Showing papers on "Quadrature amplitude modulation published in 1986"

Continuous phase modulation

Abstract: M inimum Shift Keying (MSK) [ 11 or Fast Frequency Shift Keying (FFSK) is a digital modulation technique with constant amplitude which has been studied extensively during recent years. The properties of MSK are now understood [2-51. In this article, we report on methods to improve on MSK while maintaining a constant amplitude. By improvement we mean a narrower power spectrum, lower spectral sidelobes, better error probability, or all of the above. The cost of signal processing and the speed and complexity with which it can be performed has steadily been improved over recent years. Further improvements are to be expected. Therefore, it is reasonable to study what can be accomplished with methods that might be too complex for cost effective realization today, but perhaps may be easily achievable tomorrow. In this paper, we consider a number of methods for constructing constant amplitude signals which significantly outperform MSK [5-1031. We also discuss at what level of complexity these improvements are obtained and realized.

Advanced Groupband Data Modem Using Orthogonally Multiplexed QAM Technique

Abstract: An advanced groupband data modem based on the orthogonally multiplexed QAM technique is proposed. Its performance is evaluated through experiments. In the proposed modem, data are transmitted by a number of orthogonally multiplexed parallel subchannels. Because of this orthogonally multiplexing parallelism, the proposed modem becomes more immune to Gaussian noises, impulsive noises, and channel distortions, compared to the conventional Class IV partial response VSB modem. Furthermore, the orthogonally multiplexed QAM modem retains high flexibility in multiplexing a variety of data terminals and in carrying out fallback operation to a wide range of data rates. Experimental results demonstrate the robustness of the proposed groupband data modem.

Calibration of vector modulators using a scalar detector

Abstract: Calibration of a vector modulator is done using a scalar detector to measure the amplitude of the RF output signal, phase shifters to adjust the relative phases of the I and Q components of the RF carrier, and variable attenuators in the I and Q modulation signal input lines to adjust the relative amplitude of the modulation signals. DC signal sources provide reference signals for the I and Q modulation inputs, carrier leak compensation signals, and calibration signals for balancing the amplitude of the I and Q modulation signals. An iterative four step calibration process is followed until no change in the results is observed. The quadrature phase error is minimized by adjusting the phase shifters. The carrier leakage is minimized by adjusting the carrier leak compensation sources to minimize RF output with the modulation inputs grounded. The amplitudes of the I and Q modulation signals are balanced by adjusting the attenuators until the output amplitudes are equal. Finally, the quadrature calibration signal sources are adjusted until the output amplitudes they produce are balanced.

Coded Phase/Frequency Modulation

Abstract: Channel coding combined with expanded signal sets has been shown [1] to improve error performance over uncoded modulation without expanding the bandwidth of the transmitted signals. In this paper, new coded modulation formats defined over an expanded set of signals varying both in phase and frequency are presented. The new schemes combine FSK and PSK modulation and make use of trellis coding and Viterbi decoding to improve error performance over uncoded modulation. The free Euclidean distance is calculated for several classes of codes, and upper bounds and simulation results are also presented for some simple codes. The spectral characteristics of the proposed coded modulation formats are evaluated and compared to conventional two-dimensional modulation formats. Differential encoding and various extensions of the basic scheme are also discussed.

256 QAM Modem for High Capacity Digital Radio System

Abstract: This paper presents topics related to the 256 QAM modem for high capacity digital microwave radio. Specifications for modem circuitry are derived from the allowable carrier-to-noise ratio degradation point of view. In this process, various degradation factors are categorized so that the same evaluation is possible. Two representative circuits, the automatic threshold control (ATC) and carrier recovery circuits, of particular importance for developing a 256 QAM modem, are presented. For the ATC circuit, the operation principle as well as the presence of the false-lock phenomenon and its evasion method are described. The prototype 256 QAM modem performance indicates the prospect of a 400 Mbit/s 256 QAM modem.

Self-amplitude modulation in PSK coherent optical transmission systems

Abstract: Self-amplitude modulation due to group velocity dispersion (chromatic dispersion of an optical fiber) in phase-shift keying (PSK) coherent optical transmission systems is presented. It may place a serious limitation on performance of such systems by limiting the maximum transmission distance attainable at a given bit rate. The maximum transmission distance for any prescribed degree of self-amplitude modulation is derived for PSK coherent systems. The result shows that the distance over which appreciable amplitude modulation is impressed on a phase-modulated signal is considerably less than the dispersion-limited transmission distance for intensity modulated direct detection systems.

8/10 modulation codes for digital magnetic recording

Abstract: It is difficult to record DC and low-frequency signals with the magnetic recording method. Furthermore, in high-density magnetic recording, the signal-to-noise ratio and the high-frequency output level are low and low-frequency crosstalk noise from the adjacent tracks is relatively high. A modulation code for high-density digital magnetic recording must have a large Tw and a DC-free characteristic. When we developed the R-DAT system, we developed two run-length limited 8/10 conversion rate modulation codes for use with the R-DAT. This paper discusses various possible modulation codes and confirms the superiority of one particular 8/10 modulation code.

Optical communication system utilizing phase modulation

Abstract: An optical communication system is disclosed comprising a transmission assembly including a semiconductor laser amplifier. The laser amplifier superimposes onto an amplitude modulated signal a phase modulation corresponding to the amplitude modulation. The optical communication system may further include a separation device for filtering out the amplitude modulation. A receiving assembly is provided for detecting the phase modulation of a transmitted signal.

Digital quadrature amplitude modulator

Abstract: In a quadrature amplitude modulator, two binary, synchronous data signals representing four possible phase states of a carrier signal are sampled and respectively fed into two shift registers comprising part of a finite impulse response lowpass filter. The samples in each corresponding pair of stages in the shift registers are multiplexed at twice the carrier frequency. Each multiplexer output is then added modulo-two to a binary signal at the carrier frequency using exclusive-OR gates. The gate outputs are weighted in a desired fashion and summed and converted to an analog signal which exhibits a preferred spectral shape. This approach enables the quadrature amplitude modulation function to be included within the structure of the finite impulse response lowpass filter. The binary synchronous nature of the input data enables the structure of the filter to be simplified and the number of multiplexers and modulo-two adders to be reduced to a number equal to the number of bits stored from the input sequences. An extension of this basic concept includes higher levels of encoding, specifically the case of sixteen or more possible phase and amplitude states of a carrier signal. The particular method for weighting and summing the multiplexed and modulated signals may utilize digital logic combined with resistors, switched capacitors, or a stored program combined with a digital-to-analog converter, and is suitable for integrated circuit fabrication.

Direct frequency modulation of vapor phase transported, distributed feedback semiconductor lasers

Abstract: The frequency modulation (FM) due to injection current modulation of vapor phase transported distributed feedback (VPT DFB) semiconductor lasers is measured as a function of modulation frequency from 10 kHz to 1 GHz. A large frequency modulation response of 350 MHz/mA is obtained for the modulation frequency range of 10 to 1000 MHz. Demodulation of optical frequency shift keying (FSK) at 560 Mb/s is demonstrated, indicating that frequency modulation due to thermal modulation does not pose a significant limitation at this bit rate or higher. The large FM response together with modulation bandwidths up to 8 GHz makes the VPT DFB laser an attractive source for high bit rate optical FSK transmission.

Further Results on Adaptive Equalizer Improvements for 16 QAM and 64 QAM Digital Radio

Abstract: Channel distortions due to multipath fading are a principal source of outage for high-capacity digital radio. Time domain equalization is increasingly becoming an effective means to compensate for fading induced ISI. This paper compares the performance of transversal and decision feedback equalizers used for 16 QAM and 64 QAM digital radio systems. The effect of tap spacing is examined by comparing the results of baud period spacing with fractionally spaced equalizers. The performance measure used is peak distortion. This measure is plotted against various fade parameters for the different equalizers studied, the two modulation schemes, forms of timing recovery, and the effects of joint operation of demodulation and symbol synchronization circuitry. Results clearly indicate, in terms of better signature curves, the superiority of fractionally spaced equalizers over synchronous equalizers. An additional advantage is the robustness of FSE's to timing phase errors. A decision feedback equalizer with fractional spacing on the feedforward part offers the best performance. It is shown that the performance of a DFE synchronized to the first precursor is significantly improved as compared to the conventional case of synchronizing with the main sample of the received pulse. With the new position of reference (first precursor), fractional DFE's benefit more than their T -spaced counterpart.

Minimum Mean-Square Error Timing Recovery Schemes for Digital Equalizers

Abstract: Two algorithms are presented for optimum timing recovery in digitally implemented equalizers. The first one is a polarity-type algorithm based on the conventional minimum mean-square error criterion. A theoretical analysis is made to characterize the algorithm phase detector and evaluate its steady-state phase jitter variance. Influence of various channel and system design parameters on the algorithm performance is illustrated using phase jitter probability densities obtained by means of computer simulations. Interaction of the algorithm with decision-directed carrier recovery is also examined. It is shown that interaction with carrier recovery may considerably degrade the timing acquisition performance, and a second algorithm is then presented which eliminates this interaction. The second algorithm is based on the minimization of a modified mean-square error criterion which provides a measure of the intersymbol interference, independently of the carrier phase. Decision-directed timing and carrier recoveries are thus decoupled and the system startup period is considerably reduced. Phase detector characteristic and steady-state jitter performance of the second algorithm are evaluated by analytical means and computer simulations, as in the first algorithm.

On the Performance of Combined Quadrature Amplitude Modulation and Convolutional Codes for Cross-Coupled Multidimensional Channels

Abstract: The performance of cross-coupled, M -ary quadrature amplitude modulation (QAM) systems is determined when bandwidth efficient trellis codes are used to combat interference. Performance with and without compensation for cross-coupled interference is presented. It is found that simple trellis codes can maintain the error probability at an acceptable level for cross-coupling parameters that render uncoded systems unusable. Up to two-dimensional trellis codes are considered for four-dimensional QAM signals, and possibilities of obtaining diversity advantages in the form of higher total system throughput by prolonged availability of the two signals are explored. This is accomplished through joint coding over two different constellations. The probability of the most likely error events is calculated by using the method of moments. The results are applicable to any digital communication system using multidimensional quadrature amplitude modulation, e.g., voiceband modems, cross-polarized radio systems and, to some extent, optical systems. In the paper the analysis is restricted to nondispersive cross-coupling models. In most cases the coding gain is larger than in the absence of cross-coupling interference. Specifically, it is found that simple codes have coding gains increased by at least 2 dB with cross-coupling interference relative to that obtained on the additive white Gaussian noise channel.

Differential phase shift keying demodulator

Abstract: A differential phase shift keying demodulator for demodulating a signal modulated by differential phase shift keying is arranged to compare the modulated signal with a delayed modulated signal at different points of time within a period required for transmitting one bit of data.

Performance of Receivers in Digital Radio Applications Operating in the Presence of Noise and Jamming

Abstract: The performance of digital communication systems operating in the presence of noise and jamming is analyzed and evaluated. Specifically, by modeling the jamming as additive colored Gaussian noise, and considering transmission via M-ary phase shift keyed (MPSK) modulation as well as Quadrature Amplitude Modulation (QAM), receiver performance is determined in terms of symbol error probability, P s . The receiver analyzed is optimum for the modulation used when the channel interference consists of additive white Gaussian noise (AWGN) only, and does not process signals utilizing spread spectrum modulation or forward error correction schemes. Furthermore, the derived results for P s are used in order to optimize the shape of the colored noise (jamming) spectrum so as to cause maximum receiver degradation, subject to a jamming power constraint. Results on numerical evaluations are presented graphically, thus displaying receiver vulnerability to a specific form of jamming.

Bandwidth Compressive 16-State SQAM Modems Through Saturated Amplifiers.

Abstract: The performance of new bandwidth compressive 16 SQAM (Superposed-QAM) modems, operating in a highly power-efficient saturation mode, is analyzed. Nonlinearly amplified 16 SQAM retains the out-of-band energy much lower than 4-level rectangular (i.e., 16 QAM) or sinusoidal shaped (i.e., MAMSK) QAM, and operates within 0.7 dB of theoretical (i.e., 16 QAM in Nyquist linear channel) performance at P(e) = 10^{-6} using simple 4th-order Butterworth LPF's. Effects of baseband pulse shaping, receive filter BT s , power level variations and propagation time difference (or static phase shift) of transmit HPA's are analyzed in terms of BER.

Automatic gain control circuit

Abstract: An AGC circuit for controlling an input level of a multi-level discriminatorto an optimum one and a transversal equalizer capable of fully exhibiting an equalizing function thereof, each being installed in a digital demodulation system applicable to a multi-level communication system. The AGC circuit eliminates false pull-in to promote stable pull-in operations and is applicable not only to baseband signals having two or more levels but also to a 16 QAM system in microwave digital communication. Even when a demodulator is in an asynchronous state and input signals have significant intersymbol interference, the demodulation system is capable of being restored to normal to allow the equalizer to fully exhibit its capability.

Transmit and receive filters for QPSK signals to optimise the performance on linear and hard-limited channels

Abstract: Bandlimited offset QPSK systems are analysed and attempts are made to find new combinations of transmit and receive filters which give improved performance trade-offs when compared with recently proposed filters such as quadrature overlapped raised-cosine (QORC) and squared raised-cosine (QOSRC), and superposed quadrature amplitude modulation (SQAM). The filters are based on finite impulse responses of up to five symbol periods (ten bit periods) duration. The results indicate that advantages can be gained over QORC, QOSRC and SQAM, if a good performance with linear channels is required as well as low degradation under hard limiting. This probably also implies good performance under soft limiting. The results are presented graphically, showing how the bit error rate performance with each filter pair degrades as the adjacent channel spacing of the system is reduced.

Performance of 64-QAM, 225-QPRS and 256-QAM in a cochannel interference environment

Abstract: The performance of 64-QAM (quadrature amplitude modulation), 225-QPRS (quadrature partial-response signalling) and 256-QAM in the presence of cochannel interference (CCI) and thermal noise is evaluated. It is found that the amplitude-modulated CCI causes more degradation than the single sinusoidal CCI. Measured data of a 256-QAM modem in a CCI environment show good agreement with the simulated results.

Quadrature amplitude modulation/demodulation device using multi-level digital signals

Abstract: A modulation device using multi-level digital signals includes: a pair of balanced mixers for carrying out quadrature amplitude modulation to deliver a quadrature amplitude modulated signal; input and output hybrid circuits, connected to carrier input terminals of the pair of balanced mixers, for supplying carrier inputs to the pair of balanced mixers; and a modulation characteristic compensation unit connected with input terminals of the pair of balanced mixers for supplying the pair of balanced mixers with orthogonal baseband input signals. A demodulation device with a use of multi-level digital signals including: a pair of balanced mixers for carrying out quadrature amplitude demodulation to deliver a pair of quadrature amplitude demodulated signals; first and second input hybrid circuits; and a demodulation characteristic compensation unit connected with output terminals of the pair of balanced mixers for deriving demodulation characteristic compensated orthogonal baseband signals from the pair of quadrature amplitude demodulated signals.

Method for the analogous or digital coding of information for use in angle and pulse modulation processes

Abstract: During the coding and transmission of analog or digital information, it is important to transmit this information reliably and economically, that is to say with narrow bandwidth. So that the transmission paths are better utilised, multi-value coding and/or pulse-modulation methods have therefore been developed. The multi- value coding could only be used reliably and with supportable expenditure up to 16 QAM (quadrature amplitude modulation) or up to 16 PSK (phase-shift keying modulation). In the case of the invention, the reliability of transmission is increased and the expenditure is reduced by changing the phase in the code-changing current in such a manner that an uninterrupted sequence of positive and negative halfwaves is produced (Figure 17a,b, 18a,b) and/or that transmission occurs on the basis of angle (Figure 28) or pulse modulation in a narrow-band manner or only with a code-changing current of one frequency, the code states being defined by the amplitude magnitude and/or phase steps.

Performance of 12PM3 modulation methods in digital land mobile radio with discrimination detection

Abstract: The performance of a number of narrowband continuous-phase modulation methods, belonging to the 12PM3 class (12-state, Phase Modulation with correlation over 3 consecutive bits), has been compared with reference to both AWGN (Additive White Gaussian Noise) channels and the typical propagation conditions envisaged for SCPC/FDMA digital land mobile radio systems in urban environment. Demodulation has been performed by frequency discrimination using at the receiving end pre- and post-discrimination filters optimized for the best performance. Different decision techniques have been considered, namely 2-TH (2-Threshold), 4-TH, mixed 2-TH/4-TH and MLSE (Maximum Likelyhood Sequence Estimation) techniques.

Digital demodulation system

Abstract: An AGC circuit for controlling an input level of a multi-level discriminator to an optimum one and a transversal equalizer capable of fully exhibiting an equalizing function thereof, each being installed in a digital demodulation system applicable to a multi-level communication system. The AGC circuit eliminates false pull-in to promote stable pull-in operations and is applicable not only to baseband signals having two or more levels but also to a 16 QAM system in microwave digital communication. Even when a demodulator is in an asynchronous state and input signals have significant intersymbol interference, the demodulation system is capable of being restored to normal to allow the equalizer to fully exhibit its capability.

Theoretical Evalution of Signatures and CNR Penalties Caused by Modem Impairments in Multilevel QAM Digital Radio Modems

Abstract: A calculation method for symbol error probability is proposed for multilevel QAM systems. Carrier-to-noise ratio penalties are estimated by this method for seven typical impairment factors, including recovered carrier phase error, timing error, receive filter residual delay, etc. The validity of the method is verified by experiments on penalties caused by recovered carrier phase error and timing error. Based on the new method of analysis, signatures of a 64 QAM modem are estimated theoretically, using a pseudo-two-ray model of multipath fading. The dependence of signatures on the above-mentioned impairment factors is investigated in detail. A qualitative deduction of impairment factors from a measured signature is demonstrated. The measured signature agrees well with a calculated one in which reasonable values of the impairment factors are assumed.

MSE Adaptive Baseband Canceller Performance in Dual-Polarized 16-QAM Systems

Abstract: The application of an adaptive baseband canceller to dual-polarized 16-QAM systems operating in the presence of moderate to severe crosstalk is considered Concentrating on a method which minimizes the mean square error in the estimation of received symbol values, a computational method is presented by means of which exact values of symbol error probability, P e , can be easily obtained for any specific values of relative crosstalk phase shift Using this method, results are obtained which predict P e versus crosstalk and signal-to-noise levels for uniform distributions of the crosstalk phase shift angles Results are given which show the frequency of occurrence of different P e values for uniformly distributed crosstalk phase shifts, and which show the maximum, minimum, and average values of P e over all values of relative phase angles The receiver performance is seen to be acceptable, so far as average P e is concerned, in the presence of crosstalk levels that would make the system totally inoperable if no canceller were used However, it is noted that for high crosstalk levels, values of P e can have a range of 100:1 or more as different values of relative phase shift are encountered