Cognitive radio technology: From distributed spectrum coordination to adaptive network collaboration

Abstract

This paper presents an integrated view of cognitive radio technologies for efficient wireless services in dense spectrum environments. The rationale for cognitive radio based systems is discussed, leading to an identification of the available design space that ranges from reactive interference avoidance to spectrum etiquette and eventually network collaboration. After reviewing prior work in the dynamic spectrum area, a specific distributed spectrum etiquette protocol called “common spectrum coordination channel (CSCC)” is introduced. Performance gains achieved with CSCC relative to simpler reactive time/frequency/power control algorithms are evaluated for example in WiFi/Bluetooth and WiFi/WiMax co-existence scenarios. The next level of system performance can be achieved through opportunistic collaboration between radios to form ad hoc multi-hop networks in which neighboring nodes associate with each other at high bit-rate and low power. Adaptive wireless networks of this type will require new protocol architectures which integrate flexible PHY/MAC and cross-layer capabilities with ad hoc network discovery and multi-hop routing. A specific “CogNet” protocol architecture based on the concept of a “global control plane (GCP)” is described. Major CogNet protocol modules for bootstrapping, discovery, data path setup and naming/addressing are outlined, and representative $ns\text{-}2$ simulation results are provided for validation. In conclusion, the paper gives a preview of the network-centric WiNC2R prototype under development at WINLAB as an experimental cognitive radio platform.

Introduction

Recent “Moore’s law” advances in programmable integrated circuits have created an opportunity to develop a new class of intelligent or “cognitive” radios [1], [2], [3], [4] which can adapt to a wide variety of radio interference conditions and multiple protocol standards for collaboration between otherwise incompatible systems. Such a cognitive radio would be capable of very dynamic physical layer adaptation via scanning of available spectrum, selection from a wide range of operating frequencies (possibly non-contiguous), rapid adjustment of modulation waveforms and adaptive power control. In addition, a suitably designed cognitive radio with a software-defined physical layer would be capable of collaborating with neighboring radios to ameliorate interference using higher-layer protocols. These higher-layer coordination protocols could range from multi-node signal combining and coding methods to etiquette mechanisms all the way to fully collaborative multi-hop forwarding between radio nodes. Thus, suitably designed cognitive radios have the potential for creating a next-generation adaptive wireless network, [5] in which a single universal radio device is capable of operating in a variety of spectrum allocation and interference conditions by selecting appropriate physical and network layer parameters often in collaboration with other radios operating in the same region. Such a “cognitive network” will lead to increased network capacity and user performance. Perhaps for the first time in the short history of networking, cognitive radios offer the potential for organic formation of infrastructure-less collaborative network clusters with dynamic adaptation at every layer of the protocol stack including physical, link and network layers [6], [7].

While the development of cognitive radio hardware and software, especially at the physical layer, has received considerable attention, the question of how one transforms a set of cognitive radios into a cognitive network is much less well understood, and there is a lack of research on protocols for cognitive radio networks in the community. As such, adaptive networks of cognitive radios represent an important but demanding research challenge for both the wireless and networking communities. The extreme flexibility of cognitive radios has significant implications for the design of network algorithms and protocols at both local/access network and global internetworking levels. In particular, support for cross-layer algorithms which adapt to changes in physical link quality, radio interference, radio node density, network topology or traffic demand may be expected to require an advanced control and management framework with support for cross-layer information and inter-node collaboration. At the wireless local-area network level, an important technical challenge is that of distributing and managing this inter-node and cross-layer information than using this control information to design stable adaptive networking algorithms that are not overly complex. At the global internetworking level, clusters of cognitive radios represent a new category of access network that needs to be interfaced efficiently with the wired network infrastructure both in terms of control and data. End-to-end architecture issues of importance include naming and addressing consistent with the needs of self-organizing network clusters, as well as the definition of sufficiently aggregated control and management interfaces between cognitive radio networks and the global Internet [8].

This paper presents an integrated view of cognitive radio technology as it evolves from autonomous interference avoidance methods to explicit spectrum etiquette protocols and eventually to adaptive wireless networks of collaborating radios. We start with a discussion of the rationale for cognitive radios, leading to an identification of the available design space defined in terms of hardware capabilities and protocol complexity. Different levels of spectrum coordination methods will be introduced, ranging from autonomous reactive control [9] of radio parameters (time/frequency/power) to more complex proactive coordination schemes [10] based on explicit spectrum etiquette protocols, which define rules or “etiquettes” for how to utilize and share spectrum resources between wireless devices by allowing them to exchange appropriate messages and parameters. The protocol called “common spectrum coordination channel (CSCC)” [11] is proposed as a specific spectrum etiquette solution, and is evaluated using example WiFi/Bluetooth and WiFi/WiMax co-existence scenarios. The next step up from spectrum etiquette is the concept of collaborative networks of cognitive radios, an approach which may be expected to provide significant performance gains in dense usage scenarios. In a collaborative adaptive wireless network, radio nodes avoid interference at the PHY and MAC layers by opportunistically forming or joining an ad hoc network which carries data packets (at relatively high speed and low power) over multiple radio hops. A specific protocol architecture (“CogNet”) [12] based on the concept of a cleanly separated “global control plane (GCP)” [13] is introduced as a candidate architecture for these adaptive wireless networks. The GCP supports spectrum coordination, PHY/MAC adaptation, ad hoc network discovery and cross-layer routing requirements which arise in a general adaptive wireless network scenario. This paper will provide design and validation results for a baseline CogNet protocol design that includes node bootstrapping, discovery, addressing and routing.

Another aspect of importance for the development of cognitive radio networks is the platform technology in terms of both hardware and software. A number of cognitive radio platform development projects are under way in the research community, including the Vanu SDR [14], WARP [15], KU Radio [16], WiNC2R [17] and GNU USRP [18] boards. The WINLAB network-centric cognitive radio (WiNC2R) architecture is aimed at providing a high-performance platform for experimentation with various adaptive wireless network protocols ranging from simple etiquettes to more complex ad hoc collaboration. The WiNC2R board is differentiated from other cognitive radio projects in the sense that the design uses hardware accelerators to achieve programmability and high performance at each layer of the protocol stack. We will present the results of prototype development in progress to give an idea of representative hardware architecture and implementation issues that arise in this emerging field.

The following sections provide more detail on each of the topics outlined above. Section 2 discusses the spectrum coordination problem and alternative reactive and proactive etiquette protocol based solutions. Section 3 describes a specific CogNet protocol architecture for cognitive radio networks along with some validation results. Finally, the WiNC2R hardware platform under development is briefly outlined in Section 4. Concluding remarks and future work are given in Section 5.