Time Division Multiple Access

Time Division Multiple Access

Unlike FDMA, TDMA allows access to the full bandwidth of the frequency spectrum. In TDMA, each transmitter is allocated a predefined time slot. Each transmitter receives the time slot in turn and it is allowed to transmit data for the duration of the time slot.

Time Division Multiple Access

After FDMA, TDMA is the second most popular mechanism for communication using satellites. In case of TDMA, there is no modulation of frequencies (unlike FDMA). Instead, the transmitting earth station transmits data in the form of packets of data. These data packets arrive at the satellite one by one (hence the name TDMA). By the same logic that was discussed during TDM, TDMA is also a digital form of data transmission; TDMA operates in time domain, rather than frequency domain. Bit rates of 10-100 Mbps are common for TDMA transmissions. This can be translated into roughly 1800 simultaneous voice calls using 64 Kbps PCM.
In digital systems, continuous transmission is not required because users do not use the allotted bandwidth all the time. In such cases, TDMA is a complimentary access technique to FDMA. Global Systems for Mobile communications (GSM) uses the TDMA technique. In TDMA, the entire bandwidth is available to the user but only for a finite period of time. In most cases the available bandwidth is divided into fewer channels compared to FDMA and the users are allotted time slots during which they have the entire channel bandwidth at their disposal.
TDMA requires careful time synchronization since users share the bandwidth in the frequency domain. The number of channels are less, inter channel interference is almost negligible. TDMA uses different time slots for transmission and reception. This type of duplexing is referred to as Time division duplexing(TDD).
Features of TDMA includes the following :
TDMA shares a single carrier frequency with several users where each users makes use of non overlapping time slots. The number of time slots per frame depends on several factors such as modulation technique, available bandwidth etc. Data transmission in TDMA is not continuous but occurs in bursts. This results in low battery consumption since the subscriber transmitter can be turned OFF when not in use. Because of a discontinuous transmission in TDMA the handoff process is much simpler for a subscriber unit, since it is able to listen to other base stations during idle time slots. TDMA uses different time slots for transmission and reception thus duplexers are not required. TDMA has an advantage that is possible to allocate different numbers of time slots per frame to different users. Thus bandwidth can be supplied on demand to different users by concatenating or reassigning time slot based on priority.