Showing papers on "Quadrature amplitude modulation published in 1985"

Equalizing without altering or detecting data

Abstract: For terrestrial digital radio systems that use Quadrature Amplitude Modulation, the idea of adapting equalizers to multipath distortion, without relying on accurate data estimates, is attractive. Prompt adaptation following a severe fade, when accurate data estimates are unavailable, is useful for reducing outage time. To avoid processing and administrative overhead, the adaptation method should not involve violating the transmitted signal with the insertion of equalizer training signals. We approach this kind of equalization by building on an algorithm of D. Godard (IEEE Transactions on Communications, November 1980)1 that was devised for voiceband polling networks. The method involves a very simple tap update procedure. However, the technique lacks the foundation of the years of analysis and experimentation that underlie least-mean-square adaptation algorithms. The main purpose of this paper is to present new findings, including (1) a proof that the algorithm, thought to require special equalizer initialization, converges regardless of initialization (this offers useful flexibility in digital radio systems, since, after a severe fade, the algorithm could start with any tap misalignment); (2) a preliminary look at convergence speed suggesting the possibility of significant outage reduction; (3) an algorithm that provides phase coherence (the original algorithm requires a follow-on phase-locked loop); and (4) an algorithm for cross-polarization cancellation as well as equalization.

Identification of the modulation type of a signal

Abstract: It is possible to recognize the modulation type of an unknown signal in noise by its envelope characteristics. A quantity found to be distinctive to a given modulation type is the ratio (R) of the variance of the envelope to the square of the mean of the envelope. Equations of R for four modulation types, as a function of the carrier to noise ratio, are derived and used to set up a classification scheme. The R of a received signal is first computed. Then its modulation type is determined according to the range R falls in. Simulation experiments have confirmed some of the theoretical development as well as the effectiveness of the identification scheme.

SQAM: A New Superposed QAM Modem Technique

Abstract: A spectral and power efficient modulation techniquesuperposed quadrature amplitude modulation (SQAM-is introduced. In SQAM, the premodulation baseband signal is a double-interval ( 2T_{s} ) raised-cosine pulse superposed with weighted single-interval (T s ) raisedcosine pulses. Our results indicate that SQAM has spectral advantages over OQPSK, QBL, and MSK, and better P(e) performance than MSK, IJF-OQPSK (or SQORC), and TFM.

Analysis of Four Decision-Feedback Carrier Recovery Loops in the Presence of Intersymbol Interference

Abstract: The performance of four decision-feedback carrier recovery techniques is evaluated in the presence of additive noise and intersymbol interference (ISI). For QAM signal constellations, a closed-form expression is given for the phase jitter variance (PJV) of each loop, and the loop tracking performance is examined. The analytic results are then computed in the case of a 16 QAM digital radio system subjected to multipath fading. Two cases are considered: in the first case no countermeasure techniques are used against selective fading, while in the second case a three-coefficient decision-feedback equalizer (DFE) is used. Computer simulations using a pseudorandom sequence to estimate loop performance are also reported which support the theoretical results.

SQAM - A new superposed QAM modem technique

Abstract: A spectral and power efficient modulation techniquesuperposed quadrature amplitude modulation (SQAM-is introduced. In SQAM, the premodulation baseband signal is a double-interval ( 2T_{s} ) raised-cosine pulse superposed with weighted single-interval (T s ) raisedcosine pulses. Our results indicate that SQAM has spectral advantages over OQPSK, QBL, and MSK, and better P(e) performance than MSK, IJF-OQPSK (or SQORC), and TFM.

Temperature Controlled Predistortion Circuits for 64 QAM Microwave Power Amplifiers

Abstract: This report deals with the problem of nonlinearity characteristics in microwave power amplifiers for digital radio link systems with multi-level modulation (ex. 64 QAM); and in particular With reference to their degradations vs temperature variations. Moreover various possible methods of compensation are taken into account. After a brief examination of the not temperature controlled predistortion method operating at intermediate frequency, solutions are suggested to keep the non-linearity characteristics independent from thermal conditions. By presenting the experimental results and the comparative considerations, the use of the "feed-back controlled predistorter" and the "predictive method" is analysed.

Comparison of High-Level Modulation Schemes for High-Capacity Digital Radio Systems

Abstract: This paper presents a comparative study of four 2n-state quadrature amplitude modulation techniques, namely, 16, 32, 64, and 128 QAM, in a digital radio system environment. The effects of filtering, interference, amplifier nonlinearities, and selective fading are investigated using computational and simulation methods. Increase of the carrier-to-noise ratio (CNR) at a fixed symbol error probability (10^{-3} ) is taken as a robustness criterion.

An Analysis of the Sensitivity of Digital Modulation Techniques to Frequency-Selective Fading

Abstract: This Paper investigates the sensitivity to frequency-selective fading of different modulation techniques commonly used in digital radio systems. The modulation methods considered are phase-shift-keying ( M ary PSK), nonoffset and offset quadrature amplitude modulation ( M -ary QAM), and quadrature partial response signaling (QPRS). The performances of these methods during multipath fading are compared on the basis of the "signatures" calculated for idealized systems. Results are presented from which the relative outage probability was determined, assuming a propagation channel with frequency-selective Rician fading.

A single-chip MSK coherent demodulator for mobile radio transmission

Abstract: Design and performance of a single-chip minimum shift keying (MSK) coherent demodulator fabricated by complementary metal oxide semiconductor-integrated circuit (CMOS-IC) technology is described. The demodulator consists of a phase detection circuit, carrier recovery circuit, data recovery circuit, and timing-clock recovery circuit. For the carrier recovery circuit, three types of Costas-loop are reviewed from the viewpoint that MSK modulation format has a close relationship to binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). Among these loops, a loop of center-frequency locking scheme modified from a BPSK Costas-loop, termed MSK Costas-loop in this paper, is adopted for IC implementation. Digital IC design techniques are next described. Utilizing the sample-and-hold operation of the digital devices, a divided-frequency locking scheme of the quadrature coherent demodulation is proposed. Finally, IC demodulator performance is experimentally shown in the static and Rayleigh fading environments. The bit error rate performance and error-burst characteristic are measured. It is concluded that the single-chip coherent demodulator is suitable for digital mobile radio application.

256-QAM Modem Performance in Distorted Channels

Abstract: Performance degradations of 256-QAM modems in distorted channels are studied in this correspondence. Illustrative linear, parabolic, and sinusoidal amplitude and group delay channel distortions, caused by filter imperfections and/or by selective radio fades, are investigated. To enable an easy comparison with 64-QAM systems, we present the degradations of these systems in the same figures as those of 256-QAM modems. It is shown that linear (slope) group delay and sinusoidal (ripple) amplitude distortions cause the most significant performance degradations.

Pseudobinary and Pseudoquaternary Detection Processes for Linearly Distorted Multilevel QAM Signals

Abstract: The paper describes two new detection processes suitable for QAM signals that have 16 or more levels (possible data symbol values) and that have been subjected to linear distortion, causing severe intersymbol interference in the received signal. The detectors are developments of the pseudobinary systems previously described, and are designed to operate in the presence of severe intersymbol interference. Results of computer simulation tests are presented, comparing the tolerances to additive white Gaussian noise of various arrangements of each detector with those of more conventional detectors. The tests have been carried out on a model of a 19 200 bit/s synchronous serial data transmission system, operating with a 64-level QAM signal that is transmitted, in turn, over two different telephone circuits.

Measured Frequency Diversity Improvement for Digital Radio

Abstract: This paper presents the measured frequency diversity improvement factor for 6 GHz 16-QAM 90 Mbit/s digital radio on the 26.4 mi Atlanta-Palmetto path in Georgia. Two channels with a center frequency separation of 59.3 MHz were used in a one-by-one frequency diversity experiment. The 1980 data and the 1982 data indicate a frequency diversity improvement factor of 100 and 45, respectively, at the outage threshold of 10-3BER. This is in contrast to the improvement factor of 9 predicted for analog FM radio at the same fade margin. The measured one-by-one frequency diversity improvement factor is comparable to the measured space diversity improvement factor with 30 ft antenna spacing on the same path. We conclude that 1) frequency diversity can provide a large improvement factor for digital radio, 2) as an alternative to space diversity, frequency diversity can provide substantial cost savings for digital radio routes, and 3) the frequency diversity calculation based on analog FM radio experience is too conservative (i.e., pessimistic) for digital radio application. These experimental findings are in agreement with recent advances in digital radio diversity modeling. Digital radio performance depends heavily on the multipath dispersion in the channel. The measured data indicate that the power fade depths in the two channels are highly correlated, whereas the multipath dispersion in the two channels is decorrelated. This correlation difference provides insight into the measured large frequency diversity improvement factor for digital radio.

Convolutional coding for high-speed microwave radio communications

Abstract: This paper describes the design, testing, and measured performance of a rate 11/12 self-orthogonal convolutional codec that meets the bit-error-rate objective of M-state quadrature amplitude modulation systems in terrestrial radio transmission. The objective is to reduce a bit error probability of 10−6 to 10−10 or smaller. In fact, the measured output error probability is well below 10−10 when the channel error probability is below 10−5.

A simulation study of space diversity and adaptive equalization in microwave digital radio

Abstract: In this paper we analyze the performance of M-level quadrature amplitude modulation digital radio systems subjected to microwave multipath fading. We consider two kinds of adaptive receiver techniques, either singly or in combination: dual space diversity and adaptive equalization. The space diversity is assumed to be of either the selection type or the continuous-combining type, and the equalization is assumed to be ideal. We describe a specific form of combining which is optimal when no post-combiner equalization is used. A primary aim of the study is to quantify the performance of this combining approach and to compare it with alternate strategies. The study uses Monte Carlo simulations of the dual-channel fading response functions based on a recently published statistical model. For each response pair generated, a receiver detection measure is derived analytically in terms of the system parameters and receiver approach. Probability distributions of this measure, obtained by simulating several thousand response pairs, are then computed. They can be interpreted as displaying the link outage probability as a function of the number of modulation levels (M). We find that the appropriate combining scheme can serve in some cases to avoid the need for adaptive equalization. Also, where post-combiner equalization is used, the same scheme, while no longer optimal, can sharply reduce the dispersion seen by the equalizer input.

16-QAM and trellis-coded 16-QAM on nonlinear channels

Abstract: The use of 16-QAM on bandlimited nonlinear satellite channels, in uncoded and trellis-coded form, for bandwidth efficient modulation (ideally 4 bps/Hz) is discussed To avoid severe degradation due to AM/AM and AM/PM distortion, or to avoid the need for large back-off, predistorting the 16-QAM constellation at the modulator is considered Performance for varying back-off settings for uncoded and coded 16-QAM is simulated, and it is found that the four-state trellis code has a coding gain of about 8 dB, larger than expected based on linear channel analysis, while the 16-state code gains only marginally beyond this

Digital direct modulator with type of modulation selection

Abstract: The present invention uses digital values of MSK-like waveshapes which can be stored in a storage media such as a ROM and then output in a prescribed fashion such as in phase quadrature through the use of an addressing technique to the ROM. The digital values, each of N bits, are then multiplied in quadrature times a data representative signal which is divided into phase quadrature components. These N+1 bit products are summed to produce a composite digital signal representing analog bipolar values. A digital-to-analog conversion provides the modulated analog output signal.

Performance of SQAM systems in a nonlinearly amplified multichannel interference environment

Abstract: The performance of superposed quadrature amplitude modulation (SQAM) systems, in a nonlinearly-amplified multichannel environment, in the presence of additive white Gaussian noise (AWGN), intersymbol interference (ISI), adjacent-channel interference (ACI) and cochannel interference (CCI), is investigated. This study is an extension of our previous work which was restricted to the performance study of SQAM modems in single-channel AWGN systems. Typical system configurations are analysed by means of computer simulations. Our results demonstrate that SQAM modems outperform OQPSK, MSK and IJF-OQPSK (or SQORC) modems.

Amplitude-Weighted Quadrature Phase Shift Keying Using SAW Technology

Abstract: Surface acoustic wave (SAW) convolvers are capable of performing programmable matched filtering with the desirable properties of large processing gain, good dynamic range, broad bandwidth, small size and weight, and low power requirements. Equally powerful are the SAW-based modulators, which produce the desired pseudo-random code sequence for secure transmission in a spread-spectrum system. The SAW filter provides precise reproducible pulse shaping of the coded waveform with the same advantages of the convolver. It is very important as spectrum space becomes more crowded to optimize the spectral efficiency of transmitted information. Quadrature phase (QPSK) and minimum phase shift keying (MSK) are the two most popular quadrature modulation schemes. A new modulation technique that is composed of several amplitude-weighted QPSK signals (AWQPSK) and exhibits better bandwidth efficiency than QPSK or MSK is introduced. System analysis and performance parameters are presented for evaluation. A SAW modulator implementation is discussed and proposed.

Method for pulse-code modulated transmission of radio and television signals

Abstract: The digital transmission of sound, data and images, including those for television, via radio requires many times the bandwidth required in analog transmission. For a digital television image, for example, a signal flow of 140 Mbit/s is required. Due to this enormous bandwidth requirement, digital transmission by radio has not been successful in spite of the use of quadrature amplitude modulation. In the invention, the great bandwidth requirement is avoided by providing for the digital coding of the signals a binary code which is formed from the half-waves for periods of a sinusoidal alternating current having the characteristic states of large and small amplitude values and is transmitted in an uninterrupted sequence of positive and negative half-waves. In this arrangement, a multiple utilisation of the respective transmitter is provided by time-division multiplexed and/or frequency-division multiplexed arrangement of the alternating coding currents of the various channels and modulation onto the alternating transmitting current and/or by providing the alternating transmitting current directly as alternating code current.

Method and device for digital information transmission using differential frequency shift keying

Abstract: 1. A method for the transmission of digital data, characterized in that it consists : - when transmitting, in an encoding by differential frequency shift keying about a center modulation frequency Fo , while associating with each symbol consisting of k bits to be transmitted a frequency shift DF, selected from a table containing 2**k possible frequency shifts respectively associated with 2**k possible symbols of k bits, in adding respectively substracting this shift to respectively from the frequency associated with the preceding symbol according to whether the preceding frequency is lower or higher than the modulation center frequency, and in transmitting the corresponding frequency during the period of the symbol, the modulation signal resulting from the encoding being passed on for the purpose of transmission, - when receiving, in the decoding of the associated modulation by way of estimating for the interval of the symbols contained in a signal segment, the latter being defined by means of a synchronization signal having the modulation frequency and synchronized with the received modulated signal, an assessment of the frequencies successively received, and in deriving therefrom the corresponding frequency shifts and thus the received symbols.

Single-sideband communication system

Abstract: A bandwidth reduction technique for use in digital systems wherein elements of a data signal modulate quadrature-related carriers. This modulation, referred to as quadrature amplitude modulation (QAM) or phase shift keying (PSK), generates a double-sideband signal which is transmitted in a variety of communications systems. In accordance with the present invention, the above-described double-sideband signal is filtered (103) to form a single-sideband signal prior to transmission. While this use of a single-sideband signal, in lieu of a double-sideband signal, effectively doubles the system capacity by permitting the use of two communications systems in the bandwidth previously occupied by one system, the filtering process (103) contaminates the data signal elements. To recover the data signal elements at the receiver, received signal elements are formed by extracting the carrier signals (107). Next, these received signal elements are altered by preselected quantities to form estimates of each data signal element (301, 302...307). A comparison of the formed estimates (318) against the set of permissible values for each data signal element then determines which estimate is correct.

Modem Design using Continuous Phase Modulation with Coding

Abstract: The combination of coding and low index modulation can result in both bandwidth and power efficient modem design. The low index modulation format is obtained by using a frequency division and multiplication network to alter the modulation index of ? for convertioned MSK to a lower value. This lower value modulation index in concert with restricted range encoding provides a system that can achieve a 2 bits/sec/Hz bandwidth efficiency at a power level equivalent to that required for MSK. The design concepts are such that data rates in the hundreds of megabits/second are practical.

Orthogonal amplitude modulation system transmitter

Abstract: PURPOSE:To generate an average power of non-modulation signal of a multi-value QAM system independently of a multi-number by providing a means generating the average power of (2 X2 )-set of non-modulation signals to a transmitter. CONSTITUTION:Four sets of binary digital signals S0-S3 and one set of clock signal CLK are inputted to a transmission logical circuit 1 in figure, where the logical operation such as waveform shaping, code conversion and additive operation is conducted and the signals are inputted to D-A converters 2 and 3 as two rows of parallel axis (for PCH) and two rows of vertical axis (for QCH) digital signals P, Q. The D-A converters 2, 3 input the digital signals P, Q and convert then respectively into 4(2 =4)-value of signals Sp, Sq. The four-value 2-row (4-value X 4-value) signals Sp, Sq are inputted to an orthogonal linear modulator 6, where the multi-value QAM modulation in a carrier frequency (f) is applied, the result is amplified to a prescribed level by an amplifier 7 and a desired 16QAM modulation signal Sm is obtained.

Asymmetrical Pulse Shaped QPSK Modulation Systems

Abstract: To enable differential detection of coincident, hard-limited, and reduced sidelobe QPSK type of signals, we introduce asymmetrical pulse shapes. This asymmetry in the I and Q baseband channels leads to coincident (unstaggered) QPSK system applications. We consider the bit error probability performance of these modulation schemes on a hardlimited channel in the presence of uplink and downlink additive Gaussian noise. It is found that the unstaggered QPSK modulation with the asymmetrical pulse has better bit error probability performance than QPSK and staggered QORC.

lodul Analysis of the Sensitivity of Digital .ation Techniques to Frequency-Selectj

Abstract: Abstruct-This paper investigates the sensitivity to frequency-selective fading ef different modulation techniques commonly used in digital radio systems. The modulation methods considered are phase-shift-keying (Mary PSK), nonoffset and offset quadrature amplitude modulation (M-ary QAM), and quadrature partial response signaling (QPRS). The performances of these methods during multipath fading are compared on the basis of the “signatures” calculated for idealized systems. Results are presented from which the relative outage probability was determined, assuming a propagation channel with frequency-selective Rician fading.