# Secure Inter-Frame Space Communications for Wireless LANs
## 1 Introduction  
Wireless local area networks (WLANs) such as IEEE 802.11b/g were initially deployed in 2.4-GHz bands, in which industrial, scientific, and medical (ISM) devices are allowed to operate [1]. As WLANs were deployed widely and became an essential feature of everyday life, more spectrum was required because the 2.4-GHz band has only three channels. Thus, WLANs were extended to operate in the 5-GHz (IEEE 802.11a/n/ac/ax) and Sub-1-GHz (IEEE 802.11 af/ah) ISM bands, which are also used by many other radio systems such as cordless phones, DECT (Digital Enhanced Cordless Telecommunications), and radars [2]. Because of their dependence on the same channel frequencies, the possibility of interference is higher. Several studies have investigated the interference effect, and several approaches to using new protection mechanisms in WLAN systems have been reported [3; 4; 5; 6]. However, their scope was confined to demonstrating deteriorated performance due to interference for WLANs, and coexistence by channel avoidance as a passive way [3; 4]. Some of the new protection mechanisms require modification of the current standard [5; 6]. To protect the channel medium in the current WLAN standards, WLANs basically operate in the manner of carrier detection and avoidance. For the carrier detection and avoidance operation, spectrum-sharing mechanisms, such as dynamic frequency selection (DFS) and transmit power control (TPC), are required for WLANs [7; 8]. Dynamic frequency selection and TPC allow WLANs to share channels with various radio systems, which are operating on the 5-GHz ISM channel frequencies. However, these conservative channel-access mechanisms are not efficient in terms of channel utilization and network throughput. In IEEE 802.11 ac and IEEE 802.11 ax standards, WLANs support 80-MHz and 160-MHz bandwidth operations [9; 10]. Although IEEE 802.11 ac/ax can adjust the bandwidth dynamically using pre-defined protocols between the transmitter and receiver, these wide-bandwidth modes may not work in the real world because they are operated in an \"all-or-nothing\" mode against non-802.11 interferences. In other words, WLANs may fail to communicate in the wide bandwidth if the channel is affected by other radios' interferences. For example, in the 5-GHz radar bands, WLANs should not cause interference in the radar systems. The International Telecommunication Union (ITU) Recommendation on DFS provides test patterns and threshold criteria to test DFS implementations [8]. According to the standards in WLANs and ITU Recommendation, a DFS system should meet all the test criteria, and the system should avoid the radar channel automatically. However, it is not an efficient way of communication because WLANs and radar systems can share the frequency spectrum if WLANs detect radar more accurately and transmit data frames in idle time for the shared channel. As another example, frequency hopping radios are widely used in ISM bands because of their robustness, long range, and low cost. However, if a frequency hopping radio and a WLAN device exist in the same channel, the frequency hopping signal prevents WLANs from communicating in the channel. Eventually, the WLAN devices should change the channel. Since the radar systems and frequency hopping radios can be adopted for many purposes in the modern wireless systems, secure inter-frame space communication is required to improve channel utilization and network throughput. Radars were used for decades before the same frequency bands were included in 5-GHz ISM bands that were allocated for unlicensed use. Since frequency resources have become more valuable, frequency spectrum sharing has become indispensable. The ISM bands have been adopted by many local access specifications. Radar systems emit very strong pulse signal arrays that interfere with the network communication packet signals. This scenario is less threatening than the opposite case, since WLAN communication packets are retransmitted if they are not successfully received.