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

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Chapter 8Existing Wireless Systems1G, AMPS
System

• Associate Prof. Yuh-Shyan Chen
• Dept. of Computer Science and Information
  Engineering
• National Chung-Cheng University

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Introduction

• A wireless system needs to take many factors into
  account
• Call rate, call duration, distribution of MSs,
  and traffic in an adjacent cell.
• It is important to study various characteristics
  of existing cellular systems
• How they support seamless mobile communication
• AMPS (Advanced Mobile Phone System)
• As the first representative of wireless system

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Advanced Mobile Phone System (AMPS)

• AMPS is the first-generation cellular system in
  USA
• It 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 ATT 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

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Cont.

• To explore a reuse pattern
• The 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

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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 MS
• Forward link or downlink
• The 3-KHz analog voice signal is modulated onto
  30-KHz channels
• In 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 B
• The non-wireline providers are given Band A
• Bell wireline providers are given Band B
• A total of 666 channels is divided among these
  two bands
• A cluster of seven cells allows many users to
  employ the same frequency spectrum simultaneously

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Cont.

• AMPSs 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 MSs 10-digit
  directory telephone number

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General operation of AMPS
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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

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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

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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

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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

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Forward and reverse channels
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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

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Format of FOCC
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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

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Format of RECC
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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

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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

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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 provided
• Registering for the MS with a visiting MSC
• Allowing for call origination in a foreign MSC
• Allowing the MS to roam from one foreign system
  to another

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IS-41 architecture
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Key terms and concepts
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Relationship between IS-41 and OSI protocol stack
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Various Operations Supported by IS-41

• Registration in a new MSC
• Calling an idle MS in a new system
• Call with unconditional call forwarding
• Call with no answer
• Calling a busy MS
• Handoff measurement request
• Recovery from failure at the HLR

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Internetworking of IS-41 and AMPS