Over the last two decades, dynamic and opportunistic spectrum access (DSA/OSA) has been advocated as a new paradigm for improving the utilization of the licensed spectrum below 6 GHz. Spectrum regulators (e.g., FCC, NTIA, etc.) embarked on numerous initiatives that aim at identifying underutilized portions of the licensed spectrum and exploring new models for spectrum sharing among different types of users. These initiatives include the FCC's ruling on TV broadcast channels in the 54 to 698 MHz range (so-called TV whitespaces or TVWS), the FCC's Notice of Proposed Rule Making (NPRM) on the 3.55-3.65 GHz radar band (which lays the groundwork for coexistence between small-cell LTE systems, military radar, and other incumbent systems), and others. Similar initiatives are taking place in other regions of the world, such as the European Union (EU)'s efforts to explore the 2.2-2.3 GHz band for spectrum sharing between aeronautical telemetry radar and small-cell LTE systems. The role of radio spectrum as a critical economic growth engine was highlighted in the 2012 President's Council of Advisors on Science and Technology (PCAST) report, which recommended creating "the first shared-use spectrum superhighways."
Yet, despite this strong push, commercial DSA wireless services have not proliferated. This is in part due to the limited availability of TVWS in urban areas and, more critically, their spatial non-uniformity across the Continental US. Broadcast TV stations in the US are licensed to operate over 6 MHz wide (NTSC) channels that fall into four noncontiguous segments of the spectrum (52-72 MHz, 76-88 MHz, 174-216 MHz, and 470-698 MHz). In a given urban location, the number of TV channels available for opportunistic access can be as low as 2 to 4, after accounting for FCC protection rules (e.g., power masks, adjacent-channel interference, etc.) and "TV pollution" zones, i.e., areas where TV signals are not decodable but are strong enough to disrupt secondary (opportunistic) transmissions. Not only that these channels are too few, but they often have to be shared by several secondary users (SUs), raising the prospects of SU-SU interference. The non-contiguity of available TV channels further complicates matters, as complex carrier/channel aggregation techniques must now be used over a given link to achieve WiFi-comparable channel widths (e.g., 20 MHz and higher).
The overarching goal of the underlying project is to enable DSA operation over a large swath of the opportunistic spectrum below 6 GHz, including the UHF portion of TVWS (470-698 MHz segment), the 3.5 GHz radar band, as well as other bands in the 1 to 4.4 GHz range that may later become candidates for DSA. By aggregating different (not necessarily contiguous) portions of the RF spectrum at the same wireless device, we anticipate that variations in white/grey spaces can be significantly reduced, allowing for sustained DSA operation over a large geographical area. Enabling such operation while maintaining high link throughput requires advances in the following areas:
- Electrically small tunable antennas that allow for dynamic access to narrowband (e.g., 6 MHz) channels, but yet support a broad frequency range from 470 MHz to 4.4 GHz.
- Channel/carrier aggregation techniques for noncontiguous narrowband channels.
- Multi-channel MIMO functionality that allows a given secondary link to boost its throughput (via spatial multiplexing gain) and, simultaneously, minimize interference onto other coexisting systems (via MIMO precoding techniques).
The first capability is intended to beef up the agility of the secondary radio, allowing it (for example) to switch between TVWS and the radar band, on demand. The last two capabilities are intended to significantly increase the per-link throughput and reduce SU-SU interference. Through novel advances in the above areas, we plan to design, implement, and experimentally evaluate a multi-channel 2-by-2 MIMO system for DSA with a broad RF coverage. Our research agenda involves optimizations at both the signal level (via MIMO precoders) as well as the antenna level (via highly tunable small form-factor reconfigurable antennas).
Control dimensions in a CMIMO WLAN: (a) Frequency allocation among multiple links, (b) power allocation over frequencies and antennas, and (c) beamforming-based interference suppression. The example shows a WLAN of 4 links, 2 antennas/node, and 3 channels.