Dual Mode
Bitrate and Modulation
IS-54 Time Slo t
Synchronisation
Control Channel

American digital TDMA Standard

The American AMPS system reached its capacity limits in the early 1990s. Additionally, AMPS was not very secure. Not only in terms of interception, but also in terms of authentication. Improvements were needed through digital transmission.

Like ETSI in Europe, an organization for the standardization of communication systems, the Telecommunications Industry Association, TIA, was founded in the United States in 1988. One of the first tasks of the TIA was to standardize a digital mobile communications standard. This should be based on the existing and successfully implemented AMPS standard.

The result was the IS-54 standard, also known as digital AMPS.

IS-54 Air Interface

Dual Mode

IS-54 was not a completely new standard like GSM. It was based 100% on analog AMPS. It was a standard that allowed analog AMPS and the new digital AMPS to operate in parallel on the same frequency bands around 800 MHz. New channels were not defined, but the 30 kHz channels from AMPS were reused. IS-54 gave the option to convert an analog channel into a digital channel. The capacity gain arises from an imposed TDMA structure.

Bitrate and Modulation

Since the bandwidth of AMPS was fixed at 30 kHz, the bit rate had to be significantly lower than with GSM. Actually by a factor of 7. However, a different type of modulation was chosen for AMPS than for GSM, namely Pi/4 DQPSK (Differential Quaternary Phase Shift Keying). In contrast to QPSK, which we discussed under digital modulation, the information here is not encoded in the current phase, but in the phase jumps. This makes the system less dependent on exact frequency synchronization between transmitter and receiver, but means that the process is more prone to errors.

Through the more complex modulation IS-54 had a higher bitrate than GSM

Since Pi/4 DQPSK can always encode two bits per second, a bit rate of 48.6 kbit/s could be achieved despite the low bandwidth. This means that the symbols are very long compared to GSM. Therefore, intersymbol interference was very unlikely and made the use of an equalizer unnecessary.

IS-54 Time Slots

An AMPS channel was initially divided into three time slots. A 13 kbit/s speech encoder was also used in IS-54. As speech encoders were improved over time, a 7.5 kbit/s encoder, a so-called half-rate encoder, could also be used, which allowed division into 6 time slots.

With IS-54, a time slot contains 260 bits of data. This is like GSM user data, which is protected with a convolution encoder. Also included are 12 bits for a slow associated control channel (for preparing handovers etc.) and a Coded Digital Verification Color Code Channel. This CDVCC corresponds to the analog SAT tone, which identifies the base station to which the mobile station is connected. There are 256 different CDVCCs which are protected by a Hamming code. The Hamming code generates 12 bits to protect the 8 bits for the CDVCC.

Synchronization

Each time slot has a 28 bit synchronization sequence. There is a different sequence for each of the three or six time slots. This means that a terminal device is able to distinguish between the different time slots.

Control Channel

IS-54 does not have new special control channels like GSM. Instead, the established FOCC, forward control channel and RECC reverse control channel are used. This ensures that analog and digital telephones can work in parallel. When logging in, the network can recognize whether the participant can work analog or digital. This means that if necessary, it can assign a digital channel instead of an analog channel, i.e. a channel and a time slot number.