digital_mod - Principles of Digital Modulation Dr Mike...

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1 Principles of Digital Modulation Dr Mike Fitton, mike.fitton@toshiba-trel.com Telecommunications Research Lab Toshiba Research Europe Limited
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2 Principles of Digital Modulation: Outline of Lectures l Introduction to digital modulation l Relevant Modulation Schemes (QPSK, GMSK, M-Ary Schemes) l Coherent and Differential Reception l The impact of the mobile channel on digital modulation – noise and interference – random FM (narrowband fading) – intersymbol interference (wideband fading)
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3 Digital Modulation Basics
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4 Digital Modulation Basics l The bit rate defines the rate at which information is passed. l The baud (or signalling ) rate defines the number of symbols per second. Each symbol represents n bits, and has M signal states, where M = 2 n . This is called M-ary signalling . l The maximum rate of information transfer through a baseband channel is given by: Capacity f b = 2 W log 2 M bits per second where W = bandwidth of modulating baseband signal
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5 Amplitude Shift Keying (ASK) l Pulse shaping can be employed to remove spectral spreading. l ASK demonstrates poor performance, as it is heavily affected by noise and interference. Baseband Data ASK modulated signal A cos ϖ c t A cos ϖ c t 0 0
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6 Frequency Shift Keying (FSK) l Bandwidth occupancy of FSK is dependant on the spacing of the two symbols. A frequency spacing of 0.5 times the symbol period is typically used. l FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states. Baseband Data FSK modulated signal f 1 f 1 f 0 f 0 where f 0 = A cos( ϖ c - ∆ϖ )t and f 1 = A cos( ϖ c + ∆ϖ )t
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7 Phase Shift Keying (PSK) l Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and FSK. l PSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states. l Filtering can be employed to avoid spectral spreading. Baseband Data Binary PSK modulated signal s 1 s 1 s 0 s 0 where s 0 = -A cos ϖ c t and s 1 = A cos ϖ c t
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8 l The Nyquist bandwidth is the minimum bandwidth than can be used to represent a signal. l It is important to limit the spectral occupancy of a signal, to improve bandwidth efficiency and remove adjacent channel interference. l Root raised cosine filters allow an approximation to this minimum bandwidth. +10 -60 -50 -40 -30 -20 -10 0 -4 -2 -1 0 1 Frequency Offset from Carrier (f-fc)Ts (Hz) 2 3 -3 4 Magnitude (dB) Nyquist Minimum Bandwidth Nyquist bandwidth on the QPSK spectrum
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9 Modulation - QPSK l Quadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q), and therefore exhibits the same performance but twice the bandwidth efficiency. l Quadrature Phase Shift Keying can be filtered using raised cosine filters to achieve excellent out of band suppression. l
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This note was uploaded on 12/29/2010 for the course UAE 605 taught by Professor Ttttt during the Spring '10 term at University of Arkansas – Fort Smith.

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digital_mod - Principles of Digital Modulation Dr Mike...

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