📱 AMPS Study Guide

Advanced Mobile Phone Service - First Generation Cellular Architecture

🎯 Introduction to AMPS

Advanced Mobile Phone Service (AMPS) was the first generation (1G) analog cellular phone system developed by Bell Labs and introduced in the United States in 1983. It represented a revolutionary shift from limited-capacity car phone services to a true cellular architecture with frequency reuse and handoff capabilities.

Historical Significance: AMPS was the standard that proved commercial viability of cellular telephony, paving the way for digital systems (2G) and modern mobile networks.

Key Characteristics

  • Technology: Analog FM modulation (FM) with FDMA (Frequency Division Multiple Access)
  • Frequency Bands: 824-849 MHz (Reverse/MS to BS) and 869-894 MHz (Forward/BS to MS)
  • Channel Spacing: 30 kHz per channel
  • Duplex Spacing: 45 MHz separation between forward and reverse channels
  • Total Channels: 832 full-duplex channels (416 per operator in each market)
  • Modulation: FM with ±12 kHz peak deviation
  • Cell Radius: Typically 2-20 km (configurable)
Exam Tip: Remember that AMPS uses FDMA (Frequency Division Multiple Access), not TDMA or CDMA. Each user gets a dedicated frequency pair for the entire call duration.

📊 AMPS Specifications Summary

Parameter Specification Notes
Frequency Band (Reverse) 824 - 849 MHz Mobile transmits to Base Station
Frequency Band (Forward) 869 - 894 MHz Base Station transmits to Mobile
Duplex Spacing 45 MHz Constant offset between TX/RX
Channel Bandwidth 30 kHz Per user allocation
Total Channels 832 (666 voice, 166 control) Split between two operators (A/B)
Modulation Type Analog FM ±12 kHz peak deviation
Multiple Access FDMA Frequency Division Multiple Access
Speech Coding Companding 2:1 syllabic companding

🏗️ System Architecture

AMPS employs a distributed hierarchical architecture consisting of mobile units, base stations, and mobile switching centers. This architecture enables the cellular concept of frequency reuse and seamless mobility.

AMPS Network Architecture
📞
PSTN
Public Switched Telephone Network
🖥️
MSC
Mobile Switching Center
📡
BS
Base Station
📡
BS
Base Station
📡
BS
Base Station
📱
MS
Mobile Station

1. Mobile Station (MS)

The user equipment consisting of:

  • Transceiver: Operates at 3W max ERP (Effective Radiated Power) for mobile units, 0.6W for portable
  • Control Unit: Handles signaling, scanning, and handoff procedures
  • Antenna: Typically 3dB gain for vehicular mounts
  • MIN/ESN: Mobile Identification Number and Electronic Serial Number for authentication

2. Base Station (BS)

Also known as Base Transceiver Station (BTS):

  • Cell Site: Coverage area typically 2-20 km radius
  • Transmitter Power: 100W typical per channel
  • Antenna Configuration: Sectorized (120° or 60° sectors) or omnidirectional
  • Height: 15-100m above ground level
  • Base Station Controller (BSC): Manages multiple cell towers

3. Mobile Switching Center (MSC)

The central switching office for cellular systems:

  • Switching Function: Connects mobile calls to PSTN and other mobiles
  • Handoff Control: Manages inter-cell handovers
  • Registration: Tracks mobile location for incoming calls
  • Authentication: Validates MIN/ESN pairs
  • Interconnection: Links to other MSCs and PSTN via trunks
Cellular Concept: The key innovation of AMPS was the cellular concept developed by Bell Labs - dividing coverage area into cells with frequency reuse patterns (typically 7-cell clusters) to maximize spectrum efficiency.

🔄 Signaling Protocol (SAT & ST)

AMPS uses in-band signaling with two key tones:

📡 Supervisory Audio Tone (SAT)

Continuous tone at 5970, 6000, or 6030 Hz transmitted on voice channels to:

  • Confirm channel continuity
  • Identify co-channel cells (different SAT frequencies)
  • Trigger handoff if SAT is lost

📞 Signaling Tone (ST)

Burst of 10 kbps data or 8 kHz tone used for:

  • Flash (call waiting)
  • Handoff confirmation
  • Release (call termination)
  • Answer supervision

📻 Frequency Plan & Spectrum Allocation

AMPS operates in the 800 MHz band with specific allocations for system A (non-wireline) and System B (wireline) operators.

AMPS Frequency Band Allocation
824 MHz 849 MHz 869 MHz 894 MHz
System A
Reverse
824-849
System A
Forward
869-894
System B
Reverse
824-849
System B
Forward
869-894

Reverse (Mobile Transmit) Forward (Base Transmit)

Channel Numbering

Channels are numbered 1-1023 (with some gaps):

Channel Range Usage System Frequency Calculation
1-333 Primary Control/Voice A (Block A) Reverse: 0.030N + 825.000
334-354 Control Channels A Forward: 0.030N + 870.000
355-666 Voice Channels A N = Channel Number
667-716 Control Channels B (Block B) Reverse: 0.030(N-1023) + 825.000
717-1023 Voice Channels B (For N ≥ 990)
Frequency Calculation Formula:
Reverse Channel (MHz): frev = 0.030 × N + 825.000 (for N = 1-799)
Forward Channel (MHz): ffwd = frev + 45 MHz
Note: Channels 800-1023 use extended formulas for expanded spectrum

Cell Cluster Pattern

AMPS typically uses a 7-cell reuse pattern (N=7) with:

  • Cluster size K = 7 (i² + ij + j² where i=2, j=1)
  • Co-channel reuse ratio D/R = √3K ≈ 4.58
  • Frequency reuse provides C/I (Carrier-to-Interference) ratio of ~18 dB

Click cells to see frequency reuse pattern (7-cell cluster)

📞 Channel Types & Structure

AMPS divides the 832 channels into control channels (setup) and voice channels (conversation), with dedicated channels for access and paging.

🎛️ Forward Control Channel (FOCC)

Base → Mobile

  • Continuous transmission at 10 kbps FSK
  • Contains busy/idle status
  • Pages mobile for incoming calls
  • System information and overhead messages
  • Allocated 21 channels per system

📱 Reverse Control Channel (RECC)

Mobile → Base

  • Slotted Aloha random access at 10 kbps
  • Call origination requests
  • Page response messages
  • Registration messages
  • Order confirmations

🔊 Forward Voice Channel (FVC)

Base → Mobile

  • Analog voice transmission (FM)
  • Supervisory Audio Tone (SAT) continuous
  • Blank-and-burst signaling for handoff
  • Data bursts at 10 kbps (FSK)
  • 600+ channels per system

🎤 Reverse Voice Channel (RVC)

Mobile → Base

  • Analog voice transmission (FM)
  • SAT regeneration (transponds received SAT)
  • Signaling Tone (ST) bursts
  • Data bursts for confirmation
  • Voice-activated transmission (optional)

Control Channel Message Format

FOCC Message Structure
Bit Sync (10 bits): 1010101010
Word Sync (11 bits): 11100010010
Repeat 1 (40 bits + parity)
Repeat 2 (40 bits + parity)
Repeat 3 (40 bits + parity)
Word Sync
Repeat 4 (40 bits + parity)
Repeat 5 (40 bits + parity)

5-word repetition for error correction (BCH coding)

Important: Control channels use BCH (Bose-Chaudhuri-Hocquenghem) coding for error correction. Voice channels use no error correction for speech (analog) but use Manchester encoding for digital control messages.

📱 Call Processing States

The mobile station operates in several states during idle and active modes:

Initialization State

MS selects strongest control channel, locks onto FOCC, reads system information

Idle State

Monitoring FOCC for pages, can originate calls, performs registration

Access State

Sending messages on RECC, waiting for channel assignment

Conversation State

Active call on voice channel, monitoring SAT, ready for handoff

🔄 Handover (Handoff) Procedure

Handover in AMPS is mobile-assisted but network-controlled. The process ensures call continuity as users move between cells.

Key Concept: AMPS uses a "break-before-make" hard handoff. The connection to the old base station is broken before establishing the new connection, typically causing a brief audio interruption (~100-200ms).

Handoff Triggers

  • Signal Strength: Received signal level drops below threshold (typically -100 dBm)
  • SAT Loss: Supervisory Audio Tone quality degrades
  • Traffic Balancing: Move calls to less congested cells
  • Power Budget: Better path available to different cell

Handover Procedure Steps

  1. Measurement: MSC and serving BS continuously measure RSSI (Received Signal Strength Indicator) from mobile. Mobile also measures neighboring cell signal strengths and reports to BS.
  2. Decision: When signal drops below threshold (THRESHOLD_HANDOFF, typically -100 dBm), MSC identifies candidate target cells based on measurement reports.
  3. Channel Reservation: MSC requests target BS to reserve a voice channel and prepare for handoff. Target BS selects channel and starts transmitting SAT.
  4. Handoff Order: Serving BS sends "Handoff Order" message to mobile on voice channel (blank-and-burst signaling). Message contains: New Channel Number, New SAT Color Code, New Voice Mobile Attenuation Code (VMAC).
  5. Transition: Mobile tunes to new channel, verifies SAT, and sends confirmation tone (ST). Audio path switches through MSC to new BS.
  6. Completion: MSC releases old channel, updates location records, and call continues on new base station.
Handoff Signaling Flow
MS
Measures signals
Sends RSSI
Changes channel
← Measurement Report →
← Handoff Order ←
→ Handoff Access →
BS/MSC
Evaluates measurements
Decides handoff
Coordinates switch
Exam Tip: Remember that AMPS uses MAHO (Mobile Assisted HandOff) - the mobile measures neighbor cells but the network (MSC) makes the final handoff decision. This is different from some later systems where the mobile decides.

📊 Handoff Parameters

Parameter Typical Value Description
Handoff Threshold -100 dBm Minimum acceptable signal strength
Handoff Hysteresis 6-12 dB Prevents ping-pong between cells
Handoff Delay 5-10 seconds Time to confirm signal degradation
Maximum TX Power +43 dBm (20W) Base station EIRP
Mobile TX Power +37 dBm (5W) max Adjustable in 4dB steps (11 levels)

Roaming and Registration

AMPS supports automatic roaming through:

  • Autonomous Registration: MS registers every 15 minutes or when changing location area
  • MIN-based Routing: Mobile Identification Number area code determines home system
  • IS-41 Protocol: Standard for inter-system signaling (later addition)

🧮 AMPS Frequency Calculator

Calculate forward and reverse frequencies for any AMPS channel number (1-1023).

📐 Cell Planning Calculator

Calculate reuse distance and co-channel interference for cellular planning.

📝 Quick Reference Formulas

Frequency (Standard Band):
freverse = 0.030 × N + 825.000 MHz
fforward = freverse + 45 MHz

Reuse Distance:
D = R × √(3N)
where N = i² + ij + j²

Co-channel Interference (Worst Case):
C/I ≈ 10 × log₁₀[(D/R)n / 6]

Number of Channels per Cell:
Ccell = Ctotal / N