# Secure Inter-Frame Space Communications for Wireless LANs
## 3 Secure Inter-Frame Space Communication  
### Periodic Interference Detection  
The implemented periodic interference detection method is based on the signal power and pulse periodicity. Thus far, this periodic property of radar systems has been simple and clear for most cases. This means that some simple periodic detection algorithm can perform well for most radar cases. Figure 1 shows the block diagram of the DFS algorithm implemented. To detect the periodicity of pulses, a receiver needs to measure the pulses. Since the tone frequency and pattern are not known for all the radio devices, its power is a good measure for detecting the pulses asynchronously. Of course, the sensitivity decreases with power measurement compared to synchronous tone signal measurement. Figure 2 shows the block diagram of the implemented periodic interference detector. Measuring the signal power requires averaging over a window. If this window is short, the averaging is not enough, so that its averaged power will not give a secure measurement. Sudden noise can crash the detection algorithm. Accordingly, it is important to have a long enough window size for averaging. The maximum window size is limited by the shortest pulse width, even as short as 0.5 us, which is defined in Reference [14]. With this short pulse width, only 10 samples are available with 20-MHz sampling. The designed detector uses eight samples for averaging the power, that is, a window size of 0.4 us. Thus, for every 0.4 us, the averaged power output is passed to a comparator that compares it with a threshold and determines if there is a signal. Figure 1: Block diagram of the dynamic frequency selection (DFS) algorithm. Figure 2: Block diagram of periodic an interference detector. With this many averaging samples, the power measurement has a problem in the start time. If the start time is, inopportunly, in the middle of the 0.5-\(\upmu\)s pulse, the two consecutive averaged powers will divide the total power of the pulse and fail to exceed a threshold. This problem can be eliminated with the moving average window. Since it is a moving average, a pulse will provide a plateau shape and the maximum power is always available. These moving averaged powers are first compared with a threshold to save the memory space instead of saving the power information. Here, the power threshold is set by WLAN standards as \(-\)62 dBm. Wireless local area network devices can communicate through an idle channel in the unit of the 20-MHz channel, which is not used by other radios. For example, if the WLAN's frequency bands are used by a complete 40-MHz band or upper or lower 20-MHz band, the periodic interference detection is performed for upper and lower 20-MHz bands. If WLAN uses the lower 20-MHz band and a periodic interference is detected in the same 20-MHz band, the WLAN must coordinate 20-MHz and 40-MHz transmissions so that they do not interfere with the periodic interferer. If the periodic interference is detected in the upper 20-MHz band only, the WLAN can continue to transmit 40-MHz bandwidth packets during idle time and transmit 20-MHz bandwidth packets through the lower 20-MHz band during the interference time. In the case of conventional WLANs, the WLAN must leave the lower 20-MHz band for the same conditions. Although the periodic interference detector uses moving average power and the decision is made every clock, the decision should be time-stamped and reported to the upper layer, MAC. If this does not have a period of 0.4 \(\upmu\)s, the time stamp value needs more accurate resolution to represent the time. In terms of a typical pulse period of 1 ms, 0.4 \(\upmu\)s is within an error margin and finer resolution is not required. At the report time interval 0.4 \(\upmu\)s, the periodic interference detector counts how many times the averaged power exceeds the threshold. This count value is compared with a count threshold to determine the periodic interference signal.