2012 International Conference on System Engineering and Modeling (ICSEM 2012)
IPCSIT vol. 34 (2012) © (2012) IACSIT Press, Singapore
Base Station Assisted Backoff for Broadband Wireless Access
Networks
Anbazhagan Rajesh and Rangaswamy Nakkeeran
Department of Electronics and Communication Engineering,
Pondicherry Engineering College, Puducherry, INDIA - 605014
ABSTRACT
Abstract. Contention based bandwidth request with subscriber station assisted truncated binary exponential
backoff (SAT) mechanism suffers from low contention efficiency and high access delay when transmission
failure is modeled with collision due to contention, unavailability of bandwidth and channel error. In this
paper, contention resolution with base station assisted backoff (BAB) mechanism is suggested and an
optimum contention window (OCW) is derived for bandwidth request in WiMAX networks. The
performance of WiMAX networks is studied with varying number of stations, error rates and the number of
slots allocated for bandwidth request. The contention efficiency (for q=0.60) with BAB shows an
improvement of 35.08% and 21.98% than SAT for 25% and 50% error rates, respectively. The access delay
with BAB (for q=0.60) shows an improvement of 25.42% and 22.33% than SAT with similar error rates.
Keywords: WiMAX, contention resolution, backoff mechanism, bandwidth reservation
1. Introduction
Worldwide interoperability for microwave access (WiMAX) networks has emerged as the strongest
candidate to provide last mile broadband wireless connectivity where wired infrastructure is difficult or
infeasible to deploy. Although last mile and last inch network with WiMAX aim to connect hot spots with
increased service rate, the quality of service (QoS) attribute of WiMAX does not reach the end user devices
for all class of services. WiMAX promises to offer guaranteed QoS at the media access control (MAC) layer
for real time services. One of the design challenges of MAC for non real-time services is to accommodate
high bit rates without any degradation in QoS.
The point to multipoint (PMP) WiMAX network consists of subscriber stations (SS’s) coordinated by
centralized base station (BS). The two main operations carried out by the SS are ranging and bandwidth
request. During uplink channel access (from SS to BS), the SS executes bandwidth request. The two types of
bandwidth request mechanism specified in IEEE 802.16 standards1 are unicast polling (contention free
access) and contention resolution. The main challenge in designing the backoff mechanism in contention
resolution mechanism is to avoid overlapping of backoff counter among SSs that result in transmission
failure due to collision. While designing the contention resolution in WiMAX system, the failure model has
to account all causes of transmission failure. The transmission in WiMAX systems results in failure by one
of the following events: collision due to contention, unavailability of bandwidth or channel error. The failure
models in literature2-11 account for collision with either unavailability of bandwidth or channel error. Hence,
we suggest a failure model by including these three events. While accounting these events, an effective
backoff mechanism is needed to overcome the limitation of contention resolution with existing subscriber
station assisted TBEB (SAT).
Qiang et al2 made detailed discussions on the issues of contention resolution with truncated binary
exponential backoff (TBEB) under varying error rates, saturated and unsaturated traffic load conditions with
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grouping and non-grouping schemes. Yaser et al3 presented an accurate analytical model for bandwidth
request in WiMAX network and calculated the capacity of the contention slots in delivering bandwidth
requests. In literature, several researchers2-10 carried out contention resolution with SAT. Nevertheless, there
is need for robust backoff window estimation in WiMAX due to the type its accessing mechanism. Unlike
wireless local area network (WLAN), the SS in WiMAX does not use carrier sense multiple access over
collision avoidance (CSMA/CA) before transmitting a request. Further, the computation of optimum
contention window by accounting the number of stations is very challenging with SAT. Wenyan et al11 have
proposed base station coordinated backoff to overcome the limitations of SS assisted backoff. However, the
authors have considered the failure due to collision alone. Hence, we aim to propose a base station assisted
backoff by considering all the three events of transmission failure and derive the optimum contention
window accordingly.
The rest of the paper is organized as follows: The failure models, contention resolution with the proposed
BS assisted backoff are presented in Section 2. Simulation results of the proposed and existing schemes are
given in Section 3. Finally, concluding remarks is given in Section 4.
2. Proposed Contention Resolution in WiMAX network
2.1 Unified failure model
The probability of failure of the WiMAX system can be characterized by three possible events namely:
collisions due to contention, unavailability of bandwidth and channel error. Qiang et al2 modeled the
probability of failure (Pf) with collision and channel error. According to them, Pf is given by:
(1)
P f = Pc + Pe − Pc Pe
where Pe denotes the probability of error due to channel, Pc denotes the probability of collision due to
contention. Yaser et al3 and David et al4 modeled the probability of failure with collision and unavailability
of bandwidth and hence it is given by:
Pf = Pc + (1 − Pc )(1 − q )
Tr
(2)
where q denotes the probability of BS to accept a bandwidth request and Tr denotes response time or waiting
time. However, to the best of our knowledge in WiMAX network, the transmission failure has not been
modeled with all three events. Hence, by assuming the events as mutually exclusive, we model the Pf in the
proposed system as follows,
Pf = Pc + (1 − Pe )(1 − Pc )(1 − q ) r + Pe
T
(3)
In this paper, the performance of contention resolution mechanisms in WiMAX network is performed
with the developed failure model in (3). The backoff counter in conventional truncated binary exponential
backoff (TBEB) is decremented without accounting the status of the channel or number of stations in the
network. This increases the chance of overlapping of backoff counter among the stations that leads to high
probability of collision with TBEB. The performance of TBEB degrades further under the developed failure
model given in (3). In TBEB, the backoff counter depends only on probability of collision. Nevertheless, the
backoff counter has to be varied if transmission failure is due to unavailability of bandwidth or channel error.
The contention resolution with TBEB suffers from low contention efficiency and high access delay. Hence,
an effective contention mechanism is required to improve the performance of WiMAX network.
2.2 Proposed base station assisted backoff (BAB)
To overcome the limitations of SAT with the developed failure model, base station assisted backoff
(BAB) is considered in the proposed system for bandwidth request. In BAB, the contention window size to
be chosen by the SS is broadcasted by the BS. On receiving the contention window, the SS will randomly
choose their respective backoff counters. The advantage of choosing BAB over SAT is the estimation of
optimum contention window (OCW) by the BS. The purpose of OCW is to avoid overlapping of backoff
counter by accounting all causes of transmission failure. The OCW improves the contention efficiency with
reduced access delay of the SSs. The contention process in WiMAX network is not started until uplink
channel descriptor (UCD) is received. The BS transmit UCD message at a periodic interval in order to define
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the characteristics of an uplink physical channel. The UCD message present in downlink subframe contains
uplink transmit parameters and it is broadcasted by the BS. The choice of minimum contention window
(CWmin) and maximum contention window (CWmax) by the BS, gives much flexibility in controlling the
contention resolution. The SS set the maximum number of retransmission attempts from CWmin and CWmax.
The BAB mechanism for bandwidth request is shown in Figure 1. The contention resolution algorithm with
BAB works as follows.
Figure 1. Proposed base station assisted backoff for bandwidth request
When an SS has information to send and wants to start the contention resolution process, it sets its
internal backoff window equal to the initial contention window defined in the UCD message. The SS
randomly selects a number within this backoff window. The obtained random value indicates the number of
contention transmission opportunities that the SS will defer before transmitting. An SS considers only the
contention transmission opportunities for which this transmission would have been eligible. The SS transmits
its bandwidth request when its backoff counter reduced to zero. If the SS does not receive any response from
the BS after a specific duration of time, it assumes loss of request and waits for the consecutive optimum
contention window from the BS. The BS computes optimum contention window as derived in subsection 2.3.
The optimum contention window is broadcasted over every frame and the SS will update its contention
window when the waiting time expires. The waiting time is the time over which the SS waits for response
from base station after executing the bandwidth request. Unlike TBEB, that set constant contention window
on collision, the SSs with BAB are updated with optimum contention window from BS.
2.3 Estimation of optimum contention window
The contention efficiency (Ps) is the probability that a transmission opportunity containing a successful
bandwidth request. The Ps, in the proposed system, is computed by accounting the channel error,
unavailability of bandwidth and collision and hence it can be expressed as follows,
Ps = N c P (1 − P )
N c −1
(1 − Pe ) (1 − (1 − q )
Tr
)
(4)
where Nc refers to the number of contending stations and P refers to probability of transmission. The main
challenge in BAB mechanism is the computation of optimum contention window (OCW). An appropriate
value of this contention window reduces the overlapping of backoff counter between SSs. The value of
average contention window ( W ) is computed from the estimation of contention window size with maximum
backoff stage ‘m’. The W is derived as follows,
(1 − P ) (1 − (δ P ) )
(1 − P ) (1 − (δ P ) )
m
W =W
3
f
m
f
f
f
(5)
where δ and W correspond to initial backoff factor and initial contention window size, respectively. The
geometric probability distribution is considered in the proposed system. Therefore, the average value of
contention window is given by the expected value of random variable. Thus, the probability of transmission
(P) with average contention window becomes, P =2/ W and is given by,
P=
2 (1 − Pfm ) (1 − δ Pf )
(
W (1 − Pf ) 1 − (δ Pf
)
m
(6)
)
The OCW is computed by substituting the value of P in (4), differentiating with respect to W and
equating to zero. The resultant contention window is given by:
Wopt = N c
2 (1 − δ Pf ) (1 − Pfm )
(1 − P ) (1 − (δ P )
f
f
m
if Pf <1
)
(7)
The Wopt is broadcasted to all the SS on frame basis. The BS computes Wopt with the instantaneous value
of probability of failure that is resultant of channel state information from SSs. The contention efficiency and
hence the throughput of WiMAX depends on the value of Wopt. The Wopt not only consider the number of
contenting station but also the events that cause the transmission failure. Hence, the proposed method
effectively resolves contention with increase in number of stations and diverse channel conditions.
3. Simulation Results
The contention based bandwidth request schemes with SAT and BAB has been implemented in Matlab
to verify the proposed analytical model and compare the performances of the schemes. Simulation results are
obtained by averaging over 15 runs and each run with 105 iterations. The following data are considered for
this simulation. The duration of frame is 5 ms, the number of contention slots within a physical frame for
bandwidth request is 16, minimum contention window is 8, initial value of waiting time (response time) is 8
and maximum number of backoff stage is 6. In addition, the MAC parameters are configured in accordance
with the standard1. The system performance is evaluated in terms of contention efficiency and access delay
by varying the contending stations for different values of q, namely 0.25, and 0.60. In addition, all
simulations are carried out considering error prone channel for the system with 25% and 50% error rates.
(a)
(b)
Figure 2. Contention efficiency under varying number of contending stations (a) 25% channel error and (b) 50%
channel error (SAT - SS assisted TBEB, BAB – Base station assisted backoff)
Figure 2(a) shows the contention efficiency of bandwidth request mechanism with 25% error rate. The
contention efficiency increases with increase in the number of stations, q and attains saturation. The
contention efficiency for BAB at this saturation region lies between 0.285 and 0.335 for q values of 0.25 and
0.60, respectively. Further, contention efficiency SAT lies between 0.224 and 0.248 for q values of 0.25 and
0.60, respectively. It is also evident that SAT performs better than BAB does when number of stations in the
network is less than or equal to 15. However, with increase in number of stations greater than or equal to 20,
the BAB performs better than SAT. The BAB shows an improvement of 27.23% and 35.08% than SAT for q
values of 0.25 and 0.60 respectively with 25% error rate. Simulations carried out with 50% error rate is
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shown in Figure 2(b), in which, contention efficiency lies between 0.258 and 0.283 for BAB, whereas it lies
between 0.186 and 0.232 for SAT. The BAB shows an improvement of 38.70% and 21.98% than SAT for q
values of 0.25 and 0.60 respectively.
(a)
(b)
Figure 3. Access delay under varying number of contending stations with (a) 25% channel error and (b) 50% channel
error.
The access delay of contention based bandwidth request with 25% error rate is shown in Figure 3(a). The
access delay increases with BAB when number of stations is less than 20. The increased access delay results
from reduced contention efficiency that results in larger retransmission. The access delay of the system with
SAT is 6.8401 ms and 5.8830 ms for the q value of 0.25 and 0.60 respectively, when number of stations
equals to 50, whereas the delay is reduced to 5.0600 ms and 4.3873 ms with BAB. The proposed system
shows an improvement of 26.02% and 25.42% for the q value of 0.25 and 0.60 respectively than
conventional system with SAT. Simulations are repeated with 50% error rate and the results are shown in
Figure 3(b). The access delay of the system with SAT is 7.2638 ms and 6.4150 ms for the q value of 0.25 and
0.60 respectively. The delay is reduced to 5.4213 ms and 4.9819 ms in the case of proposed BS assisted backoff. The
proposed system shows an improvement of 25.36% and 22.33% in access delay with 50% channel error.
4. Conclusion
In this paper, a contention resolution mechanism using base station assisted backoff (BAB) has been
suggested. The BAB is investigated under failure model with collision due to contention, channel error and
unavailability of bandwidth. The results are compared with conventional subscriber station assisted TBEB.
Simulation results show that the proposed BAB scheme performs better than the conventional SAT scheme
for varying error rates.
5. References
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[7] Qiang Ni and Ling Hu, “An Unsaturated Model for Request Mechanisms in WiMAX”, IEEE Communications
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