Introduction to digital communications

Prepared By Aya ElshiwiSupervisor

Dr.Noha Haikal

Outlines

1.1 Elements of a digital communication system

1.2 Communication channels and their characteristics

1.3 Mathematical models for communication channels

1.1 Elements of a digital communication system

1-Information source and input transducer

-The source output may be either an analog signal, such as an audio or video signal, or a digital signal .

2-Source encoder

- Messages produced by the source are converted into a sequence of binary digits because we seek an efficient representation of the source data that results in little or no redundancy .

-The process of efficiently converting the signal either an analog or

digital into binary digits is called source encoding or data compression.

3-Channel encoder

- The purpose of the channel encoder is to introduce in a controlled manner , some redundancy in the binary information sequence .

- Can be used at the receiver to overcome the effects of noise and interference encountered in the transmission of the signal through the channel .

- The added redundancy serves to increase the reliability of the received data and improves the fidelity of the received signal.

- In effect the redundancy in the information sequence aids the receiver in decoding the desired information sequence.

4-Digital modulator

-Serves as the interface to the communication channel.

-The primary purpose of the digital modulator is to map the binary information sequence into signal waveforms . so that the signal can be transmitted through the medium.

5-channel

-The communication channel is the physical medium that is used to send the signal from the transmitter to the receiver.

-There are two ways of transmission: -Wired transmission( wire lines, optical fibers cables ) -wireless transmission(the channel may be atmosphere or free space)

-Whatever the physical medium of transmission is , the transmitted signal is corrupted in a random manner by a variety of possible mechanisms. Such as:

-additive thermal noise generated by electronic devices -man-made noise . -atmosphere noise(thunderstorms).

6-Digital demodulator

-Processes the channel corrupted transmitted waveform and reduces it to a sequence of numbers that represent estimates of the transmitted data symbols .

7-Channel decoder- Attempts to reconstruct the original information sequence from

knowledge of the code used by the channel encoder and the redundancy contained in the received data

-Probability of error in the output is a function of [items that affect the performance]: - code characteristics

- the type of waveform used to transmit the information over the channel

- the transmitter power - the characteristics of the channel - method of demodulation and decoding.

8-Source decoder

- It accepts the output sequence from the channel decoder and from knowledge of source encoding method used ,attempts to reconstruct the original signal from the source.

- Due to the channel decoding errors and possible distortion by the source encoder or decoder , the signal at the output of the source decoder is approximation to the original source output.

- The reconstructed signal is a measure of the distortion introduced by the communication system [ efficiency ].

1.2-communication channels and their characteristics• The communication channel provides the connection between the transmitter and the

receiver.

• One common problem in signal transmission through any channel is additive noise which can be generated:

1-internally:by components such as resistors ,solid-state devices used to implement the communication system sometimes called “thermal noise”

2-Externally:such as interference from other users of the channel.

• Other types of signal degradations are: - signal attenuation -amplitude and phase distortion - multipath distortion.

• The effects of noise may be minimized by increasing the power in the transmitted signal (which is limited by: channel bandwidth , electronic components used to implement the transmitter and the receiver ).

• Communication channels : -Wireline channels (Twisted pair-coaxial cable) - Fiber optic channels - Wireless electromagnetic channels - Under water acoustic channels - Storage channels

1-Wireline channels:-Telephone network makes extensive use of wire lines for voice signal

transmission as well as data and video transmission. Twisted pair wire lines coaxial cables

Feature Twisted pair Coaxial cable

Bandwidth

Noise

1khz-100khz

Signal transmitted through it are distorted in both Amplitude and phase and further corrupted by additive noise.

Prone to cross talk interference from physical adjacent channels

1MHz-100Mhz

Signal transmitted through it are distorted in both Amplitude and phase and further corrupted by additive noise.

Frequency range for guided wire channels

2-Fiber optic channels

-Provide a larger bandwidth than the coaxial cables channels.

-The transmitter [modulator] in the optical fiber communication system is a light source either a light emitting diode or a laser.

-Information is transmitted by varying [modulating] the intensity of the light source with the message signal the light propagates through the fiber as a light wave and is amplified periodically along the transmission path to compensate for signal attenuation.

-At the receiver the light intensity is detected by a photodiode whose output is an electrical signal that varies in direct proportion to the power of light impinging on the photodiode.

-Source of Noise in optical fibers channels are photodiodes and electronic amplifiers.

3-Wireless electromagnetic channels

-In this communication system electromagnetic energy is coupled to the propagation medium by an antenna which serves as radiator .

-The physical size and the configuration of the antenna depend primarily of the frequency of the operation. To obtain an efficient radiation the antenna must be =1/10 of the wavelength.

-The mode of propagation of electromagnetic waves in the atmosphere and in free space maybe subdivided into three categories:

-ground wave propagation. -sky wave propagation - Line of sight [los]

Frequency range for wireless electromagnetic channels.

a- Ground wave propagation :

- Is the dominant mode of propagation for frequencies in the MF[Medium Frequency] band( 0.3-3 MHZ) this is the frequency band used for AM broadcasting and maritime radio broadcasting.

- Disturbances : -Atmospheric noise - man-made noise -thermal noise from the electronic devices

b-Sky wave propagation

-Results from transmitted signals being reflected from the ionosphere.-During the daytime hours the heating of the lower atmosphere by the sun causes the

formation of the lower layer at altitude below 120km . These lower layers especially D layer serves to absorb frequencies below 2MHZ. That severely limiting the sky-wave propagation of AM radio broadcast .

-During the night hours the electron density drops sharply and the frequency absorption that occurs during the daytime is significantly reduced , so powerful AM radio broadcast can propagate over large distances via sky wave over the f-layer of the ionosphere which ranges from 140-400km above the surface of the earth.

-Frequently occurring problems with electromagnetic wave propagation via sky wave in the HF Frequency range is the signal multi path. It happens when the transmitted signal arrives at the destination via multiple propagation paths with different delays.

-ceases to exist in frequencies above 30MHZ which is the end of the HF band .

c-LOS[Line of sight] propagation

-Frequencies above 30MHZ propagate through it.

-causes a little loss and make satellite and extraterrestrial communications possible.

-The dominant mode for frequencies in the VHF band and higher.

4-Under water acoustic channels

-Electromagnetic waves do not propagate over long distances under water except in very low frequencies , but the transmission of the signal in such low frequency is very expensive because of the large and powerful transmitter required.

-Is characterized as multi-path channel because of signal reflection from the surface and the bottom. This cause signal fading.

- Noise also caused by : - shrimp ,fish and various mammals. -Near harbors there are man-made noise.

5-Storage channels

- Such as Magnetic tape –Magnetic disks-Optical disks-Compact disks [are characterized as communication channels] .

-Process of storing data in any storage device is equivalent to transmitting a signal over a telephone or radio channel . And the process of reading the data is equivalent to the functions performed by the receiver to recover the transmitted signal.

-Additive noise is caused by electronic components .

1.3-Mathematical Models of communication channels

- In the design of communication systems for transmitting information throughphysical channels, we find it convenient to construct mathematical models thatreflect the most important characteristics of the transmission medium.

- Then, the mathematical model for the channel is used in the design of the channel encoder and modulator at the transmitter and the demodulator and channel decoder at the receiver.

-Models: 1-The additive noise channel 2-The linear filter channel 3-The linear time variant filter channel

1-The additive noise channel

-The simplest mathematical model.

-Signal s(t) is corrupted by noise n(t) from any source .

- If the noise introduced by electrical devices it may characterized as thermal noise, this type of noise is statistically called “Gaussian noise” process and the model is called “additive Gaussian noise channel”.

- When the signal undergoes attenuation in transmission through the channel The received signal is r (t) = αs(t) + n(t)

where α is the attenuation factor.

2-the linear filter channel

- In some channels such as wire line telephone channels filters are used to ensure that the transmitted signals do not exceed a specific bandwidth to prevent the interference.

- if the channel input is the signal s(t), the channel output is the signal

where c(t) is the impulse response of the linear filter and denotes convolution.

3-The linear time variant filter channel

-Physical channels such as underwater acoustic channels and ionospheric radio channels that result in time-variant multipath propagation of the transmitted signal may be characterized mathematically as time-variant linear filters.

- Such linear filters are characterized by a time-variant channel impulse response c(τ ; t), where c(τ ; t) is the response of the channel at time t due to an impulse applied at time t − τ . Thus, τ represents the “age” (elapsed-time) variable

-Linear time variant with additive noise

Questions ?!