Sunday, 21 August 2011

Modern Electronic Communication

Electronics is a subfield within the wider electrical engineering academic subject. An academic degree with a major in electronics engineering can be acquired from some universities, while other universitites use electrical engineering as the subject. The term electrical engineer is still used in the academic world to include electronic engineers. However, some people think the term 'electrical engineer' should be reserved for those having specialized in power and heavy current or high voltage engineering, while others believe that power is just one subset of electrical engineering (and indeed the term 'power engineering' is used in that industry) as well as 'electrical distribution engineering'. Again, in recent years there has been a growth of new separate-entry degree courses such as 'information engineering', 'systems engineering' and 'communication systems engineering', often followed by academic departments of similar name, which are typically not considered as subfields of electronics engineering but of electrical engineering.
Beginning in the 1980s, the term computer engineer was often used to refer to a subifeld of electronic or information engineers. However, Computer Engineering is now considered a subset of Electronics Engineering and computer science and the term is now becoming archaic.

Electronic engineering as a profession sprang from technological improvements in the telegraph industry in the late 19th century and the radio and the telephone industries in the early 20th century. People were attracted to radio by the technical fascination it inspired, first in receiving and then in transmitting. Many who went into broadcasting in the 1920s were only 'amateurs' in the period before World War I.[9]
The modern discipline of electronic engineering was to a large extent born out of telephone, radio, and television equipment development and the large amount of electronic systems development during World War II of radar, sonar, communication systems, and advanced munitions and weapon systems. In the interwar years, the subject was known as radio engineering and it was only in the late 1950s that the term electronic engineering started to emerge.
The electronic laboratories (Bell Labs in the United States for instance) created and subsidized by large corporations in the industries of radio, television, and telephone equipment began churning out a series of electronic advances. In 1948, came the transistor and in 1960, the integrated circuit to revolutionize the electronic industry.[11][12] In the UK, the subject of electronic engineering became distinct from electrical engineering as a university degree subject around 1960. Before this time, students of electronics and related subjects like radio and telecommunications had to enroll in the electrical engineering department of the university as no university had departments of electronics. Electrical engineering was the nearest subject with which electronic engineering could be aligned, although the similarities in subjects covered (except mathematics and electromagnetism) lasted only for the first year of the three-year course.



Saturday, 13 August 2011

Signal Analysis and Transmission

http://www.smtkmr.com/

Baseband bandwidth
A baseband bandwidth is equal to the highest frequency of a signal or system, or an upper bound on such frequencies,[1] for example the upper cut-off frequency of a passband filter. By contrast, passband bandwidth is the difference between a highest frequency and a nonzero lowest frequency.

 Baseband channel

A baseband channel or lowpass channel (or system, or network) is a communication channel that can transfer frequencies that are very near zero.[2] Examples are serial cables and local area networks (LANs), as opposed to passband channels such as radio frequency channels and passband filtered wires of the analog telephone network. Frequency division multiplexing (FDM) allows an analog telephone wire to carry a baseband telephone call, concurrently as one or several carrier-modulated telephone calls.

 Digital baseband transmission

Digital baseband transmission, also known as line coding,[3] aims at transferring a digital bit stream over base-band channel, typically an unfiltered wire, as opposed to passband transmission, also known as carrier-modulated transmission.[4] Passband transmission makes communication possible over a bandpass filtered channel, such as the telephone network local-loop or a band-limited wireless channel.
An unfiltered wire is intrinsically a low-pass transmission channel, while a line code is intrinsically a pulse wave signal that occupies a frequency spectrum of infinite bandwidth. According to the Nyquist theorem, error-free detection of the line code requires a channel bandwidth of at least the Nyquist rate, which is half the line code pulse rate.

 Baseband transmission in Ethernet

The word "BASE" in Ethernet physical layer standards, for example 10BASE5, 100BASE-T and 1000BASE-SX, implies baseband digital transmission, i.e. that a line code and an unfiltered wire are used.
This is as opposed to 10PASS-TS Ethernet, where "PASS" implies passband transmission. Passband digital transmission requires a digital modulation scheme, often provided by modem equipment. In the 10PASS-TS case the VDSL standard is utilized, which is based on the Discrete multi-tone modulation (DMT) scheme. Other examples of passband network access technologies are wireless networks and cable modems.

 Baseband signal

A baseband signal or lowpass signal is a signal that can include frequencies that are very near zero, by comparison with its highest frequency (for example, a sound waveform can be considered as a baseband signal, whereas a radio signal or any other modulated signal is not).[5]
A signal "at baseband" is usually considered to include frequencies from near 0 Hz up to the highest frequency in the signal with significant power.
In general, signals can be described as including a whole range of different frequencies added together. In telecommunications in particular, it is often the case that those parts of the signal which are at low frequencies are 'copied' up to higher frequencies for transmission purposes, since there are few communications media that will pass low frequencies without distortion. Then, the original, low frequency components are referred to as the baseband signal. Typically, the new, high-frequency copy is referred to as the 'RF' (radio-frequency) signal. A baseband signal is a low frequency signal which when modulated is transmitted on various channels.
The concept of baseband signals is most often applied to real-valued signals, and systems that handle real-valued signals. Fourier analysis of such signals includes a negative-frequency band, but the negative-frequency information is just a mirror of the positive-frequency information, not new information. For complex-valued signals, on the other hand, the negative frequencies carry new information. In that case, the full two-sided bandwidth is generally quoted, rather than just the half measured from zero; the concept of baseband can be applied by treating the real and imaginary parts of the complex-valued signal as two different real signals.

 Equivalent baseband signal

An equivalent baseband signal or equivalent lowpass signal is – in analog and digital modulation methods with constant carrier frequency (for example ASK, PSK and QAM, but not FSK) – a complex valued representation of the modulated physical signal (the so called passband signal or RF signal). The equivalent baseband signal is Z(t)=I(t)+jQ(t)\, where I(t) is the inphase signal, Q(t) the quadrature phase signal, and j the imaginary unit. In a digital modulation method, the I(t) and Q(t) signals of each modulation symbol are evident from the constellation diagram. The frequency spectrum of this signal includes negative as well as positive frequencies. The physical passband signal corresponds to
I(t)\cos(\omega t) - Q(t)\sin(\omega t) = \mathrm{Re}\{Z(t)e^{j\omega t}\}\, where ω is the carrier angular frequency in rad/s.
In an equivalent baseband model of a communication system, the modulated signal is replaced by a complex valued equivalent baseband signal with carrier frequency of 0 hertz, and the RF channel is replaced by an equivalent baseband channel model where the frequency response is transferred to baseband frequencies.

Modulation

A signal at baseband is often used to modulate a higher frequency carrier wave in order that it may be transmitted via radio. Modulation results in shifting the signal up to much higher frequencies (radio frequencies, or RF) than it originally spanned. A key consequence of the usual double-sideband amplitude modulation (AM) is that, usually, the range of frequencies the signal spans (its spectral bandwidth) is doubled. Thus, the RF bandwidth of a signal (measured from the lowest frequency as opposed to 0 Hz) is usually twice its baseband bandwidth. Steps may be taken to reduce this effect, such as single-sideband modulation; the highest frequency of such signals greatly exceeds the baseband bandwidth.
Some signals can be treated as baseband or not, depending on the situation. For example, a switched analog connection in the telephone network has energy below 300 Hz and above 3400 Hz removed by bandpass filtering; since the signal has no energy very close to zero frequency, it may not be considered a baseband signal, but in the telephone systems frequency-division multiplexing hierarchy, it is usually treated as a baseband signal, by comparison with the modulated signals used for long-distance transmission. The 300 Hz lower band edge in this case is treated as "near zero", being a small fraction of the upper band edge.
The figure shows what happens with AM modulation

Wednesday, 3 August 2011

power line communiation

Power line communication or power line carrier (PLC), also known as Power line Digital Subscriber Line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or Broadband over Power Lines (BPL) are systems for carrying data on a conductor also used for electric power transmission.
Electrical power is transmitted over high voltage transmission lines, distributed over medium voltage, and used inside buildings at lower voltages. Powerline communications can be applied at each stage. Most PLC technologies limit themselves to one set of wires (for example, premises wiring), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically the transformer prevents propagating the signal, which requires multiple technologies  to be used to form very large networks

Basics
Power line communications systems operate by impressing a modulated carrier signal on the wiring system. Different types of powerline communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power distribution system was originally intended for transmission of AC power at typical frequencies of 50 or 60 Hz, power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications.
Data rates and distance limits vary widely over many power line communication standards. Low-frequency (about 100-200 kHz) carriers impressed on high-voltage transmission lines may carry one or two analog voice circuits, or telemetry and control circuits with an equivalent data rate of a few hundred bits per second; however, these circuits may be many miles long. Higher data rates generally imply shorter ranges; a local area network operating at millions of bits per second may only cover one floor of an office building, but eliminates installation of dedicated network cabling.

Utility
Power line carrier systems have long been a favorite at many utilities because it allows them to reliably move data over an infrastructure that they control. Many technologies are capable of performing multiple applications. For example, a communication system bought initially for automatic meter reading can sometimes also be used for load control or for demand response applications.
PLC is one of the technologies used for automatic meter reading. Both one-way and two-way systems have been successfully used for decades. Interest in this application has grown substantially in recent history—not so much because there is an interest in automating a manual process, but because there is an interest in obtaining fresh data from all metered points in order to better control and operate the system. PLC is one of the technologies being used in Advanced Metering Infrastructure (AMI) systems.
In a one-way (inbound only) system, readings "bubble up" from end devices (such as meters), through the communication infrastructure, to a "master station" which publishes the readings. A one-way system might be lower-cost than a two-way system, but also is difficult to reconfigure should the operating environment change.
In a two-way system (supporting both outbound and inbound), commands can be broadcast out from the master station to end devices (meters) -- allowing for reconfiguration of the network, or to obtain readings, or to convey messages, etc. The device at the end of the network may then respond (inbound) with a message that carries the desired value. Outbound messages injected at a utility substation will propagate to all points downstream. This type of broadcast allows the communication system to simultaneously reach many thousands of devices—all of which are known to have power, and have been previously identified as candidates for load shed. PLC also may be a component of a smart power grid.

Tuesday, 2 August 2011

Digital Signal

A digital signal is a physical signal that is a representation of a sequence of discrete values (a quantified discrete-time signal), for example of arbitrary bit stream, or of a digitized (sampled and analog-to-digital converted) analog signal. The term digital signal can refer to
  1. a continuous-time waveform signal used in any form of digital communication.
  2. a pulse train signal that switches between a discrete number of voltage levels or levels of light intensity, also known as a a line coded signal, for example a signal found in digital electronics or in serial communications using digital baseband transmissionin, or a pulse code modulation (PCM) representation of a digitized analog signal.
A signal that is generated by means of a digital modulation method (digital passband transmission), produced by a modem, is in the first case considered as a digital signal, and in the second case as converted to an analog signal

WAVEFORM OF DIGITAL SIGNAL

In computer architecture and other digital systems, a waveform that switches between two voltage levels representing the two states of a Boolean value (0 and 1) is referred to as a digital signal, even though it is an analog voltage waveform, since it is interpreted in terms of only two levels.
The clock signal is a special digital signal that is used to synchronize digital circuits. The image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the rising edge or the falling edge.
The given diagram is an example of the practical pulse and therefore we have introduced two new terms that are:
  • Rising edge: the transition from a low voltage (level 1 in the diagram) to a high voltage (level 2).
  • Falling edge: the transition from a high voltage to a low one.
Although in a highly simplified and idealised model of a digital circuit we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can instantly change voltage levels. This means that during a short, finite transition time the output may not properly reflect the input, and indeed may not correspond to either a logically high or low voltage.

passband signals

In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting close to zero. Baseband can often be considered as a synonym to lowpass or non-modulated, and antonym to passband, bandpass, carrier-modulated or radio frequency (RF) signal.

Baseband bandwidth
A baseband bandwidth is equal to the highest frequency of a signal or system, or an upper bound on such frequencies,[1] for example the upper cut-off frequency of a passband filter. By contrast, passband bandwidth is the difference between a highest frequency and a nonzero lowest frequency.

 Baseband channel

A baseband channel or lowpass channel (or system, or network) is a communication channel that can transfer frequencies that are very near zero.[2] Examples are serial cables and local area networks (LANs), as opposed to passband channels such as radio frequency channels and passband filtered wires of the analog telephone network. Frequency division multiplexing (FDM) allows an analog telephone wire to carry a baseband telephone call, concurrently as one or several carrier-modulated telephone calls.

 Digital baseband transmission

Digital baseband transmission, also known as line coding,[3] aims at transferring a digital bit stream over base-band channel, typically an unfiltered wire, as opposed to passband transmission, also known as carrier-modulated transmission.[4] Passband transmission makes communication possible over a bandpass filtered channel, such as the telephone network local-loop or a band-limited wireless channel.
An unfiltered wire is intrinsically a low-pass transmission channel, while a line code is intrinsically a pulse wave signal that occupies a frequency spectrum of infinite bandwidth. According to the Nyquist theorem, error-free detection of the line code requires a channel bandwidth of at least the Nyquist rate, which is half the line code pulse rate.

 Baseband transmission in Ethernet

The word "BASE" in Ethernet physical layer standards, for example 10BASE5, 100BASE-T and 1000BASE-SX, implies baseband digital transmission, i.e. that a line code and an unfiltered wire are used.
This is as opposed to 10PASS-TS Ethernet, where "PASS" implies passband transmission. Passband digital transmission requires a digital modulation scheme, often provided by modem equipment. In the 10PASS-TS case the VDSL standard is utilized, which is based on the Discrete multi-tone modulation (DMT) scheme. Other examples of passband network access technologies are wireless networks and cable modems.

 Baseband signal

A baseband signal or lowpass signal is a signal that can include frequencies that are very near zero, by comparison with its highest frequency (for example, a sound waveform can be considered as a baseband signal, whereas a radio signal or any other modulated signal is not).[5]
A signal "at baseband" is usually considered to include frequencies from near 0 Hz up to the highest frequency in the signal with significant power.
In general, signals can be described as including a whole range of different frequencies added together. In telecommunications in particular, it is often the case that those parts of the signal which are at low frequencies are 'copied' up to higher frequencies for transmission purposes, since there are few communications media that will pass low frequencies without distortion. Then, the original, low frequency components are referred to as the baseband signal. Typically, the new, high-frequency copy is referred to as the 'RF' (radio-frequency) signal. A baseband signal is a low frequency signal which when modulated is transmitted on various channels.
The concept of baseband signals is most often applied to real-valued signals, and systems that handle real-valued signals. Fourier analysis of such signals includes a negative-frequency band, but the negative-frequency information is just a mirror of the positive-frequency information, not new information. For complex-valued signals, on the other hand, the negative frequencies carry new information. In that case, the full two-sided bandwidth is generally quoted, rather than just the half measured from zero; the concept of baseband can be applied by treating the real and imaginary parts of the complex-valued signal as two different real signals.

 Equivalent baseband signal

An equivalent baseband signal or equivalent lowpass signal is – in analog and digital modulation methods with constant carrier frequency (for example ASK, PSK and QAM, but not FSK) – a complex valued representation of the modulated physical signal (the so called passband signal or RF signal). The equivalent baseband signal is Z(t)=I(t)+jQ(t)\, where I(t) is the inphase signal, Q(t) the quadrature phase signal, and j the imaginary unit. In a digital modulation method, the I(t) and Q(t) signals of each modulation symbol are evident from the constellation diagram. The frequency spectrum of this signal includes negative as well as positive frequencies. The physical passband signal corresponds to
I(t)\cos(\omega t) - Q(t)\sin(\omega t) = \mathrm{Re}\{Z(t)e^{j\omega t}\}\, where ω is the carrier angular frequency in rad/s.
In an equivalent baseband model of a communication system, the modulated signal is replaced by a complex valued equivalent baseband signal with carrier frequency of 0 hertz, and the RF channel is replaced by an equivalent baseband channel model where the frequency response is transferred to baseband frequencies

Baseband Signals

Baseband signal
A baseband signal or lowpass signal is a signal that can include frequencies that are very near zero, by comparison with its highest frequency (for example, a sound waveform can be considered as a baseband signal, whereas a radio signal or any other modulated signal is not).[5]
A signal "at baseband" is usually considered to include frequencies from near 0 Hz up to the highest frequency in the signal with significant power.
In general, signals can be described as including a whole range of different frequencies added together. In telecommunications in particular, it is often the case that those parts of the signal which are at low frequencies are 'copied' up to higher frequencies for transmission purposes, since there are few communications media that will pass low frequencies without distortion. Then, the original, low frequency components are referred to as the baseband signal. Typically, the new, high-frequency copy is referred to as the 'RF' (radio-frequency) signal. A baseband signal is a low frequency signal which when modulated is transmitted on various channels.
The concept of baseband signals is most often applied to real-valued signals, and systems that handle real-valued signals. Fourier analysis of such signals includes a negative-frequency band, but the negative-frequency information is just a mirror of the positive-frequency information, not new information. For complex-valued signals, on the other hand, the negative frequencies carry new information. In that case, the full two-sided bandwidth is generally quoted, rather than just the half measured from zero; the concept of baseband can be applied by treating the real and imaginary parts of the complex-valued signal as two different real signals.

 Equivalent baseband signal

An equivalent baseband signal or equivalent lowpass signal is – in analog and digital modulation methods with constant carrier frequency (for example ASK, PSK and QAM, but not FSK) – a complex valued representation of the modulated physical signal (the so called passband signal or RF signal). The equivalent baseband signal is Z(t)=I(t)+jQ(t)\, where I(t) is the inphase signal, Q(t) the quadrature phase signal, and j the imaginary unit. In a digital modulation method, the I(t) and Q(t) signals of each modulation symbol are evident from the constellation diagram. The frequency spectrum of this signal includes negative as well as positive frequencies. The physical passband signal corresponds to
I(t)\cos(\omega t) - Q(t)\sin(\omega t) = \mathrm{Re}\{Z(t)e^{j\omega t}\}\, where ω is the carrier angular frequency in rad/s.
In an equivalent baseband model of a communication system, the modulated signal is replaced by a complex valued equivalent baseband signal with carrier frequency of 0 hertz, and the RF channel is replaced by an equivalent baseband channel model where the frequency response is transferred to baseband frequencies.

communiucation channels

In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel. A channel is used to convey an information signal, for example a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second
In information theory, a channel refers to a theoretical channel model with certain error characteristics. In this more general view, a storage device is also a kind of channel, which can be sent to (written) and received from (read).

Examples
A channel can take many forms. Examples of communications channels include:
  1. A connection between initiating and terminating nodes of a circuit.
  2. A single path provided by a transmission medium via either
  3. A path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths.
    • A storage which can communicate a message over time as well as space
    • The portion of a storage medium, such as a track or band, that is accessible to a given reading or writing station or head.
    • A buffer from which messages can be 'put' and 'got'. See Actor model and process calculi for discussion on the use of channels.
  4. In a communications system, the physical or logical link that connects a data source to a data sink.
  5. A specific radio frequency, pair or band of frequencies, usually named with a letter, number, or codeword, and often allocated by international agreement.
    Examples:
    • Marine VHF radio uses some 88 channels in the VHF band for two-way FM voice communication. Channel 16, for example, is 156.800 MHz. In the US, seven additional channels, WX1 - WX7, are allocated for weather broadcasts.
    • Television channels such as North American TV Channel 2 = 55.25 MHz, Channel 13 = 211.25 MHz. Each channel is 6 MHz wide. Besides these "physical channels", television also has "virtual channels".
    • Wi-Fi consists of unlicensed channels 1-13 from 2412 MHz to 2484 MHz in 5 MHz steps.
    • The radio channel between an amateur radio repeater and a ham uses two bands often 600 kHz (0.6 MHz) apart. For example, a repeater that transmits on 146.94 MHz typically listens for a ham transmitting on 146.34 MHz.
  6. A room in the Internet Relay Chat (IRC) network, in which participants can communicate with each other.
All of these communications channels share the property that they transfer information. The information is carried through the channel by a signal.

 Channel models

A channel can be modelled physically by trying to calculate the physical processes which modify the transmitted signal. For example in wireless communications the channel can be modelled by calculating the reflection off every object in the environment. A sequence of random numbers might also be added in to simulate external interference and/or electronic noise in the receiver.
Statistically a communication channel is usually modelled as a triple consisting of an input alphabet, an output alphabet, and for each pair (i, o) of input and output elements a transition probability p(i, o). Semantically, the transition probability is the probability that the symbol o is received given that i was transmitted over the channel.
Statistical and physical modelling can be combined. For example in wireless communications the channel is often modelled by a random attenuation (known as fading) of the transmitted signal, followed by additive noise. The attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model captures external interference and/or electronic noise in the receiver. If the attenuation term is complex it also describes the relative time a signal takes to get through the channel. The statistics of the random attenuation are decided by previous measurements or physical simulations.
Channel models may be continuous channel models in that there is no limit to how precisely their values may be defined.
Communication channels are also studied in a discrete-alphabet setting. This corresponds to abstracting a real world communication system in which the analog->digital and digital->analog blocks are out of the control of the designer. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. In information theory, it is common to start with memoryless channels in which the output probability distribution only depends on the current channel input.
A channel model may either be digital (quantified, e.g. binary) or analog.

Digital channel models

In a digital channel model, the transmitted message is modelled as a digital signal at a certain protocol layer. Underlying protocol layers, such as the physical layer transmission technique, is replaced by a simplified model. The model may reflect channel performance measures such as bit rate, bit errors, latency/delay, delay jitter, etc. Examples of digital channel models are:

 Analog channel models

In an analog channel model, the transmitted message is modelled as an analog signal. The model can be a linear or non-linear, time-continuous or time-discrete (sampled), memoryless or dynamic (resulting in burst errors), time-invariant or time-variant (also resulting in burst errors), baseband, passband (RF signal model), real-valued or complex-valued signal model. The model may reflect the following channel impairments:

 Types of communications channels

Communication System

In telecommunication, a communications system is a collection of individual communications networks, transmission systems, relay stations, tributary stations, and data terminal equipment (DTE) usually capable of interconnection and interoperation to form an integrated whole. The components of a communications system serve a common purpose, are technically compatible, use common procedures, respond to controls, and operate in unison. Telecommunications is a method of communication (e.g., for sports broadcasting, mass media, journalism etc.).
A communications subsystem is a functional unit or operational assembly that is smaller than the larger assembly under consideration. Examples of communications subsystems in the Defense Communications System (DCS) are (a) a satellite link with one Earth terminal in CONUS and one in Europe, (b) the interconnect facilities at each Earth terminal of the satellite link, and (c) an optical fiber cable with its driver and receiver in either of the interconnect facilities. Communication subsystem (b) basically consists of a receiver, frequency translator and a transmitter. It also contains transponders and other transponders in it and communication satellite communication system receives signals from the antenna subsystem