Wireless NETWORKS NET 434 Topic # 3 Wireless Transmission

Wireless NETWORKS
NET 434
Topic # 3
Wireless Transmission and Channel
Figure 4-6
Sine Wave
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-8
WCB/McGraw-Hill
Amplitude Change
 The McGraw-Hill Companies, Inc., 1998
Figure 4-9
Frequency Change
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-10
WCB/McGraw-Hill
Phase Change
 The McGraw-Hill Companies, Inc., 1998
Figure 4-11
Time and Frequency Domain
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-12
WCB/McGraw-Hill
Examples
 The McGraw-Hill Companies, Inc., 1998
Figure 4-13
WCB/McGraw-Hill
Signal with DC Component
 The McGraw-Hill Companies, Inc., 1998
Figure 4-14
WCB/McGraw-Hill
Complex Waveform
 The McGraw-Hill Companies, Inc., 1998
Figure 4-15
Bandwidth
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-16
WCB/McGraw-Hill
Digital Signal
 The McGraw-Hill Companies, Inc., 1998
Figure 4-17
Amplitude, Period, and Phase
for a Digital Signal
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-18
WCB/McGraw-Hill
Bit Rate and Bit Interval
 The McGraw-Hill Companies, Inc., 1998
Figure 4-19
Harmonics of a Digital Signal
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-20
Exact and Significant Spectrums
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-22
Corruption Due to Insufficient Bandwidth
WCB/McGraw-Hill
 The McGraw-Hill Companies, Inc., 1998
Figure 4-23
WCB/McGraw-Hill
Bandwidth and Data Rate
 The McGraw-Hill Companies, Inc., 1998
Relationship between Data Rate and Bandwidth
› Nyquist theorem
– In information theory, Nyquist theorem tells the maximum rate at
which information can be transmitted over a communications
channel of a specified bandwidth in the absence of noise.
Theoretical maximum data rate over a channel of Bandwidth B is
given by
𝐶 = 2𝐵𝑙𝑜𝑔2 (𝑀) (bits/sec)
– Where 𝐵 is the bandwidth of the channel. M is the number of
discrete signal or voltage levels.
› Shannon’s Capacity
– In information theory, shannon theorem tells the maximum rate at
which information can be transmitted over a communications
channel of a specified bandwidth in the presence of noise.
𝐶 = 𝐵𝑙𝑜𝑔2 (1 + 𝑆𝑁𝑅) (bits/sec)
– Where 𝐵 is the bandwidth of the channel. M is the number of
discrete signal or voltage levels.
– Where 𝑆𝑁𝑅 is given by
𝑑𝐵, 𝑑𝐵𝑚
– The decibel (dB) measures the relative strengths of two signals or
one signal at two different points.
– The (dB) is negative if a signal is attenuated and positive if the
signal is amplified.
𝑑𝐵 = 10𝑙𝑜𝑔10 (𝑃2 /𝑃1 )
– The dB tells if power is lost or gained.
– Signal to noise (decibels) 1 dB = 10 log10 S/N.
– Ex: S/N = 10 => 10 dB; S/N =100 => 20 dB,1000 is 30dB
– Decibel is used to measure signal power in milli watt. In this case it
is referred to as 𝑑𝐵𝑚 .
𝑑𝐵𝑚 = 10𝑙𝑜𝑔10 (𝑃𝑚 )
– Example:
› -3dB+7dB-3dB=1dB.
› There is an overall gain in the system
The expression 𝐸𝑏 /𝑁𝑜
›
𝐸𝑏
𝑁𝑜
– The expression
𝐸𝑏
𝑁𝑜
is the equivalent of 𝑆𝑁𝑅 for digital
communication. The parameter is the ratio of signal energy per bit to
noise power density per Hertz.
– 𝑅 is the bit rate.
– 𝐸𝑏 , the energy per bit can be represented 𝑆𝑇𝑏 . Where 𝑆 is the signal
power and 𝑇𝑏 is the time to send one bit. Watt=1J/s. 𝑅 = 1/𝑇𝑏 .
𝐸𝑏 𝑆/𝑅
=
,
𝑁𝑜
𝑁𝑜
– The metric of performance in digital communication systems is a
𝐸
plot of the bit error probability (𝑃𝑏 ) versus 𝑏 . The graph is a
𝑁𝑜
waterfall curve.
The expression 𝐸𝑏 /𝑁𝑜
– The ratio
𝐸𝑏
𝑁𝑜
is important because the bit error rate is a decreasing
function of this ratio.
–
– System describes a permissible probability of error.
– There is a given
𝐸𝑏
𝑁𝑜
for a given probability of error that has to
be achieved.
– As the bit rate 𝑅 increases, the signal power must increase
or the bandwidth of the channel W must be increased to maintain
the same
𝐸𝑏
.
𝑁𝑜
Transmission impairments
› With any communications system, the received signal
may differ from the signal that is transmitted due to
various transmission impairments.
– For analog signals, these impairments can degrade the signal
quality. For digital signals, bit errors may be introduced, such that a
binary 1 is transformed into a binary 0 or vice versa.
Transmission impairments
› Common types of transmission impairments:
– Attenuation
– Delay distortion
– Noise
› Attenuation
– The strength of a signal falls off with distance over any transmission
medium.
– For guided media, this reduction in strength, or attenuation, is
generally exponential and thus is typically expressed as a constant
number of decibels per unit distance. For twisted-pair and coaxial
cable, loss varies exponentially with distance (linear in decibels).
Transmission impairments
– For microwave (and radio frequencies), the loss is expressed as
4𝜋𝑑 2
𝐿 = 10𝑙𝑜𝑔10 (
)
λ
– where d is the distance and λ is the wavelength, in the same units.
Loss varies as the square of the distance.
– Repeaters or amplifiers may be placed farther apart for microwave
systems—10 to 100 km is typical.
– Attenuation introduces three considerations for the transmission
engineer.
› First, a received signal must have sufficient strength so that the
electronic circuitry in the receiver can detect the signal.
› Second, the signal must maintain a level sufficiently higher than
noise to be received without error.
› Third, attenuation varies with frequency.
– The first and second problems are dealt with by attention to signal
strength and the use of amplifiers or repeaters.
– For a point-to-point link, the signal strength of the transmitter must
Transmission impairments
› Distortion Delay spread
– Delay distortion occurs because the velocity of propagation of a
signal through a guided medium varies with frequency.
– For a band-limited signal, the velocity tends to be highest near the
center frequency and fall off toward the two edges of the band.
– Thus various frequency components of a signal will arrive at the
receiver at different times, resulting in phase shifts between the
different frequencies.
– This effect is referred to as delay distortion because the received
signal is distorted due to varying delays experienced at its
constituent frequencies.
Transmission impairments
– Delay distortion (Inter symbol interference) is particularly critical for
digital data.
– Consider that a sequence of bits is being transmitted. Because of
delay distortion, some of the signal components of one bit position
will spill over into other bit positions, causing inter symbol
interference, which is a major limitation to maximum bit rate over a
transmission channel.
› Noise
– For any data transmission event, the received signal will consist of
the transmitted signal, modified by the various distortions imposed
by the transmission system, plus additional unwanted signals that
are inserted somewhere between transmission and reception.
– Undesired signals are referred to as noise.
– Noise is the major limiting factor in communications system
performance.
› Noise may be divided into four categories:
Transmission impairments
– Crosstalk
– Impulse noise
› Thermal noise
– Thermal agitation of electrons. It is present in all electronic devices
and transmission media and is a function of temperature.
– Thermal noise is uniformly distributed across the bandwidths typically
used in communications systems and hence is often referred to as
white noise.
– Thermal noise cannot be eliminated and therefore places an upper
bound on communications system performance.
– Because of the weakness of the signal received by satellite earth
stations, thermal noise is particularly significant for satellite
communication.
– The amount of thermal noise to be found in a bandwidth of 1 Hz in
any device or conductor is
𝑊𝑎𝑡𝑡
𝑁𝑜𝑖𝑠𝑒 𝑃𝑜𝑤𝑒𝑟 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑁𝑜 = 𝐾𝑇(
)
𝐻𝑧
Transmission impairments
– The noise is assumed to be independent of frequency. Thus the
thermal noise in watts present in a bandwidth of B Hertz can be
expressed as: N=KTB.
› Intermodulation noise
– Intermodulation noise arises when signals at different frequencies
share the same transmission medium.
– The effect of intermodulation noise is to produce signals at a
frequency that is the sum or difference of the two original
frequencies or multiples of those frequencies.
– For example, the mixing of signals at frequencies and might
produce energy at the frequency This derived signal could interfere
with an intended signal at the frequency.
› Cross talk
– It is an unwanted coupling between signal paths. It can occur by
electrical coupling between nearby twisted pairs or, rarely, coax
cable lines carrying multiple signals.
Transmission impairments
– Crosstalk can also occur when microwave antennas pick up
unwanted signals; although highly directional antennas are used.
– microwave energy does spread during propagation.
– Typically, crosstalk is of the same order of magnitude as, or less
than, thermal noise.
› Impulse Noise
– Impulse noise, non-continuous, consisting of irregular pulses or
noise spikes of short duration and of relatively high amplitude.
– Causes, including external electromagnetic disturbances, such as
lightning, and faults and flaws in the communications system.
– Impulse noise is a minor annoyance for analog data. Example, voice
transmission may be corrupted by short clicks and crackles with no
loss of intelligibility.
– Impulse noise the primary source of error in digital data
communication. Example, a sharp spike of energy of 0.01 s duration
would not destroy any voice data but would wash out about 560 bits
of digital data being transmitted at 56 kbps.
Transmission Impairments
Wireless Line of sight
› Impairments specific to wireless line-of-sight
transmission
– Free Space Loss
– Atmospheric Absorption
– Multipath
– Refraction
› Free Space Loss
– For any type of wireless communication the signal disperses with
distance. Antenna with a fixed area farther away from the antenna
will receive less power.
– Even if no other sources of attenuation or impairment are assumed,
a transmitted signal attenuates over distance because the signal is
being spread over a larger and larger area. This form of attenuation
is known as free space loss.
–
Transmission Impairments
Wireless Line of sight
– Free space loss is expressed as a ratio of the power transmitted
from the antenna 𝑃𝑡 to the power received at the receiving antenna
𝑃𝑟 . For an isotropic antenna it is given by the following expression:
– For an antenna with a gain
Transmission impairments
› Interference
– Another source of impairment is interference. With the growing
popularity of microwave, transmission areas overlap and
interference is always a danger. Thus the assignment of frequency
bands is strictly regulated.
› Atmospheric Absorption
– An additional loss between the transmitting and receiving antennas
is atmospheric absorption. Water vapor and oxygen contribute most
to attenuation.
– Attenuation is increased with rainfall. The effects of rainfall become
especially noticeable above 10 GHz.
– At frequencies below 15 GHz, the attenuation is less.
– Rain and fog (suspended water droplets) cause scattering of radio
waves that results in attenuation. In this context, the term scattering
refers to the production of waves of changed direction or frequency
when radio waves encounter matter.
– This can be a major cause of signal loss. Thus, in areas of
Transmission impairments
› Multipath
– For wireless (Satellite/Point to
point microwave), there is a
relatively free choice of where
antennas are to be located,
they can be placed so that if
there are no nearby interfering
obstacles, there is a direct
line-of-sight path from
transmitter to receiver.
– Mobile telephony, there are
obstacles in abundance.
– The signal reflected by
obstacles resulting in multiple
copies of the signal with
varying delays. Extreme
cases, there may be no direct
– The composite signal can be
either larger or smaller than the
direct signal.(depending on diff
in the path lengths of the direct
and reflected waves)
WIRELESS PROPAGATION
› Wireless Propagation Modes
– A signal radiated from an antenna travels along one of three routes:
› Ground wave
› Sky wave,
› Line of sight (LOS).
– Ground Wave Propagation
› Ground wave propagation follows the contour of the earth.
› This effect is found in frequencies up to about 2 MHz.
› Electromagnetic waves in this frequency range are scattered by
the atmosphere and they do not penetrate the upper
atmosphere.
› The best-known example of ground wave communication is AM
radio.
Ground Wave Propagation
Sky Wave Propagation
Line of Sight Propagation
WIRELESS PROPAGATION
– Sky Wave propagation
› Sky wave propagation, a signal from an earth-based antenna is
reflected from the ionized layer of the upper atmosphere
(ionosphere) back down to earth due to refraction.
› A sky wave signal can travel through a number of hops,
bouncing back and forth between the ionosphere and the earth’s
surface.
› This effect is found in frequencies from 2-30 MHz
› With this propagation mode, a signal can be picked up
thousands of kilometers from the transmitter.
› Amateur radio, BBC world service, Voice of America
– Line of Sight propagation
› Mode of propagation above above 30 MHz,
› Neither ground wave nor sky wave propagation modes operate,
› Communication must be by line of sight.
› For satellite communication, a signal above 30 MHz is not
reflected by the ionosphere and therefore a signal can be
WIRELESS PROPAGATION
– For ground-based communication, the transmitting and receiving
antennas must be within an effective line of sight of each other.
› Optical and Radio Line of Sight
– With no intervening obstacles, the optical line of sight can be
expressed as
𝑑 = 3.57 ℎ
– where d is the distance between an antenna and the horizon in
kilometers and h is the antenna height in meters. The effective, or
radio, line of sight to the horizon is expressed by
𝑑 = 3.57 𝐾ℎ
– where K is an adjustment factor generally taken as 𝐾 = 4/3 to
account for the refraction.
– The maximum distance between two antennas for LOS propagation
is
– 3.57 𝐾ℎ1 +3.57 𝐾ℎ2
– where ℎ1 and ℎ2 and are the heights of the two antennas.
Optical and Radio Horizons
Refraction
› Velocity of electromagnetic wave is a function of
density of material
– ~3 x 108 m/s in vacuum, less in anything else
› As wave moves from one medium to another, its speed
changes
– Causes bending of direction of wave at boundary
– Towards more dense medium
– Causes sudden change of direction at transition between media
– May cause gradual bending if medium density is varying
– Density of atmosphere decreases with height
– Results in bending towards earth of radio waves