Femtosecond time division interferometry technique for measuring the tensor components of χ(3)

Nonlinear refractive-index changes in GaAs are obtained at room temperature by measuring shifts of Fabry-Perot (F.P.) transmission. The 299-Å multiple-quantum-well GaAs sample was sandwiched between two dielectric mirrors (90% reflectivity) to form a F.P. etalon (finesse =10). The pump beam was tuned to 821 nm and modulated to obtain 1-μs pulses at 10 kHz. The photoluminescence from a focused pump spot at the GaAs etalon was used as a broadband probe source. Since the photoluminescence is due to carriers generated by a pump beam, the pump and probe beams automatically align themselves. The fast electronic nonlinearity exhibited blue shifts (negative index changes) of the F.P. transmission peaks, while the slow thermal nonlinearity showed red shifts (positive index changes). Nonlinear index changes were calculated from the measured shifts of the F.P. peaks for different pump intensities. A self-consistent F.P. phase function for this calculation was obtained assuming lengths of 2.9 and 1.5 μm for the GaAs and air gap, respectively. Optical bistability was most easily observed for the region of large index change (873-878 nm). The slow nonlinear index changes below the band gap showed nonresonant characteristics rather than resonant, saturable behavior.

This paper proposes a differential modulation scheme based on quasi-orthogonal space-time block codes (STBCs) when neither the transmitter nor the receiver has knowledge of the channel. A system with four transmit antennas is considered and divided into two subsystems. Each subsystem is equivalent to a system with two transmit antennas and uses differential STBC modulation. Based on the encoding and decoding for the two subsystems, the overall encoding and decoding algorithm for the original system is derived. We also compare the performance of our differential detection with that of the corresponding coherent detection in terms of achieved signal-to-noise ratio (SNR) for a given transmission power and propose a quantitative measure to estimate the performance difference between the coherent quasi-orthogonal code and our differential detection. Furthermore, we show that our differential modulation scheme can be extended to a system with more than four transmit antennas. Compared with the existing differential modulation schemes for four and eight transmit antennas, the proposed scheme has a lower bit-error rate (BER) and provides full diversity and pair-wise decoding complexity.

In this paper, an extension of the effective index method (EIM) to waveguiding structures containing ideal or saturable third-order nonlinear materials is presented. By applying separation of variables to the dominant field component, the complete problem is subdivided into two scalar problems in the lateral and transverse direction, as in the case of the normal EIM. Making use of the strong transverse confinement, as observed in most real waveguide structures, the nonlinear index changes of the various transverse sections can be lumped into nonlinear effective indexes of the equivalent layered planar structures. By using these nonlinear effective indexes in self-consistent field calculations in the transverse direction, a complete approximate solution is obtained. In this way, the amount of computational effort required for the calculation of the effective indexes and field profiles of the waveguides can be reduced significantly.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evolutionary gravitational Neocognitron neural network based differential spatial modulation detection scheme for uplink multiple user huge MIMO systems

Recently, the differential detection scheme has attracted considerable attention as a demodulated scheme for digital mobile communications because of its robustness in the frequency nonselective fading channel. A problem of the differential detection scheme is that its bit error rate (BER) performance in the additive white Gaussian noise (AWGN) channel is inferior to that of the coherent detection scheme. To solve this problem, many methods to improve BER performance of the differential detection in the AWGN channel have been proposed. However, those methods require complex signal processings.This paper proposes a novel differential detection scheme as a simpler improvement method which estimates the differential phase of the transmitted signal by a sequence estimation scheme using the Viterbi algorithm. The proposed scheme is simplified in a manner such that its signal processing is composed of only additions and subtractions on the phase information of the received signal. Results of computer simulation show that BER performance of the proposed scheme is close to that of the ideal coherent detection in the AWGN channel. In addition, the trellis diagram of the Viterbi algorithm can be reduced without performance degradation introducing the signal processing similar to the Scarece State Transition (SST) type Viterbi decoding. The proposed scheme also is useful in the case of the fast Rician fading channel and the slow Rayleigh fading channel.

Differential modulation schemes are very relevant in receivers having power and processing limitations, as these schemes dispense with the need for knowledge of channel coefficients for symbol detection. Spatial modulation (SM) is a scheme used in multi-antenna transmission scenarios where the data is transmitted in the amplitude, phase and spatial domains through selected antennas. In the coherent domain, generalized SM (GSM) employs multiple antennas in combination during every time slot to enhance the spectral efficiency (SE). In this paper, we propose two differential schemes which activate two or more antennas at a time to transmit the modulated symbol. These schemes achieve higher SE using a lesser number of antennas and lower order modulation schemes instead of the higher number of antennas required for conventional SM schemes based on differential modulation. Simulation studies reveal that the proposed schemes have better bit error rate performance than traditional differential SM schemes. We also derive the analytical union bound for the proposed schemes and is satisfied from medium to high signal-to-noise ratio (SNR) ranges.

Variation in the round-trip group delay through an intracavity prism pair is studied as a function of index variation in the gain medium. The resulting sensitivity to index perturbation gives insight into the degree to which pump noise will transfer to timing instability through this mechanism. General expressions for group-delay time change and slope are derived. It is found that thermal and nonlinear index changes in the gain crystal result in group-delay time sensitivities of 4.34 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> s/K and 1.25 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">- 26</sup> s·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /W, respectively, for a conventional 100-MHz Kerr-lens mode-locked laser. For a nominal temperature fluctuation of 1 K and a peak intensity variation of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , this implies round-trip timing variation of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> s, which is very small, but not insignificant in today's clock technology.

This paper addresses a multiple-symbol differential detection scheme for star 16QAM. At the outset, we analytically scrutinized conventional differential detection of star 16QAM. As a result, it emerged that the energy penalty of differential amplitude detection as compared with ideal coherent detection is larger than that of phase detection. Then to improve the overall performance of star 16QAM, we propose an MLSE algorithm for a large C/N as well as a suboptimum algorithm. Both of them allow multiple-symbol differential amplitude and phase detection. Their performances are also evaluated through computer simulation.KeywordsPhase DetectionEnergy PenaltyCoherent DetectionDifferential DetectionObservation SymbolThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The paper presents a differential detection scheme for noncoherent detection of digital FM signals received over bandwidth restricted channels. The new differential detection scheme observes the phase of the FM signal at centres of signalling intervals to derive the differential phase. Differential encoding of transmitted data is required for preventing error propagation. Numerical results show that the new differential detector yields a superior performance in narrowband channels where conventional differential detection becomes unusable. Error probabilities and optimum BT values are presented assuming that bandlimiting is caused solely by IF filtering.

A variety of schemes for performing differential detection in environments characterized by frequency offset are discussed. All of the schemes involve encoding the input phase information as a second-order difference and performing an analogous second-order differential detection at the receiver. Because of the back-to-back differential detection operations at the receiver, The performance of most of the schemes is considerably degraded relative to that of first-order differential detection schemes. However, the latter are quite sensitive to frequency offset and, in many instances, cannot be used at all. It is demonstrated that via a simple enhancement of using a 2 T s (instead of T s) delay in the second stage of the encoder and first stage of the decoder, the performance degradation can be significantly reduced. This result is significant in view of the fact that it comes without any penalty in implementation complexity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Direct-sequence spread-spectrum (DSSS) with differential chip detection (DCD) schemes, chip model 1 and chip model 2, have been proposed in reference of A. Cavallini et al. (1997) and Q. Shi et al. (2002), respectively. The performances of these two DCD schemes have been studied via simulations in reference to M.M.K. Howlader and H, Chang (2003). In this paper, we derive the decision variables of these two DCD over additive white Gaussian noise (AWGN) and Rayleigh fading channel, and our results show that the noise variance in chip model 2 is twice of that in chip model 1.

Performance Analysis of Different Phase Shift Keying Modulation Schemes in Aeronautical Channels

We show that certain signal constellations invoked for classic differential encoding result in a phenomenon we term as the {\\em unbounded differential constellation size} (UDCS). Various existing differential transmission schemes that suffer from this issue are identified. Then, we propose an enhanced algebraic field extension based differential spatial modulation scheme (AFE-DSM) and its enhanced counterpart that strikes a diversity-rate trade-off (AFE-DSM-DR), both of which overcome the UDCS issue without compromising its full transmit diversity advantage. Furthermore, the proposed schemes are extended to incorporate amplitude and phase shift keying (APSK) in order to exploit all the available degrees of freedom. Additionally, we propose a pair of detection schemes specially designed for APSK aided differential transmission schemes. Explicitly, we conceive the buffered minimum mean squared error (B-MMSE) detector and buffered maximum likelihood (B-ML) detector, which exploit the knowledge of previously detected symbols in order to further improve the detection performance. Our simulation results have shown that the proposed detectors are capable of bridging the performance gap between the conventional differential detector (CDD) and the coherent detector that has full channel state information. Specifically, when employing the proposed APSK aided AFE-DSM scheme operating at a rate of 2 bits per channel use (bpcu), the B-MMSE and B-ML detectors are observed to give about 3 dB and 3.5 dB signal-to-noise ratio gain with respect to their CDD counterpart at a bit error ratio of $10^{-5}$.

In this work an Artificial Neural Network with Radial Basis Function (RBF) is employed to model a predictor, utilized in Differential Pulse Code Modulation (DPCM) scheme. The RBF predictor estimates the magnitude of signal incoming to DPCM, the error between incoming signal and estimated one is applied to quantiser unit. The resultant error contains a few bit word length and is transmitted in data format towards a receiver unit. The output of the RBF predictor is added to error at the receiver part of DPCM to produce actual output. In this study application of RBF predictor in DPCM is explained. The potential offered by DPCM scheme using RBF predictor leads to a considerable amount of reduction in word length of filter coefficients.

This paper reports measurements of intensity-induced rotation of the polarization ellipse, and measurements of the self-focusing threshold and its polarization dependence, using an unfocused laser beam in several Raman-active liquids. An attempt was made to determine the nonlinear susceptibility constants in these liquids by correlating these measurements. It is found that the self-focusing formulas for linearly polarized beams are in good agreement with the experiments, but the polarization dependence of the self-focusing threshold disagrees with the corresponding polarization dependence of the nonlinear index changes. A possible explanation for this discrepancy is discussed.