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In cognitive radio networks (CRNs), rendezvous is the vital step prior to the communication between two unlicensed secondary users (SUs), where the SUs hop on the same channel at the same time to establish a link. With the dramatic fall in the cost and size of wireless transceivers, it becomes more reasonable to apply multiple radios to achieve significant improvement in the rendezvous performance. However, most of the existing multiradio rendezvous algorithms are proposed for homogeneous CRNs where all the SUs are equipped with an equal number of radios and do not possess backward compatibility to SU with a single radio. In reality, the CRNs are heterogeneous in nature as SUs may have different numbers of radios. In this paper, a composite CH algorithm is proposed for an asynchronous and heterogeneous network to achieve blind rendezvous with full rendezvous diversity. An SU with m number radios are categorized into three groups those follow different channel hopping (CH) algorithms. The upper bound of the rendezvous latency is being evaluated with a brief theoretical and mathematical analysis. Extensive simulations have conducted for different performance metrics, and the results are compared with the state-of-art algorithms. Overall, the proposed algorithm shows better performance in heterogeneous CRNs. [ABSTRACT FROM AUTHOR]

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Cognitive radio network (CRN) has been recognized by researchers to solve the spectrum shortage problem, where the unlicensed users opportunistically exploit the idle licensed channels for data transmission. Prior to the data transmission, the SUs should rendezvous on an available idle licensed channel to establish a link or to exchange control information without causing interference to the co-located licensed channels. However, the dynamic behavior of channels and its availability make the rendezvous more challenging. A blind rendezvous on the available channels without any centralized unit, or a common control channel is preferable to address issues like long-time blocking, control channel saturation, and scalability in a congested network. In this paper, a blind rendezvous for a specific CRN is proposed, where a Role-Based Channel Hopping is introduced to achieve guaranteed rendezvous in an asynchronous environment. Analytical study shows that for N available channels, the maximum time to rendezvous is N + ⌊ N 2 ⌋ , degree of rendezvous is N, maximum conditional time to rendezvous is N 2 and Channel loading is 1 N . [ABSTRACT FROM AUTHOR]

In cognitive radio networks (CRNs), two secondary users (SUs) need to meet on a channel among multiple channels within a finite time to establish a link, which is called rendezvous. For blind rendezvous, researchers have devised ample well-grounded channel hopping (CH) sequences that guarantee smaller time-to-rendezvous. However, the best part of these works lacks the impact of network factors, particularly channel availability and collision during rendezvous. In this study, a new CH scheme is investigated by jointly considering the medium access control (MAC) protocol for single-hop multi-user CRNs. The analysis of our new variable hopping sequence (V-HS) guarantees rendezvous for the asymmetric channel model within a finite time. Although this mathematical concept guarantees rendezvous between two SUs, opportunities can be overthrown because of the unsuccessful exchange of control packets on that channel. A successful rendezvous also requires the exchange of messages reliably while two users visit the same channel. We propose a MAC protocol, namely ReMAC, that can work with V-HS and CH schemes. This design allows multiple rendezvous opportunities when a certain user visits the channel and modifies the conventional back-off strategy to maintain the channel list. Both simulation and analytical results exhibited improved performance over the previous approaches. [ABSTRACT FROM AUTHOR]

In cognitive radio networks (CRNs), if two unlicensed secondary users (SUs) want to communicate with each other, they need to rendezvous with each other on the same channel at the same time. Rendezvous is the first key step for SUs to be able to communicate with each other. Channel hopping (CH) is a representative technique to solve the rendezvous problem in CRNs. SUs equipped with multiple radios can significantly reduce the time-to-rendezvous (TTR) for several existing CH algorithms while the additional cost is low. However, several drawbacks exist in the existing CH algorithms based on multiple radios. One of the main drawbacks is that they cannot be well applied in heterogeneous CRNs. The reason is that the number of radios for different SUs is implicitly assumed same or must be more than one in the existing CH algorithms based on multiple radios, which is unrealistic for heterogeneous CRNs. In heterogeneous CRNs, SUs may be equipped with different numbers of radios including one radio. To mainly address the above issue, hybrid radios rendezvous (HRR) algorithm is proposed in this paper. Moreover, the upper bounds of maximum TTR (MTTR) for the HRR algorithm are derived by a theoretical analysis. Furthermore, extensive simulations are performed to evaluate the expected TTR (ETTR), the MTTR, and the channel qualities of the rendezvous channels for the HRR algorithm. Simulation results show that rendezvous can be guaranteed by the HRR algorithm in heterogenous CRNs. Besides, the qualities of the rendezvous channels can be improved by the HRR algorithm. In addition, our algorithms can achieve rendezvous faster than several existing algorithms.

In cognitive radio networks (CRNs), two secondary users (SUs) need to meet on a channel among multiple channels within a finite time to establish a link, which is called rendezvous. For blind rendezvous, researchers have devised ample well-grounded channel hopping (CH) sequences that guarantee smaller time-to-rendezvous. However, the best part of these works lacks the impact of network factors, particularly channel availability and collision during rendezvous. In this study, a new CH scheme is investigated by jointly considering the medium access control (MAC) protocol for single-hop multi-user CRNs. The analysis of our new variable hopping sequence (V-HS) guarantees rendezvous for the asymmetric channel model within a finite time. Although this mathematical concept guarantees rendezvous between two SUs, opportunities can be overthrown because of the unsuccessful exchange of control packets on that channel. A successful rendezvous also requires the exchange of messages reliably while two users visit the same channel. We propose a MAC protocol, namely ReMAC, that can work with V-HS and CH schemes. This design allows multiple rendezvous opportunities when a certain user visits the channel and modifies the conventional back-off strategy to maintain the channel list. Both simulation and analytical results exhibited improved performance over the previous approaches.

In channel-hopping (CH)-based distributed cognitive radio networks (CRNs), the time duration that secondary users (SUs) spend for establishing communication links is called access delay. To evaluate access delay, we propose an access delay model by jointly considering imperfect spectrum sensing and multi-channel multi-SU transmission, from the cross-layer perspective. The model considers two typical scenarios. The first scenario assumes that the SUs do not use contention scheme (CS) which indicates that the time slot is relatively shorter to just allow a transmission. The second scenario assumes that the SUs employ CS [i.e., modified Distributed Coordination Function (DCF)-based Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) in this paper], which indicates that the time slot is long enough to regulate multiple transmissions. We then propose a bio-inspired algorithm for the first scenario and a self-adaptive step-length algorithm for the second scenario to search for the optimal values of spectrum sensing parameters. The theoretical analysis and simulation results validate the proposed access delay model and show that the proposed algorithms can reduce the most redundant computation. They also show that the optimization of cross-layer parameters can significantly decrease SUs’ access delay. Moreover, we conduct a cost-benefit analysis to evaluate the performance of the two scenarios. [ABSTRACT FROM AUTHOR]

The rendezvous process is considered a key operation that allows a secondary user (SU) to access an unused authorized spectrum in cognitive radio networks (CRNs). Most existing works focused on fast guaranteed rendezvous without considering a sophisticated jamming attack environment. In this paper, I propose a fast and robust asynchronous rendezvous scheme that can improve robustness against jamming attacks under symmetric asynchronous environments in which all SUs have the same available channels. Unfortunately, in CRNs, each SU can have a different number of available channels due to their relative position to primary nodes (PUs). Therefore, I extend my fast and robust asynchronous rendezvous scheme (FRARS) to a general asymmetric scenario while preserving robustness against jamming attacks. I derive the maximum rendezvous time (MTTR) of my new algorithm and the upper bound of the expected TTR (ETTR) and compare it with the state-of-the-art algorithms such as jump-stay (JS) and Enhanced jump-stay (EJS). My numerical results show that the performance of the proposed technique is better than that of JS and EJS in terms of MTTR and ETTR. Also, the performance will be more significant when there are security concerns about a sophisticated jamming attack. [ABSTRACT FROM AUTHOR]

Asynchronous channel hopping (ACH) systems are widely used for the blind rendezvous among secondary users in cognitive radio networks without requirement for global synchronization and common control channels. We can view an ACH system with n channels as a set of sequences of a common period t on an alphabet of size n satisfying certain rotation closure properties. For any two distinct sequences u, v in an ACH system S, every l ∈{0,1,⋯, t − 1} and any letter j in the alphabet, if there always exists i such that the i-th entries of u and Ll(v) are identical to j where Ll(v) denotes the cyclic shift of v by l, then we say that S is a complete ACH system. Such a system can guarantee rendezvous between any two secondary users who share at least one common channel. It is well known that t⩾n2. When the equality holds, S is called a perfect ACH system. By applying characters of cyclic groups, we obtain a new upper bound on the number of sequences in a perfect ACH system. Furthermore, when q is a prime power and n = q − 1, we present a construction of homogeneous complete ACH systems of period t = 2n2 + o(n2). [ABSTRACT FROM AUTHOR]

Cognitive radio (CR) has emerged as an advanced and promising technology to exploit the wireless spectrum opportunistically. In cognitive radio networks (CRNs), any pairwise communicating nodes are required to rendezvous on a commonly available channel prior to exchange information. In the earlier research, the most popular method is selecting a Common Control Channel (CCC) in CRNs to establish the rendezvous. However, employing a CCC has many problems such as the control channel saturation, vulnerability to jamming attacks, and inapplicability to dynamic network scenarios. Therefore, the blind rendezvous, which requires neither CCC nor the information of the target user's available channels, has recently attracted a lot of research interests. As a contribution to this research area, in this paper we propose a Sender-Jump Receiver-Wait (SJ-RW) blind rendezvous algorithm, which has fully satisfied the following requirements: 1) guaranteeing rendezvous; 2) realizing full rendezvous diversity, i.e., any pair of users can rendezvous on all commonly available channels; 3) requiring no time-synchronization; 4) supporting both symmetric and asymmetric models; 5) supporting multi-user/multi-hop scenarios and 6) consuming short Time-to-Rendezvous (TTR). Theoretical analysis, computer simulations and experiment with testbed have validated the proposed SJ-RW algorithm. [ABSTRACT FROM PUBLISHER]

The rendezvous process is considered a key operation that allows a secondary user (SU) to access an unused authorized spectrum in cognitive radio networks (CRNs). Most existing works focused on fast guaranteed rendezvous without considering a sophisticated jamming attack environment. In this paper, I propose a fast and robust asynchronous rendezvous scheme that can improve robustness against jamming attacks under symmetric asynchronous environments in which all SUs have the same available channels. Unfortunately, in CRNs, each SU can have a different number of available channels due to their relative position to primary nodes (PUs). Therefore, I extend my fast and robust asynchronous rendezvous scheme (FRARS) to a general asymmetric scenario while preserving robustness against jamming attacks. I derive the maximum rendezvous time (MTTR) of my new algorithm and the upper bound of the expected TTR (ETTR) and compare it with the state-of-the-art algorithms such as jump-stay (JS) and Enhanced jump-stay (EJS). My numerical results show that the performance of the proposed technique is better than that of JS and EJS in terms of MTTR and ETTR. Also, the performance will be more significant when there are security concerns about a sophisticated jamming attack.

Channel rendezvous is an initial and important process for establishing communications between secondary users (SUs) in distributed cognitive radio networks. Due to the drawbacks of the common control channel (CCC) based rendezvous approach, channel hopping (CH) has attracted a lot of research interests for achieving blind rendezvous. To ensure rendezvous within a finite time, most of the existing CH-based rendezvous schemes generate their CH sequences based on the whole global channel set in the network. However, due to the spatial and temporal variations in channel availabilities as well as the limitation of SUs sensing capabilities, the local available channel set (ACS) for each SU is usually a small subset of the global set. Therefore, following these global-based generated CH sequences can result in extensively long time-to-rendezvous (TTR) especially when the number of unavailable channels is large. In this paper, we propose two matrix-based CH rendezvous schemes in which the CH sequences are generated based on the ACSs only. We prove the guaranteed and full diversity rendezvous of the proposed schemes by deriving the theoretical upper bounds of their maximum TTRs. Furthermore, extensive simulation comparisons with other existing works are conducted which illustrate the superior performance of our schemes in terms of the TTR metrics.

In cognitive radio networks, the process for any two unlicensed (secondary) users (SU) to establish links through a common channel is called a rendezvous. It is hard to employ a common control channel to solve the rendezvous problem, because of the bottleneck of the single control channel. Asynchronous channels hopping (ACH) systems have been proposed and investigated to guarantee rendezvous without requirement of global synchronization and common control channels. An ACH system with N channels can be mathematically interpreted as a set of sequences of the same period T on the alphabet $$\{0,1,\ldots , N-1\}$$ satisfying certain rotation closure properties. For each $$l\in \{0,1,\ldots , T-1 \}$$ , each $$j\in \{0,1,\ldots , N-1 \}$$ and any two distinct sequences $$\mathbf u$$ , $$\mathbf v$$ in an ACH system H, if there always exists i such that the i-th entries of $$\mathbf u$$ and $$L^{l}(\mathbf v)$$ are both identical to j where $$L^{l}(\mathbf v)$$ denotes the cyclic shift of $$\mathbf v$$ by l, then H is called a complete ACH system, which guarantees rendezvous between any two SUs who share at least one common channel. In this paper, we prove some properties of such systems. Moreover, we investigate the complete ACH systems, in each of which all SUs repeat the same (global) sequence associated with the system. One challenging research problem is to construct such ACH systems of period $$T=\beta N^2 + o(N^2)$$ such that $$\beta $$ is as small as possible. By applying mathematical tools such as group rings and (relative, relaxed) difference sets from combinatorial design theory, we obtain two constructions of them in which $$\beta =1,2$$ respectively. Finally, we also present a simple and powerful approach to combining known ACH systems to produce new ones. [ABSTRACT FROM AUTHOR]

Rendezvous is the fundamental challenge in cognitive radio networks to find each other on a specific channel and establish a communication link. The primary focus of this paper is to design an algorithm for blind rendezvous, i.e., a rendezvous without a coordinator or common control channel. Channels to jump are assumed to be homogeneous in prior work; however, in the real world, channels exhibit different conditions. Therefore, we propose an adaptive jumping pattern such that a superior channel is considered more frequently based on its noise level and users have a greater possibility to rendezvous on such a channel. Furthermore, in this paper, we investigate the rendezvous problem with multiple radio interfaces. When multiple radio interfaces are available for users, the possibility of rendezvous increases; however, the number of channels to jump for each interface may not be the same as other users’ interfaces. We propose an adaptive rendezvous algorithm that can function for multiple interfaces and different sizes of channel lists and an adaptive jumping pattern for different channel conditions. We prove that the proposed algorithm provides guaranteed rendezvous and derive the maximum time-to-rendezvous (TTR), which is also verified via simulation results. Further, our algorithm outperforms jump-and-stay in terms of TTR while enabling users to rendezvous on a better channel. [ABSTRACT FROM PUBLISHER]

Cognitive radio is a promising paradigm to address the scarcity of radio spectrum as well as for efficient usage. The rendezovus process allows two cognitive users to meet on the same channel and establish a communication link. This work focuses on the study and comparison of different rendezvous algorithms proposed in the literature. We present the major rendezvuos challenges in the context of cognitive networks as well as the main metrics used for evaluation. Through simulation we evaluate the performance of a group of rendezvous algorithms in practical scenarios and the results are contrasted with the theoretical results given by the authors. The simulation results show that for most of the rendezvous algorithms proposed in the literature the theoretical results provided by the authors are far from the real results achieved by the algorithms. Additionally, in symmetric scenarios the EJS algorithm has the better results in terms of ETTR and MTTR, while in asymmetric scenarios the FRCH and SSB algorithms outperform the others rendezvous strategies. [ABSTRACT FROM AUTHOR]

Rendezvous is a fundamental operation in cognitive radio networks (CRNs) for establishing a communication link on a commonly-available channel between cognitive users. The existing work on rendezvous implicitly assumes that each cognitive user is equipped with one radio (i.e., one wireless transceiver). As the cost of wireless transceivers is dropping, this feature can be exploited to significantly improve the rendezvous performance at low cost. In this study, we investigate the rendezvous problem in CRNs where cognitive users are equipped with multiple radios and different users may have different numbers of radios. We first study how the existing rendezvous algorithms can be generalized to use multiple radios for faster rendezvous. We then propose a new rendezvous algorithm, called role-based parallel sequence (RPS), which specifically exploits multiple radios for more efficient rendezvous. Our basic idea is to let the cognitive users stay in a specific channel in one dedicated radio and hop on the available channels with parallel sequences in the remaining general radios. We prove that our algorithm provides guaranteed rendezvous (i.e., rendezvous can be completed within a finite time) and derive the upper bounds on the maximum time-to-rendezvous (TTR) and the expected TTR. The simulation results show that i) multiple radios can cost-effectively improve the rendezvous performance, and ii) the proposed RPS algorithm performs better than the ones generalized from the existing algorithms. [ABSTRACT FROM PUBLISHER]

Rendezvous is an essential operation for secondary users communicating in a cognitive radio network (CRN). Because it is infeasible to employ a common control channel in a licensed spectrum, blind rendezvous is an expected solution and generally estimated by the criteria of asynchronous rendezvous support, guaranteed rendezvous, asymmetric model support, heterogeneity of roles, and short time-to-rendezvous (TTR). Most existing blind rendezvous methods fail to fully satisfy these criteria or have considerable TTR significantly increased with the number of channels. This paper presents a new alternate hop-and-wait channel hopping method, called E-AHW, which can support fast blind rendezvous. By assigning each secondary user a unique alternate channel hopping sequence composed of HOP/WAIT-mode subsequences, all blind rendezvous criteria can be satisfied with shorter TTR. According to simulation results, the performance of E-AHW is better than existing methods. Moreover, using E-AHW, the average value and standard deviation of time intervals between two rendezvous are also less than other methods. Thus, E-AHW can provide an efficient and stable communication environment. [ABSTRACT FROM AUTHOR]

Cognitive radio networks (CRNs) have emerged as advanced and promising paradigm to exploit the existing wireless spectrum opportunistically. It is crucial for users in CRNs to search for neighbors via rendezvous process and thereby establish the communication links to exchange the information necessary for spectrum management and channel contention, etc. This paper focuses on the design of algorithms for blind rendezvous, i.e., rendezvous without using any centralized controller and common control channel (CCC). We propose a jump-stay channel-hopping (CH) algorithm for blind rendezvous. The basic idea is to generate CH sequence in rounds and each round consists of a jump-pattern and a stay-pattern. Users “jump” on available channels in the jump-pattern while “stay” on a specific channel in the stay-pattern. We prove that two users can achieve rendezvous in one of four possible pattern combinations: jump-stay, stay-jump, jump-jump, and stay-stay. Compared with the existing CH algorithms, our algorithm has the overall best performance in various scenarios and is applicable to rendezvous of multiuser and multihop scenarios. We derive upper bounds on the maximum time-to-rendezvous (TTR) and the expected TTR of our algorithm for both 2-user and multiuser scenarios (shown in Table 1). Extensive simulations are conducted to evaluate the performance of our algorithm. [ABSTRACT FROM AUTHOR]

In Cognitive Radio networking, the blind rendezvous problem is when two or more nodes must establish a link, but where they have no predefined schedule or common control channel for doing so. The problem becomes more challenging when the information about the existence of other nodes in the network, their topology, and primary user activity are also unknown, identified here as a fully blind rendezvous problem. In this paper, a novel and fully blind multihop (FBM) rendezvous framework is proposed with an extended modular clock algorithm (EMCA). The EMCA-FBM is a fully blind multihop rendezvous protocol as it assumes the number of nodes, primary radio activity and topology information as unknown. It is shown to work with different Cognitive Radio operating policies to achieve adaptiveness towards the unknown primary radio activity, and self-organization for autonomously handling the rendezvous process by using transmission schedules. It is capable of working without any information of neighbor nodes and terminating the rendezvous process whenever all or sufficient nodes are discovered. The proposed FBM is also shown to work as a general framework to extend existing single hop rendezvous protocols to work as a multihop rendezvous protocol. In comparison with other modified blind rendezvous strategies for multihop network, the combination of the proposed EMCA-FBM protocol and operating policies is shown to be effective in improving the average time to rendezvous (up to 70%) and neighbor discovery accuracy (almost 100%) while reducing harmful interference. [ABSTRACT FROM AUTHOR]

Channel rendezvous is an initial and important process for establishing communications between secondary users (SUs) in distributed cognitive radio networks. Due to the drawbacks of the common control channel (CCC) based rendezvous approach, channel hopping (CH) has attracted a lot of research interests for achieving blind rendezvous. To ensure rendezvous within a finite time, most of the existing CH-based rendezvous schemes generate their CH sequences based on the whole global channel set in the network. However, due to the spatial and temporal variations in channel availabilities as well as the limitation of SUs sensing capabilities, the local available channel set (ACS) for each SU is usually a small subset of the global set. Therefore, following these global-based generated CH sequences can result in extensively long time-to-rendezvous (TTR) especially when the number of unavailable channels is large. In this paper, we propose two matrix-based CH rendezvous schemes in which the CH sequences are generated based on the ACSs only. We prove the guaranteed and full diversity rendezvous of the proposed schemes by deriving the theoretical upper bounds of their maximum TTRs. Furthermore, extensive simulation comparisons with other existing works are conducted which illustrate the superior performance of our schemes in terms of the TTR metrics. [ABSTRACT FROM AUTHOR]