Wireless Communications Engineering

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Wireless Communications Engineering

• Lecture 8 Cellular Fundamentals
• Prof. Mingbo Xiao
• Nov. 18, 2004

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Mobile Radio Environment
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Mobile Radio Environment

• The transmissions over the wireless link are in
  general very difficult to characterize.
• EM signals often encounter obstacles, causing
  reflection, diffraction, and scattering.
• Mobility introduces further complexity.
• We have focused on simple models to help gain
  basic insight and understanding of the wireless
  radio medium.
• Three main components Path Loss, Shadow fading,
  Multipath fading (or fast fading).

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Mobile Radio Channel Fading
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PHY Technologies

• Previous lectures have covered topics such as
  modulation, source/channel coding, equalization,
  and many more.
• These nice technologies (and others) have enabled
  reliable communication over error-prone wireless
  links.
• You may ask Isnt this the end of story?
• Unfortunately, even the modest wireless network
  needs a lot more to accomplish .

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Protocol Layers

• The previous lectures mainly deal with what is
  called the physical (PHY) layer of the networks.
• Network protocols are often organized in layers,
  with higher level of abstraction in higher layer.
• gt Simplify design and implementation.
• From now on, we assume most PHY details have been
  taken care, and focus on some higher layers, such
  as MAC and networking.

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Abstractions

• Consider only the large-scale channel behavior
• Introduce a generic concept of channel, which
  can be time slot in TDMA, frequency band in FDMA,
  or orthogonal code in CDMA systems (to be
  elaborated in next lecture).
• System performance metrics are changing from BER
  to Blocking prob., Throughput and Delay...
• Note unlike wired networks, wireless system may
  require cross-layer design.

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Radio Systems

• Fixed telephone network runs wires to every
  household
• Suppose we give every household their own
  allocation of radio spectrum using analogue
  speech of 4 kHz bandwidth (single sideband)
• 12.5 million households x 4 kHz 50 GHz!
• Clearly impractical!
• no other services possible using radio
  transmission
• whole range of radio transmission modes to
  address
• and most of the spectrum unused most of the time!
• remember traffic statistics

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Limitations of Wireless

• Channel is unreliable
• Spectrum is scarce, and not all ranges are
  suitable for mobile communication
• Transmission power is often limited
• Battery
• Interference to others

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Early Mobile Telephone Services

• First introduced in the U.S. by ATT (1946)
• Used to interconnect mobile users (in
  automobiles) to telephone networks.
• A single powerful transmitter from the BS to
  cover up to approx. 50 miles radius.
• Few channels for many people
• Early Bell Mobile Phone service in New York had
  12 channels, serving 543 customer, waiting list
  of 3,700 and market of 10 million!! - CAPACITY
  LIMITED
• Advanced systems for their time but very
  inefficient, and service was terrible (blocking
  probabilities as high as 65).

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Noise-limited System

• Range limited by thermal (and man made) noise
• Example 100 W Tx at 30m, 30 km range, 25 kHz FM,
  2 m Rx
• kTB 1.3803x10-23 x 290 x 25,000 -130 dBm.
• transmit power 10log(100/10-3) 50 dBm
• path loss over say 30 km 40 log 30,000 - 20 log
  60 143 dB
• receive signal 50 - 143 -93 dBm
• receive S/N ratio 37 dB (17 dB system plus 20
  dB fade margin)

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Advent of Cellular Systems

• Noting from the channel model, we know signal
  will attenuated with distance and have no
  interference to far users.
• In the late 1960s and early 1970s, work began on
  the first cellular telephone systems.
• The term cellular refers to dividing the service
  area into many small regions (cells) each served
  by a low-power transmitter with moderate antenna
  height.

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Cellular Networks
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Cellular Network Organization

• Use multiple low-power transmitters
• Areas divided into cells
• Each served by its own antenna
• Served by base station consisting of transmitter,
  receiver, and control unit
• Band of frequencies allocated
• Cells set up such that antennas of all neighbors
  are equidistant

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Consequences

• Transmit frequencies are re-used across these
  cells and the system becomes interference rather
  than noise limited
• the need for careful radio frequency planning
  colouring in hexagons!
• a mechanism for handling the call as the user
  crosses the cell boundary - call handoff (or
  handover)
• increased network complexity to route the call
  and track the users as they move around
• But one significant benefit very much increased
  traffic capacity, the ability to service many
  users

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Cellular System Architecture
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Cellular Systems Terms

• Mobile Station
• users transceiver terminal (handset, mobile)
• Base Station (BS)
• fixed transmitter usually at centre of cell
• includes an antenna, a controller, and a number
  of receivers
• Mobile Telecommunications Switching Office (MTSO)
  /Mobile Switch Center (MSC)
• handles routing of calls in a service area
• tracks user
• connects to base stations and PSTN

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Cellular Systems Terms (Contd)

• Two types of channels available between mobile
  unit and BS
• Control channels used to exchange information
  for setting up and maintaining calls
• Traffic channels carry voice or data connection
  between users
• Handoff or handover
• process of transferring mobile station from one
  base station to another, may also apply to change
  of radio channel within a cell

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Cellular Systems Terms (Contd)

• Downlink or Forward Channel
• radio channel for transmission of information
  (e.g.speech) from base station to mobile station
• Uplink or Reverse Channel
• radio channel for transmission of information
  (e.g.speech) from mobile station to base station
• Paging
• a message broadcast over an entire service area,
  includes use for mobile station alert (ringing)
• Roaming
• a mobile station operating in a service area
  other than the one to which it subscribes

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Steps in an MTSO Controlled Call between Mobile
Users

• Mobile unit initialization
• Mobile-originated call
• Paging
• Call accepted
• Ongoing call
• Handoff

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Frequency Reuse

• Cellular relies on the intelligent allocation and
  reuse of radio channels throughout a coverage
  area.
• Each base station is allocated a group of radio
  channels to be used within the small geographic
  area of its cell
• Neighbouring base stations are given different
  channel allocation from each other

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Frequency Reuse (Contd)

• If we limit the coverage area within the cell by
  design of the antennas
• we can re-use that same group of frequencies to
  cover another cell separated by a large enough
  distance
• transmission power controlled to limit power at
  that frequency to keep interference levels within
  tolerable limits
• the issue is to determine how many cells must
  intervene between two cells using the same
  frequency

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Radio Planning

• Design process of selecting and allocating
  channel frequencies for all cellular base
  stations within a system is known as frequency
  re-use or frequency planning.
• Cell planning is carried out to find a geometric
  shape to
• tessellate a 2D space
• represent contours of equal transmit power
• Real cells are never regular in shape

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Two-Dimensional Cell Clusters

• Regular geometric shapes tessellating a 2D space
  Square, triangle, and hexagon.
• Tessellating Hexagon is often used to model
  cells in wireless systems
• Good approximation to a circle (useful when
  antennas radiate uniformly in the x-y
  directions).
• Also offer a wide variety of reuse pattern
• Simple geometric properties help gain basic
  understanding and develop useful models.

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Coverage Patterns
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Cellular Coverage Representation
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Geometry of Hexagons
Hexagonal cell geometry and axes
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Geometry of Hexagons (Contd)

• axes u,v intersect at 60o
• unit scale is distance between cell centres
• if cell radius to point of hexagon is R
• then 2Rcos30o 1 or

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Geometry of Hexagons (Contd)

• Using this equation to locate co-channel cells,
  we start from a reference cell and move i
  hexagons along the u-axis then j hexagons along
  the v-axis. Hence the distance between cochannel
  cells in adjacent clusters is given by
• D (i2 ij j2)1/2
• where D is the distance between cochannel cells
  in adjacent clusters (called frequency reuse
  distance).
• and the number of cells in a cluster, N is given
  by D2
• N i2 ij j2

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Hexagon Reuse Clusters
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3-cell reuse pattern (i1,j1)
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4-cell reuse pattern (i2,j0)
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7-cell reuse pattern (i2,j1)
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12-cell reuse pattern (i2,j2)
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19-cell reuse pattern (i3,j2)
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Relationship between Q and N
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Proof
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Cell Clusters
since D SQRT(N)
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Cochannel Cell Location

• Method of locating cochannel cells
• Example for N19, i3, j2

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Cell Planning Example

• Suppose you have 33 MHz bandwidth available, an
  FM system using 25 kHz channels, how many
  channels per cell for 4,7,12 cell re-use?
• total channels 33,000/25 1320
• N4 channels per cell 1320/4 330
• N7 channels per cell 1320/7 188
• N12 channels per cell 1320/12 110
• Smaller clusters can carry more traffic
• However, smaller clusters result in larger
  co-channel interference

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Remarks on Reuse Ratio
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Co-channel Interference with Omnidirectional Cell
Site
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Propagation model
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Cochannel interference ratio
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Worst-case scenario for co-channel interference
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Worst-case scenario for co-channel interference
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Reuse Factor and SIR
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Remarks

• SIGNAL TO INTERFERENCE LEVEL IS INDEPENDENT OF
  CELL RADIUS!
• System performance (voice quality) only depends
  on cluster size
• What cell radius do we choose?
• Depends on traffic we wish to carry (smaller cell
  means more compact reuse or higher capacity)
• Limited by handoff

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Adjacent channel interference

• So far, we assume adjacent channels to be
  orthogonal (i.e., they do not interfere with each
  other).
• Unfortunately, this is not true in practice, so
  users may also experience adjacent channel
  interference besides co-channel interference.
• This is especially serious when the near-far
  effect (in uplinks) is significant
• Desired mobile user is far from BS
• Many mobile users exist in the cell

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Near-Far Effect
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Near-Far Effect (Contd)
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Reduce Adjacent channel interference

• Use modulation schemes which have small
  out-of-band radiation (e.g., MSK is better than
  QPSK)
• Carefully design the receiver BPF
• Use proper channel interleaving by assigning
  adjacent channels to different cells, e.g., for N
  7

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Reduce Adjacent channel interference (Contd)

• Furthermore, do not use adjacent channels in
  adjacent cells, which is possible only when N is
  very large. For example, if N 7, adjacent
  channels must be used in adjacent cells
• Use FDD or TDD to separate the forward link and
  reverse link.

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Improving Capacity in Cellular Systems

• Adding new channels often expensive or
  impossible
• Frequency borrowing (or DCA) frequencies are
  taken from adjacent cells by congested cells
• Cell splitting cells in areas of high usage can
  be split into smaller cells (microcells with
  antennas moved to buildings, hills, and lamp
  posts)
• Cell sectoring cells are divided into a number
  of wedge-shaped sectors, each with their own set
  of channels

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Sectoring

• Co-channel interference reduction with the use of
  directional antennas (sectorization)
• Each cell is divided into sectors and uses
  directional antennas at the base station.
• Each sector is assigned a set of channels
  (frequencies).

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Site Configurations
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Sectorized Cell Sites
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Worst case scenario
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Sectorizd Cell Sites
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Worst case scenario
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Illustration of cell splitting 1
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Illustration of cell splitting 2
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Illustration of cell splitting 3
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Cell Splitting
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Design Tradeoff

• Smaller cell means higher capacity (frequency
  reused more)
• However, smaller cell also results in higher
  handoff probability, which also means higher
  overhead
• Moreover, cell splitting should not introduce too
  much interference to users in other cells

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Handoff (Handover) Process

• Handoff Changing physical radio channels of
  network connections involved in a call, while
  maintaining the call
• Basic reasons for a handoff
• MS moves out of the range of a BTS (signal level
  becomes too low or error rate becomes too high)
• Load balancing (traffic in one cell is too high,
  and shift some MSs to other cells with a lower
  load)
• GSM standard identifies about 40 reasons for a
  handoff!

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Phases of Handoff

• MONITORING PHASE
• - measurement of the quality of the current
  and possible candidate radio links
• - initiation of a handover when necessary
• HANDOVER HANDLING PHASE
• - determination of a new point of attachment
• - setting up of new links, release of old
  links
• - initiation of a possible re-routing procedure

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Handoff Types

• Intra-cell handoff
• narrow-band interference gt change carrier
  frequency
• controlled by BSC
• Inter-cell, intra-BSC handoff
• typical handover scenario
• BSC performs the handover, assigns new
  radio channel in the
• new cell, releases the old one
• Inter-BSC, intra-MSC handoff
• handoff between cells controlled by
  different BSCs
• controlled by the MSC
• Inter-MSC handoff
• handoff between cells belonging to
  different MSCs
• controlled by both MSCs

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Handoff Types (contd)
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Handoff Strategies

• Relative signal strength
• Relative signal strength with threshold
• Relative signal strength with hysteresis
• Relative signal strength with hysteresis and
  threshold
• Prediction techniques

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Intra-MSC Handoff (Mobile Assisted)
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Handover Scenario at Cell Boundary
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Handoff Based on Receive Level
How to avoid ping-pong problem?
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Handoff 1G (Analog) systems

• Signal strength measurements made by the BSs and
  supervised by the MSC
• BS constantly monitors the signal strengths of
  all the voice channels
• Locator receiver measures signal strength of MSs
  in neighboring cells
• MSC decides if a handover is necessary

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Handoff 2G (Digital) TDMA

• Handoff decisions are mobile assisted
• Every MS measures the received power from
  surrounding BSs and sends reportsto its own BS
• Handoff is initiated when the power received from
  a neighbor BS begins to exceed the power from the
  current BS (by a certain level and/or for a
  certain period)

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Handoff 2G (Digital) CDMA

• CDMA uses code to differentiate users
• Soft handoff a user keeps records of several
  neighboring BSs
• Soft handoff may decrease the handoff blocking
  probability and handoff delay

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Avoiding handoff Umbrella cells
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Mixed Cell Architecture
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Handoff Prioritization

• The idea of reserving channels for handoff calls
  was introduced in the mid 1980s as a way of
  reducing the handoff call blocking probability
• Motivation users find calls blocked in
  mid-progress a far greater irritant than
  unsuccessful call attempts.
• The basic idea is to reserve a certain portion
  of the total channel pool in a cell for handoff
  users only.

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Performance Metrics

• Call blocking probability probability of a new
  call being blocked
• Call dropping probability probability that a
  call is terminated due to a handoff
• Call completion probability probability that an
  admitted call is not dropped before it terminates
• Handoff blocking probability probability that a
  handoff cannot be successfully completed

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Performance Metrics (Contd)

• Handoff probability probability that a handoff
  occurs before call termination
• Rate of handoff number of handoffs per unit
  time
• Interruption duration duration of time during a
  handoff in which a mobile is not connected to
  either base station
• Handoff delay distance the mobile moves from
  the point at which the handoff should occur to
  the point at which it does occur

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Summary

• cellular mobile uses many small cells
• hexagonal planning, clusters of cells
• cell repeat patterns 3,7,12 etc...
• re-uses frequencies to obtain capacity
• is interference not noise (kTB) limited
• S/I is independent of cell radius
• choose cell radius to meet traffic demand
• N7 is a good compromise between S/I and
  capacity.
• handoff