Transcript Radio Communications Principles

CSE5807 Wireless and Personal
Area Networks
Lecture 2
Radio Communications Principles
Chapters 2,5 and 11 Stallings
Communications Channel Capacity
• Channel capacity is the maximum rate at which data can
be transmitted over a given communication path or
channel, under given conditions of:
– Bandwidth
– Noise
– Error rate
Communications Channel Capacity
• Data Rate – the rate, in bits per second (bps), at which
data can be transmitted
• Bandwidth – the bandwidth of the transmitted signal as
constrained by the transmitter and the nature of the
transmission medium, in cycles per second, hertz
• Noise – the average white/thermal level of noise over the
communications path
• Error Rate – the rate at which errors occurs eg 10-6
Bandwidth
Telephone Channel
• Frequency range is 300Hz to 3400Hz
• Bandwidth is 3400-300 = 3100Hz
Transmitter/Receiver
• Telephone hand set
Cable
• Two pair of wires
Nyquist Bandwidth
• Noise Free channel
•The rate of signal transmission is at least twice the
bandwidth of a channel.
•C = 2*B (assumes 2-level signaling two states)
Eg Bandwidth of 3100 Hz, C= 6200 bps
Nyquist Bandwidth
•Multi-level signaling (M, more than two states)
• C = 2Blog2M
• Eg Bandwidth of 3100 Hz and M = 64,
C= 6200*log264 = 37200 bps
Nyquist Bandwidth
+ More signaling levels = more data carried each
signal interval
- Noise and transmission system effects become more
serious as M increases
Shannon
• Noise is an ever-present phenomenon in
communications
•Need a way of representing the interaction of noise and
data rate
•The amount of signal level compared to the noise is
important in considering the possible data rates
Shannon
• A higher signal to noise ratio is equivalent to a high
quality signal
• Measured in deciBels (1/10th of a Bel)
•SNRdB = 10 log10 * (signal power/noise power)
Shannon
• Maximum channel capacity (taking signal to noise ratio
into account):
• C = Blog2 (1+SNR)
Gain/Loss Measurement
power out
GaindB = 10 log10
power in
• If the measured power out is less than the power in,
then we have a loss and the formula will give a negative
result.
• Often, the negative gain result is written as a Loss
without the sign.
Gain/Loss Measurement
power out
GaindBm = 10 log10
1mW
Antenna Systems
• An antenna is a device that is used for radiating and/or
collecting electromagnetic energy
• A receiving antenna may be one or more conductors
that have electrical energy induced in them by the
passage of Electro Magnetic Radiation
•A transmitting antenna causes EMR due to the
electrical energy in the antennas conductor(s)
Antenna Systems
• An antenna will radiate power in all directions but,
typically, does not performance equally well in all
directions
• A common way to characterize the performance of an
antenna is the radiation pattern
Isotropic Antenna (Idealised)
Omnidirectional - A point source that radiates power in all
directions equally – fig 5.1(a)
Directional - preferred direction of radiation along one
axis – fig 5.1(b)
Relative Power
• To determine relative power in a given direction a line is
drawn from the antenna position at the appropriate angle,
and the point of intercept with the radiation pattern is
determined
Relative Power
• Fig 5.1 shows a comparison of two transmission angles,
A and B.
• Fig 5.1(a) vectors A and B of equal length
• Fig 5.1(b) vector B is longer then vector A
• Comments ?
Beam Width
• When we measure the directivity of an antenna, we
often describe this as the beam width. It is the angle
within which the power radiated by the antenna is
focused
• This is usually found by measuring the power in the
preferred direction and then discovering where this
power level drops to half the value of the preferred
direction
Reception Pattern
When an antenna is used for reception the radiation
pattern becomes a reception pattern
Antenna Wavelength
We usually measure the antenna by its
wavelength λ
This is calculated using the following
formula
λ=
v
f
where v is the speed of light
3 x 108 m/s
Antenna Wavelength
• Frequency of transmission f = 12 GHz
• V = 3 x 108 metres/second
• Wavelength λ = 3 x 108 /12 x 109
=- 0.025 metres
Antenna Types - Dipoles
Half-wave dipole
• Consists of two straight collinear conductors of equal
length, separated by a small gap
• The length of the antenna is one half the wavelength of
the signal that can be transmitted most efficiently
Antenna Types
Half-wave Dipole
Antenna Types - Dipoles
Quarter-wave dipole
• A vertical quarter wave antenna is the type commonly
used for car and portable radios
Antenna Types
Quarter-wave antenna
Radiation Patterns
Antenna Types
Parabolic Antenna
Radiation Pattern – Parabolic Antenna
Antenna Gain
• Is a measure of the directionality of an antenna.
• Antenna gain is defined as the power output, in a
particular direction compared to that produced in any
direction by a perfect isotropic omnidirectional antenna
• If an antenna has a gain of 3dB, that antenna
improves on the isotropic antenna in that direction by
3dB, or a factor of 2 (100.3)
• The increased power radiated in a given direction is
at the expense of other directions
Propagation Modes – Ground Wave
• Follows contour of the earth, can propagate considerable
distances
• The electromagnetic wave induces a current in the earth’s
surface, the result of which is to slow the wave front near
the earth , causing the wave front to tilt downward and
hence follow the earth’s curvature
• AM Radio
• Below 2MHz
Propagation Modes
Ground Wave
Propagation Modes – Sky Wave
• A signal from an earth based antenna is reflected from
the ionized layer of the upper atmosphere back down to
earth
• Signal can travel through a number of hops, bouncing
back and forth
• Signal can be picked up thousands of kms from
transmitter
• CB radio, BBC
• 2 to 30MHz
Propagation Modes
Sky Wave
Propagation Modes – Line of Sight
• Above 30MHz neither ground nor sky wave propagation
operate
• Signal can be transmitted between earth station and
satellite overhead that is not beyond the horizon
• For ground based communication transmitting and
receiving antennas must be within an effective line of
sight of each other
Propagation Modes – Line of Sight
• Maximum distance d between two antennas of height h1
and h2 for LOS propagation is:
d = 3.57(√Kh1 + √Kh2)
Where K=1 for optical LOS and K= 1.33 for radio LOS
What is maximum d between two antennas for LOS
if h1 = 100m and the other antenna is at ground level?
Propagation Modes
Line-of-Sight
Propagation Modes
Refraction
Polarization
• The polarization of an antenna is the orientation of the
electric field of the radio wave with respect to the Earth's
surface and is determined by the physical structure of the
antenna and by its orientation.
• A simple straight wire antenna will have one polarization
when mounted vertically, and a different polarization
when mounted horizontally
Polarization
• For line-of-sight communications for which polarization
can be relied upon, it can make a large difference in
signal quality to have the transmitter and receiver using
the same polarization
• Many tens of dB difference are commonly seen and this
is more than enough to make the difference between
reasonable communication and a broken link.
Diversity Antenna Systems
Using two antennas located a distance
apart can counteract the effect of signal
loss due to propagation problems.
Transmission Problems
Attenuation and Attenuation Distortion
Free Space Loss
Noise
Atmospheric Absorption
Multipath
Refraction
Transmission Problems - LOS
Fresnel Zone
The higher the frequency of the
transmission, the more it may be
affected by obstructing objects. The
possible interference is determined by
computing the Fresnel Zone
Transmission Problems
Fresnel Zone
Transmitter
radius
S km
D km
Receiver
Antenna Types
Yagi
A Yagi antenna consists of a driven
element (1/2 –wave dipole) and one or
more director elements (in the forward
direction) and a reflector element (behind
the driven element)