📞 Cordless Telephony Study Guide

Comprehensive Learning Resource for Undergraduate Electrical Engineering Students

Course: ECE 525E-Wireless & Mobile Communications | Topic: Personal Communications

1. Introduction to Cordless Telephony

Definition

Cordless telephony refers to a wireless communication system that provides telephone service to portable handsets within a limited coverage area (typically indoors or around a building) by replacing the traditional corded connection between the telephone base unit and the handset with a radio link.

1.1 Key Characteristics

  • Limited Coverage: Typically 50-300 meters range (indoor/outdoor)
  • Low Power: Transmit power usually 10 mW to 250 mW
  • Low Mobility: Pedestrian speed only (walking pace)
  • High Voice Quality: Comparable to wired telephony
  • Cost-Effective: Cheaper than cellular systems for fixed locations
  • Frequency Reuse: Limited due to short range

1.2 Cordless vs. Cellular Systems

Parameter Cordless Telephony Cellular Telephony
Coverage Area 50-300 meters Several kilometers
Transmit Power 10-250 mW 0.6-3 Watts
Mobility Support Low (pedestrian) High (vehicular)
Handoff Capability Limited or None Seamless handoff
Spectrum Efficiency Low priority High priority
Cost Low High
Primary Use In-home/office Wide area mobility

1.3 Frequency Bands

Cordless telephony systems operate in various frequency bands allocated by ITU (International Telecommunication Union) and regional regulatory bodies:

Common Frequency Allocations:

  • 46-49 MHz: Early analog systems (CT0)
  • 864-868 MHz: European CT1 standard
  • 944-948 MHz: CT1+ (extended range)
  • 1880-1900 MHz: DECT (Europe)
  • 1895-1918 MHz: PHS (Japan)
  • 1920-1930 MHz: PACS (USA)
  • 2.4 GHz ISM: Modern digital systems
  • 5.8 GHz: Latest generation systems

2. Evolution and History of Cordless Telephony

1980 - First Generation (CT0)

Analog systems operating at 46-49 MHz. Basic 10-channel systems with limited range and security. Simple FM modulation.

1984 - CT1 Standard

European standard using 40 channels in the 864-868 MHz band. Improved voice quality and reduced interference.

1987 - CT1+ (Plus)

Extended frequency range (944-948 MHz) providing 80 channels. Better performance but spectrum overlap with GSM in some regions.

1989 - CT2 (CT2/CAI)

First digital cordless standard using TDD (Time Division Duplex) and FDMA. 40 channels at 864-868 MHz. Common Air Interface (CAI) for interoperability.

1992 - DECT Standard

Digital Enhanced Cordless Telecommunications. Most successful European standard using TDMA/TDD with 120 carriers × 12 time slots = 1440 channels.

1995 - PHS Standard

Personal Handy-phone System developed in Japan. TDMA/TDD with 77 carriers. Supports both voice and data services.

1998 - 2.4 GHz Digital Systems

Migration to 2.4 GHz ISM band using spread spectrum technology. DSSS and FHSS implementations.

2005 - DECT 6.0 (USA)

DECT adaptation for North American market operating at 1.9 GHz to avoid WiFi interference.

2010+ - Modern Era

5.8 GHz systems, DECT-ULE (Ultra Low Energy), integration with VoIP and smart home systems.

3. Basic Principles of Cordless Telephony

3.1 System Components

PSTN / IP Network BASE UNIT (Fixed Part - FP) HANDSET (Portable Part - PP) Radio Link (Duplex)

Figure 1: Basic Cordless Telephony System Architecture

3.2 Duplexing Techniques

Cordless systems use different methods to separate transmit and receive paths:

Frequency Division Duplex (FDD)

FDD Principle:
• Transmit Frequency: f₁
• Receive Frequency: f₂
• Frequency Separation: Δf = |f₁ - f₂| (typically 1-10 MHz)
• Requires duplex filter or separate antennas

Time Division Duplex (TDD)

TDD Principle:
• Same frequency for TX and RX: f₀
• Time Slot 1: Base transmits, Handset receives
• Time Slot 2: Handset transmits, Base receives
• Switching time: typically 10-20 μs
• Advantage: No duplex filter needed, spectrum efficient

3.3 Multiple Access Techniques

Technique Principle Used In Advantages
FDMA Different frequencies for different users CT0, CT1, CT1+ Simple, no synchronization needed
TDMA Same frequency, different time slots DECT, PHS, CT2 Spectrum efficient, flexible
CDMA Same frequency/time, different codes Some 2.4 GHz systems High capacity, soft handoff

3.4 Modulation Techniques

  • Analog Systems: FM (Frequency Modulation) with 12-25 kHz deviation
  • Digital Systems:
    • GFSK (Gaussian Frequency Shift Keying) - CT2, early DECT
    • π/4-DQPSK (Differential Quadrature Phase Shift Keying) - PHS, PACS
    • π/4-QPSK - DECT
    • GFSK with BT=0.5 - Bluetooth-based systems

4. CT0 Standard (First Generation Analog)

4.1 Technical Specifications

Parameter Specification
Frequency Band 46.610 - 46.970 MHz (Base TX)
49.670 - 49.830 MHz (Handset TX)
Channel Spacing 40 kHz
Number of Channels 10 (later expanded to 25)
Modulation FM (Frequency Modulation)
Frequency Deviation ±5 kHz
Duplex Method FDD (Frequency Division Duplex)
Transmit Power 10-50 mW
Range 50-100 meters (indoor)

4.2 Channel Allocation

CT0 Channel Frequency Calculation:

Base Station Transmit: f_base = 46.610 + (n × 0.040) MHz, where n = 0, 1, 2, ..., 9

Handset Transmit: f_handset = 49.670 + (n × 0.040) MHz, where n = 0, 1, 2, ..., 9

Duplex Spacing: 3.06 MHz (constant)

Example: Channel 1
Base TX: 46.610 MHz
Handset TX: 49.670 MHz
Duplex: 3.060 MHz

4.3 Limitations

  • Limited channels (10) caused congestion in dense areas
  • No privacy - analog signals easily intercepted
  • No security - simple scanners could access calls
  • Limited range due to low power and low frequency
  • Interference from other devices (baby monitors, toys)
  • No handoff capability

5. CT1 and CT1+ Standards

5.1 CT1 Standard (1984)

Parameter CT1 Specification
Frequency Band 864.100 - 868.100 MHz
Channel Spacing 100 kHz
Number of Channels 40 duplex channels
Duplex Spacing 0 MHz (TDD not used - actually FDD with 0 spacing is impossible, this is incorrect in original)
Modulation FM (Frequency Modulation)
Transmit Power 10 mW ERP

Correction:

CT1 actually uses FDD with 0 MHz spacing is incorrect. CT1 uses FDD with specific TX/RX pairs. Actually, CT1 uses:

• Base Transmit: 864.100 - 868.100 MHz
• Handset Transmit: 864.100 - 868.100 MHz (different channels)
• Actually, CT1 uses FDD with 0 MHz spacing meaning same frequency for TX/RX but time-separated (TDD-like)

5.2 CT1+ Standard (1987)

CT1+ was developed to provide more channels by extending into the 944-948 MHz band:

Parameter CT1+ Specification
Frequency Band 944.000 - 948.000 MHz
Channel Spacing 100 kHz
Number of Channels 80 duplex channels (40 from CT1 + 40 new)
Total System Capacity 80 channels combined with CT1

5.3 Controversy and Issues

  • Spectrum Conflict: CT1+ frequencies overlapped with GSM channels used for railway communications in some European countries
  • Regulatory Problems: CT1+ was banned in several countries due to interference concerns
  • Market Confusion: Consumers couldn't distinguish between CT1 and CT1+ equipment
  • Legacy Issues: Many CT1+ phones were sold before bans were enforced, causing long-term interference problems

5.4 Channel Frequency Calculation

CT1 Channel Frequencies:
Channel n (where n = 1 to 40):
f = 864.100 + (n-1) × 0.100 MHz

CT1+ Channel Frequencies:
Channel n (where n = 1 to 40):
f = 944.000 + (n-1) × 0.100 MHz

Example: CT1 Channel 20
f = 864.100 + 19 × 0.100 = 866.000 MHz

6. CT2 Standard (Second Generation Digital)

Key Innovation

CT2 was the first all-digital cordless telephone standard, introducing the Common Air Interface (CAI) which ensured interoperability between equipment from different manufacturers.

6.1 Technical Specifications

Parameter CT2 Specification
Frequency Band 864.100 - 868.100 MHz
Channel Spacing 100 kHz
Number of Carriers 40
Duplex Method TDD (Time Division Duplex)
Access Method FDMA (Frequency Division Multiple Access)
Modulation GFSK (Gaussian Frequency Shift Keying)
BT = 0.5, h = 0.5
Bit Rate 72 kbps
Voice Coding ADPCM (Adaptive Differential PCM)
32 kbps
Frame Duration 2 ms (1 ms TX, 1 ms RX)
Transmit Power 10 mW ERP

6.2 TDD Frame Structure

DOWNLINK Base → Handset 1 ms UPLINK Handset → Base 1 ms Total Frame Duration: 2 ms Guard

Figure 2: CT2 TDD Frame Structure

6.3 Common Air Interface (CAI)

The CAI defines three types of channels:

  • Control Channel (CHM): For signaling and call setup
  • Traffic Channel (TCH): For voice/data transmission
  • Link Channel (LCH): For maintaining connection

6.4 Telepoint Service

CT2 supported a unique Telepoint service allowing outgoing calls from public base stations:

  • Users could make calls from airports, train stations, shopping centers
  • Incoming calls not supported (one-way service)
  • Required subscription to Telepoint operator
  • Competed with early cellular but lacked mobility
  • Failed commercially due to cellular competition

7. DECT Standard (Digital Enhanced Cordless Telecommunications)

Most Successful Cordless Standard

DECT is the most widely deployed cordless telephony standard globally, used in over 100 countries. It supports voice, data, and networking applications beyond simple telephony.

7.1 Technical Specifications

Parameter DECT Specification
Frequency Band 1880 - 1900 MHz (Europe)
1920 - 1930 MHz (USA - DECT 6.0)
1893 - 1906 MHz (Japan)
Carrier Spacing 1.728 MHz
Number of Carriers 10 (Europe), 5 (USA), 8 (Japan)
Duplex Method TDD (Time Division Duplex)
Access Method TDMA/FDMA
Time Slots per Frame 24 (12 downlink + 12 uplink)
Total Channels 120 (10 carriers × 12 slots)
Frame Duration 10 ms
Slot Duration 417 μs
Modulation GFSK (original)
π/4-DQPSK (enhanced data rate)
Bit Rate 1152 kbps (gross)
32 kbps (voice per slot)
Voice Coding ADPCM (G.726) 32 kbps
Transmit Power 250 mW EIRP (peak)
10 mW average
Range 50-300 meters (indoor)
Up to 1 km (outdoor)

7.2 DECT Frame Structure

DOWNLINK (Base → Handset) UPLINK (Handset → Base) Slots 0-11 Slots 12-23 Frame Duration: 10 ms Slot Duration: 417 μs

Figure 3: DECT TDMA/TDD Frame Structure (24 slots in 10 ms)

7.3 DECT Protocol Architecture

Application Layer (Session/Transport) Network Layer (Mobility Management) Data Link Control (DLC) Layer Medium Access Control (MAC) Layer Physical Layer (Radio) Call Control Connection Handover Error Control Channel Allocation Modulation/Radio

Figure 4: DECT Protocol Stack

7.4 Advanced DECT Features

  • Seamless Handover: Intra-cell and inter-cell handover without call drop
  • Dynamic Channel Selection: Automatic selection of best channel based on interference
  • Multi-Bearer Support: Can allocate multiple slots for higher data rates (up to 552 kbps)
  • Encryption: Standardized DECT Standard Authentication Algorithm (DSAA)
  • Low Power: Sleep mode for battery conservation
  • Data Services: ISDN compatibility, packet data, SMS
  • Wireless Local Loop (WLL): Can replace wired local loops

7.5 DECT Frequency Calculation

DECT Carrier Frequencies (Europe):
f_c = 1881.792 + (n × 1.728) MHz
where n = 0, 1, 2, ..., 9 (10 carriers)

Example Calculations:
Carrier 0: 1881.792 MHz
Carrier 5: 1881.792 + 5 × 1.728 = 1890.432 MHz
Carrier 9: 1881.792 + 9 × 1.728 = 1897.344 MHz

Channel Formula:
Total Channels = 10 carriers × 12 slots = 120 duplex channels

8. PHS Standard (Personal Handy-phone System)

Japanese Standard

PHS was developed in Japan as a low-cost alternative to cellular systems, supporting both voice and data services with high-quality digital transmission.

8.1 Technical Specifications

Parameter PHS Specification
Frequency Band 1895.15 - 1917.95 MHz
Channel Spacing 300 kHz
Number of Carriers 77 (originally 40, expanded to 77)
Duplex Method TDD (Time Division Duplex)
Access Method TDMA (4 slots per carrier)
Frame Duration 5 ms
Modulation π/4-DQPSK
Bit Rate 384 kbps per carrier
Voice Coding ADPCM 32 kbps
Transmit Power 10 mW (standard)
20 mW (high power mode)
Cell Radius 100-500 meters

8.2 PHS Frame Structure

Slot 0 Downlink Slot 1 Downlink Slot 2 Uplink Slot 3 Uplink Frame Duration: 5 ms (4 slots)

Figure 5: PHS TDMA/TDD Frame Structure (4 slots)

8.3 PHS Services

  • Voice Service: 32 kbps ADPCM telephony
  • Data Service: 32-64 kbps packet data (PIAFS protocol)
  • Internet Access: Early mobile internet service in Japan
  • SMS: Short message service
  • Public Access: Dense deployment in urban areas

8.4 PHS Frequency Calculation

PHS Carrier Frequencies:
f_c = 1895.15 + (n × 0.300) MHz
where n = 0, 1, 2, ..., 76 (77 carriers)

Example:
Carrier 0: 1895.15 MHz
Carrier 38: 1895.15 + 38 × 0.300 = 1906.55 MHz
Carrier 76: 1895.15 + 76 × 0.300 = 1917.95 MHz

Total Capacity:
77 carriers × 4 slots = 308 duplex channels

9. Comprehensive Technology Comparison

Feature CT0 CT1/CT1+ CT2 DECT PHS
Year 1980 1984/1987 1989 1992 1995
Technology Analog Analog Digital Digital Digital
Frequency (MHz) 46-49 864-868, 944-948 864-868 1880-1900 1895-1918
Duplex FDD FDD TDD TDD TDD
Multiple Access FDMA FDMA FDMA TDMA/FDMA TDMA/FDMA
Channels 10 40/80 40 120 308
Modulation FM FM GFSK GFSK/π/4-DQPSK π/4-DQPSK
Voice Coding FM FM ADPCM 32k ADPCM 32k ADPCM 32k
Data Rate N/A N/A 72 kbps 1152 kbps 384 kbps
Handover No No No Yes Yes
Range 50m 100m 100m 300m 200m
Power 10mW 10mW 10mW 250mW 10mW

10. System Architecture and Components

10.1 Functional Elements

FIXED PART (FP) CCF Call Control Functions RFP Radio Fixed Part Network Interface (PSTN/ISDN/IP) PORTABLE PART (PP) handset Voice Codec Keypad/Display Radio Transceiver Modem Uplink Downlink CCF: Central Control Function RFP: Radio Fixed Part PP: Portable Part

Figure 6: Generic Cordless System Architecture (DECT-based)

10.2 Connection Types

  • Point-to-Point: Single base, single handset (typical home use)
  • Point-to-Multipoint: Single base, multiple handsets (office systems)
  • Multipoint-to-Point: Multiple bases (cells), single handset (DECT)
  • Multipoint-to-Multipoint: Multiple bases, multiple handsets (enterprise)

10.3 Mobility Management

Capability Description Standards Supporting
Registration Handset registers with base station All digital standards
Authentication Security verification of handset CT2, DECT, PHS
Location Update Update position when moving DECT, PHS
Intra-cell Handover Change channel within same cell DECT, PHS
Inter-cell Handover Change to different base station DECT, PHS

11. Design Calculations and Analysis

11.1 Link Budget Analysis

Link Budget Equation:
P_r = P_t + G_t + G_r - L_p - L_m - L_c

Where:
P_r = Received power (dBm)
P_t = Transmit power (dBm)
G_t = Transmit antenna gain (dBi)
G_r = Receive antenna gain (dBi)
L_p = Path loss (dB)
L_m = Miscellaneous losses (dB)
L_c = Cable/connector losses (dB)

Minimum Required: P_r ≥ Sensitivity (typically -90 to -100 dBm)

11.2 Path Loss Calculation

Free Space Path Loss (FSPL):

L_p (dB) = 32.44 + 20log₁₀(d) + 20log₁₀(f)

Where: d = distance in km, f = frequency in MHz

Example: DECT system at 1900 MHz, 100m (0.1 km) distance

L_p = 32.44 + 20log₁₀(0.1) + 20log₁₀(1900)

L_p = 32.44 + (-20) + 65.58

L_p = 78.02 dB

Indoor Path Loss: Add 10-30 dB for walls/floors

Total loss = 78 + 20 = 98 dB (typical indoor)

11.3 System Capacity Calculation

Erlang B Formula for Trunking:

B(A, N) = (A^N / N!) / (Σ(k=0 to N) A^k / k!)

Where:
A = Offered traffic in Erlangs
N = Number of channels
B = Blocking probability (typically 1-2%)

Traffic per user: 0.02-0.03 Erlangs (business)
Traffic per user: 0.01 Erlangs (residential)

Capacity Example:

Given: DECT system with 120 channels, 2% blocking probability

From Erlang B tables: A ≈ 110 Erlangs (for N=120, B=0.02)

Number of users supported:

Business users: 110 / 0.025 = 4,400 users

Residential users: 110 / 0.01 = 11,000 users

11.4 Frequency Reuse and Cluster Size

Co-channel Reuse Ratio:
D/R = √(3N)

Where:
D = Distance between co-channel cells
R = Cell radius
N = Cluster size (1, 3, 4, 7, 9, 12...)

Signal-to-Interference Ratio:
S/I = (D/R)^n / i₀
where n = path loss exponent (2-4), i₀ = number of interferers (6)

11.5 Interactive Link Budget Calculator

DECT Link Budget Calculator

Click Calculate to see results...

12. Applications and Modern Developments

12.1 Traditional Applications

  • Residential Cordless Phones: Home telephony replacement
  • Office PBX Systems: Business communications
  • Wireless Local Loop (WLL): Rural telephony infrastructure
  • Public Telepoint: Airport/station payphones (historical)

12.2 Modern Applications

  • VoIP Integration: DECT over IP (DECT-IP)
  • Smart Home: IoT device connectivity (DECT-ULE)
  • Healthcare: Medical alert systems, nurse call systems
  • Industrial: Wireless sensors and monitoring
  • Security: Wireless alarm systems

12.3 DECT-ULE (Ultra Low Energy)

Next Generation DECT

DECT-ULE combines DECT reliability with ultra-low power consumption for IoT applications:

  • Power consumption: < 10 mW average
  • Range: Up to 1 km
  • Data rates: Up to 1 Mbps
  • Applications: Smart meters, home automation, health monitors

12.4 Comparison with Modern Technologies

Technology Use Case Advantages over DECT Disadvantages vs DECT
WiFi VoIP Smartphone apps, softphones Higher data rates, ubiquitous Higher power, interference, QoS issues
Bluetooth Headsets, short range Lower power, cheaper Shorter range, lower voice quality
4G/5G Mobile telephony Wide area mobility Higher cost, power, subscription needed
Zigbee IoT, home automation Mesh networking, very low power No native voice support, lower range

12.5 Future Trends

  • Convergence: Integration with 5G and WiFi 6
  • Cloud Integration: Cloud-based PBX and management
  • AI Enhancement: Noise cancellation, voice recognition
  • Extended Range: Long-range DECT (up to several km)
  • Unified Communications: Integration with enterprise messaging

13. Summary and Key Takeaways

Essential Points to Remember

  1. Cordless telephony provides wireless access within limited range (50-300m) using low power (10-250 mW)
  2. Evolution path: CT0 (analog) → CT1/CT1+ (analog) → CT2 (digital) → DECT/PHS (advanced digital)
  3. DECT is the most successful standard, using TDMA/TDD with 120 channels at 1.9 GHz
  4. Duplex methods: Early systems used FDD, modern systems use TDD for spectrum efficiency
  5. Voice coding: Standardized on 32 kbps ADPCM for digital systems
  6. Key differentiator from cellular: Cordless systems support low mobility only (pedestrian)
  7. Modern applications: Beyond voice to IoT, smart home, and industrial automation

Exam Preparation Checklist

  • ☐ Understand differences between cordless and cellular systems
  • ☐ Memorize frequency bands for each standard (CT0, CT1, CT2, DECT, PHS)
  • ☐ Be able to calculate channel frequencies and total capacity
  • ☐ Understand TDD vs FDD and their trade-offs
  • ☐ Know DECT frame structure (10 ms, 24 slots)
  • ☐ Be able to perform basic link budget calculations
  • ☐ Understand the concept of CAI (Common Air Interface)
  • ☐ Know the applications and limitations of each generation
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