Problem: Designing the

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Problem Designing the best channel for an
analog cellular system

• A specific example of a generic radio system
  problem, in which both signal and interference
  are controlled.
• A seemingly simple problem that turns out to be
  complex and interesting
• Goals Good voice quality spectrum efficiency

Critique the approach! Does it apply to
unregulated designs Like 802.11? How much
complexity needs to be included?
R. Frenkiel 9/18/03
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Whats this about spectrum efficiency?

• Politically important in the 60s and 70s
• Why?

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Whats this about spectrum efficiency?

• Politically important in the 60s and 70s
• Free spectrum to the best system
• efficiency implied serving more people
• BH Erlangs/MHz of spectrum/square mile
• But cell size accomplishes the same thing
• So whats the real reason to be efficient?

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Whats this about spectrum efficiency?

• But cell size accomplishes the same thing
• So whats the real reason to be efficient?
• COST!! (want to maximize channels/cell)
• Base station radios are a minor cost. The cost
  of a cell is almost entirely fixed (land,
  building, etc.), and the cost of a system
  (investment/customer) is almost entirely in the
  cells, so doubling the calls handled by a cell
  cuts the system investment almost in half.

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Whats this about spectrum efficiency?

• Today you buy the spectrum, and efficiency is
  seldom mentioned
• But the problem of being efficient remains
  but only to minimize investment (not as a
  political objective)

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Returning to the problem Designing the best
channel for an analog cellular system
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D/R4.6 N7
FM Deviation vs. Reuse Distance
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Problem Designing the best channel for analog
cellular system

• Underlying logic (circa 1970)
• Channel reuse distance determined by interference
• Greater FM deviation (wider channels) rejects
  interference better
• Thus, wider channels can be used in nearer cells
• Fewer Channels, but more channels per cell?
• Systems engineers love an optimum

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We need an objective

• Radio systems used a 5 level quality scale--
  excellent, good, fair, poor or unintelligible
• Tentative Objective 90 of calls good or
  excellent
• But cells (and therefore calls) are variable
• Re-statement quality is good at 90th -ile s/i
  of the cell i.e., the quality is good 90 of the
  time
• Is that the same?
• How do we apply this objective to the problem?

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A two-phase approach

• Model voice quality vs. s/i Use subjective
  listening tests (recorded Harvard sentences,
  recorded at different s/i), to determine (for
  each FM deviation) the minimum s/i that yields
  good quality. (Example result 12 KHz deviation
  requires 17 dB s/I for good quality)
• Model the s/i distribution for each reuse
  distance to determine 90th -ile. Propagation
  studies yield mean path loss and variance. Do we
  need simulations? (Example result a 7-cell
  pattern yields 17 dB s/i at the 90th -ile)
• Thus, combining (1) and (2), achieving good
  quality at the 90th -ile with 12 KHz deviation
  requires a 7-cell pattern

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A two-phase approach (continued)

• Repeat for other deviations. Example result
  using 5KHz deviation, we need a 16-cell pattern
• Calculate channel spacing for each deviation (how
  many total channels per MHz of spectrum)
• Calculate spectrum efficiency (channels/cell/Mhz
  of allocated spectrum)
• If 12 KHz deviation requires a 40 KHz channel
  spacing, this example yields 106/(4x104)(7) 3.6
  channels/cell/MHz
• If 5 KHz deviation requires a 25 KHz channel and
  a 16-cell pattern, we get 106/(2.5x104)(16) 2.5
  channels/cell/MHz
• 12 KHz is more efficient

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We have skipped over some significant problems

• What channel conditions are we actually recording
  for these tests? Just s/i x dB?

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Problem Rayleigh Fading

• How does fading get included in this method?
• Not realistic to use recordings at constant s/i
• Sounds too good
• How does fading get included in this method?

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Problem Rayleigh Fading

• How does fading get included in this method?
• Include fading in the recording
• Create s/i distributions based on local mean of
  fading signal and interference signals
• What fading rate?
• What other questions does this suggest?

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Problem Receiver diversity?

• Cost effective? (at base? at mobile? what
  type?)

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Problem Other radio parameters(radio is
non-linear device we need not just a radio we
need the radio)

• What does 12 KHz deviation really mean?

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Problem Radio parameters(radio is non-linear
device)

• What does 12 KHz deviation mean?
• What if I shout? (need peak limiter)
• How do we set limiter (hard vs. soft)?
• Relationship of mean to peak? (deviation vs.
  distortion)

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Problem Radio parameters(radio is non-linear
device)

• What if I whisper (need AGC)?
• parameters of AGC?
• How do we maximize deviation without distortion?

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Problem Radio parameters(radio is non-linear
device)

• What if I whisper (need AGC)?
• parameters of AGC?
• How do we maximize deviation without distortion?
• Compandor (a real breakthrough!)
• What limits the compression rule (21, 41, etc)

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Problem Channel Spacing

• 12 KHz deviation generally meant 40 KHz channel
  spacing (Carsons Rule)
• Why did cellular use only 30 KHz channel spacing?

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Problem Channel Spacing

• 12 KHz deviation generally meant 40 KHz channel
  spacing (Carsons Rule)
• Why did cellular use only 30 KHz channel spacing?
• Spacing must tolerate near/far problem
• Filter must reject adjacent channel at much
  higher level
• Cellular can use adjacent channel at different
  cell

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Problem Politics
Motorola vs. the Bell System
Political Science
Bell Labs Wider Channels reject interference so
reuse distances are reduced- more channels per
cell Motorola Narrow Channels means less
spectrum for Cellular more channels for
fleets
Conclusion Using complex technical arguments
with non-technical people for political purposes
yields major delays
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Problem Non-uniform Cell grids

• How do we account for irregular grids and
  variable terrain? (Good and bad cells?)

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Cells in the Real world
Tolerances and Propagation
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Problem Non-uniform Cell grids

• Non-uniform grids increase the spread in s/I
  (more potential dead spots)
• How do we account for irregular grids and
  variable terrain?
• Study actually said N4 would work
• N7 was based on concerns about irregularity
• Further fueled political debate (was it a ploy to
  get more spectrum?)

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Comparing deviation vs. quality over the whole
cell (not just the 90th -ile)
E
12 KHz channels
G
Q U A L I T Y
5 KHz channels
F
P
U
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22
12
S/I
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Equal at 90 percentile
Wide- 7-cells
Narrow- 16 cells
0
50
90 -ile
Wide is more efficient, but different
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Equal Efficiency
Wide with 7-cell pattern
Narrow with 12 cell pattern
Which performance is better?
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What have we ignored?
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What have we ignored?

• Variability during call
• Pedestrians vs. cars
• Harvard sentences vs conversation
• Quiet listening booths vs environmental noise

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What have we ignored?

• Variability during call
• Pedestrians vs. cars
• Harvard sentences vs conversation
• Quiet listening booths vs environmental noise
• NEW TECHNOLOGY

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And now-- the new world ofdigital quality

• Digital voice compression (more channels)
• Dramatic cost reduction (more than 50)
• Error coding to allow good performance at 12 dB
  (3-4 cell reuse patterns)
• Any Concerns?

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Digital Processing for more reuse?
4-cell w. processing?
Wide- 7-cells
Narrow- 12 cells
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So what do we conclude?

• Is such a process worthwhile?
• Is it so complex that conclusions are
  meaningless?
• Does it lead to improvements in subsystems (like
  companding)?
• Is it applicable to unregulated systems?