Digital communication basics ppt

Introduction to Digital Communications Engineering and digital communication ppt free download and digital communication techniques ppt
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Published Date:21-07-2017
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Introduction to Digital Communications Engineering I Lectures No. 1 and 2 Dr. Aoife Moloney School of Electronics and Communications Dublin Institute of TechnologyLectures No. 1 and 2: Introduction to Digital Communications Engineering I Communications System In its simplest form a telecommunications system consists of a transmitter, a channel, a receiver and two transducers. Channel Receiver Transmitter Estimate Message of message and input and output transducer transducer DT008/2 Digital Communications Engineering I Slide: 19Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Transducer • Converts the input message into an electrical signal. Ex- amples of transducers include: – Microphone – converts sound to electrical signal – Camera – converts image to electrical signal • A transducer is also used to convert electrical signals to an output message (or approximation of the input mes- sage), e.g., sound, images etc. DT008/2 Digital Communications Engineering I Slide: 20Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Transmitter • Converts electrical signal to a form that is suitable for transmission through the transmission medium or chan- nel. • Generally matching of signal to channel is done bymod- ulation. • Modulationusestheinformation(messagesignal)tovary the amplitude, frequency or phase of a sinusoidal carrier, e.g. amplitude/frequency modulation AM/FM. • The transmitter also filters and amplifies the signal. DT008/2 Digital Communications Engineering I Slide: 21Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Receiver • Recovers the message contained in the received signal • Receiver demodulates the message signal • Receiver filters signal and suppresses noise DT008/2 Digital Communications Engineering I Slide: 22Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Communication Modes There are a few basic modes of communication: • Point-to-Point: where one user wishes to communicate with one other user, or with a small group of nominated users. Examples include the telephone network or email. Communication is normally two-way. • Broadcast: Where one sender communicates with all capable receivers who cannot respond. the communica- tion is therefore normally one-way. • Multicast: Onesendercommunicateswithanominated DT008/2 Digital Communications Engineering I Slide: 23Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Methods of Data Transmission There are a few basic methods of data transmission: • Simplex: Dataistransmittedinonedirectiononly. The receiver cannot communicate with the sender. • Duplex: Data transmission can take place in both di- rections simultaneously. • Half-Duplex: Data transmission can take place in both directions but not at the same time. DT008/2 Digital Communications Engineering I Slide: 25Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Time Constraints There are generally two sets of time restraints; real-time or time-lapse: Time Lapse Real Time Data In Data In Data Out Data Out • Real-Time: Real-time communication is instant and data must be sent and received simultaneously. An ex- ample of this is the telephone network or two-way radio DT008/2 Digital Communications Engineering I Slide: 26Lectures No. 1 and 2: Introduction to Digital Communications Engineering I communications. If a conversation is to be maintained theremustbeimmediateinteractionbetweenthetalkers. Delays will make the conversation difficult or impossible. • Time-Lapse: Data may be received at any time after having been sent. Examples include email, radio and TV broadcasts. The time of receipt is not important. Consider the case of radio and TV in more detail. It does not matter when a particular program is transmit- ted - time lapse is possible. However, once transmission begins it must be continuous and at a constant rate - DT008/2 Digital Communications Engineering I Slide: 27Lectures No. 1 and 2: Introduction to Digital Communications Engineering I during reception it appears as real-time. There are also cases where time delay is not critical un- less it is excessive e.g. downloading a file from a central server or from the Internet. A delay of a few seconds or even minutes is acceptable, but a delay of several hours is not acceptable. In addition, components of a message should be received in the sequence in which they are sent (otherwise speech will be garbled). This may require that packets of data DT008/2 Digital Communications Engineering I Slide: 28Lectures No. 1 and 2: Introduction to Digital Communications Engineering I must be re-sequenced at the receiver end. DT008/2 Digital Communications Engineering I Slide: 29Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Transmission Modes All transmission is analogue, in the sense that physical quan- tities (voltage, current, electromagnetic radiation) must vary in a smooth way. However, the representation of the under- lying signals may be either analogue or digital. Analogue Digital 8,9,7,4,2,3,… Digital Analogue DT008/2 Digital Communications Engineering I Slide: 30Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Analogue versus Digital Analogue In the past most signals were generated, transmitted and re- ceived in analogue form i.e. as a sine wave or as a more complex signal which could be made up from a series of sine waves. This was done because speech is an analogue signal and it was easier to implement analogue electronic circuitry than digital. In a very simple system it is still easier to build in analogue. However, analogue has the following disadvan- tages: DT008/2 Digital Communications Engineering I Slide: 31Lectures No. 1 and 2: Introduction to Digital Communications Engineering I • Itisinflexible, inthattomakeanychangestothesystem all of the changes have to be made in hardware. This becomesmoredifficultandexpensiveasthesystemgrows in size. • It is prone to noise and distortion. • Control and manipulation of signals is difficult. The mathematical treatment of analogue signals is relatively straightforward. An analogue signal is considered to have the form of a sine wave, or a combination of sine waves, the treatment of which is well established. DT008/2 Digital Communications Engineering I Slide: 32Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Digital Computersdealin‘1s’and‘0s’. Thereforecommunicationbe- tween computers is a matter of transferring digital sequences between machines. The next step is to convert speech and other analogue signals into a digital format to permit a com- binednetwork. Thesedaysdigitalelectroniccircuitryischeaper than analogue circuitry for the implementation of complex functions. Digital has the following advantages: • Normally large scale digital systems are software con- trolledsothatitispossibletomakechangestothesystem DT008/2 Digital Communications Engineering I Slide: 33Lectures No. 1 and 2: Introduction to Digital Communications Engineering I in software and remotely. • It is less prone to noise or distortion, a ‘1’ remains a ‘1’ and will not be mistaken for a ‘0’, unless there is an extreme level of distortion. • If noise or distortion does occur, methods exist to de- termine that this has happened, and if appropriate to correct the error which has occurred. • It is relatively easy to manipulate signals. The mathematical treatment is not as straight forward as that for analogue. DT008/2 Digital Communications Engineering I Slide: 34Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Baseband and Bandpass Bit representation can be: • Sent directly e.g. voltage pulses • Modulated in some way first e.g. amplitude/frequency modulation, AM/FM In the first instance we are dealing with baseband commu- nication, in the second case bandpass communication. DT008/2 Digital Communications Engineering I Slide: 35Lectures No. 1 and 2: Introduction to Digital Communications Engineering I Digital Communications Transceiver The components of a hypothetical digital communications transceiver (transmitter/receiver) are shown below. For ex- planation purposes, the transceiver includes all the elements commonlyfoundindigitaltransceivers,however,notalltrans- ceivers will contain all of these elements. DT008/2 Digital Communications Engineering I Slide: 36Lectures No. 1 and 2: Introduction to Digital Communications Engineering I CODEC MODEM Line Error coding/ ADC Source control pulse Multiple Sampling coder coder shaping Modulation accessing PCM encoder Anti- Encryption Quantisation aliasing filter Error DAC Decision Source control Multiple circuit accessing Reconstruction decoding decoding Equalisation PCM decoder Audio- Deciphering Matched Demodulation frequency filtering amplifier DT008/2 Digital Communications Engineering I Slide: 37 Deltiplexing MultiplexingLectures No. 1 and 2: Introduction to Digital Communications Engineering I CODECs At its simplest a transceiver CODEC (coder/decoder) con- sists of an ADC (analogue to digital converter) in the trans- mitter, which converts an analogue signal into digital pulses, and a DAC (digital to analogue converter) in the receiver, which converts these digital pulses back into an analogue sig- nal. ADCs will generally consist of a sampling circuit, a quantiser and a pulse code modulator. The sampling circuit provides DT008/2 Digital Communications Engineering I Slide: 38