Digital Modulation Revision Professor Richard Harris Objectives To identify the key points from the lecture material presented in the Digital Modulation section of this paper. What is in the examination and what is not Communication Systems 143.3 - Digital Modulation Slide Presentation Outline Identifying the key points Summary on a lecture by lecture basis Supplementary material Communication Systems 143.3 - Digital Modulation Slide 3 1
Remember this! The Objectives slides in each lecture are: Intended as a guide to the things that you should be able to do as a result of Participating in the lecture and Reading the suggested reference material (Remember that merely sitting in the lectures doesn t produce learning, it is an active process!) Use the book by Leon Couch as a further guide to the material that has been presented. Tutorial exercises are also important adjuncts to the lecture material The above three things should guide you in your studies. Communication Systems 143.3 - Digital Modulation Slide 4 Lecture 01 1 A review of Analogue Modulation Techniques Introduction to Digital Modulation methods Binary Modulated Bandpass Signalling Unipolar and Bipolar Examples of OOK etc Intersymbol Interference Causes Impact on digital transmission Key Points Placing baseband signals on high frequency carriers using the process of modulation facilitates the long distance transmission of data, voice and video signals. Since they can be tied more efficiently to the communication medium, the high frequency waves travel over greater distances than could be achieved with the original message / waveform unaided. Communication Systems 143.3 - Digital Modulation Slide 5 Lecture 01 Modulation can be achieved in three distinct ways: Amplitude A can be varied in sympathy with the message Amplitude Modulation Frequency f can be varied according to the message signal Frequency Modulation Phase φ can also be varied with the message signal. Phase modulation Note that collectively, frequency and phase modulation are referred to as angle modulation. You should be able to define: Unipolar signals Bipolar signals Communication Systems 143.3 - Digital Modulation Slide 6
Lecture 01 3 You should be able to explain the difference between Non return to zero (NRZ) Return to zero (RZ) You should be able to explain the advantages and disadvantages of NRZ and RZ. You should be able to write down signal structures for signals / bit patterns coded in: Manchester Alternate Mark Inversion B1Q You should be able to state what is meant by Inter- Symbol Interference and methods for avoiding ISI. Communication Systems 143.3 - Digital Modulation Slide 7 Lecture 01 4 You should be able to explain the operation and concepts behind: OOK On-off keying or Amplitude Shift Keying (ASK) BPSK Binary Phase Shift Keying DSB-SC Double Side Band Suppressed Carrier Communication Systems 143.3 - Digital Modulation Slide 8 Lecture 0 1 Overview of Binary Keying Methods Description of the various forms of digital modulation BASK (OOK) BPSK FSK Introduction to Multi-level bandpass signalling Key Points To be able to describe the various forms of digital modulation and implement them on simple data inputs To compute the PSD for these different forms and discuss their properties. To be able to discuss the approaches required to detect each of the forms of digital modulation. Communication Systems 143.3 - Digital Modulation Slide 9 3
Lecture 0 Binary Keying definition; The bits in a message stream switch the modulation parameters (amplitude, frequency and phase) from one state to another. This process is called binary keying. Binary keying is a process that makes two values of amplitude, phase or frequency of the carrier signal change in sympathy with the values of the bits in the binary signal stream. Basic actions can be classified as: ASK Amplitude Shift Keying PSK Phase Shift Keying FSK Frequency Shift Keying Communication Systems 143.3 - Digital Modulation Slide 10 Power Spectral Density A digital waveform x(t) is modelled by n= x ( t) = a w( t nt ) n where {a n } represent data, n is an integer, w(t) is the signalling pulse shape and T is the duration of one data symbol. The power spectral density function PSD of x(t) is given by σ a Px ( f ) = W ( f ) T µ a + T k= k k W δ f T T where k is an integer, σ a and µ a are the variance and mean of the data {a n } and W(f) is the Fourier transform of w(t). Do not try to remember the formulae! Remember what the variables are and their interpretation only! Communication Systems 143.3 - Digital Modulation Slide 11 Lecture 0 4 What is the difference between coherent and noncoherent detection? Is one better than the other? Why? DPSK Should be able to show the encoded binary sequence, given a starting bit 1 or 0 as the reference bit. FSK Should be able to explain its operation in simple diagram terms. Determination of the bandwidth for FSK Note the formulae for the approximate bandwidth and raised cosine filtering cases. (Need to know these.) Communication Systems 143.3 - Digital Modulation Slide 1 4
Lecture 0 5 Multi-level M-ary Modulation What is it? How does it work? Advantages and disadvantages You should be able to state the formula for spectral efficiency You should be able to perform simple computations of the spectral efficiency. Communication Systems 143.3 - Digital Modulation Slide 13 Lecture 03 1 More detailed view of M-ary signalling schemes QPSK, MPSK QAM OQPSK, π/4 QPSK MSK, GMSK Key points To be able to describe each of the following M-ary signalling schemes and to analyse their PSD and related properties: QPSK, MPSK QAM OQPSK, π/4 QPSK MSK, GMSK Communication Systems 143.3 - Digital Modulation Slide 14 Lecture 03 You should be able to draw the signal constellations for different M-ary systems, including: QPSK signal constellations MPSK QAM (in its various forms) You should be able to compute the spectral efficiency of various M-ary signalling schemes η R log M bits/s = = = BT 1+ r 1+ r Hz Communication Systems 143.3 - Digital Modulation Slide 15 5
Lecture 03 3 You should be able to define orthogonal signalling Minimum Shift Keying is continuous phase FSK with a minimum modulation index (h=0.5) that will produce orthogonal signalling. What is Gaussian-filtered MSK? How does it work? Communication Systems 143.3 - Digital Modulation Slide 16 Lecture 04 1 Review of Digital Modulation Generic view Application areas for the different types of Digital Modulation Review of Filtering methods Raised cosine Square-root raised cosine Gaussian filters Introduction to the concepts of noise in communication systems (radio system emphasis). Key Points Discussion of why Digital Modulation is used and the factors involved in making a choice between schemes. Communication Systems 143.3 - Digital Modulation Slide 17 Lecture 04 Communication systems can be classified into one of three different categories: Bandwidth efficient Ability of system to accommodate data within a prescribed bandwidth Power efficient Reliable sending of data with minimal power requirements Cost efficient System needs to be affordable in the context of its use I/Q diagrams Filtering What is it? Why is it used? Communication Systems 143.3 - Digital Modulation Slide 18 6
Lecture 04 3 Problems with filtering Filtering may make radios more complex and larger. Filtering can create ISI problems Can occur if heavy filtering so that the symbols blur together and each symbol affects those around it. This is determined by the time response or the impulse response of the filter. What are some different types of filter, where might they be used? Raised Cosine Filters Root Cosine Filters Gaussian Filters Communication Systems 143.3 - Digital Modulation Slide 19 Lecture 04 4 Definitions of terms: AWGN Additive White Gaussian Noise (AWGN). Signal to Noise ratio The ratio of the signal strength to the noise level is called the signal-to-noise ratio (SNR) Bit Error Rate The bit error rate (BER) of a system indicates the quality of the link. Usually, a BER of 10-3 is considered acceptable for a voice link, and 10-9 for a data link. Interference is the result of other man-made radio transmissions. for example in the ISM band at.4ghz a large number of systems co-exist, such as Wireless LAN, Bluetooth, Microwave ovens, etc Adjacent channel interference occurs when energy from a carrier spills over into adjacent channels. Co-channel interference occurs when another transmission on the same carrier frequency affects the receiver. This will often arise from transmissions in another cell in their network. The ratio of the carrier to the interference (from both sources) is called the carrier-to-interference ratio (C/I). Communication Systems 143.3 - Digital Modulation Slide 0 Lecture 04 5 Multi-path problems, how do they occur? What is meant by narrowband fading? What is shadowing (slow fading)? Descriptions of various forms of interference in mobile communication systems. Communication Systems 143.3 - Digital Modulation Slide 1 7
Lecture 05 1 (Noise) Gaussian distribution Concept of white noise Introduction to the Bit Error Rate (BER) Binary Detection BER of Digital Systems Key Points You will be able to: Define what is meant by White Noise Define the Bit Error Rate Compute BER for a variety of standard Digital Modulation schemes Communication Systems 143.3 - Digital Modulation Slide Lecture 05 Thermal Noise: Produced by thermal motion of electrons in the conducting media. It corrupts the signal in an additive fashion. Thermal noise is also described as a zero-mean Gaussian random process A Gaussian process n(t) is a random function whose value n at any arbitrary time t is statistically characterised by the Gaussian Probability Density (PDF) function. You should be able to discuss the generic approach to the computation of the BER. Communication Systems 143.3 - Digital Modulation Slide 3 Lecture 05 3 You should be able to demonstrate that the threshold value of V T = ½ (s 01 + s 0 ) You should be able to quote the simple formulae for BER for standard cases. Matched filter: The BER is E d Pe = Q N0 where T E = s () t s () t dt d [ ] 1 0 Communication Systems 143.3 - Digital Modulation Slide 4 8
Lecture 05 4 You should be able to compute the BER for simple b) cases similar to those undertaken in the tutorials, see opposite for examples: c) a) Binary Digit 1 0 1 0 1 0 Transmitted Symbol A 0 A/ -A/ A sin (πt/t) -A sin (πt/t) Communication Systems 143.3 - Digital Modulation Slide 5 Lecture 06 1 Computation of BER for the following systems OOK BPSK FSK MSK QPSK Comparison of Digital Signalling schemes Key points You will be able to Compute BER for additional schemes, viz: OOK BPSK FSK MSK QPSK Discuss the choice of signalling scheme with respect to BER performance. Communication Systems 143.3 - Digital Modulation Slide 6 Examinations - Final Examination is in the same format as in the previous year. Three hour paper There are two questions from me You will need to answer five questions from the available set of eight questions. Communication Systems 143.3 - Digital Modulation Slide 7 9
Conclusions Good luck with all your examinations! See you next time. Communication Systems 143.3 - Digital Modulation Slide 8 10