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The influence of the amplitude limitation of multi-frequency signals based on the IOTA function on the power efficiency of the signals reception

DOI 10.18127/j00338486-201812-11


S.B. Makarov – Dr.Sc.(Eng.), Professor, Director of Institute of Physics, Nanotechnology and Telecommunications of Peter The Great St. Petersburg Polytechnic University
A.M. Markov – Leading Engineer, Institute of Physics, Nanotechnology and Telecommunications of Peter The Great St. Petersburg Polytechnic University

IOTA-OFDM signals is a result of modification OFDM signals by using the IOTA function. Such signals are formed using the Isotropic Orthogonal Transform Algorithm (IOTA) of the Gaussian function. The application of the IOTA-function allows to increase the spectral efficiency of multifrequency signals due to the absence of the need of use a cyclic prefix and the high decrease in the level of out-of-band emissions of such signals. Signals based on IOTA functions and offset quadrature amplitude modulation (IOTA OFDM / OQAM) are proposed for the fourth generation of cellular systems.
When IOTA-OFDM / OQAM signals are used, there is no reduction in BER performance of signals compared with classical OFDM signals. This is due that the orthogonality of such signals at different subcarrier frequencies is preserved. However, multi-frequency signals based on the IOTA function, like OFDM signals, have a significant peak to average power ratio (PAPR), which requires high linearity of the static modulation characteristic of a power amplifier.
In this work, the PAPR is reduced by limitation of amplitude (clipping). Then amplification of clipping signals to maintain a fixed peak power is used. The disadvantage of using the amplitude limitation is the expansion of the occupied bandwidth. The bandwidth of the signal is depended on the probability of limiting of the amplitude of the signals.
The aim of the paper is to quantify the spectral and peak energy efficiency by using the amplitude limitation of a random sequence of IOTA-OFDM / OQAM - 16 signals while maintaining the transmitter's unchanged peak power.
For the generation of IOTA-OFDM / OQAM signals, the sequence of bits is represented as complex values of the signal constellation, then the partition of these values into the real and imaginary parts is performed, after the rotation of their phases, serial-parallel transformation, IFFT, parallel-serial conversion are used. Then the IOTA filter is used. Then amplitude limitation of the signal samples and an amplification of the obtained values are performed. The last is to preserve the unchanged peak power of the transmitter cor-responding to the absence of clipping. At the receiver it is used the IOTA filter similar to the receiving one, then the FFT is performed, the normalization of the obtained values, the reverse phase rotation, taking the real part, mapping into complex symbols and demodulation by rounding to the nearest point of the signal constellation are performed.
In this work transmission-reception simulation was performed for a channel with constant parameters under conditions of normal white noise. It was found that clipping and increase in the average power provide increase in the peak energy efficiency of 3.7 dB with respect to receiving signals without clipping. It was found that the maximum gain conforms to a clipping probability equal to 0.07. This clipping probability corresponds to the clipping level L≈10 for N = 8 subcarriers and L≈30 for N = 64 subcarriers. At the same time, the peak factor is reduced by 4 dB. However, the use of clipping of IOTA-OFDM / OQAM-16 signals leads to an increase in out-of-band emissions by 15 dB.

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