Las misiones protestantes, en el ambiente de la modernización y las relaciones cordiales con los

In this section, the full operation of the DDH-MAC is explained with the help of a flow chart drawn in Figure 3.17. At start up, the cognitive nodes are in a steady state and the FCL is already established. Upon initialization, cognitive nodes implementing DDH- MAC use the GT to scan the GCCC for a BF. If the node does not find any BF then the node is responsible for the following three operations:

i) Deciding which white spaces are to be used as PCCH and BCCH

ii) Formation of a BF

- 65 -

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 the 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

the other CR node Switch to PCCH for

control information exchange

Select one white space as PCCH and another 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

- 66 - Any CR node which is going to launch a BF in DDH-MAC must meet the following constraint:

(3.1)

where µ is the minimum number of available empty spaces within a CR node. One of the empty spaces will be used as PCCH, another as BCCH, and the last one for the actual data transmission. It is important to note that the BCCH in DDH-MAC protocol is a reserved/secondary control channel and is used only when there is a PU re-claim on the PCCH. Dedicating a white space as BCCH may firstly give an impression of a loss of a white space, but actually it improves the overall network convergence time by simply switching to BCCH if required, and it thus help reduce the computational cost of the protocol and avoids the rescanning of the GCCC which in turn can also help CR nodes conserve energy.

Let N represent a set of cognitive nodes in the network

(3.2a) and C be a set of white spaces within each node

(3.2b) then, the FCL which the CR nodes may have can be represented as:

∑ ∑ (3.2c)

where , represents the channel sensed by node . The criterion to set one of

the channels in the FCL as PCCH or BCCH can be arbitrary or it can satisfy the following equation.

∑ ∑ (3.3) where is the function to calculate the channel grade and is defined as:

fn (CHG) =Max AB, SNR, FER {NiCh1+NiCh2 + .... NiChm | NiCh1⋂ NiCh2 ⋂ .... NiChm }i=1, 2,…,n (3.4) where AB is the available bandwidth, SNR is the signal-to-noise ratio and FER is the frame error rate. This implies that the Equation 3.4 will be used by a CR node to find the best channel amongst the channels available in its FCL with optimal values for AB, SNR and FER.

- 67 - Once the node decides about the PCCH and the BCCH, it waits for a time T before launching the BF in the GCCC, where T is the time in micro seconds and is equivalent to the time required by a node to sense at least three white spaces (Equation 3.2). This waiting time of T is there just to avoid doubling of the BF in the GCCC which might be launched by another CR node (see Figure 3.1). If the node finds the BF after scanning the GCCC, it reads the information about PCCH and BCCH, update its FCL and switches to the PCCH for exchange of subsequent control information. Otherwise, it considers itself as the starting node and launches the BF in the GCCC. More discussion on the waiting time which will be called the Pre-Transmission time, denoted as TPT , is given in the Section 3.9.1.

During the initial scanning, if the BF is successfully found by a CR node in the GCCC, it decrypts the information using the relevant decryption scheme to learn about the chosen PCCH and BCCH. Once equipped with this information, the node accordingly updates its FCL by setting the PCCH and BCCH for control information exchange and the rest of the white spaces as data channels for the subsequent data transmission. In addition to the flow chart shown in Figure 3.1 which provides a complete operation of DDH-MAC, the process of BF launch/scanning and the later FCL update by other CR nodes are shown in Figure 3.2. The communicating CR nodes always verify the re-claim of PCCH by the PU before they actually switch to it for further exchange of the FCL. After a successful exchange of the FCL on the chosen PCCH, the CR nodes eventually switch to the agreed empty space to be used as a data channel for the actual data transmission. The CR nodes may come up with a case when there is a re-claim by PU(s) on both PCCH and BCCH; and in this case the nodes have to go to the initial state where they scan the GCCC for a new BF.

Figure 3.1 provides the framework for the DDH-MAC protocol in the form of a flow chart; and Figure 3.2 explains the scenario where three CR nodes are continuously sensing the GCCC for a BF. When the CR nodes find a BF, they read the information about PCCH and BCCH, update their FCL and switch to the newly set-up PCCH for the exchange of control information in the form of RTS and CTS packets with other cooperative communicating nodes. A BCCH has also been reserved to back up cognitive functions because it is not unusual for a PU to interfere on the PCCH.

- 68 - GCCC 1 3 5 7 9 B F Node ID, PCCH, BCCH Node B PCCH Data CH FCL:1,3,5,7,9 11 1 3 6 5 9 4 7 FCL: 1,5,9,11 FCL:1,3,4,5,9 PCCH Data CH PCCH Data CH BCCH 1 5 9 11 1 3 4 5 9 3 is PCCH PCCH RTS/CTS Node C Node A

Figure 3.2. The CR nodes in DDH-MAC, receiving the BF in GCCC and switching to PCCH for the subsequent control information dialogue.

In the previous section, the operation of DDH-MAC has been discussed in detail. The next section describes the very important aspect of cognitive radio network, the time CR nodes take to exchange control information, agree on a set of communication rules and start data transmission. This time will be called pre- transmission time and will be denoted as T_PT.