Transcript Lecture 4: Cellular Fundamentals

Lecture 4: Cellular Fundamentals
Chapter 3 - Continued
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I. Adjacent Channel Interference
 Two major types of system-generated
interference:
1) Co-Channel Interference (CCI) – discussed in last
lecture
2) 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 @ 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
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
 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
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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
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
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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 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
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=λH Erlangs (also called ρ in queueing
theory)
 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)
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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] > 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 cochannel 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 @ 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
 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
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 Next topic: Mobile Radio Propagation - Largescale path loss, small-scale fading, and
multipath
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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
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