Digital Enhanced Cordless Telecommunications

Explore the TDMA/TDD architecture, dynamic channel allocation, and GFSK modulation techniques used in modern cordless telephony systems.

1880-1900 MHz Band
TDMA/TDD Access
120 Duplex Channels

Theoretical Background

1. DECT Overview

Digital Enhanced Cordless Telecommunications (DECT) is a wireless communication standard developed by ETSI in 1992 for cordless telephony. It operates in the 1880-1900 MHz band in Europe and uses a combination of FDMA, TDMA, and TDD techniques.

  • • Frequency Range: 1880-1900 MHz (Europe)
  • • Channel Spacing: 1.728 MHz
  • • Modulation: GFSK (Gaussian Frequency Shift Keying)
  • • Data Rate: 32 kbps per channel (ADPCM)

2. Multiple Access Techniques

FDMA (Frequency Division)

10 carrier frequencies spaced 1.728 MHz apart

TDMA (Time Division)

24 time slots per frame (10 ms duration)

TDD (Time Division Duplex)

Slots 0-11: Downlink, Slots 12-23: Uplink

3. Frame Structure

Parameter Value Description
Frame Duration 10 ms Total frame length
Time Slots 24 12 downlink + 12 uplink
Slot Duration 416.67 μs 10 ms / 24 slots
Bit Rate 1.152 Mbps Per carrier frequency
Guard Time ~30 μs Between time slots

4. Dynamic Channel Allocation (DCA)

DECT uses a sophisticated Dynamic Channel Allocation mechanism where the Portable Part (PP) selects the best channel based on Received Signal Strength Indication (RSSI). The PP scans all idle channels every 30 seconds to maintain an interference map.

10
Carrier Frequencies
120
Duplex Channels (10×12)
30s
RSSI Scan Interval

Interactive Simulations

Frequency Allocation & Channel Plan

1880 MHz 1890 MHz 1900 MHz
f₀: 1881.792 MHz f₅: 1888.704 MHz f₉: 1897.344 MHz

Selected Channel Info

Click on a frequency bar to view details

RSSI Monitoring

-- dBm

TDMA Frame Structure (10 ms)

Downlink (Base → Handset) Uplink (Handset → Base)
Slot 0 Slot 11 Slot 12 Slot 23
Downlink
0
Active slots (0-11)
Uplink
0
Active slots (12-23)
Efficiency
0%
Channel utilization

GFSK Modulation Visualization

Modulation Parameters

  • Modulation: GFSK (Gaussian FSK)
  • Frequency Deviation: ±288 kHz
  • BT Product: 0.5 (Bandwidth × Bit period)
  • Symbol Rate: 1.152 Mbps

Signal Properties

  • • Binary '1': Frequency increases by 288 kHz
  • • Binary '0': Frequency decreases by 288 kHz
  • • Gaussian filtering reduces spectral width
  • • Continuous phase maintains constant envelope

Dynamic Channel Allocation & Handover

Base Station Coverage

Distance (m)
FP1 (Base 1) FP2 (Base 2)

Channel Quality Monitor

Ch 5 (1888.7 MHz)
Good
Ch 3 (1892.2 MHz)
Fair
Ch 7 (1885.2 MHz)
Poor
Status: Monitoring channel quality...

Laboratory Procedure

1

Frequency Allocation Analysis

Investigate the DECT frequency band allocation and channel spacing. Understand how FDMA provides multiple carrier frequencies.

Steps:

  1. Observe the 10 carrier frequencies in the 1880-1900 MHz band
  2. Verify the channel spacing of 1.728 MHz between carriers
  3. Calculate the center frequency for each carrier using: fn = 1881.792 - n × 1.728 MHz
  4. Measure the occupied bandwidth for each channel
  5. Record the RSSI values for each carrier in the table provided
2

TDMA Frame Structure Analysis

Analyze the TDMA frame structure and understand how time slots are allocated for downlink and uplink communication.

Steps:

  1. Observe the 10 ms frame structure containing 24 time slots
  2. Identify slots 0-11 as downlink (Base to Portable) and slots 12-23 as uplink (Portable to Base)
  3. Calculate the duration of each time slot: 416.67 μs
  4. Verify the duplex spacing of 5 ms between paired slots (e.g., slot 0 downlink and slot 12 uplink)
  5. Measure the guard time between consecutive slots
  6. Calculate the total number of simultaneous calls possible: 10 carriers × 12 slots = 120 duplex channels
3

GFSK Modulation Analysis

Study the Gaussian Frequency Shift Keying modulation technique used in DECT and analyze its spectral efficiency.

Steps:

  1. Generate a random bit sequence (e.g., 10110010)
  2. Observe the GFSK waveform: mark frequency (fc + 288 kHz) for '1', space frequency (fc - 288 kHz) for '0'
  3. Measure the frequency deviation: 288 kHz
  4. Calculate the modulation index: h = 2Δf/Rb = 0.5 (where Rb = 1.152 Mbps)
  5. Observe the continuous phase property between symbol transitions
  6. Analyze the power spectral density and occupied bandwidth
4

Dynamic Channel Allocation Study

Investigate the DCA mechanism and handover procedures in DECT systems.

Steps:

  1. Monitor the RSSI values for all 10 carriers
  2. Observe the channel selection process: PP selects channel with minimum interference
  3. Simulate interference on the current channel
  4. Trigger intracell handover (change to different slot on same base) or intercell handover (change to different base)
  5. Measure the handover time and verify seamless transition
  6. Record the channel quality metrics before and after handover

Laboratory Report Guidelines

Report Structure

1. Title Page

Experiment title, student name, ID, date, course code

2. Objectives

List 3-4 specific learning objectives for this experiment

3. Theory

Brief overview of DECT technology, TDMA/TDD principles, and GFSK modulation

4. Equipment/Software

List the virtual laboratory tools and simulation parameters used

5. Procedure

Step-by-step description of experiments performed

6. Results & Observations

Tabulated data, waveforms, screenshots from simulations

7. Analysis & Discussion

Interpretation of results, comparison with theoretical values

8. Conclusion

Summary of findings and key learnings

9. References

ETSI standards, textbooks, technical papers

Data Tables & Calculations

Table 1: Frequency Allocation

Carrier ID Frequency (MHz) RSSI (dBm) Status
01881.792
11883.520
......
91897.344

Table 2: TDMA Frame Parameters

Parameter Theoretical Measured Error %
Frame Duration10 ms
Slot Duration416.67 μs
Duplex Offset5 ms

Key Calculations Required:

  • • Channel capacity: C = B log₂(1 + SNR)
  • • Spectral efficiency: η = Rb/B
  • • Modulation index: h = 2Δf/Rb
  • • Frame efficiency: ηf = (Tslot - Tguard)/Tslot × 100%

Submission Requirements

  • • Report length: 8-12 pages maximum
  • • Include at least 5 screenshots from simulations
  • • All calculations must show working steps
  • • Submit PDF format only
  • • Due date: As per course schedule

Assessment Rubric

Criteria Excellent (A) Good (B) Satisfactory (C) Poor (D/F) Weight
Theory Understanding Comprehensive explanation with diagrams Good understanding shown Basic concepts covered Major misconceptions 20%
Experimental Data Accurate, well-organized, complete Minor errors in recording Some missing data Incomplete or fabricated 30%
Analysis Critical analysis, error analysis included Good interpretation Superficial analysis No analysis provided 25%
Presentation Professional format, clear graphs Well-structured Adequate formatting Disorganized, illegible 25%