Non-orthogonal frequency-division multiplexing
A multi-carrier modulation method using non-orthogonal subcarrier spacing.
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.
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.
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.
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).
- First patent
- Russian Federation No. 2054684 (1992)
- First proposer of optimal processing aft
- Vadym Slyusar (1992)
- Proposed as alternative to ofdm
- V. Slyusar (2001)
- Priority publication
- July 2002
- Method of optimal processing without fft
- October 2003
Lore & Background
The history of N-OFDM signal theory began in 1992 with a patent from the Russian Federation (No. 2054684), in which Vadym Slyusar proposed the first method of optimal processing for N-OFDM signals after the Fast Fourier transform (FFT). In 1998, W. Kozek and A. F. Molisch wrote about N-OFDM signals with a certain subcarrier spacing that 'it is not possible to recover the information from the received signal, even in the case of an ideal channel.' In 2001, V. Slyusar proposed non-orthogonal frequency digital modulation as an alternative to OFDM for communications systems. A subsequent publication on this method has priority in July 2002, before a conference paper on SEFDM by I. Darwazeh and M.R.D. Rodrigues in September 2003. N-OFDM signals can be used in both communication and radar systems. The low-pass equivalent N-OFDM signal is expressed with data symbols, a number of sub-carriers, and a symbol time, where the sub-carrier spacing makes them non-orthogonal over each symbol period.
Reader's Guide
Despite increased demodulation complexity compared to OFDM, N-OFDM provides several advantages: higher spectral efficiency, adaptive detuning from frequency-concentrated interference by changing subcarrier nominal frequencies, the ability to account for Doppler frequency shifts when working with high-speed subscribers, and a reduction of the peak factor of the multi-frequency signal mixture. An idealized system model for a time-invariant AWGN channel describes a transmitter that sums non-orthogonal subcarriers modulated with QAM or PSK, and a receiver that uses quadrature mixing, low-pass filtering, ADC sampling, and an FFT to return parallel streams for symbol detection. The first optimal processing method after FFT was proposed in 1992; a method without FFT using ADC samples was proposed in October 2003. N-OFDM can be combined with MIMO technology using digital antenna arrays. Related techniques include Fast-OFDM (proposed in 2002), filter-bank multi-carrier modulation (FBMC) such as Wavelet N-OFDM (used for power-line communications), spectrally-efficient FDM (SEFDM), and generalized frequency division multiplexing (GFDM).
Did You Know?
- The first method of optimal processing for N-OFDM signals after FFT was proposed in a 1992 Russian Federation patent by Vadym Slyusar.
- Wavelet N-OFDM, a form of filter-bank multi-carrier modulation, has become a technique for power-line communications.
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