C2 and Payload in One Link Chances and Challenges of OFDM DGLR Symposium Datenlink-Technologien für bemannte und unbemannte Missionen 21. März 2013 Dr. Christoph Heller Christian Blümm
Outline Problem Formulation OFDM in a Nutshell Multi-Link OFDM Concept Exemplary System Implementation Use-Case SDR Platform OFDM Parameterization Conclusions Page 2
Problem Formulation Data links required for the operation of unmanned platforms: Command & Control Link Payload Link Maximum Robustness Low Data Rate C2 Payload Maximum Throughput Page 3
Typical Multi-Link Applications Data Link Performance Goals - Low Symbol Rate - Few Bits per Symbol - Low Code Rate Robustness C2 Payload - High Symbol Rate - Many Bits per Symbol - High Code Rate Throughput Conventional Solution: Multiple independent data links Different modulations/waveforms according to requirements Multiple physically independent modems Multiple antennas Page 4
OFDM in a Nutshell Orthogonal Frequency Division Multiplexing Multi-carrier transmission scheme Parallel data transmission results in long symbol duration Immune against multi-path propagation due to cyclic prefixing Channel equalization in frequency domain Individual sub-carriers are orthogonal Subcarriers may overlap in frequency domain Bandwidth efficient modulation Page 5
OFDM in a Nutshell Where is it used today? Wireless protocols: WLAN (802.11a, g, n) DAB DVB-T, DVB-H WiMAX LTE AeroMACS L-DACS1 Wired communication: Power line communication DVB-C2 Page 6
OFDM in a Nutshell Transmitter System Model Receiver System Model Page 7
Multi-Link OFDM Concept Basic Principle (also knows as OFDMA) Split set of orthogonal carrier into multiple channels Use different carrier groups with different subcarrier modulation and code rate according to link requirements Due to orthogonality no mutual influence of carriers f Page 8
Multi-Link OFDM Concept Multi-Link OFDM Transmitter Multi-Link OFDM Receiver Page 9
C2 and Payload in One Link Chances and Challenges of OFDM Carrier Allocation Concepts f f f 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 t Blocked Page 10 t Distributed Subcarrier used by Channel 1 Subcarrier used by Channel 2 Unused Subcarrier t Scattered
Multi-Link OFDM Concept Comparison of Subcarrier Modulation Schemes Modulation Amplitude Values Transmitted Bits Minimum Point Distance Normalized Min. Point Distance BPSK 2 1 2 2.45 QPSK 4 2 2 2 16-QAM 16 4 2/3 1.26 64-QAM 64 6 2/7 0.76 Page 11
Exemplary System Implementation Use-Case UAV / UGV scenario in the scope of project SiNafaR Sichere Navigation für autonome Robotik-Plattformen, Founded by the Bayerisches Staatsministerium für Wirtschaft, Infrastruktur, Verkehr und Technologie Command & Control, Waypoint Transmission to Autopilot Pan, Tilt, Zoom of Camera Transmission of Camera Images to Ground Control Station Relais UAV Unmanned Ground Vehicle (UGV) BLOS Usage Scenario Ground Control Station (GCS) Page 12
Exemplary System Implementation SDR Platform Hybrid GPP/FPGA architecture FPGA for baseband signal processing and real-time MAC control GPP for IP packet handling, routing and parameterization Frontend based on MAX2829 integrated RF transceiver (S and C band) SDR Platform Frontend Control TX I Ethernet Intel Atom Embedded- Prozessor- Board PCIe Xilinx Virtex-5 FPGA- Board Dig. I/O AD/DA Converter Board TX Q RX I RX Q Debugging via Monitor und Tastatur Debugging via JTAG (ChipScope) Page 13
Exemplary System Implementation OFDM Parameterization Sampling Frequency Channel 1 (robust) 48 MHz FFT Size 64 CP Length 16 Samples Channel 2 (throughput) No. of allotted Carriers 8 24 Subcarrier Indexes 2, 6, 10, 14, 50, 54, 58, 62 1, 3, 4, 5, 7, 8, 9, 11, 12, 13, 15, 16, 48, 49, 51, 52, 53, 55, 56, 57, 59, 60, 61, 63 Code Rate 1/2 3/4 Subcarrier Modulation BPSK 16-QAM Symbol Duration: 80 Samples/48 MHz = 1.67 µs 4 payload bits per Symbol 72 payload bits per Symbol 50 Payload Symbols per Frame, 1 Frame per 100 µs TDMA slot 1 Mbit/s 18 Mbit/s Page 14
Exemplary System Implementation OFDM Parameterization Channel 1 approx. 12 db more robust Page 15
Conclusions Pros: We have presented an approach to realize a robust C2 link and a high throughput payload link with only one waveform Big potential for weight reduction, in particular for small UAVs Cons: It still is an OFDM waveform Frequency offsets, Doppler spread critical Peak-power problem Not possible to use payload link as fall-back for C2 link Certification issues Possible Alternative: Simultaneous implementation of two different waveforms on SDR platform (in time or frequency multiplex) Page 16
Discussion Please feel free to contact me christoph.heller@eads.net +49 (89) 607-28609 Page 17