Communication Engineering

GSM FDM & TDM
Channels

Master the hybrid multiple access techniques powering 2G cellular networks. Explore Frequency Division and Time Division Multiplexing in Global System for Mobile Communications.

Why Hybrid Access?

GSM (Global System for Mobile Communications) revolutionized 2G cellular networks by combining Frequency Division Multiple Access (FDMA) with Time Division Multiple Access (TDMA). This hybrid approach maximizes spectrum efficiency while maintaining signal quality.

1

FDMA Layer

Divides 25 MHz spectrum into 124 carrier frequencies (200 kHz each)

2

TDMA Layer

Each 200 kHz carrier divided into 8 time slots (users)

3

Result

992 simultaneous users per cell (124 × 8)

GSM Spectrum Allocation 890-915 MHz / 935-960 MHz
f₁
f₂
f₃
...
f₁₂₄
200 kHz per carrier (FDMA)
Single Carrier (200 kHz) → 8 Time Slots
TS0
TS1
TS2
TS3
TS4
TS5
TS6
TS7
4.615 ms Frame Duration

Frequency Division Multiplexing

FDMA divides the available spectrum into discrete frequency bands, each serving as an independent channel.

Key Concepts

  • Each user gets exclusive frequency band
  • Guard bands prevent adjacent channel interference
  • Continuous transmission (100% duty cycle)
  • Simple hardware implementation

GSM FDMA Specs

Uplink Band 890-915 MHz
Downlink Band 935-960 MHz
Channel Spacing 200 kHz
Total Carriers 124 pairs

Interactive FDM Visualizer

User 1
f₁
User 2
f₂
User 3
f₃
User 4
f₄

Observation: Each user occupies a distinct frequency band. The spectrum shows separate peaks for each channel with guard bands in between to prevent interference.

Time Division Multiplexing

TDMA allows multiple users to share the same frequency by transmitting in assigned time slots within a repeating frame structure.

TDMA Principles

Time Slot Allocation

Each user gets a specific time slot in a repeating frame. GSM uses 8 slots per frame (0-7).

Burst Transmission

Users transmit in short bursts (≈0.577 ms) at high data rate (270.833 kbps), then remain idle.

Synchronization

Critical timing alignment ensures slots don't overlap. Guard periods (≈30 μs) absorb timing variations.

GSM TDMA Frame Structure

Frame Duration 4.615 ms
Time Slots per Frame 8
Slot Duration 0.577 ms (15/26 ms)
Bits per Slot 156.25
Data Rate 270.833 kbps

TDMA Frame Animation

Frame: 0
TS0
Control
TS1
Traffic
TS2
Traffic
TS3
Traffic
TS4
Traffic
TS5
Traffic
TS6
Traffic
TS7
Traffic

TDMA Operation: The animation shows how 8 users share one frequency carrier. Each user transmits only during their assigned slot (highlighted), allowing 8 simultaneous conversations on a single 200 kHz channel. Notice the guard periods between slots preventing overlap.

GSM Frame Hierarchy

1

Time Slot (Burst)

0.577 ms duration

156.25 bits
148 bits payload
8

TDMA Frame

4.615 ms duration

8 time slots
Basic transmission unit
26

Multiframe

120 ms duration

26 TDMA frames
Traffic channel cycle

Normal Burst Structure (Traffic Channel)

3
Tail
57
Data
S
26
Training
S
57
Data
3
Tail
8.25
Guard
0 156.25 bits →

Burst Components

Tail Bits (3+3) Burst boundary indication
Data (57+57) Encrypted user data
Stealing Flags (1+1) FACCH indication
Training Sequence (26) Synchronization & equalization
Guard Period (8.25) Timing margin (~30 μs)

Channel Types

BCH (Broadcast Channels)

FCCH, SCH, BCCH - Downlink only for system info

CCCH (Common Control)

PCH, RACH, AGCH - Call setup & access

DCCH (Dedicated Control)

SDCCH, SACCH, FACCH - Assigned to specific MS

Interactive GSM Simulator

Configure parameters and visualize the hybrid FDM/TDM access scheme

Configuration

1 3 8
1 4 8
Total Capacity
12
simultaneous users
Spectrum Used: 600 kHz
Efficiency: 2.0 users/kHz

Time-Frequency Grid

Active Idle
Frequency Spectrum
Time →
f₁: 935.2 MHz f₂: 935.4 MHz f₃: 935.6 MHz

FDM vs TDM Comparison

Characteristic FDM (FDMA) TDM (TDMA)
Resource Division Frequency bands Time slots
Signal Type Analog or Digital Digital only
Transmission Continuous Burst (intermittent)
Hardware Complexity Simple (filters) Complex (synchronization)
Efficiency Lower (guard bands) Higher (guard times)
Power Consumption Continuous (high) Intermittent (low)
GSM Application 200 kHz carriers 8 slots per carrier

Knowledge Check

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