Here, each user is allocated the full spectral occupancy of the channel, but only for a short duration of time called a time slot. TDMA divides the available spectrum into multiple time slots, by giving each user a time slot in which they can transmit or receive. Buffer zones in the form of guard times are inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arises due to system imperfections, especially in synchronization schemes. The figure below shows how the time slots are provided to users in a round robin fashion, with each user being allotted one time slot per frame.
TDMA scheme where each user is allocated a small time slot
TDMA systems transmit data in a buffer and burst method, thus the transmission of each channel is non-continuous. The input data to be transmitted is buffered over the previous frame and burst transmitted at a higher rate during the time slot for the channel. TDMA can not send analog signals directly due to the buffering required, thus is only used for transmitting digital data. TDMA can suffer from multipath effects as the transmission rate is generally very high. This leads the multipath signals into causing inter-symbol interference.
TDMA is normally used in conjunction with FDMA to subdivide the total available bandwidth into several channels. This is done to reduce the number of users per channel allowing a lower data rate to be used. This helps reduce the effect of delay spread on the transmission. The figure below shows the use of TDMA with FDMA. Each channel based on FDMA, is further subdivided using TDMA, so that several users can transmit of the one channel. This type of transmission technique is used by most digital second generation mobile phone systems.
TDMA/FDMA hybrid, showing that the bandwidth is split into frequency channels and time slots.
