The iDEN project originally began as MIRS (Motorola Integrated Radio System) in early 1991. The project was a software lab experiment focused on the utilization of discontiguous spectrum for GSM wireless. GSM systems typically require 24 contiguous voice channels, but the original MIRS software platform dynamically selected fragmented channels in the radio frequency (RF) spectrum in such a way that a GSM telecom switch could commence a phone call the same as it would in the contiguous channel scenario. The original MIRS System was renamed IDEN in 1994 by Roger Cameron Wood, a young engineering program manager leading the product development effort who also gave the handsets their distinctive industrial design, group software features, and "chirp" audio cue.
iDEN is designed and licensed to operate on individual frequencies that may not be contiguous. iDEN operates on 25 kHz channels, but only occupies 20 kHz in order to provide interference protection via guard bands. By comparison, TDMA Cellular (Digital AMPS) is licensed in blocks of 30 kHz channels, but each emission occupies 40 kHz, and is capable of serving the same number of subscribers per channel as iDEN. iDEN uses frequency-division duplexing to transmit and receive signals separately, with transmit and receive bands separated by 39 MHz, 45 MHz, or 48 MHz depending on the frequency band being used.
iDEN supports either three or six interconnect users (phone users) per channel, and six dispatch users (push-to-talk users) per channel, using time division multiple access. The transmit and receive time slots assigned to each user are deliberately offset in time so that a single user never needs to transmit and receive at the same time. This eliminates the need for a duplexer at the mobile end, since time-division duplexing of RF section usage can be performed.
The first commercial iDEN handset was Motorola's L3000, which was released in 1994. Lingo, which stands for Link People on the Go, was used as a logo for its earlier handsets. Most modern iDEN handsets use SIM cards, similar to, but incompatible with GSM handsets' SIM cards. Early iDEN models such as the i1000plus stored all subscriber information inside the handset itself, requiring the data to be downloaded and transferred should the subscriber want to switch handsets. Newer handsets using SIM technology make upgrading or changing handsets as easy as swapping the SIM card. Four different sized SIM cards exist, "Endeavor" SIMs are used only with the i2000 without data, "Condor" SIMs are used with the two-digit models (i95cl, for example) using a SIM with less memory than the three-digit models (i730, i860), "Falcon" SIMs are used in the three-digit phones, (i530, i710) and will read the smaller SIM for backward compatibility, but some advanced features such as extra contact information is not supported by the older SIM cards. There is also the "Falcon 128" SIM, which is the same as the original "Falcon", but doubled in memory size, which is used on new 3 digit phones (i560, i930).
iDEN base radio at a cell site
The interconnect-side of the iDEN network uses GSM signalling for call set-up and mobility management, with the Abis protocol stack modified to support iDEN's additional features. Motorola has named this modified stack 'Mobis'.
Each base site requires precise timing and location information to synchronize data across the network. To obtain and maintain this information each base site uses global positioning system satellites to receive a precise timing reference.