Lecture 4 Cellular Fundamentals by fjwuxn


									Lecture 4: Cellular Fundamentals

     Chapter 3 - Continued

I. Adjacent Channel Interference

 Two major types of system-generated
  1) Co-Channel Interference (CCI) – discussed in last
  2) Adjacent Channel Interference (ACI)
 Adjacent Channel Interference (ACI)
   Imperfect Rx filters allow energy from adjacent
    channels to leak into the passband of other

 desired filter response

  actual filter response

 This affects both forward & reverse links
 Forward Link → base-to-mobile
   interference @ 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 @ base station Rx from nearby mobile
    Tx when desired mobile Tx is far away from base
 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
   don’t 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

 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

 Originally 666 channels, then 10 MHz of
  spectrum was added
     666+166 = 832 channels
 395 VC plus 21 CC per service provider
  (providers A & B)
      395*2 = 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

 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 CC’s
    instead of reusing a CC every 7 cells, as for VC’s,
     reuse every 21 cells (after every three clusters)
    greater distance between control channels, so less

 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.

 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

 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

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
      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.

 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

 GOS : measure of the ability of user access to a
  trunked system during the _______ hour
    specified as probability (Pr) that call is blocked or
    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

 “Offered” Traffic Intensity (A)
    Offered? → not necessarily carried by system
     (some is blocked or delayed)
    each user Au=λH Erlangs (also called ρ in queueing
       λ = 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 = UλH Erlangs
    capacity = maximum carried traffic = C Erlangs =
     (equal to total # of available channels that are busy
     all the time)
 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

 capacities to support various GOS values

 Note that twice the capacity can support much more than
  twice the load (twice the number of Erlangs).
 Erlang C formulas
   blocked call delayed → BCD → put into holding
   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

 Graphical form of Erlang B formulas

 Graphical form of Erlang C formulas

 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?

 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:

 the allocation of channel groups can
  substantially change the # of users supported by
  trunked system
    The larger the trunking pool, the better the trunking
 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…

 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).

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
   Would like to have orderly growth
   Would like to upgrade the system instead of rebuild
   Would like to use existing towers as much as

 Cell Splitting
    subdivide congested cell into several smaller
    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

 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”

 Illustration is for towers at the corners

 advantages include:
   only needed for cells that reach max. capacity → not
    all cells
   implement when Pr [blocked call] > 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.

 complicated design process
   new base stations use lower power and antenna
   What about existing base stations?
      If kept at the same power, they would overpower new
      If reduced in power, they would not cover their own
   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.

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

 replace omni-directional antennas at base station
  with several directional antennas
   3 sectors → 3 120° antennas
   6 sectors → 6 60° antennas

 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
 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

 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

    What would the system capacity, Cnew, now be when
     120° using sectoring, as compared to the old capacity,
     Cold ?

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

 other disadvantages:
   must design network coverage with sectoring
    decided in advance
   can’t effectively use sectoring to increase capacity
    after setting cluster size N
   can’t be used to gradually expand capacity as
    traffic ↑ like cell splitting
   More Handoffs
   More antenna, more cost

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

 HW-2
  3-10, 3-13, 3-15, 3-22, 3-26


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