Coexistence server in Cognitive Networks: A real implementation
Introduction
The limited availability added to the inefficient use of the frequency spectrum demands for new mechanisms and communication paradigms that exploit the existing spectrum more efficiently [1]. Cognitive Networks, also called Cognitive Radio Networks and Next Generation Wireless Networks [2], [3], are a network technology that increases the efficiency of spectrum allocation through opportunistic access to given frequency bands. Some potential scenarios for Cognitive Networks implementation are: Vehicular Networks [4], [5], Smart Grids [6], [7], [8], Sensor Networks [9], [10], [11], UAVs (Unmanned Aerial Networks) [12], Underwater Networks [13], [14], Public Safety Communications [15] and Medical Body Area Networks [16].
Cognitive radio technology may access unused and underutilized frequency spectrum spaces, referred to as white spaces [17] (also called spectrum holes) in licensed and unlicensed bands for transmission [18], [19]. If such bands start to be used by a licensed user, all cognitive radios must change its operating channel to another spectral hole, or remain in the same band, changing its transmission power or modulation scheme in order to avoid interference.
In search of alternatives for a more efficient use of the available frequency bands, regulatory agencies worldwide began to regulate the secondary use of the TV White Spaces (TVWS) [20], [21], [22], which refer to analog TV channels being released due to the deployment of digital TV broadcasts. These regulations permit the secondary use by unlicensed wireless devices as long as they always defer channel usage to the primary licensed user. This is one of the first applications of cognitive radios.
The potential benefits brought by TVWS communications are, however, as large as the challenge it imposes. Firstly, there is the issue of efficient and organized use of the VHF/UHF spectrum, which motivated several standardization efforts, as the creation of the IEEE 802.22 Working Group [23] for WRANs (Wireless Rural Area Networks), the IEEE 802.11af amendment [24] and the ECMA-392 standard [25] for WLANs (Wireless Local Area Networks). Secondly, non-technical questions arise from TV broadcasters strongly opposing the sharing of their dedicated spectrum (licensed) with secondary users [26].
In IEEE 802.11af, spectrum sensing is used to estimate TVWS availability. In IEEE 802.22, TVWS selection is based on queries in spectrum management database. If multiples networks operate on different standards (i.e., IEEE 802.11af and IEEE 802.22) in the same geolocation and in the same frequency, severe interference can be imposed [27], [28].
Thus, a more recent challenge is to prevent harmful interference between multiple secondary networks who share the TVWS spectrum. This problem has attracted so much attention that the IEEE Wireless Coexistence Working Group (IEEE 802.19) created a subgroup specifically dedicated to define coexistence methods for TVWS, regardless of the radio technology (PHY or MAC layers) employed by the secondary user. The IEEE 802.19.1 standard [29] was published in 2014 and works in a high-level approach, in which new designs for physical and MAC layers are not considered. The standardization allows the production of compatible solutions by different vendors [30].
This article presents an architecture for coexistence of secondary TVWS wireless networks which adheres to the IEEE 802.19.1 architecture. The key element of the architecture is a central coexistence server that acts as:
- 1.
Coexistence Discovery and Information Server (CDIS), which scans and stores coexistence information from different areas.
- 2.
Coexistence Manager (CM), which redefines the channels and transmission powers of managed coexisting nodes in the network, considering radio environment characteristics, and the interference generated by elements that are not controlled by the architecture.
Practical results from a real implementation of the coexistence server are presented. Also, for contextualization, an overview of the problems of coexistence in TVWS is also provided in Section 2. The rest of this paper is organized as follows. Section 3 presents an overview of the IEEE 802.19.1 standard. Section 4 describes our real implementation of an IEEE 802.19.1 architecture, including novel algorithms to reduce network interference. Section 5 presents the results on a real testbed. A discussion about the results and the IEEE 802.19.1 standard is addressed in Section 6. Finally, Section 7 presents conclusions and future works.
Section snippets
Coexistence in Cognitive Networks
A natural consequence of multiple networks with cluttered access to TVWS is uncontrollable interference, resulting in the inability to coexist. The lack of effective coexistence can prevents the full exploitation of TVWS and significantly reduces their use.
Coexistence in cognitive networks is presented as a problem where two or more such networks use secondary spectrum bands concurrently. The major problem of the coexistence in these networks is the need to detect the primary user in the
IEEE 802.19.1
The IEEE 802.19 Working Group 1 produced a System Design Document (SDD) to describe the coexistence system [36]. The SDD includes the system requirements and the IEEE 802.19.1 architecture. The working group has finalized the standard in September 2014 [29]. In this section, we describe some relevant points of the standard.
Implementation of coexistence server
In order to make a wireless network to have a good performance, excessive interference must be avoided. Upon network deployment this interference can be avoided, for example, by choosing which channels will be used by each access points (AP).
However, a static configuration will be obsolete in a dynamic environment, where new networks may arise or be turned off. Another issue that must be considered is the difficulty of managing a network that contains many APs. As this type of equipment lacks a
Scenario
The evaluation of channel allocation and power control algorithms was performed in a real infrastructured IEEE 802.11g network. The scenario consisted of: a Coexistence Server and seven APs, three CEAPs and four UAPs. In addition to this structure, seven laptops were used as WSO devices, each one associated with a respective AP.
The basic idea was to assess a simple network scenario among several that may benefit from the use of the CS and their mechanisms of interference mitigation. The
Discussion
In IEEE 802.19.1 Standard, the CM is the fundamental piece to coordinate and maintain coexistence between devices. Furthermore, CDIS acts as a link between all WSOs, aggregating the information provided by CMs and maintaining a centralized vision of the area of operation of all CMs. CE forms a bridge between WSO and the coexistence architecture, and therefore should be implemented in devices operating in the TVWS.
In this work, the CM and CDIS were deployed as a single CS, and the CE implemented
Conclusion
This work presents an implementation of an architecture that deals with the problem of coexistence, in which a coexistence server, acting as coexistence manager and coexistence discovery and information server, redefines channels and transmission power of CEAPs, depending on the radio environment. It considers the regulations imposed by the FCC, and the IEEE 802.19.1 standard that was developed for the treatment of coexistence. Results show that the channel allocation algorithm is able to
Acknowledgments
This work is supported in part by CNPq, CAPES, FAPERJ, TBE/ANEEL and CELESC/ANEEL.
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