Control Channel Efficiency in DDH-MAC 144

The CR technology aims to integrate the human intelligence in radio devices by making them aware, adaptive, smart, and decision making capable. In order to effectively perform the CR functionality, nodes must interact and cooperate with one another. This cooperation amongst CR nodes could only be established if they are adept at exchanging the knowledge learnt with other nodes on a well-known and centrally available common control channel. Robustness and security of the control channel are the two most challenging parameters of a CR MAC protocol. The existing design constraints for the common control channel are not smart enough to cope with the real time demands on cognitive radios. For example, there is no SUs‟ last resort in case of a PU occupancy on control channel. Also, if one control channel design category is used, the benefits of other categories could not be utilized. In other words, avoiding the drawbacks of one category creates certain limitations in the selected category.

To make cognitive radio fully functional and equipped to fulfil the real time demands of a cognitive radio network, we have proposed a novel MAC scheme which integrates the best features of all design strategies for control channel. The proposed scheme is robust against PU occupancy. The security of the proposed scheme has already been discussed in Section 5.2. We will briefly review some of the features of the control channel that make the proposed scheme more efficient.

6.4.1. Availability of More than One Control Channel

Equipping the CR with more than one control channel is a novel idea which has not been previously discussed. The existing literature either uses an assumed control channel, or spends more time on finding and converging on a local control channel. The CR network deploying our proposed scheme will have access to more than one control channel simultaneously. The access to any control channel by any CR node is without obligation and nodes can operate and cooperate independently. Also, the network initialization is not subject to any parent node or master node and the operation can be performed by any node instantly.

a) Control Channel GCCC

Due to the nature of the ISM band, the GCCC is primarily available to all CR nodes. In our scheme, we only access the GCCC for BF transmission. Apart from the BF, all the communication is established over the local channels (control/data). The GCCC is scanned and searched by other CR nodes in a few extreme cases. For example,

- 145 - when network initialization is required, GCCC will be used to launch a BF by a CR node, and when there is a worst case scenario where both PCCH and BCCH are occupied, GCCC would be used to launch a new BF containing the information about the newly established PCCH and BCCH.

b) Control Channel PCCH

The core part of DDH-MAC protocol is the PCCH. Once the network is initialized, the CR nodes can dialogue the control information over the control channel. Any communication which is carried out on the PCCH is overheard by all CR nodes. This keeps all the nodes well synchronized about network adaptions. Nodes must have access to the PCCH in order to become part of the DDH-MAC functionality.

c) Control Channel BCCH

To back up the core functionality of DDH-MAC, a BCCH is always there as a standby control channel. If there is a claim on the PCCH, nodes remain calm and consistent, and simply switch to the BCCH and resume the exchange of control information. We optimize the performance of DDH-MAC by discussing the following two extreme cases:

Case I: Both the PCCH and the BCCH are amongst the white spaces sensed by the

CR nodes. It is not unusual for a PU to arrive at any time, and in this circumstance, a PU activity is always sensed prior to switching onto the PCCH or the BCCH. We have enhanced the functionality of DDH-MAC by adding the PU-activity-sensing feature. A sensor is added that continuously senses the control channel and reports any PU claims. Equipping this feature in DDH-MAC enhances the overall performance as nodes will spend less time in re-negotiations. This feature is depicted in Figure 6.2.

Case II: As previously discussed in Chapter 3 Section 3.4.2, nodes will switch to the

BCCH if there is a PU claim on the PCCH. What if the BCCH is claimed by a PU before a PU claim is made on the PCCH? To address this issue, we modify the framework of the DDH-MAC protocol. The first node in DDH-MAC is responsible for launching the BF and then periodically broadcasting the BF in GCCC at regular intervals. This node is assigned with an additional operation, i.e., observing the PU activity on the BCCH when it is not transmitting the copy of the BF in the GCCC. If some PU activity is sensed on the BCCH, this node will quickly select another white space as a BCCH and broadcast an updated copy of the BF in the GCCC which will be received by all CR nodes in the vicinity. The receiving nodes will eventually update the

- 146 - information about the newly established BCCH and will resume the exchange of control information as usual.

Scan the GCCC in ISM for Beacon

Beacon found in GCCC

Start

Wait for T time

Contend for BCCH Add PCCH/BCCH in FCL

Launch Beacon in GCCC about PCCH/BCCH

switch to chosen white space to conclude transmission Agree on a white space to select as data channel with

other CR node Switch to PCCH for control

information exchange

Select one white space as PCCH and other as BCCH Read the Beacon for

information about PCCH and BCCH PU claims on BCCH More Transactions? PU claim On PCCH PCCH/BCCH In FCL Exchange Control Information Beacon found in GCCC Yes No No Yes Yes No Yes No No Yes No Yes

PCCH : Primary Control Channel BCCH: Backup Control Channel

Existing data channel still

Free?

Conclude Transmission and keep listening to PCCH/

BCCH No

Yes

Yes

- 147 -