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

High survival HFradio 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


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


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


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 10-2

5) A fixed 4-QAM constellation is used (no autobaud capability)

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 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

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 of 56.25 Hz

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=9600 Hz

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

Subcarrier allocation


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


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

4) The signal bandwidth is chosen large enough so as to provide the system with the desired frequency diversity

5) An autobaud capability is employed to adaptively select the most appropriate constellation

Main system parameters


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. 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


HF Channel model


Channel type Mid-latitude disturbed

Mid-latitude moderate

Mid-latitude good

Delay spread(ms)

2.0 1.0 0.5

Doppler spread (Hz) 1.0 0.5 0.1

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


Voice link with good channel conditions


Data link with moderate conditions


Performance of Mode V (16-QAM)


Conclusions


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.

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

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