Assessing Routing Strategies for Cognitive Radio Sensor Networks

Suleiman Zubair, Norsheila Fisal, Yakubu Baguda, Kashif Saleem
2013 Sensors  
Interest in the cognitive radio sensor network (CRSN) paradigm has gradually grown among researchers. This concept seeks to fuse the benefits of dynamic spectrum access into the sensor network, making it a potential player in the next generation (NextGen) network, which is characterized by ubiquity. Notwithstanding its massive potential, little research activity has been dedicated to the network layer. By contrast, we find recent research trends focusing on the physical layer, the link layer
more » ... the transport layers. The fact that the cross-layer approach is imperative, due to the resource-constrained nature of CRSNs, can make the design of unique solutions non-trivial in this respect. This paper seeks to explore possible design opportunities with wireless sensor networks (WSNs), cognitive radio ad-hoc networks (CRAHNs) and cross-layer considerations for implementing viable CRSN routing solutions. Additionally, a detailed performance evaluation of WSN routing strategies in a cognitive radio environment is performed to expose research gaps. With this work, we intend to lay a foundation for developing CRSN routing solutions and to establish a basis for future work in this area. 13006 Introduction The need for efficient spectrum utilization [1] has recently brought about the new paradigm of cognitive radio sensor networks (CRSNs). The two major drives toward this paradigm are the underutilization of the spectrum below 3 GHz and the congestion problem in both licensed and unlicensed bands. As challenging as this paradigm may appear, the effort of recent studies such as [2, 3] are gradually making this paradigm a reality. Meanwhile, as the World gradually develops into an Internet of Things, the ubiquity of wireless sensor networks (WSNs) is accordingly becoming imperative. This, by implication, further complicates the issue of the congestion of the industrial, scientific and medical (ISM) spectrum and the unlicensed national information infrastructure (UNII), as evidenced by [4] [5] [6] . Notwithstanding the predicted ubiquity of WSNs, other wireless systems such as WiMAX, Bluetooth and Wi-Fi also operate in these bands, along with cordless phones and microwaves. The normal IEEE 802.15.4 standard defines 16 channels, each with a bandwidth of 2 MHz, in the 2.4-GHz ISM band, among which only four are not overlapping with the IEEE 802.11 22-MHz bandwidth channels. It should be noted that these channels sometimes overlap with the channels of IEEE 802.11. If the Wi-Fi deployment uses channels other than 1, 6 and 11, then overlapping will occur. Furthermore, a recent and practical study performed on the co-existence issue showed that, in reality, only three of these channels are actually non-overlapping [7] . In extreme cases where all other networks (e.g., medical sensor networks, security networks, disaster communications, PDAs, Bluetooth devices and many more applications envisioned in the very near future) compete for these four channels, the congestion issue becomes more urgent. The authors of [8, 9] have shown that IEEE 802.11 degrades the performance of 802.15.4 when they operate in overlapping bands, and in [7] a highly variable IEEE 804.15.4 performance drop of approximately 41% was demonstrated. Furthermore, as computing/networking heads toward ubiquity, various WSNs will form a great percentage of this phenomenon. The concept of CRSN aims to address this spectrum utilization challenge by offering sensor nodes temporary usage of vacant primary user (PU) spectra via dynamic spectrum access (DSA) with the condition that they will vacate that spectrum once the presence of the incumbent is detected. With the successful implementation of DSA via cognitive radio (CR), other advantages are exploited by the WSN. The most enticing of these advantages are that the node energy can be significantly conserved by the reduction of collisions, which invariably results in the reduction of retransmission of lost packets. Energy conservation can also be achieved by employing nodes that dynamically change their transmission parameters to suit channel characteristics, thus providing full management control of these valuable resources. This practice, in effect, can also enable the coexistence of various WSNs deployed in a spatially overlapping area in terms of communication and resource utilization. Notwithstanding the potential of this concept, the CRSN comes with its own unique challenges. For example, the practical development/implementation of a CR sensor node is still an unsolved issue. Additionally, because the DSA characteristic affects the entire communication framework of a conventional WSN [2], previous protocols proposed for classical WSNs cannot be directly applied to a CRSN, nor can the communication protocols for ad-hoc networks perfectly fit this context due to the resource constraints. Incorporating the idea of DSA into a WSN changes not only the MAC and PHY layers, but also affects all of the communication. However, the fact that WSNs still remain the launch Sensors 2013, 13 13007 pad for protocol design in CRSNs necessitates a performance study of WSN routing strategies vis-à-vis CRSN requirements [2, 10, 11] . Thus, there is a need for specially adapted communication protocols to fulfill the needs of both DSA and WSNs in a CR context. The network layer is fundamental in any network and is significantly affected by the dynamic radio environment created by CR because it addresses the peer-to-peer delivery through other nodes in a multi-hop fashion to the correct recipients in due time. The sending node must address both its dynamic radio environment and that of the next hop node. This phenomenon is otherwise referred to as the "deafness problem" and introduces a challenging scenario requiring innovative algorithms that consider the intrinsic nature of the sensor nodes. This scenario necessitates a cross-layer approach for designing spectrum-aware routing protocols. A number of researchers have proposed routing schemes for cognitive radio ad-hoc networks [12] . However, due to the differences in constraints between classical ad-hoc networks and WSNs, these solutions cannot be directly imported to solve the problem of routing in CRSNs. Although CRSNs can also be ad-hoc in nature, they differ from classical ad-hoc networks in the following ways: • Sensor networks (SNs) are usually densely deployed, with hundreds of nodes, because the harsh atmosphere to which the nodes are exposed can easily cause node failures. In contrast, ad-hoc networks are not usually densely deployed. • While SNs are highly constrained with respect to memory, energy and computation capabilities, ad-hoc networks usually do not consider these fundamental constraints. • The mode of communication in a SN is usually based on broadcast, whereas ad-hoc networks use point-to-point mode most of the time. • SNs usually have the communication goal of data aggregation, in addition to the plain communication goal of ad-hoc networks. • Addressing schemes in SNs are significantly different from those applied in traditional ad-hoc networks because of the enormous overhead of schemes such as IP addresses and GPS coordinates. • Finally, SNs have periods in which they "sleep" to conserve energy, whereas nodes in most ad-hoc networks do not have this property. To the best of our knowledge, specific attention has not been given to routing in the network layer of CRSNs, although recent research has emphasized the transport [10, 11] , MAC and physical layers [10, 12, 13] . Hence, there is the need for research to focus on this area. We present a review of WSN routing strategies vis-à-vis CRSN requirements to evaluate the strengths and weaknesses of each strategy. This review is provided to enable protocol designers to use quantitative evidence in selecting the strategies best suited to their application. The paper then discusses the factors affecting routing CRSNs, reviews recent studies in this area and categorizes them appropriately. Open issues in this respect are also identified. The paper further identifies major CRSN routing components and presents a systematic review of relevant studies in each category to reveal the open issues. The main contributions of this paper are as follows: • To identify a research gap in the network layer of CRSNs. • To evaluate WSN routing strategies vis-à-vis CRSN requirements. • To propose cross-layer and routing frameworks for routing in CRSNs. • To discuss the main components of routing in CRSNs vis-à-vis recent studies to reveal open areas.
doi:10.3390/s131013005 pmid:24077319 pmcid:PMC3859047 fatcat:xdle3amrnndxrmvjgnngfg6c5i