In GSM (Global System for Mobile Communications), multiple access techniques are employed to enable multiple users to share the available radio spectrum efficiently. The primary multiple access technique used in GSM is Time Division Multiple Access (TDMA), along with Frequency Division Multiple Access (FDMA). Here’s a brief overview of these multiple access techniques in GSM:
1. Time Division Multiple Access (TDMA):
- Description: TDMA divides the frequency band into time slots, and each user is assigned a unique time slot for transmission. The frame structure is organized into frames, each containing several time slots.
- Advantages:
- Efficient use of bandwidth.
- Simultaneous communication from multiple users in the same frequency band.
- Simple implementation.
- Drawbacks:
- Susceptible to timing synchronization issues.
- Limited scalability in terms of the number of simultaneous users.
2. Frequency Division Multiple Access (FDMA):
- Description: FDMA divides the frequency band into multiple channels, and each user is assigned a unique frequency channel for transmission. Each channel is separated by guard bands to minimize interference.
- Advantages:
- Simple and robust.
- Well-suited for handling continuous communication.
- Suitable for systems with a relatively low number of users.
- Drawbacks:
- Inefficient use of frequency spectrum compared to TDMA.
- Limited scalability in crowded network scenarios.
3. Code Division Multiple Access (CDMA) – Not Used in GSM:
- Description: CDMA allows multiple users to transmit over the same frequency band simultaneously by assigning a unique code to each user. Each user’s signal is spread over the entire frequency band using a unique code.
- Advantages:
- Increased capacity and better spectral efficiency.
- Robust against interference.
- Supports a larger number of users.
- Drawbacks:
- Complex implementation and management.
- Requires careful handling of code assignments.
GSM predominantly utilizes TDMA as its primary multiple access technique, with FDMA used to divide the frequency band into physical channels. This combination of TDMA and FDMA enables GSM to support multiple users within the same frequency band efficiently, contributing to its success as a widely adopted mobile communication standard.