# Anti-Jamming Low-Latency Channel Hopping Protocol for Cognitive Radio Networks
## 1 Introduction  
With the rapid development of mobile applications, the traffic load of the unlicensed spectrum has become excessively congested while the licensed spectrum usage remains low [1, 2]. To enhance the efficiency of the licensed spectrum, a technology named Dynamic Spectrum Access (DSA), which enables wireless devices to access the licensed spectrum, has been proposed. The DSA technology has been utilized in Cognitive Radio Networks (CRNs). In a CRN, a Primary User (PU) has the priority for channel usage, while a Secondary User (SU) uses a channel that is not occupied by any PU. In a CRN, the rendezvous problem refers to how to enable two SUs to simultaneously use the same channel. There are two types of solutions to the rendezvous problem: using a Common Control Channel (CCC) [3, 4, 5, 6, 7] or using a Channel Hopping mechanism [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. For the protocols using CCC, every SU coordinates the channel for data transmission in a specific CCC. Because using a CCC has the issue of a single point of failure and difficulty in finding an available CCC for all SUs due to dynamically changed channel conditions, designing channel hopping protocols is the mainstream solution to the rendezvous problem. An SU running a channel hopping protocol generates a channel hopping sequence to determine the order of using channels. A commonly used criterion for measuring the channel hopping protocols is the time interval between two consecutive rendezvous between two SUs (Time To Rendezvous, TTR). A lowTTR indicates a high rendezvous frequency. In addition to having a low TTR, a well-designed channel hopping protocol should ensure a rendezvous on each of the available channels between any two SUs. The rules for channel hopping for most existing channel hopping protocols are predetermined and do not change during network operation. Such a channel hopping strategy makes the next channel used by an SU predictable and is vulnerable to malicious jamming attacks. Existing rendezvous problem solutions can be divided based on the following two criteria:  
* If pre-shared secrets are necessary: Sharing secrets (such as channel hopping sequence) in advance is necessary or not. * If role pre-assignment is necessary: Assigning a particular role (receiver or sender) to an SU in advance is necessary or not. Symmetric schemes require role pre-assignment, while asymmetric schemes do not. Based on the above two factors, the classification of existing anti-jamming solutions can be found in Table 1.  
The protocols using pre-shared secrets are suitable for CRNs with fixed topology. However, because the connections between SUs are unstable, the solutions requiring secret sharing in advance are not practical. All of the asymmetric protocols have the problem that SUs of the same role may not have a rendezvous with each other [22; 23; 24]. To avoid the issues mentioned above, most anti-jamming protocols are symmetric and do not pre-share secrets. Many anti-jamming solutions use the channel hopping matrix associated with each SU \(i\) to generate its channel hopping sequence [19; 20; 21; 29; 30]. The identifier (ID) of each SU \(i\) corresponds to columns in its channel hopping matrix, with each digit of the ID corresponding to a specific column. The transmission and reception modes in the corresponding column are also determined based on the digit. If the IDs of the two SUs are similar, the arrangement of the transmission and reception mode in the two hopping matrixes are also similar. This means that these two nodes stay in the same mode most of the time, resulting in a reduced number of rendezvous. To produce more rendezvous among SUs, we propose the Anti-jamming Low-Latency channel hopping scheme (ALL) to enable a sender to adjust the arrangement of its channel hopping matrix based on its receivers' channel hopping matrixes.