iDEN is a wireless communication architecture developed by Motorola in the early 1990's. It was designed as a high-concentration replacement for the then standard DAMPS system. iDEN operates mostly in the 800MHz range, though new deployments are using the 1.5GHz range.
The core of the iDEN system is the TDMA transmission standard which Motorola also helped develop. The voice signals are coded using VSELP, or Vector Sum Excited Linear Prediction which serves for compressing the voice data before transmission. TDMA also works in concert with QAM, Quad Amplitude Modulation which allows 64kbps over the 25mHz TDMA channel.
Among other features intrinsic with the iDEN system is the capability of direct handset communications where two iDEN handsets can form communications loop between them (commonly refereed to as "radio mode" or "walkie-talkie mode"). iDEN also offers rudimentary triangularion and GPS locating facilities because the TDMA protocol works in conjunction with GPS satellite timecodes.