PAPR optimized OFDM touch engine with tone spaced windowed demodulation

A touch-sensitive apparatus, comprising:

• a panel comprising a touch surface, a plurality of drive electrodes, and a plurality of receive electrodes;
• an electronic subsystem comprising a plurality of drive amplifiers, a plurality of sense amplifiers, and an analog and digital signal processing unit;
• wherein each drive amplifier is configured to deliver to a corresponding said drive electrode an up-converted drive signal comprising a set of one or more drive frequency tones that are subcarriers around a common carrier frequency;
• and
• wherein each set of said drive frequencies is configured to be disjoint to every other set of drive frequencies of all other said drive electrodes;
• and
• wherein all said drive frequencies in all said sets are configured to be orthogonal to each other such that their separations are selected to be an integer multiple of a common frequency spacing Δf that defines the subcarrier spacing of an associated composite carrier modulated orthogonal frequency division multiplexed (OFDM) signal comprising of all said subcarrier frequencies of all the said drive unit signals at said common carrier frequency received at a said receive electrode output;
• and
• wherein each sense amplifier input is connected to a corresponding said receive electrode to receive said composite carrier modulated OFDM response signal at said common transmitter carrier frequency from its said receive electrode;
• and
• wherein each said sense amplifier output is delivered to said analog and digital signal processing unit which is configured to down-convert said composite carrier modulated OFDM signal with further means to analyze said down-converted response signal to determine therefrom a coupling capacitance between every drive electrode and its corresponding receive electrode using fast Fourier transform (FFT) or maximum likelihood estimation;
• and
• wherein said analog and digital signal processing unit is configured for up-conversion of said drive signals, wherein said up-conversion is selected from a group comprising of (a) an equivalent multiplication between real baseband signal and a real transmitter carrier signal that corresponds to creating a double side-band (DSB) modulated signal, and (b) an equivalent complex multiplication between a complex baseband signal and a complex transmitter carrier signal with the real output of the complex product taken that corresponds to creating a single side-band (SSB) modulated signal, and wherein each said complex baseband signal, said complex carrier signal and said complex product comprises of in-phase and quadrature-phase components, and (c) other known methods of up-conversion of signals including up-conversion of a SSB or DSB modulated intermediate frequency signal to a carrier frequency;
• and
• wherein said analog and digital signal processing unit is configured for down-conversion of said sense electrode signal wherein said down-conversion is selected from a group comprising (a) an equivalent multiplication between said signal at said transmitter carrier frequency and a real single-phase local un-modulated carrier signal at the same carrier frequency, followed by filtering and sampling, (b) an equivalent multiplication between said signal at said transmitter carrier frequency and a complex in-phase and quadrature-phase local un-modulated carrier signal at the same carrier frequency, followed by filtering and sampling, and (c) oversampling of said signal and separating the time samples into an in-phase and a quadrature-phase path, followed by suitable low pass filtering for removal of unwanted frequency components;
• and
• wherein a spacing of M*Δf is kept between adjacent physically used drive frequencies;
• and
• wherein M is chosen to be an integer greater than or equal 2 and is independently selected for each gap between said physically used adjacent subcarriers;
• and
• wherein a time-windowing of receive signal is implemented by said analog and digital signal processing unit for enabling amplitude modulation of the time signal at the input of said FFT block, thereby reducing spectral leakage from interfering signals and thereby improving signal to noise ratio (SNR) of used subcarriers;
• and
• wherein the length of each said time-window is selected to be one FFT duration of 1/Δf;
• and
• wherein said time-window is selected from a group consisting of sine-squared, raised-cosine, Hann, Hamming, Blackman, Chebychev, Tukey and other types of windowing functions that are generally known in the art for mitigating spectral leakage;
• and
• wherein the number of total subcarriers, length N of the number of FFT time samples and the size of the FFT, for a given number of said physically used subcarriers, are configured to be appropriately increased for accommodating an increased total number of unused subcarriers for providing said M*Δf spacing between said physically used subcarriers thereby reducing interference due to spectral leakage between them when said time-windowing is used.

A method of determining a plurality of touch locations for touches on a touch panel that are simultaneous or overlapping in time, the method comprising:

• providing a panel comprising a touch surface, a plurality of drive electrodes, and a plurality of receive electrodes;
• providing an electronic subsystem comprising a plurality of drive amplifiers, a plurality of sense amplifiers, and an analog and digital signal processing unit;
• delivering to a said drive electrode through a corresponding said drive amplifier an up-converted drive signal comprising a set of one or more drive frequency tones that are subcarriers around a common carrier frequency;
• configuring said analog and digital signal processing unit for up-conversion of said drive signals, wherein said up-conversion is selected from a group comprising of (a) an equivalent multiplication between real baseband signal and a real transmitter carrier signal that corresponds to creating a double side-band (DSB) modulated signal, and (b) an equivalent complex multiplication between a complex baseband signal and a complex transmitter carrier signal with the real output of the complex product taken that corresponds to creating a single side-band (SSB) modulated signal, and wherein each said complex baseband signal, said complex carrier signal and said complex product comprises of in-phase and quadrature-phase components, and (c) other known methods of up-conversion of signals including up-conversion of a SSB or DSB modulated intermediate frequency signal to a carrier frequency;
• selecting each set of said drive frequency to be disjoint to every other set of drive frequency of all other said drive electrodes;
• selecting all drive frequencies in all sets to be orthogonal to each other such that their separations are an integer multiple of a common frequency spacing M that defines the subcarrier spacing of an associated composite carrier modulated orthogonal frequency division multiplexed (OFDM) signal comprising of all said subcarrier frequencies of all the said drive unit signals at said common carrier frequency received at said receive electrode output;
• setting the phase of each drive frequency tone using a predetermined value based on a complementary code sequence that is chosen to reduce the peak to average power of the composite sum of all drive signals that appear at said receive electrode output as said composite carrier modulated OFDM signal;
• sensing said receive electrode at corresponding said sense amplifier input and providing corresponding said composite carrier modulated OFDM response signal at said common transmitter carrier frequency at said sensor amplifier output to said analog and digital signal processing unit;
• down-converting said composite carrier modulated OFDM signal to baseband by said analog and digital signal processing unit using a single-phase or a complex quadrature-phase down conversion method;
• analyzing said down-converted response signal to determine therefrom a coupling mutual capacitance between every drive electrode and its corresponding receive electrode using fast Fourier transform (FFT) or maximum likelihood estimation;
• introducing a spacing of M*Δf between adjacent physically used drive frequencies;
• choosing M to be an integer greater than or equal 2 and independently selecting it for each gap between said physically used adjacent subcarriers;
• performing a time-windowing of receive signal by said analog and digital signal processing unit for enabling amplitude modulation of the time signal at the input of said FFT block, thereby reducing spectral leakage from interfering signals and thereby improving SNR of used subcarriers;
• choosing the length of each said time-window to be one FFT duration of 1/Δf;
• selecting said time-window from a group consisting of sine-squared, raised-cosine, Hann, Hamming, Blackman, Chebychev, Tukey and other types of windowing functions that are generally known in the art for mitigating spectral leakage;
• configuring the number of total subcarriers, length N of the number of FFT time samples and the size of said FFT, for a given number of said physically used subcarriers such that they are all appropriately increased for accommodating an increased total number of unused subcarriers for providing said M*Δf spacing between said physically used subcarriers thereby reducing interference due to spectral leakage between them when said time-windowing is used.