Transmission Fundamentals

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Transmission Fundamentals Principles
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Class Contents

• Analogue and Digital Data Transmission

• Analogue and Digital Data
• Analogue and Digital Signals
• Analogue and Digital Transmissions

• Channel Capacity

• Data Rate Bandwidth
• Channel Capacity Nyquist and Shannon

• Transmission Media

• Guided and Unguided Media
• Wireless Transmissions and Applications

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Analogue and Digital Data Transmission
Analogue and Digital world
The terms analogue and digital, corresponds
roughly to continuous and discrete,

• Used in communications in 3 ways
• Data
• Signals
• Transmissions

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Analogue and Digital Data Transmission
Data are entries that convey meaning or
information

• Analogue Data Is data that takes on continuous
  values
• over a time interval.

Examples Voice, video, sensor readings
such as temperature

• Digital Data Is data that takes on discrete
  values
• over a time interval.

Examples Text, integers
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Analogue and Digital Data Transmission
Signals are electric or electromagnetic
representations of data
Data are propagated from one point to another by
means of electrical signals.

• Analogue signal

Is a continuously varying electromagnetic wave
that can be propagated over a variety of media.
Is a series of voltage pulses that may be
transmitted over a medium.

• Digital Signal

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Analogue and Digital Data Transmission
Media are the places used to propagate the
signals.

• Guided Media Copper Wire, twisted pair, coaxial
  cable
• optical fibre.
• Unguided Media Atmosphere, vacuum and air.

The Course Focuses in unguided media
transmissions or WIRELESS TRANSMISSIONS
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Transformations from Data to Signals
Analogue and digital data can be represented by
both analogue and digital signals
Analogue Data Analogue Signals

• Analogue data is a function of time and occupy a
  limited frequency spectrum.
• Analogue data can be directly represented by an
  electromagnetic signal occupying the same spectrum

Example Sound waves are voice data. Voice
spectrum 20 Hz 20 KHz Spectrum for Voice
Signal is 300 Hz to 3.4 KHz.
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Transformations from Data to Signals
Digital Data Analogue Signals

• A process of modulation-demodulation is
  required.
• A MODEM converts a series of binary data voltage
  pulses,
• into an analogue signal. This process is done by
  modulating a
• carrier frequency.
• The spectrum of the modulated signal is centred
  around the
• carrier frequency.

Example Most common MODEMS represent digital
data in the voice signal spectrum, this data can
then be propagated over telephone lines
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Transformations from Data to Signals
Analogue Data Digital Signals

• Process is similar to Digital Data Analogue
  Signal conversion.
• Continuous data is codified into a digital bit
  stream using a
• coding process.
• A CODEC is used to convert analogue data to
  digital signals.

Example The CODEC takes the analogue signal that
directly represents the voice data and
approximates it by a digital stream..
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Transformations from Data to Signals
Digital Data Digital Signals

• Process is equivalent to the analogue data
  analogue signal
• conversion.
• Binary data is often encoded in a more complex
  form of binary signal to improve propagation
  characteristics of the signal.

Observation Digital signals are generally
cheaper to produce and are less susceptible to
noise interferences, however, they suffer more
attenuation than their analogue counterparts.
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Analogue and Digital Signalling of Analogue and
Digital Data
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Analogue and Digital Transmissions
Analogue and digital signals may be transmitted
on suitable transmission media. The way the
signals are treated is a function of the
transmission media

• In an Analogue Transmission an analogue signal
  is
• transmitted without any regard to its content.
  Propagation
• of the signal is done through AMPLIFIERS
• Digital signals are not propagated using
  Analogue
• Transmissions

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Analogue and Digital Transmissions

• In a Digital Transmission analogue and digital
  signals are transmitted. Signal content is
  important.
• Signal Propagation Digital
• Digital signals can be propagated only a limited
  distance.
• Attenuation endangers the integrity of the
  signal
• A REPEATER is used to receive the signal,
  recover the string and generate a new signal to
  retransmit.

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Analogue and Digital Transmissions

• Signal Propagation Analogue (Constructed from
  digital data)
• Retransmitters (repeaters) are used instead of
  amplifiers.
• The repeater recovers the digital data from the
  analogue signal
• And uses it to generate a new, noise-free
  analogue signal

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Summary Table 1 Data Signals
Analogue Signal Digital Signal
Analogue Data Two alternatives a) Signal occupies same spectrum as the analogue data b) Analogue data are encoded or modulated to occupy a different portion of the spectrum Analogue data are encoded using a CODEC to produce a digital bit stream
Digital Data Digital data are encoded using a MODEM to produce analogue signal Two alternatives a) Signal consists of a two voltage levels to represent the two binary values b) digital data are encoded to produce a digital signal with desired properties
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Summary Table 2 Treatment of Signals
Analogue Transmission Digital Transmission
Analogue Signal Is propagated through amplifiers same treatment whether signal is used to represent analogue data or digital data Assumes that the analogue signal represents digital data. Signal is propagated through repeaters at each repeater, digital data are recovered from inbound signal and used to generate a new analogue outbound signal
Digital Signal NOT USED Digital signal represents a stream of 1s and 0s, which may represent digital data or may be an encoding of analogue data. Signal is propagated through repeaters at each repeater, stream of 1s and 0s is recovered from inbound signal and used to generate a new digital outbound signal.
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CHANNEL CAPACITY THEORY Data Rate
Data Rate Calculated using the time duration of
a symbol
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CHANNEL CAPACITY THEORY Bandwidth
The bandwidth depends of the signal used.
For a binary bit stream, the square pulse A,-A is
used as The elemental signal. The data takes on
the values A and A In a random way.
Fourier series expansion
where
Notice that the Bandwidth is infinite.
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CHANNEL CAPACITY THEORY Bandwidth
The nth harmonic is represented by
The amplitude of the nth harmonic When n tends
toward infinite is
The signal has a finite bandwidth as defined by
the number of harmonics taken into consideration
to build the signal.
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Examples of Data rate and bandwidth calculations
Using a square wave (Amplitude 1) with a
fundamental period of 2 m seconds, and taking
the first 2 harmonics into account (n3 and n5)
Data Rate 1 bit has a duration of 1 m sec gt
DR1 Mbps
Bandwidth Fundamental frequency 500 KHz,
frequency of the 2nd harmonic is 5f02.5
MHz BW2.5 MHz 0.5 MHz 2 MHz
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Examples of Data rate and Bandwidth calculations
Data Rate 1 Mbps Bandwidth 2 MHz
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Examples of Data rate and Bandwidth calculations
Changing the period of the signal to 1 m sec
Data Rate 2 Mbps Bandwidth 4 MHz
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Examples of Data rate and Bandwidth calculations
Keeping the period of the signal in1 m sec
Data Rate 2 Mbps Bandwidth 3 fo- fo 2 MHz
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Examples of Data rate and bandwidth calculations
Changing the period of the signal to 0.5 m sec,
and using one harmonic
Data Rate 4 Mbps Bandwidth 4 MHz
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Data Rates Bandwidth Facts

• The greater the bandwidth, the greater the data
  rate that can be achieved.
• The transmission system will limit the bandwidth
• The greater the bandwidth, the greater the cost
• The more limited the bandwidth, the greater the
  distortion and the potential for error by the
  receiver.

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Noise

• It is defined as an unwanted signal that combines
  with and hence distorts the signal intended for
  transmission and reception
• To make as efficient use as possible of a given
  bandwidth, the Maximum possible data rate must be
  achieved.
• The Limitation to this is the quantity of noise
  present in the system

Channel Capacity bps
Is the maximum data rate at which information can
be transmitted over a given communications path
or channel under given conditions.
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Channel Capacity

• There are 2 approaches in calculating Channel
  Capacity
• Nyquist Bandwidth Theorem
• Shannons Capacity Formula
• Shannons Formula takes noise into account.
• Nyquist works with multilevel signals but does
  not take noise into account.
• Both Methods give theoretical maximums to data
  rate given a bandwidth

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Nyquist Bandwidth Theorem
For a signal made of M levels, and a bandwidth of
B, using a binary transmission system, the
carrying capacity C of the system is given by
C 2.B.log2 M
Calculation of Base b logarithm
Taking logarithm base 10 in both sides
Logby x ? bx y
x log b log y ? x log y / log b
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Nyquist Bandwidth Theorem
In a binary systems (2 levels), the carrying
capacity is Twice the bandwidth
An information source is coded using a 6 bit
word that is to be propagated using a binary
system. How many levels are needed?. Find the
carrying capacity if the signal has a bandwidth
of 5 MHz.
Example
C 2 . B . log2 64 2 . B . 6 60 Mbps
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Shannons Capacity Formula
In a channel in the presence of noise, the
carrying capacity is adversely affected by the
level of noise to signal that is present in the
communications channel.
Signal to Noise Ratio
Is a parameter used to measure the immunity of
the signal power to the noise power. It is
defined as the ratio of signal power to noise
power that is present at a particular point of
the transmission
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Shannons Capacity Formula

• Signal to Noise Ratio Characteristics
• The SNR is adimensional
• It is usually expressed in dB

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Shannons Capacity Formula
The Signal to Noise Ration (SNR), imposes the
upper limit on achievable data rate in a
communications system
C B . log2(1SNR) bps
B is the signal bandwidth in Hz
This formula is also called ERROR-FREE
CAPACITY
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Shannons Capacity Formula
If the data rate of the channel is less than the
error-free capacity, then it is theoretically
possible to use a suitable code to
achieve error-free transmission through the
channel.
Observations

• The data rate could be increased by increasing
  either the signal
• strength or the bandwidth, however
• Increasing the bandwidth, increments the costs
• Increasing the signal strength, increments the
  effects of
• non-linearities in the system producing an
  increase in
• inter-modulation noise.

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Shannons Capacity Formula
Observations

• Shannon, assumes the noise to be white noise,
  therefore, the wider the bandwidth, the more
  noise is admitted into the system
• Shannons error-free capacity represents the
  theoretical maximum that can be achieved. In
  practice, only much lower rates are achieved
  because of factors such as impulsive noise,
  attenuation distortion and delay distortion, are
  not accounted for.

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Example of Calculation
The spectrum of a communications channel has
a bandwidth of 6 MHz. A signal is transmitted
through the channel and received with a SNRdB of
24 dB. Find the error-free capacity of the
channel and the number of signal levels that are
required to achieve that capacity and the number
of bits used to sample the signal.
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Transmission Media
Is the physical path between the transmitter and
the receiver in a communications system.
twisted pair, coaxial cable, optical fibre.
Guided Media
Air (atmosphere), space (vacuum)
Unguided Media
Unguided Media transmission are referred to
as WIRELESS TRANSMISSIONS
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Wireless Transmissions

• The characteristics and quality of a data
  transmission are determined by the
  characteristics of the medium and the
  characteristics of the signal.
• Guided Media The medium is more important in
  determining the limitations of the transmission
• Unguided Media The bandwidth of the signal
  produced by the transmitting antennas is more
  important than the medium in determining
  transmission characteristics.

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Unguided Media Communications

• Transmission and reception of unguided media are
  achieved by means of an antenna.
• The transmitting antenna radiates electromagnetic
  energy into the medium
• The receiving antenna picks up electromagnetic
  waves form the surrounding medium

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Frequency Directionality
Directionality is a key property of a transmitter
and is achieved by means of an antenna.
Lower Frequencies
Signal are omnidirectional in nature The
propagation occurs in all directions with the
same intensity.
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Frequency Directionality
Higher Frequencies
It is possible to focus the signal in a
directional beam
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The Frequency Spectrum
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The Frequency Spectrum

• There are several ranges that are interesting in
  wireless transmissions
• Broadcast Radio (radio range)
• Microwave Frequencies
• Terrestrial Microwave
• Satellite Microwave
• Infra-Red

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Wireless Frequency Spectrum Distributions
The Radio Range

• Transmissions in the band of 30 MHz to 1 GHz
• Suitable for omnidirectional applications (radio
  broadcast)

Applications

• FM radio
• UHF VHF television
• Some data networking applications

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Wireless Frequency Spectrum Distributions
The Radio Range
Transmission Characteristics

• Effective range for broadcast communications
• Ionosphere is transparent to radio waves above
  30 MHz
• Transmission is limited to line-of-sight.
• Distant transmitters will not interfere with
  each other due
• to reflection from the atmosphere

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Wireless Frequency Spectrum Distributions
The Radio Range
Transmission Characteristics

• Radio waves are less sensitive to attenuation
  due to rainfall
• Free space losses can be calculated using

• Wave length l can be calculated using the speed
  of light in
• vacuum

l . f c c 3x108 m/s
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Wireless Frequency Spectrum Distributions
The Radio Range
Sources of Impairment

• Multi-path interference Reflections from land,
  water and
• human made objects, create multiple
• paths between antennas

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Wireless Frequency Spectrum Distributions
The Microwave Range
Compromises frequencies between 1 GHz and 40 GHz
Possibility for highly directional beams
Mode of transmission is point to point
Classification

• Terrestrial Microwaves

• Satellite Microwaves

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Wireless Frequency Spectrum Distributions
Terrestrial Microwaves

• Typical antenna used is a parabolic dish with 3
  metres in diameter

• The antenna is fixed rigidly and focuses a
  narrow beam to achieve
• line-of-sight transmission

• Antennas are located at substantial heights
  above ground level

• To achieve long distances, microwave relays
  towers
• need to be used

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Wireless Frequency Spectrum Distributions
Terrestrial Microwaves
Applications and Frequency Bands

• Long-Haul telecommunications services (voice and
  TV)

• Short point-to-point links between buildings
  (CCTV, Data
• links between LANs

• Cellular systems and fixed wireless access

The microwave requires far fewer amplifiers or
repeaters than an equivalent coaxial cable
system over the same distance
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Wireless Frequency Spectrum Distributions
Terrestrial Microwaves
Applications and Frequency Bands
Application Band Observations
Long-Haul Telecommunications 4 GHz - 6 GHz Suffering from congestion. Increased chance for interference. 11 GHz band is coming into use.
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Wireless Frequency Spectrum Distributions
Terrestrial Microwaves
Applications and Frequency Bands
Application Band Observations
CATV Systems 12 GHz Links used to provide TV signals to local cable TV installations (CATV). Signals are distributed to subscriber via coaxial cable
Short Point-to-point links 22 GHz Used in building to building LAN applications.
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Wireless Frequency Spectrum Distributions
Terrestrial Microwaves
Transmission Characteristics

• Main source for attenuation are free space
  losses

• The higher the frequency, the higher the
  potential bandwidth,
• thus the higher the data rate for some typical
  applications

• Losses varies with the square of the distance.
  In twisted pair
• and coaxial systems, it varies logarithmically
  with the distance

• Repeater may be placed farther apart (typically
  10-100 Km)

• Attenuation is increased with rainfall
  (noticeable above 10 GHz)

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Wireless Frequency Spectrum Distributions
Satellite Microwaves
A communications satellite is a microwave relay
station used to link 2 or more earth based
microwave transmitter/receivers known as EARTH
STATIONS or GROUND STATIONS
Transmission is received in a frequency band
called UPLINK, the satellite amplifies or
repeats the signal, and transmits it back to
earth using a different frequency band called
DOWNLINK
A single satellite operates on a number of
frequency bands called TRANSPONDER CHANNELS
The electronics on the satellite that converts
uplink to downlink are called TRANSPONDER
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Wireless Frequency Spectrum Distributions
Satellite Microwaves
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Wireless Frequency Spectrum Distributions
Satellite Microwaves
Applications

• TV distribution
• Long-Distance telephone transmission
• Private Business networks

Transmission Characteristics

• Optimum frequency range 1 GHz to 10 GHz

Typical uplink 5.95 to 6.425 GHz
4/6 GHz Band
Typical downlink 3.7 to 4.2 GHz
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Wireless Frequency Spectrum Distributions
Satellite Microwaves
Transmission Characteristics Sources of
impairment

• Below 1 GHz Noise from natural sources,
  including
• galactic, solar and atmospheric noise.
• Human made interference from electronic
• devices

• Above 10 GHz Signal attenuation is severe. Also
  affected
• by precipitation and atmospheric absortion.

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Wireless Frequency Spectrum Distributions
Satellite Microwaves
Properties of satellite communications

• Propagation delay of 0.25 sec.

• Problems in the areas of Error and Flow control.

• Satellite microwave is a broadcast facility.

• Many stations can transmit to the satellite.

• Satellite transmission can be received by many
  stations.

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Infrared

• Achieved using transceivers (Tx/Rx) that modulate
  noncoherent IR light.
• Must operate within line-of-sight (directly or by
  reflection from light coloured surface)
• Does not penetrate walls ? Security and
  interference problems encountered in microwaves
  are not present.

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Recomemded Additional Reading

• Multiplexing Techniques Section 2.5 Stallings
  Wireless Communications Book
• Frequency Division Multiplexing
• Time Division Multiplexing