Showing papers on "QAM published in 1990"

Bandwidth-efficient QAM schemes for Rayleigh fading channels

Abstract: Quadrature amplitude modulation (QAM) is a bandwidth efficient transmission method for digital signals. It is expected that the available spectrum for the proposed personal communications network (PCN) will soon be at a premium as the number of subscribers increases, and changing from binary modulation to QAM may significantly ease the problem. The severe amplitude and phase changes introduced by the fading channels, however, make low error transmission of QAM difficult to achieve, unless procedures are introduced at both the transmitter and the receiver to combat the fading. The authors describe ways to improve the bit error rate (BER) of QAM by using various forms of coding on an increased symbol set. This means that the data throughput, symbol rate and transmission power are unaffected, although the transmitter and receiver are made considerably more complex. They consider Rayleigh, rather than the less severe Ricean fading channel to get worst case performance estimates of mobile radio communications.< >

Multiplexed coded modulation with unequal error protection

Abstract: Unequal error protection is provided for an HDTV signal by separately coding each one of the classes of information in the HDTV signal by using a conventional coded modulation scheme and then time-division-multiplexing the various coded outputs for transmission. In particular, each class of information is separately coded by a 4-dimensional 8-state trellis code and a uniformly-spaced (QAM) signal constellation.

Blind equalization based on radius directed adaptation

Abstract: A blind equalization algorithm, termed radius directed equalization (RDE), for quadrature amplitude modulation (QAM) signals based on the known modulus of the constellation symbol radii is described. For example, 16 QAM has three radii and 32 QAM has five radii. The algorithm uses the error between the equalizer output modulus and the nearest symbol radius to update the equalizer weights. The RDE algorithm provides faster convergence than the constant modulus algorithm (CMA) for QAM signals and is independent of the carrier offset. The algorithm is described in the context of blind carrier and baud clock recovery schemes. >

Quadrature amplitude modulation communication system with transparent error correction

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

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

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

A subband coding, BCH coding, and 16-QAM system for mobile radio speech communications

Abstract: A combined subband speech coding (SBC), Bose-Chaudhuri-Hocquenghem (BCH) error-correction coding, and 16-level quadrature amplitude modulation (16-QAM) scheme with switched diversity and speech postenhancement is proposed. The system's performance is dramatically improved by deploying some degree of fade tracking capability over fading channels. Further quality enhancement accrues by using appropriate mapping between the SBC speech codec and the Gray coded QAM words. Various BCH codes are utilized to adequately match the error-correcting power to the perceptual importance of the SBC bits. One of the proposed systems operates at 7 kBd and yields good communications-quality speech for channel signal-to-noise ratios (SNRs) in excess of 20 dB and encounters a maximum overall system delay of 55.125 ms. A more complex arrangement uses second-order switched diversity to reduce the channel SNR required to around 16 dB and the transmission rate to 5 kBd when the vehicular speed is 30 mph while the system delay is unchanged at 55.125 ms. >

Data predistortion techniques using intersymbol interpolation

Abstract: Two data predistortion techniques are presented that compensate for high-power amplifier (HPA) nonlinearities in digital microwave radio systems by employing quadrature amplitude-modulation (QAM) signal formats. The first one is a T/2-spaced predistortion technique that ensures distortion-free HPA output at two points per symbol interval T. The second is a T/3-spaced predistortion technique which cancels nonlinear distortion at the HPA output at three points per symbol interval. As opposed to conventional data predistortion, which can only compensate for warping of the signal constellation, the new techniques are effective against intersymbol interference. Using the 64- and 256-QAM signal constellations, it is shown that the proposed techniques lead to a very efficient utilization of the available HPA power. It is shown that, of the two techniques, the T/3-spaced data predistortion employs narrower transmit-pulse shaping and achieves higher protection against adjacent-channel interference at the expense of some additional complexity. >

Fiber-optic subcarrier multiplexing video transport employing multilevel QAM

Abstract: The technical feasibility of applying a fiber-optic multilevel QAM (quadrature amplitude modulation) subcarrier multiplexing scheme to a multichannel conventional TV signal transport is explored. This transport scheme has some advantageous features. Because a digital subcarrier modulation scheme is used, a high-quality transmission performance is expected to be achieved, if the bit error is sufficiently small over a transmission line and the required carrier-to-noise ratio (CNR) is relatively small among the many candidates of subcarrier modulation schemes. The use of multilevel modulation allows the bandwidth to be efficiently utilized; consequently, up to 40-channel uncompressed digital TV signals are estimated to be accommodated in the frequency region of the VHF and UHF bands, where well-developed and inexpensive electronics can be easily applied. >

Fiber-optic transmission of microwave 64-QAM signals

Abstract: Experimental results on fiber-optic transmission of microwave 64-level quadrature amplitude modulated (QAM) signals at a 1.3 mu m wavelength at a 90 Mb/s transmission rate are presented. Two important methods of improving the system performance are discussed and demonstrated: laser intensity noise minimization and error-correction coding using a self-orthogonal convolutional code. The applicability of this transmission technique to the distribution of digital TV services is assessed. >

Volterra linearizer for digital transmission

Abstract: of the Disclosure Characterized nonlinearities in a QAM transmission channel are removed by this linearizer, in which selected bits from a sequence of received symbols on both rails form the address of a PROM. Stored values represent the Volterra expansion of the response of the channel, including the nonlinearity, to the symbol sequence. Symmetries in the Volterra expansion allow the same stored value to be used to correct both AM/AM same rail and AM/PM opposite rail distortion.

Performance of multi-level QAM with maximal ratio combining space diversity for land mobile radio communications

Abstract: A novel type of space diversity with maximal ratio combining for multilevel quadrature amplitude modulation (QAM) in land mobile communications to improve transmission performance is proposed. In order to realize maximal ratio combining space diversity for QAM, a novel type of diversity combining method using a pilot symbol insertion type fading estimator is applied. The proposed diversity combining is carried out at the baseband using digital signal processing, simplifying the diversity receiver hardware. Computer simulation and laboratory experiments confirm that the multilevel QAM using the proposed method is effective for improving the transmission performance of land mobile communications. >

Performance of Optimum and Adaptive Frequency-Shift Filters for Cochannel Interference and Fading

Abstract: Analog and digital carrier modulated signals, such as AM, digital QAM, PSK, and FSK, exhibit correlanon among spectral components separated by multiples of the keying rate and separated by the doubled carrier frequency plus multiples of the keying rate. This spectral redundancy can be exploited to facilitate rejection of cochannel interference, while maintaining minimal signal distortion. It also can be exploited to mitigate the effects of frequency-selective fading with minimal noise amplification. The spectral redundancy is exploited by filtering and adding frequency-shifted versions of the corrupted data. This paper presents the results of a study to evaluate the performance capabilities of optimum and adaptive frequencyshift filters for severely corrupted carrier-modulated signals including AM, BPSK, and QPSK.

Multichannel physiological monitor plus simultaneous full-duplex speech channel using a dial-up telephone line

Abstract: A physiological monitoring system which can transmit up to six low-frequency data channels in the presence of a full-duplex speech channel using one telephone line is described. Bandlimiting the speech signal to 2.5 kHz leaves a bandwidth of about 900 Hz for the six data channels. To make best use of this allocation quadrature amplitude modulation (QAM) was chosen as the main transmission mode. Coherent detection must be used at the receiver to extract the data. The center frequencies for the carriers were set at 2800, 3050, and 3300 Hz. The system was tested on a local public switched telephone network line over a distance of approximately 40 km, and a sample of the signals received is shown. These were recorded at 10.00 am on a weekday when the noise level on the line can be expected to be higher than normal. The output ECG and blood pressure waveforms proved to be clinically acceptable. >

Carrier acquisition scheme for QAM and QPSK data

Abstract: The invention involves an apparatus for recovering the carrier signal in OPSK or QAM data. First an energy directed phase detector controls a voltage controlled oscillator so that the output signal has a phase that is within a few degrees of the carrier phase. Following this coarse acquisition, the system clock timing is recovered and established. A decision directed phase detector, using the system clock information, then controls the VCO to adjust the output signal phase to within a very small tolerance of the actual carrier phase.

An introduction to cable television in the United States

Abstract: The history of cable television in the US is reviewed. The opportunity for reuse of frequencies assigned for other purposes in the over-the-air environment is discussed in the context of the crowding of the radio spectrum. The problem of loss in coaxial systems is briefly examined. The tree-and-branch topology used by cable systems is described. Signal quality issues are addressed. Tradeoffs in cable system design and cable system economics are discussed. >

Further results in the unified analysis of digital communication systems

Abstract: Stein's (1964) method can be extended to the analysis of the bit error probability (BEP) of quadrature phase shift keying (QPSK), staggered QPSK, and minimum shift keying (MSK) communication systems. The resultant noisy reference BEP waterfall curves are presented. The numerical advantages of this technique and some practical results are discussed. In a parallel manner, the BEP for quadrature amplitude modulation (QAM) communication systems can be analyzed. This technique is numerically more intensive but is used to generate noisy reference BEP waterfall curves for 16-QAM and 64-QAM modulations. Unfortunately, few carrier synchronizers produce a complex Gaussian reference signal, but pragmatically many reference signals can be accurately approximated by a complex Gaussian at moderate to high SNR. Actual BEP performance and the approximate results are compared. >

Specifications for tuner design for use in cable ready television receivers and VCRs

Abstract: It is pointed out that, with the expansion of the number of cable channels from 12 or fewer to the current 78, the realization has come that television tuner performance is not always satisfactory. Some of the performance parameters important to tuners operating on cable systems are described, and tuner performance is compared with set-top converter performance. The following tuner issues affecting performance on cable television systems are discussed: direct pick-up, local oscillator emission, distortion, image rejection, adjacent channel rejection, and noise figure. >

A 10-14 GHz linear MMIC vector modulator with less than 0.1 dB and 0.8 degrees amplitude and phase error

Abstract: The design, fabrication, and performance of a GaAs monolithic linear vector modulator in the 10-14 GHz band is described. The circuit exhibits sideband and carrier rejections of more than 45 dB with third-order intermodulation signals at -40 dBc. Such performance has never been obtained in hybrid or monolithic technology. The design is based on symmetrical circuits. The vector modulator comprises two monolithic binary-phase-shift keying (PSK) modulators, a monolithic quadrature power divider, and a Wilkinson power combiner. The modulator is ideal for direct 16-quadrature amplitude modulation (QAM) in X-Ku-band digital radio links and satellite communications. Due to its linearity, amplitude, and phase precision, it has numerous applications, such as for an analog phase shifter for phased-array antennas or for a single-sideband up converter. >

Cancellation of nonlinearities in bandpass QAM systems

Abstract: An orthogonalized input-output characterization of nonlinear bandpass quadrature amplitude modulation systems with memory is developed. The orthogonal expansion is used with a nonlinear canceller structure to counteract the effect of nonlinear distortion introduced by high-power amplifiers at the transmitter. It is shown through simulations that when the transmitted symbols are known at the canceller, relatively simple structures can eliminate the effect of the nonlinearity. Even when the transmitted symbols are not known at the canceller, the improvement in performance with the nonlinear canceller, with respect to a linear receiver, is significant. >

Rayleigh fading compensation method for 16qam modem in digital land mobile radio systems

Abstract: To cope with the increasing demands for the land mobile communication with a limited frequency band, it is necessary to improve the spectral efficiency beyond the present state. QAM (quadrature amplitude modulation) is one of the effective means for this purpose. When QAM is applied to land mobile communication, however, a great distortion is produced in the envelope and the phase due to fading fluctuation. Because of this property, few discussions have been made on the application of QAM to land mobile communication. This paper proposes the following system for such an application. The fading distortion is measured from the periodically inserted known frame symbol. By interpolating the obtained time-series, the fading distortion for all symbols is estimated. Then the distortion is compensated. The performance when the proposed method is applied to 16QAM is examined by computer simulation. It is seen as a result that the fading distortion can be compensated with high accuracy, making 16QAM applicable to land mobile communications.

Analysis and performance evaluation studies of high bit rate digital subscriber lines (HDSL) using QAM and PAM schemes with ideal decision feedback equalization (DFE) within a carrier serving area (CSA)

Abstract: QAM and PAM technologies are compared for possible HDSL application. Analysis and simulation studies with ideal decision feedback equalization (DFE) on a sample of loops within a carrier serving area (CSA) are presented. Self near-end crosstalk (NEXT) has been assumed to be the dominant source of external impairment. Simulation studies using a 64-point symbol rectangular constellation with a transmission rate of 1.6 Mb/s (in the vicinity of DS1 rate of 1.544 Mb/s) with a carrier frequency of 153.5 kHz and 10th-order Butterworth transmit and receive filters are presented for a sample of loops extracted from Bellcore's 1983 loop survey database. It is shown that a large improvement in SNR is required to make 1.544-Mb/s 64-point QAM a viable transmission technology for HDSL application in a CSA environment. The baseband PAM with 800-kb/s and 4-level code (2B1Q) has a performance which is, on the average, 5 dB superior to that of the simulated QAM across the tested loops. >

CMOS digital adaptive decision feedback equalizer chip for multilevel QAM digital radio modems

Abstract: The design of a complex-valued single-chip digital adaptive decision feedback equalizer for 256 quadrature amplitude modulation (QAM) radio modems is described. The chip contains 62000 transistors on a silicon area of 75 mm/sup 2/ and is designed for operation at frequencies of up to 70 MHz, in a 1.5- mu m CMOS technology. In high-speed applications clocking is a very critical issue in the systems design. The high operating frequency is achieved by performing a proper nonuniform redistribution of the delays over the data paths. The limitations are discussed for such a nonuniform distribution with respect to a maximum operating frequency required to be independent of the clocking scheme (i.e. nonoverlap of complementary two-phase clock system, etc.). >

Classification of voiceband data signals

Abstract: The viability of a voiceband signal classification technique which can service as a front-end for digital signal interpolation is demonstrated. The technique uses second-order statistical parameters of the complex envelope of the input signal. The input signal is assumed to be in mu -law PCM (pulse code modulation) form. For observation windows of at least 128 ms, it is shown that this method correctly classifies the input baud (e.g. 2400, 1600, 1200, 600 and 300 symbols/s) and modulation type (e.g. QAM, PSK, and FSK). This information allows the determination of the bit rate of most voiceband data signals. Coupled with a digital signal interpolator and variable rate coding, an increased compression ratio on long-haul digital trunks can be achieved. >

Carrier recovery for 16 level QAM in mobile radio

Abstract: Carrier recovery for quadrature amplitude modulation (QAM) has proved to be considerably more difficult than for binary modulation. One of the simplest effective solutions is the use of phase locked loops (PLLs) but with the phase detector replaced by a circuit capable of producing phase error signals from a received QAM phasor with varying phase and amplitude. Circuits to do this have been suggested but do not operate on all received signals. A circuit which can operate on all incoming signals thus increasing the stability of clock recovery is presented. >

Synchronization techniques for all digital 16-ary QAM receivers operating over land mobile satellite links

Abstract: The performance of a low bit rate (64 Kb/s) all digital 16-ary Differentially Encoded Quadrature Amplitude Modulation (16-DEQAM) demodulator operating over a mobile satellite channel, is considered. The synchronization and detection techniques employed to overcome the Rician channel impairments, are described. The acquisition and steady state performance of this modem, are evaluated by computer simulation over AWGN and RICIAN channels. The results verify the suitability of the 16-DEQAM transmission over slowly faded and/or mildly faded channels.

Coded QAM using a binary convolutional code

Abstract: The motivation is to utilize commercially available high-speed, low-cost, single-VLSI-chip binary-encoding/Viterbi-decoding systems (codecs). A Gray code is used to map the encoder output to the QAM constellation, transforming the large Hamming distance (HD) of the binary code into a large squared Euclidian distance of (SED) of CQAM and providing the basis for a simple, effective way to generate the binary metrics used for binary Viterbi decoding. Although the scheme may be generalized to any 2/sup 2k/-QAM, the discussion focuses on the design of 16-ary CQAM with a rate-3/4 punctured convolutional code of constraint length 7. A binary metric generation method is used so that no modification of the existing VLSI codec chips is required. This method, in conjunction with eight-level quantization for the binary metric, is shown to be suboptimum with little loss in signal-to-noise ratio compared to direct use of M-ary unquantized metric. Since the overall system is nonlinear, both worst-case and average performance are considered. Analytical and Monte Carlo simulation results are presented. >

Performance of trellis coded modulation using multi-frequency channels in land mobile communications

Abstract: A multifrequency channel transmission method for trellis-coded quadrature amplitude modulation (QAM) that uses an interleaving function to improve bit error rate (BER) performance under Rayleigh fading environment conditions is described. A multifrequency channel transmission method for trellis-coded modulation (TCM) that uses interleaving is proposed to distribute the burst error that occurs in the time and frequency domains, and its performance is examined by computer simulation. Computer simulation shows that this method should be useful for land mobile communications. >

Polyphase FFT filter bank for QAM data transmission

Abstract: Polyphase fast Fourier transform (FFT) filter banks for quadratic amplitude modulation (QAM) data transmission are proposed A previously proposed multiple carrier data transmission method is discussed along with its transient response The transient response of the proposed polyphase filters is represented in a two-dimensional mesh plot with high resolution Experimental responses are shown and discussed >

Performance comparison of LMS, diagonalizer and bootstrapped adaptive cross-pol cancellers for M-ary QAM

Abstract: In microwave radio communication networks, the transmission capacity can be doubled by using orthogonally polarized quadrature amplitude modulated (QAM) carriers. The orthogonally polarized waves can suffer degradation due to carrier-to-cross-polarization interference (C/XPI). Particularly during multipath fading, such degradation would become intolerable. Different canceller structures are proposed to mitigate the effect of cross-polarization. Among these are the diagonalizer, the LMS, and the bootstrapped cancellers. The bootstrapped canceller has been implemented in different applications such as satellite dual-polarized communication, tactical communication, and QAM microwave radio. The probability of error performance of the bootstrapped cross-polarization canceller (BXPC) is studied and compared to the performance of other cancellers. The performance of the three cancellers is compared numerically for the 16 QAM with r=-10 dB, and -5 dB. It is shown that the LMS canceller outperforms the diagonalizers, particularly when the cross-coupling is high. Except for a very low signal-to-noise ratio, the BXPC outperforms the LMS with 64 QAM, more that with 16 QAM. >

150 KGate general-purpose high-coding-gain TCM VLSIs for high-speed multi-level QAM systems

Abstract: Multilevel trellis-coded QAM (quadrature amplitude modulation) VLSIs were designed and developed These VLSIs realize a coding gain of 79 dB with SPORT-256 QAM The VLSIs contain an encoder, a mapper, and a decoder for trellis-coded 16, 32, 64, 128, and 256 QAM utilizing general mapping The VLSIs also function as a phase ambiguity rejection circuit for a recovered carrier and error pulse generator Using small-size and low-power path memory, 150K gates are achieved utilizing two chips with die sizes of 8*11 mm/sup 2/ and 14*14 mm/sup 2/ The VLSIs realize maximum data throughput rates of 17 Mb/s (guaranteed) and 30 Mb/s The devices are designed with BiCMOS 08 mu m, utilizing the HSL-FX language for functional design and ALPHA2 for layout design schemes Experimental results on bit error rate, signature performance, and synchronization performance for phase ambiguity rejection are presented The developed VLSIs are effective for outage-free digital microwave radio systems >