Time Division Multiple Access, GSM and Code Division Multiple Access Air Interfaces

All digital wireless services use multiplexing techniques to carry more calls on the same amount of spectrum as analog cellular service. Multiplexers are devices that transmit signals from two or more devices over a single channel. Thus, while all digital services have more capacity than analog cellular, the ways they add capacity are incompatible with each other. That is why callers cannot necessarily use their cellular telephones when they travel. The three most prevalent multiplexing techniques in the world are time division multiple access (TDMA), code division multiple access (CDMA) and Global System for Mobile Communications (GSM). When used in wireless networks, multiplexing techniques are referred to as air interfaces. They are methods used to access (share) spectrum between the wireless device (handset or wireless computer) and the provider's base station.

These different multiplexing schemes are:

• CDMA (code division multiple access) used by Sprint PCS, Verizon Wireless and South Korea Telecom and other parts of Asia. Thirty percent of cell phones in the U.S. and Canada operate on CDMA.

• TDMA (time division multiple access), implemented by digital cellular providers such as Cingular Wireless and AT&T Wireless.

• GSM, originally Groupe Speciale Mobile, but now known as Global System for Mobile Communications. A standard set by the European Union and used in Europe, the Far East, much of South America, New Zealand and Australia. The AT&T Wireless TDMA scheme is similar to GSM's TDMA. In the U.S., VoiceStream uses GSM and AT&T Wireless and Cingular Wireless are in the process of installing it alongside their existing TDMA service.

Code Division Multiple Access

U.S.-based Qualcomm developed CDMA, which was invented by the company's founder, Irwin Jacobs, in the late 1980s. The promise of more capacity and a requirement for fewer base stations made by CDMA proponents steered many U.S. digital wireless companies, particularly the Bell telephone companies and Sprint Communications, away from the time division multiple access (TDMA). The Telecommunications Industry Association in 1989 agreed upon the TDMA standard. Qualcomm was started in 1985 and introduced code division multiple access in 1989. Service using CDMA was targeted for implementation in 1992. However, technical glitches delayed actual implementation until 1996.

Code division multiple access is a “spread spectrum” technology. Each conversation transmitted is spread over multiple frequencies as it is sent. This is accomplished through the use of unique 40-bit codes assigned to each telephone transmission. These codes are called Walesh codes. Having a unique code assigned to each data or voice transmission allows multiple users to share spectrum or air space. CDMA has more capacity than TDMA.

In addition to capacity, CDMA handsets use low amounts of power. This can be significant in light of consumer concerns about cellular handsets causing cancer. Lower power emissions translate to less threats of wireless service causing cancer. Finally, calls are transferred (handed off), from cell to cell by a soft handoff method superior to TDMA and analog cellular's handoff. With a soft handoff, a call is rarely dropped during the handoff. For a short period of time during the handoff or transfer, the call is held as it is received and as the cell hands it off. Unfortunately, the decision to use what they perceived as a superior multiplexing method cost many carriers in the U.S. a high price in lost compatibility.

Time Division Multiple Access and GSM

The standard that had been agreed upon by the TIA when CDMA was promulgated is time division multiple access, or TDMA. In TDMA, the transmission channel is broken into a number of time slots—for example, six. Three of the time slots carry traffic from three devices, and three are not used. The three unused time slots ensure that there is no interference between traffic on the time slots carrying traffic. Time division multiple access has three to five times the capacity of analog cellular service.

While time division multiple access is used both in the U.S. and the rest of the world, the methods do not interoperate with each other. The standard in Europe and most of the rest of the world is called GSM (Groupe Speciale Mobile). GSM works by dividing channels of 200-kilohertz spectrum into eight time slots. Seven of the time slots carry traffic and the eighth carries control signals. The U.S. has settled on a standard called IS-136. It is also used in Latin America, Russia, parts of Asia and the Ukraine.