What Is Time Division Multiple Access?

Time division multiple access (TDMA) is a channel access method that allows multiple users to share a single radio-frequency carrier by dividing transmission time into a repeating sequence of fixed-length frames, each subdivided into a number of time slots, and assigning each user an exclusive slot within every frame. The user transmits a compressed burst during its assigned slot and remains silent during the slots allocated to others. To the end device the link appears continuous, even though it occupies the shared channel only for a fraction of each frame period. TDMA is one of three principal multiple-access techniques in wireless communications, alongside frequency division multiple access (FDMA) and code division multiple access (CDMA).

The technique emerged in the 1970s and was formalized in a series of second-generation (2G) cellular standards during the late 1980s and early 1990s. GSM, the most widely deployed 2G standard, divides each 200 kHz carrier into eight time slots and assigns up to eight users per carrier. D-AMPS (IS-136) uses three time slots on a 30 kHz carrier, tripling the capacity of the earlier analog AMPS system. Digital Enhanced Cordless Telecommunications (DECT) and satellite-based systems such as Iridium also rely on TDMA. An overview of how TDMA compares to FDMA and CDMA in spectrum utilization is provided in the Murata Manufacturing article on multiplexing and multiple access.

Frame and Slot Structure

A TDMA frame consists of a fixed number of time slots, and the duration of a slot is determined by the frame duration and the number of slots per frame. In GSM, the TDMA frame repeats every 4.615 milliseconds and contains eight slots of approximately 577 microseconds each. Within each slot, the user transmits a burst that carries 116 bits of encrypted voice or data payload, along with training sequences and tail bits that help the receiver synchronize to the burst and estimate the channel impulse response. The training sequence, known as the midamble in some standards, allows the base station's equalizer to compensate for multipath distortion introduced by reflections in the propagation environment. Slot boundaries must be preserved with high precision because guard periods of only a few tens of microseconds separate consecutive users; a burst that runs long will collide with the next user's transmission.

Synchronization and Guard Times

Precise timing is the defining operational requirement of any TDMA system. Each mobile terminal must advance its transmission by an amount equal to the propagation delay between the handset and the base station, so that its burst arrives at the base station within the correct slot boundary. This timing advance is measured by the base station, which observes the arrival position of the training sequence relative to the expected slot boundary and commands the mobile to shift its transmit time accordingly. In GSM, the timing advance command covers delays up to about 233 microseconds, limiting the maximum cell radius to roughly 35 kilometers. Guard periods of a few microseconds on either side of each burst absorb residual timing uncertainty and prevent adjacent-slot interference. The Tualcom analysis of TDMA in modern data link systems details how guard periods and timing control are implemented in military and commercial applications.

Spectrum Efficiency and Handoff

TDMA improves spectral efficiency over analog FDMA by digitizing speech and compressing it before transmission, allowing more users per unit of spectrum. Because the transmitter is active only during its assigned slot, the power amplifier duty cycle is reduced, which extends battery life in portable devices. The burst nature of TDMA transmission also facilitates seamless handoff measurements: during idle slots the handset can monitor signals from neighboring base stations without interrupting its active call, enabling the mobile-assisted handoff procedure defined in the GSM technical specifications maintained by ETSI.

Applications

Time division multiple access has applications in a wide range of fields, including:

• Second-generation cellular telephony in GSM and D-AMPS networks
• Cordless telephony standards including DECT and PHS
• Satellite communication systems for voice and narrowband data services
• Combat-net radio and tactical data links in military communications
• Passive optical networks for upstream burst transmission from subscriber premises