Real-World Experimentation of Distributed DSA Network Algorithms

Introduction

The scarcity of frequency spectrum resources and their increasing fragmentation are among the main issues hindering the achievement of very large data rates in future wireless communication networks. In this context the improvement of network capacity by exploiting large continuous spectrum chunks is becoming increasingly challenging. The traditional rigid and exclusive assignation of frequency bands to licensed users may represent an obstacle to the efficient utilization of the spectrum. Discussions were raised for example, concerning the under-utilization of UHF frequency channels and about the limited adaptability of standard frequency reuse schemes to the operations in dynamic scenarios. Dynamic Spectrum Access (DSA) techniques have drawn considerable attention, by both industry and academia, providing increased opportunities for the sharing and reusing of spectrum resources among multiple and heterogeneous wireless networks. The concept of DSA may be discussed within the broader research paradigm of Cognitive Radio (CR)(Haykin, 2005). Cognitive radios are devices able to “learn” from the surrounding environment and reconfigure their transmission parameters in order to optimize their communication capabilities.

A large variety of CR/DSA solutions has been presented in literature, covering a wide set of application areas. CR concepts for military or public safety purposes for example, focus on the setup of a communication network in challenging transmission conditions and/or deployment scenarios. The exploitation of TV white spaces instead, aims at enabling opportunistic spectrum access to secondary users. DSA concepts can also be applied to commercial mobile networks. Future generation wireless standards such as Long Term Evolution – Advanced (LTE-A) (3GPP, 2008) are expected to cope with the dramatic increase of demand in data services. Very high data rate in local-area (LA) is foreseen as a challenging requirement in relation to the limited and very costly resources in current licensed bands. In a potential operative scenario, operators may share frequency resources in order to enable the usage of a large transmission bandwidth. Moreover, the setup of a network in indoor home/office environments is expected to be characterized by dense and uncoordinated deployment of access points (APs)(Chandrasekhar, Andrews, & Gatherer, 2008). A planned reuse of frequencies under these circumstances is likely to be uneffective and thus the network may experience unbearable inter-cell interference. The exploitation of DSA and CR techniques in such scenarios can provide significant advantages. By dynamically adapting the allocated resources by the APs, the generated interference is reduced and an increased spectrum utilization can be achieved. In addition, if considering the applicability of these concetps over wide network deployments, distributed algorithmic solutions typically provide a larger scalability in respect to pure centralized approaches.

Current research activities with CR/DSA algorithms for resource management at network level mainly rely on the extensive usage of Monte Carlo simulations for validation purposes. The benefits of simulation-based studies are evident in terms of reduced implementation effort and statistical coverage of scenario parameters. In simulators it is however challenging to accurately model the propagation environment; simplified deployment scenarios and standard channel models are therefore commonly utilized (e.g., WINNER model(Hentilä, Kyösti, Käske, Narandzic, & Alatossava, 2012)). Despite the predominance of simulation-based works in literature, there is an increasing interest for more tangible evidence of the effectiveness of CR concepts. Experimental activities in realistic deployment environments, including terminals mobility and human presence, can provide the missing proofs for a complete validation of the proposed solutions. The setup of experiments for distributed network algorithms requires the realization of a testbed and the definition of adequate experimental procedures. In literature a considerable amount of work has been proposed in the field of platforms and development tools for CR testbeds. The early interest by the research community for the opportunistic usage of the spectrum, and military applications, resulted in the early development of testbed platforms primarly focusing on specific physical (PHY) layer features such as spectrum sensing, waveform adaptation and reconfigurable transceivers. The related experimental experiences mainly consider a limited number of nodes in the network testbeds and focus on point-to-point communication aspects.