Efficient Pulse Shaping Using MSK or PSK Modulation

The increasing requirement for high data rate, bandwidth efficient digital radio systems has led to the development of MSK-type modulation methods designed to achieve a compact signal spectrum. These modulation methods include sinusoidal frequency shift keying (SFSK), special MSK-type pulse shapes, and multiamplitude minimum shift keying (MAMSK). When more compact signal spectra are required, bandlimited filtering must be introduced. This note considers the use of conventional MSK or PSK modulators followed by newly developed bandlimited pulse shaping filters. With this approach, it is shown that MSK and offset QPSK modulators yield identical signals on the channel when filtered by properly designed bandlimited pulse shaping networks.     

Amplitude modulation with a noise carrier

Consider an ideal amplitude modulation system in which the usual sinusoidal carrier is replaced by a narrow band of random noise, of midband frequency f0and half-bandwidth B; the spectral density and the total power of the output noise of this system are determined. An optical system of this type would use an incoherent spectral line as a carrier, rather than the coherent output of an optical maser. Considerable improvement is obtained by shifting a baseband modulating signal to a frequency band much lower in frequency than f0but much higher than B, before modulating the noise carrier. In an optical system this translated modulating signal might lie in the microwave region. Even so W, the bandwidth of the modulation, must be very small compared to B to attain a high output signal-to-noise ratio.     

Naturally sampled triangle carrier PWM bandwidth limit and output spectrum

This paper first presents a general theory predicting harmonic components in naturally sampled pulse-width modulation (PWM) output signals, so that any input signal with a Fourier series representation may be handled. This theory provides a basis for the mathematical analysis of PWM systems, such as converters transmitting a main signal and a broadband of feedback or feedforward signals, or Class D amplifiers. Ultimately, the results of this theory are applied to two concrete problems, and conditions similar to the Nyquist theorem or Carson's rule for FM modulation are derived for the cases of recovery of an input signal consisting of a main signal and a bandwidth of small feedback or feedforward signals and a bandlimited signal consisting of several harmonics of comparable magnitude. A rule of thumb, and conservative estimate of the bandwidth in both cases is one third of the carrier frequency (/spl omega//sub upper/<(1/3)/spl omega//sub c/). This research has been based on the premise that the results obtained will provide valuable insight into the general behavior of PWM systems, and provide a supporting theory for concrete systems utilizing signals of this kind, such as PWM converters whose purpose is to ensure a dominant sinusoidal waveform along with a broadband frequency channel for small feedback signals.     

Fundamental Limits for Radio Signals with Large Bandwidth

Radio services have traditionally used narrow frequency bands individually assigned. More recently, the concept of sharing very wide frequency bands by several users has been advanced, and this opens the door for the use of much larger bandwidths than in the past. This paper investigates the limits imposed by nature on the bandwidth of line-of-sight radio services operating in the earth's atmosphere. Furthermore, it investigates the limits for time resolution of radio signals, as well as the related limit of the angular resolution of a line array of sensors that receive signals with large bandwidth, and compares it with the classical resolution angle that holds for sinusoidal signals with vanishing bandwidth. Finally, an example is given where the concept of a practically finite bandwidth of a signal reaches its limit, and a more rigorous specification of the signal is required.     

Broadcast-Signal Bandwidth Measurements Using Real-Time Data Averaging

The bandwidth of a signal may be analyzed in terms of what is defined as an observable, or detectable, signal level, the frequency range over which this detectable signal occurs, and the percentage of time that the sigal exceeds defined limits. This paper presents a technique for observing and quantitating in real time the frequency excursion of radio signals in the time domain. A scanning spectrum analyzer interfaced to a minicomputer data-acquisition system is used to collect spectral bandwidth data. The concept of percentage band-width occupancy is presented for evaluating the broadcast-signal modulation character and representative data are given for signals in the FM broadcast band.     

The effects of polarization rotation and phase delay with frequency on ionospherically propagated signals

When a CW skywave signal is received on a linearly polarized antenna, polarization (Faraday) rotation produces a variation of received signal strength with radio frequency. The resulting dependence of received signal amplitude on radio frequency may impose a bandwidth limitation on pulsed signals where waveform preservation is important. A measure of this limitation, termed polarization bandwidth, is defined to correspond to the bandwidth in which the plane of polarization rotates90deg. Computer ray-tracing calculations were performed using a single Chapman-layer ionospheric model to determine the 1-hop polarization bandwidth as a function of geomagnetic azimuth and radio frequency. The polarization bandwidth was found to decrease with increasing radio frequency and with increasingly close alignment of the propagation path with the longitudinal component of the earth's magnetic field. Assuming a critical frequency of 9 MHz and a path length of 2000 km, the polarization bandwidth increased from a minimum of 140 kHz at 10.5 MHz and from a minimum of 70 kHz at 17.5 MHz, as the propagation direction varied from geomagnetic north to east. A model for the 1-hop ionospheric signal channel is proposed whose parameters are the rate of change of polarization rotation with frequency and the phase versus frequency characteristic of the path. These two parameters are shown to be readily determined from FM-CW or equivalent oblique-path sounding records. Using this model, predictions are made of the effects of polarization rotation with frequency, and also of ionospheric dispersion or phase distortion, on the envelope shape of short-pulse signals (of from 1.5 to50 mus duration). A pronounced waveshape distortion due to the effects of polarization rotation on the pulse envelope was observed when the signal bandwidth appreciably exceeded the "polarization bandwidth" for the path.     

Reliable adaptive modulation and interference mitigation for mobile radio slow frequency hopping channels

The long range fading prediction algorithm for Slow Frequency Hopping (SFH) systems is proposed and demonstrated to enable combined adaptive modulation and adaptive frequency diversity to mitigate the effects of fading and partial-band interference. Significant performance gains are demonstrated relative to non-adaptive methods in realistic mobile radio SFH channels where the total bandwidth does not exceed approximately 15 times the coherence bandwidth.     

过调幅AM信号的参数测量与计算

讨论了过调幅AM(调幅系数ma>1)信号调幅系数的测量,过调幅信号功率参数的计算,频谱和频带宽度以及抗噪声性能等问题。     

Single-channel blind equalization for GSM cellular systems

This paper focuses on the study of blind equalization global system for mobile communications (GSM) systems using a single antenna. In order to utilize the well-known linear system model in conventional studies of blind equalization, an equivalent baseband quadrature amplitude modulation (QAM) approximation is used for the nonlinear GMSK signal in GSM systems. Since the GMSK signal in GSM has very little excess bandwidth to warrant oversampling, a derotation scheme is developed to create two subchannels for each received GMSK signal sampled at the baud rate. Linear approximation of the GMSK signal makes the traditional QAM blind equalization system model applicable for GSM. Derotation induces channel diversity without an additional antenna and reduces the number of necessary radio frequency (RF) receivers (sensors) without increasing hardware or computational costs. Several second-order statistical and higher order statistical methods of blind equalization are adopted for GSM signals     

A closed form to predict outage of digital radio

A simplified formula for predicting the outage probability of an unprotected high capacity line-of-sight digital radio link operating under frequency-selective fading conditions is presented. First, the propagation anomaly is modeled as a linear in-band amplitude dispersion in conjunction with a transmission minimum (notch) frequency located within or outside the signal band of interest. A simplified expression for the outage probability of the radio system under these conditions is then obtained. This expression is based on the fade occurrence factor in the climatic region of interest, the bandwidth of the transmitted signal, the carrier frequency of operation, the modulation scheme employed and the length of the link under consideration. The formula is applied to predict the outage probability of 4 phase shift keying (PSK), 8-PSK, and 16 quadrature amplitude modulation (QAM) high capacity radio links with specified parameters. The results obtained compare favorably with measured data     

A Simplified Method for Prediction of Multipath Fading Outage of Digital Radio

We propose a simplified method based on the correlated Rayleigh probability distribution of in-band radio signals. It can be efficiently applied to the calculation of the outage for various radio path length, propagation terrain, radio frequency, bandwidth, and digital modulation techniques. Predicted results are compared to various propagation data.     

基于椭圆球面波信号的连续相位调制及其频谱性能分析

椭圆球面波信号（PSWF）是时频域最佳能量聚集性信号,探索性地将PSWF信号应用于连续相位调制（CPM）技术,对基于椭圆球面波信号的CPM调制信号产生过程进行了描述;给出了采用自相关函数法对基于PSWF的CPM调制信号功率谱密度计算方法;结合数值计算、分析对比了最小频移键控、正弦频移键控、高斯最小频移键控以及基于高斯函数的CPM调制信号功率谱密度及其信号占用带宽等特性。数值计算结果表明椭圆球面波信号相比矩形脉冲、升余弦脉冲、高斯脉冲及（类）高斯脉冲作为CPM的基带调频信号,可以获得频谱性能、能量聚集性较好的CPM信号。     

Passive demodulation of optical interferometric sensors

Two techniques for passive (no-feedback) demodulation of signals from a remote two-beam interferometric sensor are discussed. Termed "synthetic heterodyne" and "quadrature recombination," both methods are based on forced sinusoidal modulation of phase within the interferometer. If the phase modulation is generated by linearly proportional frequency modulation of the laser light source, then the interferometer, deployed by means of optical fibers, can be a completely passive remote sensor, and an array of identical sensors can be operated from one light source. The former technique, proposed by Cole, Danver, and Bucaro permits standard FM demodulation. The latter technique, proposed by Cable and Green, employs arithmetic operations that are appropriate to digital implementation. Constraints on application of the technique, especially frequency distribution and amplitude of phase noise (out-of-band) versus signal amplitude (in-band), are analyzed and an implementation is proposed that is consistent with Contemporary analog-to-digital conversion technology.     

Frequency correlation of line-of-sight signal scintillations

The frequency autocorrelation of amplitude and phase scintillations of radio signals propagated over turbulent line-of-sight paths is calculated theoretically. The single scattering (Born) approximation to the electromagnetic response of the turbulent irregularities is used, limiting the results to small amplitude and phase variations. However, the results are valid for both Fresnel (near zone) and Fraunhofer (far zone) scattering. The calculations are made for an arbitrary model of the turbulent irregularities by using the spectrum method, which postpones the specialization of the analysis to a particular turbulence model until the wave-propagation aspects of the problem are completed. It is shown that the signal variations on adjacent carrier frequencies have high correlation for frequency separations comparable to the carrier frequency for all propagation conditions, indicating that the "medium bandwidth" for line-of-sight paths is very large. This also means that there should be negligible pulse distortion for high-speed data links or high resolution radars operating on line-of-sight paths. These predictions are compared with the few available experimental results.     

Acoustic noise reduction in sinusoidal PWM drives using a randomlymodulated carrier

Acoustic noise in an inverter-driven electric machine can be reduced by avoiding the concentration of harmonic energy in distinct tones. One method to spread out the harmonic spectrum without the use of programmed PWM (pulse width modulation) is to make the switching pattern random. It is proposed that the switching pattern can be randomized by modulating the triangle carrier in sinusoidal PWM with bandlimited white noise. All of the advantages of sinusoidal PWM are preserved with this technique. These include, real-time control, linear operation, good transient response, and a constant average switching frequency. By controlling the bandwidth and RMS value of the bandwidth limited noise modulation, it is shown that the instantaneous variation in switching frequency and the bandwidth of the energy spectrum in the machine can be specified within predetermined limits. Experimental results show the absence of acoustic noise concentrated at specific tones, which is present in conventional sinusoidal modulation     

Multicarrier constant envelope OFDM signal design for radar applications

Orthogonal frequency division multiplexing (OFDM) radar signals have been introduced for high range resolution radars. These signals have prominent properties such as favorable ambiguity function, high bandwidth efficiency, and possibility of use in dual mode radar/communication systems. But the large amplitude fluctuations of the OFDM signal make it susceptible to system nonlinearities. To alleviate this problem, constant envelope OFDM (CE-OFDM) signal has been introduced which combines orthogonal frequency division multiplexing and phase modulation or frequency modulation. Although several works have been reported on OFDM radar signal design, there is no a systematic approach for designing CE-OFDM signals for radar applications. In this paper we will focus on CE-OFDM signal design for radar applications. Two different methods for designing a CE-OFDM signal with favorable ambiguity functions are introduced. The first one is based on modulating a complementary set of sequences on different sub-carriers while the second is based on using a proper single carrier coded signal and then extracting its most similar multicarrier OFDM or CE-OFDM coded signal.     

A real zero SSB transceiver for land mobile radio: A simple method of demodulating SSB signals without an envelope

A real zero single sideband (RZ SSB) transceiver is newly proposed for mobile radio services. The necessary transmission bandwidth of RZ SSB is similar to that of a conventional SSB. In this new transceiver, an amplitude limiter, which can remove the amplitude impairment, can be used without degrading the quality of detected output signals in the receiver. Furthermore, since RZ SSB signals possess FM-like characteristics, they can be demodulated with frequency detection as well as product detection.     

Impact of nonlinear distortion in radio over fiber systems with single-sideband and tandem single-sideband subcarrier modulations

In radio over fiber (RoF) systems, two subcarrier modulations (SCMs), i.e., single-sideband (SSB) and tandem single-sideband (TSSB), have been widely used. Both SSB and TSSB SCMs can be obtained using optical Mach-Zehnder modulators. In this paper, the authors theoretically investigate the impact of harmonic distortion and intermodulation distortion in RoF systems for one wavelength carrying two radio frequency (RF) signals with either SSB or TSSB SCM. It is found that nonlinear distortion can be reduced when the frequency difference of two RF signals is /spl sim/ 1 GHz compared with /spl sim/ 2 GHz and higher for both TSSB and SSB SCMs, particularly for large modulation indexes. When the frequency difference of RF signals is /spl sim/ 2 GHz and beyond, either a small modulation index or a medium modulation index combined with a minimum RF of 6 GHz must be used, and a large modulation index cannot be used due to serious nonlinear distortion. Comparison of SSB and TSSB SCMs with uniform distribution of optical signal subcarriers in a dense wavelength-division-multiplexing RoF system with channel spacing of 12.5 GHz shows by simulation that SSB SCM is better (worse) than TSSB SCM for small (large) modulation indexes.