MULTIMEDIA UNIVERSITY OF KENYA
ETI 2511 - WIRELESS SYSTEMS II
ADVANCED MOBILE PHONE SERVICE (AMPS)

 

Advanced Mobile Phone Service (AMPS) is a first generation (analogue) cellular communication system developed by American Telephone and Telegraph (AT & T) company for use in North America. AMPS had a maximum coverage of  about 20 km. It was  launched in Chicago in 1983.

 

AMPS uses a total 50 MHz bandwidth, i.e 25 MHz in the downlink and 25 MHz in the Uplink. Uplink RF carrier frequency is 824MHz to 849MHz.  while Downlink RF carrier frequency is 869MHz to 894MHz as shown below.

 

Figure. 1. AMPS frequency plan.

 

Two operators were licensed to offer AMPS cellular services. Each  operator was  allocated 12.5 MHz (half of 25MHz) both in the uplink and downlink, i.e the frequency band was allocated to two operators. Channel bandwidth was 30KHz, which give about 833 total channels in 25MHz span. Hence each operator supports about 416 channels for traffic as well as control functions. 21 channels are dedicated for control signaling channels per operator and the rest is used for traffic purpose.

 

AMPS Architecture

The AMPS architecture formed the foundation of 2G systems. It consisted of base tranceiver stations and  Mobile Telecommunication Switching Office (MTSO). The MTSO performed the following functions:

(a) Interconnecting calls within thin the Cellular network and to other PSTNs,

(b) Compiling billing information,

 (c) registration,

(d) authentication,

(e) location updating, and

(f) Call routing.

 

 

Figure. 2. AMPS Architecture-The Base Trasnsceiver Station (BTS) was retained in DAMPS and GSM

 

Modulation

In AMPS, voice traffic is modulated using FM modulation with peak deviation of about 12 KHz. Control channel information is modulated using FSK modulation with peak deviation of about 8 KHz. AMPS uses BCH as forward error correction technique. It supports BCH (48, 36, 5) and BCH (40, 28, 5) flavours.

 

AMPS Call setup Procedure

The Amps call set up procedure is outlined in the figure below.

Figure. 3. AMPS call setup procedure

 

The call is initiated by dialing telephone number of the called mobile(SUB B). The calling mobile (SUB A)  is first authenticated by Mobile Telephone Switching Office(MTSO) and assigned traffic channels for further use. After this MTSO sends ring tone to SUB A and in turn circuit connection is established between SUB A and SUB B. The call teardown can be initiated by either SUB A or SUB B

 

AMPS Handoff Procedure

AMPs was designed to initiate hand-off as follows.

1. The home base station (BS1) notices mobile station’s (MSs)  signal is weakening (when the received signal strength goes below a certain threshold value).

2. BS1 sends a handoff measurement request message to its MTSO.

3. The MTSO requests neighbour base stations to report their reception of mobile’s signal strength.

4. The MTSO picks neighbour base station with highest received signal strength.

5. The MTSO instructs BS1 and BS2 to commence hand-off

6. BS1 instructs the MS to turn off its transmission on channel f1 and tune to new channel f2.

7. MS confirms it has tuned to f2

8. BS2 confirms to the MSC that it has tuned to channel f2

9. Conversation progresses on channel f2

 

 

 

Figure. 4. Hand-off detection. As the MS moves away from BS1, the its received signal strength becomes low while that from BS2 increases.

 

Figure. 5. Flow-chart of hand-off process initiation and measurement

 

 

Figure. 6. Flow-chart of the AMPS hand-off procedure - seting up of new connection.

 



                                                                                                                                                        ©  Prof. James Kulubi May 2016