Software Receiver Technology

Copyright © 2004 NordNav Technologies AB

Software Receiver Technology

Glenn MacGouganGlenn [email protected]

+46 703113880

30-September-2004Tampere University of Technology

-The GALILEO & GPS Software Receiver Company


• Unique Competence GPS & GALILEO Real-time Software Receiver

Technology

• Headquarter in Luleå, Sweden Luleå University of Technology Office in Stockholm

• Developing and licensing real-time GPS & GALILEO software receivers

NordNav Technologies AB


Agenda

• GNSS Receiver Technology– Traditional GNSS Receiver Technology– Software Radio Technology– Software GNSS Receiver Technology

• Demonstration– NordNav-R30 Software GPS Receiver– New Features

• GPS and Galileo

• Conclusion


GNSS Receiver – 3 Steps• Search phase (acquisition)

– Satellite, Carrier Frequency, Code phase

• Track satellites (tracking)– Adjust local replica signal using

two coupled loops• Code - Delay-Lock-Loop • Carrier - Phase-Lock-Loop

– Decode Data message

• Navigation computation (navigation)– ”Triangulate” position

• Distance to satellites known and their precise position

– X,Y,Z, Velocity and Time

Acq

Start GPSt0

t1

t3

Tracking

TOW decoded ?

Ephemeris decoded ?

Yes

Yes

No

Not2

0-2 sec

1.2-6 sec

18-30 sec4 satellites ?

Yes

No

Acq

Pos Fix

Start GPSt0

t1

t3

Tracking

TOW decoded ?

Ephemeris decoded ?

Yes

Yes

No

Not2

0-2 sec

1.2-6 sec

18-30 sec4 satellites ?

Yes

No


Traditional GNSS Receiver Architecture

• Block diagram of a typical GNSS receiver

Pre Amp(LNA)

AGC

DownConverter

A/DConverter

UserInterface

ReferenceOscillator

FrequencySynthesizer

Antenna Analog RF Front End ASIC

N

2

DigitalReceiver

Channel 1

Baseband ASIC

PowerSupply

Analog IF Digital IFRF

AcquisitionTracking

NavigationProcessing

Microprocessor

Low Speed Comm Link


Software Radio• The software radio concept is built upon two basic principles

1. Move the analog-to-digital converter (ADC) as close to the antenna as possible

2. Process the resulting samples using a programmable processor

Microprocessor(AssemblyLanguage)

Microprocessor(High LevelLanguage)

Microprocessor(Simulation

Tool)FPGAASIC

Available Processing Rate

Level of Flexibility high

lowhigh

low

Analog filtering

AmplificationAnalog to digital

Conversion (ADC)

Antenna

Programmableelement


• Current technology simply does not meet the needs for the ”ideal” software radio

– High-end analog-to-digital converter (ADC) examples• Maxim MAX104: 8 Bits; 1.0 Gsps; 2.2 GHz Analog Input BW• Analog Devices:AD6645: 14 Bits; 0.105 Gsps; 0.200 GHz Analog Input BW

– High performance processor element examples• Intel Pentium IV Processor @ 3.4 GHz clock• Xilinx Virtex II Pro FPGA (up to four embedded PowerPC 405 processors)

• Impractical to sample wide spectrum and digitally filter, decimate, and process bands/signals of interest

– It is possible to construct multiple front ends and use software to process the output of each

– It is possible to have a single front end and use software to provide an efficient, flexible, and dynamic signal processing solution

• Such an ”ideal” radio would not be cost-effective

Software Radio: Myths & Truths


• The typical GPS receiver design, with a combination of hardware and software signal processing, is well engineered design

– The high speed signal processing deals with a samples on the order of 4-20 Msps, while the low speed programmable processor deals with pre-processed samples on the order of 1 Ksps

• Current technology allow for the implementation of a real time GNSS software receiver

– Flexible signal processing• Possible to use for new signals and the

• Hybrid GPS/Galileo receivers

– Potenial low-cost alternative for system integrators

– Bandwidth of the signals [sampling frequency] the most important parameter

• Moore’s law can be interpreted to show processing power has and continues to increase exponentially since the 1970’s – so tradeoff changes perspective

Software GNSS Receiver: Feasibility & Comments


A Feasible Commercial Software GNSS Receiver Architecture

Pre Amp(LNA)

AGC

DownConverter

A/DConverter

ReferenceOscillator

FrequencySynthesizer

Analog RF Front EndN

2

DigitalBasebandChannel 1

Analog IF Digital IF

Antenna

AcquisitionTracking

NavigationProcessing

Microprocessor/DSP

• Downconversion is used – ADC is situated after the IF stage - Ideally programmable bandwidth & frequency band

• Signal processing function after IF stage are realized in software increased flexibility


Two product lines:• PC-based GNSS Receiver :

NordNav-Rxx– Specialized customer applications– High end receiver – End customers in R & D– R25/R30 being shipped now!

• Embedded Receiver : NordNav-Exx Family– Single point fixes/continuous tracking– Designed for a DSP/Embedded processors– Extremely cost effective

(re-use existing processing power in mobile terminal)

General DSP or Microprocessor

Automotive

Mobile Terminals

ExxSW

NordNav Soft GPS


NordNav-RXX characteristics

• Complete receivers targeted towards R&D and Test & Verification market segments– Desktop research– Desktop verification

• Specialized customer applications

• Designed to run on an PC platform

• Multiple sensor integration (GPS/INS/dead reckoning), interference investigations, antenna arrays/beamforming etc.

• Record raw IF samples & replay samples


NordNav-RXX Architecture

AcqusitionEngine

CorrelatorEngine

DataInterface

Acqusition & Tracking

Navigation

Harddrive

APIUserApp.

SampleStreamer GUI Receiver GUI

GPS Antenna

RF

USBv2

Multibit L1Front End

IF Samples

Microprocessor


NordNav-R30 Demonstration

• Receiver will be run on Pentium 1.7 GHz Notebook PC– Replay a recorded datafile from Stockholm

• Unique features briefly demonstrated– 24 channels (typically 14-16 realtime depending on configuration)– Configurable parameters– Add multiple correlators – New feature!– Tracking loop framework – Updated framework– Signal Injection – example study interference effects


Baseband Configuration


Receiver GUI Examples

Monitor the antenna frequency spectrumMonitor AGClevel

Horizontal scatter plot


Real-Time GUI Correlator Plot

• Add multiple correlator pairs

• Each channel can be individually configured

• User can set the tracking pairs & spacing


Impact of Tracking Loop Parameters

10 Hz PLL 20 Hz PLL


External Tracking Loop Framework 1(2)

NordNavR30

Receiver CloseLoops.dll

CloseLoops API

NordNavR30 GUI

NordNav R30 API

Visual C Framework• User implemented code - dll• Example implementation included

• The user can implement its own discriminators for code & carrier

• Implement its own code and carrier tracking loop

• Excellent for ”aiding” of tracking loops by for example IMU


External Tracking Loop Framework 2(2) Updated and added functionality

• Updated values every navigation update rate (not every ms as the accumulators):– Satellite positions– Receiver position & velocity

• Indicator to tell the receiver to NOT try and extract data– For low C/No studies


Signal Combiner 1(4)

• Allows to inject a simulated signal into real GPS samples prior receiver processing

• Possibility to study the effect interference signals and jamming scenarios

• The user can implement any signal structure, even GPS signals which the receiver can track– Simulated file : each sample stored as signed char (byte)


GPS Signal

Antenna

Front end A/D

Stored GPS samples

Stored external signal samplesExample : Simulated CW, Noise

Multiplicationfactor

R30 SoftwareReceiver

Signal Combiner

SampleStreamer



Signal Combiner 3(4)>> cw_gen(5e5, 1e6, 0.05,

'cw_500kHz.sim')

Included example signal

generation scripts

CW tonenoise



Example of GPS L1 frequency spectrum with a injected 20 dB CW tone (sinusoid) at 500 KHz off L1 frequency


New features in this software release

• Fault Detection and isolation• Improved Dynamic performance• Velocity output• Troposphere (same as WAAS model)• 2-D navigation (height fixing)• Almanac• Configuration per channel basis

– Correlators (spacing and numbers)– Tracking loop parameters– Acquisition parameters

• External Tracking Loop Framework updated


Fault Detection Example (severe multipath reflection)

Normal Processing – R30 Reference Receiver ProcessingWith Fault Detection Processing – R30


Next Major Software Release

• SBAS– Support for WAAS/EGNOS

• Scheduled IF recording• Improved Sensitivity• External Position API

• Next Next Major Software release– Galileo L1 (software IF signal generator & processing)


Galileo• Galileo – European ”GPS”. Designed to be independent but

compatible with GPS– Same frequency band as GPS– Different signal structure

• Operational 2008 [2010] – Civil system Great asset for all users with hybrid GPS/Galileo

receivers!• Increase service availability drastically!

• Five different service categories– Open Service (OS) - Free of charge!– Safety of Life (SoL), Commercial services (CS), Search and Rescue (SAR),

Public Regulated Service (PRS)


GNSS Frequency Spectrum

Modernized GPS and Glonass signals not included


Carrier at 1575.42 MHz (L1)1227.60 MHz (L2)

19 cm (L1)

300 m (CA)

Code at 1.023 Mcps (C/A)10.23 Mcps (P(Y))

Navigation Data at 50 bps 6000 km

GPS Signals


Galileo – Open Service SignalL1 Band, BOC(n,m)

PRN Signal

Square Wave

Resulting BOC(m,m) Signal

t

t

t

1

1

1

-1

-1

-1

PRN Signal

Square Wave

Resulting BOC(m,m) Signal

t

t

t

1

1

1

-1

-1

-1


GPS and Galileo Sharing L1 Spectrum : C/A and BOC(1,1)

Galileo BOC(1,1) (data bearing signal)• Code length 8184 chips• 1.023 Mhz base frequency (8 ms period time)• 125 Hz data rate (1 code period per data bit)• ~85 % of signal power within ~4 Mhz bandwidth

GPS C/A• Code length 1023 chips• 1.023 MHz chipping rate (1 ms period time)• 50 Hz data rate (20 code periods per data bit)• ~90 % of signal power within ~2 MHz bandwidth

00

0.2

0.4

0.6

0.8

1

Nor

mal

ized

Mag

nitu

de

GPS C/A and GALILEO BOC(1,1) spectrum

GPS C/AGALILEO BOC(1,1)

-6000 -4000 -2000 0 2000 4000 6000-35

-30

-25

-20

-15

-10

-5

0

Frequency Offset (kHz)

Nor

mal

ized

PS

D (d

B)

GPS C/A and GALILEO BOC(1,1) Power Spectral Density

GPS C/AGALILEO BOC(1,1)

~4 MHz BW


Conclusion• ”Ideal Software Receiver” is still a dream

– Current technology do not allow for such designs

• However for bandlimited signals, such as GPS/GNSS, software receiver are commercially feasible– Downconversion front end used – Process digital IF samples in software

• Software receivers are receiving market acceptance– Technology not only for research in a laboratory

• Although fantastic for this purpose!– More and more feasible as alternative to traditional Rx– Multi-channnel receivers exists today

• Important technology for Galileo– Hybrid GPS/Galileo L1 receiver for mass market

Software Receiver Technology

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