Cognitive Radios have been advanced as a technology for the opportunistic use of under-utilized spectrum since they are able to sense the spectrum and use frequency bands if no Primary user is detected. However, the required sensitivity is very demanding since any individual radio might face a deep fade. We propose light-weight cooperation in sensing based on hard decisions to mitigate the sensitivity requirements on individual radios. We show that the "link budget" that system designers have to reserve for fading is a significant function of the required probability of detection. Even a few cooperating users (~10-20) facing independent fades are enough to achieve practical threshold levels by drastically reducing individual detection requirements. Hard decisions perform almost as well as soft decisions in achieving these gains. Cooperative gains in a environment where shadowing is correlated, is limited by the cooperation footprint (area in which users cooperate). In essence, a few independent users are more robust than many correlated users. Unfortunately, cooperative gain is very sensitive to adversarial/failing Cognitive Radios. Radios that fail in a known way (always report the presence/absence of a Primary user) can be compensated for by censoring them. On the other hand, radios that fail in unmodeled ways or may be malicious, introduce a bound on achievable sensitivity reductions. As a rule of thumb, if we believe that 1/N users can fail in an unknown way, then the cooperation gains are limited to what is possible with N trusted users.

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Cooperative Sensing among Cognitive Radios

The new paradigm of dynamic spectrum access (DSA) networks aims to provide opportunistic access to large parts of the underutilized spectrum. The majority of research in this area has focused on free-for-all, uncoordinated access methods common in ad-hoc military applications (Horne, W. 2003; Leaves, P. et al., 2002; Lehr, W. et al., 2002; Schafer, D.J.; To/spl uml/njes, R., 2002). We argue that a simpler pragmatic approach that offers coordinated, spatially aggregated spectrum access via a regional spectrum broker is more attractive in the immediate future. We first introduce two new concepts, coordinated access band (CAB) and statistically multiplexed access (SMA), to the spectrum. We describe their implementation in the new DIMSUMnet (dynamic intelligent management of spectrum for ubiquitous mobile-access network) architecture consisting of four elements: base stations; clients; a radio access network manager (RAN-MAN) that obtains spectrum leases; a per-domain spectrum broker that controls spectrum access. We also discuss in detail various issues in the design of spectrum brokers and spectrum allocation policies and algorithms.

DIMSUMnet: new directions in wireless networking using coordinated dynamic spectrum

This article discusses recent standardization efforts related to cognitive radio focusing on the work of IEEE Standards Coordinating Committee 41, formerly known as IEEE 1900. Some important tasks to be performed by the CR standardization community also are presented. These tasks will expedite the introduction of CR devices to the market while promoting a fair use of scarce radio resources. Some avenues for using the currently available standards for rapid deployment of CR devices, such as ISO standards, also are discussed.

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Cognitive functionality in next generation wireless networks: standardization efforts

In future applications, next generation spectrally agile software defined radios (SASDR) will be equipped with advanced radio frequency (RF) sensors that will enable them to sense their environment and adaptively select the best and most stable portions of the spectrum to establish communications. This paper presents a robust method for estimating locally the unused or underutilized regions of RF spectrum to optimize dynamic spectrum usage. These adaptive algorithms are described and results of "open spectrum" estimation performed on real RF spectrum measurements illustrate the robust performance of the approach. Simulation results are shown along with results from real measurements that describe the RF resources in rural, suburban and urban areas in and around Philadelphia in the VHF and UHF bands. The measurements represent a good dynamic data set including motion and spatio-temporal diversity. The effect on reducing the risk of interference by collaborating across multiple nodes that provides increased robustness of sensing of open spectrum through spatial diversity is also discussed and quantitative values for the interference mitigation under collaborative behaviors are also presented

A scalable dynamic spectrum allocation system with interference mitigation for teams of spectrally agile software defined radios

We consider the concept of opportunistic spectrum access (OSA) -- whereby radios identify unused portions of licensed spectrum, and utilize that spectrum without adverse impact on the primary licensees. OSA allows both dramatically higher spectrum utilization and near-zero deployment time, with an obvious and significant impact on both civilian and military communications. We discuss two broad classes of challenges to OSA: spectrum agility, which involves wideband sensing, opportunity identification, coordination and use; and policy agility, which enables regulatory policies to be applied dynamically using machine understandable policies. Focusing on spectrum agility, we present an architecture based on an OSA adaptation layer. We describe protocols for OSA, including a hole information protocol, idle channel selection and use, and an access protocol for the coordination channel. We present a simulation study, discuss insights, and show that even a simple protocol for opportunistic spectrum allocation can provide an order-of-magnitude performance improvement in throughput over a legacy system.

Opportunistic spectrum access: challenges, architecture, protocols

Measurement Procedure and Results Using a Rohde & Schwarz EPSI Spectrum Analyzer connected to a laptop for data gathering purposes, several hours of data were collected over a contiguous range of frequencies between 30MHz and 3GHz. The measurement equipment was placed in an electromagnetically shielded enclosure to ensure the cleanest possible spectrum readings. Two types of antennae, each with particular benefits for use at frequencies above and below 1GHz, were mounted above the highest point of the roof, feeding signals to the spectrum analyzer via several meters of RG-8 coaxial cable.

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Mark McHenry

Papers

Dupont Circle Spectrum Utilization During Peak Hours