GSM (Global System for Mobile Communications) is a standard for digital cellular networks that allows mobile devices to communicate with each other and with the network infrastructure. It employs the Time Division Multiple Access (TDMA) technique to transmit signals efficiently.
In TDMA, the available frequency spectrum is divided into time slots, and each user is allocated a specific time slot for transmitting and receiving data. In the case of GSM, the frequency band is divided into 200 kHz carrier frequencies, and each carrier is divided into eight time slots. Each time slot has a duration of 577 microseconds and is known as a burst.
Here’s how GSM utilizes narrowband TDMA:
- Time Division Multiplexing: GSM divides each carrier frequency into eight time slots using Time Division Multiplexing (TDM). This means that eight different users can share the same carrier frequency simultaneously. Each time slot is assigned to a specific user.
- Transmission in Bursts: Data transmission in GSM occurs in bursts. Each burst has a duration of 577 microseconds, and it consists of several parts: the training sequence, the guard period, and the information bits. The training sequence helps in synchronization and channel estimation, while the guard period provides separation between bursts to avoid interference.
- Time Slot Assignment: In GSM, the base station allocates specific time slots to each user based on demand and available resources. This allocation is dynamic and can change as users make or receive calls. The base station manages the allocation of time slots to ensure efficient utilization of the available bandwidth.
- Frequency Reuse: GSM employs a technique called frequency reuse to increase the capacity of the network. By dividing the available frequency band into smaller cells and reusing the same frequency in non-adjacent cells, multiple users can use the same frequency simultaneously without interference.
By using narrowband TDMA, GSM achieves efficient use of the frequency spectrum by allowing multiple users to share the same carrier frequency. This enables a large number of users to communicate simultaneously within a limited frequency band, making GSM an efficient and widely adopted cellular communication standard.