SOFTWARE DEFINED RADIOUSR SDR WORKSHOP, SEPTEMBER 2017

PROF. MARCELO SEGURA

SESSION 2: AM MODULATION, REVIEW OF CONCEPT

REAL IMPLEMENTATION

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USC SDR WORKSHOP

AM

• AM is poor efficiency since it use double of bandwidth.

• But is on of the simplest modulation systems, AM-DSB-SC.

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QUADRATURE AMPLITUDE MODULATION

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AM

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BANDWIDTH EFFICIENCY

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AM

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DSB-SC

Pros: very simple implementation

Cons: Require coherent demodulator, phase and frequency.

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AM

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DSB-SC

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AM

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DSB-SC BASE BAND

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AM

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DSB-TC

m: modulation index

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AM

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DSB-TC

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AM

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MODULATION INDEX

Pros: NOT require coherent demodulator

Cons: bad spectral efficiency

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AM

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AM-SSB

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QAM

• Advantages of QAM, we have better spectral efficiency.

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QAM MODULATOR

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QAM

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QAM DEMODULATOR

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QAM

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I/Q PERFECT DEMO

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QAM

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I/Q PERFECT DEMO

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QAM

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PHASE SHIFT

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QAM

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PHASE OFFSET

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QAM

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FREQUENCY OFFSET

WE HAVE TO CORRECT OFFSET BEFORE

START WORKING WITH SDR

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QAM

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COMPLEX EXPONENTIAL

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QAM

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COMPLEX EXPONENTIAL

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QAM

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REAL HARDWARE VS COMPLEX NOTATION

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AM

• The simplest receiver will be a DSB-SC, because we just need to multiply by the carrier.

• This will works ONLY if no phase or frequency offset.

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THEORETICAL DEMODULATION ON DSB-SC

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AM

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REAL COHERENT DEMODULATOR DSB-TC

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AM

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REAL COHERENT DEMODULATOR

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AM

• Real demodulator, w/offset

• Perfect demo

• Because is hard to get “perfect tuning” on the carrier, we need a coherent receiver with frequency and

phase recovery to avoid this problems.

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PERFECT DEMODULATOR

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AM

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DEMODULATED RECEIVED SPECTRUM

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SSB

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GENERATION AM-SSB

EASY way is to filter AM-DSB-SC, BUT filter needs very large attenuation and rolloff factor

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SSB

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SSB TRANSMITTER

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VSB

• It was created to reduce the bandwidth on analog TV channels that use DSB-TC.

• The modulator is DSB-TC, but with a pass-band filter at the output. This filter allows to pass the carrier plus

a portion of the lower band, the vestigial band.

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VESTIGIAL SIDE BAND (VSB)

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VSB

• Transmitter

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VSB

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AM

• Classical detector, only works for DSB-TC

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NON-COHERENT DEMO: ENVELOPE DETECTOR

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AM TRANSMISSION

• USRP hardware support package is required.

• By default only digital up-conversion is done on the FGPA

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USRP TX

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AM TRANSMISSION

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USRP MATLAB PARAMETERS

LO Offset

• TX Gain: expressed as scalar. Max 90dB

• RX Gain: 73dB

• Clock Freq: 5Mhz a 56Mhz

• DUC: default 512, correspond to simple rate of 1e8/512=195Khz

• Underrun: “+” not enough samples, “0” no data loss

• Enable burst: no underrun /overrun, fixed number of frames

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AM TRANSMISSION

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USRP TX AM DSB-SC

Only send real data!!!

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AM TRANSMISSION

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USRP TX AM DSB-TC

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AM TRANSMISSION

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AM-SSB

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AM TRANSMISSION

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AM-SSB

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AM RECEIVER

• Demo Ideal

• PROBLEM: it is probable that we have frequency offset, so the AM will be modulated but a low frequency

near DC. Envelop works if fc>>fi.

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RECEIVER DSB-TC WITH ENVELOPE DETECTOR

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AM RECEIVER

• SOLUTION: Use an offset intermediate freq.

• Demodulated signal will be on IF area, after that envelope detector can be applied.

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RX NON-COHERENT

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AM RECEIVER

• We cannot recover DSB-SC signal we envelope detector.

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RX NON-COHERENT

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AM RECEIVER

• We need a coherent receiver that use a PLL to recover the phase and frequency.

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RX COHERENT

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FDM

• FDM allows to send multiple signals at the same time, sharing the available spectrum.

• The first application of FDM was on wired telephony, where each user has 3.4Khz of the total bandwidth.

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FDM INTRO

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FDM

• DSB-TC modulation with real carrier

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FDM TX

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FDM

• Quadrature modulation, complex carrier.

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FDM TX

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FDM

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FDM- TX USRP

USC SDR WORKSHOP

FDM

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FDM TX USRP

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FDM

Intermediate frequency receiver.

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FDM RX