This research involves the development, installation, and verification of a collection of error control codes that are employed across base stations to vastly improve the performance of public safety radio systems (i.e. those used by police officers and firefighters). In particular, recent events have demonstrated the stringent demands placed on public safety radio systems in times of extreme crisis. Such systems must handle high-priority delay-sensitive voice transmissions that often need to be broadcast to a large number of receivers over the challenging wireless communications channel. The wireless channel can introduce significant signal fading, and the narrowband frequency allocations and current collection of single-antenna base stations in public safety radio systems make it difficult to combat the effects of such fading using previously considered techniques. Employing a single error control code across multiple base stations can improve the broadcast to a group of system users in the area, but there are a number of technical barriers to such an approach.
The fundamental problem is that, because of the large geographical area covered by the transmitting base stations, the relative time of arrival at the receiver of signals from different base stations can vary greatly, and, since the application is partial broadcast, no amount of relative transmission timing can remedy such. The research team has demonstrated that many standard space-time schemes lose diversity versus signal fading in such a situation; however, the team is developing families of codes that are robust to this offset, thus preserving diversity against fading across the coverage area. These codes are termed "macroscopic space-time codes," which, in addition to being developed, are being verified on the OpenSky testbed of partner M/A-COM, Inc.
