Lecture 4: Cellular Fundamentals

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Lecture 4 Cellular Fundamentals

• Chapter 3 - Continued

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I. Adjacent Channel Interference

• Two major types of system-generated interference
• Co-Channel Interference (CCI) discussed in last
  lecture
• Adjacent Channel Interference (ACI)
• Adjacent Channel Interference (ACI)
• Imperfect Rx filters allow energy from adjacent
  channels to leak into the passband of other
  channels

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• desired filter response

• actual filter response

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• This affects both forward reverse links
• Forward Link ? base-to-mobile
• interference _at_ mobile Rx from a ______ Tx
  (another mobile or another base station that is
  not the one the mobile is listening to) when
  mobile Rx is ___ away from base station.
• signal from base station is weak and others are
  somewhat strong.
• Reverse Link ? mobile-to-base
• interference _at_ base station Rx from nearby mobile
  Tx when desired mobile Tx is far away from base
  station

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• Near/Far Effect
• interfering source is near some Rx when desired
  source is far away
• ACI is primarily from mobiles in the same cell
• some cell-to-cell ACI does occur as well ? but a
  secondary source
• Control of ACI
• dont allocate channels within a given cell from
  a contiguous band of frequencies
• for example, use channels 1, 4, 7, and 10 for a
  cell.
• no channels next to each other

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• maximize channel separation
• separation of as many as N channel bandwidths
• some schemes also seek to minimize ACI from
  neighboring cells by not assigning adjacent
  channels in neighboring cells

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• Originally 666 channels, then 10 MHz of spectrum
  was added
• 666166 832 channels
• 395 VC plus 21 CC per service provider
  (providers A B)
• 3952 790, plus 42 control channels
• Provider A is a company that has not
  traditionally provided telephone service
• Provider B is a traditional wireline operator
• 21 VC groups with 19 channels/group
• at least 21 channel separation for each group

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• for N 7 ? 3 VC groups/cell
• For example, choose groups 1A, 1B, and 1C for a
  cell so channels 1, 8, 15, 22, 29, 36, etc. are
  used.
• ? 57 channels/cell
• at least 7 channel separation for each cell group
• to have high quality on control channels, 21 cell
  reuse is used for CCs
• instead of reusing a CC every 7 cells, as for
  VCs, reuse every 21 cells (after every three
  clusters)
• greater distance between control channels, so
  less CCI

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• use high quality filters in base stations
• better filters are possible in base stations
  since they are not constrained by physical size
  and power as much as in the mobile Rx
• makes reverse link ACI less of a concern than
  forward link ACI
• also true because of power control (discussed
  below)
• choice of modulation schemes
• different modulation schemes provide less or more
  energy outside their passband.

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• Power Control
• technique to minimize ACI
• base station MSC constantly monitor mobile
  received signal strength
• mobile Tx power varied (controlled) so that
  smallest Tx power necessary for a quality reverse
  link signal is used (lower power for the closer
  the mobile is to the base station)
• also helps battery life on mobile

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• dramatically improves adjacent channel S / I
  ratio, since mobiles in other cells only transmit
  at high enough power as transmitter controls (not
  at full power)
• most beneficial for ACI on reverse link
• will see later that this is especially important
  for CDMA systems

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III. Trunking Grade of Service (GOS)

• Trunked radio system radio system where a large
  of users share a pool of channels
• channel allocated on demand returned to channel
  pool upon call termination
• exploit statistical (random) behavior of users so
  that fixed of channels can accommodate large
  of users
• Trade-off between the number of available
  channels that are provided and the likelihood of
  a particular user finding no channels available
  during the busy hour of the day.

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• trunking theory is used by telephone companies to
  allocate limited of voice circuits for large
  of telephone lines
• efficient use of equipment resources ? savings
• disadvantage is that some probability exists that
  mobile user will be denied access to a channel
• blocked call access denied ? blocked call
  cleared
• delayed call access delayed by call being put
  into holding queue for specified amount of time

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• GOS measure of the ability of user access to a
  trunked system during the _______ hour
• specified as probability (Pr) that call is
  blocked or delayed
• designed to handle the busiest hour ? typically
  ______
• Erlang unitless measure of traffic intensity
• e.g. 0.5 erlangs 1 channel occupied 30 minutes
  during 1 hour
• Table 3.3, pg. 78 ? trunking theory definitions

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• Offered Traffic Intensity (A)
• Offered? ? not necessarily carried by system
  (some is blocked or delayed)
• each user Au?H Erlangs (also called ? in
  queueing theory)
• ? traffic intensity (average arrival rate of
  new calls, in new requests per time unit, say
  calls/min).
• H average duration of a call (also called 1/ µ
  in queueing theory)
• system with U users ? A UAu U?H Erlangs
• capacity maximum carried traffic C Erlangs
  (equal to total of available channels that are
  busy all the time)

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• Erlang B formula
• Calls are either admitted or blocked
• A total offered traffic
• C channels in trunking pool (e.g. a cell)
• AMPS designed for GOS of 2
• blocked call cleared (denied) ? BCC

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• capacities to support various GOS values
• Note that twice the capacity can support much
  more than twice the load (twice the number of
  Erlangs).

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• Erlang C formulas
• blocked call delayed ? BCD ? put into holding
  queue
• GOS is probability that a call will still be
  blocked even if it spends time in a queue and
  waits for up to t seconds
• equations (3.17) to (3.19) in book

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• Graphical form of Erlang B formulas

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Graphical form of Erlang C formulas
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• Example Find how many users can be supported in
  a cell containing 50 channels for a 2 GOS
  (Blocked Calls Cleared) if the average user calls
  twice/hr with an average call duration of 5
  minutes.
• What is the corresponding C from the figure?
• What is A (Traffic Intensity) from the figure?
• So, how many users can be supported?

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• Trunking Efficiency
• measure of the of users supported by a specific
  configuration of fixed channels, efficiency in
  terms of users per available channel U / C
• Table 3.4, pg. 79 ? assume 1 GOS
• Assume Au 0.2
• 1 group of 20 channels
• 2 groups of 10 channels, with equal number of
  users per group

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• the allocation of channel groups can
  substantially change the of users supported by
  trunked system
• The larger the trunking pool, the better the
  trunking efficiency.
• as trunking pool size ? then trunking efficiency
  ?
• What is the relationship between trunking pool
  size, trunking efficiency, received signal
  quality, and cluster size?
• As cluster size decreases

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• Note Trunking efficiency is an issue both in
  FDMA/TDMA systems and in CDMA systems (where the
  capacity limit is the number of possible codes
  and the interference levels).

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IV. Improving Cellular System Capacity

• A cellular design eventually (hopefully!) becomes
  insufficient to support the growing number of
  users.
• Need to provide more channels per unit coverage
  area
• Would like to have orderly growth
• Would like to upgrade the system instead of
  rebuild
• Would like to use existing towers as much as
  possible

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• Cell Splitting
• subdivide congested cell into several smaller
  cells
• increases number of times channels are reused in
  an area
• must decrease antenna height Tx power
• so smaller coverage per cell results
• and the co-channel interference level is held
  constant

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• each smaller cell keeps same of channels as
  the larger cell, since each new smaller cell uses
  the same number of frequencies
• this means that we keep that same cluster size
• capacity ? because channel reuse ? per unit area
• smaller cells ? micro-cells

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• Illustration is for towers at the corners

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• advantages include
• only needed for cells that reach max. capacity ?
  not all cells
• implement when Pr blocked call gt acceptable GOS
• system capacity can gradually expand as demand ?
• disadvantages include
• handoffs/unit area increases
• umbrella cell for high velocity traffic may be
  needed
• more base stations ? for real estate, towers,
  etc.

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• complicated design process
• new base stations use lower power and antenna
  height
• What about existing base stations?
• If kept at the same power, they would overpower
  new microcells.
• If reduced in power, they would not cover their
  own cells.
• One solution Use separate groups of channels.
• One group at the original power and another group
  at the lower power.
• New microcells only use lower power channels.
• As load growth continues, more and more channels
  are moved to lower power.

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• Sectoring
• cell splitting keeps D / R unchanged (same
  cluster size and CCI) but increases frequency
  reuse/area
• alternate way to ? capacity is to _____ CCI
  (increase S / I ratio)

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• replace omni-directional antennas at base station
  with several directional antennas
• 3 sectors ? 3 120 antennas
• 6 sectors ? 6 60 antennas

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• cell channels broken down into sectored groups
• CCI reduced because only some of neighboring
  co-channel cells radiate energy in direction of
  main cell
• center cell labeled "5" has all co-channel cells
  illustrated
• only 2 co-channel cells will interfere if all are
  using 120 sectoring
• only 1 co-channel cell would interfere when using
  60 sectoring
• If the S/I was 17 dB for N 7 and n 4, what is
  the S / I now with 120 sectoring?
• 24.2 dB

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• How is capacity increased?
• sectoring only improves S/I which increases voice
  quality, beyond what is really necessary
• by reducing CCI, the cell system designer can
  choose smaller cluster size (N ?) for acceptable
  voice quality
• smaller N ? greater frequency reuse ? larger
  system capacity
• What would the system capacity, Cnew, now be when
  120 using sectoring, as compared to the old
  capacity, Cold ?

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• much less costly than cell splitting
• only require more antennas _at_ base station vs.
  multiple new base stations for cell splitting
• primary disadvantage is that the available
  channels in a cell are subdivided into sectored
  groups
• trunked channel pool ?, therefore trunking
  efficiency ?
• There are more channels per cell, because of
  smaller cluster sizes, but those channels are
  broken into sectors.

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• other disadvantages
• must design network coverage with sectoring
  decided in advance
• cant effectively use sectoring to increase
  capacity after setting cluster size N
• cant be used to gradually expand capacity as
  traffic ? like cell splitting
• More Handoffs
• More antenna, more cost

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• Next topic Mobile Radio Propagation -
  Large-scale path loss, small-scale fading, and
  multipath
• Free space propagation loss
• Reflections
• 2-ray model
• Diffraction
• Fading
• Multipath

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• HW-2
• 3-10, 3-13, 3-15, 3-22, 3-26