With the last large spectrum auction at 700 MHz almost behind us, the current spectrum supply and demand model threatens to constrain not only our appetite for new broadband applications but also restrict innovative companies from launching new wireless services. Enter a new approach known as dynamic spectrum access (DSA) that was first demonstrated in 2006 by the Defense Advanced Research Projects Agency (DARPA) and Shared Spectrum Company (SSC) of Vienna, Va. DSA software technology enables users of virtually any modern radio device to utilize dynamic spectrum access techniques and thereby dramatically improve spectrum efficiency, communications reliability, and deployment time. Fundamentally, a device running DSA software dynamically senses and adapts to its radio frequency (RF) environment to maintain reliable communications with other DSA-enabled devices, and it does so without interference with non-DSA-enabled systems (i.e., non-cooperative or legacy radios). Furthermore, a DSA-enabled radio operates within prescribed policy constraints, which may vary depending upon geographic location, frequency band, time of day, legacy radio activity and other anticipated or unanticipated factors. DSA presents a real opportunity to harvest licensed, underutilized spectrum to enhance everything from mission critical wireless access to consumer wireless consumption. DSA technology can change our use of spectrum assets the way the IP protocol changed our use of traditional switched network communications^[5].

The U.S. Government has long been concerned about spectrum depletion of available spectrum in communities assigned stove-piped frequencies from a larger frequency pool. For example, the Federal Aviation Administration (FAA) has estimated that its VHF spectrum will be exhausted by 2015 due to growth in spectrum usage^[1]. The White House has been concerned enough to start a Presidential Spectrum Policy Initiative and create a multi-agency Federal Government Spectrum Task Force^[2] to focus on interagency initiatives to use spectrum better and more efficiently. This calls attention to the fact that spectrum is a finite resource of great economic value. National Science Foundation (NSF)-funded spectrum usage measurements have shown that the actual amount of spectrum used, averaged over time, space, and frequency, is about 14%, with about a 2-3% swing depending on the location measured^[3,4]. This is the busy-period percentage during the day, when wireless traffic is at its peak, and is true in the SSC has taken spectrum measurements at several major U.S. cities and determined that at times some licensed bands are rarely used at all (Figure 01). This means that spectrum can be re-used in real time if radios were sufficiently nimble and well designed, and if re-use protocols could be developed that could take advantage of the remaining 86%.

On Aug. 15-17, 2006, Shared Spectrum Company and the U.S. Department of Defense's Defense Advanced Research Projects Agency (DARPA) demonstrated, for the first time, a multinode network of neXt Generation (XG) radios capable of using spectrum over a wide range of frequencies on a secondary basis. The cognitive radios making up several formations of XG networks sensed radio signals over 225-600 MHz and adapted frequencies automatically to prevent interfering with existing military and civilian radio systems. The XG networks were tested on a military test range at Fort A.P. Hill in Bowling Green, VA, in front of an audience of more than one hundred military and government spectrum management agency representatives from the Army, Navy, Air Force, Coast Guard, Joint Spectrum Center, the Federal Communications Commission and the National Telecommunications and Information Administration^[5].

DARPA, recognizing the convergence of radio technology advances with increased need for better use of spectrum, developed the XG radio communications program^[5], which focused on the transformation of spectrum assignment technology for DoD applications through dynamic spectral use. The burden of sharing spectrum falls on XG radios, which must detect legacy radio systems (that are “primary” users of the frequencies in terms of regulatory priority) and not interfere with them, without a priori knowledge of the legacy radio waveform, MAC layer protocol, radio positions, transmit powers, or receiver sensitivities. Since legacy radio systems have primary access rights to the spectrum, many were not designed to cooperate or interoperate with secondary users like the XG radios; hence, these legacy radios are often referred to as “non-cooperative^[5].” The program is currently in the Phase III transition stage and SSC is working with Harris Corporation and Thales Communications to test and integrate Dynamic Spectrum Access into their widely deployed Harris PRC152 Falcon III handheld and Thales PRC148 JTRS-enabled MBITR radios.

The SSC dynamic spectrum access solution is a radio software solution comprising the core components depicted in Figure 2. The key features of how dynamic spectrum access both learns and adapts are grouped into four key categories: frequency detection, spectrum management and neighbor discovery and channel maintenance.

Frequency detection: DSA software uses information from the radio detector(s) and/or the policy subsystems to manage spectrum usage. DSA maintains reliable communications while avoiding interference and ensuring compliance with the existing constraints. Ultra-high sensitivity detectors are used for a variety of different bands of interest. In cases where detectors are not sufficient or prohibited DSA software uses geo-location and group-sensing techniques. The scalability and seamless integration into existing network architectures is achieved through the frequency topology module that combines the detector information, network information as well as policy information to continuously track networks of interest in frequency.

Spectrum manager: The spectrum manager module uses information from both the detector and the spectrum policy module to dynamically manage spectrum access to maintain reliable communications while avoiding interference and ensuring compliance with prescribed policy constraints.

Channel maintenance and neighbor discovery modules: The channel maintenance and neighbor discovery modules establish an operating channel by determining and negotiating an operating frequency for a particular network of DSA-enabled radio nodes. In this process, the radios establish a means of rendezvous when one or more DSA-enabled radios are impacted by frequency interference.

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