Passband modulation |
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Analog modulation |
Digital modulation |
Hierarchical modulation |
Spread spectrum |
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Non-orthogonal frequency-division multiplexing (N-OFDM) is a method of encoding digital data on multiple carrier frequencies with non-orthogonal intervals between frequency of sub-carriers.[1][2][3] N-OFDM signals can be used in communication and radar systems.
The low-pass equivalent N-OFDM signal is expressed as:[3][2]
where are the data symbols, is the number of sub-carriers, and is the N-OFDM symbol time. The sub-carrier spacing for 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 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:
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.
is a serial stream of binary digits. By inverse multiplexing, these are first demultiplexed into 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, , respectively. These signals are then summed to give the transmission signal, .
The receiver picks up the signal , which is then quadrature-mixed down to baseband using cosine and sine waves at the carrier frequency. This also creates signals centered on , so low-pass filters are used to reject these. The baseband signals are then sampled and digitised using analog-to-digital converters (ADCs), and a forward FFT is used to convert back to the frequency domain.
This returns parallel streams, which use in appropriate symbol detector.
The 1st method of optimal processing for N-OFDM signals after FFT was proposed in 1992.[1]
The method of optimal processing for N-OFDM signals without FFT was proposed in October 2003.[3][9] In this case can be used ADC samples.
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]
Filter-bank multi-carrier modulation (FBMC) is.[14][15][16] As example of FBMC can consider Wavelet N-OFDM.
N-OFDM has become a technique for power-line communications (PLC). In this area of research, a wavelet transform is introduced to replace the DFT as the method of creating non-orthogonal frequencies. This is due to the advantages wavelets offer, which are particularly useful on noisy power lines.[17]
To create the sender signal the wavelet N-OFDM uses a synthesis bank consisting of a -band transmultiplexer followed by the transform function
On the receiver side, an analysis bank is used to demodulate the signal again. This bank contains an inverse transform
followed by another -band transmultiplexer. The relationship between both transform functions is
N-OFDM is a spectrally efficient method.[6][18] All SEFDM methods are similar to N-OFDM.[6][19][20][21][22][23][24]
Generalized frequency division multiplexing (GFDM) is.