Chapter 3 Analog Cellular Communications AMPS System

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Chapter 3Analog Cellular Communications AMPS
System

• This chapter covers the first-generation cellular
  technology focusing on AMPS (Advanced Mobile
  Phone System).
• AMPS delivers basic telephony and supplement
  services (voice mail and call forwarding).

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Network Elements

• The AMPS specification refers to terminals as
  mobile stations and to base station as land
  stations.
• The common terminology for an AMPS switch is
  mobile telephone switching office (small and
  large MTSO).
• The communication links between the base stations
  and switch are labeled land lines (copper wires,
  optical fibers or microwave systems)

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AMPS Identification Codes

• Mobile Identification Number (MIN)
• Area code (3 digits), Exchange number (3 digits)
  and subscriber number (4 digits)
• Electronic Serial Number (ESN)
• System Identifier (SID)
• Station Class Mark (SCM)
• Indicates capabilities of a mobile station
• Supervisory Audio Tone (SAT)
• Digital Color Code (DCC)
• Help mobile stations distinguish neighboring base
  stations from one another

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Frequency Bands and Physical Channels

• The band for forward transmissions, from cell
  site to mobile station, is 870-890 MHz.
• The reverse band, for transmissions by mobiles,
  is 45 MHz lower.
• An AMPS physical channel occupies two 30 KHz
  frequency bands, one for each direction.

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Radiated Power

• An AMPS terminal is capable of radiating signals
  at 6 or 8 different power levels (6 mW to 4W).
• 10 log 4000 36 dBm
• The radiated power at a a base station is
  typically 25 W.
• Discontinuous transmission (DTX)
• Speech activity detector
• ON-OFF state
• Power saving and Interference reducing

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Analog Signal Processing

• Compression and pre-emphasis are established
  techniques for audio signal transmission.
• An amplitude limiter confines the maximum
  excursions of the frequency modulated signal to
  12 KHz.
• Low pass filter Attenuates signal components at
  frequencies above 3 KHz, refer to Fig. 3.5.
• The notch (at 6KHz) removes signal energy at the
  frequencies associated with the 3 SAT of the AMPS
  system.

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SAT and ST

• The SAT (Supervisory Audio Tone) transmitted with
  user information serves to identify the base
  station assigned to a call.
• Each base station has its own SAT- at 5970 Hz,
  6000 Hz, or 6030 Hz.
• An analog signals from AMPS terminals can also
  contain a 10 KHz sine wave referred to as a ST
  (Supervisory Tone).
• On-hook and Off-hook indications signaling
• The channel reuse principles (Section 9.3.2)

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Digital Signals

• AMPS also transmits important network control
  information in digital form.
• AMPS digital signal are sine waves either 8 KHz
  above or 8 KHz below the carrier.
• The signal format is Manchester coded binary
  frequency shift keying at a rate of 10 Kbps

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Spectrum Efficiency

• Frequency modulation in 30 KHz physical channels
• Signal-to-Interference ratio (SIR)
• SIR gt (SIR)req 18 dB
• Reuse factor N 7 (Figure 9.9)
• Spectrum efficiency
• E395 /725 2.26 conversations/cell/MHz
• 395 traffic channels, 25 MHz/system, 7 cells in a
  cluster

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Logical Channel Categories

• FOCC Forward (Downlink) Control Channel
• Carries the same information from one base
  station to all of the mobile terminals
  (Broadcast)
• RECC Reverse (Uplink) Control Channel
• Carries information from many mobile terminals
  that do not have voice channel (Random access)
• FVC Forward Voice Channel (Dedicated)
• RVC Reverse Voice channel (Dedicated)
• Forward and reverse traffic channel
• User information (Dedicated)

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Tasks Performed by Terminals

• Initialization mode
• The terminal turns the power on
• A conversation ends
• Loses contact with the current base station
• Idle mode
• Access mode (from Idle mode)
• The terminal presses the SEND button
• An incoming call request detected (MIN)
• A registration event stimulated
• Conversation mode

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Capacity

• There are 3 ways to increase the capacity
• Operate with smaller cells
• Obtain additional spectrum allocations
• Improve spectrum efficiency
• NAMPS (Narrowband-AMPS)
• Messages similar to AMPS
• Synchronization sequences
• Digital versions of the SAT and ST

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Review Exercises

• What is the purpose of the busy / idle bits in
  the FOCC? Why are they not used in the other
  control channel formats?
• Explain how the AMPS system users supervisory
  audio tones (SAT) and a digital color code (DCC).
  Why are both required?
• Explain why it is sometimes desirable for the
  AMPS system to set up a call through a base
  station that is not the nearest base station to
  the terminal. How does the AMPS system achieve
  this effect?