Non-orthogonal frequency-division multiplexing

Non-orthogonal frequency-division multiplexing (N-OFDM) is a technique for encoding digital data across multiple carrier frequencies, where the spacing between sub-carrier frequencies is non-orthogonal. This approach is used in both communication and radar systems.

Subcarriers system

The low-pass equivalent N-OFDM signal is defined by a set of data symbols, a number of sub-carriers, and a symbol time. The sub-carrier spacing is set so that the sub-carriers are not orthogonal over each symbol period.

History

The theoretical foundation of N-OFDM signals began with a 1992 Russian Federation patent, in which Vadym Slyusar introduced the first method for optimal processing of N-OFDM signals after a Fast Fourier transform (FFT). In 1998, W. Kozek and A. F. Molisch wrote that, for certain N-OFDM signals, it is not possible to recover information from the received signal, even over an ideal channel. In 2001, V. Slyusar proposed non-orthogonal frequency digital modulation (N-OFDM) as an alternative to OFDM for communications. A subsequent publication on this method, from July 2002, predates a September 2003 conference paper on SEFDM by I. Darwazeh and M.R.D. Rodrigues.

Although demodulating N-OFDM signals is more complex than demodulating OFDM, the use of non-orthogonal sub-carrier spacing offers several advantages: higher spectral efficiency, which reduces the signal's frequency band and improves electromagnetic compatibility among many terminals; adaptive detuning from frequency-concentrated interference by adjusting sub-carrier nominal frequencies; the ability to account for Doppler frequency shifts in sub-carriers when serving high-speed mobile subscribers; and a reduction in the peak factor of the multi-frequency signal mixture.

Transmitter N-OFDM signals

In an idealized N-OFDM system model for a time-invariant additive white Gaussian noise (AWGN) channel, the transmitter generates a carrier signal as the sum of non-orthogonal sub-carriers. Each sub-carrier is independently modulated in baseband, typically using quadrature amplitude modulation (QAM) or phase-shift keying (PSK). This composite baseband signal then modulates a main RF carrier. A serial stream of binary digits is demultiplexed into parallel streams, each mapped to a symbol stream using a modulation constellation (which may differ between streams, allowing some to carry higher bit rates).