Tactical Targeting Networked Technology
Project work for DARPA’s Tactical Targeting Networked Technology (TTNT) consists of researching new waveforms for use in air-to-air networks of high-speed aircraft. This entails designing robust schemes for processing the user data at the transmitter and receiver, at the physical layer, and also includes algorithms for modulation, coding, and spread spectrum transmission and reception.
We are evaluating the performance of new multi-carrier direct-sequence spread spectrum (DS-SS) waveforms and receiver techniques for improving performance in the presence of interference, both same-system and outside-system interference. In addition, we are investigating new approaches for rapidly synchronizing the receiver to the incoming signal (“acquisition”), so as to minimize the delay between actual message transmission and reception. These acquisition schemes employ interference canceling in strong “near-far” power imbalances, to actively remove interference and allow for acquisition and demodulation of “weak” signals. A receiver technique we are exploring, called multi-user detection, treats the local interference as a structured, rather than random, disturbance, and exploits this signal structure to enhance performance. The multi-carrier techniques also enable a large degree of waveform flexibility, which is advantageous in crowded spectral regions, and for adapting transmissions in the presence of hostile interference.
Figure 1: Diagram of Multi-Carrier/Multi-tone Direct Sequence
Spread Spectrum Transmitter
Figure 2: Diagram of Multi-Tone Direct Sequence SS Spectra
A third area that is being investigated for the TTNT project is an error-control scheme known as M-ary Turbo Block-Coded Modulation (MTBCM). MTBCM is a forward-error correction (FEC) code uniquely matched to the conditions and requirements of a spread spectrum (SS) communication system operating in a severe anti-jam and/or a low probability of intercept/detection environment. MTBCM integrates coding and modulation into a single operation for maximum power efficiency, and fully exploits the wide SS bandwidth by using a very low rate code. The system borrows the best features from other coding methods (block, trellis, turbo), and achieves performance enhancement by matching coding to orthogonal waveform dimensions. Additionally, MTBCM can achieve “turbo code” performance while using a relatively small number (approximately 50-100) of information bits.
Figure 3: System-level depiction of MTBCM FEC coding scheme
Members of the Tactical Targeting Networked Technology Team
- TTNT article published in October in the Military Information Technology magazine