By: Dr. N. Ioannides (Feb 2010) CT0004N – L.04 – Wireless Comms & Mobile Telephony - pp 1/32 Wireless Communications & Wireless Communications & Mobile Telephony Mobile Telephony Saroj Regmi Saroj Regmi Lecture 04 Lecture 04 CT0004N CT0004N Principles of Comms Systems Principles of Comms Systems
Transcript
Slide 1
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 1/32 Wireless Communications &
Mobile Telephony Saroj Regmi Lecture 04 CT0004N Principles of Comms
Systems
Slide 2
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 2/32 Last Lecture: 03
Telecommunications Systems Systems and System Specifics,
Telecommunications Systems, Fundamental Blocks of Communication,
Bandwidth in Analogue & Digital Systems, Transmission Media,
Attenuation, The Decibel, Noise in Analogue & Digital Systems,
Multiplexing Techniques (FDM, WDM & TDM), Modulation (AM &
FM).
Slide 3
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 3/32 Todays Lecture: 04 Wireless
Communications & Mobile Telephony Introduction to Wireless
Communications. Radio Wave Frequency Bands. Modes of Propagation of
Signals. Mobile Radio Systems.
Slide 4
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 4/32 Offer good broadcasting ability
and enable true mobile communications. Use unguided media as the
transmission medium: The transmission between transmitter and
receiver is achieved via electromagnetic (E-M) wave propagation
through free space. Electromagnetic (E-M) waves in free space
travel at the velocity of light (3 x 10 8 m/s). For terrestrial
communications the unguided medium is the atmosphere and its
layers, and for inter-satellite communications it is outer space.
Wireless (or Radio) Communications
Slide 5
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 5/32 Works by: modulating the input
signal with a suitable carrier, amplifying the modulated carrier
wave, radiating it in free space with the use of an antenna. A
receiver tuned to the right carrier frequency can recover the
transmitted information. An antenna can be used to both transmit
and receive E-M waves. A Basic Wireless Communications System
Slide 6
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 6/32 Antennas in Wireless
Communications Systems Antennas are essential components of both
the transmitter and receiver of a radio communications system. When
transmitting, the antenna radiates energy from a transmission line
into free space in the desired direction. When receiving, the
antenna picks up the signals transmitted towards it. The signals
received are then guided by a transmission line (usually coaxial
cable) to the receiver circuit for demodulation.
Slide 7
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 7/32 Radio Wave Frequency Bands The
usable radio frequencies span from 3 kHz to 100 GHz. The radio
spectrum has been divided into a number of bands. Classification
Frequency Range Very Low FrequencyVLF3 kHz - 30 kHz Low
FrequencyLF30 kHz - 300 kHz Medium FrequencyMF300 kHz - 3 MHz High
FrequencyHF3 MHz - 30 MHz Very High FrequencyVHF30 MHz - 300 MHz
Ultra High FrequencyUHF300 MHz - 3 GHz Super High FrequencySHF3 GHz
- 30 GHz Extra High FrequencyEHF30 GHz - 300 GHz
Slide 8
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 8/32 Modes describe the way that E-M
waves propagate in free space. Various physical characteristics
affect the way by which E-M waves propagate in free space. Radio
waves may be: Reflected: From the Earth's surface. Refracted: Using
upper layers of the atmosphere, to increase range beyond optical
horizon. Diffracted: This enables long to medium wave bands to
propagate around obstacles. The above physical effects can also be
put to good use and help the propagation of radio waves in free
space depending on the specific need. Modes of Radio Wave
Propagation
Slide 9
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 9/32 Most popular Mobile systems
include: Radio Paging, Cellular Radio, Radio Data Service (RDS),
Mobile Satellite Communications, Location & Navigation Systems.
Mobile Radio Systems
Slide 10
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 10/32 Most mobile systems use VHF and
UHF bands. In VHF and UHF the E-M waves are very directional and
diffraction is poor, therefore radio shadows can occur. Even in
flat terrain a shadow begins beyond the radio horizon (30% further
than the visible horizon since upper atmospheric refraction is
lower than at ground level). LF radio waves are able to diffract
around buildings and natural obstacles but are not used for mobile
systems. Mobile Radio Systems Frequency Bands
Slide 11
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 11/32 Transmission in mobile bands at
distance appreciably beyond the horizon is greatly reduced. This is
used to advantage for Frequency Re-use. Coverage area of an antenna
is approximately proportional to its height. Although it may be
tempting to reduce system costs by using tall antennas, the scope
for frequency re-use diminishes and there is an optimum
relationship between height, coverage area, frequency reuse and
cost. Shadowing is greatly mitigated in built-up areas by
reflections off hard-surfaced objects such as buildings. Often no
line-of-sight path exists in urban areas and reception relies upon
such reflection. Mobile Radio Systems Frequency Bands (2)
Slide 12
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 12/32 Multipath effects often come into
play due to a number of reflections appearing at a receive antenna.
Multipath effects mean that the resultant signal is the sum of the
individual rays. May be additive or subtractive depending upon
their respective phases. Sometimes subtraction may be so severe
that the signal is lost in noise or at least is weaker than the
receiver sensitivity (known as a 'fade). Propagation within
buildings is desirable, particularly for cellphones / mobiles and
pagers. The mechanism is via doors, windows and thin non-metallic
roofs. Buildings increase loss. Other factors to consider are
clothing and body orientation. Mobile Radio Systems Frequency Bands
(3)
Slide 13
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 13/32 Co-Channel Interference Occurs
when the various frequencies transmitted by the various
transmitters interfere with each other. To avoid co-channel
interference various techniques may be employed: Frequency Re-Use:
The solution favoured by cellular systems. Explained later on.
Sequential Transmission: While one transmitter is in use, all
others are switched off. At any one time, only one transmitter is
operating. Each transmitter operates in turn. Simulcast: All
transmitters within the network operate on the same frequency. It
is then arranged that they are all modulated in virtual synchronism
in time.
Slide 14
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 14/32 Basic Structure of Cellular or
Mobile Networks
Slide 15
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 15/32 A cellular / mobile
communications system uses a large number of low-power wireless
transmitters to create cells. Cells are the basic geographic
service area of a wireless communications system. Cells can have
variable power levels which allows them to be sized according to
the subscriber density and demand within a particular region.
Mobile Communications System Structure
Slide 16
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 16/32 Each mobile uses a separate,
temporary radio channel to talk to the cell site. The cell site
talks to many mobiles at once, using one channel per mobile.
Channels use a pair of frequencies for communication. One
frequency, the forward link, is used for transmitting from the cell
site, Another frequency, the reverse link, is used for the cell
site to receive calls from the users. Mobile Communications
Channels
Slide 17
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 17/32 Structured similar to television
broadcasting: Uses one very powerful transmitter. The transmitter
is located at the highest spot in an area. The transmitter would
broadcast in a radius of up to fifty kilometers. Traditional Mobile
Service
Slide 18
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 18/32 Different from the Traditional
Mobile Structure. Many low-power transmitters instead of a powerful
one. Placed throughout a coverage area. The Cellular Concept
Slide 19
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 19/32 The cellular concept employs
variable low-power levels which allow cells to be sized according
to the subscriber density and demand of a given area. Cells can be
added on demand to accommodate population grow. Conversations can
be handed off from cell to cell to maintain constant phone service
as the user moves between cells. The cellular radio equipment (base
station) can communicate with mobiles as long as they are within
range. The Cellular Concept (2)
Slide 20
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 20/32 Provisioning for each region is
planned according to an engineering plan that includes: Cells,
Clusters, Frequency Re-Use, Handovers. The Cellular Concept
(3)
Slide 21
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 21/32 Cells A cell is the basic
geographic unit of a cellular system. The term "cellular" comes
from the honeycomb shape of the areas into which a coverage region
is divided. Cells are base stations transmitting over small
geographic areas that are represented as hexagons. Each cell size
varies depending on the landscape. Because of constraints imposed
by natural terrain and man-made structures, the true shape of cells
is not a perfect hexagon. Cells are divided / classified based on
their size.
Slide 22
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 22/32 Cell Size Common Use Coverage
Pico-cellsWithin buildingsA few metres Nano-cellsWithin
buildings< 10 m In-building Micro-cellsOffice floor< 100 m
Nodal CellsHigh capacity network node< 300 m Street
Macro-cellsPedestrian mobiles10 m - 400 m Street Macro-cells
Vehicular mobile 300 m - 2 km Mini-cellsPedestrian mobiles500 m - 3
km Macro-cellsCities and suburbia1 km - 5 km Large CellsSuburbia
and rural5 km - 35 km Mega CellsCities to counties20 km - 100 km
Satellite CellsSatellites> 300 km Cell Classification
Slide 23
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 23/32 Clusters Clusters are groups of
Cells. Cells are constructed as polygons and their shape (number of
sides) is chosen so that they tessellate (overlap) in order to
prevent gaps occurring in the service area offered by a cluster.
Tessellations may be arranged to produce a wide range of cluster
sizes.
Slide 24
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 24/32 Clusters may have the following
number of cells: 3, 4, 7, 9, 12, etc. Within a particular system
the number of cells in a cluster (cluster size) is usually fixed.
No channels are reused within a cluster. In the UK hexagonal cells
have been chosen. Such cells are attractive because they
approximate to a circle. Clusters (2)
Slide 25
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 25/32 The solution the industry adopted
to reuse radio channels in order to carry more than one
conversation at a time. WHY? Because only a small number of radio
channel frequencies were available for mobile systems. Also called
frequency planning. Frequency Re-Use Frequency re-use was
implemented by restructuring the mobile telephone system
architecture into the cellular concept. The Cellular Concept:
Slide 26
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 26/32 The concept of frequency reuse is
based on assigning to each cell a group of radio channels used
within a small geographic area. Cells are assigned a group of
channels that is completely different from neighbouring cells. The
coverage area of cells is called the footprint. The footprint is
limited by a boundary so that the same group of channels can be
used in different cells that are far enough away from each other so
that their frequencies do not interfere. Frequency Re-Use (2)
Slide 27
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 27/32 Cell Splitting Cell splitting is
the splitting of a single area into smaller ones. WHY? Because
economic considerations made the concept of creating full systems
with many small areas impractical. To overcome this difficulty,
system operators developed the idea of "cell splitting".
Slide 28
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 28/32 As a service area becomes full of
users, this approach is used to split a single area into smaller
ones. Urban centers can be split into as many areas as necessary in
order to provide acceptable service levels in heavy-traffic
regions. Larger, less expensive cells can be used to cover remote
rural regions. Cell Splitting (2)
Slide 29
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 29/32 Handoff or Handover Handoff or
Handover relates to the process of transferring a call from one
cell to another when a mobile subscriber travels from one cells
coverage area to anothers during a call. Handoff occurs when the
mobile telephone network automatically transfers a call from radio
channel to radio channel as a mobile crosses adjacent cells. WHY?
Because dropping the call is unacceptable. Adjacent areas do not
use the same radio channels so a call must either be dropped or
transferred from one radio channel to another.
Slide 30
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 30/32 During a call, two parties are on
one voice channel. When a mobile unit moves out of the coverage
area of a given cell site, the reception becomes weak. At this
point, the cell site in use requests a handoff. The system switches
the call to a stronger-frequency channel in a new site without
interrupting the call or alerting the user. The call continues as
long as the user is talking, and the user does not notice the
handoff. If handoff does not take place then the call is dropped. A
call is dropped only if there is no channel available, i.e. full
utilisation of the cell. Handoff Process
Slide 31
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 31/32 Handoff Process (2)
Slide 32
By: Dr. N. Ioannides (Feb 2010)CT0004N L.04 Wireless Comms
& Mobile Telephony - pp 32/32 Summary Introduction to Wireless
Communications. Radio Wave Frequency Bands. Modes of Propagation of
Signals. Mobile Radio Systems.