Voice Service Support over Cognitive Radio Networks
†
Ping Wang† , Dusit Niyato† , and Hai Jiang§
Centre For Multimedia And Network Technology (CeMNeT), School of Computer Engineering,
Nanyang Technological University, Singapore
§ Department of Electrical & Computer Engineering, University of Alberta, Canada
Abstract—In this paper, quality of service (QoS) provisioning
for voice service over cognitive radio networks is considered. As
voice traffic is sensitive to delay, the presence of primary users
and the requirement that secondary users should not interfere
with them pose many challenges for QoS support for secondary
voice users. Two cognitive medium access schemes are proposed
in this paper for the secondary voice users to access the available
channel. An analytical model is developed to obtain the voice
service capacity (i.e., the maximum number of voice users that
can be supported with QoS guarantee) for the secondary users,
taking the impact of primary users’ activities into consideration.
The analytical model is validated by the simulation. The analytical
results will be useful to support voice service in cognitive radio
networks.
Keywords – cognitive radio, medium access control, quality of
service (QoS), voice service capacity, delay, packet loss.
I. I NTRODUCTION
Cognitive radio, the idea firstly introduced by Mitola [1], [2]
and recently promoted by the U.S. Federal Communications
Commission (FCC) [3], provides an effective and efficient
solution for the paradox between the shortage of the wireless
spectrum resources and the under-utilization of the licensed
spectrum. An opportunistic (or cognitive) spectrum access
approach has been proposed to allow the unlicensed users (also
called secondary users) to exploit the spectrum that is not
being used by the licensed users (also called primary users)
[4]. In this manner, a highly economical and efficient usage
of the frequency spectrum can be achieved while allowing
primary users to enjoy their licensed spectrum without facing
any interference from the secondary users. Because of this
property, cognitive radio has recently drawn a lot of attention
in academia [5], [6], [7], [8], [9]. Many research efforts focus
on addressing the cooperative sensing of the primary users’
activities at the physical layer [6], [7], and little work has
been done at the medium access control (MAC) layer. In this
work, our goal is to support the quality of voice service for
secondary users at the MAC layer, and we assume that perfect
channel sensing can be achieved at the physical layer. The
presence of primary users and the requirement of secondary
users not interfering with them pose many challenges for
quality of service (QoS) provisioning for secondary users.
First, an efficient and low-complexity cognitive medium access
control scheme is needed for secondary users to share the
available spectrum unoccupied by the primary users. Second,
in order to guarantee the QoS of voice service for secondary
users, it is critical to obtain the voice service capacity (i.e., the
maximum number of voice users that can be supported), taking
the impact of primary users’ activities into consideration.
In this paper, we propose two cognitive medium access
schemes for the secondary voice users to access the wireless
spectrum, and we also develop an analytical model to obtain
the voice service capacity for these two cognitive medium
access schemes. The analytical results are validated by simulations. The analytical results reveal how the activities of the primary users and the different cognitive medium access schemes
affect the cognitive voice user capacity. The analytical model
can be used for the cognitive radio resource management and
call admission control.
II. T HE S YSTEM M ODEL
Cognitive communication technology has been studied for
different wireless networks including wireless metropolitan
area networks (WMANs) and wireless local area networks
(WLANs) [10], [11]. In this paper, we consider a WLAN,
where the activities of all the primary users can be sensed
by all the secondary users. The network has a single wireless
channel, which is shared by all the primary and secondary
users. The channel is time-slotted. We assume that at each
time slot, a primary user will not occupy the channel with
probability Pi . At the beginning of each slot, the secondary
users sense the activity of the primary users. If the channel is
sensed idle, the secondary users can exploit the availability of
the channel.
Constant rate voice traffic is supported for the cognitive
users. As voice traffic is sensitive to delay, a voice packet with
a large delay will be considered useless. Therefore, if a voice
packet cannot be delivered successfully during the delay bound
after its generation, it will be dropped by the voice sender. In
order to maintain satisfactory voice quality, the voice packet
dropping probability should not be higher than a threshold Pl .
Usually the threshold is set to be 1%.
III. C OGNITIVE M EDIUM ACCESS
In cognitive networks, a cognitive medium access control
scheme is needed for the secondary users to efficiently share
the available wireless channel when the primary users are
not active. A cognitive medium access control scheme has
two basic functions. The first is to ensure that the secondary
users will not interfere with the primary users. The second is
to achieve low-complexity, highly efficient, and fair medium
access among secondary users. Different cognitive medium
access schemes may have different performance (e.g., resource
utilization), leading to different system capacities for secondary users. In this section, two cognitive medium access
control schemes are proposed. One is contention based, and
the other is contention-free. The system capacities (i.e., the
voice service capacity) for these two schemes are analyzed in
Section IV.
????
??
??
time slot
sensing part
?
??
channel time
contention part
Fig. 1.
transmission part
ACK part
A time slot structure.
A. Contention-Based Medium Access
A time slot is further divided into four parts, as shown in
Fig. 1. The first part is called sensing part, which is used for
all the secondary users to sense the activities of the primary
users. If the channel is sensed busy, no secondary user should
contend for that slot. The second part is called contention part,
consisting of a number of mini-slots. Each secondary user has a
contention window. Prior to every contention, each secondary
user randomly chooses a backoff timer from the contention
window. Then the secondary user starts to sense the channel.
If the channel has been sensed idle for a duration of the backoff
timer (in unit of mini-slots), the secondary user will transmit its
packet; Otherwise, it will quit the contention for the current
slot. Thus, for each contention, the secondary user with the
smallest backoff timer will win and transmit its packet in the
third part (i.e., transmission part) of the slot. Note that it is
possible that more than one secondary user may choose the
same smallest backoff timer, resulting in a collision. In order
to determine whether or not a packet has been successfully
transmitted, a receiver sends acknowledgment at the fourth part
of each slot (which is at the end of each slot) to the sender
upon a successful packet reception.
B. Contention-free Medium Access
Similar to the contention-based medium access, a time slot
is also divided into four parts in the contention-free medium
access. The difference is that in the second part, the secondary
users do not follow the backoff mechanism. Instead, each minislot in the second part is assigned to a secondary user in a
deterministic way (the mini-slot assignment procedure is to be
discussed). A secondary user (say user A) with mini-slot index
i first senses the channel from mini-slot 1 to mini-slot i − 1.
If the channel keeps idle (i.e., no secondary user with minislot index smaller than i has packet to transmit), then user
A can start transmission from mini-slot i till the end of the
transmission part. If the channel becomes busy from any minislot prior to mini-slot i, which indicates that another secondary
user with a smaller mini-slot index has already started its
transmission, user A should not transmit in the current slot.
Since a single secondary user will be assigned to one minislot, a collision-free medium access can be achieved. Note that
the chance that one user transmits in a slot largely depends
on its mini-slot index. The smaller the index, the larger the
chance. In order to maintain fair medium access among all
the secondary users, the mini-slot index will be rotated after
each slot. For example, the user associated with the first minislot in the current slot will have the last mini-slot index in the
next slot, and the user associated with the second mini-slot in
the current slot will have the first mini-slot index in the next
slot, and so on.
If the number of secondary users in the cognitive network
is fixed, the mini-slot assignment procedure can be done once
at the initialization of the network. If the secondary users
dynamically join or leave the cognitive network, a mini-slot
assignment procedure needs to be performed upon every user
joining and leaving events. Any existing secondary user can
be designated to perform the mini-slot assignment procedure.
We denote the secondary user who is in charge of the minislot assignment as MSA (Mini-Slot Assigner). When a new
secondary user wants to join the cognitive network, it first
broadcasts a JOIN message. Upon receiving the JOIN message,
the MSA sends a JOIN-ACK message, which includes the
assigned mini-slot index, to the new user. Similarly, when
a user leaves the cognitive network, it also broadcasts a
LEAVE message. The MSA will then re-assign the mini-slots
to all the existing secondary users, and broadcast the new
assignment result to all the secondary users. When the MSA
leaves the cognitive network, it designates another existing
user to perform the mini-slot assignment task before leaving,
and includes in the LEAVE message the new MSA ID and
information of other existing users. Upon receiving the LEAVE
message from the current MSA, the new MSA will then
re-assign the mini-slots and broadcast the new result to all
the existing secondary users. The JOIN/JOIN-ACK/LEAVE
message is given high priority to be sent, compared with voice
packets. To achieve this, the first mini-slot (we call it mini-slot
0) in the second part of a slot is dedicated to the users with
JOIN/JOIN-ACK/LEAVE message. A user with JOIN/JOINACK/LEAVE message can transmit starting from the first minislot, while the users with voice packets have to monitor the
channel in the first mini-slot. If the channel is busy, the users
with voice packets should not transmit in the current slot.
As JOIN/JOIN-ACK/LEAVE message is sent infrequently,
collisions caused by more than two simultaneous JOIN/JOINACK/LEAVE transmissions in one slot are negligible.
IV. A NALYTICAL M ODEL
A. Voice service capacity analysis
In order to guarantee QoS of voice traffic, it is critical to
have appropriate call admission control. Call admission control
is responsible for admitting or rejecting a new voice call based
on the available resources, to ensure that the QoS requirements
(e.g., delay and packet loss rate) of all the admitted voice
calls are satisfied. To facilitate call admission control, it is
essential to obtain the system capacity. In cognitive networks,
the system capacity for the secondary users is related to the
number of primary users and their activities, and also related
to the performance of the cognitive medium access scheme. In
this section, we present an analytical model to derive the voice
service capacity for aforementioned two proposed cognitive
medium access schemes. For simplicity, we assume that the
channel sensing at the physical layer always provides a correct
outcome (our model can be easily extended to the case where
0,0
1
Ps
Ps
1- P s
1,0
Ps
1- P s
1,1
1,2
. . .
1- P s
1,T a-1
. . .
1- P s
2,2T a-1
Ps
Ps
Ps
1- P s
2, T a
2,T a+1
1- P s
1- P s
2,T a+2
Ps
Ps
1- P s
3,2T a
.
.
.
.
.
.
n -1,T i
1- P s
n -1,T i +1
...
n-1, T b -T a
Ps
n ,T j
..
n -1,T j -1
1
1- P s
n ,T j +1
Fig. 2.
...
n,T b -1
The state transition diagram.
the channel sensing error exists). Without loss of generality,
we assume that one time slot is used to transmit one voice
packet. Let Ts denote the time of one slot. For presentation
simplicity, we set Ts = 1. Denote the voice packet inter-arrival
time and the voice packet delay bound as Ta and Tb (both in
the unit of time slot), respectively.
We arbitrarily choose a secondary voice user as the tagged
user. Define state (n, t), where n is the number of voice packets
in the queue of the tagged user, and t is queueing delay (in the
unit of time slot) experienced by the voice packet that is at the
head of the queue of the tagged user. The initial state is (0,0),
indicating that there is no voice packet at the tagged user. As
constant rate voice traffic is considered, after a certain time
period (no more than the voice packet inter-arrival time Ta ),
the first voice packet will arrive at the tagged user. Therefore,
the state will move to (1,0) with probability 1. Since then,
after each time slot, the state will evolve, moving to another
state. The state transition process is modeled by a discretetime Markov chain, shown in Fig. 2. To describe this Markov
chain, we use state (i, tk ) (i > 1) as an example.
• When tk < i · Ta − 1, its next state is (i, tk + 1) if the
tagged user does not successfully transmit a voice packet
in the current slot, (i − 1, tk − Ta + 1) if the tagged user
successfully transmits a voice packet in the current slot.
Note that since voice traffic has a constant rate, the voice
packet inter-arrival time Ta is a fixed value. For any two
consecutive packets in the queue, the difference of their
queuing delay (i.e., the waiting time in the queue, not
including the time in packet transmission) equals to Ta .
• When tk = i · Ta − 1, its next state is (i + 1, tk + 1) if
the tagged user does not successfully transmit a voice
packet in the current slot, (i, tk − Ta + 1) if the tagged user
successfully transmits a voice packet in the current slot.
This is because when the delay of the packet at the head
of queue equals to i · Ta − 1, a new voice packet will arrive
after one time slot.
Let Ps denote the probability that the tagged secondary user
successfully transmits a voice packet in a randomly chosen
time slot, given that the tagged user has packets to send,
and P (n1 , t1 |n0 , t0 ) denote the transition probability from state
(n0 , t0 ) to state (n1 , t1 ). In the Markov chain, the one-step
transition probabilities are

P (1, 0|0, 0) = 1







P (1, tj + 1|1, tj ) = 1 − Ps ,






P (0, 0|1, tj ) = Ps ,






P (2, tj + 1|1, tj ) = 1 − Ps ,






P (1, 0|1, tj ) = Ps ,



P (i, tj + 1|i, tj ) = 1 − Ps ,





P
(i − 1, tj + 1 − Ta |i, tj ) = Ps ,






P (i + 1, tj + 1|i, tj ) = 1 − Ps ,






P (i, tj + 1 − Ta |i, tj ) = Ps ,






P
(i − 1, Tb − Ta |i, Tb − 1) = 1,





P (i, Tb − Ta |i, Tb − 1) = 1,
tj < T a − 1
tj < T a − 1
tj = Ta − 1
tj = Ta − 1
tj < iTa − 1, i ≥ 2
(1)
tj < iTa − 1 , i ≥ 2
tj = iTa − 1, i ≥ 2
tj = iTa − 1, i ≥ 2
if
Tb < iTa − 1
if
Tb = iTa − 1.
Given all the one-step transition probabilities of the Markov
chain listed in (1), the steady state probability vector of the
Markov chain can be obtained. Denote π(i, tj ) as the steady
state probability of state (i, tj ).
Next, we derive Ps for the two proposed cognitive medium
access schemes. The voice packet arrival rate of each secondary user is T1a packets per slot. The voice packet service
rate of each secondary user is denoted as µ packets per slot.
Thus, the queue utilization of the secondary user is given by
ρ = T 1·µ . As aforementioned in Section II, at each time slot, a
a
primary user will not use the channel with probability Pi . Thus,
the probability that the channel is available for the secondary
p
user to access Pidle
= (Pi )Np , where Np is the total number of
primary users.
For the contention-based medium access, Ps is given by
p
Ps = Pidle
·
CW
X
k=1
1
CW − k ρ·(Ns −1)
·(
)
CW
CW
(2)
where Ns is the total number of secondary users, and CW is
the contention window size of the secondary users. The term
in the summation indicates the probability that the tagged user
chooses a backoff timer value k, and other active secondary
users (i.e., the users whose queues are not empty) choose
backoff timer values larger than k.
For the contention-free medium access, Ps is given by
p
Ps = Pidle
·
Ns
X
1
· (1 − ρ)i−1
N
s
i=1
(3)
where the term in the summation indicates the probability that
for a randomly chosen slot, the tagged user has mini-slot index
i, and all other secondary users with mini-slot index smaller
than i have no packet to transmit.
To determine ρ, we need to characterize the average service
time of a voice packet µ1 . Given that the voice packet at the
head of the queue has already waited tj slots, the average time
(in the unit of slot) needed to serve this packet Ts (tj ), i.e., the
time to let the packet leave the queue (either due to successful
transmission or due to packet dropping), is given by
X
(1 − Ps )k−1 · Ps · k + (1 − Ps )Tb −tj −1 · (Tb − tj ).
(4)
k=1
The average service time of a voice packet
1
=
µ
1
µ
is derived as
P Tb −1
tj =0 π(m(tj ), tj ) · Ts (tj )
P Tb −1
tj =0 π(m(tj ), tj )
(5)
Pdrop =
tj =0
π(m(tj ), tj ) · (1 − Ps )Tb −tj
.
P Tb −1
tj =0 π(m(tj ), tj )
(6)
Note that Pdrop is a function of Ns , denoted by Pdrop (Ns ). In
order to guarantee the QoS of voice traffic, Pdrop (Ns ) should
not exceed the voice packet dropping rate bound Pl . Thus, the
capacity for voice service of secondary users is the maximum
integer Ns (denoted by Ns∗ ) satisfying Pdrop (Ns ) ≤ Pl .
B. Average overhead
In addition to voice service capacity, the other important
performance metric of the proposed cognitive medium access
is the average overhead, which is measured as the average
number of mini-slots needed in each slot before a successful
transmission.
Let Oc and Of denote the average overhead of the
contention-based and the contention-free medium access
schemes, respectively. We have
Oc =
Ns X
Ns i=1
i
· ρi · (1 − ρ)Ns −i ·
CW
X
j=1
i·
CW − j i−1
1
·(
)
·j ,
CW
CW
Ns where the term i ·ρi ·(1−ρ)Ns −i indicates the probability that
out of Ns , i secondary users are active in a random slot, and
the term in the second summation indicates that j mini-slots
are needed before a successful transmission when an active
secondary user chooses a backoff timer value j , and other
active secondary users choose backoff timer values larger than
j.
For the contention-free medium access, we have
Of =
P Ns
i=1
P Ns
ρ · (1 − ρ)i−1 · i
i=1
ρ · (1 − ρ)i−1
25
20
15
10
5
where m(tj ) is the corresponding number of voice packets in
the queue, given that the queueing delay of the voice packet at
the head of the queue is tj . With a constant voicelpacket
m arrival
. Given
rate T1a , it is straightforward to have m(tj ) = tjT+1
a
equations (1), (4), and (5), along with equation (2) or (3),
Ps can be solved numerically for the two proposed cognitive
medium access schemes. The steady state probability vector
for the Markov chain can further be obtained.
Let Pdrop denote the voice packet dropping probability. Pdrop
can be expressed as
P Tb −1
Contention, Analysis
Contention, Simulation
Contention−free, Analysis
Contention−free, Simulation
30
The cognitive voice capacity
Tb −tj −1
Ts (tj ) =
35
,
where the denominator is the probability of a successful
transmission in a randomly chosen slot, and the term in the
summation in the numerator indicates that the number of minislots needed before a successful transmission is i when all the
0
0
Fig. 3.
5
10
15
20
25
30
35
40
The number of primary users
45
50
55
The cognitive voice capacity with Pi = 95%.
secondary users with mini-slot index smaller than i have no
packet to send, and the secondary user with mini-slot index i
has packet to send.
V. N UMERICAL R ESULTS AND D ISCUSSIONS
We validate our analytical results by simulations using
Matlab. The simulation for each run consists of 10000 time
slots. Without loss of generality, we choose Ta = 40 and
Tb = 450. The voice packet dropping rate bound Pl is set as
1%. We vary the other parameters such as Np , Pi , and CW to
investigate their impact on the voice service capacity.
A. Voice Service Capacity
First, we fix the value of Pi as 95% and the contention
window size CW as 40 mini-slots, and vary the number Np
of primary users in the system. The cognitive voice service
capacity (i.e., the maximum number Ns of secondary voice
users that can be supported with QoS guarantee) is calculated
by using the proposed analytical model. The analytical results
are shown in Fig. 3. It can be seen that when the number of
primary users increases, the capacity of secondary voice users
decreases due to the reduced available channel resources. The
capacity of the contention-free medium access is larger than
that of the contention-based medium access. The reason is
that the contention-free medium access utilizes the channel
more efficiently than the contention-based medium access
by eliminating collisions, resulting in a larger capacity. The
simulation results are also shown in Fig. 3. In the simulation,
if a voice packet cannot be delivered within the delay bound Tb ,
it will be dropped. Voice capacity is obtained as the maximum
number of voice users such that the voice packet dropping
rate is less than Pl . As shown in Fig. 3, the simulation results
match well with the analytical results in all cases.
Next, we change Pi from 95% to 92%, the corresponding
capacities of the two proposed medium access schemes are
shown in Fig. 4. It is obvious that with a lower Pi , fewer
channel resources are left for the secondary users, leading to a
smaller capacity. Again, the simulation results conform to the
analytical results.
For the contention-based medium access, the choice of CW
also has impact on the capacity. Fig. 5 compares the capacity
TABLE I
30
Contention, Analysis
Contention, Simulation
Contention−free, Analysis
Contention−free, Simulation
The cognitive voice capacity
25
T HE AVERAGE OVERHEAD OF THE TWO PROPOSED COGNITIVE MEDIUM
ACCESS
Ns
Contention based
CW = 20
Contention based
CW = 40
20
15
10
Contention-free
Analysis
Simulation
Analysis
Simulation
Analysis
Simulation
5
10.2
9.9
19.9
20.7
2.9
2.6
10
9.6
9.4
18.9
18.9
5.2
5.2
15
8.8
8.9
17.3
16.0
7.2
7.2
20
6.9
6.8
14.4
13.3
8.8
9.2
5
0
0
Fig. 4.
5
10
15
20
25
30
35
40
The number of primary users
45
50
55
The cognitive voice capacity with Pi = 92%.
30
CW=40, Analysis
CW=40, Simulation
CW=20, Analysis
CW=20, Simulation
The cognitive voice capacity
25
20
15
10
VI. C ONCLUSION
5
0
0
medium access, the average overhead decreases when the
number of secondary users increases. The reason is that with a
fixed CW size, the larger the number of contenders, the more
chances that the winner (i.e., the contender with the smallest
backoff timer) will choose a smaller backoff timer. For the
similar reason, with a smaller CW size, the average overhead
for the contention-based medium access will be smaller, as
shown in Table I. Note that this result does not mean that a
small CW is favored for the contention-based medium access.
As small CW may cause serious collisions, the resources
wasted by collisions should also be taken into account when
choosing CW .
10
20
30
40
The number of primary users
50
Fig. 5. The cognitive voice capacity of the contention-based medium access
with Pi = 95% and different CW
for the contention-based medium access with two different CW
sizes. When the number of primary users is small, a relatively
large number of secondary users can be admitted. A larger CW
can effectively reduce the collisions among them, resulting in a
slightly larger capacity. However, with the increase of primary
users, less secondary users can be admitted into the system
(e.g., less than 7 secondary users can be admitted when the
number of primary users is larger than 30 from Fig. 5). With a
small number of contenders, collisions rarely occur. Therefore,
a larger CW cannot help to increase the capacity. Note that
with a small size of CW , the collision probability may be high,
while a large size of CW can alleviate the collisions. However,
the overhead also increases. Therefore, there exists a tradeoff.
How to choose an optimal CW size is an interesting problem
and will be studied in our further research.
B. Average Overhead
To obtain the average overhead (i.e., the average number of
mini-slots needed before a successful transmission), we fix the
value of Np as 5, Pi as 95%, and vary the number of secondary
users in the system. Table I compares the analytical and
simulation results of the average overhead of the two proposed
medium access schemes. For the contention-free medium
access, when the number of secondary users increases, the
average overhead increases since more mini-slots are needed
to accommodate the secondary users. For the contention-based
In this paper, we have proposed two different cognitive
medium access schemes to support the cognitive voice service in presence of primary users. The analytical model is
developed to investigate the performance of the two cognitive
medium access schemes, including voice service capacity and
average overhead. The correctness of the analytical model
is verified by the simulation. This work can provide helpful
insights to voice service support over cognitive radio networks.
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