Objective. A new training-free framework was proposed for target detection in steady-state visual evoked potential (SSVEP)-based brain–computer interfaces (BCIs) using joint frequency-phase coding. Approach. The key idea is to transfer SSVEP templates from the existing subjects to a new subject to enhance the detection of SSVEPs. Under this framework, transfer template-based canonical correlation analysis (tt-CCA) methods were developed for single-channel and multi-channel conditions respectively. In addition, an online transfer template-based CCA (ott-CCA) method was proposed to update EEG templates by online adaptation. Main results. The efficiency of the proposed framework was proved with a simulated BCI experiment. Compared with the standard CCA method, tt-CCA obtained an 18.78% increase of accuracy with a data length of 1.5 s. A simulated test of ott-CCA further received an accuracy increase of 2.99%. Significance. The proposed simple yet efficient framework significantly facilitates the use of SSVEP BCIs using joint frequency-phase coding. This study also sheds light on the benefits from exploring and exploiting inter-subject information to the electroencephalogram (EEG)-based BCIs.

http://iopscience.iop.org/article/10.1088/1741-2560/12/4/046006/pdf

Enhancing performances of SSVEP-based brain–computer interfaces via exploiting inter-subject information

Steady-state visual evoked potential (SSVEP)-based brain-computer interfaces (BCIs) have potential to realize high-speed communication between the human brain and the external environment. Recently, multiple access (MA) methods in telecommunications have been introduced into the system design of BCIs and showed their potential in improving BCI performance. This study investigated the feasibility of hybrid frequency and phase coding methods in multi-target SSVEP-based BCIs. Specifically, this study compared two hybrid target-coding strategies: (1) mixed frequency and phase coding, and (2) joint frequency and phase coding. In a simulated online BCI experiment using a 40-target BCI speller, BCI performance for both coding approaches were tested with a group of six subjects. At a spelling speed of 40 characters per minute (1.5 seconds per character), both approaches obtained high information transfer rates (ITR) (mixed coding: 172.37±28.67 bits/min, joint coding: 170.94±28.32 bits/min) across subjects. There was no statistically significant difference between the two approaches (p>0.05). These results suggest that the hybrid frequency and phase coding methods are highly efficient for multi-target coding in SSVEP BCIs with a large number of classes, providing a practical solution to implement a high-speed BCI speller.

Hybrid frequency and phase coding for a high-speed SSVEP-based BCI speller.

M-ary on-off frequency-shift keying (OOFSK) is a digital modulation format in which M-ary FSK signaling is overlaid on on/off keying. This paper investigates the potential of this modulation format in the context of wideband fading channels. First, it is assumed that the receiver uses energy detection for the reception of OOFSK signals. Capacity expressions are obtained for the cases in which the receiver has perfect and imperfect fading side information. Power efficiency is investigated when the transmitter is subject to a peak-to-average power ratio (PAR) limitation or a peak power limitation. It is shown that under a PAR limitation, it is extremely power inefficient to operate in the very-low-SNR regime. On the other hand, if there is only a peak power limitation, it is demonstrated that power efficiency improves as one operates with smaller SNR and vanishing duty factor. Also studied are the capacity improvements that accrue when the receiver can track phase shifts in the channel or if the received signal has a specular component. To take advantage of those features, the phase of the modulation is also allowed to carry information.

/pdf/on-off-frequency-shift-keying-for-wideband-fading-channels-3ynciv8wvm.pdf

On-off frequency-shift keying for wideband fading channels

A structure and a method for transmitting digital data modulate the amplitude and the phase of a set of orthogonal band-limited finite duration waveforms. In one embodiment, a group of information symbols modulate in parallel a set of spectrally efficient orthogonal waveforms, known as prolate spheroidal wave functions. The modulated orthogonal waveforms are summed, and the resulting composite signal then modulates a carrier signal for transmission. The compressed spectrum modulated signal provides a robust data waveform which reduces the spectral bandwidth requirements compared to digital modulation formats employed in the prior art. The transmitter and the receiver for modulating and demodulating the orthogonal waveforms for transmission operate at slower speeds than corresponding equipment of existing modulation schemes of comparable symbol rate. Consequently, both simplicity of design and cost savings are realized in a compressed spectrum modulation scheme.

Method for signal transmission using spectrally efficient orthogonal modulation

This paper studies the design and performance analysis of a noncoherent hybrid modulation scheme derived from multifrequency and multiphase signals, and referred to as joint frequency-phase modulation (JFPM). This modulation class includes many formats, which can be classified as efficient bandwidth-limited modulation or power-limited modulation techniques. A noncoherent receiver structure for JFPM signals is introduced, and an exact expression for the probability of a bit error over AWGN channels is obtained. The results show that noncoherent JFPM perform better than M-ary DPSK and/or noncoherent M-ary FSK systems. The power spectral density and the spectral efficiency of JFPM are analyzed. Fractional out-of-band power containment bandwidths are obtained numerically for different JFPM formats. These analytical results show that the power spectral density of JFPM does not contain a discrete component and the same bandwidth efficiency can be obtained by using different JFPM formats. Overall, JFPM can be classified as a bandwidth- and power-efficient modulation technique. >

Bit error rate performance and power spectral density of a noncoherent hybrid frequency-phase modulation system

The performance of a hybrid modulation technique derived from binary frequency shift keying is presented. In the hybrid scheme, each frequency is allowed to be phase modulated with any of p discrete, equally spaced phase shifts. The spectral separation between frequencies is chosen to make the carriers orthogonal, thus generating a multidimensional modulation scheme. The performance of complete and expurgated phase codes is examined for the Gaussian and Rayleigh fading channels, and their spectral characteristics are determined. >

/pdf/on-the-performance-of-a-hybrid-frequency-and-phase-shift-azscb7uew0.pdf

On the performance of a hybrid frequency and phase shift keying modulation technique

Steiner systems constitute an example of codes that achieve asymptotic error-free transmission for the Gaussian channel. In contrast to simplex and orthogonal codes which also have this property, these codes make more efficient use of the bandwidth by reducing the required dimensionality to generate the M signals. Tight upper bounds for the symbol error probability are derived for coherent and noncoherent detection that include previously reported results as special cases. The relative bandwidth efficiency with respect to orthogonal codes is presented, and a combined performance plot is given that allows a meaningful comparison between the different coding schemes considered. >

R.A. Khalona

Papers

On the performance of a hybrid frequency and phase shift keying modulation technique

Steiner system signal sets over the Gaussian channel