This system is most often referred to as TDMA. That name is based on the acronym for time division multiple access, a common multiple access technique which is used by multiple protocols, including GSM, as well as in IS-54 and IS-136. However, D-AMPS has been competing against GSM and systems based on code division multiple access (CDMA) for adoption by the network carriers, although it is now being phased out in favor of GSM/GPRS and CDMA2000 technology.
D-AMPS uses existing AMPS channels and allows for smooth transition between digital and analog systems in the same area. Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots (hence time division) and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analog scanners could not access digital signals. Calls were encrypted, although the algorithm used (CMEA) was later found to be weak.
IS-136 added a number of features to the original IS-54 specification, including text messaging, circuit switched data (CSD), and an improved compression protocol. SMS and CSD were both available as part of the GSM protocol, and IS-136 implemented them in a nearly identical fashion.
Former large IS-136 networks included AT&T; in the United States, and Rogers Wireless in Canada. AT&T; and Rogers Wireless have upgraded their existing IS-136 networks to GSM/GPRS. Rogers Wireless removed all 1900 MHz IS-136 in 2003, and has done the same with their 800 MHz spectrum as the equipment failed. Rogers deactivated their IS-136 network (along with AMPS) on May 31, 2007. AT&T; soon followed in February 2008, shutting down both TDMA and AMPS.
Alltel, who primarily uses CDMA2000 technology but acquired a TDMA network from Western Wireless, shut down their TDMA and AMPS networks in September 2008. US Cellular, who now also primarily uses CDMA2000 technology, shut down their TDMA network in February 2009.
IS-54 is the first mobile communication system which had provision for security, and the first to employ TDMA technology.
IS-54 maintains compatibility with Advanced Mobile Phone System (AMPS) in many ways. It is a digital extension of AMPS and so it is also quite widely known as Digital AMPS (D-AMPS). Another name for IS-54 is United States Digital Cellular (USDC). But sometimes D-AMPS and USDC also refers to the other prominent interim standard IS-136.
Out of the 1G standards, the most successful was the AMPS system. Despite the Nordic countries' cooperation, European engineering efforts were divided among the various standards, and the Japanese standards did not get much attention. Developed by Bell Labs in the 1970s and first used commercially in the United States in 1983, AMPS operates in the 800 MHz band in the United States and is the most widely distributed analog cellular standard. (The 1900 MHz PCS band, established in 1994, is for digital operation only.) The success of AMPS kick-started the mobile age in the North America.
The market showed an increasing demand because it had higher capacity and mobility than the then existing mobile communication standards. For instance, the Bell Labs system in the 1970s could carry only 12 calls at a time throughout all of New York City. AMPS used Frequency Division Multiple Access FDMA which meant each cell site would transmit on different frequencies, allowing many cell sites to be built near each other.
However, AMPS had many disadvantages too. Primarily, it did not have the potential to support the increasing demand for mobile communication usage. Each cell site did not have much capacity for carrying higher numbers of calls. It also had a poor security system which allowed people to steal a phone's serial code to use for making illegal calls. All of these triggered the search for a more capable system.
The quest resulted in IS-54, the first American 2G standard. In March 1990, the North American cellular network incorporated the IS-54B standard, the first North American dual mode digital cellular standard. This standard won over Motorola's Narrowband AMPS or N-AMPS, an analog scheme that increased capacity by cutting down voice channels from 30 kHz to 10 kHz. IS-54, on the other hand, increased capacity by digital means using TDMA protocols. This method separates calls by time, placing parts of individual conversations on the same frequency, one after the next. TDMA tripled call capacity.
Using IS-54, a cellular carrier could convert any of its system's analog voice channels to digital. A dual mode phone uses digital channels where available and defaults to regular AMPS where they are not. IS-54 was, in fact, backward compatible with analog cellular and indeed co-exists on the same radio channels as AMPS. No analog customers were left behind; they simply could not access IS-54's new features. IS-54 also supported authentication, a help in preventing fraud.
The IS-54 standard specifies 84 control channels, 42 of which are shared with AMPS. To maintain compatibility with the existing AMPS cellular telephone system, the primary forward and reverse control channels in IS-54 cellular systems use the same signaling techniques and modulation scheme (binary FSK) as AMPS. An AMPS/IS-54 infrastructure can support use of either analog AMPS phones or D-AMPS phones.
The access method used for IS-54 is Time Division Multiple Access (TDMA), which was the first U.S. digital standard to be developed. It was adopted by the TIA in 1992. TDMA subdivides each of the 30 kHz AMPS channels into 3 full-rate TDMA channels, each of which is capable of supporting a single voice call. Later, each of these full-rate channels was further sub-divided into two half-rate channels, each of which, with the necessary coding and compression, could also support a voice call. Thus, TDMA could provide 3 to 6 times the capacity of AMPS traffic channels. Time Division Multiple Access or TDMA was initially defined by the IS-54 standard and is now specified in the IS-13x series of specifications of the EIA/TIA.
The channel transmission bit rate for digitally modulating the carrier is 48.6 kbit/s. Each frame has six time slots of 6.67-ms duration. Each time slot carries 324 bits of information, of which 260 bits are for the 13-kbit/s full-rate traffic data. The other 64 bits are overhead; 28 of these are for synchronization, and they contain a specific bit sequence known by all receivers to establish frame alignment. Also, as with GSM, the known sequence acts as a training pattern to initialize an adaptive equalizer.
The IS-54 system has different synchronization sequences for each of the six time slots making up the frame, thereby allowing each receiver to synchronize to its own preassigned time slots. An additional 12 bits in every time slot are for the SACCH (i.e., system control information). The digital verification color code (DVCC) is the equivalent of the supervisory audio tone used in the AMPS system. There are 256 different 8-bit color codes, which are protected by a (12, 8, 3) Hamming code. Each base station has its own preassigned color code, so any incoming interfering signals from distant cells can be ignored.
The modulation scheme for IS-54 is 7C/4 differential quaternary phase shift keying (DQPSK), otherwise known as differential 7t/4 4-PSK or π/4 DQPSK. This technique allows a bit rate of 48.6 kbit/s with 30 kHz channel spacing, to give a bandwidth efficiency of 1.62 bit/s/Hz. This value is 20% better than GSM. The major disadvantage with this type of linear modulation method is the power inefficiency, which translates into a heavier hand-held portable and, even more inconvenient, a shorter time between battery recharges.
IS-54 security features is also a matter of interest as it was the first standard to specify some security measures. IS-54 uses the CAVE (Cellular Authentication, Voice Privacy and Encryption) algorithm for authentication and the CMEA (Cellular Message Encryption Algorithm) for encryption.
The technical specifications can be summarized as below:
Time slots for the mobile-to-base direction are constructed differently from the base-to-mobile direction. They essentially carry the same information but are arranged differently. Notice that the mobile-to-base direction has a 6-bit ramp time to enable its transmitter time to get up to full power, and a 6-bit guard band during which nothing is transmitted. These 12 extra bits in the base-to-mobile direction are reserved for future use.
Once a call comes in the mobile switches to a different pair of frequencies; a voice radio channel which the system carrier has made analog or digital. This pair carries the call. If an IS-54 signal is detected it gets assigned a digital traffic channel if one is available. The fast associated channel or FACCH performs handoffs during the call, with no need for the mobile to go back to the control channel. In case of high noise FACCH, embedded within the digital traffic channel overrides the voice payload, degrading speech quality to convey control information. The purpose is to maintain connectivity. The slow associated control channel or SACCH does not perform handoffs but conveys things like signal strength information to the base station.
The IS-54 speech coder uses the technique called vector sum excited linear prediction (VSELP) coding. This is a special type of speech coder within a large class known as code-excited linear prediction (CELP) coders. The speech coding rate of 7.95 kbit/s achieves a reconstructed speech quality similar to that of the analog AMPS system using frequency modulation. The 7.95-kbit/s signal is then passed through a channel coder that loads the bit rate up to 13 kbit/s. The new half-rate coding standard reduces the overall bit rate for each call to 6.5 kbit/s, and should provide comparable quality to the 13-kbit/s rate. This half-rate gives a channel capacity six times that of analog AMPS.
On May 31, 2007 Rogers Wireless decommissioned its D-AMPS and AMPS networks and moved the remaining customers on these older networks onto its GSM network.
Alltel has completed their shutdown of their D-AMPS and AMPS networks in September 2008.
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