Digital Modulation Lecture 01 Review of Analogue Modulation Introduction to Digital Modulation Techniques Richard Harris
Objectives You will be able to: Classify the various approaches to Analogue Modulation Discuss the basic methodologies involved in Digital Modulation Amplitude Modulation Frequency Modulation Phase Modulation Describe the concepts behind binary modulated bandpass signalling Communication Systems 143.332 - Digital Modulation Slide 2
Presentation Outline Review of Analogue Modulation Techniques Introduction to Digital Modulation methods Binary Modulated Bandpass Signalling Unipolar and Bipolar Examples of OOK etc Communication Systems 143.332 - Digital Modulation Slide 3
Review of Analogue Modulation - 1 We have found that 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. Modulation: The signal processing technique where, at the transmitter one signal (the modulating signal) modifies a property of another signal (the carrier signal) so that a composite wave (the modulated wave) is formed. Communication Systems 143.332 - Digital Modulation Slide 4
Review of Analogue Modulation - 2 Demodulation At the receiver, the modulating signal is recovered from the modulated wave (demodulation). The bandwidth of the modulated wave is equal to, or greater than the bandwidth of the modulating signal. Since the modulated wave has a higher frequency it can be launched from: Practical sized antennas Moderate sized cables or waveguides Each symbol represents a specific sequence of bits and the symbol set covers all possible bit combinations. The maximum symbol rate is determined by the passband of the bearer and associated equipment. Communication Systems 143.332 - Digital Modulation Slide 5
Analogue Modulation Analogue modulation combines a higher frequency sinusoidal carrier with a lower frequency signal carrying the message. Such carriers can be modulated 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. Communication Systems 143.332 - Digital Modulation Slide 6
What is Digital Modulation? Digital Modulation combines a high frequency sinusoidal carrier signal and a digital data stream to create a modulated wave that assumes a limited number of states. As for Amplitude Modulation, we can modulate the wave in sympathy with the digital data stream in three basic 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 Communication Systems 143.332 - Digital Modulation Slide 7
Why Digital Modulation - 1 Most 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 Communication Systems 143.332 - Digital Modulation Slide 8
Why Digital Modulation - 2 Move from Analogue (AM) to Digital (DM) since it provides better information capacity, higher data security, better quality communications. Industry trends: Signal/System Complexity AM, FM Scalar signals QAM, FSK, QPSK Vector signals TDMA, CDMA Time variant signals Required Measurement Capability Communication Systems 143.332 - Digital Modulation Slide 9
Why Digital Modulation - 3 Another layer of complexity in many new systems is multiplexing. Two principal types of multiplexing (or multiple access ) are TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). These are two different ways to add diversity to signals allowing different signals to be separated from one another. Communication Systems 143.332 - Digital Modulation Slide 10
Transmitting Information A pure carrier is generated at the transmitter. The carrier is modulated with the information to be transmitted. Any reliably detectable change in signal characteristics can carry information. At the receiver the signal modifications or changes are detected and demodulated. Modulation Modify a signal Demodulation Detect the modifications Communication Systems 143.332 - Digital Modulation Slide 11
Polar Display - 1 Polar display - magnitude and phase represented together A simple way to view amplitude and phase is with the polar diagram. The carrier becomes a frequency and phase reference and the signal is interpreted relative to the carrier. The signal can be expressed in polar form as a magnitude and a phase. The phase is relative to a reference signal, Usually the carrier in most communication systems. Communication Systems 143.332 - Digital Modulation Slide 12
Polar Display - 2 Magnitude is represented as the distance from the centre and phase is represented as the angle. Amplitude modulation (AM) changes only the magnitude of the signal. Phase modulation (PM) changes only the phase of the signal. Amplitude and phase modulation can be used together. Frequency modulation (FM) looks similar to phase modulation, though frequency is the controlled parameter, rather than relative phase. Communication Systems 143.332 - Digital Modulation Slide 13
I/Q Formats In digital communications, modulation is often expressed in terms of I and Q. This is a rectangular representation of the polar diagram. On a polar diagram, the I axis lies on the zero degree phase reference, and the Q axis is rotated by 90 degrees. The signal vector s projection onto the I axis is its I component and the projection onto the Q axis is its Q component. Communication Systems 143.332 - Digital Modulation Slide 14
I and Q in Transmitter I/Q diagrams are useful since they mirror the way in which digital communication signals are created using an I/Q modulator. In the transmitter, I and Q signals are mixed with the same local oscillator. A 90 o phase shifter is placed on one of the paths. Signals that are at 90 o are said to be orthogonal to each other or in quadrature. Communication Systems 143.332 - Digital Modulation Slide 15
Transmitter Side Q Local Oscillator 90 o Phase Shift Σ Composite output signal I Signals that are in quadrature are independent and do not interfere with each other. Simplifies digital radios and similar devices Communication Systems 143.332 - Digital Modulation Slide 16
Receiver Side Quadrature component Composite input signal Local Oscillator 90 o Phase Shift In-phase component On the receiver side, the combined signals are easily separated out Communication Systems 143.332 - Digital Modulation Slide 17
Why use I/Q? Digital Modulation is easy to accomplish with I/Q modulators. Most modulators map data onto a number of discrete points on the I-Q plane. Points are known as constellation points. As the signal moves from one point to another, simultaneous amplitude and phase modulation usually takes place. Difficult to achieve in conventional phase modulators. Communication Systems 143.332 - Digital Modulation Slide 18
Application Areas Modulation Format MSK, GMSK BPSK QPSK and ¼p DQPSK OQPSK FSK, GFSK 8, 16 VSB 8PSK 16 QAM 32 QAM 64 QAM 256 QAM Application GSM and CDPD Deep space telemetry, cable modems Satellite, CDMA, NADC, TETRA, PHS, PDC, LMDS CDMA, satellite DECT, paging, RAM mobile data, AMPS, CT2, land mobile and public safety North American digital TV Satellite, aircraft Microwave digital radio, modems, DVB-C, DVB-T Terrestrial microwave, DVB-T DVB-C modems, set top boxes MMDS Modems, Digital video (USA) Communication Systems 143.332 - Digital Modulation Slide 19
MSK and GMSK Minimum-shift keying (MSK) is a type of continuous phase frequency-shift keying. Similarly to OQPSK, MSK is encoded with bits alternating between quarternary components, with the Q component delayed by half a bit period. However, instead of square pulses as OQPSK uses, MSK encodes each bit as a half sinusoid. This results in a constant-modulus signal, which reduces problems caused by non-linear distortion. The resulting signal is represented by the formula where a I (t) and a Q (t) are the square pulses as shown in QPSK. A similar modulation scheme is Gaussian minimum shift keying, which uses Gaussian instead of sinusoidal pulse shapes. Retrieved from "http://en.wikipedia.org/wiki/minimum-shift_keying" Communication Systems 143.332 - Digital Modulation Slide 20
Digital Modulation The modulating signal m(t) is a digital signal given by Binary line codes or Multi-level line codes Correspondingly, the bandpass signals are also given by Binary line codes or Multi-level line codes Communication Systems 143.332 - Digital Modulation Slide 21
Binary Modulated Bandpass Signalling Examples 1 We shall illustrate a number of binary signal formats in the following sequence of slides. Unipolar A 1 is represented by a current of 2A signal units and a 0 is represented by a current of zero signal units. Communication Systems 143.332 - Digital Modulation Slide 22
Binary Modulated Bandpass Signalling Examples 2 Unipolar actually can occur in two forms, viz: Non return to zero (NRZ) Current maintained for entire bit period (time slot) In a long sequence with equally likely 1s and 0s, power is ½ (2A 2 ) or 2A 2 signal watts Return to zero (RZ) Currents are maintained for a fraction of the time slot. If we assume that the current is maintained for ½ the time slot and the symbols are equally likely, then the power in this case is ½ x 2A 2 = A 2 signal watts. Consider the sequence 101100111000 and view the following diagrams to compare the two cases. Communication Systems 143.332 - Digital Modulation Slide 23
Binary Modulated Bandpass Signalling Examples 3 With Non Return to Zero operation: Long sequences of 0s produce periods where there is no current generated Long sequences of 1s produce periods where positive current is generated When the 1s and 0s are equally likely, the mean value is A signal units. Each of the above conditions can cause problems for an electronic receiver, viz: When a constant current or no current flows there is no timing information and synchronisation is difficult. Unipolar (Non Return to Zero) Communication Systems 143.332 - Digital Modulation Slide 24
Binary Modulated Bandpass Signalling Examples 4 With Return to Zero operation: Long sequences of 0s produce periods where there is no current generated Long sequences of 1s produce periods where positive current is generated for a fraction of the time and hence a change can be detected by the receiver. When the 1s and 0s are equally likely and the pulses are ½ T wide, the mean value is A/2 signal units. So RZ eliminates the timing problem, but not the problem of long term level shifts. Communication Systems 143.332 - Digital Modulation Slide 25
Bipolar operation: Binary Modulated Bandpass Signalling Examples 5 A 1 is represented by a current of +A signal units A 0 is represented by a current of A signal units Two modes of operation, once again: Non Return to Zero Currents maintained for entire time slot Power needed for equally likely symbols is A 2 signal watts Return to Zero Currents maintained for fraction of time slot Power needed for equally likely symbols is A 2 /2 signal watts Communication Systems 143.332 - Digital Modulation Slide 26
Binary Modulated Bandpass Signalling Examples 6 Bipolar Non Return to Zero Bipolar Return to Zero Communication Systems 143.332 - Digital Modulation Slide 27
Binary Modulated Bandpass Signalling Examples 7 Long strings of 1s or 0s produce constant currents in NRZ bipolar and these represent a problem for electronic circuits once again. For RZ bipolar, these problems are basically eliminated because the receiver detects the return to zero in each pulse period. When 1s and 0s are equally likely, the mean signal value is just zero. Communication Systems 143.332 - Digital Modulation Slide 28
Biphase (or Manchester) Binary Modulated Bandpass Signalling Examples 8 A 1 is a positive current of amplitude A signal units that changes to a negative current pulse of equal magnitude and a 0 is a negative pulse that changes to a positive current pulse of equal magnitude. The change-over occurs at the midpoint of the timeslot. This type of coding is used between equipment that operates at a high speed and requires close synchronisation. Communication Systems 143.332 - Digital Modulation Slide 29
Binary Modulated Bandpass Signalling Examples 9 Alternate Mark Inversion (AMI) 1s are represented by return to zero current pulses of equal magnitude A that alternate between positive and negative. 0s are represented by the absence of current pulses. Power requirements are A 2 /4 which is half of RZ bipolar and one eighth of NRZ bipolar. Since the polarity alternates, almost all the power is contained within a bandwidth equal to the bit rate expressed in Hz. With a pulse shape that is approximately the same as a raised cosine, AMI is used extensively in the USA T1 carrier systems. Communication Systems 143.332 - Digital Modulation Slide 30
Binary Modulated Bandpass Signalling Examples 10 Two binary, one quaternary (2B1Q) Four signal levels (±3 and ±1) each represent a pair of bits. Of each pair, the first bit determines whether the level is positive or negative (1 = +ve, and 0 = -ve). 101100111000 Communication Systems 143.332 - Digital Modulation Slide 31
Comments on 2B1Q Signalling amplitude 11 10 01 00 time Usually described as distance 2 : -3, -1, +1, +3 2B1Q signalling is used for BISDN basic rate services (at 160kbps) and ISDN digital subscriber loop services. For long sequences of 1s and 0s, or alternating 1s and 0s (ie 1010101010 ) 2B1Q signalling produces constant currents and synchronisation is impossible. Since the frequency power density spectrums of 2B1Q, AMI and Raised Cosine are lower, they are employed in bandwidth limited environments such as telephone connections. Manchester is used in LANs and other applications where precise synchronisation is important and bandwidth is available. Communication Systems 143.332 - Digital Modulation Slide 32