EEC4113 Data Communication & Multimedia System Chapter 3: Broadband Encoding by Muhazam Mustapha, October 2011

Learning Outcome By the end of this chapter, students are expected to be able to explain link level broadband encoding for transmission

Chapter Content Amplitude Shift Keying Frequency Shift Keying Phase Shift Keying Pulse Width Modulation Quadrature Modulation Spread Spectrum Technology

Amplitude Shift Keying CO1, CO2

Amplitude Shift Keying (ASK) Values represented by different amplitudes of carrier frequency It is similar to Amplitude Modulation (AM) in analog communication, but with only two levels of amplitude Usually, one amplitude is zero – i.e. presence and absence of carrier CO1, CO2

Amplitude Shift Keying (ASK) Susceptible to sudden changes in gain Up to 1200bps on voice grade lines Used over optical fiber CO1, CO2

Amplitude Shift Keying (ASK) 1 1 1 1 CO1, CO2

Frequency Shift Keying CO1, CO2

Frequency Shift Keying (FSK) Different frequency used to represent data Two types: Binary FSK (BFSK) Multiple FSK (MFSK) CO1, CO2

FSK on Voice Grade Line Signal strength Frequency (Hz) 1170 2125 Spectrum of signal transmitted in one direction Spectrum of signal transmitted in opposite direction Frequency (Hz) 1170 2125 Full Duplex FSK Transmission on a Voice Grade Line CO1

Multiple FSK More than two frequencies used Each signaling element represents more than one bit Example: 3 bits per signal element, 8 signal elements, 8 different frequencies Advantage: More bandwidth efficient Disadvantage: More prone to error CO1, CO2

Binary Frequency Shift Keying (BFSK) Most common form of FSK Two binary values are represented by two different frequencies It is similar to Frequency Modulation (FM) in analog communication, but with only two frequencies Less susceptible to error than ASK CO1, CO2

Binary Frequency Shift Keying (BFSK) Up to 1200bps on voice grade lines High frequency radio transmission (3 to 30 MHz) Even higher frequency on LANs using coaxial cable CO1, CO2

Binary Frequency Shift Keying (BFSK) 1 1 1 1 CO1, CO2

Phase Shift Keying CO1, CO2

Phase Shift Keying (PSK) Phase of carrier signal is shifted to represent data Broadband signaling ASK FSK PSK QAM CO1, CO2

Binary Phase Shift Keying (BPSK) Two phases represent two binary digits 1 1 1 1 CO1, CO2

Differential Phase Shift Keying (DPSK) Binary 1: Phase change Binary 0: No phase change 1 1 1 1 CO1, CO2

Pulse Width Modulation CO1, CO2

Pulse Width Modulation (PWM) The width (duration) of the pulse is used to represent data In analog communication, PWM needs continuous width values to represent the analog waveform at certain sampling instances In data (digital) communication, the pulses can have discrete values of width CO1, CO2

Pulse Width Modulation (PWM) An integrator or timing circuit can be used to decode the carried bits Data 10 01 11 01 00 Pulse CO1, CO2

Quadrature Modulation CO1, CO2

Quadrature PSK (QPSK) More efficient use by each signal representing more than one bit e.g. for shifts of π/2 (90°), each element represents two bits 135°:01 45°:11 225°:00 315°:10 Constellation diagram for QPSK CO1, CO2

Quadrature PSK (QPSK) Alternative choice of phases: Dibit Phase 00 01 90°:01 Dibit Phase 00 01 90 10 180 11 270 180°:10 0°:00 270°:11 Constellation diagram CO1, CO2

Quadrature PSK (QPSK) 00 11 01 00 10 CO1, CO2

Phase Detector Signal Signal Ref Signal Ref Signal Ref Ref CO1 phase leading Signal phase lagging Signal Ref phase leading Signal Ref phase leading phase lagging phase leading Signal phase lagging Ref phase lagging Ref CO1

OQPSK (Offset / Orthogonal) QPSK sometimes causes a phase change of 180° (π). Phase change of 180° sometimes means discontinuity jump, and it is the largest possible jump. The jump causes a large amplitude of high frequency and small amplitude of low frequency in transmission, and if it is low pass filtered, then the signal will experience a large fluctuation during the phase change. CO1

Examples: QPSK & OQPSK Waveforms bit number 1 2 3 4 5 6 7 8 9 10 value 1 −1 1 1 −1 −1 −1 1 1 1 I Q I Q I Q I Q I Q input signal I(t) 1 3 5 7 9 Q(t) 2 4 6 8 10 phase of output signal −π/4 π/4 −3π/4 3π/4 π/4 Q(t−Tb) phase of output signal −π/4 −π/4 π/4 3π/4 −3π/4 −3π/4 3π/4 π/4 π/4 CO1

OQPSK OQPSK was designed to reduce this fluctuation by delaying one of the signal combined in the modulator. Revisit the example: Transition between bit 3-4 & 5-6 causes 180° in QPSK, but OQPSK signal made a gradual 2 steps of 90°. This reduces the fluctuation. The signal would be strobed at the right time to get the right binary combination. CO1

QPSK and OQPSK Modulators Offset QPSK (orthogonal QPSK) Delay in Q stream I(t) an = ±1 R/2 bps carrier frequency binary input 2 bit serial to parallel conversion Σ output s(n) π/2 phase shift R/2 bps Delay Tb Q(t) bn = ±1 OQPSK only CO1

OQPSK 1 3 5 7 9 2 4 6 8 10 −π/4 π/4 −3π/4 3π/4 π/4 −π/4 −π/4 π/4 3π/4 input signal I(t) 1 3 5 7 9 Q(t) 2 4 6 8 10 phase of output signal −π/4 π/4 −3π/4 3π/4 π/4 180° transition Q(t−Tb) phase of output signal −π/4 −π/4 π/4 3π/4 −3π/4 −3π/4 3π/4 π/4 π/4 two 90° transitions CO1

Multilevel PSK More levels taking more than 2 bits at a time e.g. Transmit 3 bits at a time by using 8 phase angles (8-PSK) Further, each angle can have more than one amplitude (QAM) Tribit Phase 000 001 45 010 90 011 135 100 180 101 225 110 270 111 315 010 011 001 100 000 101 111 110 CO1, CO2 Constellation diagram

Quadrature Amplitude Modulation (QAM) QAM used on Asymmetric Digital Subscriber Line (ADSL) and some wireless standards Combination of ASK and PSK Logical extension of QPSK Send different signals simultaneously on same carrier frequency Signals are distinguished by phase and amplitude difference CO1, CO2

8-QAM 2 amplitudes 4 phases QAM Constellations 011 01 00 010 101 100 000 001 10 11 110 111 4-QAM 1 amplitude 4 phases 8-QAM 2 amplitudes 4 phases CO1, CO2

QAM Constellations CO1, CO2 2 amplitudes 8 phases Standard 9600 bps modem use 12 angles, four of which have two amplitudes for a total of 16 different signal elements CO1, CO2

Spread Spectrum Technology CO1

Spread Spectrum Technology Spread spectrum technology is a method whereby a signal is being transmitted over a bandwidth much wider than the required bandwidth to transmit the intended information. The main reason for the technology is security. The signal that has been scattered over a large bandwidth can easily be scrambled and become hard to detect or interpret CO1

Frequency Hopping Spread Spectrum (FHSS) The communication is done over many carrier frequencies that is changed randomly Sequence: Initiating side sends a request via a predefined frequency The receiving side sends a number, known as seed The initiating side uses the number to calculate a sequence of frequencies to be used The initiating side sends a synchronization signal through the first frequency in the sequence Both sides would then continue communication via those frequencies at known timing CO1

Frequency Hopping Spread Spectrum (FHSS) Signal strength Carrier frequency hops from channel to channel Frequency (Hz) CO1

Frequency Hopping Spread Spectrum (FHSS) Carrier frequencies are chosen at random over times Carrier Frequency 8 7 6 5 4 3 2 1 Time 1 2 3 4 5 6 7 8 9 10 11 12 13 14 CO1

Direct Sequence Spread Spectrum (DSSS) The communication is done over many phase modulations that is applied randomly The original modulated signal is further phase modulated at random phases – called chips The rate of chips are much higher than the data rate The receiver should know the same sequence of chips The original data can then be retrieved by removing the chips CO1

Direct Sequence Spread Spectrum (DSSS) Signal strength Narrow Band Spectrum Spread Spectrum Noise Level Frequency (Hz) CO1

Other Spread Spectrum Technologies Time Hopping Spread Spectrum (THSS) Data is burst at random times bursts Time CO1

Other Spread Spectrum Technologies Chirp Spread Spectrum (CSS) A chirp is sinusoidal signal that increases or decreases over some period of time CSS data is transmitted over a pulse that is modulated by chirp (time varying carrier frequency) data spectrum at start of pulse data spectrum at end of pulse Frequency (Hz) CO1