Distributed Digital Radios and WLAN Interoperability

In addition to the speaker and the microphone, the radio handset contains the baseband electronics for a digital radio. This includes an Ethernet jack, a full network protocol stack, an OFDM baseband modem and a user interface to place calls: one-to-one (private line) or one-to-many (talk groups). The handset plugs into an ordinary land mobile radio that is used only for up conversion to RF and power amplification. No modification to the radio is needed. Using existing radios means the existing infrastructure and existing channel frequencies are also used. This is a significant cost savings to achieve interoperability, and it immediately puts the technology into the marketplace. A distributed digital radio (DDR) is a 6-way radio: voice/RF, voice/Ethernet and Ethernet/RF. In computer network terminology, a DDR is a hybrid router - hybrid is used because the PHY layer at each of the ports is different: audio, RF or Ethernet. A public safety radio network with DDRs becomes a wireless local area network (WLAN). The WLAN is not WiFi; it is a client mesh with an Ethernet backplane. The DDR provides 2G communications and cost effective IP-based interoperability to all first responders - even to the local and rural public safety agencies with conventional analog radio networks. This paper discusses WLAN interoperability architectures, and it traces the development of the DDR.