Presentation on theme: "Chapter : Digital Modulation 4.2 : Digital Transmission"— Presentation transcript:
1Chapter 4 4.1 : Digital Modulation 4.2 : Digital Transmission 4.3 : Multiple Access Methods
24.1 Digital Modulation Outlines Introduction Information capacity, Bits, Bit Rate, Baud,M-ary EncodingDigital Modulation Techniques- ASK, FSK, PSK, QAM
3Introduction to Digital Modulation Is the transmittal of digitally modulated analog signals between two or more points in a communications system.Can be propagated through Earth’s atmosphere and used in wireless communication system - digital radio.Offer several outstanding advantages over traditional analog system.Ease of processingEase of multiplexingNoise immunity
4Cont’d... Applications: Low speed voice band data comm. modems High speed data transmission systemsDigital microwave & satellite comm. systemsPCS (personal communication systems) telephone
5Why digital modulation? The modulation of digital signals with analogue carriers allows an improvement in signal to noise ratio as compared to analogue modulating schemes.
6Important Criteria High spectral efficiency 2. High power efficiency 3. Robust to multipath4. Low cost and ease of implementation5. Low carrier-to-co channel interference ratio6. Low out-of-band radiation
7Cont’d… 7. Constant or near constant envelop 8. Bandwidth Efficiency Ability to accommodate data within a limited bandwidthTradeoff between data rate and pulse width9. Power EfficiencyTo preserve the fidelity of the digital message at low power levels.Can increase noise immunity by increasing signal power
8Forms of Digital Modulation If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK)If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)
9Cont’d…If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK)If both the amplitude and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)
11Example 1For the digital message , sketch the waveform for the following:a. ASKb. FSKc. PSKd. QAM
12Simplified block diagram of a digital modulation system
13Cont’d…Precoder performs level conversion & encodes incoming data into group of bits that modulate an analog carrier.Modulated carrier filtered, amplified & transmitted through transmission medium to Rx.In Rx, the incoming signals filtered, amplified & applied to the demodulator and decoder circuits which extracts the original source information from modulated carrier.
14Information capacity, Bits & Bit Rate Bit Rate, Baud, M-ary EncodingInformation capacity, Bits & Bit RateInformation capacity is a measure of how much information can be propagated through a communication system and is a function of bandwidth and transmission time.represents the number of independent symbols that can be carried through a system in a given unit of time.Basic digital symbol is the binary digit or bit.Express the information capacity as a bit rate.
15Hartley’s LawWhereI = information capacity (bps)B = bandwidth (Hz)t = transmission time (s)From the equation, Information capacity is a linear function of bandwidth and transmission time and directly proportional to both.
16Shannon’s FormulaWhereI = information capacity (bps)B = bandwidth (Hz)= signal to noise power ratio (unitless)The higher S/N the better the performance and the higher the information capacity
17Example 2By using the Shannon’s Formula, calculate the information capacity if S/N = 30 dB and B = 2.7 kHz.
18fs is equal or greater than 2fm fs >= 2fm Nyquist Sampling Ratefs is equal or greater than 2fmfs >= 2fmfs = minimum Nyquist sample rate (Hz)fm = maximum analog input frequency (Hz)
19Example 3Determine the Nyquist sample rate for a maximum analog input frequency 7.5 kHz.
20M-ary EncodingIt is often advantageous to encode at a level higher than binary where there are more then two conditions possible.The number of bits necessary to produce a given number of conditions is expressed mathematically asWhere N = number of bits necessaryM = number of conditions, level or combinations possible with N bits.
21Cont’d…Each symbol represents n bits, and has M signal states, where M = 2N.Find the number of voltage levels which can represent an analog signal witha. 8 bits per sampleb. 12 bits per sample
22Baud & Minimum BWBaud refers to the rate of change of a signal on the transmission medium after encoding and modulation have occurred.Wherebaud = symbol rate (symbol per second)ts = time of one signaling symbol (seconds)
23Cont’d… Minimum Bandwidth Using multilevel signaling, the Nyquist formulation for channel capacityWhere fb= channel capacity (bps)B = minimum Nyquist bandwidth (Hz)M = number of discrete signal or voltage levels
24Cont’d… For B necessary to pass M-ary digitally modulated carriers Where N is the number of bits encoded into each signaling element.
26Amplitude Shift Keying (ASK) A binary information signal directly modulates the amplitude of an analog carrier.Sometimes called Digital Amplitude Modulation (DAM)Where vask (t) = amplitude shift keying wavevm(t) = digital information signal (volt)A/2 = unmodulated carrier amplitude (volt)ωc = analog carrier radian frequency (rad/s)
28Frequency Shift Keying (FSK) Called as Binary Frequency Shift Keying (BFSK)The phase shift in carrier frequency (∆f) is proportional to the amplitude of the binary input signal (vm(t)) and the direction of the shift is determined by the polarityWhere vfsk(t) = binary FSK waveformVc = peak anlog carrier amplitude (volt)fc = analog carrier center frequency (Hz)∆f = peak shift in analog carrier frequency (Hz)vm(t) = binary input signal (volt)
32Phase Shift Keying (PSK) Another form of angle-modulated, constant amplitude digital modulation.Binary digital signal input & limited number of output phases possible.M-ary digital modulation scheme with the number of output phases defined by M.The simplest PSK is Binary Phase-Shift Keying (BPSK)N= 1, M=2Two phases possible for carrier with one phase for logic 1 and another phase for logic 0The output carrier shifts between two angles separated by 180°
33Cont’d...a) Truth Table b) Phasor Diagram c) Constellation Diagram
36CONSTELLATION DIAGRAM Definition : A graphical representation of the complexenvelope of each possible symbol state.The x-axis represents the in-phase componentand the y-axis the quadrature component of thecomplex envelopeThe distance between signals on a constellationdiagram relates to how different the modulationwaveforms are and how easily a receiver candifferentiate between them.
42Cont’d...Amplitude and phase shift keying can be combined to transmit several bits per symbol.Often referred to as linear as they require linear amplification.More bandwidth-efficient, but more susceptible to noise.For M = 4, 16QAM has the largest distance between points, but requires very linear amplification. 16PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise.High level M-ary schemes (such as 64-QAM) are very bandwidth-efficient but more susceptible to noise and require linear amplification
43Bandwidth EfficiencyUsed to compare the performance of one digital modulation technique to another.Bη = Transmission bit rate (bps)Minimum bandwidth (Hz)
44Example 5For 16-PSK system, operating with an information bit rate of 32 kbps, determine:a. Baudb. Minimum bandwidthc. Bandwidth efficiency
45Solution:16PSK= log2(M)a) baud= 32000/4=8000b) minimum bandwidth = 32000/4 =8000c)Bandwidth efficiency =transmission bit rate/minimum bandwidth= 32000/8000=4 bits per cycle
46CONCLUSIONTo decide which modulation method should be used , we need to make considerations ofa) Bandwidthb) Speed of Modulationc) Complexity of Hardware