TDMA: Time Division Multiple Access Explained

TDMA, or Time Division Multiple Access, is an access technique employed to share a common resource within a wireless network. A prime example is GSM or a VSAT network, where a single RF carrier is shared among multiple mobile users or remote VSATs at different points in time.

Types of TDMA

There are primarily two main types of TDMA implementations:

1. Classic TDMA

In classic TDMA, a single modulated carrier occupies the full transponder bandwidth. This approach is generally favored from a capacity standpoint, as it dedicates the entire bandwidth to the TDMA scheme.

2. Narrowband TDMA

Narrowband TDMA involves a single modulated carrier signal occupying a sub-band of the total transponder bandwidth. This approach is commonly used for smaller TDMA networks that share the transponder bandwidth with other carriers in an FDMA (Frequency Division Multiple Access) configuration.

Image showing a general TDMA Operation.

TDMA Operation with VSATs

Let’s delve into the TDMA concept within a VSAT (Very Small Aperture Terminal), Hub station, and satellite network context.

As illustrated in the figure, all VSATs receive a reference burst (RB) from the Hub station. This reference burst is crucial for synchronization.

The VSATs then use this reference burst to update their internal timers. These timers serve as the baseline for initiating their own transmissions.

Consequently, all VSAT transmissions become aligned “in the air,” meaning they’re synchronized in time.

For instance, VSAT 1 might begin its transmission after 1 ms, VSAT 2 after 2 ms, and VSAT 3 after 3 ms, and so forth.

In this example, it’s assumed that the reference burst (RB) has a duration of 1 ms. The “TB” in the context refers to a traffic burst.

Disadvantages of TDMA

Now, let’s explore some of the disadvantages of TDMA, particularly in the context of cellular systems:

• Call Dropping During Handoff: Because each user has a predefined time slot, a call could be dropped if a user moves from one cell to another. This is due to the potential for timing conflicts during the handoff process.

• Limited Capacity in Fixed TDMA: Consider a scenario where 8 time slots are assigned to a cell in a GSM network on a single RF carrier. If all 8 slots are occupied, a 9th user attempting to dial in might not even receive a dial tone from the GSM network. They’re essentially locked out until a time slot becomes available.

• Inefficient Use of Resources: In some systems, fixed time slots are reserved for specific users. This means that even if a user isn’t actively using the network service, their assigned time slots remain unoccupied. This leads to wasted bandwidth and inefficient resource utilization.

This problem is addressed in advanced TDMA or Enhanced TDMA systems, where time slots are dynamically assigned to users based on their actual needs.

Enhanced TDMA

In an Enhanced TDMA system, subscribers are connected to a buffer queue. When a subscriber has data to transmit, they place a flag in the buffer queue.

The system then scans the buffer, detects that the user has data to transmit, and allocates bandwidth accordingly.

If a subscriber has nothing to transmit, the queue simply moves on to the next subscriber. This dynamic allocation of time slots, rather than a static assignment, allows for more efficient use of network resources.