S. M. ELRABIEI ET AL.649
2. Battery Drain of RAs in Battery Unaware
Network
To achieve a reliable downlink transmission for MBMS
traffic and to extend the coverage outside the trans-
mission area, three energy efficient re-multicasting
techniques were proposed in [5] for properly selecting
RAs in a two phase cooperative transmission model. At
phase I, the BS multicasts data to all SSs at high
transmission rate R1, where only subscribers in a good
channel state (SSGCS), e.g., as defined by their CINR
threshold, can successfully receive the data, and the
remaining group of subscribers in a bad channel state
(SSsBCS) fail to receive the data. BS preselects some of
SSGCS to be RAs using one of the selection algorithms in
Elrabiei et al. [5]. Upon receiving signals from BS, each
RA decodes the received signals and then forwards them
to SSsBCS at a proper rate R2 in phase II. By exploiting the
channel state information (CSI) and the location based
service (LBS), a Nearest-Neighbor Discovery Protocol
(NNP), and two other optimized versions of it, based on
RA’s transmission range and instantaneous CSI, were
simulated and studied in the context of WiMAX single
frequency networks. These protocols are described brief-
ly below as follow:
2.1. NNP RA Selection Scheme
The proposed relay agent selection protocol is based on
geographical positioning of users and on the instanta-
neous channel condition of the SSs. Since we have as-
sumed that BS knows the location of each SS, the as-
signed RA is chosen to be the nearest located SSGCS
(neighbor) to the SSBCS of interest, independently of
other SSsBCS. The NNP can be executed periodically
according to the mobility of the users and how often
they change their locations.
2.2. Transmission Radius RA Selection Scheme
The proposed NNP selects a RA for each SSBCS inde-
pendently of other RAs selection, by finding the nearest
SSGCS to the SSBCS from the same MGroup. In worst case
scenario, there is independent RA for each SSBCS, (i.e.,
number of RAs equal number of SSsBCS). Therefore, to
reduce the number of RAs selected, a scheme optimiza-
tion is proposed. If there are two or more SSsBCS, which
are in close proximity to each other, the BS can select
one RA for all of them if they are located within the
RA’s transmission radius. In other words, BS chooses
the RA with Transmission Radius arg min
di
to
the SSsBCS. For instance, if we consider da1, da2, are the
distances between RAa and SS1BCS, SS2BCS respectively,
and db1, db2, are the distances between RAb and SS1BCS,
SS2BCS respectively, then we select the RA which has:
min(da1 + da2, db1 + db2). The objective of the optimiza-
tion is to minimize the number of RAs as much as possi-
ble, by selecting the SSsGCS that can support re-mul- ti-
casting to the largest number of SSsBCS within its cover-
age transmission range, and not necessary be the nearest
SSGCS neighbor to each SSBCS. The template is used to
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2.3. SS-SS Inter-Link CSI RA Selection Algrithm
The proposed Transmission Radius RA selection algo-
rithm proposed in 2.2 section above reduces further the
number of RAs involved in the re-multicasting process
but at the expense of algorithm reliability. This is be-
cause some of the SSs previously enjoying their own
nearby RA are now located at the edge of transmission
range of the new RA. Adding that to the varying channel
state, it would result into less reliable delivery. In order
to compensate for that and to further reduce the number
of RAs involved, each SS in the cell is assumed to report
also its instantaneous channel quality between itself and
its neighboring SSs (i.e., SS-SS inter-link CSI) that can
overhear in the cell. Such SS-SS inter-link CSI aggrega-
tion can help the BS to select the proper SSsGCS, that have
maximum number of SSsBCS within in its transmis-
sion range and have good inter-link CSI, to support the
re-multicasting. Such technique requires further modify-
cations to the Wimax Channel Quality Indicator Channel
(CQICH) but the implementation is out of the scope of
this paper.
The RAs are selected so that channel condition
between them and all SSsBCS has to be in a good state.
Using SS-SS instantaneous Inter-link CSI, R2 in phase II
is determined based on computing the sum of the instan-
taneous received power from all corresponding RAs
using the large-scale and small-scale fading channel
model, by selecting the Adaptive Modulation and Coding
(AMC) level that support the worst CSI between SSsBCS
and the RAs. Further details can be found in [5].
The above presented schemes were found to consid-
erably reduce the amount of energy consumed [5], provi-
ding a lower cost coverage solution with no dereliction in
achieved throughput for all multicast group members. In
this study, we have assumed throughout our proposed
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