Chapter 8: Existing Wireless Systems: 1G, AMPS System

Yschen, CSIE, CCU 1

Chapter 8:Existing Wireless Systems:

1G, AMPS System

Associate Prof. Yuh-Shyan Chen

Dept. of Computer Science and Information Engineering

National Chung-Cheng University

Yschen, CSIE, CCU 2

Introduction

A wireless system needs to take many factors into accountCall rate, call duration, distribution of MSs, and

traffic in an adjacent cell.

It is important to study various characteristics of existing cellular systemsHow they support seamless mobile communication

AMPS (Advanced Mobile Phone System)As the first representative of wireless system

Yschen, CSIE, CCU 3

Advanced Mobile Phone System (AMPS)

AMPS is the first-generation cellular system in USAIt transmits speed signals employing FM and

important control information is transmitted in digital form using FSK (Frequency shift keying)

• Is used for modulating a digital signal over a carrier (or two carriers) by using a different frequency for a “1” or a “0”.

AMPS is the first cellular phone technology created by AT&T Bell labs.

• With the idea of dividing the entire service area into logical division called cells

• Each cell is allocated one specific band in the frequency spectrum

Yschen, CSIE, CCU 4

Cont.

To explore a reuse patternThe frequency spectrum is divided among seven

cells• Improving the voice quality as each user is given a

higher bandwidth

AMPS uses a cell radius of 1 to 16 miles• Larger cells tend to have more thermal noise and less

interference• Smaller cells have more interference and less termal

noise

One important aspect of AMPS is that it allows both cell sectoring and splitting

Yschen, CSIE, CCU 5

Cont.

It is also sufficient to have a lower-power MS (4 Watts or less) and a medium-power BS (about 100 Watts)

AMPS is capable of supporting about 100,000 customers per city

• The system is aimed to reduce blocking probability to about 2% during busy hours

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Characteristics of AMPS

AMPS uses frequency band from 824 MHz to 849 MHz for transmissions from MSs to the BS Reverse link or uplink

Frequency band between 869 MHz to 894 MHz from the BS to MSForward link or downlinkThe 3-KHz analog voice signal is modulated onto

30-KHz channelsIn transmitting data

• The system uses Manchester frequency modulation at the rate of 10 kbps

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Band Allocation in AMPS

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Cont.

Separate channels are used for transmitting control information and data

In AMPS, there are one control transreceiver for every eight voice transreceivers

Frequency allocation in AMPS is done by dividing the entire frequency spectrum into two bands – Band A and Band BThe non-wireline providers are given Band ABell wireline providers are given Band BA total of 666 channels is divided among these two bandsA cluster of seven cells allows many users to employ the

same frequency spectrum simultaneously

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Cont.

AMPS’s use of directional radio propagation enables different frequencies to be transmitted in different directions

• Thereby, reducing radio interference considerably.

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Operations of AMPS

A general state diagram of how an AMPS system handles and various other responsibilities is shown in Fig. 10.1

Three identification numbers are included in the AMPS system to perform various functions

• Electronic serial number (ESN): A 32-bit binary number uniquely identifies a cellular unit of an MS

and is established by the manufacturer at the factory

• System identification number (SID): A unique 15-bit binary number assigned to a cellular system The SID serves as a check and can be used in determining if a

particular MS is registered in the same system or if it is just roaming

• Mobile identification number (MIN): A digital representation of MS’s 10-digit directory telephone

number


General operation of AMPS


General working of AMPS phone system

When a BS powers up (enter idle task), it has to know its surroundings before providing any service to the MSs

It scans all the control channels and tunes itself to the strongest channel

It sends its system parameters to all the MSs present in the service area

Each MS updates its SID and establishes its paging channels only if its SID matches the one transmitted by the BS

Then, the MS goes into the idle state, responding only to the beacon and page signals


Cont.If a call is placed to an MS, the BS locates the MS

through the IS-41 message exchange• Then the BS pages the MS with an order

If the MS is active, it responds to the page with its MIN, ESN, and so on

• The BS then sends the control information necessary for the call

• For which the MS has to confirm with a supervisory audio tone (SAT), Indicating completion of a call

If a call is to be placed from an MS, the MS first sends the origination message to the BS on the control channel


Cont.

The BS passes this to IS-41 and sends the necessary control signal and orders to the MS

Thereafter, both MS and BS shift to the voice channel

An FVC and RVC control message exchange follows to confirms the channel allocation

Thus the actual conversation starts


How an MS know when it receives a call ?

The answer lies in the messages passed on the control channels

Whenever the MS is not in service, it tunes to the strongest channels to find out useful control information

• The same happens at the BS as well

The various channels used by the AMPS are as follows

• FOCC• RECC• FVC• RVC


Forward and reverse channels


Forward control channel (FOCC)

FOCC is primarily used by the BS to page and locate the MSs using the control information in three way time division multiplexing mode

The busy/idle status shows if the RECC is busy, and stream A and stream B allow all the MSs to listen to the BS


Format of FOCC


Reverse control channel (RECC)

One or more MSs using the RECC channel• This could be in response to the pages by the BS• There could be several MSs responding to quesies

A simple mechanism to indicate whether RECC is busy or idle is to model it after the slotted ALOHA packet radio channel

The seizure precursor fields are used for synchronization ad identification

For a multiple-word transmission following the seizure precursor, the first RECC message word repeats itself fie times; then the second RECC message word is repeated five times


Format of RECC


Forward voice channel (FVC)

FVC is used for one-to-one communication from the BS to each individual MS

A limited number of messages can be sent on this channel

A 101-bit dotting pattern represents the beginning of the frame

The forward channel supports two different tones• Continuous supervisory audio

The BS transmits beacon signal to check for the live MSs in the service area

• The discontinuous data stream BS sends orders or new voice channel assignment

to the MS


Reverse voice channel (RVC)

Reverse voice channel is used for one-to-one communication form MS to the BS during calls in progress and is assigned by the BS to an MS for its exclusive use


IS-41

IS-41 is an interim standard that allows handoff between BSs under control of different MSCs and allows roaming of a MS outside its home system

In order to facilitate this, the following services need to providedRegistering for the MS with a visiting MSCAllowing for call origination in a foreign MSCAllowing the MS to roam from one foreign system to

another


IS-41 architecture


Key terms and concepts


Relationship between IS-41 and OSI protocol stack


Various Operations Supported by IS-41

Registration in a new MSCCalling an idle MS in a new systemCall with unconditional call forwardingCall with no answerCalling a busy MSHandoff measurement requestRecovery from failure at the HLR


Internetworking of IS-41 and AMPS

Chapter 8: Existing Wireless Systems: 1G, AMPS System

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