Showing papers on "Quadrature amplitude modulation published in 1988"

Digital filter and square timing recovery

Abstract: The digital realization of timing recovery circuits for digital data transmission is considered. A digital algorithm is proposed that can be implemented very efficiently even at high data rates. The resulting timing jitter has been computed and verified by simulations. In contrast to other known algorithms, the one presented here allows free-running sampling oscillators and a novel planar filtering method that prevents synchronization hangups. >

Multiple trellis coded modulation (MTCM)

Abstract: The authors demonstrate a trellis coded modulation technique referred to as multiple trellis coded modulation (MTCM) wherein more than one channel symbol per trellis branch is transmitted. They have found simple two-state trellis codes for symmetric MPSK multiple phase-shift keying and AM modulations that can achieve 3-dB gain over uncoded modulation at very high signal-to-noise ratios without bandwidth expansion or reduction in information bit rate. The gain of these codes with respect to previously reported two-state trellis codes is between 1 and 2 dB at very high signal-to-noise ratios, depending on the number of bits per Hertz transmitted. These gains are achieved for those of the equivalent conventional trellis codes with the same number of states in the trellis diagram. The authors note that additional computations per branch are needed for the multiple trellis coding scheme. The concept can be extended to a higher number of states and other types of modulations. >

New phase and frequency detectors for carrier recovery in PSK and QAM systems

Abstract: Phase and frequency detectors (PFDs) are presented that considerably extend the acquisition range of carrier-recovery loops in digital communication systems. Based on a simple modification of conventional phase detectors (PDs), the proposed detectors are applicable to a large variety of modulation schemes, including the popular PSK and QAM signal formats. Their application to QPSK and 16- and 64-QAM is extensively discussed, and simulated frequency-detector (FD) characteristics, as well as acquisition behavior of several PFDs, are reported for QPSK and 16 QAM. The results of an experimental evaluation using a 16-QAM laboratory modem are also reported which show that the detectors increase the acquisition range achievable by conventional PDs by more than one order of magnitude. In PSK, the improved acquisition performance is obtained with no penalty in steady-state phase jitter. In combined amplitude- and phase-shift keying, it generally leads to increased jitter, but this is easily avoided by incorporating a lock indicator and switching back to the original PD after lock is acquired. >

Sensitivity penalty in multichannel coherent optical communications

Abstract: The error rates and sensitivity penalties for multichannel coherent optical communications systems are evaluated for amplitude-shift keying (ASK), phase-shift keying (PSK), and frequency-shift keying (FSK) modulation, taking into account adjacent channel interference. Both time-domain and frequency-domain analysis are used, the latter being based on a Gaussian approximation. Both techniques yield similar results for sensitivity penalties below 1 dB. For FSK systems, larger values of the modulation index Delta do not necessarily lead to larger channel spacings. ASK and PSK systems both require larger channel spacings than FSK systems with Delta =1. The study was conducted for sources with linewidths narrow enough so that phase noise does not degrade the performance of receivers with matched filter demodulators. >

Method and apparatus for two stage automatic gain control

Abstract: The present invention is a method and apparatus for implementing a two stage AGC circuit. In the preferred embodiment, the present invention is used as part of a receive channel in a modem. The first stage of the AGC is a "coarse" AGC and is used to track large signal transients of an input signal. The coarse AGC locks on to transient signals without excessive settling time. In operation, the coarse AGC acquires a new signal by using a nonlinear clipped feedback loop technique supported by a linearized feedback loop. The coarse AGC stage uses an error signal derived from the noncoherent power fluctuations of the incoming signal. The second stage of the AGC circuit is a "fine" AGC using a decision-directed coherent amplitude error signal and a quick linear feedback loop to correct for finer signal level fluctuations. The fine AGC has a high pass characteristic which decouples its response from that of the equalizer for stability reasons. The present invention avoids performance and response limitations of prior art AGC's in that the coarse stage is not required to have wide band response and associated noisy tracking response. Further, when the input signal is in quadrature amplitude modulation (QAM) the coarse stage tracks by using a nonlinear power detection algorithm which removes the effect of data power modulation. The fine AGC stage utilizes a decision-directed (coherent) error signal and a linear feedback loop with zero excess delay so that wideband response can be achieved without introducing amplitude modulation error in the signal path. As a result, the two stage scheme of the present invention can track rapid gain changes and restore correct data detection within a few baud, limiting error corruption to only one data block.

Method and apparatus for quadrature modulation

Abstract: A quadrature amplitude modulation system utilizes two subcarrier signals (11, 12) each quadrature modulated with information signals (S1, S2, S3, S4) which are quadrature modulated onto the RF carrier (22). This provides a hole in the center of frequency spectrum, permitting carrier and bit sync signals can be provided in the center area of the signal spectrum.

The implementation of a 16-QAM mobile data system using TTIB-based fading correction techniques

Abstract: The transmission of high-speed data to and from a mobile terminal is considered to be far from simple. Practical results for one solution to the problem, namely reference-base channel sounding, are presented. A 9600-b/s 16-QAM data system is used together with the transparent tone-in-band (TTIB) reference processing technique. Excellent performance under both static and fading conditions is observed. >

Fourier-Bessel error performance analysis and evaluation of M-ary QAM schemes in an impulsive noise environment

Abstract: The authors derive closed-form expressions for the BER (bit error rate) performance of coherent M-ary quadrature amplitude modulated (QAM) system in the presence of an additive combination of Gaussian and highly impulsive noise by using a Fourier-Bessel series expansion method. The model used for the impulse noise, which closely models atmospheric and/or ignition noise, consists of a stream of Poisson impulses with areas distributed according to a bilateral Rayleigh probability distribution function. Although the methodology is general enough to accommodate any M-ary QAM scheme, they present analytical as well as some computer simulated results for 16-, 64-, and 256-QAM systems. An almost insignificant increase of the truncation error with the increase of the modulation levels verifies that the proposed method of analysis can be a useful tool in evaluating the performance of very large-alphabet M-ary QAM systems, e.g. M>or=256. >

Specification and development of an equalizer-demodulator for wideband digital microwave radio signals

Abstract: The authors describe the specification and implementation of a sophisticated digital signal processor to be used for equalizing and demodulating high-rate, high-order quadrature-amplitude-modulated (QAM) signals used in digital radio systems. The processor uses state-of-the-art algorithms as well as the latest high-speed CMOS VLSI technology to achieve a processing rate in excess of 10 gigamultiply/adds per second. The system exhibits several unique features; in particular, 40 Mband signals can be processed with 64-tap equalizer and the functions of baud and carrier recovery are implemented digitally. The signal processing problems to be solved are described, and from them a set of requirements is developed. A summary of the algorithms needed to recover symbol timing, equalize the incoming signals, and remove carrier offset is provided. A brief discussion of the hardware tradeoffs is followed by a detailed description of the system design. >

Performance of a novel selection diversity technique in an experimental TDMA system for digital portable radio communications

Abstract: The authors present the measured average bit error and block error ratio performance of a coherent burst time-division multiple-access radio link in a simulated flat Rayleigh fading environment using a unique technique for implementing two-branch selection diversity The link consists of a 500-kb/s quadrature amplitude modulation burst transmitter and a coherent receiver with a fast carrier recovery circuit The receiver is switched between two signals which are corrupted by uncorrelated Rayleigh fading If the transmitter is on before the actual data burst, received signal measurement and selection can be performed just before the reception of a data burst This way, when data is to be demodulated, the receiver is already switched to the branch with the higher power With this technique, the performance of selection diversity is achieved with only one receiver chain Experimental results are presented showing link performance as functions of Rayleigh fading rate and the time delay between signal measurement/diversity selection and the actual data burst >

Blind channel identification using the constant modulus adaptive algorithm

Abstract: A technique is presented for identifying the impulse response of a nonminimum phase channel based solely on observations of the channel output. The technique uses a two-step estimation procedure whereby the channel input is first estimated and then used in conjunction with the observed channel output to determine a least-squares model of the actual channel. Estimation of the channel input is accomplished through equalisation, demodulation, and remodulation of the channel output. The constant-modulus adaptive algorithm (CMA) is used in the equalization process. The technique is applicable to channels carrying quadrature amplitude modulation (QAM), frequency modulation (FM), and frequency-shift keyed (FSK) modulation. In addition to identifying the channel response, the technique also generates an estimate of the additive cochannel interference. >

Cross-polarization interference cancellation in the presence of delay effects

Abstract: It is shown how a fractionally spaced transversal canceler with five taps is capable of providing large improvement factors in channel situations of practical significance, including large delay differences. How the capability of counteracting the depolarization depends on the performance of the adaptive equalizer in the system is shown using a realistic channel model with bidirectional crosstalk. For an optimum system configuration, a matching between the equalizer and canceler capability is necessary. This has been verified experimentally for a 140 Mb/s system employing 16 QAM and operating with plesiochronous data streams. >

A 2- mu m CMOS digital adaptive equalizer chip for QAM digital radio modems

Abstract: The design and fabrication of a fully digital adaptive equalizer chip for QAM (quadrature amplitude modulation) digital radio modems is reported. The chip contains 107936 transistors on a silicon area of 94.6 mm/sup 2/. The chip was designed in a 2- mu m CMOS technology for a clock and sampling rate of 23.5 MHz. Accordingly, the functional throughput rate per chip area is 6.7 10/sup 11/ eq. gates Hz/cm/sup 2/. The inputs and outputs of the chip are ECL compatible, using a control unit compensating the influence of transistor parameter variations. For proper communication between chips having different technology parameters, a matched clocking scheme for synchronization was developed. A complex-valued equalizer was realized with four chips and tested in a 16-QAM digital radio modem, running at 35-MHz clock frequency. >

Spectrum shaping SAW filters for high-bit-rate digital radio

Abstract: The structure of a QAM radio system is explained. Some novel designs that demonstrate the current status of spectral shaping SAW (surface acoustic wave) filters are presented. Compensation of second-order effects is discussed. Experimental results for a filter for a 140-Mb/s 16 QAM cochannel system with a roll-off factor of 0.19 are shown. Two other examples of filters for a 140-Mb/s, 64 QAM system with a 35% relative bandwidth are also presented. >

Sphere-packing upper bounds on the free distance of trellis codes

Abstract: Sphere-packing arguments are used to develop upper bounds on the free distance of trellis codes. A general bounding procedure is presented. Sphere packing bounds, including bounds on the density of infinite regions, packings for hypercubes, and packings on the surface of a unit sphere, are then used to produce bounds for a wide variety of trellis codes. Among the applications are convolutional codes, Ungerboeck codes for phase-shift keying and quadrature amplitude modulation, coset codes, and continuous phase modulation codes. The new bounds are significantly tighter than existing bounds in many cases of practical interest. >

Digital demodulator apparatus

Abstract: A digital demodulation apparatus which has a detector (11) which receives transmission signals (Sin) modulated under a multivalue quadrature amplitude modulation method having a honeycomb structure signal point arrangement and which produces I channel and Q channel demodulation signals and a discriminator (13) which converts the analog demodulation signals to digital demodulation signals, a memory (21) being introduced which successively receives said digital demodulation signals and reproduces corresponding original data, said read only memory (21) storing data premapped in the same way as the honeycomb structure signal point arrangement and storing polarity bits (P) and error bits epsilon of the signal points.

Trellis-coded modulation on digital microwave radio systems-simulations for multipath fading channels

Abstract: Extensive simulations have been made to examine the performance of digital microwave radio systems based on TCM (trellis-coded modulation). Two coded systems have been simulated: the coded 1024-QAM (quadrature amplitude modulation) system uses separate TCM schemes for the I and Q channels, while the coded 512-CR system treats I and Q channels together. Theoretically predicted lower bounds on error performance were approached by both systems for additive white Gaussian noise channels. With residual ISI (intersymbol interference) the coded system achieved larger coding gains than with additive white Gaussian noise alone, due to the ability of the Viterbi decoder to correct short burst errors. The coded 512-CR system performed better than the coded 1024-QAM system in all circumstances. This is attributed to the use of combined I and Q channel coding and the more complex convolutional encoder used in the coded 512-CR system. It is concluded that TMC used in conjunction with adaptive equalization can achieve significant improvements in error performance on digital microwave radio systems. >

Modulator producing phase modulation by combining amplitude modulated signals

Abstract: A system is described for using the modulation of the phase of a RF carrier as means of transmitting information. The RF input is split into four quadrature components each of which is passed through individual amplitude control attenuators and then summed in a combiner to create the required phase-controlled RF signal. The resultant signal is then amplified in an RF signal amplifier chain as required to achieve the desired output level. The RF output level is sampled and routed back to an in-phase splitter the output of which is compared to the corresponding individual quadrature components of the unmodulated RF signal to generate a signal which is compared with a processor generated modulation reference signal in a control loop amplifier and the resulting error signal is applied to the individual attenuators to adjust the output amplitude of each quadrature component. The quadrature components are then combined to obtain the desired phase-modulated RF output signal.

Performance of trellis codes for a class of equalized ISI channels

Abstract: The performance of trellis codes is examined for a class of intersymbol interference (ISI) channels that occur in high-frequency radio systems. The channels considered are characterized by in-band spectral nulls and by a rapid time variation. The baseline modulation technique is 4QAM (four-point quadrature amplitude modulation). When spectral nulls are absent, performance of fractionally spaced linear equalizers and trellis decoders is found to be near ideal and to be better than using symbol-spacing in the equalizer. However, error propagation in the feedback path, resulting from equalizer-based decisions, ruins the performance of the combination of decision-feedback equalizers and trellis decoders when spectral nulls are present. Their performance can be improved by using fractionally spaced feedforward equalizer sections and by designing the decoder to compensate for ISI. Rate 2/3 codes are found to outperform rate 1/2 codes in error performance. >

Trellis-coded MPSK modulation for highly efficient military satellite applications

Abstract: Trellis-coded multiple-phase-shift-keyed (MPSK) modulation is an effective technique for increasing the bandwidth efficiency of an existing channel while maintaining at least a moderate degree of power efficiency through coding. The authors consider the application of this technique to increase markedly the capacity of a 25-kHz military satellite channel. It is shown that with only minor modifications to the QPSK modem to incorporate 8 PSK and 16 PSK modulation and no modification to the rate-1/2 coder used to transmit 16 kb/s over this channel, transmission rates of 32 kb/s and 48 kb/s, can be supported at E/sub b//N/sub 0/ levels only moderately higher than for the lower data rate. In fact, it is demonstrated that 48 Kb/s can be transmitted within exactly the same bandwidth and at the same E/sub b//N/sub 0/ levels as that required to transmit uncoded QPSK at 32 kb/s at bit error rate of 10/sup -5/. >

An NTSC compatible wide screen television system with evolutionary extendibility

Abstract: A review is presented of the role of the inverse Nyquist filter in quadrature amplitude modulation (QAM) technology and an outline of studio and network issues of this system is given. For two of the problems concerned, i.e., (a) crosstalk caused by ghosting and (b) interference from conventional TV receivers, the authors propose a two-dimensional waveform equalizer, with processing in the time and frequency domain at a transmitter. The encoding and decoding for this wide-screen television system, and the evolutionary extendibility of QAM technology toward further improvement of image and sound quality in a compatible fashion are also discussed. >

Application of trellis coding to digital microwave radio

Abstract: The authors examine the application of trellis coding to bandwidth-efficient digital microwave radio systems. Specifically, the authors study trellis codes carrying four information bits per transmitted symbol, as in uncoded-16 QAM (quadrature amplitude modulation), and codes carrying six information bits per symbol, as in uncoded-64 QAM. In addition to their performance at high SNRs (signal-to-noise ratios) the authors report their effective performance at threshold BER (bit error rate) values of 10/sup -4/ and 10/sup -3/ in a multipath fading environment, using the noiseless system signature and the equivalent CNR (carrier-to-noise ratio) degradation as performance measures. >

Hybrid frequency shift keying modulator and method

Abstract: A hybrid frequency shift keying modulator and method produces binary FSK by simultaneously applying a digital modulating signal to the DDS section of a frequency synthesizer along with a coordinated analog modulating signal as direct frequency modulation of the accompanying PLL section. Low frequency performance is derived from the characteristics of the DDS modulation and the high frequency performance and accurate wave shape reproduction are derived from the modulation of the direct FM section.

An adaptive modulation scheme for bandwidth-limited meteor-burst channels

Abstract: The author investigates the performance of an adaptive information rate technique that is particularly well suited to the bandwidth-limited meteor burst channel. This technique uses the quadrature amplitude signal sets common to digital radio and is called adaptive QAM. Improvements in throughput that are possible with the proposed approach are examined. The results are pertinent to the use of meteor-burst channels for military applications. >

256 QAM 400 Mb/s microwave radio system with DSP fading countermeasures

Abstract: A 256 QAM 400 Mb/s system and repeater configuration, using fading countermeasures with digital signal processing (DSP) technologies, is described. A one-chip transversal filter is developed. This large-scale integration chip is used as a fully digital transversal equalizer and a cross-polarization interference canceller offering compactness, low power consumption, and no tuning. An advanced control method called the shift-bit select and the maximum level error method are introduced to improve control response, acquisition and steady performance. The field test confirms good performance of the fading countermeasures with DSP technologies. >

A new carrier regeneration scheme for QAM signals

Abstract: Customary processes for regenerating the carrier of a QAM (quadrature amplitude modulation) signal utilize the sampled receive signal in the vicinity of the nominal states of the state plane. Thus, once synchronization has occurred, good control action in the vicinity of the synchronization point is achieved. However, for higher-level QAM signals the synchronization process is not perfect. The author proposes a method in which the information contained in the amplitude and the relative phase position of the sampled signal values is used to calculate the most probable phase deviation of the local carrier signal in relation to the transmitted carrier signal. This provides better acquisition characteristics without impairing the good tracking characteristics obtained with other methods. Computer simulations suggest that relatively quick and secure synchronization will be achieved with higher-level QAM systems. >

Phase-dominant spatial light modulators

Abstract: Computer simulation results are given for an optical correlator in which the spatial light modulator has a strong phase modulation and a weak proportional amplitude modulation at each pixel. The signal/noise ratio on the correlation plane is found to be much higher than that of a matched filter, and the residual signal-dependent amplitude modulation improves the system performance in some cases.

Performance of a 16-ary DEQAM modem employing a baseband or RF predistorter over a regenerative satellite link

Abstract: The paper presents the results of a series of computer-simulation tests that determine the effects of bandlimiting (nonlinear distortion with and without predistortion) on the tolerance to additive white Gaussian noise of a digital modem. The modem transmits a 16-ary differentially encoded quadrature amplitude modulation (16-ary DEQAM) signal over a regenerative satellite link, where the high power amplifier (HPA) on board the satellite transmitter may introduce AM-AM and AM-PM conversion effects into the 16-ary DEQAM signal. Four different bandwidths of the transmitted signal are tested, together with different levels of output back-off in the HPA at the transmitter. The arrangements achieving the most efficient use of the power are selected. There sults show that a surprisingly good improvement in performance can be achieved by using predistortion of the 16-ary DEQAM signal to reduce the nonlinear distortion of the high power amplifier.

A technique for multidimensional symbol and multiplexing frame synchronization in multidimensional trellis coded modems using non-standard baud

Abstract: Multidimensional trellis-coded modems use symbols that are formed from multiple two-dimensional QAM (quadrature amplitude modulated) symbols. The receiver must have the ability to determine the start point of the multidimensional symbol. In modems that use a baud other than 2400 it may be necessary to multiplex channels at standard bit rates. This requires synchronization of frames of symbols that contain a number of bits that is evenly divisible by the number of multiplexed channels. An algorithm has been developed which solves both problems simultaneously. The algorithm is presented along with an explanation of its operation. >

QAM modulator/demodulator

Abstract: A digital signal transmission system adopting a quadrature amplitude modulation and demodulation principle and feasible for large capacity data transmission in which a number of levels are used. Each of a modulating and a demodulating device of the system is constructed to shift the phase difference between two carrier waves by a suitable angle of Δθ radian from π/2 radian. The tuning phase in the demodulating device is limited to one so as to allow a bit error rate characteristic to remain constant.