TDMA

Time Division Multiple Access or TDMA is the access technique used to share common resource in the wireless network. For example, in GSM or VSAT network, one RF carrier is shared by many Mobile users or remote VSATs at different times.

Types of TDMA

Here single modulated carrier will occupy full transponder Bandwidth. It is popular for capacity point of view.

Here single modulated carrier signals occupy sub band of the total transponder bandwidth. It is used for smaller TDMA networks which share the transponder bandwidth with other carriers in the FDMA configuration.

Operation

Let us understand TDMA concept with VSATs, Hub station and satellite network. As shown in the figure, all the VSATs receive reference burst (RB) from Hub station. All the VSATs update their timers accordingly which will be used as reference to start their own transmission. Hence all the VSAT transmissions are aligned in the air. VSAT 1 starts their transmission after 1ms, VSAT 2 after 2 ms and VSAT 3 after 3 ms and so on. Here is has been assumed that RB is of 1ms duration. TB is traffic burst here.

Disadvantages of TDMA

Let us understand disadvantages with respect to cellular system, here each user has predefined time slot, hence if user enters from one cell to another cell, a call might well be dropped.
-Also if 8 time slots assigned to one cell as in GSM on one RF carrier, in this case if all slots are occupied and 9th user dials in he will not even receive dial tone from the GSM network.
-Also in some of the systems fixed time slots are reserved to particular users; in this case even if user is not using network service particular time slots will be remain unoccupied. This problem is avoided in advanced TDMA or Enhanced TDMA where subscriber is assigned the slots dynamically.
-In Enhanced TDMA system, all the subscribers are connected with buffer queue. Here when subscribers have something to transmit, they put one bit in the buffer queue. The system scans the buffer, notices that the user has something to transmit & allocates bandwidth accordingly. If a subscriber has nothing to transmit, the queue simply goes to the next subscriber. So instead of being static assignment of time slots, it will be allocated dynamically.

RF and Wireless Terminologies