Software Defined Radio

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Software Defined

Radio

GNU Radio and the

USRP

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Overview



What is Software Defined Radio?



Advantages of Software Defined Radio



Traditional versus SDR Receivers



SDR and the USRP



Using GNU Radio

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Introduction

 What is Software Defined Radio (SDR)?

 Getting code as close to the antenna as possible

 Replacing hardware with software for

modulation/demodulation

 Advantages:

 Makes communications systems reconfigurable

(adapting to new standards)

 Flexible (universal software device - not special

purpose)

 Filters/Other Hardware

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Traditional Receiver

Local Oscillator RF Amplifier IF Amplifier x Demod-ulator fc fLO |fLO-fc| fLO+fc f (KHz) 530 ₉₈₀ 1700 10 KHz f (KHz) 530 ₉₈₀ 1700 10 KHz 455 f (KHz) 10 KHz fLO=1435 KHz 455 f (KHz) 10 KHz f (KHz) 5 5

Traditional vs. SDR Receiver

RF Amplifier IF Amplifier x Demod-ulator Local Oscillator

Receiver Front End Traditional

/ Hardware Receiver

Software

Receiver Front End ADC

Current SDR Receiver Software ADC Future SDR Receiver ? 6

SDR Receiver Using the USRP

Receiver

Front End ADC FPGA ControllerUSB PC

Daughterboard Motherboard

similar to traditional front end with fIF = 0

Decimation, MUX, +

Interface to PC GNU Radio software

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Quadrature Signal

Representation

The received signal, S(t), may be represented as follows:

S(t)= I(t)cos(2! fct)+ Q(t)sin(2! fct)

fc = carrier frequency

I(t) = in-phase component Q(t) = quadrature component

Contain amplitude and phase information of baseband signal •GNU Radio software uses I and Q components to demodulate signals

•USRP front end translates the signal to zero frequency and extracts I and Q

f (MHz) 446 16 KHz

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Extracting I(t) from S

S(t)= I(t)cos(2! fct)+ Q(t)sin(2! fct) S(t)cos(2! fct)= I(t)cos 2_{(2! f} ct)+ Q(t)sin(2! fct)cos(2! fct) =I(t) 2

[

1+ cos(4! fct)

]

+ Q(t) 2

[

sin(4! fct)+ sin(0)

]

=1 2I(t)+ 1 2I(t)cos(4! fct)+ + 1 2Q(t)sin(4! fct)

Multiplying both sides by cos(2πfct):

Applying this signal to a low pass filter, the output will be: 1

2I(t)

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Extracting Q(t) from S

S(t)= I(t)cos(2! fct)+ Q(t)sin(2! fct)

S(t)sin(2! fct)= I(t)cos(2! fct)sin(2! fct)+ Q(t)sin

2_{(2! f} ct) =I(t) 2

[

sin(4! fct)" sin(0)

]

+ Q(t) 2

[

1" cos(4! fct)

]

=1 2I(t)sin(4! fct)+ 1 2Q(t)" 1 2Q(t)cos(4! fct)

Multiplying both sides by sin(2πfct):

Applying this signal to a low pass filter, the output will be: 1

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USRP Receiver Front End

RF Amplifier x LPF 90° x LPF LO fc I Q 11

Analog to Digital Converter

(ADC)



12 bit A/D Converter (2

12

levels)



2 volt peak-peak maximum input



64 Msamp/second

ADC ∆t !t = 1 64" 106= 0.0156µS !v = 2 212= 0.488mV Quantization Levels: Sampling Interval: 12

Decimation

 Original sampling rate is 64Msamp/sec

 Converts a portion of spectrum 32 MHz wide

 Generally we are interested is a narrower portion of the spectrum requiring a lower sampling rate

 USB cannot handle that high data rate

 Occurs in the FPGA of the USRP

f 32MHz LPF f 250KHz Downsample divide by 128 f 250KHz

fs = 64Msamp/sec fs = 64Msamp/sec fs = 500Ksamp/sec

64M 500K= 128

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SDR Receiver with USRP

ADC FPGA (Decimator, MUX, etc.) USB Controller PC I Q Daughterboard Motherboard GNU Radio Software 14

USRP

-Motherboard/Daughterboard

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GNU Radio Software

 Community-based project started in 1998

 GNU Radio application consists of sources (inputs), sinks (outputs) and transform blocks

 Transform blocks: math, filtering, modulation/demodulation, coding, etc.

 Sources: USRP, audio input, file input, signal generator, …

 Sinks: USRP, audio output, file output, FFT, oscilloscope, …

 Blocks written in C++

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Design of a Receiver

 USRP: Set frequency of local oscillator (receive

frequency), gain of amplifier, decimation factor

 GNU Radio application: use Python to specify

and connect blocks that perform demodulation and decoding

USRP GNU Radio Application

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Example: 400 - 500 MHz NBFM

Receiver



Problem: Receive an audio signal (up to 4

KHz) transmitted at 446 MHz using

narrowband FM (NBFM) with a 16 KHz

transmission bandwidth

USRP GNU Radio Application

f (MHz) 400 446 500 16 KHz a f 4KHz 18

Design Procedure

1.

Plan the block diagram of system

components

2.

Determine block parameters

3.

Determine decimation rates

4.

Write Python script to specify the blocks

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NBFM Receiver: Block

Diagram/Parameters

a f (MHz) 400 446 500 16 KHz Daughterboard fc = 446 MHz f (MHz) 16 KHz ADC 64 Msamp/sec f (MHz) 16 KHz 32 -32 FPGA Dec. factor = ? f (MHz) 16 KHz ? -? Channel Filter cutoff = 8KHz Dec. factor = ? f (KHz) 16 KHz ? -? -88 FM Demodulator Dec. factor = ? f (KHz) 4 -4 Audio FM USRP PC 20

Determining the Decimation

Factors

64Msamp/sec 8Ksamp/sec

Total Decimation factor = 8000 = D1D2D3

a f (MHz) 16 KHz 32 -32 FPGA Dec. factor = D1 Channel Filter cutoff = 8KHz Dec. factor = D2 FM Demodulator Dec. factor = D3 -44 f (KHz) Audio 21

FPGA Decimation Factor, D

₁

•Total Decimation factor = 8000 = D1D2D3

•Maximize the decimation in FPGA •Maximum decimation factor in FPGA = 256 •Select D1 = 250 (factor of 8000)

•Output sample rate = 64Ms/s / 250 = 256Ks/s

f (MHz) 16 KHz 32 -32 FPGA Dec. factor = D1 Channel Filter cutoff = 8KHz Dec. factor = D2 FM Demodulator Dec. factor = D3 -44 f (KHz) Audio

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Channel Filter Specification

•Maximum frequency = 16 KHz Reduce sample rate to 32 Ks/s •256Ks/s / 32Ks/s D2 = 8 a f (MHz) 16 KHz 32 -32 FPGA Dec. factor = 250 Channel Filter cutoff = 8KHz Dec. factor = D2 FM Demodulator Dec. factor = D3 -44 f (KHz) Audio f (KHz) 16 KHz 128 -128 64Ms/s 256Ks/s a H (dB) f (KHz) -60 -16 -9-8 89 16 -0.1 0 Channel Filter 23

FM Demodulator

•Maximum frequency = 4 KHz Reduce sample rate to 8 Ks/s •32Ks/s / 8Ks/s D3 = 4

•FM Demodulator block “extracts” audio signal from FM waveform by operating on I and Q a f (MHz) 16 KHz 32 -32 FPGA Dec. factor = 250 Channel Filter cutoff = 8KHz Dec. factor = 8 FM Demodulator Dec. factor = D3 -44f (KHz) Audio f (KHz) 16 KHz 128 -128 64Ms/s 256Ks/s f (KHz) 16 KHz -16-88 FM 16 32Ks/s 24

Complete Application Design

•Total decimation ratio = 250*8*4 = 8000

•Problem: The audio card requires an input sample rate ≥ 44.1 Ks/s •Solution: Use a Resampler to increase the output sample rate

f (MHz) 16 KHz 32 -32 FPGA Dec. factor = 250 Channel Filter cutoff = 8KHz Dec. factor = 8 FM Demodulator Dec. factor = 4 f (KHz) 4 -4 Audio f (KHz) 16 KHz 128 -128 64Ms/s 256Ks/s f (KHz) 16 KHz -16-88 FM 16 32Ks/s 8Ks/s

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Final Application Design

•Audio Card requires a sample rate ≥ 44.1 Ks/sec. Use 48 Ks/sec. •Modify FM Demodulator to have a decimation factor of 1 (no change) •Increase the sample rate to 48 Ks/sec with Resampler (x 3/2)

a FPGA Dec. factor = 250 Channel Filter cutoff = 8KHz Dec. factor = 8 FM Demodulator Dec. factor = 1 48Ks/s Resampler mult by 3 div by 2 32Ks/s 32Ks/s 256Ks/s 64Ms/s 26

Implementing the Design



Create a Python script to specify and

connect the various GNU radio blocks



Blocks are already written in C++



USRP parameters are set within Python

script



# indicates that the line is a comment



Refer to nbfm.py script

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Setting the USRP Parameters



The following code sets the USRP

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Channel Filter Design



The following code specifies the channel

filter and computes the coefficients

a H (dB) f (KHz) -60 -16 -9-8 89 16 -0.1 0 29

Channel Filter Creation

 The following code creates the channel filter

using the coefficients computed:

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FM Demodulator

 The following code creates the FM demodulator.

 The demodulator block also includes a low pass

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Resampler

 The following code creates the resampler.

 The resampler decimates and/or interpolates the

data to adjust the sample rate.

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Connecting the Blocks



The following code connects the blocks:

Or, a single connect statement:

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Final Thoughts



Demonstration



Storing/creating data



Transmitters



Installing GNU radio

