Signal Encodin Techniques Review/Recap - Lecture 20

SIGNAL ENCODIN TECHNIQUESReview/RecapLecture 20OverviewDifferential EncodingModem FunctionsDigital Vs Analog (Advantages of Digital Over Analog)Binary Amplitude Shift Keying (Limitations)NRZ-LQPSK Vs.OQPSKQAM2Differential EncodingQ:- What is differential encoding?3Way of looking at techniquesDataMediumDigitalAnalogDigitalAnalogNRZManchesterDifferential ManchesterPulse Coded Modulation(digitized voice)ASKFSKPSKmodemsAM/FM radioTelevisionEncoding TechniquesEncoding schemes deal with how to transport bits over the physical mediaWe must deal and manage many issues to well represent and interpret the bits: Timing of bits (start and end, duration, Signal levels) Clocking ( Synchronizing transmitter and receive, External clock, Sync mechanism based on signal)Error detection, Signal interference and Noise immunityCost and complexity5Encoding TechniquesEncoding techniques depend on the type of data to transmit and the medium being used:Digital data on digital signal (our focus in this brief description...)Analog data on digital signalDigital data on analog signalAnalog data on analog signal6Digital Data, Digital SignalDigital signal:Discrete, discontinuous voltage pulsesEach pulse is a signal elementBinary data encoded into signal elements7A Simple Encoding Scheme0 is Vo (some voltage)1 is V1Example : let us encode 100111018Clock VoV1Problem?Let us now try to encode two bytes that come one 2 seconds after the other: 10011101 and 00001101What is the problem with this encoding scheme?What is the solution?9Bipolar Encoding (RZ Signal)1 is 0 is Example : let us encode 011000101000Clock +0.85V-0.85VHightV + NoV –» 1bitLowV + NoV –» 0bitProblem?The problem with this encoding scheme is that we have to return to 0 each time the transmission is done which generates some complexity and cost in implementing and managing this techniqueWe should look for a better technique We have 2 alternatives:Encoding of 0110001011Clock Alternative 1: Non-Return to ZeroTwo different voltages for 0 and 1 bitsVoltage constant during bit intervalno transition i.e. no return to zero voltagee.g. Absence of voltage for zero, constant positive voltage for oneMore often, negative voltage for one value and positive for the otherThis is NRZ also known as Manchester Encoding...12Alternative 1: NRZ-Manchester Encoding1 is 0 is Example : let us encode 011000101300Clock +0.85V-0.85VHightV + LowV –» 1bitLowV + HighV –» 0bitAlternative 2: Differential EncodingData represented by changes trends rather than levels of voltageMore reliable detection of transition rather than levelIn complex transmission layouts it is easy to lose sense of polarity14Differential Manchester Encoding0 is 1 is A transition in the middle of the bit is required anyway Example : let us encode 1001110115Clock 00(Presence of transition)(Abscence of transition)16101011001UnipolarNRZNRZ-Inverted(DifferentialEncoding)BipolarEncodingManchesterEncodingDifferentialManchesterEncodingPolar NRZDifferential EncodingData represented by changes rather than levelsMore reliable detection of transition rather than levelIn complex transmission layouts it is easy to lose sense of polarity17Pro and ConsIf you have a DC component (i.e. NRZ), you must preserve it and therefore your transmission channel must have a DC (galvanic) path. Try that with a satellite! That's a pretty long cable out into space. There are work-arounds; DC restorers and such for NRZ signals, but there must still be limits placed on the amount of zeros that can be transmitted at-a-time. Coding schemes that have a clock embedded into the signal (i.e. manchester) have no DC component but require a higher bandwidth channel because both the signal and the clock take up bandwidth. These signals can be passed through transformers, AC coupled amplifiers, and other non-galvanic paths. This is important if you need isolation (i.e. for safety) or if the transmitter and receiver are at locations where different power sources exist (common mode rejection). AC coupled amplifiers are almost always cheaper than DC coupled amplifiers. 18Pros and ConsEmbedded clocks provide very good sync between Tx and Rx, because you can extract the exact clock you need for the signal. NRZ does not have an embedded clock so it is difficult to regenerate a clock at the Rx end. Miller coding also contains an embedded clock, but uses only half the bandwidth as manchester coding. In general the differential types offer better noise immunity than the non-differential types. The drawback is that you need differential Tx and Rx amplifiers, which are generally more expensive. 19Differential EncodingQ:- What is differential encoding?Ans:- In differential encoding, the signal is decoded by comparing the polarity of adjacent signal elements rather than determining the absolute value of a signal element.2021Modem FunctionalityQ:- What function does a modem perform?22Digital to AnalogMany carrier facilities are analogMany transmission media are also analog (microwave, radio)We can carry digital values over analog signalsWe must ‘encode’ the digital data over the digital signalThis is what is done with modemsModemdigitalAnalogModem = Modulation/DemodulationModem Hardware for Modulation and Demodulation A mechanism that accepts a sequence of data bits and applies modulation to a carrier wave according to the bits is called a modulatorA mechanism that accepts a modulated carrier wave and recreates the sequence of data bits that was used to modulate the carrier is called a demodulatorTransmission of data requires a modulator at one end of the transmission medium and a demodulator at the otherMost communication systems are full duplexwhich means each location needs both a modulator to send data and a demodulator to receive dataModem Hardware for Modulation and Demodulation Users would like to keep cost low and make the pair of devices easy to install and operateManufacturers combine modulation and demodulation mechanisms into a single device called a modem (modulator and demodulator)Figure 10.9 illustrates how a pair of modems use a 4-wire connection to communicateModems are designed to provide communication over long distancesModem Hardware for Modulation and Demodulation Optical and Radio Frequency Modems Modems are also used with other mediaincluding Radio Frequency (RF) transmission and optical fibersA pair of RF modems can be used to send data via radioA pair of optical modems can be used to send data across a pair of optical fibersModems can use entirely different media, but the principle remains the same:at the sending end, a modem modulates a carrierat the receiving end, data is extracted from the modulated carrierDialup Modems A dialup modem uses an audio toneas with conventional modems, the carrier is modulated at the sending end and demodulated at the receiving endA dialup modem uses data to modulate an audible carrierwhich is transmitted to the phone systemThe chief difference between dialup and conventional modems arises from the lower bandwidth of audible dialup modemsInterior of a modern telephone system used today is digitalThe phone system digitizes the incoming audioTransports a digital form internallyConverts the digitized version back to analog audio for deliveryThe receiving modem demodulates the analog carrierExtracts the original digital dataDialup Modems Figure illustrates the ironic use of analog and digital signals by dialup modemsa dialup modem is usually embedded in a computerTerm internal modem to denote an embedded deviceTerm external modem to denote a separate physical deviceQAM Applied to Dialup QAM is also used with dialup modems as a way to maximize the rate at which data can be sentFigure 10.11 shows the bandwidth available on a dialup connectionMost telephone connections transfer frequencies between 300 and 3000 HzA given connection may not handle the extremes wellThus, to guarantee better reproduction and lower noise, dialup modems use frequencies between 600 and 3000 HzIt means the available bandwidth is 2400 HzA QAM scheme can increase the data rate dramaticallyQAM Applied to Dialup V.32 and V.32bis Dialup ModemsConsider the V.32 and V.32bis standardsFigure on next page illustrates the QAM constellation for a V.32 modem that uses 32 combinations of ASK and PSK to achieve a data rate of 9600 bps in each direction  a V.32bis modem uses 128 combinations of ASK and PSKto achieve a data rate of 14,400 bps in each directionFigure 10.13 illustrates the constellationSophisticated signal analysis is needed to detect the minor change that occurs from a point in the constellation to a neighboring pointQAM for V.32 Dialup ModemsConsider the V.32 and V.32bis standardsFigure on next page illustrates the QAM constellation for a V.32 modem that uses 32 combinations of ASK and PSK to achieve a data rate of 9600 bps in each direction  a V.32bis modem uses 128 combinations of ASK and PSKto achieve a data rate of 14,400 bps in each directionQAM for V.32bis Dialup ModemsFigure illustrates the constellationSophisticated signal analysis is needed to detect the minor change that occurs from a point in the constellation to a neighboring point35ModemsEncode digital values over analog circuitsTo encode data, modems use combinations ofAmplitude Shift Keying (ASK)Frequency Shift Keying (FSK)Phase Shift Keying (PSK)Typically, a carrier frequency is provided and various SHIFT KEYING is appliedFor half duplex we could have only one carrierFor full duplex, we need two carriers, one for each directionModems (Standards)There have been many modem standards over timeEarly modems used a form of frequency shifting – Ex 300 bps full duplex modemOriginating modemSine wave at 1070 Hz for a 0 bitSine wave at 1270 Hz for a 1 bitAnswering modemSine wave at 2025 Hz for a 0 bitSine wave at 2225 Hz for a 1 bitModemsLater modems used combinations of shift keying by combining PSK and ASKSome common 9600 bps modems used12 phase shifts at 1 amplitude4 phase shift at a second amplitudeCombination of 16 different statesCalled Quadrature Amplitude Modulation (QAM)Baud rate was 2400 cycles per secondEach of the 16 states represented 4 bitsModem ConstellationModemsStandard Baud Bit Modulation rate rate TechniqueV.21 300 300 FSKV.22 600 600/1200 PSKV.22bis 600 1200/2400 QAMV.23 1200 1200 FSKV.32 2400 4800/9600 QAM/TCMV.34 2400 28,800 V.90 2400 56,000V.92 2400 56,000ModemsTo improve performance, compression and error correction standards developedTwo compression standards in in vogueV.42bisMNP 5Two error correction standardsV.42MNP 4Modem FunctionalityQ:- What function does a modem perform?Ans:- A modem converts digital information into an analog signal, and conversely.4243Digital Vs. AnalogQ:- Indicate three major advantages of digital transmission over analog transmission.44Analog vs. Digital TransmissionTransmissions can also be either analog or digital.Analog transmissions, like analog data, vary continuously. Examples of analog data being sent using analog transmissions are broadcast TV and radio.Digital transmissions are made of square waves with a clear beginning and ending. Computer networks send digital data using digital transmissions.Data can be converted between analog and digital formats. When digital data is sent as an analog transmission modem (modulator/demodulator) is used. When analog data is sent as a digital transmission, a codec (coder/decoder) is used.45Data Type vs. Transmission TypeAnalogTransmissionDigital TransmissionAnalogDataRadio, Broadcast TVPCM & Video standards using codecsDigital DataModem-basedCommunicationsLAN Cable Standards46Advantages of Digital TransmissionDigital transmission:produces fewer errors than analog transmission. Because the transmitted data is binary (1s and 0s), it is easier to detect and correct errors.permits higher transmission rates. Optical fiber, for example, is designed for digital transmission.is more efficient. It’s possible to send more data through a given circuit using digital rather than analog transmission.is more secure since it is easier to encrypt.Integrating voice, video and data on the same circuit is also far simpler with digital transmission since signals made up of digital data are easier to combine.47Digital Over AnalogDigital signals do not get corrupted by noise etc. You are sending a series of numbers that represent the signal of interest (i.e. audio, video etc.)Digital signals typically use less bandwidth. This is just another way to say you can cram more information (audio, video) into the same space.Digital can be encrypted so that only the intended receiver can decode it (like pay per view video, secure telephone etc.) 48Analog Signals49Digital data, analog signal: Some transmission media, such as optical fiber and satellite, will only propagate analog signals.Analog data, analog signal: Analog data are easily converted to an analog signal.Digital Signals50Digital data, digital signal: In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to analog Equipment.Analog data, digital signal: Conversion of analog data to digital form permits the use of modern digital transmission and switching equipment for analog data.Data51Analog and Digital TransmissionQ:- Indicate three major advantages of digital transmission over analog transmission.Ans:- Cost, capacity utilization, and security and privacy are three major advantages enjoyed by digital transmission over analog transmission.5253Amplitude Shift KeyingQ:- How are binary values represented in amplitude shift keying, and what is the limitation of this approach?54Types of digital-to-analog conversionTypes of Analog to Digital ConversionAmplitude Shift Keyingencode 0/1 by different carrier amplitudesusually have one amplitude zerosusceptible to sudden gain changesinefficientused for:up to 1200bps on voice grade linesvery high speeds over optical fiberAmplitude Shift KeyingValues represented by different amplitudes of carrierUsually, one amplitude is zeroi.e. presence and absence of carrier is usedSusceptible to sudden gain changesInefficient57Example of ASK 58Bit ValuesAmplitude00A101A210A311A459Amplitude Shifting Keying (four amplitudes), two bits per baudExample of ASKAmplitude Shift Keying (ASK)ASK is implemented by changing the amplitude of a carrier signal to reflect amplitude levels in the digital signal.For example: a digital “1” could not affect the signal, whereas a digital “0” would, by making it zero. The line encoding will determine the values of the analog waveform to reflect the digital data being carried.Bandwidth of ASKThe bandwidth B of ASK is proportional to the signal rate S.B = (1+d)S“d” is due to modulation and filtering, lies between 0 and 1.Binary amplitude shift keyingBinary ASKImplementation of binary ASKBinary ASKASKQ:- How are binary values represented in amplitude shift keying, and what is the limitation of this approach?Ans:- With amplitude-shift keying, binary values are represented by two different amplitudes of carrier frequencies. This approach is susceptible to sudden gain changes and is rather inefficient.6465NRZ-LQ:- What is NRZ-L? What is a major disadvantage of this data encoding approach?66Line coding schemesLine EncodingLine EncodingError detection - errors occur during transmission due to line impairments.Some codes are constructed such that when an error occurs it can be detected. For example: a particular signal transition is not part of the code. When it occurs, the receiver will know that a symbol error has occurred.Line EncodingNoise and interference - there are line encoding techniques that make the transmitted signal “immune” to noise and interference.This means that the signal cannot be corrupted, it is stronger than error detection.Line EncodingComplexity - the more robust and resilient the code, the more complex it is to implement and the price is often paid in baud rate or required bandwidth.UnipolarAll signal levels are on one side of the time axis - either above or belowNRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission.Scheme is prone to baseline wandering and DC components. It has no synchronization or any error detection. It is simple but costly in power consumption.Unipolar NRZ schemeUnipolar NRZPolar - NRZThe voltages are on both sides of the time axis.Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0.There are two versions: NZR - Level (NRZ-L) - positive voltage for one symbol and negative for the otherNRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not. Polar NRZ-L and NRZ-I schemesNRZ-L and NRZ-IIn NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the inversion or the lack of inversion determines the value of the bit.NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.NRZ-L and NRZ-I both have a DC component problem and baseline wandering, it is worse for NRZ-L. Both have no self synchronization &no error detection. Both are relatively simple to implement. 75NRZ-L and NRZ-INonreturn to Zero-Level (NRZ-L)easiest way to transmit digital signals is to use two different voltages for 0 and 1 bitsvoltage constant during bit intervalno transition (no return to zero voltage)absence of voltage for 0, constant positive voltage for 1more often, a negative voltage represents one value and a positive voltage represents the other(NRZ-L)NRZ-LQ:- What is NRZ-L? What is a major disadvantage of this data encoding approach?Ans:- Non return-to-zero-level (NRZ-L) is a data encoding scheme in which a negative voltage is used to represent binary one and a positive voltage is used to represent binary zero. A disadvantage of NRZ transmission is that it is difficult to determine where one bit ends and the next bit begins.7778QPSK and OQPSKQ:- What is the difference between QPSK and offset QPSK?79Quadrature PSKmore efficient use if each signal element represents more than one bituses phase shifts separated by multiples of /2 (90o)each element represents two bitssplit input data stream in two and modulate onto carrier and phase shifted carriercan use 8 phase angles and more than one amplitude9600bps modem uses 12 angles, four of which have two amplitudesQPSK and OQPSK ModulatorsQPSK and OQPSKIn QPSK signaling, the bit transitions of the even and odd bit streams occur at the same time instants. but in OQPSK signaling, the even and odd bit Streams, mI(t) and mQ(t), are offset in their relative alignment by one bit period (half-symbol period)QPSK and OQPSKQ:- What is the difference between QPSK and offset QPSK?Ans:- The difference is that offset QPSK introduces a delay of one bit time in the Q stream8384QAMQ:- What is QAM?85PSKPSK Constellation4-PSK4-PSK Characteristics8-PSK CharacteristicsPSK BandwidthThe minimum BW for PSK transmission is the same as that required for ASK transmissionMax baud rates of ASK and PSK are the same for a given BW, but the bit rates could be 2 or more times greaterQAMQuadrature Amplitude ModulationCombined ASK and PSKIf there are x variations in phase and y variations in amplitude, it will give us x times y possible variations.4-QAM and 8-QAM Constellations4 possible variations8 possible variationsTime domain for 8-QAM Signal16-QAM Constellation – Different configurations16 out of 36/32 possible variations are utilized – to ensure readabilityGreater ratio of phase shift to amplitude handles noise best1st figure – ITU-T recommendation2nd figure – OSI recommendationQuadrature Amplitude ModulationQAM used on asymmetric digital subscriber line (ADSL) and some wirelesscombination of ASK and PSKlogical extension of QPSKsend two different signals simultaneously on same carrier frequencyuse two copies of carrier, one shifted 90°each carrier is ASK modulatedtwo independent signals over same mediumdemodulate and combine for original binary outputQAMQ:- What is QAM?.Ans:- QAM takes advantage of the fact that it is possible to send two different signals simultaneously on the same carrier frequency, by using two copies of the carrier frequency, one shifted by 90° with respect to the other. For QAM, each carrier is ASK modulated.9798SummaryDifferential Encoding ??Functions Performed by ModemDigital Advantages Over Analog)Limitations of Binary Amplitude Shift KeyingDisadvantages of NRZ-LQPSK and OQPSK DifferenceWhat is QAMComplimentary Session for (Signal Encoding & Antennas) 100Complimentary Session(Multiple FSK (MFSK))101WhereQuestion102103Q:- A sine wave is to be used for two different signaling schemes: (a) PSK; (b) QPSK. The duration of a signal element is 10-5 s. If the received signal is o :' the following formand if the measured noise power at the receiver is 2.5 X 10-8 ,,vatts, determine the Eb/N0 (in dB) for each case.Solution:-104QQ:- What SNR ratio is required to achieve a bandwidth efficiency , of 1.0 for ASK, FSK, PSK, and QPSK? Assume that the required bit error rate is 10-6105Q/A106107NRZ-LQ:- An NRZ-L signal is passed through a filter with r = 0.5 and then mqdulated onto a carrier. The data rate is 2400 bps. Evaluate the bandwidth for ASK and FSK. For FSK assume that the two frequencies used are 50 kHz and 55 kHz.Sol108109BandwidthQ:- Assume that a telephone line channel is equalized to allow bandpass data transmission over a frequency range of 600 to 3000 Hz. The available bandwidth is 2400 Hz. For r = 1 , evaluate the required bandwidth for 2400 bps QPSK and 4800-bps, eight level multilevel signaling. Is the bandwidth adequate?110Bandwidth111Q:- Assume that a telephone line channel is equalized to allow bandpass data transmission over a frequency range of 600 to 3000 Hz. The available bandwidth is 2400 Hz. For r = 1 , evaluate the required bandwidth for 2400 bps QPSK and 4800-bps, eight level multilevel signaling. Is the bandwidth adequate?112QQ:- A microwave transmitter has an output of 0.1 W at 2 GHz. Assume that this transmitter is used in a microwave communication system where the transmitting and receiving antennas are parabolas, each 1 .2 m in diameter.What is the gain of each antenna in decibels?Taking into account antenna gain, what is the effective radiated power of the transmitted signal?If the receiving antenna is located 24 km from the transmitting antenna over a free space path, find the available signal power out of the receiving antenna in dBm units113114Q:- A microwave transmitter has an output of 0.1 W at 2 GHz. Assume that this transmitter is used in a microwave communication system where the transmitting and receiving antennas are parabolas, each 1 .2 m in diameter.What is the gain of each antenna in decibels?Taking into account antenna gain, what is the effective radiated power of the transmitted signal?If the receiving antenna is located 24 km from the transmitting antenna over a free space path, find the available signal power out of the receiving antenna in dBm units115QQ:- Determine the height of an antenna for a TV station that must be able to reach customers up to 80 km away.Ans:- 116