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
(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
λ = 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 = Boltzman’s constant = x 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
λ = 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 = Boltzman’s constant = x 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. Doppler’s Shift
When a client is mobile, the frequency of received signal could be less or more than that of the transmitted signal due to Doppler’s effect
If the mobile is moving towards the direction of arrival of the wave, the Doppler’s shift is positive
If the mobile is moving away from the direction of arrival of the wave, the Doppler’s shift is negative

16. Doppler’s Shift where fd =change in frequency due to Doppler’s shift
v = constant velocity of the
mobile receiver
λ = wavelength of the transmission θ X Y

17. Doppler’s shift f = fc + fd where f = the received carrier frequency
fc = carrier frequency being transmitted
fd = Doppler’s 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 (Cont’d)
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 (Cont’d)
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 re–use 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 (Cont’d)
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. Hexagonal cell geometry and axes
Geometry of Hexagons
Hexagonal cell geometry and axes

39. Geometry of Hexagons (Cont’d)
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 = R√3)
N = number of cells in repetitious pattern (Cluster) Reuse factor
Each cell in pattern uses unique band of frequencies

40. Geometry of Hexagons (Cont’d)
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 D=√3R.
(D/d)2 = j2 cos2(30) + (i+ jsin30)2
= i2 + j2 +ij = N
D=Dnorm x √3 R =(√3N)R
In general a candidate cell is surrounded by 6k cells in tier k.

41. Geometry of Hexagons (Cont’d)
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 co–channel cells in adjacent clusters is given by:
D = (i2 + ij + j2)1/2
where D is the distance between co–channel 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 (i=1,j=1)

44. 4-cell reuse pattern (i=2,j=0)

45. 7-cell reuse pattern (i=2,j=1)

46. 12-cell reuse pattern (i=2,j=2)

47. 19-cell reuse pattern (i=3,j=2)

48. Relationship between Q and N

49. Proof

50. Cell Clusters
since D = SQRT(N)

51. Co–channel Cell Location
Method of locating co–channel cells
Example for N=19, i=3, j=2

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
N=4 channels per cell = 1320/4 = 330
N=7 channels per cell = 1320/7 = 188
N=12 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 (Cont’d)

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 (Cont’d)
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

77. 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 Inter-cell, intra-BSC handoff
– narrow-band interference => 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 (cont’d)

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 (Cont’d)
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
N=7 is a good compromise between S/I and capacity.
handoff