# Fast and Robust Asynchronous Rendezvous Scheme for Cognitive Radio Networks
## 2 Related Work  
[PERSON] et al. [7] provides a comprehensive review of CRN existing rendezvous algorithms and categorizes those algorithms into two groups--centralized and decentralized. In a centralized system [11; 12], a centralized controller manages the rendezvous of all SUs in the networks. [PERSON] et al. [13] introduced a systematic construction of common channel-hopping rendezvous strategy to guarantee that every node should be able to rendezvous in all common available channels. However, these algorithms have limitations discussed in the previous section. In a decentralized system, each SU attempts rendezvous without any help from a controller. Recently, many studies have focused on decentralized rendezvous system without using common control channel, i.e., blind rendezvous. A typical method of blind rendezvous is the CH technique. The taxonomy of rendezvous algorithms [7; 14] shows three categories for blind rendezvous; random algorithms, synchronous algorithms, and asynchronous algorithms. The asynchronous algorithms can be further classified into two sub-categories--a symmetric model and an asymmetric model. A purely random [15] or an improved random algorithm [16] provide a trivial CH algorithm where SUs rendezvous by chance. However, these random algorithms cannot guarantee a bounded TTR. Both synchronous and asynchronous algorithms provide guaranteed rendezvous. Several synchronous algorithms are proposed with an assumption of global time synchronization [17; 18; 19]. [PERSON] et al. [17] introduced a Slotted Seeded Channel Hopping to increase the capacity of an IEEE 802.11 network and [PERSON] et al. [18] proposed a deterministic approach in which each SU can dynamically calculate a common set of channels globally. [PERSON] et al. [PERSON] et al. (2019) introduced two quorum-based CH methods (M-QCH and L-QCH) that can guarantee rendezvous between time-synchronized SUs. These synchronous systems are usually unfeasible for blind rendezvous and the impact of a jamming attack can be significant.  
Most of the recently proposed algorithms ([PERSON] et al., 2010; [PERSON] et al., 2011; [PERSON] et al., 2012; [PERSON] et al., 2013; [PERSON] et al., 2014; [PERSON] et al., 2015) consider asynchronous scenarios without time synchronization. [PERSON] and [PERSON] [PERSON] and [PERSON] (2010) proposed a generated orthogonal sequence (GOS) algorithm that uses interspersed permutation channels to guarantee rendezvous. [PERSON] et al. [PERSON] et al. (2011) introduced modular clock and modified modular clock algorithms in which each SU generates its CH sequence from pre-defined modulo operations. [PERSON] et al. [PERSON] et al. (2012) proposed a ring walk (RW) algorithm by using the idea of velocity. Thus, the rendezvous is achieved when the SU with lower velocity is caught by the SU with higher velocity. [PERSON] et al. proposed two significant algorithms, namely deterministic rendezvous sequence (DRSEQ) ([PERSON] et al., 2010) and channel rendezvous sequence (CRSEQ) ([PERSON] et al., 2010), which provide fast asynchronous rendezvous under symmetric and asymmetric models, respectively. The performance of CRSEQ is good under the asymmetric model but it does not perform well under the symmetric model. [PERSON] et al. [PERSON] et al. (2015) designed an efficient dynamic rendezvous algorithm when the status of the channels varies dynamically for both synchronous and asynchronous users. Among the rendezvous algorithms that are applicable to asymmetric scenario, the JS ([PERSON] et al., 2010) and EJS ([PERSON] et al., 2010) are known to have the overall best performance. However, most of the aforementioned algorithms including the JS and EJS are significantly vulnerable to a sophisticated jamming attack. In this paper, I propose a FRARS algorithm to overcome the vulnerability problem by employing a randomized permutation technique while preserving fast guaranteed rendezvous. I also derive the upper bounds on MTTR and EITR of the FRARS for both symmetric and asymmetric systems.