The Cellular Concept System Design Fundamentals

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Chapter 3

• The Cellular Concept - System DesignFundamentals

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I. Introduction

• Goals of a Cellular System
• High capacity
• Large coverage area
• Efficient use of limited spectrum
• Large coverage area - Bell system in New York
  City had early mobile radio
• Single Tx, high power, and tall tower
• Low cost
• Large coverage area - Bell system in New York
  City had 12 simultaneous channels for 1000 square
  miles
• Small users
• Poor spectrum utilization
• What are possible ways we could increase the
  number of channels available in a cellular
  system?

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• Cellular concept
• Frequency reuse pattern

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• Cells labeled with the same letter use the same
  group of channels.
• Cell Cluster group of N cells using complete set
  of available channels
• Many base stations, lower power, and shorter
  towers
• Small coverage areas called cells
• Each cell allocated a of the total number of
  available channels
• Nearby (adjacent) cells assigned different
  channel groups
• to prevent interference between neighboring base
  stations and mobile users

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• Same frequency channels may be reused by cells a
  reasonable distance away
• reused many times as long as interference between
  same channel (co-channel) cells is lt acceptable
  level
• As frequency reuse? ? possible simultaneous
  users?? subscribers ?? but system cost ? (more
  towers)
• To increase number of users without increasing
  radio frequency allocation, reduce cell sizes
  (more base stations) ?? possible simultaneous
  users ?
• The cellular concept allows all mobiles to be
  manufactured to use the same set of freqencies
• A fixed of channels serves a large of
  users by reusing channels in a coverage area

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II. Frequency Reuse/Planning

• Design process of selecting allocating channel
  groups of cellular base stations
• Two competing/conflicting objectives
• 1) maximize frequency reuse in specified area
• 2) minimize interference between cells

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• Cells
• base station antennas designed to cover specific
  cell area
• hexagonal cell shape assumed for planning
• simple model for easy analysis ? circles leave
  gaps
• actual cell footprint is amorphous (no specific
  shape)
• where Tx successfully serves mobile unit
• base station location
• cell center ? omni-directional antenna (360
  coverage)
• not necessarily in the exact center (can be up to
  R/4 from the ideal location)

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• cell corners ? sectored or directional antennas
  on 3 corners with 120 coverage.
• very commom
• Note that what is defined as a corner is
  somewhat flexible ? a sectored antenna covers
  120 of a hexagonal cell.
• So one can define a cell as having three antennas
  in the center or antennas at 3 corners.

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III. System Capacity

• S total of duplex channels available for use
  in a given area determined by
• amount of allocated spectrum
• channel BW ? modulation format and/or standard
  specs. (e.g. AMPS)
• k number of channels for each cell (k lt S)
• N cluster size ? of cells forming cluster
• S k N

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• M of times a cluster is replicated over a
  geographic coverage area
• System Capacity Total Duplex Channels C
• C M S M k N
• (assuming exactly MN cells will cover the area)
• If cluster size (N) is reduced and the geographic
  area for each cell is kept constant
• The geographic area covered by each cluster is
  smaller, so M must ? to cover the entire coverage
  area (more clusters needed).
• S remains constant.
• So C ?
• The smallest possible value of N is desirable to
  maximize system capacity.

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• Cluster size N determines
• distance between co-channel cells (D)
• level of co-channel interference
• A mobile or base station can only tolerate so
  much interference from other cells using the same
  frequency and maintain sufficient quality.
• large N ? large D ? low interference ? but small
  M and low C !
• Tradeoff in quality and cluster size.
• The larger the capacity for a given geographic
  area, the poorer the quality.

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• Frequency reuse factor 1 / N
• each frequency is reused every N cells
• each cell assigned k ? S / N
• N cells/cluster
• connect without gaps
• specific values are required for hexagonal
  geometry
• N i2 i j j2 where i, j ? 1
• Typical N values ? 3, 4, 7, 12 (i, j) (1,1),
  (2,0), (2,1), (2,2)

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• To find the nearest co-channel neighbors of a
  particular cell
• (1) Move i cells along any chain of hexagons,
  then (2) turn 60 degrees and move j cells.

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IV. Channel Assignment Strategies

• Goal is to minimize interference maximize use
  of capacity
• lower interference allows smaller N to be used ?
  greater frequency reuse ? larger C
• Two main strategies Fixed or Dynamic
• Fixed
• each cell allocated a pre-determined set of voice
  channels
• calls within cell only served by unused cell
  channels
• all channels used ? blocked call ? no service
• several variations
• MSC allows cell to borrow a VC (that is to say, a
  FVC/RVC pair) from an adjacent cell
• donor cell must have an available VC to give

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• Dynamic
• channels NOT allocated permanently
• call request ? goes to serving base station ?
  goes to MSC
• MSC allocates channel on the fly
• allocation strategy considers
• likelihood of future call blocking in the cell
• reuse distance (interference potential with other
  cells that are using the same frequency)
• channel frequency
• All frequencies in a market are available to be
  used

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• Advantage reduces call blocking (that is to say,
  it increases the trunking capacity), and
  increases voice quality
• Disadvantage increases storage computational
  load _at_ MSC
• requires real-time data from entire network
  related to
• channel occupancy
• traffic distribution
• Radio Signal Strength Indications (RSSI's) from
  all channels

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V. Handoff Strategies

• Handoff when a mobile unit moves from one cell
  to another while a call is in progress, the MSC
  must transfer (handoff) the call to a new channel
  belonging to a new base station
• new voice and control channel frequencies
• very important task ? often given higher priority
  than new call
• It is worse to drop an in-progress call than to
  deny a new one

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• Minimum useable signal level
• lowest acceptable voice quality
• call is dropped if below this level
• specified by system designers
• typical values ? -90 to -100 dBm

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• Quick review Decibels
• S Signal power in Watts
• Power of a signal in decibels (dBW) is Psignal
  10 log10(S)
• Remember dB is used for ratios (like S/N)
• dBW is used for Watts
• dBm dB for power in milliwatts 10 log10(S x
  103)
• dBm 10 log10(S) 10 log10(103) dBW 30
• -90 dBm 10 log10(S x 103)
• 10-9 S x 103
• S 10-12 Watts 10-9 milliwatts
• -90 dBm -120 dBW
• Signal-to-noise ratio
• N Noise power in Watts
• S/N 10 log10(S/N) dB (unitless raio)

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• choose a (handoff threshold) gt (minimum useable
  signal level)
• so there is time to switch channels before level
  becomes too low
• as mobile moves away from base station and toward
  another base station

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• Handoff Margin ?
• ? Phandoff threshold - Pminimum usable signal
  dB
• carefully selected
• ? too large ? unnecessary handoff ? MSC loaded
  down
• ? too small ? not enough time to transfer ? call
  dropped!
• A dropped handoff can be caused by two factors
• not enough time to perform handoff
• delay by MSC in assigning handoff
• high traffic conditions and high computational
  load on MSC can cause excessive delay by the MSC
• no channels available in new cell

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• Handoff Decision
• signal level decreases due to
• signal fading ? dont handoff
• mobile moving away from base station ? handoff
• must monitor received signal strength over a
  period of time ? moving average
• time allowed to complete handoff depends on
  mobile speed
• large negative received signal strength (RSS)
  slope ? high speed ? quick handoff
• statistics of the fading signal are important to
  making appropriate handoff decisions ? Chapters 4
  and 5

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• 1st Generation Cellular (Analog FM ? AMPS)
• Received signal strength (RSS) of RVC measured at
  base station monitored by MSC
• A spare Rx in base station (locator Rx) monitors
  RSS of RVC's in neighboring cells
• Tells Mobile Switching Center about these mobiles
  and their channels
• Locator Rx can see if signal to this base station
  is significantly better than to the host base
  station
• MSC monitors RSS from all base stations decides
  on handoff

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• 2nd Generation Cellular w/ digital TDMA (GSM,
  IS-136)
• Mobile Assisted HandOffs (MAHO)
• important advancement
• The mobile measures the RSS of the FCCs from
  adjacent base stations reports back to serving
  base station
• if Rx power from new base station gt Rx power from
  serving (current) base station by pre-determined
  margin for a long enough time period ? handoff
  initiated by MSC

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• MSC no longer monitors RSS of all channels
• reduces computational load considerably
• enables much more rapid and efficient handoffs
• imperceptible to user

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• A mobile may move into a different system
  controlled by a different MSC
• Called an intersystem handoff
• What issues would be involved here?
• Prioritizing Handoffs
• Issue Perceived Grade of Service (GOS) service
  quality as viewed by users
• quality in terms of dropped or blocked calls
  (not voice quality)
• assign higher priority to handoff vs. new call
  request
• a dropped call is more aggravating than an
  occasional blocked call

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• Guard Channels
• of total available cell channels exclusively
  set aside for handoff requests
• makes fewer channels available for new call
  requests
• a good strategy is dynamic channel allocation
  (not fixed)
• adjust number of guard channels as needed by
  demand
• so channels are not wasted in cells with low
  traffic

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• Queuing Handoff Requests
• use time delay between handoff threshold and
  minimum useable signal level to place a blocked
  handoff request in queue
• a handoff request can "keep trying" during that
  time period, instead of having a single block/no
  block decision
• prioritize requests (based on mobile speed) and
  handoff as needed
• calls will still be dropped if time period
  expires

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VI. Practical Handoff Considerations

• Problems occur because of a large range of mobile
  velocities
• pedestrian vs. vehicle user
• Small cell sizes and/or micro-cells ? larger
  handoffs
• MSC load is heavy when high speed users are
  passed between very small cells

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• Umbrella Cells
• Fig. 3.4, pg. 67
• use different antenna heights and Tx power levels
  to provide large and small cell coverage
• multiple antennas Tx can be co-located at
  single location if necessary (saves on obtaining
  new tower licenses)
• large cell ? high speed traffic ? fewer handoffs
• small cell ? low speed traffic
• example areas interstate highway passing thru
  urban center, office park, or nearby shopping mall

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• Cell Dragging
• low speed user w/ line of sight to base station
  (very strong signal)
• strong signal changing slowly
• user moves into the area of an adjacent cell
  without handoff
• causes interference with adjacent cells and other
  cells
• Remember handoffs help all users, not just the
  one which is handed off.
• If this mobile is closer to a reused channel ?
  interference for the other user using the same
  frequency
• So this mobile needs to hand off anyway, so other
  users benefit because that mobile stays far away
  from them.

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• Typical handoff parameters
• Analog cellular (1st generation)
• threshold margin ? 6 to 12 dB
• total time to complete handoff 8 to 10 sec
• Digital cellular (2nd generation)
• total time to complete handoff 1 to 2 sec
• lower necessary threshold margin ? 0 to 6 dB
• enabled by mobile assisted handoff

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• benefits of small handoff time
• greater flexibility in handling high/low speed
  users
• queuing handoffs prioritizing
• more time to rescue calls needing urgent
  handoff
• fewer dropped calls ? GOS increased
• can make decisions based on a wide range of
  metrics other than signal strength
• such as also measure interference levels
• can have a multidimensional algorithm for making
  decisions

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• Soft vs. Hard Handoffs
• Hard handoff different radio channels assigned
  when moving from cell to cell
• all analog (AMPS) digital TDMA systems (IS-136,
  GSM, etc.)
• Many spread spectrum users share the same
  frequency in every cell
• CDMA ? IS-95
• Since a mobile uses the same frequency in every
  cell, it can also be assigned the same code for
  multiple cells when it is near the boundary of
  multiple cells.
• The MSC simultaneously monitors reverse link
  signal at several base stations

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• MSC dynamically decides which signal is best and
  then listens to that one
• Soft Handoff
• passes data from that base station on to the PSTN
• This choice of best signal can keep changing.
• Mobile user does nothing for handoffs except just
  transmit, MSC does all the work
• Advantage unique to CDMA systems
• As long as there are enough codes available.

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VII. Co-Channel Interference

• Interference is the limiting factor in
  performance of all cellular radio systems
• What are the sources of interference for a mobile
  receiver?
• Interference is in both
• voice channels
• control channels
• Two major types of system-generated interference
• 1) Co-Channel Interference (CCI)
• 2) Adjacent Channel Interference (ACI)

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• First we look at CCI
• Frequency Reuse
• Many cells in a given coverage area use the same
  set of channel frequencies to increase system
  capacity (C)
• Co-channel cells ? cells that share the same set
  of frequencies
• VC CC traffic in co-channel cells is an
  interfering source to mobiles in Several
  different cells

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• Possible Solutions?
• 1) Increase base station Tx power to improve
  radio signal reception? __
• this will also increase interference from
  co-channel cells by the same amount
• no net improvement
• 2) Separate co-channel cells by some minimum
  distance to provide sufficient isolation from
  propagation of radio signals?
• if all cell sizes, transmit powers, and coverage
  patterns same ? co-channel interference is
  independent of Tx power

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• co-channel interference depends on
• R cell radius
• D distance to base station of nearest
  co-channel cell
• if D / R ? then spatial separation relative to
  cell coverage area ?
• improved isolation from co-channel RF energy
• Q D / R co-channel reuse ratio
• hexagonal cells ? Q D/R

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• Fundamental tradeoff in cellular system design
• small Q ? small cluster size ? more frequency
  reuse ? larger system capacity great
• But also small Q ? small cell separation ?
  increased co-channel interference (CCI) ? reduced
  voice quality ? not so great
• Tradeoff Capacity vs. Voice Quality

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• Signal to Interference ratio ? S / I,
  ____________
• S desired signal power
• Ii interference power from ith co-channel cell
• io of co-channel interfering cells

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• Approximation with some assumptions
• Di distance from ith interferer to mobile
• Rx power _at_ mobile

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• n path loss exponent
• free space or line of sight (LOS) (no
  obstruction) ? n 2
• urban cellular ? n 2 to 4, signal decays faster
  with distance away from the base station
• having the same n throughout the coverage area
  means radio propagation properties are roughly
  the same everywhere
• if base stations have equal Tx power and n is the
  same throughout coverage area (not always true)
  then the above equation (Eq. 3.8) can be used.

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• Now if we consider only the first layer (or tier)
  of co-channel cells
• assume only these provide significant
  interference
• And assume interfering base stations are
  equidistant from the desired base station (all at
  distance D) then

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• What determines acceptable S / I ?
• voice quality ? subjective testing
• AMPS ? S / I ?18 dB (assumes path loss exponent n
  4)
• Solving (3.9) for N
• Most reasonable assumption is io of
  co-channel interfering cells 6
• N 7 (very common choice for AMPS)

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• Many assumptions involved in (3.9)
• same Tx power
• hexagonal geometry
• n same throughout area
• Di D (all interfering cells are equidistant
  from the base station receiver)
• optimistic result in many cases
• propagation tools are used to calculate S / I
  when assumptions arent valid

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• S / I is usually the worst case when a mobile is
  at the cell edge
• low signal power from its own base station high
  interference power from other cells
• more accurate approximations are necessary in
  those cases

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N 7 and S / I 17 dB
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• Eq. (3.5), (3.8), and (3.9) are (S / I) for
  forward link only, i.e. the cochannel base Tx
  interfering with desired base station
  transmission to mobile unit
• so this considers interference _at_ the mobile unit
• What about reverse link co-channel interference?
• less important because signals from mobile
  antennas (near the ground) dont propagate as
  well as those from tall base station antennas
• obstructions near ground level significantly
  attenuate mobile energy in direction of base
  station Rx
• also weaker because mobile Tx power is variable ?
  base stations regulate transmit power of mobiles
  to be no larger than necessary

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• HW1
• 1-9, 1-11, 1-18, 3-5, 3-7