Outline : Wireless Networks Lecture 6: Physical Layer Coding and Modulation 1. Basic Modulation Techniques. From Signals to Packets.

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Outline 18-759 : Wireless Networks Leure 6: Physial Layer Coding and Modulation 1 Peter Steenkiste Departments of Computer Siene and Elerial and Computer Engineering Spring Semester 2016 http://www.s.mu.

1 Outline : Wireless Networks Leure 6: Physial Layer Coding and Modulation 1 Peter Steenkiste Departments of Computer Siene and Elerial and Computer Engineering Spring Semester Peter A. Steenkiste 1 RF introduion Modulation and multipleing Channel apaity Antennas and signal propagation Equalization and diversity Modulation and oding» Coding and modulation» Amplitude, frequeny, phase» Code division multiple aess» OFDM Some newer tehnologies Sperum aess Peter A. Steenkiste 2 From Signals to Pakets Basi Modulation Tehniques Paket Transmission Pakets Bit Stream Digital Signal Analog Signal Sender Reeiver Header/Body Header/Body Header/Body Peter A. Steenkiste 3 Enode digital data in an analog signal Amplitude-shift keying (ASK)» Amplitude differene of arrier frequeny Frequeny-shift keying (FSK)» Frequeny differene near arrier frequeny Phase-shift keying (PSK)» Phase of arrier signal shifted Peter A. Steenkiste 4 Page 1

2 Amplitude-Shift Keying Modulator & Demodulator One binary digit represented by presene of arrier, at onstant amplitude Other binary digit represented by absene of arrier Aos 2f s t 0 where the arrier signal is Aos(2πf t) Ineffiient beause of sudden gain hanges» Only used when bandwidth is not a onern, e.g. on voie lines (< 1200 bps) or on digital fiber A an be a multi-bit symbol Peter A. Steenkiste 5 Modulate os(2f t) by multiplying by for T seonds: os(2f t) Y i (t) = os(2f t) Transmitted signal during kth interval Demodulate (reover ) by multiplying by 2os(2f t) for T seonds and lowpass filtering (smoothing): Y i (t) = os(2f t) Reeived signal during kth interval 2os(2f t) Filter (Smoother) X i (t) 2 os 2 (2f t) = {1 + os(22f t) +..} Peter A. Steenkiste 6 Binary Frequeny-Shift Keying (BFSK) Multiple Frequeny-Shift Keying (MFSK) Two binary digits represented by two different frequenies near the arrier frequeny s t where f 1 and f 2 are offset from arrier frequeny f by equal but opposite amounts Less suseptible to error than ASK Sometimes used for radio or on oa Demodulator looks for power around f 1 and f 2 Peter A. Steenkiste 7 A 1 A 2 os 2f t os 2f t More than two frequenies are used More bandwidth effiient but more suseptible to error s t A os 2 f t 1 i i f i = f + (2i 1 M)f d f = the arrier frequeny f d = the differene frequeny M = number of different signal elements = 2 L L = number of bits per signal element Eah symbol represents L bits» Symbol length is T s =LT seonds, where T is bit period Peter A. Steenkiste 8 i M Page 2

3 Multiple Frequeny-Shift Keying (MFSK) Phase-Shift Keying (PSK) Peter A. Steenkiste 9 Two-level PSK (BPSK)» Uses two phases to represent binary digits Aos 2f s t A os 2 f t Differential PSK (DPSK)» Phase shift with referene to previous bit Binary 0 signal of same phase as previous signal burst Binary 1 signal of opposite phase to previous signal burst Aos 2f Aos 2f t Peter A. Steenkiste 10 Phase-Shift Keying (PSK) Quadrature Amplitude Modulation (QAM) Four-level PSK (QPSK)» Eah element represents more than one bit s t Aos2f Aos2f Aos2f t 00 4 A os 2f t 10 4 Peter A. Steenkiste 11 QAM uses two-dimensional signaling» modulates in-phase os(2f t)» modulates quadrature phase sin(2f t)» Transmit sum of inphase & quadrature phase omponents Y i (t) = os(2f t) os(2f t) sin(2f t) Y q (t) = sin(2f t) Y i (t) and Y q (t) both oupy the bandpass hannel QAM sends 2 pulses/hz + Y(t) Transmitted Signal Peter A. Steenkiste 12 Page 3

4 QAM Demodulation Signal Constellations Y(t) 2os(2f t) 2sin(2f t) filter (smoother) 2os 2 (2f t)+2 os(2f t)sin(2f t) = {1 + os(4f t)}+ {0 + sin(4f t)} Peter A. Steenkiste 13 smoothed to zero B filter k (smoother) 2 sin 2 (2f t)+2 os(2f t)sin(2f t) = {1 - os(4f t)}+ {0 + sin(4f t)} smoothed to zero Eah pair (, ) defines a point in the plane Signal onstellation set of signaling points (-A,A) (-A,-A) (A, A) (A,-A) 4 possible points per T se. 2 bits / pulse Peter A. Steenkiste possible points per T se. 4 bits / pulse Some Eamples Gaussian Frequeny Shift Keying (GFSK) Gaussian Frequeny Shift Keying.» 1/-1 is a positive/negative frequeny shift from base» Gaussian filter is used to smooth pulses redues the speral bandwidth pulse shaping» Used in Bluetooth Differential quadrature phase shift keying.» Variant of regular frequeny shift keying» Symbols are enoded as hanges in phase» Requires deoding on pi/4 phase shift» Used in b networks Quadrature Amplitude modulation.» Combines amplitude and phase modulation» Uses two amplitudes and 4 phases to represent the value of a 3 bit sequene Peter A. Steenkiste 16 FSK an result in frequeny deviations» Uses more sperum GFSK first passes the binary signal through Gaussian filter to smoothen it and then uses it to modulate the signal Used in Bluetooth Gaussian Minimum Shift Keying (GMSK) is a variant that keeps the frequeny differene to a minimum» Used in GSM Peter A. Steenkiste 17 Page 4

5 Adapting to Channel Conditions Channel onditions an be very diverse» Affeed by the physial environment of the hannel» Changes over time as a result of slow and fast fading Fied oding/modulation sheme will often be ineffiient» Too onservative for good hannels, i.e. lost opportunity» Too aggressive for bad hannels, i.e. lots of paket loss Adjust oding/modulation based on hannel onditions rate adaptation» Controlled by the MAC protool» E.g a: BPSK QPSK 16-QAM 64 QAM Bad Good Peter A. Steenkiste 18 Page 5

Outline / Wireless Networks and Applications Lecture 5: Physical Layer Signal Propagation and Modulation

Outline / Wireless Networks and Applications Lecture 5: Physical Layer Signal Propagation and Modulation Outline 18-452/18-750 Wireless Networks and Applications Lecture 5: Physical Layer Signal Propagation and Modulation Peter Steenkiste Carnegie Mellon University Spring Semester 2017 http://www.cs.cmu.edu/~prs/wirelesss17/