High survival HF radio network Michele Morelli, Marco Moretti, Luca Sanguinetti CNIT- PISA

Outline 2SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 1) Selection of the modulation technology 2) System design for voice transmission 3) System design for data transmission

Selection of the modulation technology 3SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 Most military HF standards employ a serial-tone waveform with a powerful FEC code and time interleaving to exploit the time-diversity of the HF channel The use of a time 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 propagation In this case the transmission bandwidth must greatly exceed the channel coherence bandwidth and the received signal will be affected by ISI Multi-tone transmission in the form of OFDM is the most appropriate technology for low-complexity multipath mitigation

Advantages of the OFDM technology 4SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 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)

Requirements of the digital voice link 5SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 1) It will support interactive voice communications. Interactivity is a basic design constraint 2) 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 3) Time interleaving cannot be used due to the strict requirement in terms of overall delay 4) In order for the system to be applicable to commercial vocoders, the bit rate should be 2400 bps with a BER lower than ) A fixed 4-QAM constellation is used (no autobaud capability)

Guidelines for the design of the digital voice link 6SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 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 must be much smaller than the channel coherence bandwidth B coh 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

Design of the main system parameters 7SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 The sampling frequency f s 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 of Hz The number of modulated subcarriers is N u =171, while the number of null subcarriers placed at the spectrum edges is N v =N-N u =85 The signal bandwidth is B=9600 Hz

Transmitter structure for the voice link 8SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 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

Subcarrier allocation 9SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 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

Requirements of the data link 10SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 1) The data link provides non-delay sensitive services, meaning that we can relax the interactivity constraint 2) Channel coding and frequency interleaving are necessary to provide sufficiently low packet error rate 3) CRC and ARQ are requested for error-free packet delivery 1) The signal bandwidth is chosen large enough so as to provide the system with the desired frequency diversity 1) An autobaud capability is employed to adaptively select the most appropriate constellation

Main system parameters 11SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

Transmitter structure for the data link 12SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 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. 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.

Data link waveforms 13SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

HF Channel model 14SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 Channel typeMid-latitude disturbed Mid-latitude moderate Mid-latitude good Delay spread (ms) Doppler spread (Hz) 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

Voice link with moderate channel conditions 15SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

Voice link with good channel conditions 16SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

Data link with moderate conditions 17SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

Performance of Mode V (16-QAM) 18SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

Conclusions 19SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 We have designed a multicarrier system for voice and data transmission in the HF band which can exploit the frequency diversity of the multipath channel The voice link employs a fixed 4 QAM constellation with bit interleaved coded modulation without any autobaud capability The data link employs CRC and an ARQ protocol for error-free packet delivery. Six transmission modes are available to achieve the desired trafe-off between reliability and data rate. Numerical analysis indicate that the multicarrier system operates satisfactorily in both moderate and good channel conditions.