1 ECE513 RF Design for Wireless MODULE 1 RF Systems LECTURE 1 Modulation Techniques Chapter 1, Sections 1.1 1.3 Professor Michael Steer http://www4.ncsu.edu/~mbs
2 Module 1: RF Systems Amplifiers, Mixers & Oscillators Antennas and RF Link Radio Frequency Systems Aim is to develop a full appreciation of an RF communication, focus is on a cellular radio system. System requirements drive component and subsystem development. Also capabilities of components and subsystems drive systems.
3 Module 1: Lecture 1 Amplifiers, Mixers & Oscillators RF Front Ends RF signals Analog Modulation Digital Modulation
4 Lecture Goal Develop an understanding of modulation techniques used in analog and digital cellular radio. Lecture Outline Analog Modulation: AM, FM, PM Introduction to Digital Modulation: ASK, FSK, PSK Advanced Modulation Concepts Occupied Bandwidth Performance of Modulation Types used in Cellular Radio Analog FM, DQPSK, FSK, GMSK
5 A Modulator I K WAVEFORM SHAPING I i(t) a(t) SERIAL BIT STREAM SERIAL TO PARALLEL VCO 90 s(t) Q K WAVEFORM SHAPING q(t) Q b(t)
6 Performance of Different Modulation Types Each type of modulation has advantages and drawbacks: Necessary bandwidth How wide is the signal? How much spectrum is needed? How big a "guard band" is needed between channels? Relative vulnerability to interference What C/I ratio is required for good system performance? Relative difficulty of implementation Is complex equipment required? Implementation cost? Maintainability
7 Analog Modulation Modulation is the process of varying some characteristic of a radio signal in order to convey information Time This analog waveform modulates a sinewave. The basic, unchanging, steady radio signal without modulation is called a carrier. Characteristics that can be modulated: Amplitude e.g. AM radio broadcasting Frequency e.g. FM broadcasting, Voice transmission in AMPS cellular Phase Note that frequency and phase modulation look very similar with this kind of input.
8 Amplitude Modulation (AM) [ ] xt ( ) = A 1+ mcosω t cosω t c m c The First Radio System to Transmit Voice were based on amplitude modulation.
9 Peak-to-Average Ratio CW Envelope AM (100%) ENVELOPE v peak v average = ½ v peak PAR = 4.26 db RF Power FM PAR = 0 db With FM amplitude distortion does not matter as there is no information in the amplitude of the signal.
10 PAR Peak-to-Average Ratio of AM [ ] xt ( ) = A 1+ mcosω t cosω t c m c PAR is an important metric for modulation formats Determines how amplifier must be designed for specified distortion. [ ] xpeak ( t ) = Ac 1 + m cos ω t c
11 PAR 1/4 [ ] x ( ) 1 cos PEAK t = Ac + m ωct
12 PAR 2/4 τ 1 2 avg lim τ 2τ ( ) τ P = Gv t. dt.
13 PAR 3/4 [ ω ] xt ( ) = A 1+ mcos t cosω t c m c
14 PAR 4/4 [ ω ] xt ( ) = A 1+ mcos t cosω t c m c
15 Analog FM How Much Bandwidth is Required? Time Carrier -3f i Input Signal -2f i -f i c +f i f i = input signal frequency frequency deviation Frequency Sidebands +2f i +3f i Frequency As time passes, the carrier moves back and forth in frequency in exact step with the input signal frequency deviation is proportional to the input signal voltage a group of many sidebands is created, spaced from carrier by amounts N x f i relative strength of each sideband N depends on Bessel function N of (input signal freq./freq. deviation) strength of individual sidebands far away from the carrier is proportional to (freq. deviation x input frequency) Carson s Rule Bandwidth Required = 2 x (highest input frequency + frequency deviation)
16 Analog Modulation and Bandwidth Time-Domain (as viewed on an Oscilloscope) Time Frequency-Domain (as viewed on a Spectrum Analyzer) 0 Lower Sideband Frequency Upper Sideband The bandwidth occupied by a signal depends on: input information bandwidth modulation method Information to be transmitted, called input or baseband bandwidth usually is small, much lower than frequency oarrier Unmodulated carrier the carrier itself has Zero bandwidth!! AM-modulated carrier Notice the upper & lower sidebands total bandwidth = 2 x baseband FM-modulated carrier Many sidebands! bandwidth is a complex mathematical function PM-modulated carrier Many sidebands! bandwidth is a complex mathematical function
17 Analog Modulation and PAR Time 0 Frequency CARRIER 100% AM Lower Sideband Upper Sideband FM PM
18 Narrow Band Communication The majority of modulation formats result in narrow band communication systems AM These were the easiest modulation schemes for most of the 20 th century and dictated the assignment of radio into narrow band channels. FDMA AMPLITUDE FREQUENCY 1912: Regulation began with the sinking of the Titanic.
19 Frequency Modulation & Bandwidth Time-Domain (as viewed on an Oscilloscope) Time Frequency-Domain (as viewed on a Spectrum Analyzer) 0 Frequency The bandwidth occupied by a signal depends on: input information bandwidth modulation method Information to be transmitted, called input or baseband bandwidth usually is small, much lower than frequency oarrier FM-modulated carrier Many sidebands! bandwidth is a complex mathematical function The bandwidth requirement for a frequency modulated signal is difficult to determine for arbitrary modulating signals as the frequency spectrum of this signal extends indefinitely but the amplitude of the spectrum falls off rapidly.
20 Analog FM How Much Bandwidth is Required? Time Carrier -3f i Input Signal -2f i -f i c +f i f i = input signal frequency frequency deviation Frequency Sidebands +2f i +3f i Frequency As time passes, the carrier moves back and forth in frequency in exact step with the input signal frequency deviation is proportional to the input signal voltage a group of many sidebands is created, spaced from carrier by amounts N x f i relative strength of each sideband N depends on Bessel function N of (input signal freq./freq. deviation) strength of individual sidebands far away from the carrier is proportional to (freq. deviation x input frequency) Carson s Rule Bandwidth Required = 2 x (highest input frequency + frequency deviation)
21 Narrowband and Wideband FM The difference is between what are called Narrowband FM and Wideband FM. The best rule of thumb formula is Carson s Rule: Bandwidth Required = B R = 2 x (highest input frequency + frequency deviation) Let's replace this by B R = Bandwidth Required F = frequency deviation f m = highest input frequency (bandwidth of modulating signal) So B R = 2 * ( F + f m ) Narrowband FM is when F << f m and then it can be shown that the bandwidth required is 2 * f m but only for Frequency Shift Keying (FSK) which is a two state form of FM. Wideband FM is when F >> f m and then it can be shown that the bandwidth required is 2 * F.