Mini Project Communication Link Simulation Digital Modulation Techniques Lecture

Mini Project – Communication Link Simulation Digital Modulation Techniques Author: University of Hertfordshire Date created : Date revised : 2009 Abstract The following resources come from the 2009/10 BEng ( Hons ) in Digital Communications & Electronics (course number 2ELE0064) from the University of Hertfordshire. All the mini projects are designed as level two modules of the undergraduate programmes. The objective of this module is to have built communication links using existing AM modulation, PSK modulation and demodulation blocks, constructed AM modulators and constructed PSK modulators using operational function blocks based on their mathematical expressions, and conducted simulations of the links and modulators, all in Simulink®. Use Matlab®/ Simulink® to design a communication link for AM audio broadcasting. The message signal is a mono audio signal although you may not be able to transmit the full audio frequency range that is normally required for high quality sound. In addition to the resources found below there are supporting documents which should be used in combination with this resource. Please see: Mini Projects - Introductory presentation. Mini Projects - E-Log. Mini Projects - Staff & Student Guide. Mini Projects - Standard Grading Criteria. Mini Projects - Reflection. You will also need the ‘Mini Project- Communication Link Simulation’ text file and the lecture presentation on ‘Channels and Noise’. © University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License .

Contents Digital Bandpass Modulation Modulation Types of modulation Digital Modulation Digital Modulation – Carrier Four main modulation techniques Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) PSK: Phasor or vector diagrams (constellation diagram) BPSK: Phasor or vector diagram (constellation diagram) Quadrature Phase Shift Keying (QPSK) - Phasor or vector d... M- ary Phase Shift Keying (MPSK) - Phasor or vector diagram PSK – General Expression QPSK – Implementation Reliability & Efficiency Spectral efficiency and transmitted power trade-off Quadrature Amplitude Modulation (QAM) Reading list Credits

Digital Bandpass Modulation Digital modulation techniques Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (BPSK, QPSK) Quadrature Amplitude Modulation (QAM) Comparison with regards to: Reliability (power), Efficiency (bandwidth)

Modulation What is modulation? Modulation is the process by which message signals are transformed into higher frequency waveforms that are compatible with the characteristics of the channel

Why modulate? Message signals need to be matched to the characteristics of channels Subsequent advantages of modulation: Enables efficient economic communication methods to be used as the sharing of communication resources is made possible (signals can be combined using frequency division multiplexing - FDM) Efficient antennas of reasonable physical size to be constructed for radio communication systems

Types of modulation Modulation techniques for analogue signals Modulation techniques for digital signals

Digital Modulation Digital modulation is the process by which digital symbols are transformed into waveforms that are compatible with the characteristics of the channel To carry out digital modulation, we need: A digital message or information or modulating signal, and A sinusoid carrier wave or simply a carrier N.B.: The carrier is always of much higher frequency than the modulating signal

Digital Modulation - Carrier General form of the carrier wave is where A c = amplitude in volts (V)  c = angular or radian frequency in rads -1  c = phase in radian (rad) Alternatively, since where f c = frequency in hertz (Hz)

Digital Modulation - Carrier In digital modulation, one of the properties of the carrier (amplitude, frequency or phase) is changed according to the modulating (or information or message) signal +A -A T c t c(t)

Four main modulation techniques Changing amplitude (A c ) of carrier according to modulating signal Changing phase (  c ) of carrier according to modulating signal Changing frequency (f c ) of carrier according to modulating signal Combination of ASK and PSK

Amplitude Shift Keying (ASK) m(t): modulating signal (baseband signal) c(t): carrier wave (high frequency cosine) y(t): modulated signal – ASK signal (bandpass signal) ASK modulator can be represented by the schematic diagram on the right ASK  amplitude of carrier is changed according to the modulating signal m(t) y(t) c(t)

Amplitude Shift Keying (ASK) ctd… Binary ASK also called on-off keying (OOK) Information or message or baseband data Carrier wave or carrier Data stream: OOK waveform (bandpass signal) 0 1 1 0 1 0 0 1

Frequency Shift Keying (FSK) FSK  frequency of carrier is changed according to modulating signal Binary FSK (BFSK) represents ones and zeros by carrier pulses of two distinct frequencies, f 1 and f 2 Binary zero  frequency f 1 Binary one  frequency f 2

Frequency Shift Keying (FSK) ctd… Information or message or baseband data Carrier wave or carrier Data stream: BFSK waveform (bandpass signal) 0 1 1 0 1 0 0 1

Frequency Shift Keying (FSK) ctd… BFSK signal can be considered as the combination of two OOK signals: One representing the baseband data stream {m(t)}modulated onto a carrier with frequency f 1 , and One representing the inverse data stream {m’(t)} modulated onto a carrier with frequency f 2 c 1 (t)=A cos(2  f 1 t) c 2 (t)=A cos(2  f 2 t) BFSK signal m(t) m’(t) Schematic of BFSK modulator: as the combination of two OOK signals

Phase Shift Keying (PSK) PSK  phase of carrier is changed according to modulating signal One period,T c Equivalent to a complete turn phase angle 1 complete turn phase angle = 2  rad (=360  ) +A -A T c t c(t)

Phase Shift Keying (PSK) ctd…  c = 0 rad (=0  )  c =  rad (=180  ) t t c(t) c(t+  ) t t  c = 3  /2 rad (=270  )  c =  /2 rad (=90  ) c(t+ 3  /2) c(t+  /2)

Phase Shift Keying (PSK) ctd… Binary PSK (BPSK) represents ones and zeros by shifting the phase by  1 and  2 Binary zero  phase  1 (0 rad or 0  ) Binary one  phase  2 (  rad or 180  ) PSK  phase of carrier is changed according to modulating signal

Phase Shift Keying (PSK) ctd… Information or message or baseband data Carrier wave or carrier Data stream: BPSK waveform (bandpass signal) 0 1 1 0 1 0 0 1

PSK: Phasor or vector diagrams (constellation diagram)  =0 rad =0  = 2  rad =360   =  /2 rad =90   =  rad =180   =3  /2 rad =270 

BPSK: Phasor or vector diagram (constellation diagram) m 1 m 2 Binary: two possible states m 1 and m 2 Euclidean distance: distance between two message points  =0  =  /2  =   =3  /2 Decision region 1 Decision region 2 Decision boundary

Quadrature Phase Shift Keying (QPSK) - Phasor or vector diagram  =0  =  /2  =   =3  /2 m 1 m 2 Quadrature: four possible states m 1 , m 2 ,m 3 and m 4 m 4 m 3 Decision region 1 Decision region 2 Decision region 3 Decision region 4 Decision boundary Decision boundary

M-ary Phase Shift Keying (MPSK) - Phasor or vector diagram  =0  =  /2  =   =3  /2 m 1 m 3 M-ary: M possible states m 1 , m 2 , m 3 , … m M m 7 m 5 m 8 m 6 m 4 m 2 Signal constellation for 8-PSK Region 1 Region 8 Region 4 Region 2 Region 7 Region 3 Region 5 Region 6

PSK – General Expression The general analytic expression of PSK is more popularly written as E is the symbol energy and T is the information signal’s symbol time duration. i=1, 2, ..M. Phase term  i (t) has M discrete values given by BPSK, M=2; QPSK, M=4; 8-PSK, M=8; etc

PSK – Coding BPSK: each state (m1, m2) is represented by one digit (0, 1) QPSK: each state (m1, m2, m3, m4) is represented by two digits (00, 01, 10, 11) 8PSK: each state is presented by three digits (000, 001, 010, 011, 100, 101, 110, 111) Etc…

QPSK – Implementation By expanding the general expression, QPSK can be implemented in the following way. In QPSK the information bit stream is divided to form two streams, in-phase (I) and in quadrature (Q), comprising of the even and odd bits of the original information signal respectively Since each transmitted symbol is represented by two successive binary pulses, the symbol rate of the I and Q waveforms is half the bit rate of the information signal ( Rs=Rb /log 2 M). Subsequently the bipolar I and Q streams are used to modulate the components of a carrier frequency in quadrature Modulation of each orthogonal carrier follows a DSB-SC-AM mode resulting in two BPSK signals

QPSK Circuit Diagram Two carriers are inphase quadrature. In the case of the inphase data stream, the phase of the cosine carrier is shifted, at symbol transitions, between 0 o and 180 o Equivalently the quadrature data stream shifts the phase of the sine function between 90 o and 270 o The modulated signals are combined linearly to produce the QPSK waveform θ (t)= 0 o , 90 o , 180 o and 270 o

Reliability & Efficiency Reliability of scheme: how likely are errors; this is related to the Euclidean distance Expressed by the BER versus SNR (Eb/No): What is the probability of error? Efficiency: measure of the data rate Expressed by the number of bits per symbol

Reliability & Efficiency ctd… As M increases, the Euclidean distance decreases Hence, the probability of error increases; therefore the reliability decreases As M increases, data rate increases Hence the efficiency increases Trade-off between reliability and efficiency to be considered

Spectral efficiency and transmitted power trade-off For the same system bandwidth a quadrature modulation scheme can transmit twice the data rate achievable with its binary counterpart The superior performance of M -level signaling by means of higher achievable transmission rates for a given channel bandwidth is achieved in the expense of increased transmitted power (better SNR) for a required reliability (BER).

Quadrature Amplitude Modulation (QAM) Also known as Amplitude Phase Keying (APK) Combination of ASK and PSK 8-QAM 16-QAM

Reading list Sklar, B., (2001), “Digital Communications: Fundamentals and Applications”, Prentice Hall, 2 nd Edition: sections 4.1 – 4.2 Glover, I.A & Grant P.M., (2004), Digital Communications”, Pearson Prentice Hall, 2 nd Edition: sections 11.1 – 11.3

This resource was created by the University of Hertfordshire and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. © University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License . The name of the University of Hertfordshire, UH and the UH logo are the name and registered marks of the University of Hertfordshire. To the fullest extent permitted by law the University of Hertfordshire reserves all its rights in its name and marks which may not be used except with its written permission. The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher.

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