Conventional Modulation Schemes Research Articles

Blind estimation of modulation schemes according to noise powers in the presence of carrier frequency offset

AbstractA method for identifying modulation schemes is a vital elemental technology of adaptive software‐defined radio that switches its software in response to the environment and the application objective. The problems faced in conventional modulation scheme estimation are synchronization and robustness in a fading environment. In this paper, we address the problem of the degradation in the accuracy of modulation scheme estimation due to the presence of carrier offsets and the fluctuations in the carrier‐to‐noise power ratio (CNR). The former is addressed by implementing offset cancellation that uses the temporal phase difference information in one symbol. The latter is addressed by using the estimates found by a blind noise power estimation algorithm to implement modulation scheme estimation that exhibits excellent estimation accuracy even with a fluctuating CNR. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(12): 76–88, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10001

Multiple-Symbol Decision-Feedback Space-Time Differential Decoding in Fading Channels

Space-time differential coding (STDC) is an effective technique for exploiting transmitter diversity while it does not require the channel state information at the receiver. However, like conventional differential modulation schemes, it exhibits an error floor in fading channels. In this paper, we develop an STDC decoding technique based on multiple-symbol detection and decision-feedback, which makes use of the second-order statistic of the fading processes and has a very low computational complexity. This decoding method can significantly lower the error floor of the conventional STDC decoding algorithm, especially in fast fading channels. The application of the proposed multiple-symbol decision-feedback STDC decoding technique in orthogonal frequency-division multiplexing (OFDM) system is also discussed.

Electromagnetic interference of switching mode power regulator with chaotic frequency modulation

In this work, we propose an improved switching scheme (called chaotic frequency modulation (CFM)) for switched-mode power supplies to suppress the electromagnetic interference (EMI) noise source. The basic principle of CFM is to use a chaotic signal to modulate the switching signal so that the harmonics of noise power is distributed evenly over the whole spectrum instead of concentrated at the switching frequency. When compared with the conventional pulse width modulation (PWM) scheme, significant improvements in both conducted and radiated EMI noise levels were found with the proposed CFM method. For conducted EMI, the peak noise level was reduced by 25 dB_V. For radiated EMI, we found that the noise was found mainly in the frequency range of 30 MHz to 230 MHz and the CFM scheme would help to reduce the peak noise level in this frequency range by 22 dB_V.

CHAOTIC MODULATION FOR ROBUST DIGITAL COMMUNICATIONS OVER MULTIPATH CHANNELS

This tutorial work identifies the characteristic properties of chaotic digital modulation schemes and shows how the unique features of chaotic basis functions can be exploited to competitive advantage in selected application domains. Two robust noncoherent chaotic digital modulation schemes (DCSK and FM-DCSK) are described in detail and the performance of FM-DCSK is examined critically for communication channels where systems using conventional coherent and narrow-band modulation schemes fail. Since the FM-DCSK signal is a wide-band signal and its demodulation is performed without synchronization, the FM-DCSK modulation scheme is especially suitable for operation in multipath channel such as Wireless Local Area Networks (WLANs).

Multilevel inverter modulation schemes to eliminate common-mode voltages

It is well known that conventional two-level pulsewidth modulated (PWM) inverters generate high-frequency common-mode voltages with high dv/dt. Similarly, commonly used multilevel inverter modulation schemes generate common-mode voltages. Common-mode voltages may cause motor shaft voltages and bearing currents and conducted electromagnetic interference (EMI). Premature motor bearing failures and electronic equipment malfunctions have been reported to be directly related to bearing currents and EMI. In this paper, approaches to eliminating common-mode voltage when using multilevel PWM inverters are presented. It is shown that inverters, which have an odd number of levels, will generate zero common-mode voltage by switching among certain states. Therefore, motor bearing currents will be eliminated and conducted EMI will be reduced. Both sinusoidal PWM and space-vector modulation (SVM) schemes are discussed and detailed comparative simulation results between conventional and novel modulation schemes are provided. The value of the proposed technique is demonstrated experimentally by applying the novel SVM approach to a conventional multilevel inverter.

Strategies for realizing optical CDMA for dense, high-speed, long span, optical network applications

Since the mid 1990s, the role of optical CDMA has expanded from local area networks to longer span, telecommunication-type networks. In order to play a significant role in these longer span, denser, higher data rate networks, optical CDMA code set must (1) have at least as many codes as dense wavelength division multiplexing (WDM) (i.e., more than eight codes); (2) operate at high data rates (i.e., greater than 2.5 Gb/s); and (3) propagate with high fidelity over the installed or installable fiber links. Most approaches to optical CDMA require narrow pulses, which are more susceptible to fiber impairments and may have lower spectral efficiency than conventional WDM modulation schemes such as non-return-to-zero (NRZ), so they do not meet these new requirements. Therefore, we have formulated a strategy which simultaneously increases the number of good codes (resulting in higher density) and reduces their code length (i.e., decreasing the number of time slots required thus enabling higher data rates for a given chip time): the strategy of matrix codes. In this paper, we describe the design of a set of eight matrix codes for operation at 2.5 Gb/s and evaluate their propagation over an existing 214 km network link by means of computer simulation. The results indicate that the codes propagate well if dispersion management is used. The paper also discusses a strategy for managing the multiaccess interference (MAI) in a bursty traffic environment.

Optical up-conversion on continuously variable true-time-delay lines based on chirped fiber gratings for millimeter-wave optical beamforming networks

An electrooptical up-converting modulation scheme has been considered on chirped-fiber-grating-based (CFG) true-time-delay lines for optical beamforming networks. A rigorous analytical study about the effect of both the up-conversion process and CFG dispersion on the amplitude and delay of detected signal is presented. Simulations and measurements show that the bandwidth limitation of the delay line due to the fiber grating dispersion is clearly improved if compared to the conventional intensity modulation scheme, while the time-delay performance over a 2-30 GHz bandwidth is maintained (/spl sigma//sub max/=/spl plusmn/1 ps). This new concept is used on an optical beamforming network architecture for a millimeter-wave (15-25 GHz) phased array antenna showing a promising performance.

Electro-optical upconversion in chirped fibre gratingbased true time delay lines

An investigation is presented into the use of an electro-optical upconverting modulation scheme on chirped fibre grating based true time delay lines, and analyses made of both the amplitude and delay of detected microwave signals. The bandwidth limitation of the delay line due to the fibre grating dispersion is clearly improved over that in conventional modulation schemes, while the time delay performance is kept constant (/spl sigma//sub max/=/spl plusmn/0.9 ps).

Performance of high-diversity multidimensional constellations

We evaluate analytically the performance of component-interleaved multidimensional constellations over the Rayleigh fading channel. The concept of modulation diversity was introduced by Boutros and Viterbo (see IEEE Information Theory Workshop, Haifa, Israel, 1996) as a way to enhance the error performance of communication systems over the Rayleigh fading channel. Several multidimensional signal sets with high diversity order have been proposed to obtain substantial gains over conventional modulation schemes in a fading environment. Modulation diversity can also be viewed as a special form of signal space coding producing highly efficient (in terms of bandwidth and power) schemes over the fading channel.

Direct sequence spread spectrum Walsh-QPSK modulation

We present Walsh-quadrature phase-shift keying (Walsh-QPSK) pseudonoise (PN) modulation schemes for both coherent and noncoherent direct-sequence code-division multiple-access (DS-CDMA) systems, wherein the PN spreading sequences for in-phase and quadrature data in a conventional QPSK PN modulation scheme are coded by Walsh sequences indexed by a special rule to reduce the envelope variation of the transmitted signal. The signal characteristics of the two schemes are analyzed when a rectangular-shaped PN chip pulse is used, and it is shown that the proposed coherent DS-CDMA system has a constant envelope even in the presence of a transmitted phase reference. We simulate the signals to obtain the envelope variations when a spectrally efficient shaped PN chip pulse is used, and compare the results with those of conventional QPSK and orthogonal QPSK (OQPSK) PN modulation schemes. The results show that both the noncoherent and coherent Walsh-QPSK schemes have smaller envelope variations than the conventional noncoherent QPSK and OQPSK PN modulation schemes, even though in the coherent Walsh-QPSK scheme the pilot channel is added to the signal channel.

Optimal chip waveforms for DS‐CDMA with generalized OQPSK modulation

AbstractThe effect of chip waveform shaping on both the error and spectral performance of DS‐CDMA communication systems using generalized offset quadrature modulation formats is investigated. A solution to the problem of optimizing the chip waveform shape to achieve minimum multiple access interference variance at a given in‐band power bandwidth occupation and with a continuous phase constraint is presented. Several design examples with various bandwidth occupancies are used to illustrate the gains achieved by the optimized chip waveforms over other conventional modulation schemes such as OQPSK and MSK.

Spectrum shaping for land mobile radio systems

AbstractSpectral efficiency for land mobile radio channels is primarily limited by adjacent channel interference. Conventional digital modulation schemes do not achieve current specifications for adjacent channel interference at high data rates. The adjacent channel interference can be mitigated by the intelligent choice of transmitter components such as the modulation scheme and the transmit filter. We present a trellis coded modulation (TCM) scheme that can provide high data rates and at the same time reduce adjacent channel interference. We present methods to choose an appropriate TCM scheme under constraints of Euclidean distance and peak‐to‐average power. We also present methods to optimize transmit filters to minimize adjacent channel interference under various constraints. Examples are provided to illustrate the gains that can be obtained.

I-Q TCM: reliable communication over the Rayleigh fading channel close to the cutoff rate

This paper presents some trellis codes that provide high coding gain to channels with slow, non frequency-selective Rayleigh fading. It is shown that the use of two encoders in parallel-used to specify the in-phase and quadrature components of the transmitted signal-results in greater minimum time diversity than the conventional design in which a single encoder is used. Using this approach-which we label "I-Q TCM"-codes with bandwidth efficiencies of 1, 2, and 3 bits/s/Hz are described for various constraint lengths. The performance of these codes is bounded analytically and approximated via simulation; the results show a large improvement in the bit error rate (BER) when compared with conventional trellis-coded modulation (TCM) schemes when perfect channel state information (CSI) is available to the receiver. Indeed, when this approach is applied to channels with independent Rayleigh fading, the resulting coding gain is close to that implied by the cutoff rate limit, even for only moderately complex systems. The proposed codes are also simulated under less ideal assumptions. For instance, results for a 1-bit/s/Hz IQ-TCM code without CSI show a significant gain over conventional coding. Finally, simulations over channels with correlated fading were undertaken; it is concluded that an interleaver span of 4/spl nu/ yields performance close to what is achieved with ideal interleaving.

M -ary nonlinear continuous phase frequencyshiftkeying

The authors present a new approach for designing a continuous phase frequency shift keying (CPFSK) system which is a nonlinear mapping of the M-ary symbols into the N-ary CPFSK. Using this approach they have designed an M-ary nonlinear CPFSK (NCPFSK) system of simple structure. The proposed M-ary NCPFSK system is a modification of the multi-h CPM (MHCPM) scheme. The proposed modulation gives improved performance over conventional modulation schemes.

Spectral analysis of M-ary PSK modulation with smooth and monotonic phase variations

Abstract Computer simulation has shown that smooth and monotonic phase variations in constant envelope Af-ary PSK modulation offer a significant reduction in the mainlobe width of the trasmitted signal, as compared with alternative conventional PSK modulation schemes. In this paper it is shown that the above modulation scheme can be considered as a special case of CPFSK signals with constant modulation index equal to I/M. A closed-form expression for the power density spectrum is theoretically derived, and spectral comparisons to conventional PSK schemes with abrupt phase variations are performed, Theoretical results are verified using computer simulation, for M equal to 2, 4 and 8.

1 MHz bandwidth true NMR SQUID amplifier

We have developed an integrated dc SQUID magnetometer with additional positive feedback (APF) for low frequency true NMR applications. The APF scheme allows direct coupled read out from the SQUID to room temperature electronics and eliminates the need for the conventional modulation scheme, thereby greatly simplifying the flux-locked loop electronics. We have configured our SQUID system for the specific needs of sensitive NMR measurements which include large bandwidth and high slew rate. We have achieved a bandwidth of 1.2 MHz and a slew rate greater than 105Φ0/s for frequencies above 10 kHz.

Modulation methods for PCNs

The interference levels that may be expected for a range of conventional cellular and microcellular clusters used in personal communication networks (PCNs) and personal communication systems (PCSs) are discussed. Simulation results show that for conventional size cells, 32- and 64-level quadrature amplitude modulation (QAM) schemes are preferred for bit error rates (BER) above 1*10/sup -2/, while 4PSK or variable-rate QAM schemes are better for lower BER. For microcells with communications at both 900 MHz and 1.8 GHz, four- or six-cell clusters are advocated, depending on the SNR expected. Based on the expected signal-to-noise ratio (SNR) and interference levels, it is argued that variable-rate QAM schemes are superior to the other modulation schemes considered. For low BERs 4PSK may often provide the best performance, whereas for high BERs, particularly when the SNR is high, 32- and 64-level star QAM are the most suitable.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic spectral broadening in digitally modulated lasers

It is demonstrated that Gbit/s digitally modulated, voltage controlled singlemode multiple quantum well semiconductor lasers with an intracavity saturable absorber show reduced spectral broadening compared to conventional high contrast modulation schemes. The intracavity loss modulated laser linewidth is also calculated using experimentally determined voltage and intensity dependence of absorption in MQW modulators and excellent agreement with experiment is found.

Calculated chirp in intracavity loss modulated DFB lasers

Calculations of modulation induced line-width broadening in 1.55 μm DFB lasers are reported. Voltage controlled intracavity loss modulation has significant performance advantages compared with conventional large signal current modulation schemes.

Multi-H phase-coded modulations with asymmetric modulation indexes

Multi-H phase-coded modulation (MHPM) is a bandwidth-efficient modulation scheme which offers substantial coding gain over conventional digital modulation schemes. MHPM with asymmetric modulation indices corresponding to the bipolar data +1 and -1 is considered, and numerical results for the minimum Euclidean distances are provided. It is shown that performance improvements on the error probability over conventional MHPM are gained with essentially the same bandwidth and a very slight modification in implementation. The upper bounds on the error probabilities as functions of observation intervals and received E/sub b//N/sub 0/ are also investigated in detail. It is concluded that the concept of asymmetric modulation indices for MHPM is attractive for bandwidth and power-efficient modulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>