communications i introduction

Dr. Talal Skaik 2016 IUG

EELE 3370

Communications I

Introduction

Dr. Talal Skaik 2016

Islamic University of Gaza

Electrical Engineering Department

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• Communications is the Process of Transmitting Information from a Source to a Destination

• Example of typical communication systems

– Wire-line telephone – Cellular phone

– TV broadcasting system.

– Wireless computer network

– Satellite Communications.

Communications

2 Dr. Talal Skaik 2016 IUG

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Some examples of communications systems.


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Input Transducer

Transmitter Channel

Noise

Receiver

Output transducer

Input Signal

Input Message

Transmitted Signal

Received Signal

Output Signal

Output Message

Communication System Model


• Source: originates a message (human voice, a television picture, an email, or data message, or data

• Input Transducer: Converts nonelectric (human voice, email text, TV video) into an electric waveform called a message or baseband signal using physical devices (microphone, a computer keyboard, or a camera)

• Transmitter: Modifies the baseband signal for efficient transmission and may consist of A/D converter, an encoder, and a modulator. [Receiver- Demodulator, decoder and D/A.]

• Channel: Medium that convey the electric signals at the transmitter output over a distance. Examples:

• Twisted copper wire – telephone and DSL,

• Coaxial Cable – television and internet,

• Optical fiber or radio link

Communication System Model (cntd)

Dr. Talal Skaik 2016 IUG 5

• Receiver: Reprocesses signals received from the channel by reversing signal modifications made at the transmitter. Removal of noise due to channel.

• Output Transducer: Converts electric signal to its original form (Message)

• Destination: Unit to which message is communicated.



• The Signal is distorted by Channel and Noise, which are random and unpredictable .

• NOISE comes from external and internal sources

• External sources comes from : Interference signals transmitted on nearby channels, lightning, electrical equipment, cell phones emission, etc…

• Internal noise results from thermal motions of electrons in conductors.

• SNR: The signal to noise ratio is defined as the ratio of the signal power to the noise power. The channel distorts the signal and the noise accumulates along the path.



• The signal strength decreases while the noise level remains steady with distance from the transmitter.

• Thus SNR is continuously decreasing along the channel.

• For good results, SNR is supposed to be high. In other words signal power should be high compared to noise power.



Digital Communication System

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Source Coder

Channel Coder

Modulation

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Source decoder

Channel decoder

demodulation

Distortion and noise

transmitter

channel

receiver

Source input

Reconstructed Signal output


Analog and Digital Signals

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Analog signals values vary continuously. Digital signals value limited to a finite set.

Binary signals have 2 possible values used to represent bit values Bit time T needed to send 1 bit Data rate R = 1/T bits per second


Analog and Digital Messages

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Messages could be digital or Analog. Digital Message: Ordered combinations of finite symbols. Example: text document in English, morse code (dash and dot). Analog Messages: Characterized by data whose values vary over a continuous range of time e.g temperature, atmospheric Pressure, speech waveform. Why are digital technologies better and Why are they replacing the analog technologies? Enhanced Immunity to noise and interference. Message extraction from received signal is easier for digital signal since digital decision must belong to the finite-sized alphabet. Detail of received signal is not an issue. In contrast: The waveform shape in analog message carries the needed information, and even slight distortion or interference in the waveform will show up in the received signal.


Analog and Digital Messages

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(a) Transmitted signal. (b) Received distorted signal (without noise). (c) Received distorted signal (with noise). (d) Regenerated signal


Analog to Digital conversion (A/D)

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Two steps in A/D conversion – sampling and quantization Continuous time signal is sampled into discrete time signal (DTS). Continuous amplitude of the DTS is quantized into discrete level signal. Sampling Theorem: if the highest frequency in signal spectrum is B (in Hz), the signal can be reconstructed from its discrete samples taken uniformly at a rate not less than 2B samples per second.



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Quantization: Sample is approximated or “rounded off” to the nearest quantized level. Given signal m(t), whose amplitude range (–mp, mp), quantizer partitions the signal range into L intervals. Each sample amplitude ~ midpoint of interval. Each sample is represented by one of the L numbers (digitized). Quantized signal is an approximation of original signal. Accuracy improves with increase in L Voice signal L = 8 or 16 is sufficient. Commercial use, L = 32 is minimum, telephone L 128 or 256 commonly used.



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Analog-to-digital conversion of a signal.


Pulse Coded Modulation

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The task of Communication system is to transmit the quantized samples (done with the aid of PCM).

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, Compact Discs, digital telephony and other digital audio applications.

In PCM, each quantized sample is represented by an ordered combination of pulses p1(t) and p2(t) where p1(t) represents 1 and p2(t) represents 0.

Each of the L possible samples can be written as a bit string of length log2L. Example: if L=16, each quantized level is described by 4 bits.

The binary case of great practical importance bacause of its simplicity and ease of detection.


Pulse Coded Modulation


Channel Error

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If there is too much channel distortion or noise, receiver may make a mistake, and the regenerated signal will be incorrect. Channel coding is needed to detect and correct the message.


Bandwidth and Power

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• The fundamental parameters that control the rate and quality of information transmission are the channel bandwidth and the signal power S.

Channel Bandwidth

The bandwidth (BW) of a channel is the range of frequencies that it can transmit with reasonable fidelity .

OR: Difference between the highest and the lowest frequencies in the specific range of frequencies.

• Example: if channel can transmit with reasonable fidelity a signal whose frequency components vary from 0 Hz (dc) up to maximum 5000 Hz (5 KHz), the Channel bandwidth B is 5 KHz.


Bandwidth and Power

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• The signal power S plays an important role in information transmission.

• Increasing S reduces the effect of channel noise and thus accurate data is received.

• Signal to Noise Ratio (SNR) means, the higher (strength) the value of the signal compared to Noise, the quality of the signal would be better over a longer distance.

• However, a certain minimum SNR is necessary for communication.


Channel Capacity, Shannon’s equation

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• Shannon Theory It establishes that given a noisy channel with

information capacity C and information transmitted at a rate R, then:

– if R<C, there exists a coding technique which allows the probability of error at the receiver to be made arbitrarily small. This means that theoretically, it is possible to transmit information without error up to a limit, C.

– If R>C, the probability of error at the receiver increases as the rate is increased. So no useful information can be transmitted beyond the channel capacity

• Shannon Capacity (equation)

C = B log2(1+ SNR) Dr. Talal Skaik 2016 IUG

Channel Capacity, Shannon’s equation

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• C – Channel Capacity and is the upper bound of the rate of information transmission per second or is the maximum number of bits that can be transmitted per second with probability of error arbitrarily close to zero.

• Not possible to transmit at a rate higher than this without incurring any error.

• Practical system operate at rates below the shannon rate.

• B and SNR demonstrate ultimate limitation on the rate of communication.


Example

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Find the Shannon channel capacity for a telephone channel with BT = 3400 Hz and SNR = 10000

C = 3400 log2 (1 + 10000)

= 3400 log10 (10001)/log102 = 45200 bps

Note that SNR = 10000 corresponds to

SNR (dB) = 10 log10(10001) = 40 dB


•Message from source – baseband signal because they are

lowpass in nature.

•Baseband signals are not always suitable for direct

transmission over the channel.

•Messages will need to be moved to the right channel

frequency band.

•Modulation: baseband signal is used to modify (i.e modulate),

some parameter of RF carrier signal.

•Carrier: Sinusoid of high frequency.

Modulation and Detection


In modulation, one of carrier sinusoidal parameter such as

amplitude, frequency, or phase is varied in proportion to the

baseband signal m(t).

Fig. 1.6 shows a baseband m(t) and the corresponding

Amplitude modulation (AM) and Frequency Modulation (FM)

waveforms after modulation.

Demodulation – Reversal process of modulation to

reconstruct the baseband signal.



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Figure 1.6 Modulation: (a) carrier; (b) modulating (base band) signal; (c) amplitude-modulated wave; (d) frequency-modulated wave.


Reason for modulation Ease of radiation/Transmission: radiating antenna is an order of a fraction or more of the wavelength of the driving signal.

Wavelength are usually high for reasonable antenna dimension for baseband signal.

E.g) power in a speech signal is concentrated in the range of 100-3000 Hz, wavelength is 100 to 3000 km which will result in impractically large antenna.

By modulating a high frequency carrier, the signal spectrum is translated to the neighborhood of the carrier frequency that corresponds to a much smaller wavelength.



Reason for modulation Avoid Interference If several signals (for example, all radio stations), each occupying the same frequency band, are transmitted simultaneously over the same transmission medium, they will all interfere. Difficult to separate or retrieve them at a receiver. One solution is to use modulation whereby each radio station is assigned a distinct carrier frequency.



Reason for modulation Each station transmits a modulated signal, thus shifting the signal spectrum to its allocated band, which is not occupied by any other station. When you tune a radio or television set to a particular station, you are selecting one of the many signals being received at that time. Since each station has a different assigned carrier frequency, the desired signal can be separated from the others by filtering.



Demodulation

Requires bandpass filter at the receiver to select the modulated signal at a predetermined frequency band specified by the transmission station or channel.

The carrier variation of amplitude, frequency or phase is converted back into baseband signal voltage.

Demodulator can downconvert the modulated (RF) signals back into baseband signals that represent original source message.



Radio Frequency Bands

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Classification Band Initials Frequency Range Characteristics

Extremely low ELF < 300 Hz

Ground wave

Infra low ILF 300 Hz - 3 kHz

Very low VLF 3 kHz - 30 kHz

Low LF 30 kHz - 300 kHz

Medium MF 300 kHz - 3 MHz Ground/Sky wave

High HF 3 MHz - 30 MHz Sky wave

Very high VHF 30 MHz - 300 MHz

Space wave

Ultra high UHF 300 MHz - 3 GHz

Super high SHF 3 GHz - 30 GHz

Extremely high EHF 30 GHz - 300 GHz

Tremendously high THF 300 GHz - 3000 GHz


Morse Code


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