Advanced Mobile Phone System (AMPS)

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Chapter 6

• Advanced Mobile Phone System (AMPS)

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Preliminary

• Technology Tutorials

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Multiple Access

• Frequency Division Multiple Access (FDMA)
• AMPS and CT2
• Time Division Multiple Access (TDMA)
• Hybrid FDMA/TDMA
• Code Division Multiple Access
• a physical channel corresponds to a binary code

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CDMA

• Each station has its own unique chip sequence
  (CS)
• All CS are pair-wise orthogonal
• For example (codes A, B, C and D are pair-wise
  orthogonal)
• A 00011011 gt (-1-1-111-111)
• B 00101110 gt (-1-11-1111-1)
• C 01011100 gt (-11-1111-1-1)
• D 01000010 gt (-11-1-1-1-11-1)

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CDMA

• AB (11-1-11-11-1) 0
• BC (1-1-1-111-11) 0
• Example if station C transmits 1 to station E,
  but station B transmits 0 and station A transmits
  1 simultaneously then the signal received by
  station E will become S (-11-33-1-1-11). E
  can convert the signal S to SC
  (11331-11-1)/8 1

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Mobile Radio Signals

• Four main effects produced by physical
  conditions
• Attenuation that increases with distance
• Random variation due to environmental features,
  i.e., shadow fading.
• Signal fluctuations due to the motion of a
  terminal, i.e., Rayleigh fading.
• Distortion due to that the signal travels along
  different paths, i.e., multi-path fading.

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Attenuation Due to Distance

• The signal strength decreases with distance
  according to the relationship

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Slow/Shadow Fading

• Random Environmental Effects
• As a terminal moves, the signal strength
  gradually rises and falls with significant
  changes occurring over tens of meters.
• Let P (received power) be a log-normal
  distributed random variable with mean Preceive
  and S (signal strength in dBm), i.e.,
  S10log10(1000P) dBm.
• The log-normal of P implies that S is normal
  distributed.

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Fast/Rayleigh Fading

• Fast (Rayleigh) Fading Due to Motion of Terminals
• As the terminal moves, each ray undergoes a
  Doppler shift, causing the wavelength of the
  signal to either increase or decrease
• Doppler shifts in many rays arriving at the
  receiver cause the rays to arrive with different
  relative phase shifts
• At some locations, the rays reinforce each other.
  At other locations, the ray cancel each other
• These fluctuations occur much faster than the
  changes due to environmental effects

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Multi-path Propagation

• There are many ways for a signal to travel from a
  transmitter to a receiver (see Fig 9.5)
• Multiple-path propagation is referred to as
  inter-symbol interference (see Fig. 9.6)
• Path delay the maximum delay difference between
  all the paths

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Technology Implications

• Systems employ power control to overcome the
  effects of slow fading
• Systems use a large array of techniques to
  overcome the effects of fast fading and
  multi-path propagation
• Channel coding
• Interleaving
• Equalization
• PAKE receivers
• Slow frequency hopping
• Antenna diversity

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Spectrum Efficiency
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Spectrum Efficiency (Contd)

• Compression Efficiency and Reuse Factor
• Compression Efficiency C conversations/per MHz
  (one-cell system)
• If N is the number of reuse factor, spectrum
  efficiency E C/N conversations per base station
  per MHz
• A measure of this tolerance is the
  signal-to-interference ratio S/I
• A high tolerance to interference promotes
  cellular efficiency
• S/I is an increasing function of the reuse factor
  N

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Spectrum Efficiency (Contd)

• Channel Reuse Planning
• A channel plan is a method of assigning channels
  to cells in a way that guarantees a minimum reuse
  distance between cells using the same channel.
• N 1/3(D/R)2 where D is the distance between a
  BS and the nearest BS that use the same channel
  and R is radius of a cell.
• Practical value of N range from 3 to 21.

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Slow Frequency Hopping

• The signal moves from one frequency to another in
  every frame
• The purpose of FH is to reduce the transmission
  impairments
• Without FH, the entire signal is subject to
  distortion whenever the assigned carrier is
  impaired

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RAKE Receiver

• Synchronization is a major task of a SS receiver
• Difficulty multi-path propagation
• Solution Multiple correlator (demodulator) in
  each receiver
• Each correlator operates with a digital carrier
  synchronized to one propagation path

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Channel Coding

• Channel codes protect information signals against
  the effects of interference and fading
• Channel coding decrease the required
  signal-to-interference ratio (S/I)req and the
  reuse factor N
• Channel coding will decrease the compression
  efficiency C
• The net effect is to increase the overall
  spectrum efficiency
• Channel codes can serve two purposes
• error detection and forward error correction (FEC)

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Block Codes

• Block code (n, k, dmin)
• Used to Protect The Control Information
• n is the total number of transmitted bits per
  code word
• k is the number of information bits carried by
  each code word
• dmin the minimum distance between all pairs of
  code word
• Ex n 3, k 2, dmin 2 (000, 011, 101, 110)
• Code rate rk/n.

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Block Codes

• When dmin 5, there are three possible decoder
  actions
• The decoder can correct no errors and detect up
  to four errors
• It can correct one error and detect two or three
  errors
• It can correct two errors, three or more bit
  errors in a block produce a code word error

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Convolutional Codes

• Each time a new input bit arrives at the encoder,
  the encoder produces m new output bits
• the encoder obtains m output bits by performing m
  binary logic operations on the k bits in the
  shift register
• The code rate is r 1/m

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Example
V1 R1 V2 R1 ? R2 ? R3 V3 R1 ? R3
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Interleaving

• Most error-correcting codes are effective only
  when transmission error occurs randomly in time.
• To prevent errors from clustering, cellular
  systems permute the order of bits generated by a
  channel coder.
• Receivers perform the inverse permutation.

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Interleaving

• Example
• WHAT I TELL YOU THREE TIMES IS TRUE
• If there are four consecutive errors in the
  middle, the result is
• WHAT I TELL YBVOXHREE TIMES IS TRUE
• Alternatively, it is possible to interleave the
  symbol using a 5 x 7 interleaving matrix (See pp.
  364-365)
• WHOT I XELL YOU THREE TIMEB IS VRUE

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Adaptive Equalization

• An adaptive equalizer operates in two modes
• Training mode Modem transmits a signal, referred
  to as a training sequence, that is known to
  receiver. The receiving modem process the
  distorted version of training sequence to obtain
  a channel estimate
• Tracking mode The equalizer uses the channel
  estimate to compensate for distortions in the
  unknown information sequence

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Walsh Hadamard Matrix

• The CDMA system uses a 64 x 64 WHM in two ways
• In down-link transmissions, it used as an
  orthogonal code, which is equivalent to an
  error-correcting block code with (n, k dmin)
  (64, 6 32)
• In up-link transmissions, the matrix serve as a
  digital carrier due to its orthogonal property

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Walsh Hadamard Matrix

• W1 0

0 0 0 1
W2
0 0 0 1
0 0 0 1
W4
0 0 0 1
1 1 1 0
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AMPS System

• The first generation cellular phone system

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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?

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References

• D.J. Goodman, Wireless Personal Communications
  Systems, Ch9 and Ch3.
• Ch9 Preliminary
• Ch3 AMPS system