Blind Rendezvous Problem

Abstract

Rendezvous is the fundamental process to establish a communication link between a pair of neighboring entities. In traditional multichannel wireless networks and cognitive radio networks, rendezvous is the prerequisite for communication, via which the users try to choose the same channel for data transmission. Here, we introduce the blind rendezvous problem, where blind means the entities or the users in the system do not know the others’ information and they have to make decisions completely locally. This definition makes a distinction away from centralized rendezvous where a central unit is used to provide the port or channel information to the users [1, 3], or some local common control channel is established and maintained to control and simplify the rendezvous process [2, 5]. Blind rendezvous draws a lot of attention from both academic and industrial areas due to its scalability, flexibility and robustness in implementing large scale distributed systems. We depict the blind rendezvous problem in Figs. 5.1 and 5.2. Consider a cognitive radio network which is composed of several secondary users (SUs) and several primary users (PUs). Because of the PUs’ occupancy on the licensed channels, the SUs can only have opportunistically a portion of the licensed spectrum. Suppose user A has three channels $\{1,2,6\}$ that are not used by the PUs, while user B can access channels $\{3,5,6\}$ after spectrum sensing. If they try to communicate with each other, they should choose an available channel for their communication attempt. However, neither of them knows the other SU’s information about the licensed channels, so they have to apply rendezvous strategies in a distributed “blind” way. Consider a simple algorithm: each SU accesses the available licensed channels in a round robin way, i.e. user A accesses channels by repeating the sequence $\{1,2,6\}$:

$$\begin{aligned} \{1,2,6,1,2,6,1,2,6,1,2,6,\ldots \} \end{aligned}$$

(5.1)

and user B accesses channels by repeating the sequence $\{3,5,6\}$:

$$\begin{aligned} \{3,5,6,3,5,6,3,5,6,3,5,6,\ldots \} \end{aligned}$$

(5.2)

As shown in Fig. 5.1, users A and B can rendezvous on channel 5 at time 3 if they start at the same time, which is the synchronous setting. However, if user A is two time slots earlier than user B, which is the asynchronous setting, as shown in Fig. 5.2, they will never rendezvous. In this chapter, we first present the system model of blind rendezvous in Sect. 5.1, and then we introduce two important metrics: expected time to rendezvous and maximum time to rendezvous in Sect. 5.2. The problem definition is given in Sect. 5.3 and the challenges are discussed in Sect. 5.4. Finally, Sect. 5.5 summarizes the chapter.