Chapter 44.1 : Digital Modulation4.2 : Digital Transmission4.3 : Multiple Access Methods 4.1 Digital ModulationOutlinesa.Introductionb.Information capacity, Bits, Bit Rate, Baud, M-ary Encodingc.Digital Modulation Techniues - !"#, $"#, %"#, &!M Digital modulationIs the transmittal of digitally modulated analog signals et!een to or more points in a communications system."an e propagated through #arth$s atmosphere and used in !ireless communication system % digital radio.&ffer se'eral outstanding ad'antages o'er traditional analog system. #ase of processing #ase of multiple(ing )oise immunity Contd...Applications:*o! speed 'oice and data comm. modems+igh speed data transmission systemsDigital micro!a'e , satellite comm. systems-". (personal communication systems) telephone Why digital modulation?The modulation of digital signals !ith analogue carriers allo!s an impro'ement in signal to noise ratio as compared to analogue modulating schemes. Important Criteria1.High spectral efficiency2. High power efficiency3. Robust to multipath4. Low cost and ease of implementation5. Low carrier-to-co channel interference ratio. Low out-of-band radiation Contd!. "onstant or near constant en#elop$. %andwidth &fficiency'bility to accommodate data within a limited bandwidth(radeoff between data rate and pulse width). *ower &fficiency(o preser#e the fidelity of the digital message at low power le#els."an increase noise immunity by increasing signal power Forms of Digital Modulation) 2 sin( ) ( + ft V t vIf 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) ContdIf the phase, 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, of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude odulation (QA) Contd... Example 1 /or the digital message 1101 1100 10101 s2etch the !a'eform for the follo!ing:a. A.3. /.3c. -.3d. 4AM Block DiagramSimpli!ed bloc" diagram of a digital modulation system Contd-recoder performs le'el con'ersion , encodes incoming data into group of its that modulate an analog carrier.Modulated carrier filtered1 amplified , transmitted through transmission medium to 5(.In 5(1 the incoming signals filtered1 amplified , applied to the demodulator and decoder circuits !hich e(tracts the original source information from modulated carrier. Information capacity1 6its , 6it 5ate75epresents the numer of independent symols that can e carried through a system in a gi'en unit of time.76asic digital symol is the binary digit or bit.7#(press the information capacity as a bit rate. artleys !a"t B I 'hereI ( information capacity )bps*B ( band+idth ),-*t ( transmission time )s*From the equation, Information capacity is a linear function of bandwidth and transmission time and directly proportional to both. #hannons Formula) 1 ( log 2 .) 1 ( log1! 2NSNSB I or B I + + 'hereI ( information capacity )bps*B ( band+idth ),-*( signal to noise po+er ratio )unitless*The higher "./ the better the performance and the higher the information capacityNS Example 2 6y using the .hannon$s /ormula1 calculate the information capacity if .8) 9 30 d6 and 6 9 2.: 2+;. Nyquist Sampling Rate fs is e