transmission fundamentals & principles. analogue and digital data transmission analogue and...

Transmission Fundamentals & Principles

• 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

Class Contents:

Analogue and Digital world:

The terms analogue and digital, corresponds roughly to continuous and discrete,

Analogue and Digital Data Transmission

Used in communications in 3 ways:DataSignalsTransmissions

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


Signals are electric or electromagnetic representations of data

• Analogue signal:

• Digital Signal:

Data are propagated from one point to another by means of electrical signals.

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.


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 orWIRELESS TRANSMISSIONS


Analogue and digital data can be represented by bothanalogue 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

Transformations from Data to SignalsDigital 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 voicesignal spectrum, this data can then be propagated over telephone lines

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..


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 andare 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 signals may be transmitted on suitabletransmission 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 it’s content. Propagation of the signal is done through AMPLIFIERS

• Digital signals are not propagated using Analogue Transmissions

Analogue and Digital 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

Analogue Signal Digital Signal

Analogue Data

Two alternatives:a) Signal occupies same spectrum as the analogue datab) 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 valuesb) digital data are encoded to produce a digital signal with desired properties

Summary Table 1: Data & 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.

Summary Table 2: Treatment of Signals

Data Rate: Calculated using the time duration of a symbol

CHANNEL CAPACITY THEORY: Data Rate

CHANNEL CAPACITY THEORY: BandwidthThe 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 –AIn a random way.

Fourier series expansion:

where

Notice that the Bandwidth is infinite.

CHANNEL CAPACITY THEORY: Bandwidth

The amplitude of the nth harmonicWhen n tends toward infinite is:

The signal has a “finite bandwidth” as defined by thenumber of harmonics taken into consideration to buildthe signal.

The nth harmonic is represented by: tfnn

Atxn

02sin4

)(

Using a square wave (Amplitude 1) with a fundamental period of 2 m seconds, and taking the first 2 harmonics into account (n=3 and n=5):

Data Rate: 1 bit has a duration of 1 m sec ==> DR=1 Mbps

Bandwidth: Fundamental frequency = 500 KHz, frequency of the 2nd harmonic is 5f0=2.5 MHz

BW=2.5 MHz – 0.5 MHz = 2 MHz

Examples of Data rate and bandwidth calculations:

Data Rate: 1 MbpsBandwidth: 2 MHz

Examples of Data rate and Bandwidth calculations:

Changing the period of the signal to 1 m sec:

Data Rate: 2 MbpsBandwidth: 4 MHz


Keeping the period of the signal in1 m sec:

Data Rate: 2 MbpsBandwidth: 3 fo- fo = 2 MHz


Changing the period of the signal to 0.5 m sec, and using one harmonic:

Data Rate: 4 Mbps Bandwidth: 4 MHz

Examples of Data rate and bandwidth calculations:

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.

Channel Capacity [bps]

Is the maximum data rate at which information can be transmittedover a given communications path or channel under given conditions.

NoiseIt 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

There are 2 approaches in calculating Channel Capacity:

Nyquist Bandwidth Theorem Shannon’s Capacity Formula

Shannon’s 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

Channel Capacity

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:

Logby = x bx = y Taking logarithm base 10 in both sides:

x log b = log y x = log y / log b

Nyquist Bandwidth Theorem

In a binary systems (2 levels), the carrying capacity isTwice the bandwidth

Example: An information source is coded using a 6 bit wordthat is to be propagated using a binary system. How many levels are needed?. Find the carrying capacityif the signal has a bandwidth of 5 MHz.

C = 2 . B . log2 64 = 2 . B . 6 = 60 Mbps

Nyquist Bandwidth Theorem

bits per word levels of the system

M = 26 = 64

Binary System is represented by base 2

In a channel in the presence of noise, the carrying capacityis 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 powerto 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:

Shannon’s Capacity Formula

Signal to Noise Ratio Characteristics:

• The SNR is adimensional• It is usually expressed in dB


The Signal to Noise Ration (SNR), imposes the upper limit onachievable data rate in a communications system

C = B . log2(1+SNR) [bps]

B is the signal bandwidth in Hz

This formula is also called:

ERROR-FREE CAPACITY


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 achieveerror-free transmission through the channel.

Observations

The data rate could be increased by increasing either the signalstrength 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.


Shannon, assumes the noise to be “white noise”, therefore, the wider the bandwidth, the more noise is admitted into the system

Shannon’s 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.

Observations


The spectrum of a communications channel has abandwidth of 6 MHz. A signal is transmitted throughthe channel and received with a SNRdB of 24 dB. Find the error-free capacity of the channeland the number of signal levels that are required to achieve that capacity and the number of bits used to sample the signal.

Example of Calculation

Is the physical path between the transmitter and the receiverin a communications system.

Guided Media:

Unguided Media:

twisted pair, coaxial cable, optical fibre.

Air (atmosphere), space (vacuum)

Unguided Media transmission are referred to as:

WIRELESS TRANSMISSIONS

Transmission Media

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

Directionality is a key property of a transmitter and is achieved by means of an antenna.

Lower Frequencies

Frequency & Directionality

Signal are omnidirectional in nature: The propagationoccurs in all directions with the same intensity.

Higher Frequencies

It is possible to focus the signal in a directional beam:

Frequency & Directionality

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

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


The Radio Range:


• 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


The Radio Range:

Sources of Impairment:

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


The Microwave Range:

Classification:

Compromises frequencies between 1 GHz and 40 GHz

Possibility for highly directional beams

Mode of transmission is point to point

• Terrestrial Microwaves

• Satellite Microwaves



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


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





Application Band Observations

Long-Haul Telecommunications

4 GHz - 6 GHz Suffering from congestion. Increased chance for interference.11 GHz band is coming into use.



Application Band ObservationsCATV 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.




• 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 DistributionsSatellite Microwaves:

A communications satellite is a microwave relay station usedto link 2 or more earth based microwave transmitter/receiversknown 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 backto 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

Satellite Microwaves:


ApplicationsSatellite Microwaves:


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

• Optimum frequency range: 1 GHz to 10 GHz

Typical uplink: 5.95 to 6.425 GHz

Typical downlink: 3.7 to 4.2 GHz4/6 GHz Band


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.


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 BookFrequency Division MultiplexingTime Division Multiplexing

transmission fundamentals & principles. analogue and digital data transmission analogue and...

Documents