Showing papers on "Quadrature amplitude modulation published in 1996"

Variable-rate variable-power MQAM for fading channels

Abstract: We propose a variable-rate and variable-power MQAM modulation scheme for high-speed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then derive the spectral efficiency of our proposed modulation. We show that there is a constant power gap between the spectral efficiency of our proposed technique and the channel capacity, and this gap is a simple function of the required bit-error rate (BER). In addition, using just five or six different signal constellations, we achieve within 1-2 dB of the maximum efficiency using unrestricted constellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequency for these constellation sets. We also obtain the exact efficiency loss for smaller constellation sets, which may be required if the transmitter adaptation rate is constrained by hardware limitations. Our modulation scheme exhibits a 5-10-dB power gain relative to variable-power fixed-rate transmission, and up to 20 dB of gain relative to nonadaptive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conclude with a discussion of coding techniques and the relationship between our proposed modulation and Shannon capacity.

OFDM channel estimation by singular value decomposition

Abstract: We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).

Analysis of the effect of impulse noise on multicarrier and single carrier QAM systems

Abstract: The paper first shows the equivalence of the Bernoulli-Gaussian impulse noise model in the discrete time domain to the continuous-time model of Poisson arriving delta functions with random area distributed according to the power Rayleigh probability density function. This equivalence is then used to develop a closed form expression for the probability of error for single carrier QAM that is easily evaluated. Furthermore, the performance of multicarrier modulation (MCM) is also analyzed using the same impulse noise model and it is shown that in most cases MCM performs better than single carrier systems, specifically when the probability of an impulse is not too high and the impulse power is moderate.

CD3-OFDM: a novel demodulation scheme for fixed and mobile receivers

Abstract: This paper describes a novel channel estimation scheme identified as coded decision directed demodulation (CD3) for coherent demodulation of orthogonal frequency division multiplex (OFDM) signals making use of any constellation format [e.g., quaternary phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM]. The structure of the CD3-OFDM demodulator is described, based on a new channel estimation loop exploiting the error correction capability of a forward error correction (FEC) decoder and frequency and time domain filtering to mitigate the effects of noise and residual errors. In contrast to the conventional coherent OFDM demodulation schemes, CD3-OFDM does not require the transmission of a comb of pilot tones for channel estimation and equalization, therefore yielding a significant improvement in spectrum efficiency (typically between 5-15%). The performance of the system with QPSK modulation is analyzed by computer simulations, on additive white Gaussian noise (AWGN) and frequency selective channels, under static and mobile reception conditions. For convolutional coding rate 1/2, the results indicate that CD3-OFDM allows one to achieve a very fast adaptation to the channel characteristics in a mobile environment (maximum tolerable Doppler shift of about 80 Hz for an OFDM symbol duration of 1 ms, as differential demodulation) and an E/sub b//N/sub 0/ performance similar to coherent demodulation (e.g., E/sub b//N/sub 0/=4.3 dB at bit-error rate (BER)=2/spl middot/10/sup -4/ on the AWGN channel). Therefore, CD3-OFDM can be suitable for digital sound and television broadcasting services over selective radio channels, addressed to fixed and vehicular receivers.

OFDM codes for peak-to-average power reduction and error correction

Abstract: Orthogonal frequency division multiplexing (OFDM) is a promising way to provide large data rates at reasonable complexity in wireless fading channels. However, a major disadvantage of OFDM is its large peak-to-average power ratio, which significantly decreases the efficiency of the transmitter power amplifier and hence forms a major obstacle to implementing OFDM in portable communication systems. This paper shows the possibility of using complementary codes for both decreasing the peak-to-average power (PAP) ratio and error correction. Set sizes and minimum distance properties of these codes are derived. It is shown that specific subsets of complementary codes have a minimum distance of up to half the code length, while their PAP ratio is only 3 dB. Simulation results demonstrate the viability of using these codes in multipath fading channels. It is currently planned to implement OFDM with complementary codes in the Wireless ATM Network Demonstrator (WAND), a joint European ACTS program.

Parallel concatenated trellis coded modulation

Abstract: In this paper, we propose a new solution to parallel concatenation of trellis codes with multilevel amplitude/phase modulations and a suitable bit by bit iterative decoding structure. Examples are given for throughput 2 and 4 bits/sec/Hz with 8 PSK, 16 QAM, and 64 QAM modulations. For parallel concatenated trellis codes in the examples, rate 2/3 and 4/5, 8, and 16-state binary convolutional codes with Ungerboeck mapping by set partitioning (natural mapping), a reordered mapping, and Gray code mapping are used. The performance of these codes is within 1 dB from the Shannon limit at a bit error probability of 10/sup -7/ for a given throughput, which outperforms the performance of all codes reported in the past for the same throughput.

RF power amplifier linearization through amplitude and phase predistortion

Abstract: This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other well-known predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity.

Pilot spacing in orthogonal frequency division multiplexing systems on practical channels

Abstract: The performance of two different channel interpolation methods to be used with orthogonal frequency division multiplexing systems are investigated. The considered schemes use constant pilot frequencies for channel response estimation. The interpolation techniques are piecewise-constant and piecewise-linear methods which due to their inherent simplicity are straightforward to implement. The results are given as error probability vs. pilot separation for a channel with exponential type power-delay profile and M-ary quadrature amplitude (MQAM) submodulation with M=4, 16 and 64.

Full-range, continuous, complex modulation by the use of two coupled-mode liquid-crystal televisions

Abstract: Switchable, continuous, complex-amplitude modulation is demonstrated with two cascaded, twisted nematic liquid-crystal televisions (LCTV's), both operating in phase- and amplitude-coupled modulation modes The condition for full-range complex modulation is that one of the LCTV's must provide a 2π-range phase modulation A look-up table encoding method is proposed that permits the compensation of phase-amplitude coupling and nonlinearity in the two individual LCTV modulations Experimental techniques for determining the LCTV-device parameters, for maximizing the phase-mostly modulation range and the amplitude-mostly modulation contrast, and for testing the complex-amplitude modulation are developed Optical complex-amplitude Fresnel holograms are shown

A novel bandwidth efficient coding scheme employing turbo codes

Abstract: We have investigated a novel bandwidth efficient channel coding scheme that has a code structure similar to binary turbo codes but employs Ungerboeck codes as component codes. The combination of turbo codes with powerful bandwidth efficient component codes leads to a straightforward encoder structure and allows iterative decoding in analogy to the binary turbo decoder. However, certain special conditions need to be met at the encoder, and the iterative decoder needs to be adapted to the decoding of the component Ungerboeck codes. The scheme has been investigated for 8 PSK and 16 QAM modulation, for which a first attempt at component code optimization was performed. Simulation results are presented and the scheme is compared with Ungerboeck codes on their own and also turbo codes with Gray mapping. The results show that the novel scheme is markedly superior at comparable complexity.

Intrinsic envelope fluctuations and modulation-detection thresholds for narrow-band noise carriers.

Abstract: A model is presented which calculates the intrinsic envelope power of a bandpass noise carrier within the passband of a hypothetical modulation filter tuned to a specific modulation frequency. Model predictions are compared to experimentally obtained amplitude modulation (AM) detection thresholds. In experiment 1, thresholds for modulation rates of 5, 25, and 100 Hz imposed on a bandpass Gaussian noise carrier with a fixed upper cutoff frequency of 6 kHz and a bandwidth in the range from 1 to 6000 Hz were obtained. In experiment 2, three noises with different spectra of the intrinsic fluctuations served as the carrier: Gaussian noise, multiplied noise, and low-noise noise. In each case, the carrier was spectrally centered at 5 kHz and had a bandwidth of 50 Hz. The AM detection thresholds were obtained for modulation frequencies of 10, 20, 30, 50, 70, and 100 Hz. The intrinsic envelope power of the carrier at the output of the modulation filter tuned to the signal modulation frequency appears to provide a good estimate for AM detection threshold. The results are compared with predictions on the basis of the more complex auditory processing model by Dau et al. [J. Acoust. Soc. Am. 99, 3615–3622 (1997)].

Wireless transmission using fiber link

Abstract: A wireless transmission and reception system includes a mixing section using a phase shift keying or quadrature amplitude modulation scheme to modulate a baseband signal, representing voice or data, onto a carrier frequency within the range of standard (cable television format) AM-VSB frequencies. A combiner section combines the modulated carrier signal into a channel of a multiplexed cable television (CATV) signal having two or more channels. The CATV signal is used to modulate a laser source. The laser source generates a laser signal based on the CATV signal. The laser signal is optically coupled into a bidirectional fiber link. The fiber link transmits the laser signal and ultimately converts the laser signal into the electrical CATV signal. A splitting section coupled to the bidirectional fiber link splits the modulated carrier signal from the CATV signal. A demodulating section coupled to the splitting section recovers the baseband signal from the modulated carrier signal. The baseband signal is remodulated onto a carrier signal having a designated wireless frequency and is broadcast into free space using an antenna. Signals received by the antenna from free space are amplified and passed through the bidirectional fiber link.

Performance of component-interleaved signal sets for fading channels

Abstract: The authors calculate the exact (pairwise) error probability for an arbitrary component-interleaved 2-D constellation over the Rayleigh fading channel. Using this result, they improve the accuracy of the performance analysis at high error rates. For illustration, this is applied to signal sets derived from 4-PSK and 16-QAM over the Rayleigh fading channel.

Optimization of symbol timing recovery for QAM data demodulators

Abstract: This paper describes an effective technique for the optimization of the clock recovery circuit in an all-digital modem for linearly modulated signals. Starting from the concept of prefiltering of the data signal (already pursued by these authors in the context of analog data receivers), it is shown how to design an optimum digital prefilter for the minimization of jitter due to both Gaussian and pattern noise in the closed-loop clock recovery scheme by Gardner (1986). The numerical results of the theoretical analysis, obtained after iterative resolution of a constrained-minimum problem via the Lagrange multiplier method, are checked by simulation and can be nicely justified by the consideration of the frequency response of the optimum prefilter. The key outcome of such an approach is the demonstration of a substantial performance improvement in terms of steady-state clock jitter, even with remarkably simple FIR prefilters with a small number of taps.

Transmission performance analysis of Multi-Carrier Modulation in frequency selective fast Rayleigh fading channel

Abstract: In this paper, we discuss the transmission performance of Multi-Carrier Modulation (MCM) in frequencyselective fast Rayleigh fading channels First, we optimize the transmission parameters of MCM withM-ary differential phase shift keying/differential detection (DPSK):the guard duration andthe number of sub-carriers for frequency-selective fast Rayleigh fading channels, and then show the bit error rate (BER) performance of the optimizedM-ary DPSK MCM Next, we propose an MCM with pilot-assistedM-ary quadrature amplitude modulation/coherent detection (QAM), and discuss the BER performance when we reduce the number of pilot signals from the view-point of frequency-time utilization efficiency Finally, we propose a two-stage frequency offset compensation method

The error probability of M-ary PSK block coded modulation schemes

Abstract: A tight upper bound on the decoding error probability is derived for block-coded modulation structures where an M-ary phase shift keying (M-PSK) signal constellation is employed. This bound, called a tangential sphere bound, is tight for very low (as well as for high) signal-to-noise ratios (SNRs). Berlekamp's tangential union bound, previously derived for binary codes, can be derived for an M-PSK block coded modulation structure as well. However, it is proven that our tangential sphere bound is tighter than Berlekamp's (1980) tangential bound. For particular schemes, it is shown that for low SNRs our bound is considerably tighter than the tangential bound. As one of the examples, a multistage decoder is considered.

Blind phase recovery in QAM communication systems using higher order statistics

Abstract: We present a method for phase recovery in QAM communication systems based on higher order statistics. Under the assumption that the QAM signals have independent, identically distributed (i.i.d.) in-phase and quadrature components, we derive the relation between the phase error and the fourth-order cumulant of the output. Numerical performance of the approach is demonstrated by computer simulation.

Decision feedback differential detection of differentially encoded 16APSK signals

Abstract: Multiple-symbol differential detection (DD) with reference signal estimation based on the feedback of past detected symbols is presented for differentially encoded 16-level amplitude/phase shift keying (16DAPSK). A suboptimal decision is derived. The approximate analysis of the bit-error rate (BER) performance taking into account the decision error propagation effect is presented in an additive white Gaussian noise (AWGN) channel and the BER performance is compared with those of 16DPSK and 16 quadrature amplitude modulation (16QAM).

Bit-error-rate performance of lightwave hybrid AM/OFDM systems with comparison with AM/QAM systems in the presence of clipping impulse noise

Abstract: Theoretical analysis is presented for the BER performance of hybrid AM/OFDM systems in the presence of clipping impulse and Gaussian noise. In particular, the OFDM BER has been compared with that of conventional QAM. It is found that, due to the dilution effect of the subchannels, the impact of the clipping impulse noise on BER for OFDM systems is virtually zero for reasonably large laser modulation depths. Therefore, OFDM is more suitable to the lightwave transmission of digital video.

Multicarrier coherent communications for the underwater acoustic channel

Abstract: We propose a multicarrier transmission scheme for severely spread channels-but not overspread. The underwater acoustic channel is a good representative of this kind of channel. The technique of orthogonal data and pilot symbols allows coherent detection and therefore the use of QAM signal constellations. Some simulated performances of the proposed scheme are presented in both Rayleigh and Rician multipath fading. Acceptable error rate floors are shown to be possible for channel spread factor reaching 0.15.

On the uniform ADC bit precision and clip level computation for a Gaussian signal

Abstract: The problem of computing the required bit precision of analog-to-digital converters is revisited with emphasis on Gaussian signals. We present two methods of analysis. The first method fixes the probability of overload and sets the dynamic range of the quantizer to accommodate the worst-case signal-to-quantization noise ratio (SQNR). The second method sets the clipping level of the quantizer to meet a desired overload distortion level, using knowledge of the input probability density function. New closed-form expressions relating the distortion-minimizing clip level of the uniform quantizer and the input bit rate are derived and shown to give remarkably close results to the optimum ones obtained using numerical iterative procedures devised elsewhere.

DVB-T: the COFDM-based system for terrestrial television

Abstract: In early 1996, the European DVB Project finalized what is now called the “Common 2k/8k Specification”. This specification is another important corner-stone of digital video broadcasting in Europe and numerous other countries in the world. The terrestrial system, termed DVB-T, is based on the channel-coding algorithms the DVB Project had earlier devised for digital television on cable and satellite. The modulation scheme that is used for DVB-T is OFDM (orthogonal frequency division multiplexing), which has been assigned the name of COFDM in conjunction with the channel coding. Two variants are described, one employing 1705 individual carriers in an 8 MHz channel (“2k variant”), the other employing 6817 carriers (“8k variant”). For each of the variants four specific types of guard interval have been defined. Each individual carrier can be modulated either by QPSK, 16QAM, 64-QAM or hierarchical modulation.

Performance evaluation of OFDM transmission with conventional and two-branch combining power amplification schemes

Abstract: It is recognized that OFDM schemes (orthogonal frequency division multiplexing) offer many advantages for signalling at high data rates over time dispersive channels. However, they exhibit strong envelope fluctuations, which is considered to be an important drawback over many single-carrier schemes, since the inherently high peak-to-mean envelope power ratio leads to heavy power amplification requirements. We present a new approach to the performance evaluation of OFDM transmission with nonlinear power amplification. This approach can be employed for both conventional, single-branch, amplification and two-branch combining amplification, whenever a high number of subcarriers is assumed, and relies on the decomposition of the power amplified signals into uncorrelated "useful" and "self-interference" components. A simple analytical method is presented for evaluating BER performances, within the proposed approach, for a selected OFDM/QAM multichannel system. The analytical results are in good agreement with simulation results and allow simple optimization of the operating points for the power amplification within the system, as well as simple comparisons of the several amplification schemes.

SER Analysis of QAM with Space Diversity in Rayleigh Fading Channels

Abstract: This paper derives the symbol error probability for quadrature amplitude modulation(QAM) with L-fold space diversity in Rayleigh fading channels. Two combining techniques, maximal ratio combining(MRC) and selection combining(SC), are considered. The formula for MRC space diversity is obtained by averaging the symbol error probability of M-ary QAM in an additive white Gaussian noise(AWGN) channel over a chi-square distribution with 2L degrees of freedom. The obtained formula overcomes the limitations of the earlier work, which has been limited only to deriving the symbol error rate(SER) of QAM with two branch MRC space diversity. The formula for SC space diversity is obtained by averaging the symbol error probability of M-ary QAM in an AWGN channel over the distribution of the maximum signal-to noise ratio among all of the diversity channels for SC space diversity has been reported yet. Analytical results show that the probability of error decreases with the order of diversity gain per additional branch decreases as the number of branches becomes larger. On the other hand, the performance of 16 QAM with MRC becomes much better than that of SC as the number of branches becomes larger. By giving the order of diversity, L, and the number of signal points, M, we have been able to obtain the SER performance of QAM with general space diversity. These results can be used to determine the order of diversity to achieve the desired SER in land mobile communication system employing QAM modulation.

Adaptive equalization using the complex backpropagation algorithm

Abstract: For decreasing intersymbol interference (ISI) due to band-limited channels in digital communication, the uses of equalization techniques are necessary. Among adaptive equalization techniques, because of their ease of implementation and nonlinear capabilities, the neural networks have been used as an alternative for effectively dealing with the channel distortion, especially the nonlinear distortion. The complex backpropagation (BP) neural networks are proposed as nonlinear adaptive equalizers that can deal with both QAM and PSK signals of any constellation size (e.g. 32-QAM, 64-QAM and MPSK), and the complex BP algorithm for the new node activation functions having multi-output values and multi-saturation regions is presented. We also show that the proposed complex BPN provides, compared with the linear equalizer using the least mean squares (LMS) algorithm, an interesting improvement concerning bit error rate (BER) when channel distortions are nonlinear.

Selection of an optimal modulation scheme for digital communications over low voltage power lines

Abstract: There is an ever increasing demand for environmental management of buildings, office automation, security monitoring or remote control of customer appliances and remote meter reading. The AC low voltage power line would be an ideal communications medium for digital data. Several systems for power line communications (PLC) were presented. The reliability of PLC is strongly influenced by time-variant and frequency-selective attenuation and interference on the power line channel. Various experiments were made to evaluate different PLC systems and set up a channel model. This paper gives a short summary of typical transfer characteristics and interference to be observed at power line networks. It is shown that at transmission over a single phase of the power supply system the maximum phase jitter of the time-variant transfer function is in most cases below 90/spl deg/. In the second part important modulation schemes are evaluated for applicability to power line communications. This evaluation considers chirp modulation, direct sequencing phase shift keying modulation (PN/PSK) and hopping techniques with spectral redundancy given by carriers with M different frequencies. The selection of the optimal modulation technique considers the European standard EN 50065, which restricts both the frequency range and the maximum signal amplitude. This paper deals with modulation schemes, using M frequencies, namely classic frequency hopping modulation (MFH) and the multifrequency,phase modulation (MFH/PSK and MF/PSK). Significant advantages of M frequency phase modulation in comparison with classic frequency hopping are pointed out.

Multistage interference cancellation in multirate DS/CDMA on a mobile radio channel

Abstract: A novel hybrid interference cancellation (IC) scheme is proposed for the uplink of multirate direct-sequence code division multiple access (DS/CDMA) systems communicating over mobile radio channels. The hybrid IC scheme includes both non-decision directed and decision directed IC, joined together with a modified RAKE combiner for utilization of the diversity. The performance is evaluated via computer simulations for two multiple data rate schemes, mixed modulation and parallel channels, in a multipath environment. Both the case of known channel parameters and channel estimation using pilot symbols are considered. An analytical performance assessment is also given using a Gaussian approximation.

Performance of modulation‐level‐controlled adaptive modulation systems

Abstract: The multivalued quadrature amplitude modulation (QAM), such as 16 QAM, is a very useful technique in land mobile communication since a high-speed transmission can be realized without enlarging the transmission band. The technique has a problem, however, in that it is not robust against the decrease of the received signal level due to fading or the effect of the delayed waves, which easily deteriorates the transmission quality. To solve such a problem, this paper proposes an adaptive modulation system where the number of modulation levels is adjusted according to the condition of the channel. In the proposed system, the instantaneous C/N0 and the delay spread are estimated as the condition of the channel, using a method based on time division multiple access/time division duplex (TDMA/TDD). When the channel is in a good situation, a highspeed transmission is executed using a larger number of modulation levels such as 256 QAM. When the channel is in a bad condition, a modulation method with a small number of modulation levels, such as quadraphase-shift keying (QPSK), is used, which is robust against the noise or the delayed waves. By this scheme, it is possible to realize a high-speed transmission without deteriorating the transmission quality, compared to the conventional system with the fixed number of modulation levels. The transmission performance is evaluated by a computer simulation, and it is shown that a high-speed transmission can be realized without deteriorating the transmission quality, compared to the QPSK system, in the uniform Rayleigh fading condition as well as in the situation containing delayed waves.

Fast adaptive polynomial I and Q predistorter with global optimisation

Abstract: High spectral efficiency modulation methods such as quadrature amplitude modulation are of considerable interest for future mobile communication systems. These methods are highly sensitive to transmitter power amplifier nonlinearities. The linearity of a power amplifier can be improved by using an adaptive nonlinear predistorter. A modified version of a previously reported polynomial lineariser is presented which has a faster rate of convergence by an order of magnitude. Faster convergence is achieved using postdistorter polynomial functions of two variables. The price is 30% higher computation load for a predistorter having six coefficients. It is shown analytically that the gain and phase errors in the output demodulator used for adaptation of the predistorter simply result in a phase offset equal to the phase error. Adaptability of the proposed lineariser with changes in the power amplifier, predistorter quadrature modulator and the output demodulator are tested. Results show that the lineariser can adapt to these changes. The effect of the rotation of the demodulated output signal phase on convergence is studied. It is shown that the optimum value for this rotation is in the range of 15 to 70°. Spectrum-spreading results for different values of output back-off are included.

Quadtree-based reconfigurable cordless videophone systems

Abstract: Arbitrarily programmable, but fixed-rate quadtree (QT) decomposed, parametrically enhanced videophone codecs using quarter common intermediate format (QCIF) video sequences are proposed as a direct replacement for mobile radio voice codecs in second generation systems, such as the Pan-European GSM, the American IS-54 and IS-95, as well as the Japanese systems. The corresponding bit rates are 13, 8, 9.6 and 6.7 kb/s, respectively. As an example, the proposed 11.36 kb/s prototype Codec 1 and the 11 kb/s Codec 2 are embedded in the adaptively reconfigurable wireless videophone systems 1-4 featured in a table and their video quality, bit rate, robustness, and complexity issues are investigated. Coherent reconfigurable 16 or four-level pilot symbol assisted quadrature amplitude modulation (PSAQAM) is used and the system's robustness is improved by a combination of diversity and automatic repeat request (ARQ) techniques. When using a bandwidth of 200 kHz, as in the Pan-European GSM mobile radio system, the number of videophone users supported varies between three and 16, while the minimum required channel signal to noise ratio over Gaussian and Rayleigh channels is in excess of 6 and 8 dB, respectively, assuming a noise-limited, rather than interference-limited scenario. The salient system features are also summarized in a table.