Wireless Communications Engineering

1
Wireless Communications Engineering

• Cellular Fundamentals

2
Definitions Wireless Communication

• What is Wireless Communication?
• Ability to communicate via wireless links.
• Mobile Communication ?

3
Wireless Communication

• Wireless Communication are of two types
• Fixed Wireless Communication
• Mobile Wireless Communication.

4
Mobile Wireless Communication

• Mobile Wireless Communication
• (Infrastructured Network)
• Single Hop Wireless Link to reach a
• mobile Terminal.

Mobile Communication ?
5
Mobile Ad Hoc Networks

• Infrastructureless or Adhoc Network
• Multihop Wireless path from source to
• destination.

6
Mobile Radio Environment
7
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).

8
Free Space loss

• Transmitted signal attenuates over distance
  because it is spread over larger and larger area
• This is known as free space loss and for
  isotropic antennas
• Pt power at the transmitting antenna
• Pr power at the receiving antenna
• ? carrier wavelength
• d propagation distance between the antennas
• c speed of light

9
Free Space loss

• For other antennas
• Gt Gain of transmitting antenna
• Gr Gain of receiving antenna
• At effective area of transmitting antenna
• Ar effective area of receiving antenna

10
Thermal Noise

• Thermal noise is introduced due to thermal
  agitation of electrons
• Present in all transmission media and all
  electronic devices
• a function of temperature
• uniformly distributed across the frequency
  spectrum and hence is often referred to as white
  noise
• amount of noise found in a bandwidth of 1 Hz is
• N0 k T
• N0 noise power density in watts per 1 Hz
  of bandwidth
• k Boltzmans constant 1.3803 x 10-23 J/K
• T temperature, in Kelvins
• N thermal noise in watts present in a
  bandwidth of B
• kTB where

11
Free Space loss

• Transmitted signal attenuates over distance
  because it is spread over larger and larger area
• This is known as free space loss and for
  isotropic antennas
• Pt power at the transmitting antenna
• Pr power at the receiving antenna
• ? carrier wavelength
• d propagation distance between the antennas
• c speed of light

12
Free Space loss

• For other antennas
• Gt Gain of transmitting antenna
• Gr Gain of receiving antenna
• At effective area of transmitting antenna
• Ar effective area of receiving antenna

13
Thermal Noise

• Thermal noise is introduced due to thermal
  agitation of electrons
• Present in all transmission media and all
  electronic devices
• a function of temperature
• uniformly distributed across the frequency
  spectrum and hence is often referred to as white
  noise
• amount of noise found in a bandwidth of 1 Hz is
• N0 k T
• N0 noise power density in watts per 1 Hz
  of bandwidth
• k Boltzmans constant 1.3803 x 10-23 J/K
• T temperature, in Kelvins
• N thermal noise in watts present in a
  bandwidth of B
• kTB where

14
Data rate and error rate

• Bit error rate is a decreasing function of Eb/N0.
• If bit rate R is to increase, then to keep bit
  error rate (or Eb/N0) same, the transmitted
  signal power must increase, relative to noise
• Eb/N0 is related to SNR as follows
• B signal bandwidth
• (since N N0 B)

15
Dopplers Shift

• When a client is mobile, the frequency of
  received signal could be less or more than that
  of the transmitted signal due to Dopplers effect
• If the mobile is moving towards the direction of
  arrival of the wave, the Dopplers shift is
  positive
• If the mobile is moving away from the direction
  of arrival of the wave, the Dopplers shift is
  negative

16
Dopplers Shift
S

• where
• fd change in frequency
• due to Dopplers shift
• v constant velocity of the
• mobile receiver
• ? wavelength of the transmission

?
X
Y
17
Dopplers shift

• f fc fd
• where
• f the received carrier frequency
• fc carrier frequency being transmitted
• fd Dopplers shift as per the formula in the
  previous slide.

18
Multipath Propagation

• Wireless signal can arrive at the receiver
  through different paths
• LOS
• Reflections from objects
• Diffraction
• Occurs at the edge of an impenetrable body that
  is large compared to the wavelength of the signal

19
Multipath Propagation (source Stallings)
20
Mobile Radio Channel Fading
21
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

22
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.

23
Cell Concept

• Cell
• A cell is a small geographical area served by
  a singlebase station or a cluster of base
  stations
• 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

24
Cellular Networks
25
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

26
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

27
Cellular System Architecture
28
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

29
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

30
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

31
Steps in an MTSO Controlled Call between Mobile
Users

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

32
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

33
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

34
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

35
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.

36
Coverage Patterns
37
Cellular Coverage Representation
38
Geometry of Hexagons
Hexagonal cell geometry and axes
39
Geometry of Hexagons (Contd)

• D minimum distance between centers of cells
  that use the same band of frequencies (called
  co-channels)
• R radius of a cell
• d distance between centers of adjacent cells
  (d Rv3)
• N number of cells in repetitious pattern
  (Cluster) Reuse factor
• Each cell in pattern uses unique band of
  frequencies

40
Geometry of Hexagons (Contd)

• The distance between the nearest cochannel cells
  in a hexagonal area can be calculated from the
  previous figure
• The distance between the two adjacent co-channel
  cells is Dv3R.
• (D/d)2 j2 cos2(30) (i jsin30)2
• i2 j2 ij N
• DDnorm x v3 R (v3N)R
• In general a candidate cell is surrounded by 6k
  cells in tier k.

41
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

42
Hexagon Reuse Clusters
43
3-cell reuse pattern (i1,j1)
44
4-cell reuse pattern (i2,j0)
45
7-cell reuse pattern (i2,j1)
46
12-cell reuse pattern (i2,j2)
47
19-cell reuse pattern (i3,j2)
48
Relationship between Q and N
49
Proof
50
Cell Clusters
since D SQRT(N)
51
Cochannel Cell Location

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

52
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

53
Remarks on Reuse Ratio
54
Co-channel Interference with Omnidirectional Cell
Site
55
Propagation model
56
Cochannel interference ratio
57
Worst-case scenario for co-channel interference
58
Worst-case scenario for co-channel interference
59
Reuse Factor and SIR
60
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

61
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

62
Near-Far Effect
63
Near-Far Effect (Contd)
64
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

65
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.

66
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

67
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).

68
Site Configurations
69
Sectorized Cell Sites
70
Worst case scenario
71
Sectorizd Cell Sites
72
Worst case scenario
73
Illustration of cell splitting 1
74
Illustration of cell splitting 2
75
Illustration of cell splitting 3
76
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

78
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!

79
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

80
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

81
Handoff Types (contd)
82
Handoff Strategies

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

83
Intra-MSC Handoff (Mobile Assisted)
84
Handover Scenario at Cell Boundary
85
Handoff Based on Receive Level
How to avoid ping-pong problem?
86
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

87
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)

88
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

89
Avoiding handoff Umbrella cells
90
Mixed Cell Architecture
91
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.

92
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

93
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

94
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