Communication time over control and data channels

Figure 6-2 shows the communication time over the control and data channels for each pair of CUs. Each pair exchanges 1500 bytes of payload after the data channel is determined. Therefore, the total time to successfully exchange the control information within the RTS and CTS and 1500 bytes of data for the first pair of CUs (CU1 and CU2), who first won in the channel contention process, is 1242.82µsecs.

However, the second pair of CUs (CU9 and CU10), who the second winner in the control channel contention, requires a waiting time equal to 109.91µsecs to access the CCC as it is reserved for the first pair of CUs and the channel is vacated after this period of time. Then both the sender (CU9) and receiver (CU10) necessitate 1242.82µsecs to successfully complete the data communication.

The waiting time to launch the RTS frame belonging to Group 3 of CUs (CU11 and CU12) is doubled to 2*109.91µsecs since the control channel is busy exchanging four control frames belonging to Groups 1 and 2. In addition to this time, the third pair of CUs also requires 1242.82µsecs to complete their entire communication over both channels. This process is repeated for the remaining 7 groups of CUs including the waiting time for the control channel to be vacated and available for the next pair of CUs in order to avoid any collisions.

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In order to calculate the total required time to exchange the control and data phases of the MCRN protocol (TTMCRN) between senders and receivers

successfully the following Equation 8 is applied.





MCRN DIFS RTS CTS Data ACK SIFS

T  T  T  T T T  T

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However, Figure 6-3 demonstrates a scenario in which the communication activities of 10 pairs of CUs, and the prediction activities of LUs take place over 10 data channels. In each data channel, the x-axis explains which LUs are busy and which have free time; the green areas show the busy signals for the LUs and the remainder are available to the CU. Therefore, the red areas represent the CUs activities in the white space in each channel. However, the y-axis shows the amplitude of the LUs’ and CUs’ activities (the signal strength is represented by the ASCII format).

As discussed in (Hussein, et al., 2013) CUs are allowed to share the spectrum with LUs with some restrictions such as the transmitted power’s limitation. Thus 20 CUs are involved in the communication and initially two users, who won in the channel contention process perform the successful exchange of the control information over the CCC and select a data channel based on the highest available time as shown in channel 1 while the rest of CUs wait until the first group moved from the control to the data channels. Then, again contention process starts and only one CU wins the contention, which then leads to exchange the control information between the next pair of CUs and so on. Generally, channel 1, which is occupied by the first group of CUs; CU1 and CU2, has the maximum time of availability since LUs utilise the current channel after a period of time equal to 1.9*104µses. This makes this channel is the most reliable data channel for the first pair of CUs to transmit the data. In contrast, channel 10, which is occupied by the last pair of users, CU11 and CU12, has the lowest time availability in which the LU is predicted to appear in approximately 0.2*104µses and this results in channel 10 having the lowest priority in terms of data channel selection criteria. Although the time of the CU activities over these channels is equal, since they have the same size of data to exchange, their communication process is initiated at different times based on the waiting time of the control channel’s availability. For instance, the first pair of CUs utilises the CCC immediately after the successful contention process for the channel utilisation while the last pair of CUs (CU11 and CU12) had to wait 9*109.91µsecs to content the CCC and initiate their communication over CCC and SLDCH respectively. The time required over the

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control channel would influence the data channel availability since the LUs have priority to utilise the licensed data channel at any time.

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However, Figure 6-4 shows the overall time in microseconds which is required for 20 CUs to successfully complete the communication process in the MCRN protocol. This time refers to both the period over the CCC for control frames exchange and the period over the SLDCHs for data transmission for 10 pairs of CUs. Both the number and the sizes of the control frames significantly affect the time of the frames exchange between two CUs. Moreover, the overall time increases with the number of participating CUs. This is common sense where each sender wins contention for accessing the CCC to launch their RTS and CTS frames for channel selection and this requires 109.91µsecs where each pair needs 1132.91µsecs to exchange their data and ACK frames over the SLDCH.

Figure ‎6-4: Total communication time of 20 pair of CUs in MCRN