Showing papers on "Demodulation published in 1990"

Multistage detection in asynchronous code-division multiple-access communications

Abstract: A multiuser detection strategy for coherent demodulation in an asynchronous code-division multiple-access system is proposed and analyzed. The resulting detectors process the sufficient statistics by means of a multistage algorithm based on a scheme for annihilating successive multiple-access interference. An efficient real-time implementation of the multistage algorithm with a fixed decoding delay is obtained and shown to require a computational complexity per symbol which is linear in the number of users K. Hence, the multistage detector contrasts with the optimum demodulator, which is based on a dynamic programming algorithm, has a variable decoding delay, and has a software complexity per symbol that is exponential in K. An exact expression is obtained and used to compute the probability of error is obtained for the two-stage detector, showing that the two-stage receiver is particularly well suited for near-far situations, approaching performance of single-user communications as the interfering signals become stronger. The near-far problem is therefore alleviated. Significant performance gains over the conventional receiver are obtained even for relatively high-bandwidth-efficiency situations. >

Near-far resistance of multiuser detectors in asynchronous channels

Abstract: Consideration is given to an asynchronous code-division multiple-access environment in which receiver has knowledge of the signature waveforms of all the users. Under the assumption of white Gaussian background noise, the authors compare detectors by their worst case bit error rate in a near-far environment with low background noise, where the received energies of the users are unknown to the receiver and are not necessarily similar. Conventional single-user detection in a multiuser channel is not near-far resistant, and the substantially higher performance of the optimum multiuser detector requires exponential complexity in the number of users. The authors explore suboptimal demodulation schemes which exhibit a low order of complexity while not exhibiting the impairment of the conventional single-user detector. It is shown that there exists a linear detector whose bit-error-rate is independent of the energy of the interfering users. It is also shown that the near-far resistance of optimum multiuser detection can be achieved by a linear detector. The optimum linear detector for worst-case energies is found, along with existence conditions, which are always satisfied in the models of practical interest. >

Performance of coherent optical receivers

Abstract: Coherent optical communications, an area of research that shows great promise for future high-bandwidth and long-haul applications, is reviewed. Coherent optical receivers, which add light to the received signal as part of the detection process, have numerous advantages over direct-detection receivers, most notably increased sensitivity and increased selectivity, at the cost of increased complexity. The performance of coherent optical receivers under shot-noise-limited conditions is reviewed for a variety of modulation and demodulation formats. In addition, laser phase noise is discussed, and its effect on receiver performance is analyzed. >

Variable rate cardiac pacer

Abstract: An endocardial lead having first and second spaced apart electrodes resides in a patient's heart. The first electrode is a sensing electrode and the second electrode is a carrier signal driving electrode. The lead has a conductor coupling a source of alternating current carrier signals of a predetermined frequency to the second electrode. A third electrode is in electrical contact with body tissues. A cardiac pacer apparatus includes a pacer can which functions as a fourth electrode and has a plastic top wherein the third electrode is located. Said third electrode acts in cooperation with the first electrode to form a pair of sensing electrodes. The sensing electrode pair is further coupled to a sense amplifier for receiving an amplifying modulated electrical signals developed across the sensing electrode pair. A demodulator and filters circuit for demodulating the modulated carrier signal and recovering the modulating signal therefrom is connected to the output of the sense amplifier. The modulating signal is proportional to instantaneous stroke volume of the patient's heart and the demodulator and filters circuit develops a control signal therefrom called a stroke volume signal. The control signal is applied to the pulse generator so as to control the rate of stimulating pulses.

Method and apparatus for the reception and demodulation of spread spectrum radio signals

Abstract: A spread spectrum receiver with filters matched to transmitter chip codes are implemented in digital circuits along with a digital circuit for acquisition and tracking of the arrival times of the chip codes. The digital circuit implementations are used for the noncoherent demodulation of pulse position spread spectrum modulation signals where the pulse is a carrier modulator by a chip code and for the noncoherent demodulation of multiple chip code modulation signals where each information symbol is represented by one of several chip codes modulating a carrier.

Frequency domain optical imaging using diffusion of intensity modulated radiation

Abstract: Arrangements are disclosed for producing images based upon diffusional wave theory and frequency domain analysis. A medium to be imaged is illuminated with amplitude modulated radiation, and diffusional radiation transmitted or reflected by the medium is detected at a plurality of detection locations, as by a television camera. The phase and also the amplitude demodulation of the amplitude modulated diffusional radiation is detected at each detection location. A relative phase image and also a demodulation amplitude image of the medium are then generated from respectively the detected relative phase values and the detected demodulation amplitudes of the diffusional radiation at the plurality of locations. The present invention is particularly suited for medical applications for generating images of internal anatomical details of the body by using a near infrared amplitude modulated source for illumination of the body. In such medical applications, the body is illuminated with near infrared radiation having a wavelength between 600 and 1200 nanometers which is amplitude modulated at a frequency in the megahertz to gegihertz range, and internal images of the patient are generated for medical diagnosis. In preferred embodiments, a laser is utilized to generate the near infrared radiation near or below a 10 watt power level which is coupled by fiber optic cables to the body of the patient. Detection is also with fiber optic cables placed in direct contact with an opposed surface of the body which couple the transmitted diffusional radiation to an image intensifier placed in front of a television camera detector. The gain of the image intensifier is modulated at a frequency to obtain a heterodyned output frequency of less than 60 hertz which is within the detection bandwidth of the television camera.

Automatic modulation classification using zeroç crossing

Abstract: A modulation recogniser that automatically reports modulation types of constant-envelope modulated signals is developed using zero-crossing techniques. The zero-crossing sampler, as a signal conditioner, has the advantage of providing accurate phase transition information over a wide dynamic frequency range. Signal parameters such as zero-crossing variance, carrier-to-noise ratio (CNR) and carrier frequency are estimated. Phase difference and zero-crossing interval histograms play the role of features for modulation recognition. The classifier performance is given in the form of a confusion matrix. The simulation results obtained demonstrate that a reasonable average probability of correct classification is achievable for CNR ≥ 15 dB.

A 100-channel optical FDM transmission/distribution at 622 Mb/s over 50 km

Abstract: A 100-channel optical frequency-division multiplexing (FDM) transmission/distribution experiment at 622 Mb/s is demonstrated for a fiber length of 50 km. The feasibility of a polarization-insensitive waveguide frequency selection switch for 10-GHz intervals and a frequency-shift-keying (FSK) direct-detection scheme employing a Mach-Zehnder filter is verified. The demodulation circuit employs a Mach-Zehnder filter and a balanced receiver, which utilizes optical power more efficiently than the Fabry-Perot filter. No receiver sensitivity degradation is observed due to interchannel crosstalk of the 128-channel tunable waveguide frequency selection switch (FS-SW) or fiber four-wave mixing for transmissions over a 50-km-long nondispersion-shifted (NDS) fiber and a 26-km-long dispersion-shifted (DS) fiber. >

Quadrature amplitude modulation communication system with transparent error correction

Abstract: In a multi-level QAM communication system, Reed-Solomon encoders and Reed-Solomon decoders are employed for error correction purposes. The phase ambiguity of the received signal is eliminated with differential coding. The multi-level QAM communication system utilizing n bits ("n" being an integer) QAM signal having 2 n signal points, comprises: a quadrature differential encoder/decoder unit for differentially encoding/decoding n pieces of input digital signal series to produce n pieces of differentially coded signal series; an error correction unit including a Reed-Solomon encoder and a Reed-Solomon decoder, provided inside the quadrature differential encoder/decoder unit along a signal processing path of the input digital signal series, for error-correcting the n pieces of differentially-coded signal series by utilizing at least one of the digital signal series with employment of a Reed-Solomon code; and, a QAM modulator/demodulator unit for QAM-modulating/demodulating n pieces of error-corrected signal series so as to produce 2 n QAM signals.

Modulation and demodulation techniques in optical heterodyne PSK transmission systems

Abstract: Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described. >

Frequency conversion circuit

Abstract: A circuit array for frequency translation by means of quadrature heterodyne signals has very low quadrature errors even at very high frequencies and is monolithically integratable with little external circuitry. The circuit array includes a first mixer which receives a first portion of an input signal, a second mixer which receives a second portion of the input signal, and a heterodyne signal generator which receives a local oscillator signal and supplies quadrature heterodyne signals to the mixers. The heterodyne signal generator includes a control loop to ensure a 90° phase shift between the quadrature heterodyne signals. The circuit array can be used in a modulator for a transmitter or in a demodulator for a receiver.

Low-overhead symbol timing and carrier recovery for TDMA portable radio systems

Abstract: Simulation and experimental results for a hardware implementation of a low-overhead digital coherent burst demodulator for time-division multiple-access (TDMA) radio systems using short bursts are analyzed. This implementation is based on digitizing a receiving signal after a low-frequency IF amplifier and performing all demodulation functions using digital signal processing techniques. Demodulation with very low overhead for TDMA is made possible by storing a burst in memory. A novel forward loop plus backward loop structure performs carrier recovery. Symbol-timing and carrier-frequency-offset estimations are performed by block processes, using the error signal resulting from differential demodulation. Bursts as short as 32 symbols can be demodulated without overhead. Since the demodulator is entirely digital and has the potential for low-power VLSI implementation, it is suitable for a portable radio system. Other advantages of this implementation include the availability of a channel quality measure derived from symbol timing estimation and the possibility of providing differential detection when it is desirable. >

Multiple-access capability of frequency-hopped spread-spectrum revisited: an analysis of the effect of unequal power levels

Abstract: A method for the evaluation of the probability of error of uncoded asynchronous frequency-hopped spread-spectrum multiple-access communications is presented. For systems with binary FSK modulation this method provides an accurate approximation and a tight upper bound to the bit error probability; for systems with M-ary FSK modulation, it provides tight upper bounds to the symbol error probability. The method enables the computationally efficient averaging of the error probability with respect to the delays, phase angles, and data streams of the different users. It relies on the integration of the product of the characteristic function of the envelope of the branch of the BFSK demodulator, which carries the desired signal, and of the derivative of the characteristic function of the envelope of the other branch. For sufficient frequency separation between the BFSK tones, the method can achieve any desirable accuracy. Moreover, the computational effort required for its evaluation grows linearly with the number of interfering users. In the M-ary case, tight upper bounds based on the union bound and the results of the binary case are derived. The method allows the effect of unequal power levels on other-user interference in FH/SSMA systems to be quantified accurately for the first time. The results indicate that the FH/SSMA systems suffer from the near-far problem, although less seriously than direct-sequence SSMA systems. >

Burst demodulator for establishing carrier and clock timing from a sequence of alternating symbols

Abstract: OF THE DISCLOSURE In a burst demodulator, an APSK burst signal containing a preamble of alternating "1" and "-1" symbols, followed by a data field is received and quasi-coherently demodulated into a baseband complex signal, which is converted to digital form such that each symbol of the complex signal yields N digital samples, with N being selected such that at least one of the N digital samples is closest to a signal point of the burst signal. A clock recovery circuit is responsive to the arrival of the burst for estimating the symbol timing of the burst signal from digital samples of the preamble. A digital sample is extracted from every N samples of the preamble in response to the estimated symbol timing so that it is most likely to be closest to the signal point. A carrier recovery circuit responds to the estimated symbol timing by estimating the carrier frequency and phase of the burst signal from the extracted digital samples to produce a complex carrier signal, which is multiplied with the extracted digital samples to recover the original signal. Memories are preferably provided to store the received burst signal. By repeatedly reading the preamble from the memories, the same symbols of the preamble can be used for clock timing and carrier phase recovery operations.

Multi-carrier high speed modem

Abstract: A modem system includes a transmitter section having generators for successively generating sets of carrier signals where each carrier signal of a set has a different frequency and a modulator for modulating each carrier signal of a set with a different portion of digital data to be transmitted where all such portions used to modulate a set of carrier signals comprise a frame of digital data. Also included are an adder for successively combining together the modulated carrier signals of each set to produce a frame waveform, an inverse Fast Fourier Transform circuit for transforming each waveform from a frequency domain signal to a time domain signal, a rotate and match buffer circuit for rotating each transform waveform so that its beginning point amplitude and slope substantially match the ending point amplitude and slope of the immediately preceding transformed waveform, and a transmitter for successively and continuously transmitting the transformed and rotated waveforms over a telephone channel. The modem system also includes a receiver section having a circuit for receiving from the telephone channel transmitted signals, a rerotate circuit for rerotating each received waveform substantially to its original condition before rotating, a Fast Fourier Transform circuit for transforming each received, rerotated waveform from a time domain signal back to a frequency domain signal, and a demodulator for demodulating each received and rerotated waveform to recover the digital data transmitted.

Adaptive predistortion circuit for a digital transmission system

Abstract: An adaptive predistortion circuit for a digital transmission system includes a predistortion circuit (52 1 , 52 2 , 52 3 ) for predistorting the input data before they pass through a modulator (56) and then through an amplifier (57), and an adaptation circuit (61 1 , 61 2 , 61 3 ) for continuously adapting the predistortion circuit to the stream of input data in response to a demodulation of the stream of transmitted data. The predistortion circuit further includes an encoder (51) which, on a first path, in response to digital data a k , generates digital data b k which leave a first predistortion circuit (52 1 ) in a predistorted manner in-phase with the symbol clock, on a second path, digital data c k which leave a second distortion circuit (52 2 ) in a predistorted manner phase-shifted by T/3 relative to the symbol clock and on a third path, digital data d k which leave a third predistortion circuit (52 3 ) in a predistorted manner phase-shifted by 2T/3 relative to the symbol clock. The signals b k , c.sub. k and d k are obtained by means of a polynomial encoding. The encoding may be performed by shift registers (62 1 . . . 62 I ) addressing a memory (63) or, by a transversal filter (71 1 , 71 2 , 71 3 ). It may also be obtained by means of an analog transmission filter (81) followed by samplers (81 1 , 82 2 , 82 3 ).

Video signal processing circuit for extracting a chroma signal from a luminous signal

Abstract: A video signal processing circuit for reproducing an FM video signal. A video reproduction signal output from a magnetic head is supplied to a low-pass filter, which extracts a chroma signal. The video reproduction signal is also supplied to a chroma signal removing circuit through a high-pass filter of low degree and sharpness. The chroma signal removing circuit removes the chroma signal component. The output of the chroma signal removing circuit is amplitude-limited to a predetermined degree by a limiter, and then supplied to a demodulator which extracts a luminance signal.

Charge-coupled device image sensor as a demodulator in a 2-D polarimeter with a piezoelastic modulator

Abstract: It is shown how demodulation of rapidly modulated light beams can be achieved within a single charge-coupled device (CCD). Two interlaced image planes are created by optically masking every second CCD row and transferring the charges back and forth between the two image planes in synchrony with the modulation. The method has been successfully tested for modulation frequencies of 50 and 100 kHz, using integration times up to 1 s. No significant accumulated charge transfer losses are seen for integration times as long as 10(5) modulation cycles (1 s). This demonstrates the feasibility of a CCD polarimeter using piezoelastic modulation of the state of polarization.

Interrogation signal processor for air traffic control communications

Abstract: The present invention is directed to a Mode S uplink or interrogation signal demodulation system which can quickly recognize the Mode S signal and also filter out of noise present in the Mode S uplink or interrogation signal, thereby reducing the bit error rate. To realize this goal the present invention includes a digital differential phase-shift keyed demodulator to demodulate the differential phase-shift keyed data, thereby reducing the time needed to acquire the Mode S uplink or interrogation signal. This digital demodulator also reduces the noise present in the Mode S uplink or interrogation signal and provides an integrated system which is small in structure that can be easily implemented in an aircraft. This Mode S system also includes a preamble and sync phase reversal detection circuit to recognize if the transmitted signal is a Mode S signal. This signal also utilizes Mode A and Mode C detection devices to make the system compatible with present communication systems.

Telecommunications system and method

Abstract: A teleconferencing system includes desktop computers, external cameras anderipheral apparatus to allow interconnection of two or more terminals to engage in teleconferencing via digital data networks. A frame grabber interfaces each camera with its computer. Signals output from the computer are encoded and compressed before being input to a telephone line via a modulator/demodulator. A decoder is connected between the modulator/demodulator and the computer for decoding compressed video signals received by the modulator/demodulator means from the telephone line so that the signals may be displayed on a video display connected to the computer.

Impact of phase noise in weakly coherent systems: a new and accurate approach

Abstract: Coherent optical systems for future broadband local loops may use lasers with significant phase noise, manifest as broad linewidths. This phase noise can be accommodated if the receiver is correctly designed, i.e. if nonsynchronous (envelope or square-law) IF demodulation is used and sufficient IF bandwidth is provided. It is difficult to analyze the performance of a coherent optical receiver when the signals are corrupted by phase noise. The central theoretical problem arising from filtering a signal with phase noise is defined in a particular form which permits the derivation of the forward or Fokker-Planck partial differential equation for probability density of the output voltage of the receiver. The results are used to discuss the IF bandwidth required for optical heterodyne receivers for amplitude-shift-keying (ASK) signals. >

System for receiving TV signals retransmitted by satellites

Abstract: A system intended for the simultaneous reception of several TV channels retransmitted by satellites. This system comprises a multi-frequency microwave frequency head, installed in an external unit, whose inputs are connected to the satellite antennas. Wide band demodulation is made possible by a microwave frequency source controlled by a synthesizer. Digital signals transmitted by a link cable ensure programming of the synthesizer and therefore of the selected TV channels. A remodulation in amplitude modulation is performed for supplying signals in the 40-860 MHz band, carried by an existing link cable for terrestrial antennas. The system finds particular application to the simultaneous reception of several TV channels.

Demodulation apparatus incorporating adaptive equalizer for digital communication

Abstract: A demodulation apparatus incorporating an adaptive equalizer, capable of realizing a highly reliable communication, lower power consumption, and a compact size. In the demodulation apparatus, an output of one of two demodulators, at least one of which including an equalizer, is selected according to a presence or absence of multipath which is determined from a non-coincidence of bit codes, eye apertures, bit error rates, or a matched filter output, while the operation of the other demodulator is stopped for reduction of power consumption, by stopping a supply of power or clock signals to the other demodulator. In a case using a frequency offset detection unit and a frequency offset memory, one of these is selected according to a convergence or divergence of an error signal from an equalizer, while the operation of the other one is stopped for reduction of power consumption.

Decimation for bandpass sigma-delta analog-to-digital conversion

Abstract: The authors explain their view of Sigma Delta modulation and use this understanding to develop bandpass Sigma Delta from lowpass Sigma Delta . The problem of narrow bandpass filtering and decimation of a bit stream is examined. It is shown that with the proposed scheme the digital filter is only marginally more complex than in the low-pass case. The scheme does filtering and translation to baseband, thus simultaneously accomplishing channel-selection, demodulation, and analog-to-digital conversion of AM signals. >

Space diversity tdma receiver

Abstract: In a space diversity receiver, multiple signal paths are connected respectively to antennas (10a, 10b) for multipath reception. Each signal path includes a digital demodulator (11a, 11b) coupled to the antenna and an impulse response detector (15a, 15b) or autocorrelator connected to the demodulator for deriving a channel impulse response of each one of the multiple paths. A maximum response detector (16a, 16b) is connected to each impulse response detector for detecting equalizable delay components which give a maximum response value and a nonequalizable delay components. A power ratio of the nonequalizable delay components to the equalizable delay components is derived for each signal path and a minimum value of the power ratios is determined and one of the signal paths in which the minimum value is detected is selected. The output signal of the demodulator of the selected signal path is applied to one input of an equalizer, or a maximum likelihood sequence estimator (23) and the equalizable delay components of the selected signal path are applied to a second input of the equalizer.

A VLSI architecture for a high-speed all-digital quadrature modulator and demodulator for digital radio applications

Abstract: An all-digital architecture is presented for implementing the front-end signal-processing functions in a quadrature modulator and demodulator for high bit-rate digital radio applications. A pair of CMOS chips has been designed and submitted for fabrication in a 1.25- mu m process and is expected to accommodate symbol rates up to 35 MBd. The modulator chip accepts a pair of 8-b in-phase and quadrature data streams and generates a bandlimited IF output with an excess bandwidth factor of 35%. The demodulator chip accepts a digitized IF input signal and generates a pair of filtered in-phase and quadrature baseband signals. The modulator and demodulator chips each incorporate 40-tap multiplierless FIR (finite-impulse response) square-root Nyquist matched filters, and the cascade of the two chips achieves a peak intersymbol interference distortion of -54 dB. The modulator chip can generate any arbitrary signal constellation within a rectangular grid of 256*256 points. Thus, the all-digital implementation results in a generic chip set suitable for a wide variety of high bit-rate digital modem designs using formats such as M-ary PSK and QAM. >

Automatic gain control

Abstract: An automatic gain control (AGC) for a demodulator which receives digitally modulated burst signals time-serially from a plurality of terminals. The AGC has a series connection of a variable attenuator and a limiter that precedes a demodulator. The variable attenuator is controlled such that when a burst signal is absent, the noise power level inputted to the limiter coincides with a predetermined reference value, whereby when burst signal is present, the limiter controls the burst signal to a predetermined level. As a result, when burst signals greatly differ in level from one another, they are controlled to the predetermined level in their preamble word portions. This insures stable demodulation of data signals.

Method of and system for power line carrier communications

Abstract: The transmitter in a power line communications system applies to the power line encoded logical-one and logical-zero bit signals consisting of signals formed respectively of predetermined different sequences of frequencies. The receiver, coupled to the power line, includes a first homodyne, non-coherent, quadratic demodulator for demodulating the logical-one bits and a second for demodulating the logical-zero bits. The first demodulator produces, locally, and in first and second channels, respectively, a first reference signal that is a replica of the logical-one bit sequence of frequencies and a second reference signal that is the replica in quadrature. The second demodulator produces, locally, and in third and fourth channels, respectively, corresponding third and fourth reference signals for the logical-zero bit sequence. Each of the four reference signals is applied to a corresponding analog multiplier for multiplication by the input signal in each channel. The output of each multiplier is applied to a low pass filter that passes only the dc component of the product signal which in turn is squared. The resultant signals in the first and second channels are summed to provide a first output signal, and the resultant signals in the third and fourth channels are summed to provide a second output signal. Compensation circuitry cancels error voltages developed in the demodulation channeis as a result of component offset, and the two output signals are integrated over a bit period and then compared to each other to detect a logical-one or logical-zero bit.

Csk communication system

Abstract: The present invention provides new arrangements for various components of a CSK communication system: modulator, correlators, demodulator, carrier detection circuit, and synchronization control circuit. The various improvements provide more accurate demodulation of data from the received signal in the presence of noise and signal deterioration. A modulator configuration permits the CSK system to use two M series codes in which the second code is simply a phase shifted version of the first code. The PN correlators, via shift-registers, efficiently output two correlation signals. The demodulator accepts two correlation signals from the correlators, detects auto-correlation peaks in the correlation signals, and uses the peaks to accurately determine whether the received bit has a value of "1" or "0". The carrier detecting circuit detects the presence of data in the received signal. A carrier detecting circuit conveniently allows demodulation of data without exact synchronization of the center of demodulator's monitoring window to auto-correlation peaks. The synchronization control circuit generates accurate timing pulses that mark the start and end of each data bit. If auto-correlation peaks appear consistently in one location for the duration of N consecutive data bits, subsequent timing pulses are delayed in accordance with weighted averages of the stored locations of auto-correlation peaks. In one embodiment, separate correlation signal monitoring windows for demodulation and synchronization tracking may be set independently of each other, enabling the demodulator to clearly distinguish between an actual auto-correlation peaks and an inter-correlation peak, and the synchronization control circuit to attain stability by "averaging" out temporal effects of noise.

Spread spectrum communication system

Abstract: A spread spectrum communication system includes a transmitter and a receiver, and an on-off keying signal is inputted to the transmitter. A carrier signal is modulated by the on-off keying signal, and a spread spectrum signal is produced by multiplying a modulated signal by a first PN code (16) which is repetition of 3-bit data. The spread spectrum signal is transmitted through a transmission antenna (19). In the receiver, a second PN code which is repetition of 4-bit data larger than the first PN code by 1 bit is generated. Therefore, a phase of the second PN code becomes coincident with a phase of the first PN code every 12 bits. Then, a spread spectrum demodulation is performed by multiplying (24) a received spread spectrum signal by the second PN code to reproduce the carrier signal being modulated by the on-off keying signal. The on-off keying signal is restored by envelope -detecting a reproduced carrier signal.