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Rome, February 14, 2013

Status of the Project Report on the first year activities. With the support of the Prevention, Preparedness and Consequence Management of Terrorism and other Security-related Risks Programme European Commission - Directorate-General Home Affairs. Rome, February 14, 2013.

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Rome, February 14, 2013

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  1. Status of the Project Report on the first year activities With the support of the Prevention, Preparedness and Consequence Management of Terrorism and other Security-related Risks Programme European Commission - Directorate-General Home Affairs Rome, February 14, 2013

  2. PHY layer architectureof the SWING system 1) Selection of the modulation technology 2) System design for voice transmission 3) System design for data transmission

  3. Selection of the modulation technology • Most military HF standards employ a serial-tone waveform with a powerful FEC code and temporal interleaving to exploit the time-diversity of the HF channel • The use of a temporal interleaver with an interleaving depth greater than the HF channel coherence time poses a serious problem in terms of overall link latency • The alternative approach to increase the system reliability is to exploit the frequency diversity offered by the multipath phenomenon

  4. Selection of the modulation technology • In this case the transmission bandwidth must greatly exceed the channel coherence bandwidth and the received signal will be affected by ISI • The common approach to mitigate ISI in serial-tone waveforms is the use of a channel equalizer in the form of a tapped delay line. In case of severe multipath distortion, the number of required taps is very high and the equalizer cannot be implemented with affordable complexity • Multi-tone transmission in the form of OFDM is the most appropriate technology for low-complexity multipath mitigation

  5. Advantages of the OFDM technology • The channel distortion appears as a multiplicative factor which can be compensated for through a bank of complex multipliers • Increased spectral efficiency due to partially overlapping subbands in the frequency domain • Simple digital implementation by means of DFT/IDFT operations • Increased resilience against narrowband interference, which only hits a small portion of the signal spectrum • Possibility of adaptively selecting the constellation size on each subband (autobaud capability)

  6. Requirements of the digital voice link It will support interactive voice communications. Interactivity is a basic design constraint The maximum accepted delay is around 120 ms so as to guarantee a whole delay observed by the user below the subjective limit of 250 ms Temporal interleaving cannot be used due to the strict requirement in terms of overall delay In order for the system to be applicable to commercial vocoders, the bit rate should be 2400 bps with a BER lower than 10-2 A fixed 4-QAM constellation is used (no autobaud capability)

  7. Guidelines for the design of the digital voice link • The signal bandwidth B must exceed the channel coherence bandwidth so as to capture most of the frequency diversity offered by the HF channel • The subcarrier spacing f must be much smaller than the channel coherence bandwidth Bcoh so as to make the channel response nearly flat over each subcarrier and much larger than the Doppler spread in order to avoid significant channel variations over one OFDM block

  8. Design of the main system parameters • The sampling frequency fs is fixed to 14.4 kHz, which seems reasonable for implementation on commercial HW platforms • The IDFT/DFT size is fixed to N=256. This value results into a subcarrier distance f =56.25 Hz • Assuming a maximum delay spread max =5 ms, the number of samples in the cyclic prefix is fixed to Ng= maxfs=72 • The number of modulated subcarriers is Nu=171, while the number of null subcarriers placed at the spectrum edges is Nv=N-Nu=85 • The signal bandwidth is B=Nu f =9600 Hz

  9. Pilot insertion in the voice link A total of 35 pilot subcarriers are inserted in each OFDM block for channel estimation This results into 136 data subcarriers divided into 34 chunks, each containing 4 data subcarriers. The baud rate is 5970 baud

  10. Transmitter structure for the voice link • FEC is accomplished by means of the industry-standard convolutional encoder with rate 1/2 and constraint length 7 • Bit interleaving is accomplished by means of a block interleaver matrix • Interleaved bits are mapped onto 4-QAM symbols without any autobaud capability

  11. Subcarrier allocation: mode I • Mode I is suggested in case of harsh channel conditions • A total of 136 coded bits are mapped onto 68 channel symbols, which are next repeated and allocated over each subband (repetition factor Rf=2)

  12. Subcarrier allocation: mode II • Mode II is suggested in case of better channel conditions • A total of 272 coded bits are mapped onto 136 channel symbols, which are next allocated over the 136 available data subcarriers without any repetition

  13. Requirements of the data link The data link provides non-delay sensitive services, meaning that we can relax the interactivity constraint Channel coding and frequency interleaving are necessary to provide sufficiently low packet error rate The signal bandwidth is chosen large enough so as to provide the system with the desired frequency diversity CRC and ARQ are requested for error-free packet delivery An autobaud capability is employed to adaptively select the most appropriate constellation

  14. Design of the main system parameters • The subcarrier distance is f =56.25 Hz as in the voice link • To account for the severe constraint on the achievable data rate, the IDFT/DFT size is fixed to N=2048. • Assuming a maximum delay spread max =5 ms, the number of samples in the cyclic prefix is fixed to Ng= maxfs=576 • The number of modulated subcarriers is Nu=1728, while the number of null subcarriers placed at the spectrum edges is Nv=N-Nu=320 • The signal bandwidth is B=Nu f =97.25 kHz

  15. Main system parameters

  16. Pilot grid for the data link • The available subcarriers are divided into clusters, where each cluster contains 9 subcarriers and spans over 3 adjacent OFDM blocks • In each cluster there are 8 pilot symbols and 19 data subcarriers • A total of 192 clusters are present in three adjacent OFDM blocks, corresponding to 3648 data subcarriers

  17. Transmitter structure for the data link • A 16-bit CRC is appended to each data packet • FEC and bit interleaving as in the voice link • The overall bandwidth is divided into 8 subbands, each containing 24 adjacent clusters and 456 data subcarriers. A different constellation size can be used on different subbands (autobaud) • The interleaved bits are mapped onto 4QAM, 16QAM or 64QAM constellation symbols, which are transmitted within one single subband.

  18. Data link waveforms

  19. HF Channel Model • Coherence bandwidth can range from less than 100 Hz to more than 20 kHz • Coherence time can range from 1 second to more than 10 seconds

  20. Voice link with moderate channel condition

  21. Voice link with good channel condition

  22. Data link with moderate channel condition

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