comm basics

8/10/2019 Comm Basics

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COMMUNICATION

Communication can be defined as the

exchangeofinformation between two

terminals, using the suitable mediumandwith the usage of the right set of

protocols, along with the utilization of the

proper switchingfacilities.


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TECHNICAL GOALS

1. To reduce the overall size

2. To increase the memory capacity

3. To increase the operating speed

4. To reduce the power consumption

5. To covey large amount of information

6. To transmit less amount of signal

7. To consume lesser power

8. To cover larger area


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The terminals

The terminals can be either humans or thecorresponding equipments / machines. To connect the

two distant terminals, there is the necessity of a

network, and this network must consist of the

switching facilities, to connect as well as to disconnectthe link for the respective terminals.

TERMINAL

1

SWITCHING

NETWORK

TERMINAL

2


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INFORMATION

Information can be of various types such as

voice (300-3400 Hz), sound (20-20,000 Hz),music, picture, video, text, data etc.

Further, the communicating medium (also called

as channel) can be of various types namelycopper wire, coaxial cable, optical fiber,

waveguide, terrestrial space (microwave link),

extra-terrestrial space (satellite link) etc.

Hence, depending upon the medium being used,the signal can be electrical (current), optical

(light)or electromagnetic (radio wave).


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Networks

In the case of bus, ring and star networks, the

no. of links is equal to the no. of terminals. In thecase of a mesh, the no. of links is equal ton(n-1) / 2, where "n" is the no. of terminals. Forthousands of nodes, mesh network is impossible

to implement and manage.

Star network structure is adopted for wiredtelephone exchanges (PSTN). The bus and ring

structures are used for LAN (Local AreaNetwork); Ethernet and Token-ring are theexamples respectively.


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Communication types

Communication is of 3 types namely - simplex, half-duplexand full-duplex.

Simplex is one-way (radio/TV broadcast, pager etc).

Half-duplex is conditional two-way (walkie-talkie, HAMradio, FAX etc).

Full-duplex is complete two-way (telephone, videoconferencing etc).

Thus, the exact type of communication depends on theparticular application.


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INFORMATION SWITCHING

CIRCUIT SWITCH PACKET SWITCH

Traffic Voice Data

Connection Dedicated SharedConcept Space division Time division

System Real-time Delayed

Information Analog / digital Only digitalRouting through Numbering plan Address space

Cost Expensive Economical


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SIGNAL Signal is a single-valued function of time that conveys

information. It can also be defined as the movement of

energy in the medium.

The movement of energy is due to the vibration of therespective particles. Lower frequency implies bigger

sized particles and higher frequency implies smaller

sized particles. Hence, the higher frequency signals

tend to be narrower, and with LOS propagation.

As the particles are spherical in shape, the signal

remains essentially sinusoidal, or its variations.


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THE SPECTRUM

Audio Radio Thermal Video

10 Hz - 100kHz - 1THz - 100THz

The radio band:VLF LF MF HF VHF UHF SHF EHF

3k 30k 300k 3M 30M 300M 3G 30G 300G

LW1 LW2 MW SW1 SW2 mW1 mW2 mW3

When size of the oscillating object / particle reduces:- frequency and speed increase.

- energy content and spreading decrease.


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PROPERTIES OF ENERGY

1. LAW OF CONSERVATION: Energy can neither be

created nor be destroyed, but can only be transformed.

2. LAW OF MASS ACTION: Energy can be converted into mass,and mass into energy.

3. LAW OF REASON: Energy in dynamic form, always tries tomove back to its own source.

4. LAW OF OBSTRUCTION: Energy, while returning to itssource, moves more through the path of least resistance.

5. LAW OF CONVERSION: Energy, when proper path is notavailable, gets stored, converted and returned, in parts.


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NEED FOR MODULATION

1. To match the signal characteristics to the

channel characteristics.

2. To reduce the antenna height.

3. To increase the coverage area.4. To overcome the effects of noise and

interference.

5. To reduce the equipment size.6. To overcome the equipment limitations.

7. To enhance the multiplexing scheme.


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TYPES OF MODULATION

ANALOG DIGITAL

Amplitude Angle Binary Hybrid

AM FM ASK FSK PSK QAMPM

DSBFC MSK BPSK 16-QAM

DSBSC GMSK QPSK 64-QAMSSB FFSK 8-PSK 256-QAM

VSB DPSK 1024-QAM OQPSK 4096-QAM


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Types of modulationcontd

PULSE MODULATION

ANALOG DIGITAL

PAM PCM

PWM

PPM DPCM

ADPCMDM

ADM


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NOISE

External Internal(channel) (electronic)

Atmospheric noise Thermal noise

Galactic noise Shot noise

Industrial noise Flicker noise

Burst noise

Avalanche noise

Transit-time noise


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Noise continued

- Most of the atmospheric noise is below VHF range.

- Thermal noise is called as agitation noise, and also asJohnson noise, after its discoverer.

- Shot noise is generated by the current flowing across ajunction. This noise increases with decrease in current.

- The psd of the thermal noise and shot noise is independent of

frequency; hence both of them come under white noise.

- The psd of the thermal noise and shot noise follows theGaussian distribution.


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Noise continued

- All the other types of noises are non-white as well asnon-gaussian.

- Flicker noise is also called as 1/f noise, due to itsreduction with increase in frequency.

- This noise results due to the imperfections in thecrystalline structure.

- It is also called as pink noise, as similar psd atvisible frequencies would appear pink in color.


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Noise continued

- Burst noise is characterized by discrete high-frequency pulses,and hence it is also called as popcorn noise.

- This noise reduces with increase in frequency, and its responseis inversely proportional to the square of the frequency; hence it

is further called as 1/f noise.

- Avalanche noise occurs in the reverse breakdown mode of thePN junction. This type of noise as well as the burst noise occurin the brownregion of the psd.

- Transit-time noise occurs when the transit-time of the chargecarriers approaches the signal period. This type of noise occursin the VHF range.


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ANALOG MODULATION

Instantaneous voltage of the carrier is given by,

ac= Ac sin(2fct + c)

- With AM, Acis modulated. The modulated carrier is

represented as: acm= [Ac+ am] sin(2fct)Or, acm= [Ac+ Am sin(2fmt)] sin(2fct)

- The modulation index is given by, m = Am/Ac,with 0 < m < 1. The modulated signal will have

only two sidebands, and the bandwidth of the

modulated signal is 2fm.


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Analog modulation - contd

- The total power in an AM wave is given by,

Pt= Pc[1 + sqr(m)/2] = Pc+ PSB

- If the carrier is modulated by several sine waves

simultaneously, then the total modulation index

is given by the square root of the sum of thesquares of the individual modulation indices.


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Analog modulation - contd

- With FM, fcis modulated. The modulated carrieris represented as:

acm= Acsin(2[fc +fcsin(2fmt)] t)

- The modulation index is given by, m =fc/fm, with

0 < m < . The value of fcdepends on am. With

increase in m, there will be increase in thebandwidth as well, as the modulated signal will

have increased number of sidebands.


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Analog modulationcontd

- With PM, cis modulated. The modulated carrier

is represented as:

acm= Acsin(2fct +msin(2fmt))

- Here,mis the maximum change in phase and

this itself is the modulation index. But the smallchanges in phase cannot be transmitted anddetected with accuracy. Hence, PM is not usedfor the transmission of speech and music.


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Advantages of digital methods

1. Increased immunity to noise and interference.

2. Flexible operation of the system, due to the

multiplexing methods.

3. Common format for the transmission of differenttypes of message signals.

4. Improved security through the use of encryption.

5. Due to channel coding, errors can be detected

and corrected in the receivers.


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Disadvantages

1. Increased transmission bandwidth.

2. Increased system complexity.

- First one can be overcome with the

usage of channels such as OFC and

satellite links. The second one is

overcome with the advent of VLSI.


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Design goals for the selection of

a DIGITAL modulation scheme Maximum data-rate

Maximum immunity to interference Minimum transmit power

Minimum channel bandwidth

Minimum probability of symbol error Minimum circuit complexity


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Comparison of

digital modulation methods

Method Bits per symbol Modulation scheme

GMSK 1 2F

BPSK 1 2P

QPSK 2 4P8-PSK 3 8P

4-QAM 2 2Ax2P

16-QAM 4 2Ax8P / 4Ax4P

64-QAM 6 4Ax16P / 8Ax8P

256-QAM 8 8Ax32P / 16Ax16P

1024-QAM 10 16Ax64P / 32Ax32P

4096-QAM 12 32Ax128P / 64Ax64P


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No. of phase Phase-shift

changes in degrees

2 180

4 90

8 45

16 22.5

32 11.25

64 5.625

128 2.8125


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Properties of human speech

1. The speech consists of 56% pause, 22%

repetition and 22% essentials. The speech

can be divided as voiced speech and

unvoiced sounds.

2. The voiced speech is produced by forcing air

through the glottis, by means of which thevocal cords vibrate.


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Properties of human speechcontd

3. The unvoiced sounds are produced by a

turbulence, which results due to the forcing of

air through a constriction within the vocal tract.

These are also called as fricatives. e.g.: ss,sh, phoo, whistle, whisper etc.

4. The voiced sounds are quasi-periodic in

nature, and hence this periodicity permits the

prediction of pitch frequency.


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Properties of human speechcontd

5. The vocal tract tube resonates at some

particular frequencies, called as formants. A

large coding error can be tolerated near the

formants. (e.g: 500 Hz, 1500 Hz etc.)

6. The human ear does not perceive the noise, if

the noise is 15 dB below the signal level in that

band. This phenomenon, known as noise

masking is a characteristic of human hearing.


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SPEECH CODING PROCESS

The advanced coding methods take all the above facts

into consideration. The sounds are modeled as the

response of the vocal tract filter: voiced sounds are

excited with a periodic sequence of impulses, andunvoiced ones with a white noise sequence. Thus the

bit-rate gets drastically reduced.

The main disadvantage of these voice coding methods

is the processing delay of about 25 ms.


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SIGNAL DOMAINS

TIME DOMAIN FREQUENCY DOMAIN

1. Amplitude vs. time Amplitude vs. frequency

2. Frequency is linear Frequency is angular

3. For pure waveforms For complex waveforms

4. Via differential equations Via algebraic equations

5. Tougher to analyze Easier to analyze

Note: A signal cannot be strictly limited both in time and

in frequency. A strictly finite time function will have indefinite

spectrum, and vice versa. e.g.: rect(f) sinc(f)

SPEECH CODING


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SPEECH CODING

WAVEFORM CODERS SOURCE CODERS

Time Frequency Time Frequency

domain domain domain domain

PCM DPCM SBC ATC LPC Vocoder

DM ADPCM ASBC

ADM APC

CVSDM

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