Development and operation of such communication systems that are designed to work in complex electromagnetic environment is a very acute challenge accounted for by a rapid growth of mutually interfering electronic systems (especially in big cities) and by existence of a great amount of electromagnetic noise from diverse sources. One of the directions to be followed while addressing the problem of operation of communication systems under the conditions of external noise presence is creation of novel types of noise-immune waveforms that secure their operability.

The objective of this work is to develop a system of broadband noise-immune communication links based on employment of fractal signals. The fractal signals are drastically different from other signal types that are used in the existent communication systems by their waveform, spectral characteristics and time response. These performances are self-similar, i.e. they are repeated on different temporal and frequency scales, which allows, by using them, for the creation of air and cable communication links that are protected from unauthorized access and suitable for an efficient operation in complex electromagnetic environment. The employment of a system of orthogonal fractal waveforms will also enable to increase the amount of information transmitted in the same frequency range.

	a)	b)
	Fig.1. Fractal signal (a) and its spectrum (b).

Fig.1 illustrates theoretical time response realization of one of the types of the fractal signals and its spectrum. The experiments resulted in generation of fractal signals at the carrier frequency 2 GHz. The experimental spectra of these signals are given in Fig.2.

	Fig.2. Characteristic spectra of fractal signals.

The fractal signals belong to the type of noise-immune signals. The outstanding feature of the fractal signals is the precision of the boundaries of their fractal spectra.

Our proprietary system of fractal communications employs fractal waveforms instead of radio-frequency pulses for data transmission (Fig.3).

	Fig.3. Fractal waveforms employed in communication systems.

The block diagram of the fractal communication system is shown in Fig.4.

	Fig.4. Block diagram of fractal communication system.

1 - computer; 2 - converter USB - RS485; 3 – fractal signal encoder; 4 – fractal signal decoder; 5 – electronic switch; 6 – antenna unit. The antenna unit incorporates wide-range fractal antennas (Fig.5).

	Fig.5. Wide-range fractal antennas.

There are no immediate analogs elsewhere for this proprietary system, yet by certain performances it comes rather close to the state-of-the-art system of chaotic communications. The principal parameters of the fractal communication system (FCS) are given in Table 1.

Table 1. FCS performances

FCS parameters	Fractal communications system
Data transmission rate, Мb/s	1
Broadband operability, MHz	500 - 600
Type of modulation	АМ, PhM
Signal output, dBm	1-15
Sensitivity, dBm	-130
Operating frequency range, GHz	2 - 10,1
Transmission distance, m	150

The FCS adapter is connected to the computer USB port and to a similar adapter connected to the USB inled of another computer, allowing user to transmit or receive data over wireless or cable network. The adapter is made as individual module with autonomous supply (Fig.6).

	Fig.6. FCS adapter.

At a dedicated stand we carried out successful experiments on fractal signal noise immunity and establishing of computer communication air links under the conditions of external noise presence.

	a)	b)
	Fig.7. Stand for simulation of FCS operation under conditions of external noise: a) stand receiver part, b) stand transmitter part.

Fig. 8 shows experimental results in studies on extraction of the fractal signals out of the external noise.

	Fig.8. Unimpeded fractal signal and fractal signal against noise.

We used the stand (Fig.7) to run experiments on establishing computer air communication links, using the radio-frequency pulses and fractal signals.

	Fig.9. Pattern distortions during data transmission via radio-frequency pulses through noise..

	Fig.10. FCS stable operation against external noise.

Our experiments have proven experimentally the superiority of FCS over the conventional communication systems for data transmission under the conditions of external noise presence. In the above figures (Figures 9 and 10), one can clearly see that radio-frequency pulses carry data with distortions, while, using the fractal signals, the data is transmitted unimpeded despite the interfering external noise action.