# Presentation: 402134

## 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 Subdivision
- The Aerospace Corporation

## Outline

- Background
- Simulation Overview and Results
- Hardware Overview and Results
- Summary and Conclusions

## 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

## Sample Correlation Data Taken with R/C Code POC Hardware

- Sample Correlation Data Taken with R/C Code POC Hardware

- Measured Correlation
- is In Agreement
- With Theoretical
- Reduction in
- Non-Prompt Correlation
- for a Single Satellite

## 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

## 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

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

## Zoom in to TDMA/CA Results

## 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

## 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

## 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

## Aerospace’s FlexGPS Prototyping System

## Rapid-Prototype BoardClose-Up of Modulator and Receiver

- Tx

- Rx

- 40.92 MHz I/Q ADCs

- I/Q DACs

## Tx/Rx Signal Probe and Control

- Logic Analyzer Probes

- GUI Interface Control and I/O

## 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

## 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)

## Hardware Measured Power Spectral Density

- Hardware Measured Power Spectral Density

- TDMA Signal PSD
- Follows Same Sinc2
- Response as C/A
- Without Observable
- Spectral Lines

## Measured Spectral Lines

- 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

## Measured R/C vs C/A Correlation and Pseudorange 

- Measured R/C vs C/A Correlation and Pseudorange

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

## 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)

## 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

## 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 

**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 Brassboard

- FPGA

- HP Logic Analyzer

- Synthesize VHDL to EDIF/XNF

**Compile FPGA File**

- Sun Workstation

**LAN**

**Concept**
**     ****to **
**Hardware**