# Anti-Jamming Low-Latency Channel Hopping Protocol for Cognitive Radio Networks
## 2 Related Work  
Pre-shared secrets have been utilized by several researchers to propose anti-jamming channel hopping protocols [25; 26]. The circular dependency issue may arise in these protocols, as the exchange of pre-shared secrets between SUs may be vulnerable to jamming attacks [30]. The majority of anti-jamming channel hopping protocols do not pre-share secrets and pre-assign roles. To resist jamming attacks, those protocols usually use the available channels randomly. In the UFH protocol, the sender and the receiver choose the channel being used randomly, while a communication link can be built when they simultaneously stay at the identical channel [27]. An SU adjusts the channel hopping sequence based on the strategy of the attacker in the MDP protocol. When the attack strategy is unknown, SUs utilize maximum likelihood estimation and Q-learning to infer the attack strategy [28]. Both the UFH and MDP protocols are able to resist jamming attacks. However, it cannot be ensured that all pairs of SUs will meet within a specific time period. A rendezvous between any two SUs within a time period is guaranteed in the Sec-CH protocol [29]. SUs running Sec-CH can operate in either transceiving-mode random jump pattern (\(T\) mode) or reception-mode stay pattern (\(R\) mode). When operating in \(T\) mode, an SU is capable of both transmitting and receiving packets, whereas, in \(R\) mode, only receiving is allowed. The generation of the channel hopping sequence for an SU \(a\) relies on the use of its corresponding channel hopping matrix, denoted as \(M_{a}\). \(M_{a}\) is constructed such that each of its columns alternates between \(T\)-sub-columns (when SU \(a\) is in \(T\)-mode) and \(R\)-sub-columns (when SU \(a\) is in \(R\)-mode). The two distinct types of columns in \(M_{a}\) are bit-0 column (which begins with a \(T\)-sub-column) and bit-1 column (which begins with an \(R\)-sub-column). Each SU is assigned a unique binary ID, and the value of each digit within the ID of SU \(a\) corresponds to a specific column within \(M_{a}\). Specifically, if the value of the digit is 0, a bit-0 column is selected, and if the value of the digit is 1, a bit-1 column is chosen. In a load-balanced environment, Sec-CH has demonstrated high performance. The Tri-CH protocol, an improved version of the Sec-CH protocol, reduces the Maximum Time To Rendezvous (MTTR) of Sec-CH [30]. Every SU that operates using Tri-CH constructs its unique hopping matrix by utilizing its respective ternary ID. Both the \(J\) mode and \(S\) mode are utilized to create each sub-column within the hopping matrix. Each digit in the ID of an SU corresponds to one of three different sub-columns, with possible values of 0, 1, and 2. Like Sec-CH, Tri-CH may not perform well in a network where the traffic load is imbalanced. In networks with unbalanced traffic loads, the LAA protocol has demonstrated good performance [19]. An SU running LAA constructs its hopping matrix based on the extended Langford pairing (ELP) [33]. When an ELP sequence has an order of \(n_{e}\), an SU that utilizes the LAA protocol is given an \((n_{e}+1)\)-ary ID. The construction of a channel hopping matrix \(M_{a}\) involves forming each of its columns by combining a transmission part denoted as the \(T\) frame, and a reception part denoted as the \(R\) frame. The ratio and arrangement of these two parts are determined by the traffic load of SU \(a\) as well as the specific digit in its \((n_{e}+1)\)-ary ID. Both the OLAA_T and OLAA_R protocols are improved versions of the LAA protocol. Each SU running OLAA_T and OLAA_R adjusts its usage probability for each channel based on the probability of PU occupancy on that channel, with OLAA_T focusing on the transmission part and OLAA_R focusing on the reception part. Such a strategy makes OLAA_T and OLAA_R practical anti-jamming solutions [21]. Specifically, the ratio of the \(T\) frame to the \(R\) frame is adjusted based on their own traffic load.