BSs) and mobile
stations (MSs) can be improved without high infrastructure
deployment costs. In particular, it becomes possible to forward
data to an MS using a high transmission rate in line-of-sight
(LOS) conditions through an MR path to avoid the nonlineof-
sight (NLOS) direct (i.e., single-hop) transmission with
bad channel quality. In addition, spatial reuse [19] is another
promising approach that can be employed in IEEE 802.16j
MR networks to improve spectral efficiency. Based on the
centralized scheduling, spatial diversity gain can be achieved if
multiple simultaneous transmissions using the same bandwidth
resources are realized within the same BS cell.
Frame Structure
The frame structure of IEEE 802.16j MR systems is extended
from that of IEEE 802.16e networks, which also adopt orthogonal
frequency-division multiple access (OFDMA) as the
primary channel access mechanism for NLOS communication.
The basic unit of resource for allocation in OFDMA is a slot,
which comprises a number of symbols in the time domain
and one subchannel in the frequency domain. The timeline is
divided into contiguous frames, each of which further consists
of a DL and an uplink (UL) subframes. In IEEE 802.16j, the DL
and UL subframes shall include one access zone for MR-BS↔
RS and MR-BS↔MS transmissions and may include one relay
zone for RS ↔ subordinate MS transmissions, respectively.
IEEE 802.16j RELAY STATION
GROUPING MECHANISM
Although deploying RSs in IEEE 802.16 networks can provide
significant throughput or coverage enhancements, several
issues regarding the relaying architecture of IEEE 802.16j
should be addressed. These issues include frequent handoffs,
redundant control overhead, and low spectral efficiency. It is
perceived that these issues will result in unpredictable performance
reduction for IEEE 802.16j MR networks. Therefore,
the IEEE 802.16j standard provides the optional RS grouping
mechanism to reduce the impacts of these issues. The concept
of an RS grouping is that adjacent RSs could be grouped
together as an RS group, which acts as a virtually regular
RS to its associated MSs.
System Assumptions and the Concept
of Our RS Grouping Algorithm
To accommodate general scenarios, our algorithm makes
no assumptions of the underlying IEEE 802.16j MR network
topology, the user mobility behavior, and/or the packet traffic
pattern. Specifically, within a considered BS, the IEEE 802.16j
RSs can arbitrarily be deployed, and the coverage area of each
RS can be irregular. In addition, the MSs within the considered
BS can randomly move. In such an arbitrary environment, we
only require that the handoff-rate information between each two
RS cells should be available. The handoff rate between two RS
cells, i.e., RS cell 1 and RS cell 2, is the total rate that the
resident MSs hand off from RS cell 1 to RS cell 2 or from RS
cell 2 to RS cell 1. Note that the handoff-rate information can
simply be derived from the statistical data that are collected by
the network service providers.
MULTIHOP CENTRALIZED DOWNLINK
SCHEDULING POLICIES
In the scheduling-simulation phase of our RS grouping algorithm
described in Section IV, centralized DL scheduling
policies are required to evaluate the performance gain of a given
RS grouping layout. In this section, we first define the scheduling
problem for RS-grouping-enabled IEEE 802.16j MR networks.
Then, the system description of our considered multihop
network is addressed. Finally, we propose two centralized DL
scheduling policies for IEEE 802.16j MR networks under RS
grouping and spatial-reuse assumptions, with the objectives
of maximizing the system throughput and minimizing the DL
traffic delay, respectively.
Effects of RS Group Sizes on Handoff Frequency
Fig. 8 individually evaluates the handoff frequency of each
grouping in Fig. 7 under different user densities (specifically,
100, 500, and 1000 users in our experiments). The results
indicate that, using the greedy grouping policy of our RS
grouping algorithm, the handoff frequencies of the groupings
gradually decline, regardless of the user density, as the group
size increases. The main reason is that larger group sizes generally
lead to lower handoff probabilities. It could be concluded
that it is reasonable to choose larger group sizes if higher user
mobility speeds are observed.
Effects of RS Grouping Patterns
In Figs. 10 and 11, we also observe that the performance
under the case of group size 3 is worse than those under the
other cases. Two primary reasons for this phenomenon are
given here. First, the activation sets used in this case are more
than those used in all the other cases (see Fig. 7). As discussed
in Section IV-C, the more activation sets there are, the less the
spatial diversity gain. Therefore, poorer throughput and delay
performance results are expected for group size 3. Second,
since the group size in this case is small, the packet losses
from handoff events would also influence the throughput performance.
Under the joint impacts of low spatial diversity gain
and high packet loss rate, the worst performances are observed
for the case of group size 3. This special case implies that, if
the RS grouping patterns cannot appropriately be determined,
the system performance would significantly be affected. The
simulation results shown in Figs. 10 and 11 demonstrate that
our proposed RS grouping algorithm can derive suitable RS
grouping patterns in most cases.
CONCLUSION
The IEEE 802.16j MR standard has been developed to provide
performance enhancement to the existing IEEE 802.16e
network. However, issues such as frequent handoffs and low
spectrum utilization, which were not encountered in IEEE
802.16e, may occur in IEEE 802.16j. The RS grouping is
one optional mechanism in the IEEE 802.16j MR standard to
overcome these problems. This paper has investigated the RS
grouping performance enhancement in terms of throughput and
handoff frequency. An RS grouping algorithm was designed
by utilizing a greedy grouping policy: RS pairs with higher
handoff rates will have higher priority to be selected. The
simulation results have shown that the handoff frequency of
the considered MR network can significantly be reduced, and
suitable RS grouping patterns with determined activation set
assignments can be derived using our grouping algorithm.
In addition, we have proposed the TF and DF centralized
scheduling policies to maximize the system throughput and to
minimize the average packet delay.