R/C Simulation and Hardware Proof of Concept Development

Dr. Philip A. Dafesh,Dr. R. T. Bow, Mr. G. Fan and Mr. M. Partridge

Communication Systems SubdivisionThe Aerospace Corporation

2

Outline

• Background

• Simulation Overview and Results

• Hardware Overview and Results

• Summary and Conclusions

3

Background• As part of the R/C code Development, Constellation

Simulations and Hardware Proof of Concept (POC) Demonstrations Were Completed Signal Processing Worksystem (SPW) Simulation of L5 and

Newly Proposed TDMA Code HW POC was Developed as a Modification to The Aerospace

Corporation's "FlexGPS" FPGA-Based Prototyping System IIRM Modulator Implementation of C/A, P and M-code on L2 Single Channel Receiver Hardware (Code Corr. and Tracking Loops) Implemented R/C Transmitter and Receiver as Modification to

C/A Hardware in 2 Weeks from Receipt of the R/C Specification

• Simulated and Measured Correlation Response and DLL Tracking Loop Response for R/C Codes Alternatives

4

+/- 25.6 Chips

Receiver Code Delay (s)

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

Nor

mal

ized

Cor

rela

tion

Vol

tage

0.0

0.2

0.4

0.6

0.8

1.0

RC_TDMA 2CM CA Code

TDMA Code Scaled by 2

Sample Correlation Data Taken with R/C Code POC Hardware

Measured Correlationis In AgreementWith Theoretical Reduction in Non-Prompt Correlation for a Single Satellite

5

Simulation• Implements Multiple R/C Code Satellites Using the

Signal Processing WorkSystem (SPW) Simulates the Effect of Random Code Phases and Carrier

Doppler for Each Satellite 10 Satellites in View at Varying Power Levels

• Signal Parameters are Adjusted to Mimic the IIRM Implementation Interplex Modulation of C/A or TDMA, P(Y)

and M-Code Relative Power Adjusted to IIRM Levels

6

Caveats

• M-Code Implemented as a 20 TAP M-Sequence Actual Code is Nonrepeating

• Used Actual P-Code• Used Actual C/A and TDMA Codes• Simulation Sample Rate = 4.092x107

40 Samples/TDMA Chip (80 Samples/Bit)

M0L0M1L1M2L2M3L3M4L4

1Chip~ 1 s

1 Bit ~ 2 s

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Normalized Correlation of SV1 with Constellation (with P and M)

Crosscorrelation of CA/TDMA/2CM/2CL with a Multiple-Satellite SignalDATA=1, With P_code & M_code

-0.1000

0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

0.7000

0.8000

0.9000

1.0000

0 511.5 1023 1534.5 2046 2557.5 3069 3580.5 4092

TDMA CA 2CM 2CL

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Crosscorrelation of CA/TDMA/2CM/2CL with Multiple-satellites SignalDATA=1, With P_code & M_code

-0.0800

-0.0600

-0.0400

-0.0200

0.0000

0.0200

0.0400

0.0600

0.0800

0 200 400 600 800 1000 1200 1400 1600 1800 2000

TDMA CA 2CM 2CL

Zoom in to TDMA/CA Results

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Summary of Observations

• R/C Code Correlation Results in Fewer and Smaller Size Non-Prompt Code-Cross Correlations with other Satellites than Does C/A Code

• Results Indicate that R/C Code Should Result in Smaller CDMA Noise Contributions During Tracking and Fewer False Alarms During Acquisition Even when Constellation of Satellites is Considered Including

Effects of Doppler, Amplitude and Phase Distribution

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Hardware Proof of Concept Description

• Implementation of a Single IIRM Satellite with Switches to Select Different R/C Code Options on the L2 Carrier C/A, L5, TDMA

• Receiver Can Select Between Correlation of Entire TDMA or Individual 2CM and 2CL Codes Selectable DLL Early-Late Code Tracking at Spacing

from +/- 0.05 chips to +/- 0.5 chips SVi to SVi Autocorrelation and SVi to SVk Cross-Correlation

• New TDMA Code was Implemented in FlexGPS Transmitter and Receiver Correlator within 2 Weeks Relatively Straightforward Modification to C/A Receiver

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Hardware Description (Cont.)• All Tests Conducted at L2 Frequency• Correlation Sweep and DLL Tracking at Selectable

Early-Late Spacing, Loop Parameters• No Navigation Processing• I and Q Sample Rate = 40.92 MHz • Receiver Bandwidth = 24 MHz• Data Measurement and Diagnostic Capability

Time-Domain, Frequency-Domain & Logic-Level Oscilloscope, Spectrum Analyzer, Logic Analyzer

PR Residual and Code Correlation Data Software GUI Control and Data Acquisition Interface

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Aerospace’s FlexGPS Prototyping System

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Rapid-Prototype BoardClose-Up of Modulator and Receiver

Tx

Rx

40.92 MHz I/Q ADCs

I/Q DACs

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Tx/Rx Signal Probe and Control

Logic Analyzer Probes GUI Interface Control and I/O

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What's Different from Prior C/A-Code Prototype

• What was Implemented SV Selectable C/A, L5 or TDMA Code at Transmitter Selectable 2CM, 2CL, L5-I or C/A Codes in Receiver 511.5 kHz Square-Wave Gated Integrate and Dump Clock

("0" of square-wave corresponds to 2CM Accumulator Clock and "1" Corresponds to 2CL Accumulator Clock) Square-wave Gated Correlator With Standard Bi-Phase (+1,- 1)

Reference Used Instead of Three-Level (+1,0,-1) Reference Code No Significant Changes to C/A Code DLL Tracking Channel

Additional Details to be Published at ION GPS 2001

• What was Not Implemented (Planned) Independent 2CM and 2CL Tracking Channels Viterbi decoding

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R/C TDMA Specific Additions • Difference Between C/A and TDMA Receiver Hardware

Requires 2 Code Generators Same Hardware Initialized Differently and Short Cycled

at Different Points to Generate 2CM and 2CL Codes Requires Different Correlation Method

Implemented as Time Gating or 3-Level Reference Requires Min of 1 and Max of Two New Tracking

Channels for L2 Reception• Additional Changes Include Standard Viterbi Decoding

and Carrier PLL Carrier Tracking as would be Implemented for the L5 Signal Associated Data Demodulation at 25 Hz Rate (50 Hz Coded)

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Measured PSD of TDMA Signal

Frequency (MHz)

-15 -10 -5 0 5 10 15

Pow

er S

pect

ral D

ensi

ty (d

Bm

)

-100

-90

-80

-70

-60

-50

-40

TDMA PSD

Hardware Measured Power Spectral Density

TDMA Signal PSDFollows Same Sinc2

Response as C/AWithout Observable Spectral Lines

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Measured Spectral LinesPower Spectral Density Comparison

Frequency (Hz)-1500 -1000 -500 0 500 1000 1500

Pow

er S

pect

ral D

ensi

ty (d

Bm

)

-105

-100

-95

-90

-85

-80

-75CA SV1, = 4 dbmL5 SV1, = 5 dbmTDMA SV1, = 2 dbm

1 KHz

100 Hz

C/A

L5

Line Structure of TDMA R/C Code is Within Resolution of Spectrum Analyzer< 1 Hz Line Spacing

TDMA R/C Code Has P-Code Like Line Structure:

Better Interference Rejection Properties

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Measured DLL Pseudorange Residual for Ti=20 ms

Time (sec)

0 2 4 6 8 10 12 14 16 18

PR

Res

idua

l (C

/A C

hips

)

0.40

0.45

0.50

0.55

0.60

CA_Pseudorange2CM Pseudorange2CL Pseudorange

Measured R/C vs C/A Correlation and Pseudorange

Measured C/A and R/C Code Correlation

Received Code Delay (ns)

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

Nor

mal

aize

d C

orre

latio

n A

mpl

itude

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

2CM Correlation2CL CorrelationC/A Correlation

Measured C/A and Scaled R/C Code Correlation

Received Code Delay (ns)

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

Nor

mal

aize

d C

orre

latio

n A

mpl

itude

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

2CM Correlation x2 2CL Correlation x2C/A Correlation

Measured Mean PR Residual of 803PR I&D Samples (in Chips):

2CL

2CM

2C/A

0.48680.48670.4866

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Summary of Hardware Measurement Observations

• TDMA Code Has an Undetectable Spectral Line Structure More P-Code Like and Potentially Better Immunity to

Co-Channel Interference than C/A or L5• TDMA Correlation Response

Comparable to C/A Code but 1/2 the Magnitude for Correlation Over Fixed Integration Period In agreement with Theoretically Expected Multiplexing Ratio

Significantly Smaller Amplitude Non-Prompt Peaks Better Acquisition Performance, Smaller CDMA Noise

• TDMA Code Pseudorange Response Does Not Exhibit any Biases Relative to C/A Code Agreement to 0.0001 C/A Chip (within Experimental Error)

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Planed Hardware Additions• Implementation of Code and Carrier Tracking

for Various Combinations Code and Carrier Tracking 2CM Code with Data

Early-Late Code Tracking, AFC and PLL Carrier Tracking with Data removal

Code and Carrier Tracking 2CL Code without Data Early-Late Code Tracking, AFC and PLL Carrier Tracking without

Data removal Acquisition Performance of C/A vs 2CM vs 2CL

• Industry Inputs Welcome To Address Specific Concerns with TDMA Receivers

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Summary and Conclusions• C/A, L5 (1.023 MCPS) and TDMA Code R/C Options

were Compared Using a Constellation Simulation and a FPGA Hardware Proof of Concept New L2 TDMA R/C Code Exhibited Superior Characteristics

• A Proof of Concept was Developed for Both Transmitter and Receiver Implementations of the L2 R/C Code Small Change to Aerospace's FlexGPS Receiver Implemented

by Time Gated Correlator with 511.5 kHz Square-wave• Simulation and Hardware Measurements are Constant

with Theoretical Expectations for TDMA R/C Codes Detailed Results to be Published at ION GPS 2001

Backup

Current Prototype Development Approach

SPW/C Floating Point Simulation

HDS (Fixed Pt. SPW ) Model Development

Integrated SPW w/ Hardware Development System (HDS) Simulation: Cycle Correct Fixed Point (Integer) Simulation

HDS Subsystem Optimization (Digital Design)

HDS: Optimized Prototype Design

APTIX Software: Place and Route FPGA Components, Interconnect Routing, Test Points Selection

VHDL Code Generation

APTIX Programmable BrassboardFPGA

HP Logic Analyzer

Synthesize VHDL to EDIF/XNFCompile FPGA File

Sun Workstation

LAN

Concept

to

Hardware