AMPS Tutorial: Basics, Frame Structure, and Call Setup

This AMPS Tutorial covers AMPS basics, AMPS frequency spectrum, AMPS system specifications, AMPS frame structure, and the AMPS call setup procedure.

AMPS Basics

AMPS is the acronym for Advanced Mobile Phone Service. This system was developed by AT&T and is considered a 1G analog system, most commonly used in North America. It supports coverage of about 20 km and a data rate of approximately 10kbps.

AMPS Frequency Spectrum and System Specifications

The AMPS spectrum allocated in North America consists of a total of 50 MHz, with 25 MHz in the downlink and 25 MHz in the Uplink. Downlink refers to the direction from the Base station (BS) to the Mobile station (MS), and Uplink refers to the direction from the Mobile station to the Base station.

The Downlink RF carrier frequency ranges from 869MHz to 894MHz, and the Uplink RF carrier frequency ranges from 824MHz to 849MHz. One operator is allocated half of the 25MHz (i.e., a 12.5MHz band) in both the uplink and downlink.

The channel bandwidth is about 30KHz, which provides approximately 833 total channels in the 25MHz span. Therefore, each operator supports about 416 channels for traffic and control functions. 21 channels are dedicated to control signaling channels per operator, and the rest are used for traffic purposes.

In AMPS, voice traffic is modulated using FM modulation with a peak deviation of about 12 KHz. Control channel information is modulated using FSK modulation with a peak deviation of about 8 KHz.

AMPS uses BCH as a forward error correction technique and supports BCH (48, 36, 5) and BCH (40, 28, 5) flavors.

AMPS Frame Structure

As shown in the figure, the AMPS frame structure differs for forward (BS to MS) and reverse (MS to BS) control channels, with about 21 of each. During a connected state, control information is carried over the normal voice channel used for conversation.

Reverse Control channel frame structure:

• Precursor: Total length: 48 bits, composed of a sync field (30 bits), a word sync field (11 bits), and a digital color code (7 bits).
  • A sync field consists of alternating 1s and 0s.
  • The word sync field is a fixed pattern of 11100010010.
  • DCC is unique for each Base station and helps deliver RCC to the correct Base station from MS.
• Following the precursor, 1 to 6 words of data will be sent. Each word is about 36 data bits and encoded to form 48 bits using a BCH shortened version. The original BCH is of polynomial (63,51,5).

To increase the robustness of the air interface and make the link more reliable, each word with the same information is transmitted 5 times in each frame. For example, w1 is mentioned 5 times.

Forward Control channel frame structure:

The forward control channel has a bit sync (10 bits), a word sync (11 bits), and two words of data bits. Each word is about 40 bits (including 28 bits of data and 12 redundant bits). For the reliability of the frame, each word is sent 5 times over the radio link.

AMPS Call Set Up Procedure

The call setup procedure is outlined in Figure 2. As shown, the call is initiated by dialing the telephone number of the called mobile (subscriber #2). The requested mobile (here, Subscriber #1) is first authenticated by the MTSO (Mobile Telephone Switching Office) and assigned traffic channels for further use. After this, the MTSO sends a ring tone to SS #2, and a circuit connection is established between SS #1 and SS #2. The call disconnection can be initiated by either SS #1 or SS #2.