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ECE 4371, Fall, 2014 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 13 Oct. 8th, 2014 Outline Partial Response Carrier systems – ASK, OOK, MASK – FSK, MFSK – BPSK, DBPSK, MPSK – MQAM, MQPR – OQPSK, – Continuous phase modulation (CPM): MSK, GMSK Partial Response Signals Previous classes: Sy(w)=|P(w)|^2 Sx(w) – Control signal generation methods to reduce Sx(w) – Raise Cosine function for better |P(w)|^2 This class: improve the bandwidth efficiency – Widen the pulse, the smaller the bandwidth. – But there is ISI. For binary case with two symbols, there is only few possible interference patterns. – By adding ISI in a controlled manner, it is possible to achieve a signaling rate equal to the Nyquist rate (2W symbols/sec) in a channel of bandwidth W Hertz. Example Duobinary Pulse – p(nTb)=1, n=0,1 – p(nTb)=1, otherwise Interpretation of received signal – 2: 11 – -2: 00 – 0: 01 or 10 depends on the previous transmission Duobinary signaling Duobinary signaling (class I partial response) Duobinary signal and Nyguist Criteria Nyguist second criteria: but twice the bandwidth Differential Coding The response of a pulse is spread over more than one signaling interval. The response is partial in any signaling interval. Detection : – Major drawback : error propagation. To avoid error propagation, need deferential coding (precoding). Modified duobinary signaling Modified duobinary signaling – In duobinary signaling, H(f) is nonzero at the origin. – We can correct this deficiency by using the class IV partial response. Modified duobinary signaling Spectrum Modified duobinary signaling Time Sequency: interpretation of receiving 2, 0, and -2? Pulse Generation Generalized form of correlative-level coding (partial response signaling) Tradeoffs Binary data transmission over a physical baseband channel can be accomplished at a rate close to the Nyquist rate, using realizable filters with gradual cutoff characteristics. Different spectral shapes can be produced, appropriate for the application at hand. However, these desirable characteristics are achieved at a price : – A large SNR is required to yield the same average probability of symbol error in the presence of noise. Exercise What is the differential Duobinary output What is the modified Duobinary output and decoded signal? Interpretation of receiving 2, 0, and -2? What is the modified differential Duobinary output Other types of partial response signals paper Type r0 r1 r2 r3 r4 ideal 1 I 1 1 II 1 2 1 5 III 2 1 -1 6 IV 1 0 -1 3 V -1 0 2 p(t) P(W) Levels 2 3 0 -1 5 ASK, OOK, MASK The amplitude (or height) of the sine wave varies to transmit the ones and zeros One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation) Binary amplitude shift keying, Bandwidth d ≥ 0 related to the condition of the line B = (1+d) x S = (1+d) x N x 1/r Implementation of binary ASK OOK and MASK OOK (On-OFF Key) – 0 silence. – Sensor networks: battery life, simple implementation MASK: multiple amplitude levels Pro, Con and Applications Pro – Simple implementation Con – Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude – Susceptible to sudden gain changes – Inefficient modulation technique for data Applications – On voice-grade lines, used up to 1200 bps – Used to transmit digital data over optical fiber – Morse code – Laser transmitters Example We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What are the carrier frequency and the bit rate if we modulated our data by using ASK with d = 1? Solution – The middle of the bandwidth is located at 250 kHz. This means that our carrier frequency can be at fc = 250 kHz. We can use the formula for bandwidth to find the bit rate (with d = 1 and r = 1). Frequency Shift Keying One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation) Represent each logical value with another frequency (like FM) A cos2f1t s t A cos2f 2t binary1 binary 0 FSK Bandwidth Limiting factor: Physical capabilities of the carrier Not susceptible to noise as much as ASK Applications – On voice-grade lines, used up to 1200bps – Used for high-frequency (3 to 30 MHz) radio transmission – used at higher frequencies on LANs that use coaxial cable Example We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What should be the carrier frequency and the bit rate if we modulated our data by using FSK with d = 1? Solution – This problem is similar to Example 5.3, but we are modulating by using FSK. The midpoint of the band is at 250 kHz. We choose 2Δf to be 50 kHz; this means Multiple Frequency-Shift Keying (MFSK) More than two frequencies are used More bandwidth efficient but more susceptible to error si t A cos2f i t 1 i M f i = f c + (2i – 1 – M)f d f c = the carrier frequency f d = the difference frequency L M = number of different signal elements = 2 L = number of bits per signal element Phase Shift Keying One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation) binary1 A cos2f ct s t binary 0 A cos2f ct DBPSK, QPSK Differential BPSK – 0 = same phase as last signal element – 1 = 180º shift from last signal element Four Level: QPSK s t A cos 2f c t 4 3 A cos 2f c t 4 3 A cos 2f c t 4 A cos 2f c t 4 11 01 00 10 QPSK Example Bandwidth Min. BW requirement: same as ASK! Self clocking (most cases) Concept of a constellation diagram MPSK Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved R R D L log2 M – D = modulation rate, baud – R = data rate, bps – M = number of different signal elements = 2L – L = number of bits per signal element QAM – Quadrature Amplitude Modulation Modulation technique used in the cable/video networking world Instead of a single signal change representing only 1 bps – multiple bits can be represented buy a single signal change Combination of phase shifting and amplitude shifting (8 phases, 2 amplitudes) QAM QAM – As an example of QAM, 12 different phases are combined with two different amplitudes – Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations – With 16 signal combinations, each baud equals 4 bits of information (2 ^ 4 = 16) – Combine ASK and PSK such that each signal corresponds to multiple bits – More phases than amplitudes – Minimum bandwidth requirement same as ASK or PSK QAM and QPR QAM is a combination of ASK and PSK – Two different signals sent simultaneously on the same carrier frequency st d1 t cos2f ct d2 t sin 2f ct – M=4, 16, 32, 64, 128, 256 Quadrature Partial Response (QPR) – 3 levels (+1, 0, -1), so 9QPR, 49QPR Offset quadrature phase-shift keying (OQPSK) QPSK can have 180 degree jump, amplitude fluctuation By offsetting the timing of the odd and even bits by one bit-period, or half a symbol-period, the in-phase and quadrature components will never change at the same time. Continuous phase modulation (CPM) CPM the carrier phase is modulated in a continuous manner constant-envelope waveform yields excellent power efficiency high implementation complexity required for an optimal receiver minimum shift keying (MSK) – 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. ECE 4371 Fall 2008 Gaussian minimum shift keying GMSK is similar to MSK except it incorporates a premodulation Gaussian LPF Achieves smooth phase transitions between signal states which can significantly reduce bandwidth requirements There are no well-defined phase transitions to detect for bit synchronization at the receiving end. With smoother phase transitions, there is an increased chance in intersymbol interference which increases the complexity of the receiver. Used extensively in 2nd generation digital cellular and cordless telephone apps. such as GSM