Showing papers on "Modulation published in 1994"

A technique for orthogonal frequency division multiplexing frequency offset correction

Abstract: This paper discusses the effects of frequency offset on the performance of orthogonal frequency division multiplexing (OFDM) digital communications. The main problem with frequency offset is that it introduces interference among the multiplicity of carriers in the OFDM signal. It is shown, and confirmed by simulation, that to maintain signal-to-interference ratios of 20 dB or greater for the OFDM carriers, offset is limited to 4% or less of the intercarrier spacing. Next, the paper describes a technique to estimate frequency offset using a repeated data symbol. A maximum likelihood estimation (MLE) algorithm is derived and its performance computed and compared with simulation results. Since the intercarrier interference energy and signal energy both contribute coherently to the estimate, the algorithm generates extremely accurate estimates even when the offset is far too great to demodulate the data values. Also, the estimation error depends only on total symbol energy so it is insensitive to channel spreading and frequency selective fading. A strategy is described for initial acquisition in the event of uncertainty in the initial offset that exceeds 1/2 the carrier spacing, the limit of the MLE algorithm. >

Analysis of a simple successive interference cancellation scheme in a DS/CDMA system

Abstract: Compensating for near/far effects is critical for satisfactory performance of DS/CDMA systems. So far, practical systems have used power control to overcome fading and near/far effects. Another approach, which has a fundamental potential in not only eliminating near/far effects but also in substantially raising the capacity, is multiuser detection and interference cancellation. Various optimal and suboptimal schemes have been investigated. Most of these schemes, however, get too complex even for relatively simple systems and rely on good channel estimates. For interference cancellation, estimation of channel parameters (viz. received amplitude and phase) is important. We analyze a simple successive interference cancellation scheme for coherent BPSK modulation, where the parameter estimation is done using the output of a linear correlator. We then extend the analysis for a noncoherent modulation scheme, namely M-ary orthogonal modulation. For the noncoherent case, the needed information on both the amplitude and phase is obtained from the correlator output. The performance of the IC scheme along with multipath diversity combining is studied. >

Turbo-codes and high spectral efficiency modulation

Abstract: This paper presents a new coding scheme based on the association of a turbo-code with a bandwith efficient modulation. It is shown that the new coding scheme provides a substantial coding gain both on Gaussian channels and Rayleigh channels. On a Gaussian channel, it outperforms 64-state trellis-coded modulation (TCM) by 2.5 dB at the bit error rate (BER) of 10/sup -6/. On a Rayleigh fading channel, it outperforms 64-state TCM optimized for that environment. >

Multiple-subcarrier modulation for non-directed wireless infrared communication

Abstract: We examine multiple-subcarrier modulation (MSM) schemes for wireless infrared digital communication in the indoor environment. Intensity modulation with direct detection (IM/DD) is employed, which results in equivalent baseband channels with a nonnegativity constraint on the input. The power efficiencies of modulation schemes are compared at 30 Mb/s and 100 Mb/s over an ensemble of experimentally determined multipath channels. Carrier selection and power shaping are examined as methods for improving MSM performance. It is found that MSM schemes can allow operation at higher data rates than single-carrier modulation schemes without equalization. Moreover, MSM schemes can be more bandwidth-efficient and also can provide a simple and flexible method for multiple access to the channel. However, they are not as power efficient as single-carrier schemes, and this will limit their use to applications which are not power limited.

Theoretical description of frequency modulation and wavelength modulation spectroscopy.

Abstract: We present a comprehensive theory for heterodyne absorption spectroscopy with phase-modulated light. The general equations presented allow for an arbitrary modulation index and an arbitrary modulation frequency. We use this description for three purposes: First, we review the special cases of so-called frequency modulation and wavelength modulation spectroscopy. Second, we present the additional case of large-index, high-frequency modulation. Third, we present an overview of how the absorption signal depends on the experimental parameters of modulation frequency and modulation index. This overview may be helpful to experimentalists in choosing these parameters, for it provides a systematic understanding of how moving around in parameter space changes certain features of the signal, while leaving other features invariant.

Method and apparatus for signal transmission and reception

Abstract: A method and modem for communicating serial input data over a transmission link. Serial input data is partitioned into parallel data elements prior to rotation by an invertible linear mapping. Resulting frames of parallel signal elements sequentially modulate a carrier, which is then transmitted over the link. After receipt of the modulated carrier from the link, the signal is demodulated and assembled into frames of parallel signal elements which are de-rotated by an inverse linear mapping. Thresholding the result of the inverse mapping recovers the parallel data elements, which are then re-assembled into serial output data. The linear mapping employs: 1) commuting rotation matrices for convolutionally rotating data vectors into signal vectors and vice-versa; 2) filter bank polyphase rotation matrices; or 3) computationally efficient multi-rate wavelet filter banks. Transmitter pre-emphasis places most of the information in lower baseband frequencies; complimentary de-emphasis occurs in the receiver. Logarithmic amplification of the baseband signal prior to carrier modulation improves modulation gain and transmit channel noise attenuation. Coefficients of the rotation matrix of the receiver are adaptively equalized to correct for transmission path distortion. FM double-side band is employed in systems requiring minimized cost and complexity. FM single-side band is employed in systems in which bandwidth reduction is desirable. AM is also employable.

OFDM synchronization demodulation circuit

Abstract: An OFDM synchronization demodulation circuit includes a receiving circuit for receiving an orthogonal frequency division multiplexing (OFDM) modulation signal. An orthogonal detection circuit orthogonally detects the OFDM modulation signal from the receiving circuit using a detected carrier. A demodulation circuit orthogonally demodulates the detected OFDM multiplex modulation signal from the orthogonal detection circuit by sampling the OFDM multiplex detection signal using a detected clock. A constellation analyzing circuit analyzes the constellation of demodulated symbols of predetermined carriers in the demodulated OFDM multiplex signal signal. A carrier frequency deviation detection circuit detects a frequency deviation of the detected carrier based on a rotation of the constellation of the demodulated symbol detected by the constellation analyzing circuit. A carrier detection circuit responsive to the frequency deviation detected by the carrier frequency deviation detection circuit controls the detected carrier. A clock frequency deviation detection circuit detects a frequency deviation of the detected clock based on rotations of the constellations of at least two demodulated symbols detected by the constellation analyzing circuit. A clock detection circuit controls the detected clock based on the frequency deviation detected by the clock frequency deviation detection circuit.

Digital to analog converter

Abstract: A segmented DAC is described in which the outputs of a pair of subword DAC circuits are summed by modulating the offset voltage of a differential buffer amplifier. Also described are various alternative DAC embodiments and an operational amplifier input stage in which modulation of the offset voltage of a differential amplifier responsive to a digital signal is accomplished using interpolation techniques for eliminating errors in linearity and monotonicity arising from component inaccuracies.

Multicarrier frequency hopping communications system

Abstract: A multicarrier frequency hopping spread spectrum communications system wherein a transmitting station assigns encoded data symbols to particular ones of a predetermined series of fixed carrier frequencies, the frequency assignments being periodically mixed according to a frequency hopping code, and the encoded data symbols are combined with their assigned carrier frequencies in accordance with an inverse Fourier transform so as to multiplex the encoded data symbols by orthogonal frequency division modulation. A receiving station performs demultiplexing and de-mixing of such multiplexed signals in accordance with the inverse of the multiplexing and mixing performed at the transmitter. The system thereby provides frequency hopping operation without requiring a rapidly tunable and accurately phase controlled oscillator at each of the transmit/receive stations.

A CMOS area image sensor with pixel-level A/D conversion

Abstract: Charge-coupled devices (CCD) are at present the most widely used technology for implementing area image sensors. However, they suffer from low yields, consume too much power, and are plagued with SNR limitations due to the shifting and detection of analog charge packets, and the fact that data is communicated off chip in analog form. This paper describes an area image sensor that can potentially circumvent the limitations of CCDs and their alternatives. It uses a standard CMOS process and can therefore be manufactured with high yield. Digital circuitry for control and signal processing can be integrated with the sensor. Moreover, CMOS technology advances such as scaling and extra layers of metal can be used to improve pixel density and sensor performance. The analog image data is immediately converted to digital at each pixel using a one-bit sigma-delta modulator. The use of sigma-delta modulation allows the data-conversion circuitry to be simple and insensitive to process variations. A global shutter provides variable light input attenuation to achieve wide dynamic range. Data is communicated off chip in a digital form, eliminating the SNR degradation of analog data communication. To demonstrate the viability of the approach, an area image sensor chip is fabricated in a 1.2 /spl mu/m CMOS technology. The device consists of an array of 64x64 pixel blocks, a clock driver, a 6:64 row address decoder, 64 latched sense amplifiers, and 16 4:1 column multiplexers. The chip also contains data compression circuitry. >

System with chip to chip communication

Abstract: A system for wireless communication between a plurality of semiconductor chips is disclosed Each data line in the present invention is coupled with a transmitter for transmitting information to any other semiconductor chip Furthermore, each data line is coupled with a receiver for receiving information transmitted by any transmitter The system also comprises multiple antennas, fabricated from the chip's metalization layer Nonetheless, separate antennas within the chip packaging can also be used The antenna unit comprises a dipole and a loop antenna in a planar arrangement, thereby forming a spherical electromagnetic pattern of coverage and making the orientation between semiconductor chips for transmission purposes substantially irrelevant Each transmitter in the system comprises a modulator for modulating the information being transmitted, while each receiver comprises a demodulator for demodulating the information transmitted Several modulation schemes can be employed, though amplitude modulation is preferred, whereby each transmitter has a distinct carrier frequency within the operative radio frequency spectra--preferably above 900 MHz Each transmitter and receiver is coupled to a power source comprising a capacitor, as a signal generator, and a rectifying circuit To ensure that the communication between chips is noise/interference free, the entire system is shielded with a metal housing

Receiver having for charge-coupled-device based receiver signal processing

Abstract: This invention relates to a novel receiver architecture, the Charge-Coupled-Device Integrated Receiver (IR), which simplifies electronic circuitry requirements and reduce baseband processing rates through efficient signal processing. The focus of this receiver's design is its use of Charge-Coupled-Device (CCD) technology. By using CCD's, in conjunction with other key technologies, to implement the signal processing techniques disclosed herein, it is possible to provide a receiver that is suitable for a wide range of applications (e.g., communications, sonar, radar, etc.). Information is presented which outlines the fundamental receiver architecture that is appropriate for a wide range of commercial services (e.g., AMPS, NAMPS, Digital Cellular, GSM, PCS, ISM, CT1, CT2, etc.) Furthermore, tile versatility of this architecture makes it suited for uses ranging from simple AM/FM receivers to complex high-order modulation TDMA/CDMA receivers.

Portable digital signal transceiver providing communication via a terrestrial network and via a satellite network

Abstract: A dual mode portable digital radio transceiver for communicating via a terrestrial network (first mode) and via a satellite network (second mode) synthesizes a modulation first frequency for modulation of signals transmitted in both modes and a conversion second frequency for demodulation of signals received in the two modes. It divides the conversion second frequency supplying a conversion third frequency for demodulation of signals received in one of the modes using a signal receive frequency band far away from the other frequency bands used. It is therefore possible, using a single synthesized pure carrier, to carry out direct conversion in a first mode and intermediate frequency conversion followed by the same direct conversion in the second mode.

Performance analysis of DS/CDMA with noncoherent M-ary orthogonal modulation in multipath fading channels

Abstract: The performance of a DS/CDMA system using M-ary orthogonal modulation with noncoherent demodulation is evaluated. The system operates in a multipath fading channel. A RAKE receiver structure with equal gain combining is used for demodulation. An approximation to the bit error probability is given which depends only on the first- and second-order moments of the multipath energies. The analysis results are compared with the results from computer simulations. It is seen that the approximation is accurate for multipath energies with realistic coefficient of variation. The system performance is also evaluated in terms of the capacity, which is defined as the number of users that can be supported at a given bit error probability. The approximation is used to evaluate the capacity reductions due to power variations caused by multipath fading. >

Dependence of cross-phase modulation on channel number in fiber WDM systems

Abstract: The phase term appearing in the expression for cross-phase modulation due to the optical Kerr effect depends on the sum of the powers carried by each wavelength channel. For this reason, one might expect that the amount of cross-phase modulation would increase with increasing channel number, causing increased interference among channels and hence limiting the total number of channels that a WDM system can support. However, computer simulations of multichannel systems have shown no change in signal distortion as the number of wavelength channels is increased from four to eight. In a simulated three-channel system, the signal distortion of the central channel approaches that of a single-channel system as the wavelength separation is increased to approximately 2 nm. Thus, even a moderate amount of dispersion tends to cancel out the influence of cross-phase modulation, so that beyond a certain wavelength spacing, additional channels do not interfere with the channel under consideration. From these observations, we conclude that cross-phase modulation does not limit the number of wavelength channels that a single optical fiber can support. However, self- and cross-phase modulation are not the only nonlinear effects influencing fiber lightwave systems. Stimulated Raman scattering tends to transfer optical power from short-wavelength channels to channels operating at longer wavelength, degrading their signal-to-noise ratio. The efficiency of this process increases with increasing wavelength spacing. Clearly, a compromise needs to be reached between the conflicting requirements imposed by the optical Kerr effect and by stimulated Raman scattering. >

Optical-feedback-induced chaos and its control in multimode semiconductor lasers

Abstract: The effects of optical feedback in multilongitudinal mode semiconductor lasers are studied through computer simulations. Two separate regimes are found based on the length of the external cavity. For long external cavities (external-cavity mode spacing larger than the relaxation-oscillation frequency), the laser follows a quasi-periodic route to chaos as feedback is increased. For short external cavities, the laser can undergo both quasi-periodic and period doubling routes to chaos. When the laser output becomes chaotic, the relative-intensity noise is greatly increased (by more than 20 dB) from its solitary-laser value. Considerable attention is paid to the effects of optical feedback on the longitudinal-mode spectrum. The stabilization of the mode spectrum and the reduction of the feedback-induced noise through current modulation are studied and compared with experimental results. Current modulation eliminates feedback-induced chaos when the modulation frequency and depth are suitably optimized. This technique of chaos control has applications in optical data recording. >

On sampling rate, analog prefiltering, and sufficient statistics for digital receivers

Abstract: We consider the joint sequence estimation, timing and phase recovery for linear modulation. The paper differs from the classical ones in the sense that time-discrete algorithms suitable for fully digital receivers are discussed. Sufficient conditions are given such that the signal samples represent sufficient statistics. These conditions involve signal bandwidth, sampling/symbol rate and the analog prefilter characteristics. It is shown that the sampling rate need not be an exact multiple of the symbol rate, i.e., the samples can be taken from a free-running oscillator. All subsequent signal processing operations in the receiver then operate with the clock of this free-running oscillator. Timing recovery is then performed by a time-variant linear digital interpolator and a decimator. Carrier recovery and sequence estimation are performed at an average rate of one symbol per sample. The digital matched filter for this case is derived for an arbitrary colored noise spectrum. >

A third-order sigma-delta modulator with extended dynamic range

Abstract: Oversampling modulators based on high-order sigma-delta modulation provide an effective means of achieving high-resolution analog-to-digital conversion in VLSI technology. Because high-order noise shaping greatly reduces the quantization noise in the signal band, the dynamic range of these modulators tends to be bounded by the thermal noise of the input stage and the maximum voltage swing in the signal path. This paper introduces a third-order cascaded sigma-delta modulator that uses a modified cascaded architecture and reduced gain in the first integrator to increase the dynamic range. An experimental modulator fabricated in a 1-/spl mu/m CMOS technology attains a resolution of 17 b for a 25-kHz signal bandwidth while operating from a single 5-V supply. With an oversampling ratio of 128 and a clock frequency of 6.4 MHz, the modulator achieves a 104-dB dynamic range and a peak signal-to-noise+distortion ratio (SNDR) of 98 dB. As indicated by both measurements and simulations, the cascaded architecture also greatly reduces the discrete noise peaks that can be present in a single-stage architecture. >

Controlled equalization of multi-carrier CDMA in an indoor Rician fading channel

Abstract: In this paper, a novel digital modulation technique called Multi-Carrier Code Division Multiple Access (MC-CDMA) is analyzed. With MC-CDMA, each data symbol is transmitted at multiple subcarriers with each subcarrier modulated by "1" or "-1" based on a spreading code. Analytical results are presented on the performance of this modulation/multiple access scheme in the downlink of an indoor wireless Rician fading channel. The performance of a controlled equalization technique that attempts to restore the orthogonality between users is evaluated. >

Low power GPS receiver

Abstract: A low power GPS receiver, including an RF section to receive a GPS signal having information for determining a location fix and to issue an IF signal, a correlator system to correlate an internal replica signal to the IF signal and to issue a correlation signal including location and signal strength information, a microprocessor system to receive the correlation signal, to read the signal strength, and to provide a location fix, and an on/off modulator to receive a duty cycle factor from the microprocessor system and to on/off modulate the operating power to the RF section to provide a minimum signal strength for a valid location fix. The off time duration of the modulation is shorter than the time duration of the interval used to correlate the IF signal enabling the GPS receiver to continue to correlate the IF signal and to provide the location fixes while the RF section operating power is reduced by the duty cycle factor.

A high-quality analog oscillator using oversampling D/A conversion techniques

Abstract: This paper describes a high-quality analog oscillator for low-frequency applications, which uses a combination of over-sampling and delta-sigma modulation. With the exception of a lowpass filter and a 1-bit D/A, the proposed circuit is entirely digital, providing accurate control over the oscillation frequency and amplitude. At the core of the oscillator is a digital simulation of an LC-tank circuit consisting of two cascaded integrators. This arrangement guarantees oscillation by constraining the poles of the resonator to locations on the z-plane unit-circle, even in a finite-precision implementation. To minimize circuit complexity, the entire oscillator is operated at the oversampled rate, thereby eliminating the associated interpolation filter. Furthermore, the incorporation of a delta-sigma modulator inside the resonator loop leads to a very efficient implementation requiring only 4 multi-bit adders and a 2-input multiplexor. The desired analog signal may be recovered by lowpass filtering the 1-bit output of the delta-sigma modulator. Experiments performed thus far have indicated an effective dynamic range exceeding 80 dB. >

Method and apparatus for providing adaptive modulation in a radio communication system

Abstract: A method for providing adaptive modulation in a radio communication system employs a plurality of data modulation techniques (207). A two-way communication unit (100) automatically selects (318) from amongst the plurality of data modulation techniques (207), and uses the selected data modulation technique to transmit (308) data.

Method for providing and selecting amongst multiple data rates in a time division multiplexed system

Abstract: In a Radio Frequency (RF) communication system using Time Division Multiple Access (TDMA) having time slots of a common duration, a quantity of information bits to be transmitted is provided. Based at least in part on the number of information bits to be transmitted, a modulation technique is selected from a plurality of modulation techniques. Based at least in part on the modulation technique selected and the common duration of the time slots, the information bits are formatted into blocks, each block containing an equal number of information bits. The blocks are transmitted in the time slots such that a predetermined symbol rate is maintained.

Optical transmission of narrow-band millimeter-wave signals by resonant modulation of monolithic semiconductor lasers

Abstract: We demonstrate the use of monolithic semiconductor lasers as efficient narrow-band (20-300 MHz) millimeter-wave optical transmitters at a cavity round-trip resonant frequency of 45 GHz. The modulation efficiency within the passband is >10 dB above that of a conventional laser diode at frequencies below relaxation oscillation. At an input RF drive power of 10 dBm, a carrier-to-noise ratio of 90 dB (1 Hz) is achieved. We also present a system demonstration of this technique in which 50 Mb/s digital data is transmitted at a 45 GHz subcarrier over optical fiber. This is the highest subcarrier frequency transmission reported to date. >

A simplified continuous phase modulator technique

Abstract: A low cost GMSK (Gaussian minimum shift keying) modulation technique is presented. GMSK is chosen as an example although the technique is suitable for any continuous phase constant envelope modulation with discriminator, differential or coherent detection in the targeted receiver. In this technique, a fractional-N synthesizer is used to control the instantaneous frequency and phase of the phase locked synthesizer output. As a result of this approach, no in-phase and quadrature mixer or D/A converters are required. Furthermore, the desired signal can be generated directly at RF with no IF conversion stages. The look-up table to generate the modulation is only one bit wide. /spl Delta/-/spl Sigma/ techniques are used to obtain high accuracy through the long term average of a sequence of the single bits. The narrow band filtering of the transmit data is accomplished in two parts, a precise digital linear phase band-reject part using the one bit stored ROM look-up table and a less precise low pass analog part inherent to the PLL of the synthesizer. >

Space vector modulated, zero-voltage transition three-phase to DC bidirectional converter

Abstract: Operation and implementation of the novel, space vector modulated, zero-voltage transition, three phase voltage source inverter/boost rectifier is presented. The converter is intended for high performance, medium power applications requiring bidirectional power flow. The proposed modified space-vector modulation allows all switches to be operated with soft-switching and constant frequency. The modulation algorithm also eliminates any low frequency distortion caused by the zero-voltage transitions and can be applied to any soft switching PWM three-phase converters. A simple, digital signal processor based implementation of the modulator and current regulators, operating with 30 /spl mu/s sampling time, is described. Measured efficiency of the 30 kHz, IGBT prototype is around 95%, and distortion of the three-phase currents is extremely small. >

Effects of carrier frequency and background noise on the detection of mixed modulation

Abstract: This article is concerned with the mechanisms underlying the detection of amplitude modulation (AM), frequency modulation (FM), and mixed modulation (MM), i.e., simultaneously occurring AM and FM. In a previous study [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119–3131 (1992)], psychometric functions were measured for the detection of AM alone and FM alone, using a 10‐Hz modulation rate and a 1‐kHz carrier frequency. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. The detectability of the combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently. Significant effects of relative modulator phase were found when detectability was relatively high, but these effects were not correctly predicted by either of two excitation‐pattern models considered. The first experiment reported here was similar to the earlier experiment, ...

Comparison of hybrid FDMA/CDMA systems in frequency selective Rayleigh fading

Abstract: A hybrid FDMA/CDMA scheme operating over a frequency selective Rayleigh fading channel is described and analyzed. The performance of the hybrid system is then compared with that of a wideband CDMA system occupying the same total bandwidth. Both coherent and noncoherent modulation formats are investigated; it is found that, for coherent modulation with a RAKE receiver, wideband CDMA has greater capacity than does the hybrid. However, for the noncoherent modulation formats (either DPSK or square-law detected orthogonal signalling), a hybrid system can have a greater capacity if a high channel error rate can be tolerated. Otherwise, a wideband noncoherent system remains optimal. >