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

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 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

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 = 4.092x10 7  40 Samples/TDMA Chip (80 Samples/Bit) M0L0M1L1M2L2M3L3M4L4M0L0M1L1M2L2M3L3M4L4 1Chip ~ 1  s 1 Bit ~ 2  s

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

8 Zoom in to TDMA/CA Results

9 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

10 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 +/ chips to +/- 0.5 chips  SV i to SV i Autocorrelation and SV i to SV k Cross-Correlation New TDMA Code was Implemented in FlexGPS Transmitter and Receiver Correlator within 2 Weeks  Relatively Straightforward Modification to C/A Receiver

11 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 = 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

12 Aerospace’s FlexGPS Prototyping System

13 Rapid-Prototype Board Close-Up of Modulator and Receiver Tx Rx MHz I/Q ADCs I/Q DACs

14 Tx/Rx Signal Probe and Control Logic Analyzer Probes GUI Interface Control and I/O

15 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  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

16 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)

17 Hardware Measured Power Spectral Density TDMA Signal PSD Follows Same Sinc 2 Response as C/A Without Observable Spectral Lines

18 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

19 Measured R/C vs C/A Correlation and Pseudorange Measured Mean PR Residual of 803 PR I&D Samples (in Chips):

20 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 C/A Chip (within Experimental Error)

21 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

22 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 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

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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