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Transmission Fundamentals Principles

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

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

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

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

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

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.

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

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..

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.

Analogue and Digital Signalling of Analogue and

Digital Data

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

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.

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

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

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.

CHANNEL CAPACITY THEORY Data Rate

Data Rate Calculated using the time duration of

a symbol

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.

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.

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

Examples of Data rate and Bandwidth calculations

Data Rate 1 Mbps Bandwidth 2 MHz

Examples of Data rate and Bandwidth calculations

Changing the period of the signal to 1 m sec

Data Rate 2 Mbps Bandwidth 4 MHz

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

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

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.

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.

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

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

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

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

Shannons Capacity Formula

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

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

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.

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.

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

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.

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

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.

Frequency Directionality

Higher Frequencies

It is possible to focus the signal in a

directional beam

The Frequency Spectrum

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

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

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

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

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

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

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

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

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.

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.

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)

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

Wireless Frequency Spectrum Distributions

Satellite Microwaves

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

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.

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.

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.

Recomemded Additional Reading

- Multiplexing Techniques Section 2.5 Stallings

Wireless Communications Book - Frequency Division Multiplexing
- Time Division Multiplexing