# Fast and Robust Asynchronous Rendezvous Scheme for Cognitive Radio Networks
## 1 Introduction  
Unlicensed spectrum bands that can be used without complying with the regulations applicable to licensed services have already become globally overcrowded. In contrast, large portions of the licensed spectrum bands are severely underused as shown in federal communications commission report [1]. To overcome this problem, a method has been widely used in which a secondary user (SU) can use an authorized spectrum that is not in use opportunistically without interfering with primary users (PUs). Cognitive radio networks (CRNs) have been recognized as a promising paradigm to improve the spectrum efficiency of wireless communications [2; 3; 4]. [PERSON] et al. [3] highlight some of the recent information theoretic limits, models, and design of these promising CRNs. [PERSON] et al. [2] provide a systematic overview of CRNs and communications by looking at the key functions of the physical, medium access control, and network layers involved in a CR design and how these layers are crossly related. In CRNs, a pair of SUs wishing to communicate with each other must establish a link, i.e., two SUs must simultaneously exchange handshake information on a common channel and this process is called the rendezvous process. Most existing works [5; 6] focus on implementing a rendezvous process by using dedicated common control channel (CCC) for the sake of simplicity. However, CCC is probably occupied by a PU that is an incumbent license holder of a frequency band, and maintaining a CCC may result in a bottleneck problem as well as creating a single point of failure. Therefore, I study a different scheme to rendezvous without using a centralized controller. In this approach, each SU uses a channel-hopping (CH) sequence with guaranteed rendezvous during the CH sequence. This process is known as blind rendezvous. In most CH algorithms, time is dividedinto slots of constant size, and the number of time slots required for two SUs to achieve a rendezvous after all SUs start the rendezvous process is defined as rendezvous time (TTR). If a pair of SUs achieve successful rendezvous on the same channel at the same time, they can exchange control messages and select one of the commonly available channels to start data transmission. The MTTR and the EITR are two important metrics commonly used to evaluate performance of proposed CH schemes. In CH algorithms, each SU in the CRN periodically senses the spectrum of the PU to determine the available channels for the rendezvous process. Each SU can have a different set of channels because geographic locations of SUs are different. If all SUs have the same available channels, this is called a symmetric system, otherwise asymmetric system, i.e., different SUs have different numbers of available channels. There has been much work in the literature to guarantee blind rendezvous, but most work focuses on symmetric scenarios due to time synchronization constraints. Moreover, few of those works consider security attacks such as jamming. One of the most notable works, as addressed in a recent survey paper [7], is the jump-stay (JS) rendezvous algorithm [8]. The same author revises the original configuration and suggests an improved jump-stay (EJS) algorithm [9] to improve performance for the asymmetric scenario. It is known that the EJS is one of the best blind rendezvous algorithms for CRNs based on non-deterministic CH sequences and the guaranteed rendezvous times for both symmetric and asymmetric models. The basic idea in both JS and EJS algorithms is to generate CH sequences based on jump and stay patterns to ensure blind rendezvous without time synchronization. These schemes can be applied to the rendezvous of multi-user and multi-hop scenarios. Nevertheless, those are significantly vulnerable to a hostile environment, i.e., the CH sequences can be easily detected and jammed by the channel-detecting jamming attack (CDJA) [10].