Non-orthogonal frequency-division multiplexing

Subcarriers system of N-OFDM signals after FFT

The low-pass equivalent N-OFDM signal is expressed as:[3][2]

${\displaystyle \ \nu (t)=\sum _{k=0}^{N-1}X_{k}e^{j2\pi \alpha kt/T},\quad 0\leq t<T,}$

where ${\displaystyle {X_{k}}}$ are the data symbols, ${\displaystyle N}$ is the number of sub-carriers, and ${\displaystyle T}$ is the N-OFDM symbol time. The sub-carrier spacing ${\displaystyle \alpha /T}$ for ${\displaystyle \alpha <1}$ makes them non-orthogonal over each symbol period.

The history of N-OFDM signals theory was started in 1992 from the Patent of Russian Federation No. 2054684.[1] In this patent, Vadym Slyusar proposed the 1st method of optimal processing for N-OFDM signals after Fast Fourier transform (FFT).

In this regard need to say that W. Kozek and A. F. Molisch wrote in 1998 about N-OFDM signals with ${\displaystyle \alpha <1}$ that "it is not possible to recover the information from the received signal, even in the case of an ideal channel." [4]

In 2001 V. Slyusar proposed non-orthogonal frequency digital modulation (N-OFDM) as an alternative of OFDM for communications systems .[5]

The next publication about this method has priority in July 2002[2] before the conference paper regarding SEFDM of I. Darwazeh and M.R.D. Rodrigues (September, 2003).[6]

Despite the increased complexity of demodulating N-OFDM signals compared to OFDM, the transition to non-orthogonal subcarrier frequency arrangement provides several advantages:

1. higher spectral efficiency, which allows to reduce the frequency band occupied by the signal and improve the electromagnetic compatibility of many terminals;
2. adaptive detuning from interference concentrated in frequency by changing the nominal frequencies of the subcarriers;[7]
3. an ability to take into account Doppler frequency shifts of subcarriers when working with subscribers moving at high speeds;
4. reduction of the peak factor of the multi-frequency signal mixture.

This section describes a simple idealized N-OFDM system model suitable for a time-invariant AWGN channel[8]

An N-OFDM carrier signal is the sum of a number of not-orthogonal subcarriers, with baseband data on each subcarrier being independently modulated commonly using some type of quadrature amplitude modulation (QAM) or phase-shift keying (PSK). This composite baseband signal is typically used to modulate a main RF carrier.

${\displaystyle \scriptstyle s[n]}$ is a serial stream of binary digits. By inverse multiplexing, these are first demultiplexed into ${\displaystyle \scriptstyle N}$ parallel streams, and each one mapped to a (possibly complex) symbol stream using some modulation constellation (QAM, PSK, etc.). Note that the constellations may be different, so some streams may carry a higher bit-rate than others.

A Digital Signal Processor (DSP) is computed on each set of symbols, giving a set of complex time-domain samples. These samples are then quadrature-mixed to passband in the standard way. The real and imaginary components are first converted to the analogue domain using digital-to-analogue converters (DACs); the analogue signals are then used to modulate cosine and sine waves at the carrier frequency, ${\displaystyle \scriptstyle f_{c}}$, respectively. These signals are then summed to give the transmission signal, ${\displaystyle \scriptstyle s(t)}$.

The 1st method of optimal processing for N-OFDM signals after FFT was proposed in 1992[1]

N-OFDM+MIMO system model

The combination N-OFDM and MIMO technology is similar to OFDM. To the building of MIMO system can be used digital antenna array as transmitter and receiver of N-OFDM signals.

Fast-OFDM[10][11][12] method was proposed in 2002.[13]

FBMC is Filter-Bank Multi-Carrier Modulation.[14][15][16] As example of FBMC can consider Wavelet N-OFDM.

N-OFDM is a spectrally efficient method.[6][18] All SEFDM methods are similar to N-OFDM.[6][19][20][21][22][23][24]

GFDM is generalized frequency division multiplexing.

• OFDM
• COFDM