Research Paper

1. Adaptive Admission Control and Packet Scheduling Schemes
for QoS Provisioning in Multihop WiMAX Networks
Perumalraja Rengaraju, Chung-Horng Lung
Department of Systems and Computer Engineering
Carleton University, Ottawa, Ontario, Canada
{rpraja, chlung}@sce.carleton.ca
Anand Srinivasan
EION Inc.
Ottawa, Ontario, Canada
anand@eion.com
Abstract— Selection of Call Admission Control (CAC) and packet
scheduling are crucial for multihop WiMAX networks to satisfy
the Quality of Service (QoS) for end users. In this paper, we
propose an adaptive CAC method and two different scheduling
schemes for multihop WiMAX networks. The proposed CAC in
the multihop Base Station (BS) reserves some bandwidth (BW)
for the mobile users and changes the BW reservation adaptively
based on most recent requests from the handover users. When
there are few or no handover users exist in a network, the
remaining reserved BW is allocated to low priority Best Effort
(BE) users for effective BW utilization. While admitting the New
Calls (NCs) or Handover Calls (HCs), the BS verifies both BW
and multihop delay requirements to satisfy the QoS of the call.
Next, we propose two downlink scheduling algorithms (P+E) and
(P+TB) for the BS in multihop networks. The (P+E) scheduler
combines the Priority and Earliest Due Date (EDD) scheduling
methods, while the (P+TB) scheduler combines the Priority and
Token Bucket (TB) scheduling methods. When the network is
lightly or moderately loaded, the (P+E) scheduler performs well
for both single and multihop users but the performance of real
time services are highly affected under high load conditions. On
the other hand, the (P+TB) scheduler has very good QoS
performance for real time services under high load conditions
and also has closer QoS performance to (P+E) scheduler for
multihop users under low and moderate load conditions. The
simulation results for CAC show that the proposed CAC have
lower Call Drop Probability (CDP) for HCs than existing fixed
BW reservation policy. The CAC admits more NCs when fewer
number of HCs arriving at the BS. The simulation results for
scheduling show that the (P+E) scheduler outperforms the
(P+TB) scheduler when the system is moderately loaded and the
(P+TB) scheduler outperforms the (P+E) scheduler when the
system is fully loaded.
Keywords- Call Admission Control, Packet Scheduling,
multihop WiMAX,
I. Introduction
The IEEE 802.16 standards, also known as Worldwide
Interoperability for Microwave Access (WiMAX) [1], are
designed for long distance broadband communication. It
defines the physical and Medium Access Control (MAC)
layers and their functionalities. The objective of IEEE802.16j
standard, released in 2009 is to increase the system throughput
and to extend the network coverage by more than one hop
using the Relay Stations (RSs). The aim of recently released
IEEE802.16m standard is to support the requirements of
International Mobile Telecommunications- Advanced systems.
Since the IEEE802.16j and IEEE802.16m standards are
multihop nature, assuring QoS including delay performance
are the major challenging task for the end customers.
Although the WiMAX standard defines the signaling
messages for MAC operations, RRM protocols for CAC,
scheduling and many other aspects are left open for
innovations. The CAC is performed on NCs and HCs to
determine whether the call should be admitted to the network
or not, based on the availability of radio resources and
evaluation of QoS requirements. In general, the forced
termination of a call in progress is more frustrating than
blocking of a new call. So the HCs are given higher priority
than NCs while admitting into the network. Once the call has
been admitted, the scheduler efficiently allocates the radio
resources based on the QoS parameters set. In IEEE 802.16
standards, five different types of services are defined for
mobile and multihop WiMAX networks. They are Unsolicited
Grant Service (UGS), real time Polling Service (rtPS),
extended real time Polling Service (ertPS), non-real time
Polling Service (nrtPS) and BE Service. The QoS parameters
specified for the WiMAX service classes are Maximum
Sustained Traffic Rate (MSTR), Minimum Reserved Traffic
Rate (MRTR), Maximum Latency (ML), Tolerated Jitter (TJ),
and Request/Transmission Policy.
In this paper, we explore the problem of CAC and packet
scheduling for multihop WiMAX networks. In the design of
existing CAC methods [5], BW is reserved for HCs to reduce
the CDP but the reserved BW is not fully utilized when there
are few or no HCs exist in a network. For effective BW
utilization, we allocate the unused reserved BW for NCs who
need a BE services. If any HCs arrive after that, the allocated
BW to BE service is re-allocated to HCs to reduce the CDP.
Thus the proposed CAC assures the effective BW utilization
and minimizes the CDP for HCs.
Similarly, in the design of existing schedulers for WiMAX
network [2, 6, 7], the delay requirement for multihop users
may not be satisfied. In addition to that, when the system
reaches the full load condition, providing both QoS
differentiation between real time and non-real time services
and minimizing delay performance of multihop users are not
considered. The proposed (P+E) scheduler guarantees the
delay requirement of multihop users but the QoS
differentiation is highly affected under high load condition. On
978-1-4577-1379-8/12/$26.00 ©2012 IEEE 866

2. the other hand the (P+TB) scheduler guarantees the QoS
differentiation for WiMAX users, but the delay performance
of multihop users is slightly reduced when compared to (P+E)
schedulers.
The rest of the paper is organized as follows. The existing
works related to CAC and packet scheduling are discussed in
Section II. The proposed adaptive CAC and dynamic packet
scheduling schemes are described in Section III and Section
IV, respectively. The simulation environment and the results
are demonstrated in Section V. Lastly, Section VI is the
conclusion.
II. RELATED WORKS AND MOTIVATION
For both CAC and Scheduling, many research efforts have
been conducted for fixed and mobile WiMAX networks. Some
important works on CAC to minimize the CDP and packet
scheduling methods to assure BW and delay guarantee for
mobile WiMAX networks are listed below:
In [2], a scheduling algorithm and an admission control
policy for fixed WiMAX network were proposed. The
connections are admitted only when it satisfies the necessary
condition for delay guarantee. The CAC methods proposed in
[3] and [4], HCs are given high priority. In [3], BW
degradation policy was introduced to admit the HCs. The BW
assigned to individual active service flow is reduced during
the CAC process, which is called “BW degradation”. The
CAC proposed in [5] reserves an adaptive temporal channel
BW for mobile users based on most recent requests. But the
scheme does not provide any delay guarantee and the reserved
BW are wasted when there are few or no HC exist in a
network. In [6], the CAC scheme for mobile WiMAX was
proposed to satisfy both BW and delay guarantee. The
proposed CAC scheme provides higher priority to HCs and
UGS calls, but there is no reservation for HCs.
In our previous work [7], we compared the different
scheduling schemes for fixed and mobile WiMAX networks
and we simulated the (P+E) scheduler for uplink scheduling.
The (P+E) scheduler is simple, but it provides BW and delay
guarantee. Authors in [8] proposed the TB based CAC and
uplink scheduling for the fixed WiMAX networks. It provides
the BW and delay guarantee for real time traffic.
To the best of our knowledge, the existing CAC does not
consider the multihop delay guarantee to admit the
connections. In this paper we proposed an adaptive CAC and
scheduling schemes for multihop WiMAX networks in the
downlink. The proposed CAC considers the BW reservation
policy from [5] and delay guarantee from [2]. But the BW
reservation algorithm is modified to avoid the BW wastage
and the delay guarantee is extended to include the multihop
delay. Finally, the CAC considers the BW degradation policy
on BE traffic, only when the reserved BW for HCs is allocated
to the BE connections. For the packet scheduling, we
modified our earlier (P+E) scheduler [7] to work for multihop
downlink traffic. If the BW degradation policy is used in
CAC, the packet scheduling should ensure the BW
degradation on BE traffic only. For this reason, we propose the
(P+TB) scheduler in this paper to allocate the BW for real
time traffic first for the generated tokens. Then the remaining
BW is allocated for BE traffic.
III. ADAPTIVE CAC FOR MULTIHOP WIMAX NETWORKS
The CAC procedure is same for both uplink and downlink
traffic. Hence, we consider only the downlink and also we
make the following assumptions in our proposed CAC
scheme: As per the standard, the BE connections do not
provide any QoS guarantees. But in real world, the
subscription available from Internet Service Providers are
mostly the BE connections for home users and non-BE
connections for corporate users. Therefore, in order to satisfy
the BE users, we recommend to provide a portion of the BW
in the MSTR, i.e., k × MSTR, where 0<k≤1 for BE
connections. For readability, the terminology used in this
paper is as follows.
f : frame duration (ms)
B: Total downlink BW available at the BS (Mbps)
and
: Total BW allocated for New Calls (NCs) and
Handover Calls (HCs), respectively.
,

3. ,

4. ,

5. and
: BW allocated for NCs
of total UGS, rtPS, e-rtPS, nrtPS and BE calls, respectively
,

6. ,

7. ,

8. and
: BW allocated for HCs
of total UGS, rtPS, e-rtPS, nrtPS and BE calls, respectively
di : maximum latency/delay requirement of connection i (ms).
: modified latency, which includes multihop delay (ms).
bi : BW required for connection i.
ri : token (packet) arrival rate of a connection i (Kbps).
Ti : TB size of a connection i (Kbits).
mi = di /f , mi must be an integer.
un : number of UGS connections admitted into the network.
rn : number of rtPS connections admitted into the network.
nn : number of nrtPS connections admitted into the network.
th : BW reservation threshold for handover users.
The flow chart for the proposed CAC is shown in Figure 1.
The main idea of the CAC scheme in the BS is described as
follows. Whenever a NC or HC arrives at the BS, first it will
verify whether sufficient amount of BW is available to
allocate and whether the connection meets the multihop delay
requirement or not. The key functions for CAC are highlighted
as follows:
Arriving calls priority: If more than one call arrives at the
BS, the CAC module assigns priority for the incoming calls.
The real time HCs are given high priority when more than one
call arrives at the BS. The real time NCs are given the next
priority. Within the real time calls, the priority order of the
calls is UGS, ertPS and rtPS. Then the non-real time HCs
(nrtPS and BE) and finally the non-real time NCs (nrtPS and
BE) will be processed for call admission.
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9. BW reservation: The CAC reserves an adaptive BW for
HCs by defining two levels of threshold, ‘thmin’ and ‘thmax’,
where thmin thmax B. Let ‘th’ be some value such that thmin
th thmax. Initially, the CAC module reserves the minimum
BW for HCs, which is th = thmin. Later, the BW reservation is
adaptively changed based on most recent requests and leaving
of HCs.
BW verification: If a connection ‘i’ is a HC with requested
BW equal to ‘bi’, the CAC module at the BS checks whether
the remaining BW is greater than or equal to the requested
BW (bi ≤ B - Bt). If the remaining BW is enough to admit, the
CAC accepts the connection for the next stage (delay
guarantee). Otherwise, it checks if the reserved BW ‘th’ is
allocated to any BE connections for NCs by checking the
condition (bi + BHC
≤ th). If the condition (bi + BHC
≤ th) is
true, then it is clear that the reserved BW is allocated to BE
connections for NCs. Therefore the CAC applies BW
degradation on the BE traffic to accept the connection ‘i’ for
the next stage. Else, the CAC rejects the connection.
Figure 1. Proposed Adaptive CAC – Flow chart
If a connection ‘i’ is a NC with requested BW equal to ‘bi’,
the CAC module at the BS first verifies the BW requirement.
In BW verification process, the CAC checks whether the
remaining BW (B- (BNC
+ th)) is greater than or equal to the
requested BW (bi). The remaining BW calculation includes the
adaptive BW reservation ‘th’. If the remaining BW is enough
to admit, the CAC accepts the connection for the next stage.
Otherwise the CAC checks, whether the reserved BW (th) is
not fully utilized by the HCs or and whether the connection is
BE traffic or not. If both conditions are true, the CAC accepts
the BE call for efficient BW utilization. For BE traffic, the
delay verification is not necessary. In the BW verification
process, the BW requested by the connection ‘i’ is equal to
‘bi’.
Where bi =
for UGS traffic
for rtPS, ertPS and nrtPS traffic
( × for BE, where 0 ( ≤ 1
Delay guarantee: The next stage of the CAC process is to
verify the multihop delay guarantee. In a multihop WiMAX
network, once the BS schedules the packet for transmission,
the subordinate RS can forward the packet in the next frame
itself. The packet forwarding in next frame is possible
because the RSs BW capacity is the same as the BS and
downlink traffic is only from BS. With this assumption, we
modified the delay guarantee scheme from [2] that was based
on the TB model. Here, the delay condition is verified for real
time and non-real time traffic, not for UGS and BE traffic.
Since the UGS traffic is scheduled first, the delay performance
is always within the limit but for BE traffic, delay guarantee is
not necessary. For deriving multihop delay guarantee
equation, we modified the delay guarantee equation for single-
hop networks, given in equation (1) [2].
bi ≤ [(mi - 1). (1+(BnrtPS)/BrtPS) -1] . ri f (1)
In equation 1 [2], the authors considered the UGS, rtPS
and nrtPS traffic. Here, we also considered the BE traffic to
provide some BW assurance for BE users. The multihop delay
guarantees of rtPS and nrtPS connections are satisfied
according to equations (2) and (3).
bi ≤ [(mi - nh). (1+(BnrtPS+BBE)/BrtPS) -1] . ri f (1)
bi ≤ [(mi - nh). (1+(BrtPS+BBE)/BnrtPS) -1] . ri f (2)
Where, nh is number of hops from the BS.
IV. PS SCHEMES FOR MULTIHOP WIMAX NETWORKS IN
DOWNLINK
The main objective of scheduling and BW management is
to efficiently allocate the radio resources based on the set of
connection QoS parameters. The (P+E) scheduler [7]
combines the Earliest Due Date (EDD) scheduling and priority
scheduling that is shown in Figure 2.a. The inner EDD
scheduler first schedules the rtPS, ertPS, nrtPS and BE traffic
into EDD queue. The outer priority scheduler schedules the
UGS queue first and then the EDD queue for the remaining
BW in each frame. The priority scheduler ensures the packet
deadlines while scheduling the packet and it drops the packet
868

10. if the deadline time exceeds the limit. The packet deadlines
are modified according to number of hops from the BS to the
user node (li=di – nh × f). Hence, the BS schedules the packet
before the modified latency (li) and the packet can reach the
destination within the specified maximum latency (di).
Figure 2. (a) P+E Scheduler, (b) P+TB Scheduler
The (P+TB) scheduler combines the TB scheduling and
priority scheduling that is shown in Figure 2.b. The inner TB
scheduler generates tokens for ertPS, rtPS, nrtPS and BE
traffic. The outer priority scheduler schedules the UGS queue
first and then for the remaining BW; it schedules the ertPS,
rtPS, nrtPS and BE queues for the available tokens in a TB
scheduler. In a TB scheduler, a packet is not allowed to be
transmitted until the scheduler possesses a token. Therefore,
over a period of time t, the maximum data allowed by the TB
scheduler is ri × t + Ti [8].Since the CAC verifies the delay
guarantee for UGS, rtPS and nrtPS using the token bucket
algorithm, the (P+TB) scheduler assures the BW and delay
guarantee for UGS, rtPS and nrtPS services. But the QoS
performance of BE services are highly affected.
V. SIMULATION ENVIRONMENT AND RESULT ANALYSIS
In this section, the preliminary simulation results of
adaptive CAC and the PS schemes are presented. The system
parameters considered for the BS and the traffic model
considered for the simulations are given in Table I and II [7].
TABLE I. SYSTEM PARAMETERS
Parameter Value
Physical Layer Wireless MAN-OFDMA, TDD
No of OFDM symbols and
subchannels (DL)
24, 35
BW and Frame duration 10MHz and 5msec
Minimum resource allocation
unit for downlink ( slot )
2 OFDM symbols in time,
1 subchannel in frequency
TABLE II. TRAFFIC MODEL
Application Latency MRTR MSTR Pkt. Size
Voice (UGS) 30msec 64Kbps 64Kbps 40 Bytes
Voice (ertPS) 30msec 40Kbps 64Kbps 40 Bytes
Video (rtPS) 50msec 512Kbps 0.8Mbps 2400 Bytes
Audio (nrtPS) 80msec 320Kbps 400Kbps 800 Bytes
Data (BE) 200msec NA 192Kbps 120 Bytes
In addition to that, the following points are considered for
both the CAC and scheduling simulation.
• The maximum number of hops considered for this
simulation is two for simplicity.
• In generated calls, even numbers of calls are assigned to
multihop SS (two hops).
• While scheduling the packets, if a packet exceeds the
maximum latency, then the packet is dropped before the
transmission.
• The minimum packet loss requirement for real time
services is 1E-04.
• The packet loss and delay performances of UGS calls are
same for both (P+E) and (P+TB) scheduling as they
schedule the UGS traffic first.
To observe the effective performance of proposed CAC, we
compared the proposed adaptive BW reservation with fixed
BW reservation scheme for three different cases. In that, the
threshold of the fixed scheme is set at thmim, while the th of the
adaptive scheme varies between thmim (20 percent) and thmax
(90 percent) [5]. Call arrivals and departures are Poisson
distributed. In a call generation process, the applications are
randomly distributed with equal probabilities and the NCs and
HCs are randomly distributed with a probabilities of p and q,
where p + q = 1. In case I, the connection request contains
larger NCs (p=0.7) and smaller HCs (q= 0.3). In case II, the
connection request type is reversed, i.e., the connection
request contains larger HCs with q=0.7. In case III, NCs and
HCs have equal probability (p=q=0.5).
For the PS, we studied various schedulers for mobile
WiMAX networks and verified the performance of (P+E)
schedulers in [7] for uplink traffic. The (P+E) scheduler design
is simple in nature and it also avoids the drawback of real time
priority scheduler and the EDD scheduler. In this simulation,
we compared the QoS performance of individual (P+E) and
(P+TB) scheduler.
A. Result analysis for the proposed CAC
The results presented in Figures 3 - 5 show the CDP of
HCs and Call Blocking Probability (CBP) of NCs for case I,
case II and case III. For case I as shown in Figure 3, the
following points are observed:
• When the system is lightly loaded, the CDP and CBP of
both the fixed and adaptive schemes perform the same.
• However, when the system is moderately loaded (call
arrival rate is between 12 and 18), both CBP and CDP
869

11. of the adaptive scheme is better. Here, the unused
reserved BW is utilized by the BE calls to improve the
CBP. The adaptive BW reservation scheme and BW
degradation on BE calls to admit the HCs improve the
CDP.
• When the system is fully loaded (the call arrival rate is
18,) the system blocks the NCs but a few HCs are
admitted using the degradation policy that improves the
CDP of HCs.
Figure 3. CBP and CDP performance for case I (p=0.7)
For case II, as shown in Figure 4, it is clear that the
performance results of CDP and CBP are similar to case I.
But, when the system is fully loaded, both CBP and CDP are
slightly increased, because the system has lower probability of
NCs and the degradation policy on BE calls to accept further
calls is reduced.
Figure 4. CBP and CDP performance for case II (p=0.3)
In case III, as shown in Figure 5, both CBP and CDP
performance results are closer to that of case II at full load
condition (At the call arrival rate of 19, the CBP for case I, II
and III are 0.85, 0.94, and 0.91, respectively). From the CAC
simulation results the following points are observed.
• When the connection requests contain smaller HCs, the
reserved bandwidth is not fully utilized but the unused
reserved bandwidth is allocated to BE traffic to improve
the CBP. Later, the reserved bandwidth allocated to BE
calls are re-allocated to HCs to improve the CDP.
• The bandwidth reservation for HCs is adaptively
changed to improve the CDP.
Figure 5. CBP and CDP performance for case II (p=0.5)
B. Result analysis of (P+E) and (P+TB) scheduling
schemes
For the PS, we analyzed the packet loss and delay
performances of the multihop SS as shown in Figures 6 and 7.
The packet loss performance measures the total number of
packets dropped from each service class of multihop SS. From
Figure 6, packet loss performance, the following points are
observed:
• When the system is lightly loaded, the performance of
both (P+E) and (P+TB) scheduling are same.
• When the system is moderately loaded (call arrival rate
is between 12 and 18) the packet drops in (P+TB)
scheduling in ertPS, rtPS and nrtPS services are very
small and within the required limit (1E-04), but BE
packets are dropped more. In (P+E) scheduling, the
packet loss for all service classes are similar and the
packet loss for the real time services are within the
required limit.
• When the system is fully loaded (call arrival rate is
18), most of the BE packets are dropped in (P+TB)
scheduling and the packet drop from the real time
services are moderate but within the required limit. In
(P+E) scheduling the packet drops in ertPS, rtPS and
nrtPS are bit higher than (P+TB) and not within the
limit (1E-04).
Figure 6. Packet Loss performance
2 4 6 8 10 12 14 16 18 20 22
0
0.2
0.4
0.6
0.8
1
1.2
Call Arrival Rate
Call
Blocking
and
Drop
Probability
CDP and CBP Performance (p = 0.7)
CDP Proposed Scheme
CDP Fixed Reservation
CBP Fixed Reservation
CBP Proposed Scheme
2 4 6 8 10 12 14 16 18 20 22
0
0.2
0.4
0.6
0.8
1
1.2
Call Arrival Rate
Call
Blocking
and
Drop
Probability
CDP and CBP Performance (p = 0.3)
CDP Proposed Scheme
CDP Fixed Reservation
CBP Fixed Reservation
CBP Proposed Scheme
2 4 6 8 10 12 14 16 18 20 22
0
0.2
0.4
0.6
0.8
1
1.2
Call Arrival Rate
C
all
Blocking
and
D
rop
P
robability
CDP and CBP Performance (p = 0.5)
CDP Proposed Scheme
CDP Fixed Reservation
CBP Fixed Reservation
CBP Proposed Scheme
2 4 6 8 10 12 14 16 18 20 22
0
20
40
60
80
100
Call Arrival Rate
Num
ber
of
Packets
dropped
Packet Loss Performance
rtPS - (P+TB) Scheduling
rtPS - (P+E) Scheduling
ertPS - (P+TB) Scheduling
ertPS - (P+E) Scheduling
nrtPS - (P+E) Scheduling
nrtPS - (P+TB) Scheduling
BE - (P+E) Scheduling
BE - (P+TB) Scheduling
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12. Figure 7. Latency performance
Latency: The latency or average delay performance is
measured by dividing the total delay of individual transmitted
packet by total transmitted packets. When the packet exceeds
the deadline time period, the packet is dropped. So the delay
of the individual packet is lesser than packet deadline length.
The average delay is given by,
2345674 8469 :; =4?@ =
∑ 8C : ; =4?@
C
Where, 8C is the delay of the ith
transmitted packet from
jth traffic class (UGS, rtPS, nrtPS and BE) and C is the total
transmitted packets for the jth
traffic class. From Figure 7, the
latency performance of multihop SS, the following points are
observed:
• When the system is lightly loaded, the latency for both
(P+E) and (P+TB) scheduling are same and the latency
is approximately two frames duration (~10msec).
• When the system is moderately loaded, the delay for
real time services in (P+TB) scheduling is slightly
higher than (P+E) scheduling. Because the (P+E)
scheduler considers the multihop frame delay, but
(P+TB) considers only the tokens. The latency for BE
services in (P+TB) scheduling is moderately higher
than (P+E) scheduling.
• When the system is fully loaded, the latency of rtPS and
nrtPS for (P+E) scheduling is higher than that for
(P+TB) scheduling. On the other hand, the latency of
BE services in (P+TB) scheduling is drastically
increased.
VI. CONCLUSIONS AND FUTURE WORK
This paper proposed an adaptive CAC scheme and two
packet scheduling policies for multihop WiMAX networks.
The performance for both CBP and CDP of the adaptive CAC
is better than the fixed BW reservation scheme. The CBP is
improved by utilizing the unused reserved BW of HCs by the
NCs BE traffic. On the other hand, the CDP is improved by
the adaptive BW reservation scheme and BW degradation
policy. If the BW degradation is applied, the PS should
maintain the required QoS level. For this reason we have
conducted the (P+E) and (P+TB) scheduling policies. Next,
we are interested in dynamic selection of (P+E) scheduling
when the system is lightly or moderately loaded, and switch to
the (P+TB) scheduling when the system is fully loaded. In this
paper, the CAC design does not consider the RS mobility.
Now we are working on RS mobility for CAC design and
dynamic selection of scheduling algorithms to achieve high
QoS performance.
ACKNOWLEDGEMENTS
This project is partially funded by EION Wireless and
Ontario Centres of Excellence (OCE), Canada.
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0
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Call Arrival Rate
L
atency
(m
sec) Latency Performance
rtPS - (P+TB) Scheduling
rtPS - (P+E) Scheduling
ertPS - (P+TB) Scheduling
ertPS - (P+E) Scheduling
nrtPS - (P+E) Scheduling
nrtPS - (P+TB) Scheduling
BE - (P+E) Scheduling
BE - (P+TB) Scheduling
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