Question

Which access technique is primarily used in the Global System for Mobile communications (GSM)?

• A. CDMA
• B. TDMA
• C. FDMA
• D. WDMA

Hint:

Remember how different generations of cellular networks map to their primary multiple access technologies: - 1G (Analog): FDMA exclusively. - 2G (GSM): TDMA combined with FDMA channels. - 3G Systems: CDMA. - 4G and 5G Systems: OFDMA (Orthogonal Frequency Division Multiple Access).

Answer 1

The Correct Option is B

Concept: Multiple access techniques allow multiple mobile users to share a allocated block of wireless frequency spectrum simultaneously without causing mutual interference. The Global System for Mobile Communications (GSM) is a foundational 2G digital cellular standard developed to optimize spectrum efficiency by combining two clean separation techniques: Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).

Step 1: Identifying the Core Access Method
While GSM uses FDMA to divide its allocated frequency bands into smaller carrier channels (spaced $200\,\text{kHz}$ apart), its defining multiple access feature is how it shares each individual carrier channel among users:
• Each carrier frequency channel is subdivided in the time domain using Time Division Multiple Access (TDMA).
• Time is structured into recurring intervals called frames. Each individual TDMA frame is split into exactly 8 distinct time slots.

Step 2: Operating Principle of TDMA in GSM
When a call is connected, an individual user is assigned one specific time slot within the frame. The mobile device transmits data in quick, compressed bursts only during its assigned time slot. By cycling through the slots sequentially, up to 8 separate users can share the exact same frequency channel at the same time without overlapping. Because this time-slot sharing is the defining feature of user separation in the standard, GSM is classified primarily as a TDMA-based system.

Step 3: Disproving alternative choices

• CDMA (Code Division Multiple Access): Separates users by assigning unique orthogonal codes rather than separate time slots. This is the core technology behind 3G systems (IS-95, UMTS), not GSM.
• FDMA: Used on its own in old 1G analog systems (like AMPS) to give each user a dedicated frequency channel for the duration of a call.
• WDMA (Wavelength Division Multiple Access): Used to split channels by wavelength in fiber-optic communications, not over wireless radio networks.