К.V. Gorokhov – Leading Engineer, JSC «RPC «Polyot» (N. Novgorod)
А.V. Kolobkov – Ph.D. (Eng)., Deputy Head of the Scientific and Technical Center, JSC «RPC «Polyot» (N. Novgorod)
D.V. Khiteva – Post-graduate Student, Software Engineer, JSC « RPC «Polyot» (N. Novgorod)
Multipath signal propagation leads to signal distortion in HF channel. These distortions are characterized by the dispersion of the signal in time and frequency. This degrades the reliability of the transmission due to errors that are grouped into packets. Interleaving allows converting group errors into single errors that are easily corrected by noise immunity coding. The aim of the paper is to
determine the best way of interleaving in HF OFDM system with PSK and convolution coding. In the paper the influence of the length and type of the interleaver on bit-error-rate performance of OFDM system is investigated by simulation.
We simulate OFDM system with 140 subcarriers in the band of 3100 Hz from which only 128 active subcarriers are modulated with PSK. The convolution encoder has octal constitutive coefficients of (133, 171) and the code constraint length of 7. HF channel is simulated within the framework of two-path Watterson model. In the receiver, after corresponding deinterleaving of the soft decisions of the demodulator, they are decoded by the Viterbi algorithm.
In the paper block and random interleavers are investigated. Increasing the interleaver length improves bit-error-rate performance of OFDM system. But a large interleaver length results in a large delay. The optimal interleaver length must be determined to provide a trade-off between bit-error-rate and channel delay. It is established that the optimal interleaver length is equal to ten times of coherence time. The optimal interleaver lengths are 32, 64 and 256 OFDM symbols for the Doppler spread 6,6, 3,3 and 1 Hz, respectively. Using interleavers longer than 10 seconds at lower fading rates is not advisable because of the excessive increase in the block of transmitted data. PSK constellation size does not affect the choice of interleaver characteristics.
Frequency selectivity of the signal affects the block interleavers. It is necessary to adapt block interleavers parameters to the multipath interval. Consequently, it is necessary to build a system with feedback, which greatly complicates the OFDM system. Random interleavers do not need this.
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