Macroscopic Space-Time Codes for Homeland Security

这项研究包括开发、安装和验证一组错误控制代码，这些代码用于各个基站，以极大地提高公共安全无线电系统（即警察和消防员使用的无线电系统）的性能。特别是，最近的事件表明，在极端危机时期对公共安全无线电系统提出了严格的要求。这种系统必须处理高优先级的延迟敏感语音传输，这些语音传输通常需要通过具有挑战性的无线通信信道向大量接收器广播。无线信道可以引入明显的信号衰落，而公共安全无线电系统中单天线基站的窄带频率分配和电流收集使得使用先前考虑的技术难以对抗这种衰落的影响。在多个基站之间使用单一错误控制码可以改善对该地区的一组系统用户的广播，但是这种方法存在许多技术障碍。最根本的问题是，由于发射基站所覆盖的地理区域很大，来自不同基站的信号到达接收器的相对时间可能相差很大，而且由于应用是局部广播，再多的相对发射时间也无法弥补这一点。研究小组已经证明，在这种情况下，许多标准的时空方案由于信号衰落而失去了分集；然而，该团队正在开发对这种偏移具有鲁棒性的代码族，从而在覆盖区域内保持多样性。这些代码被称为“宏观时空代码”，除了正在开发之外，还正在合作伙伴M/A-COM， Inc.的OpenSky测试平台上进行验证。

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.