Distributed Coordination protocol for Adhoc cognitive radio networks

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JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 14, NO. 1, FEBRUARY 2012 51

Distributed Coordination Protocol for Ad Hoc Cognitive

Radio Networks

Mi-Ryeong Kim and Sang-Jo Yoo

Abstract: The exponential growth in wireless services has resulted

in an overly crowded spectrum. The current state of spectrum al-

location indicates that most usable frequencies have already been

occupied. This makes one pessimistic about the feasibility of inte-

grating emerging wireless services such as large-scale sensor net-

works into the existing communication infrastructure. Cognitive

radio is an emerging dynamic spectrum access technology that

can be used for flexibly and efficiently achieving open spectrum

sharing. Cognitive radio is an intelligent wireless communication

system that is aware of its radio environment and that is capa-

ble of adapting its operation to statistical variations of the radio

frequency. In ad hoc cognitive radio networks, a common control

channel (CCC) is usually used for supporting transmission coor-

dination and spectrum-related information exchange. Determining

a CCC in distributed networks is a challenging research issue be-

cause the spectrum availability at each ad hoc node is quite differ-

ent and dynamic due to the interference between and coexistence

of primary users. In this paper, we propose a novel CCC selection

protocol that is implemented in a distributed way according to the

appearance patterns of primary systems and connectivity among

nodes. The proposed protocol minimizes the possibility of CCC dis-

ruption by primary user activities and maximizes node connectiv-

ity when the control channel is set up. It also facilitates adaptive

recovery of the control channel when the primary user is detected

on that channel.

Index Terms: Ad hoc networks, cognitive radio, common control

channel (CCC), coordination protocol.

I. INTRODUCTION

Wireless spectrum licensing on a long-term basis is currentlyunder way over vast geographical regions. In order to address

the critical problem of spectrum scarcity, the federal commu-

nications commission (FCC) has recently approved the use of

unlicensed devices in licensed bands [1]. The report published

by the spectrum policy task force of the FCC in 2002 [2], which

was aimed at improving the way of utilizing the spectrum re-source, catalyzed intensive research activities in this new field

of open spectrum sharing. Consequently, dynamic spectrum ac-

cess (DSA) techniques have been proposed to solve spectrum

inefficiency problems. Cognitive radio (CR), a term first coinedby J. Mitola III in 1999 [3], is a promising approach for flexi-

bly and efficiently achieving open spectrum sharing [4], [5]. A

CR system is an intelligent wireless communication system that

is aware of its radio environment and is capable of adapting its

Manuscript received April 27, 2010; approved for publication by BrookeShrader, Division III Editor, October 20, 2010.

This work was supported by the Inha University Research Grant.The authors are with the Graduate School of Information Technology and

Telecommunications, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon402-751, Korea, email: [email protected], [email protected].

operation to statistical variations of the radio freqeuncy of in-

coming signals. IEEE 802.22 [6] is the first standard based on

CR. IEEE 802.16h [7] is expected to implement CR functions

in worldwide interoperability for microwave access (WiMAX)networks for facilitating the coexistence of homogeneous and

heterogeneous networks. A number of cognitive radio test-beds

based on different architectures and radio technologies have

been developed [8], [9]. Since most of the useful spectrum is

already assigned to primary systems (licensed systems), to uti-lize the spectrum holes of the licensed bands, CR devices should

be capable of detecting primary user signals on the licensed pri-

mary channels and not lead to harmful interference to the pri-

mary system users. Research on CR covers a wide range of ar-

eas, including spectrum analysis, channel estimation, spectrumsharing, medium access control (MAC), and routing. However,

CR networks pose unique challenges owing to high fluctuations

in the available spectrum as well as the diverse quality of service

(QoS) requirements [5]. Specifically, in ad hoc networks based

on CR, the distributed multi-hop architecture, the dynamic net-

work topology, and the variation of the spectrum availability

with time and location are key distinguishing factors. CR net-

works are different from traditional ad hoc networks in the sense

that the CR networks can opportunistically utilize various spec-

tral holes for smooth peer-to-peer communications by virtue ofthe unique CR functionalities [10].

The ad hoc cognitive radio network requires reliable con-

trol and spectrum management message exchanges between

neighbor nodes to communicate local sensing results, set updata channels, immediately notify incumbent detection events,

announce spectrum handover, and so on. These message ex-

changes are usually assumed to be carried out on a dedicated

control channel, the so-called common control channel (CCC)

[5]. However, in many cases, we cannot guarantee that dedi-cated CCCs can be set up for all different cognitive radio net-

works, and further, dedicated control channels are wasteful of

channel resources when there is no CR user. Therefore, a dy-

namic and distributed method is required for CCC selection inad hoc CR networks. In this paper, we propose a novel CCCselection protocol (distributed coordination protocol for a CCC

(DCP-CCC)) in a distributed way, based on the appearance pat-

terns of the primary system (PS) and network connectivity. The

proposed protocol provides a decentralized cluster-based archi-

tecture to form a large-scale network, and it provides mecha-nisms to adapt the network topology to network and radio en-

vironment changes. Preliminary results of the proposed proto-

col were presented in [11]. In this paper, we have extended the

cluster management methods (cluster merge, cluster intercon-

nection, and common channel change schemes) and have inten-

sively studied the performance of the proposed protocol for var-

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52 JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 14, NO. 1, FEBRUARY 2012

ious network conditions.The rest of this paper is organized as follows. Related work

is discussed in Section II. The proposed DCP-CCC protocol is

presented in detail in Section III. Cluster management is pre-

sented in Section IV. The performance of the proposed protocol

is studied in Section V. Finally, we conclude this paper in Sec-tion VI.

II. RELATED WORK

Most of the existing distributed CR network protocols for

MAC and routing assume that there exists a dedicated CCC.However, such protocols are in conflict with the opportunistic

nature of cognitive radio, and moreover, most of the available

spectrum has already been allocated to existing communication

systems. Therefore, it may not be feasible to assign dedicated

CCCs to all possible types of CR networks in the future. SomeCR systems use an unlicensed band, such as industrial, scien-

tific, and medical (ISM) bands, or ultra-wide bands (UWBs) for

the control channel. In this case, the unlicensed band is alreadyovercrowded and suffers strong interference from existing un-

licensed band users; therefore, reliability, which is one of themost important features of the control channel, cannot be guar-

anteed [12]. In the absence of a dedicated CCC, a distributed

and dynamic self-configurable procedure is required for setting

up a common channel.

A CCC can be classified as an in-band CCC or out-of-band

CCC, depending on whether the control channel shares the data

channel or uses a dedicated spectrum, respectively [13]. In the

in-band CCC, the current data transmission channel is also usedfor exchanging control messages. Since each node pair may use

a different channel for communication, the CCC is generallylimited to the corresponding communication pair (i.e., local cov-

erage). Therefore, the out-of-band CCC is widely considered for

ad hoc CR networks. Some studies assume that there exists acommon channel with global coverage available for all nodes in

ad hoc CR networks. The network nodes also have the capabil-

ity determine and use an alternate global control channel if the

original control channel is jammed [14][16]. Two neighboring

nodes can exchange available channel information or negotiatechannel assignments of a link via the control channel. In [16]

and [17], the authors propose a global control channel to carry

out the MAC-layer control mechanisms in the open spectrum

sharing paradigm which is so popular so that it is not required to

be explained further. The C-MAC protocol presented in [18] isbased on the dynamic channel assignment protocol [19]. These

protocols use the packet-based channel assignment approach.

However, it is difficult to find a globally common channel in

a network, and there are frequent individual spectrum changes

even if a CCC covering the entire network is found. Changingthe CCC frequently leads to drastic communication overhead.

There is high possibility that the global CCC can suffer denial of

service (DoS) attacks [20]. Therefore, global CCC approaches

are unlikely to be applicable to real CR network scenarios.

Since the probability that a global CCC is available to ev-

ery node in the network is small, CR users can be grouped into

clusters and a CCC may be used for the nodes in the same clus-ter. In this case, the selected CCC has only group coverage.

Fig. 1. Proposed ad hoc CR network architecture.

In [14], Zhao et al. proposed a distributed coordination scheme

for spectrum allocations that addresses the spectrum variability

problem without using a global control channel. Zhao et al.s

scheme uses a group coordination channel instead. In [21], a

close group of users form a sub ad hoc network and select achannel for communicating control information. If the primary

user of the channel returns, then a different channel that is avail-

able to everyone in the available sub-group channels is chosen.

It is assumed that one of the members of the group has the ca-

pability to connect to the neighboring groups. In CogMesh [10],

a cluster-based framework is used to form a wireless mesh net-

work in the context of open spectrum sharing. The study in [10]

investigates the issues involved in setting up an ad hoc open

spectrum sharing network that coexists with primary users of

the spectrums and proposes a decentralized cluster-based archi-tecture to form a large-scale network. The basic unit of the net-

work is a cluster, which is a sub-network formed by a group of

neighbor nodes sharing common channels and coordinated by aselected node (called cluster head) in the cluster.

On the other hand, some proposed MAC protocols do not re-

quire separate control channels for the purpose of control signal

exchange. Using a single CCC may introduce control channelsaturation that degrades the overall performance of the network.

In [22], SYN-MAC was proposed for avoiding the use of the

CCC. The main idea is to divide the total time into fixed-time

intervals, each corresponding to one of the available channels.

At the beginning of each time slot, all nodes in the network lis-

ten to a channel whose time slot is equal to the time required

for the exchange of control messages. However, this scheme

requires global time synchronization. Synchronization in an ad

hoc network is a challenge owing to the absence of centralized

coordination.

III. DCP-CCC: DISTRIBUTED COORDINATION

PROTOCOL FOR COMMON CONTROL

CHANNEL SELECTION

A. Network Architecture and Design Requirements

Fig. 1 shows the network environments of cognitive radio

ad hoc networks. We assume that there is no fixed dedicated

CCC and that the average spectrum idle time of the PSs is much

longer than the packet transmission time. As shown in Fig. 1,each node may have a different set of available channels after


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KIM AND YOO: DISTRIBUTED COORDINATION PROTOCOL FOR AD-HOC... 53

local spectrum sensing. In the proposed common channel selec-tion and maintenance protocol, we form clusters with desired

sizes, and within the cluster range, an optimal CCC is selected.

In order to choose a CCC, the DCP-CCC takes into account not

only the locally available spectrum bands of each node, but also

the temporal and spatial variations in the primary user spectrumactivities. In Fig. 1, the cluster head (CH) is a node that initi-

ates cluster formation for selecting the CCC. The actual cluster

nodes that share the same CCC are determined by the initially

desired cluster size and each nodes available channel set. Mul-

tiple clusters can be merged or interconnected by using gatewaynodes.

The proposed DCP-CCC has the following unique design fea-tures.

i) No global time synchronization: Achieving time synchro-

nization in ad hoc networks is not a trivial task and generally

requires complex device implementation or synchronization

protocols. Therefore, we assume that time synchronization isnot provided in the proposed method.

ii) Configurable cluster size: A cluster is a set of nodes that usethe same control channel. To increase the network connectiv-

ity, a cluster should include as many CR nodes as possible.

If the cluster size is too small, then an entire CR network canbe partitioned into many small clusters, and they may not be

connected to exchange the control messages of the MAC and

of the routing operations. Even if some of them can be inter-

connected by gateway nodes, the control channel switching

overheads of the interconnected clusters, which use differ-ent CCCs, increase the packet delivery time and decrease the

throughput. In this paper, the cluster size is adaptively con-

figurable when the CH initiates the control channel setup pro-

cedure. The actual cluster size can be smaller than the desiredsize when the optimal channel in terms of the proposed per-

formance criteria (average primary system appearance prob-

ability and idle time) is not available for all nodes in the de-

sired cluster.

iii) Fast common channel setup: In CCC-based MAC opera-

tions of ad hoc CR networks, before the CCC is setup, eachnode cannot send data packets to or receive data packets from

neighbor nodes. Exchanges pertaining to the data channel ne-

gotiation and transmission time schedule are performed on

the control channel. Therefore, the control channel setup time

should be minimized. Since nodes may have different sets ofavailable channels and since they can tune into different chan-

nels at a given time, ensuring fast message delivery for settingup a common channel is not a trivial task.

iv) New CCC selection criteria: A single cluster should include

as many nodes as possible while having a size less than thedesired cluster size, and the selected channel should have low

PS activities.

v) Common channel change mechanism upon the primary sig-

nal detection: When some of the cluster nodes have detected

the primary signals of the current CCC, then this informationshould be delivered to the neighbor nodes in the cluster to

make them perform local sensing. By considering the number

of interfering CR nodes, the cluster should decide whether all

cluster nodes should reselect the control channel or whetherthe interfering nodes should be isolated.

Fig. 2. CC IVT message format.

vi) Cluster merge and interconnection: For control message ex-

change or packet delivery between any nodes in the entiread hoc network, the clusters should be able to communicate

through CCCs. Clusters with the same CCC are merged, and

the clusters with different CCCs are interconnected by gate-

way nodes in the proposed protocol.

The proposed DCP-CCC can set up a wide-area CCC thatcan reach many CR nodes with a single control channel without

channel switching. In CogMesh [10], which is also a cluster-

based common channel CR network, a CH constructs a clus-

ter only with single-hop neighbors, and therefore, many gate-way nodes are required to interconnect other neighboring clus-

ters. Since gateway nodes should manage the data transmission

schedule for multiple clusters and frequently switch channels,

their use results in a decrease in the channel throughput. In

DCP-CCC, the CH selects an optimum control channel on thebasis of primary signal activities and the number of possibly

covered nodes, unlike CogMesh.

B. Common Control Channel Setup Procedure

To set up a CCC, CR nodes follow six steps: 1) Local sensing

and scanning, 2) common channel invitation, 3) cluster tree con-

struction, 4) common channel reporting, 5) CCC decision, and6) common channel advertisement. In the following subsections,the operations are explained in detail.

B.1 Local Sensing and Scanning

Each node i in a ad hoc CR network periodically or aperiod-

ically performs local sensing to determine the available chan-

nel set (Ca(i)) and primary channels. If a node needs to findan existing cluster to share the common channel, then it shouldscan all channels to possibly receive a common channel beacon

(CC BC) message from a neighbor node; CC BC is periodi-

cally broadcast by a CH. A node should listen to one channel

for a duration of at least Tp to receive CC BC. Tp is the time in-terval taken for beacon transmission from the CH. The CC BCincludes the CCC information. Therefore, once a node receives

the message, it can immediately tune to the indicated CCC and

start exchanging control messages. In this case, there is no ex-

plicit procedure for connecting to the CH. The local sensing andscanning of each channel may be done simultaneously.

B.2 Common Channel Invitation

If a node does not receive any CC BC during the scanning

procedure, then it can be inferred that there is no CCC or that the

existing CCCs are not available for the node. In this case, the

CR node initiates the cluster construction process by broadcast-ing a common channel invite (CC IVT) message to its neighbor


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Fig. 3. Broadcasting CC IVT via the CH.

nodes using carrier sense multiple access with collision avoid-

ance (CSMA/CA). The CC IVT message format is defined as in

Fig. 2. The CC IVT can be rebroadcast until the desired clustersize is reached.

Transmitting node ID: ID of the node that transmits theCC IVT.

CH node ID (cluster head node ID): ID of the node that ini-tially generated the CC IVT. When a CH sends the CC IVT

at first, Transmitting node ID and CH node ID are the

same.

SEQ number (sequence number): Whenever a CH sends aCC IVT to find a new CCC, it increases the SEQ number

by 1. This number is for uniquely identifying CC IVT mes-

sages; periodic CC IVT repetitions have the same SEQ num-ber.

Hop count: The desired maximum hop counts (cluster size)from the CH to retain the same CCC. When a node receives

a CC IVT, it decreases the hop count by 1, and it broadcasts

it again only if the count is not zero.

Candidate channel list (CCL): This list contains the candidatechannels that can possibly be used as the CCC. Whenever a

node rebroadcasts a CC IVT, the CCL can be updated.

If a CH node broadcasts the CC IVT message only on a sin-gle channel, then some neighbor nodes cannot receive the mes-

sage when the channel used to broadcast the CC IVT is not

available to these neighbor nodes. Further, because each CRnode tunes into one of the available channels at a given time,

the neighbor nodes may not be able to receive the message even

though the channel is available to the neighbor nodes. Therefore,

a CH sequentially broadcasts a CC IVT on each available chan-

nel using a single transceiver, as is shown in Fig. 3. If the CHhas multiple transceivers, then the available channels are subdi-

vided among the different transceivers. After a duration ofTp,

the CH rebroadcasts the CC IVT with the same SEQ number.

Each neighbor node should wait at least for a duration ofTp foreach of its available channels. If a neighbor node does not re-

ceive the CC IVT, then it switches to the next channel on its

local available channel list.

B.3 Common Channel Cluster Tree Construction

After a node receives a CC IVT from its neighbor node, itstores the candidate channel list of the CC IVT and the node

ID of the transmitting node. When node i receives a CC IVT

from neighbor node j, it determines the overlapping common

channel set Cc(i, j) from its local available channel list Ca(i)and the CCL of the received CC IVT, CIVTc (j), from node j.

Cc(i, j) = Ca(i) CIVT

c (j). (1)

If node i first receives a CC IVT message from node j and

Cc(i, j) = , then node i records nodej as a parent node. Notethat within a cluster, each node can reach the CH through its par-

ent node using one of the overlapping common channels. Then,

node i rebroadcasts the CC IVT with an updated CCL as indi-

cated in (2).

CIVTc (i) = Cc(i, j) = Cc(i, j) = Ca(i) CIVTc (j) (2)

where Cc(i, j) is the set of channels in the CCL field rebroad-cast by node i after receiving a CC IVT from node j. After de-

creasing the hop count of the received CC IVT, unless the hop

count is zero, node i sequentially and periodically rebroadcasts

the CC IVT message on all available channels. If there are nooverlapping channels between its locally available channels and

the received CCL from node j, then it indicates that node i and

node j do not have any common channels; therefore, they are

not able to setup a CCC. In this case, node i simply discards the

CC IVT.

If node i receives additional CC IVT messages satisfying thefollowing three conditions from any other neighbor node k, then

it simply discards them.

i) The same CH node ID with the previously received CC IVT

from node j.ii) The same SEQ number.

iii)

Cc(i, k) for all parent nodes j

of node i

Cc(i, j). (3)

If some channels ofCc(i, k) were not included in the previ-ous CC INV forwarded, then it indicates that node i can have

new channels that can be used as a CCC; these new channels are

not selected on the basis of CC IVTs sent by the previous par-

ent nodes. Therefore, if the condition in (4) is satisfied despitethe CH node ID and SEQ number of the CC IVT from node ibeing the same as those in the CC IVT from the previous parent

nodes, then node k becomes a new parent node of node i and

node i rebroadcasts the CC IVT from node k to its neighbor

nodes with the CCL in (5).

Cc(i, k) for all parent nodes j

of node i

Cc(i, j), (4)

CIVTc (i) = Cc(i, k)

= {Ca(i) CIVT

c (k)} for all parent nodes jof node i Cc(i, j).(5)

Fig. 4 explains the CC IVT message transmissions and

presents an exampleof tree construction. As shown in thefigure,

the CC IVT message is propagated until the desired cluster size

is reached, and the common channel cluster tree is constructed

accordingly. When there is no neighbor node to rebroadcast

the CC IVT or when the neighbor nodes cannot receive theCC IVT, the message cannot be propagated further even if the

desired cluster size is not reached. It should be noted that un-

like the conventional multicasting tree, a node can be involved

in multiple tree paths because the common control tree is builtaccording to channel availability. The numbers in brackets for


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Fig. 4. CC IVT transmission and tree construction.

each node are the available channels of that node. Assume thatnode A is the CH that generated the CC IVT message. Node B

receives the CC IVT message from node A on one of its avail-

able channels (channel 1 or channel 2) and computes the can-

didate channel list, which consists of the overlapping available

channels between node A and node B, as indicated in (2). Then,node B rebroadcasts the CC IVT message (in Fig. 4, CC IVT

(3) message). For channel 1 and channel 2, node B records node

A as a parent node. On the other hand, node D cannot receive

the CC IVT from node A because it does not have any avail-

able channels overlapping with node A. When node B receivesCC IVT (2) from node C, since node B has already set up a

common channel tree for the channel set {1, 2}, node B simplyignores the message.

As shown in Fig. 4, node H receives two CC IVT messages.We assume that the CC IVT from node F arrives later than the

CC IVT from E. Node H discovers that the CH node ID and

the SEQ number of each of the CC IVTs are the same, but the

CCLs are different. At node H, first, the candidate channel list

with parent node E is derived as {1, 2} using (2), and when nodeH receives the list a little later in the CC IVT from F, the CCL

with parent node F is newly derived as {3} using (5). Therefore,node H has two parent nodes: Node E for channel set {1, 2}and node F for {3}. When node K finally receives the CC IVT,the hop count reaches 0, and therefore, node K stops CC IVT

broadcasting.

B.4 Common Channel Reporting

Common channel report (CC RPT) message is used to de-

liver CCC information on the path through which the CC IVT

is delivered, to the CH to help in deciding the final common

channel. For the following two cases, a node stops broadcastinga CC IVT and periodically sends back a CC RPT message to

Fig. 5. CC RPT message format.

its parent nodes.1) A node receives a CC IVT and the hop count reaches zero.2) A node has broadcast a CC IVT, but has not received any

CC RPT message during a certain time TR

TR = (R Hop Count)TW (6)

where R Hop Count is the hop count at the node when it hasbroadcast the CC IVT and TW indicates the maximum waiting

time to receive a CC RPT from one hop neighbor node. The

CC RPT message format is illustrated in Fig. 5. Transmitting node ID: ID of the node that transmits the

CC RPT.

CH node ID: ID of the CH node that initially generatedCC IVT.

SEQ number: The same SEQ number as that of the receivedCC IVT.

Tuple {Ch, Pp, Tpi, NUMC}: For each candidate com-mon channel on the path to the CH, the following definitions

hold.

Ch: Candidate CCC Pp: Accumulated PS appearance probability on the chan-

nel Ch Tpi: Accumulated average PS idle time on the channelCh

NUMC: Accumulated number of connected nodes thatcan use the channel Ch as a CCC

Each node in ad hoc CR networks senses the channels and

maintains a channel status table that contains the PS appearanceprobability (Pp) and the average PS idle time (Tpi) of each chan-

nel. In this paper, we utilize the PS appearance probability and

idle time statistics for the CCC decision. As we mentioned in

the subsection on design requirements, to ensure that the com-

mon channel in a cluster is used by as many nodes of the clusteras possible, information on the number of nodes on the channel

tree to the cluster head is included in the CC RPT message.Fig. 6 shows an example of a CC RPT delivery procedure.

When nodes I and K receive CC IVT messages, the hop count

reaches zero, so nodes I and K cease to broadcast the CC IVT

and respond with CC RPT messages to their parent nodes. Af-

ter node C sends a CC IVT, it does not receive any CC RPT

during a certain time (i.e., 2TW). Therefore, node C initiatesa CC RPT response message. The CC PRT message is deliv-

ered to the parent node on the channel cluster tree to the CH.Note that the tree formation from the CH to nodes with a hop

count within a predetermined value is already performed upon

the delivery of the CC IVT message, so that nodes can send a

CC RPT to only their stored parent nodes on available chan-nels. Each node on the common channel cluster tree adds the


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Fig. 6. CC RPT transmission example.

PS appearance probability and the average primary idle time of

each channel to the received CC RPT message. In Fig. 6, node

B adds its local measurement information on channel 1 to that

of CC RPT (2) from node G, so that the tuple of channel 1 ofCC RPT (3) is {1, 0.8, 25, 3}. Node H has two different parentnodes, E and F, for the candidate channel sets {1, 2} and {3},respectively. Therefore, the channel information on {1, 2} and{3} is delivered to the CH through node E and node F, respec-tively. In the proposed mechanism, the channel and connectiv-ity information of each node inside the desired cluster size is

delivered to the CH through a unique path so that each nodes

information is not counted as duplicate content. Fig. 7 shows the

pseudocode for our proposed CC RPT transmission scheme.

B.5 Common Control Channel Decision

A CH maintains a candidate CCC status table. Whenever the

CH receives CC RPT messages, it updates the table. For eachchannel in the received CC RPT messages, Pp, Tpi, and the

connected nodes are accumulated. Finally, the CH divides Ppand Tpi by the number of connected nodes for each channel after

the CH has received a sufficient number of CC RPT messages,

or after the maximum waiting time has expired. The cluster head

can estimate the average PS appearance probability, average PS

idle time, and the total number of possibly connected nodes in-

cluding CH itself for each candidate CCC. Therefore, the CHcan identify a CCC based on the PS activities and determine the

number of nodes that can be connected together via a common

channel. Table 1 shows the status of each candidate channel for

the example scenario in Fig. 6.Depending on the importance of each parameter, the CH can

Fig. 7. Pseudocode for the common channel reporting procedure.

Table 1. Candidate CCC status for a scenario corresponding to the

table in Fig. 6.

Channel ID Average Pp Average Tpi Connected nodes

1 0.21 8 8

2 0.24 7.6 5

3 0.18 7 5

Fig. 8. Procedure for the CCC decision.

determine the optimal CCC. Various decision rules can be ap-

plied (e.g., a cost function with different weights for three pa-

rameters). In this paper, the purpose of determining a CCC is

that as many CR nodes as possible should be able to share the

same CCC as long as the primary activity values are less than thepre-defined thresholds. As shown in Fig. 8, in step 1, the chan-

nel set CSP is determined by considering the PS appearanceprobability, and the average PS idle time is considered in step

2. From the channel set CST, the channel with the largest num-

ber of connected nodes is finally selected for use as the CCC.After step 1 or step 2, if the number of channels determined is

less than 2, then the channel with the minimum Pp (in step 1) or

maximum Tpi (in step 2) is selected.

B.6 Common Control Advertisement

After the CH determines the optimal CCC, information on the

selected CCC is sequentially broadcast with a common channel

advertisement (CC ADV) message to the nodes in the clusterhop count on all available channels. After a sufficient number of

CC ADV transmissions have been completed, the CH switches

to the CCC and periodically broadcasts CC BC. The CC BC is

a message to indicate activation of the new CCC. The neigh-bor nodes that receive CC ADV sequentially rebroadcast the


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CC ADV along with their available channels and then switchto the CCC. Therefore, switching to the new CCC is performed

sequentially in the cluster. When a node receives a CC ADV

message, the selected CCC may not be available to the node.

In this case, the node discards the CC ADV and tries to find or

construct a new cluster by starting from procedure A (scanning).It should be noted that since ad hoc nodes can move without

notifications, the CH should periodically reinitiate the common

channel setup procedure.

In this paper, the proposed CCC selection algorithm relies on

many broadcast messages. Since radio signals are likely to over-lap with other signals in a geographical area, straightforward

broadcasting usually results in serious redundancy and collision,

which is referred to as the broadcast storm problem. To reduce

the number of broadcast packets and possible packet losses re-

sulting from collisions, several mechanisms have been proposedin the literature, such as a probabilistic scheme, a counter-based

scheme, and spanning tree clustering [23], [24]. Any broad-

cast storm avoidance mechanism can be used with the proposed

method.

IV. CLUSTER MANAGEMENT

A. Cluster Merge and Interconnection

Multiple CCC clusters organized in the network can be

merged or interconnected by gateway nodes. Cluster merge and

interconnection mechanisms facilitate seamless data exchanges

between any pair of nodes in the network by enhancing the net-

work connectivity. A node that already uses a CCC of a cer-

tain cluster may overhear another clusters CC ADV or CC BC

messages through one of the available channels during its nor-

mal operation. When both clusters use the same control channel,the overhearing node can be a gateway node to merge two clus-ters, as shown in Fig. 9(a). In this case, all ad hoc nodes in the

two clusters can exchange control messages by using a single

common channel. If the CCC in the overheard messages is dif-

ferent but one available to the node, then the node can intercon-

nect the two clusters via control channel switching, as shown in

Fig. 9(b).

In the case of cluster interconnection, if a single transceiver

is used to access a common control channel, the gateway node

needs to divide its resources between both clusters, as shown in

Fig. 9(b), so the neighbor nodes (nodes B and G) of the gatewaynode should know the exact time schedule of the gateway node.

The time schedule indicates when the gateway node switches tothe neighbor clusters CCC. Therefore, the greater the number

of gateway nodes in the network, the higher the cluster connec-

tion overheads and the lower the control channel throughput.In the proposed procedure for CCC selection, a CH selects a

CCC that can reach many nodes without gateway nodes. When

the CH selects a new control channel, it considers the number

of nodes connected to each candidate CCC, which is obtained

from NUMc in the CC RPT message. Another method to re-duce the gateway nodes in this paper is to broadcast a gateway

node announcement to its neighbors. If there are many overlap-

ping nodes between two clusters, we may have multiple candi-

date gateway nodes. Once any node assumes the gateway role,it broadcasts a gateway advertisement (GATE ADV) message

(a)

(b)

Fig. 9. Cluster merge and interconnection: (a) Cluster merge and (b)cluster interconnection.

to its neighbor nodes, and any neighbor node that receives the

message will not be a gateway node. The GATE ADV message

includes the CH IDs of both clusters.

B. Common Channel Change

Even though the CH selects the CCC after proper considera-

tion of the average PS appearance probability and idle time, thePS may suddenly use the CCC. Since the cognitive radio net-

work should avoid harmful interference with PSs, when nodes

detect the primary signal on the current CCC, the detection in-

formation should be conveyed to the neighbors. In addition, thedetecting nodes should stop using the CCC, to protect the PSs.

In the proposed mechanism, the primary system detection in-

formation is delivered to the CH, and the CH decides whether

the common channel should be changed. This decision will be

made on the basis of the cost of changing the CCC (i.e., mes-sage exchange overhead and delay) and the number of detecting

nodes in the cluster. If the CCC is not changed, then the de-

tecting nodes simply leave the cluster without exchanging mes-

sages. As mentioned in Section III, the detecting nodes may try

to form a new cluster or join one of the other existing clusters.

B.1 Primary Detection Notification

When a node detects a primary signal on the current CCC, it

sends a common channel detection (CC DET) message to the

CH on the present CCC and immediately returns to listen mode.

In this mode, a node can listen to a common channel signal, but

it is prohibited from sending messages on the channel. Even

though it is not on the routing path to the CH, any neighbor

node that overhears the CC DET message performs local sens-ing to determine whether it is inside the primary signal area. The

CC DET message includes the CH node ID, the SEQ number

(the same as the SEQ in CC ADV), information on the primary

detected CCC, and NUMd. NUMd is the number of detectingnodes on the routing path. At the first detecting node, NUMd is


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Fig. 10. Forwarding CC DET.

set to 1.

B.2 Forwarding CC DET Message

When a node receives a CC DET message, if it is the next

node on the routing path, then it forwards the message to the CH

node. If it is not on the routing path and has simply overheardthe message, then it performs local sensing. If the node detects

a primary signal on the CCC as a result of local sensing, then itinitiates the primary detection notification procedure by sending

a CC DET message. If the node is on the path and has also

detected a primary signal and if it has not previously forwarded

any CC DET message with the same SEQ and CH node ID,

then it increases NUMd by 1 and forwards the CC DET to theCH. If the node has already transmitted the CC DET with the

same SEQ and CH node ID to the same CH node or if it does

not detect a primary signal on the CCC, then it simply forwards

the message to the CH without increasing NUMd.Fig. 10 shows the CC DET forwarding procedure. Nodes A,

B, C, D, E, F, and G belong to the cluster using channel 1 asthe CCC; node A is the CH of that cluster. As soon as node F

detects the primary signal of channel 1, it transmits a CC DET

to its neighbor node E that is on the routing path to node A. Sincenode E is also within the range of the primary signal, it increases

NUMd by1. NodeB isnot influenced by the primary signal, andtherefore, it forwards the CC DET to node A without increasing

NUMd.

B.3 Common Channel Change

On the basis of the received CC DETs, the CH decideswhether it should change the cost CCC. To change the current

CCC, the CH should take into account of CCC in terms of thenumber of control message exchanges and the delay in form-

ing a new cluster. The percentage ofNUMd in the total numberof nodes in the cluster can be used to determine CCC changecriteria. If the CH decides to change the current CCC, then it

broadcasts a common channel change (CC CNG) message in

all available channels, including the new CCC. The new chan-

nel is selected from the candidate common channel set shown in

Table 1. If there is no candidate channel for the new CCC, thenthe CH initiates a new common channel construction procedure

by sending a CC IVT. When a node receives a CC CNG, it de-

creases the hop count and checks whether the hop count is zero.

If it is not zero, the node rebroadcasts the CC CNG on all avail-able channels. Then, the node switches to the new CCC.

Table 2. Simulation parameters.

Parameter Value Parameter Value

PS appearanceON-OFF model

CR nodes40 m

pattern transmission range

Back-offCSMA/CA

Number of50600

mechanism CR nodes

Number of grids 36 grids Channel pool 120 channelsSize of one grid 100 m 100 m Cluster size 15 hops

PS interference 4 neighboring, 2.0, 0.2

range grids

Number of14

% ofNUMd20%

primary systems for CCC change

B.4 Cluster Division

If a node in the listen mode is not able to hear the CC CNG

message from the CH until the CC CNG waiting time has ex-

pired or if the new CCC of the received CC CNG is not avail-

able to the node, then the node starts a new CCC setup proce-dure as discussed in Section III. Before it initiates a new cluster,

it should wait to check whether it receives any CC BCs from

neighboring clusters for joining one of the existing clusters. If

there is no CC BC matching the available channels, then it startsto create a new cluster by sending a CC IVT message. In this

case, cluster division can occur.

V. SIMULATION RESULTS

In this section, simulation experiments conducted to evalu-

ate the performance of the proposed algorithm are discussed.

For the simulation study, we implemented a CR network simu-lator completely in object-oriented C++, and the simulator in-

cludes a multi-channel system environment, channel sensing,

and CSMA/CA based data transmission. We employ the com-

mon channel selection algorithm of CogMesh [10], [25] for per-formance comparison. CR network devices are randomly placed

in a two-dimensional area 600 m 600 m, and the maximumtransmission range of a CR node is set to 40 m. We use the

ON-OFF model for the PS appearance pattern. The CSMA/CA

back-off mechanism is used when each node sends control mes-sages. The entire area is subdivided into 36 grids (66 grids), asshown in Fig. 11. PSs are located at the cross points of the grids,

and they affect the 4 neighboring grids. The channels used by

the PSs are randomly selected from the channel pool (CP). Thespecific simulation parameters are shown in Table 2. Fig. 11shows an example of a topology (300 CR nodes in the entire

network and 5 channels in the CP) considered in our simulation

study. Fig. 12 shows the corresponding PS appearances on grids

G1, G15, and G25. From the ON-OFF model, the activation and

idle time of the PSs are found to follow an exponential distribu-

tion with and . We set as 0.2 and as 0.4.

In the first experiment, we observe the average number ofclusters for different numbers of nodes. As shown in Fig. 13,

when the cluster size (CS) is set to 1, the proposed scheme

shows performance similar to that of CogMesh. However, as

we increase the cluster size, the number of generated clustersdecreases in the proposed method. Therefore, many CR nodes


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Fig. 11. Example of a simulation topology.

Fig. 12. PS appearances according to the ON-OFF model.

can have the same common channel. The number of channels in

the CP is fixed at 5 in Figs. 13 and 14.

The number of single-node clusters is compared for the differ-

ent methods in Fig. 14. A single-node cluster has only a single-

node that is acting as a CH. It can be considered as an isolatednode, and it cannot exchange control messages with neighbor

nodes. Therefore, a smaller percentage of single-node clusters

indicates better network connectivity. As shown in Fig. 14, the

proposed method results in a lesser number of isolated nodes.

In the CCC selection, it is desirable that the selected CCC

should be stable for the longest time possible. This stability canbe measured by determining the average PS appearance proba-

bility and the average idle time of the PS of the chosen CCC.For different CP sizes, ranging from 1 to 20 channels, the sta-

bility factors are compared (Figs. 15 and 16). The number of

nodes in the network is set to 150. Regardless of the total num-ber of channels, PS appearance probabilities determined by us-

ing CogMesh do not show a specific trend with increasing CP

for the selected CCC because it is not aware of the PS statis-

tics. When the network has a single channel, the two algorithms

show similar performances in selecting the CCC. However, asthe number of channels in the CP increases, the proposed proto-

col shows outstanding performance with regard to not only the

PS appearance probability but also the CCC idle time. In other

words, the proposed method needs less frequent CCC changescompared to the conventional method.

Fig. 13. Number of clusters.

Fig. 14. Number of isolated nodes.

Fig. 17 presents the number of clusters for different CP sizes.When the size of the CP is one, the network is reduced to a

single-channel network. Upon increasing the CP size, CogMesh

retains a similar number of clusters. On the other hand, the num-

ber of clusters is significantly reduced as the CPs are increased

in the proposed protocol because if the number of channels in-creases, then each cluster can find a CCC that can cover many

CR nodes with high possibility.In Fig. 18, we show the average delay of cluster formation. To

successfully set up a CCC for a cluster, some control message

exchanges (CC IVT, CC RPT, and CC ADV) are required in

the proposed method. When the cluster size is 1, the required

time is relatively short and similar to that of CogMesh. How-ever, as the cluster size increases, the path from a CH to the

last node on the cluster boundary also increases, so the average

delay increases. However, the larger the cluster size, the more

the nodes that can use the CCC and the lesser the number of

gateway nodes required.

Figs. 19 and 20 show the number of gateway nodes for clus-

ter merging and cluster interconnection, respectively. As thenumber of nodes in the network increases, we can observe that


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Fig. 15. Average PS appearance probability for the selected CCC.

Fig. 16. Average primary system idle time for the selected CCC.

the number of gateway nodes also increases. In the proposedmethod, as the CS increased, the required number of gateway

nodes for a given number of nodes in the network decreases

since each cluster contains more member nodes. The gateway

node should perform common channel switching, resource shar-

ing between multiple clusters, and frequent control message ex-

changes. Therefore, a small number of gateway nodes are re-quired if reliable network connectivity is supported. The pro-

posed method (with CS 2) always shows a lesser number ofgateway nodes than CogMesh, as can be observed in Figs. 19and 20, and the network connectivity is also superior to that of

CogMesh, as shown in Fig. 14.

Fig. 21 explains the CCC change trials performed by the CH

when the nodes detect PS appearances. In this experiment, the

number of nodes is 300, and if the number of detected nodes ina certain cluster is 5, 10, 15, 20, 25, or 30% of the total number

of nodes in the cluster, then the CH tries to change the exist-

ing CCC. In the proposed method, upon detecting PS appear-

ances, the detecting node in a certain cluster sends a CC DETmessage to its CH. After the CH receives several CC DETs,

(a)

(b)

Fig. 17. The number of clusters in different CPs: (a) CogMesh and (b)proposed.

Fig. 18. Average delay for cluster formation.

it should decide on the basis of the number of detected nodes

whether the cluster should change the current CCC. If a large

number of nodes detect the PS, then the CH selects a new CCCusing the aforementioned CCC selection algorithm. The x-axis


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Fig. 19. Number of gateway nodes that perform cluster merging.

Fig. 20. Number of gateway nodes that perform cluster interconnection.

of Fig. 21 represents the threshold percentage of detecting nodes

required to effect a CCC change. As expected, a lower thresh-

old percentage results in more CCC change trials. In fact, a CCC

change involves high cost in terms of convergence time, clusterre-formation, and message exchanges. Therefore, if the area of

the primary detected signal is relatively small, then local CCC

repair is preferred. In local CCC repair, if the nodes that trans-mitted CC DET do not receive any CCC change message within

a predetermined time, then they simply join one of the neighborclusters or start a new cluster formation process.

Fig. 22 shows the CCC switching delay. When a CH decides

to change the current CCC, it transmits a CC CNG message

and the message is sent to all the nodes in the cluster. In this

experiment, we measured the delay up to the instant at which

a node successfully changed the control channel after the CHannounced a CCC change. As can be observed in Fig. 22, as the

cluster size increases, the average delay also increases. Since the

CC CNG is sent from the CH, a node that is closer to the CH

can switch to the new control channel much faster than clusterboundary nodes.

Fig. 21. CCC change trials performed by the CH upon detecting the PS.

Fig. 22. Common control channel switching delay.

VI. CONCLUSION

In this paper, we have proposed an efficient common controlchannel selection protocol for ad hoc networks in the context of

CR. The control channel for CR operation is constructed in a

distributed way. To determine an optimal common control chan-

nel, we have defined a channel-based tree from the cluster head

to the nodes within a predetermined number of hops. The pro-posed distributed coordination protocol takes into account the

primary system activities of each channel and node connectivityfor common channel selection. We have also proposed a pri-

mary detection notification procedure to efficiently change the

common control channel. In a simulation study, we have eval-uated the protocol performance under various network condi-

tions. Compared with the conventional CogMesh CR clustering

method, the proposed DCP-CCC requires a smaller number of

clusters, and therefore, many CR nodes can communicate with

the same control link. Further, the proposed DCP-CCC enhancesthe control channel connectivity at the cost of an increase in the

delay in cluster formation. The most important contribution of

this paper is that the common control channel selected by us-

ing the proposed protocol is more reliable and stable since it isselected by considering channel activities of the primary sys-


12/12


tems. The proposed protocol also reduces the frequency withwhich control channels are changed and cluster re-formation.

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Mi-Ryeong Kim received the B.S. degree in Com-puter Science Engineering and M.S. degree in In-formation and Telecommunication Engineering fromInha University, Incheon, Korea, in 2008 and 2010,respectively. Her research interests include cognitiveradio network protocols and wireless sensor networks.Since 2010, she has been with Hyundai Mobis.

Sang-Jo Yoo received the B.S. degree in ElectronicCommunication Engineering from Hanyang Univer-sity, Seoul, Korea, in 1988 and the M.S. and Ph.D.degrees in Electrical Engineering from the Korea Ad-

vanced Institute of Science and Technology (KAIST),in 1990 and in 2000, respectively. From 1990 to 2001,he was a member of technical staff at the Korea Tele-com Research and Development Group, where heworked in the communication protocol conformancetesting and network design fields. From September1994 to August 1995 and from January 2007 to Jan-

uary 2008, he was a guest researcher at the National Institute of Standards andTechnology (NIST), USA. Since 2001, he has been with the Graduate Schoolof Information Technology and Telecommunications, Inha University, where heis currently a Professor. His current research interests include cognitive radionetwork protocols, seamless network mobility control, wireless network QoS,and wireless sensor networks.

Documents

Distributed Coordination protocol for Adhoc cognitive radio networks