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Lecture 2Communication Terminology and
Channel Concepts
Lecturer Madeeha Owais
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What is Wireless Communication?
• Any form of communication that does not require the transmitter
and receiver to be in physical contact through guided media
• Electromagnetic wave propagated through free-space
Communication Channel
• Refers to the medium used to convey information from a sender (or
transmitter) to a receiver
• A path for conveying electrical or electromagnetic signals, usually
distinguished from other parallel paths
• A specific radio frequency, pair or band of frequencies, usually
named with a letter, number, or codeword, and often allocated by
international agreement
• e.g. Wi-Fi consists of unlicensed channels 1-13 from
2412MHz to 2484MHz in 5MHz steps
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Types of communications channels
• Simplex Communication
– Communication is possible in only one direction . The systems in
which messages are received but not acknowledged.
• Paging systems
• TV broadcast
• Duplex Communication
– A duplex communication system is a system composed of two
connected parties or devices which can communicate with one another
in both directions.
– (The term duplex is not used when describing communication between
more than two parties or devices. It is then called multiplex)
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Types of communications channels
• Half Duplex Communication
– Allow two –way communication ,but use the same radio channel for
both transmission and reception( not simultaneously)
– At any given time ,a user can only transmit or receive information
• "walkie-talkie" style two-way radio
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Types of communications channels
• Full Duplex Communication
– Unlike half-duplex, full duplex systems allows radios transmission and
reception to happen simultaneously.
• Land-line telephone networks or Mobile phone are full-duplex since they
allow both callers to speak and be heard at the same time.
– How can duplex communication be achieved?
• Simultaneous communication possible by
– providing two separate radio channels (Frequency Division
Duplex) or
– adjacent time slots on a single radio channel (Time Division
Duplex)
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Types of communications channels
• Frequency Division Duplex
– Frequency-division duplexing means that the transmitter and receiver
operates at different carrier frequencies.
– A pair of simplex channels with a fixed and known frequency
separation is used (5% of the nominal RF frequency)
– Based station to mobile user
Forward Channel/Downlink
– Mobile user to base station
Reverse Channel/Uplink
– At base station
• separate transmit and receive antennas
– At subscriber unit
• single antenna for transmission/reception
• Duplexer inside the unit to enable the antenna to be used for both purposes
simultaneously
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FDD
Frequency Separation
Figure and explanation in Chapter 1-Rappaport
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Types of communications channels
• Time Division Duplex (TDD)
– Uses time instead of frequency to provide both forward and reverse
channels.
– In TDD, multiple users share a signal radio channel by taking turns in the
time domain i-e portion of time is used to transmit from BS to mobile and
remaining is used to transmit from mobile to BS
– Individual users are allowed to access the channel in assigned time slots.
Each duplex channel has both downlink and uplink time slots.
– Time separation between downlink and uplink is necessary.
– TDD only possible with digital transmission formats and modulation
– It is very sensitive to timing.
– It is for this reason that TDD has been recently used
– And only for indoor or small area wireless applications where physical
coverage distance(radio propagation delay) are much smaller than many
kms used in conventional cellular systems.
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TDD
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Symmetric Transmission vs. Asymmetric
Transmission
• Data transmission is symmetric if the data in the downlink and the data in
the uplink is transmitted at the same data rate. This will probably be the
case for voice transmission - the same amount of data is sent both ways.
• For internet connections or broadcast data (e.g., streaming video), it is
likely that more data will be sent from the server to the mobile device (the
downlink).
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Asymmetric Transmission in TDD
• FDD transmission is not so well suited for asymmetric
applications as it uses equal frequency bands for the uplink
and the downlink (a waste of valuable spectrum).
• On the other hand, TDD does not have this fixed structure,
and its flexible slots assignment is well-suited to
asymmetric applications, e.g., the internet
• For example, TDD can be configured to provide 384kbps
for the downlink (the direction of the major data transfer),
and 64kbps for the uplink (where the traffic largely
comprises requests for information and acknowledgements)
• Asymmetric transmission to be used in 3G Technologies
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Decibel
• The decibel (dB) is a logarithmic unit of
measurement that expresses the magnitude of a
physical quantity (usually power or intensity)
relative to a specified or implied reference level.
• Since it expresses a ratio of two quantities with
the same unit, it is a dimensionless unit. A
decibel is one tenth of a bel (B).
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Other related absolute measurements
• dBm or dBmW
– dB(1 mW) —> power measurement relative to 1
milliwatt.
– XdBm = XdBW + 30.
• dBW
– dB(1 W) —> similar to dBm, except the reference
level is 1 watt.
– 0 dBW = +30 dBm;
– −30 dBW = 0 dBm;
– XdBW = XdBm − 30.
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Presentation Credits for part that
follows…
• William Stallings.
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Antenna Basics
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An antenna is an electrical conductor or
system of conductors
– Transmission - radiates electromagnetic
energy into space
– Reception - collects electromagnetic
energy from space
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In two-way communication, the same
antenna can be used for transmission and
reception
• Radiation pattern
– Graphical representation of
radiation properties of an
antenna
– Depicted as two-dimensional
cross section
• Beam width (or half-power
beam width)
– Measure of directivity of
antenna
• Reception pattern
– Receiving antenna’s equivalent
to radiation pattern
Types of Antennas
• Isotropic antenna (idealized)
– Radiates power equally in all
directions
• Dipole antennas
– Half-wave dipole antenna (or
Hertz antenna)
– Quarter-wave vertical antenna
(or Marconi antenna)
• Parabolic Reflective Antenna
Antenna Gain and Effective Area
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Antenna gain
– Power output, in a particular direction, compared to that produced in any
direction by a perfect omni-directional antenna (isotropic antenna)
– Self reading at Antenna Gain
http://radarproblems.com/chapters/ch06.dir/ch06pr.dir/c06p6.dir/c06p6.htm
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Antenna Effective area or Effective Aperture
– Functionally equivalent area from which an antenna directed toward the
source of the received signal gathers or absorbs the energy of an incident
electromagnetic wave.
– Related to physical size and shape of antenna
– Relationship to antenna gain
Aeff=(λ2*Gant)/(4π)
Unit of effective area of antenna: m2
Gant: antenna gain (not in decibels)
λ: wavelength
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Wireless Transmission Impairments
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Path loss or Attenuation
Free space loss
Noise
Atmospheric gaseous absorption
Rain effects
Multipath
Reflection
Refraction
Diffraction
Scattering
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Path loss or Attenuation
• Reduction of signal strength/intensity during transmission
• It is normal when a signal is sent from one point to another.
• Strength/power loss is a function of transmission method and circuit
medium.
• Received signal must have sufficient strength so that circuitry in the
receiver can interpret the signal
• Signal must maintain a level sufficiently higher than noise to be received
without error
• Attenuation is greater at higher frequencies, causing distortion
• What can cause energy reduction?
Everything. It is important to understand that most of the calculation will
show theoretical information. It is very difficult to predict the real reduction
of energy since there are many factors that can cause reduction in energy.
Terrain contours, environment (urban or rural, vegetation and foliage),
propagation medium (dry or moist air), the distance between the transmitter
and the receiver, and the height and location of antennas.
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Free Space Loss
• “The reduction in power density (attenuation) of an
electromagnetic wave as it propagates through space" In other
words it means how much energy an electromagnetic wave
lose when its traversing through space.
• Free space is a region where these is nothing - the vacuum of
outer space is a fair approximation for most purposes.
• There are no obstacles to get in the way, no gases to absorb
energy, nothing to scatter the radio waves.
• Unless you are into space communications, free space is not
something you are likely to encounter, but it is important to
understand what happens to a radio wave when there is
nothing to disturb it.
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Free Space Loss
• Free space loss, ideal isotropic antenna
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Ptx = signal power at transmitting antenna
Prx = signal power at receiving antenna
 = carrier wavelength
r = propagation distance between antennas
where r and  are in the same units (e.g., meters
• Free Space Loss = 32.5 + 20log(d) + 20log(f) dB, Where d is the distance
in km and f is the frequency in MHz
• Read derivation from Free Space Propagation
• http://www.mike-willis.com/Tutorial/free_space.htm
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Free Space Loss
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Categories of Noise
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Thermal Noise
Intermodulation noise
Crosstalk
Impulse Noise
In wireless systems(unguided media),thermal
noise is major concern.
• Rest of the categories are prevalent
significantly in guided media transmission.
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Thermal Noise
• Thermal noise due to agitation of electrons
• Present in all electronic devices and transmission media & cannot be
eliminated.
• Function of temperature
• Particularly significant for satellite communication because of weakness of
signal received by satellite earth stations.
• Amount of thermal noise to be found in a bandwidth of 1Hz in any device
or conductor is:
N0=kT(W/Hz)
• N0 = noise power density in watts per 1 Hz of bandwidth
• k = Boltzmann's constant = 1.3803 ´ 10-23 J/K
• T = temperature, in kelvins (absolute temperature)
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Thermal noise present in a bandwidth of B Hertz (in watts):
or, in decibel-watts
N  kTB
N  10 log k  10 log T  10 log B
 228.6 dBW  10 log T  10 log B
Other Categories
• Inter-modulation noise –special type of crosstalk. Signals from two
circuits inter-modulate and form a new signal that falls into a
frequency band that is reserved for another signal
• Crosstalk – unwanted coupling between signal paths
• Impulse noise – irregular pulses or noise spikes
– Short duration and of relatively high amplitude
– Caused by external electromagnetic disturbances, or faults and
flaws in the communications system
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• Fading is the distortion that a carrier-modulated
telecommunication signal experiences over
certain propagation media.
Types of Fading shall be studied in
detail in later part of course.
Small scale and Large Scale Fading
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Phenomena that cause fading:
• Multipath – obstacles reflect signals so that multiple copies with varying delays are
received
• Reflection - occurs when signal encounters a surface that is large relative to the
wavelength of the signal
• Diffraction - occurs at the edge of an impenetrable body that is large compared to
wavelength of radio wave
• Scattering – occurs when incoming signal hits an object whose size is the order of
the wavelength of the signal or less and where number of obstacles per unit volume
is large
• Refraction – bending of radio waves as they propagate through the atmosphere
• Atmospheric absorption – water vapor and oxygen contribute to attenuation
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The Effects of Multipath Propagation
• Multiple copies of a signal may arrive at different phases
– If phases add destructively, the signal level relative to noise
declines, making detection more difficult
– The received signal is made up of a sum of attenuated, phase
shifted and time delayed versions of transmitted signal
• Intersymbol interference (ISI)
– One or more delayed copies of a pulse may arrive at the same
time as the primary pulse for a subsequent bit.
The Effects of Multipath Propagation
• The received signal is made up of a sum of attenuated,
phase shifted and time delayed versions of transmitted
signal
Perfect channel
White noise
Phase jitter
Statistical Channel Models
• AWGN Channel Model
• Rayleigh Channel Model
• Rician Channel Model
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Additive White Gaussian Noise Channel
• The Gaussian channel can be considered the ideal channel, and it
is impaired by additive white Gaussian noise(AWGN) developed
internally by the receiver
• There may also be contributions from interferers.
• The AWGN channel is the “best case” channel.
• The ideal Gaussian channel is very difficult to achieve in mobile
radio environment
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Rayleigh Channel
• The Rayleigh channel is at the other end of the line, often referred
to as a worst-case channel.
• In a non-line-of-sight multipath propagation direct wave from the
transmitter to the receiver is blocked.
• The multipath reception has many components.
• Each of the waves(multipath component) has a different phase and
this phase can be considered as an independent uniform
distribution, with the phase associated with each wave being
equally likely to take on any value.
• If each multipath component is independent, the PDF(power
distribution function) of its envelope is Rayleigh
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Rician Channel
• The characteristics of a Rician channel are in-between those of a
Gaussian and those of a Rayleigh channel.
• In a line-of-sight(LOS) situation, the received signal is composed
of a random multipath component( with multipath energy from
local scatterers),plus a coherent LOS component which has
essentially constant power.
• The power of LOS component will be usually greater than the
total multipath power.The theoretical distribution which applies to
this case is the Rice distribution.
• The Rician channel can be characterized by a function K.It is
defined as
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• K=0
Rayleigh(i.e,the
numerator is 0 and all
the received energy
derives from scattered
paths)
• K=∞
The channel is
Gaussian and the
denominator is zero
Channel Capacity
• Variety of impairments distort the signal and limit the data rate for digital
data.
• Channel Capacity-The maximum rate at which data can be transmitted over
a given communication path , under given conditions.
• Four concepts to relate:
– Date rate: The rate, in bits per second(bps),at which data can be
communicated.
– Bandwidth :The bandwidth of the transmitted signal as constrained by
the transmitter and the nature of transmission medium, expressed in
cycles per second or Hertz
– Noise: This is the average level of noise over the communication systems
– Error rate : This is the rate at which errors occur
• Error = transmit 1 and receive 0; transmit 0 and receive 1
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Relationship between Data Rate and Bandwidth
• The greater the bandwidth, the higher the information-carrying
capacity
• BUT the transmission system will limit the bandwidth that can be
transmitted.
• Limitations arise from physical properties of transmission medium or
from deliberate limitations at the transmitter on the bandwidth to
prevent interference from other sources.
• AND, for any given medium, the greater the bandwidth transmitted,
the greater the cost
• HOWEVER, limiting the bandwidth creates distortions
• Trade Off: Aim is to get as high date rate as possible at a particular
limit of error rate for a given bandwidth
Nyquist Bandwidth
• If rate of signal transmission is 2B then signal with frequencies no greater
than B is sufficient to carry signal rate
• Given bandwidth B, highest signal rate is 2B
• Given binary signal(two voltage levels), data rate supported by B Hz is 2B
bps
• When using more than two voltage levels(each signal representing more
than one bit, Nyquist formulation is
C= 2B log2M,where M is # of discrete signal or voltage levels
• For a given bandwidth, data rate can be increased by increasing # of
different signalling elements however this places an increased burden on
the receiver.
• Instead of distinguishing one of two possible signal elements during each
signal time, it must distinguish one of M possible signal elements.
• Noise and other impairments on the transmission line will limit the
practical value of M.
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Shannon Capacity Formula
• Consider data rate, noise and error rate
• Signal to noise ratio(in decibels):Ratio of the power in a
signal to the power contained in the noise that’s present at a
particular point in the transmission.Typically measured at the
receiver.
• SNRdb=10 log10 (signal/noise)
• SNR is important in transmission of digital data because it sets
the upper bound on achievable date rate.
• Capacity C=B log2(1+SNR),C is in bps,B is in Hz
• Represents theoretical maximum that can be achieved.
• This is error free capacity
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Bit Energy to Noise Ratio
• Eb/N0
– Energy per bit over the noise power spectral density
– Related to SNR power ratio
– Standard quality measure for digital communications
• The bit error rate(BER) for digital data is a function of Eb/N0
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Relate Eb/N0 to SNR
• SNR=S/N (Watt/Watt)
• Eb=STb
– Eb: energy per bit (J)
– S: signal power (carrier power) (W)
– Tb: duration of a bit (s)
• Eb/N0=(S/N0)*Tb = (S/N0)*(1/R )
• N0=N/B
– N: total noise power (W)
– B: bandwidth (Hz)
• Eb/N0=(S/N)(B/R)
• The S/N (or Eb/N0) at the input to the receiver will determine
the system performance
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