Spectral Efficient Coding Schemes in Optical Communications

Abstract Achieving high spectral efficiency in optical transmissions has recently attracted much attention, aiming to satisfy the ever increasing demand for high data rates in optical fiber co mmun ications. Therefore, strong Forward Error Correct ion (FEC) coding in co mb ination with mult ilevel modulat ion schemes is mandatory to approach the channel capacity of the transmission link. In this paper we g ive design rules on the joint optimization of coding and signal constellations under practical considerations. We give trade-offs between spectral efficiency and hardware complexity, by comparing coding schemes using capacity achieving constellations with bit-interleaved coded modulation and iterative decoding (BICM-ID) against applying conventional square quadrature amp litude modulation (QAM) constellations but emp loying powerful low co mplexity lo w-density parity-check (LDPC) codes. Both schemes are suitable for optical single carrier (SC) and optical orthogonal frequency-division mu ltiplexing (OFDM) transmission systems, where we consider the latter one in this paper, due to well-studied equalizat ion techniques in wireless communications. We numerically study the performance of different coded modulation formats in optical OFDM transmission, showing that for a fiber optical transmission lin k of 960 km reach the net spectral efficiency can be increased by ≈0.4 bit/s/Hz to 8.61 b it/s/Hz at a post FEC BER of <10 -15 by using coded optimized constellations instead of coded 64-QAM

Automatic focus algorithms for TDI X-Ray image reconstruction

In food industry, most products are checked by X-rays for contaminations. These X-ray machines continuously scan the product passing through. To minimize the required X-ray power, a Time, Delay and Integration (TDI) CCD-sensor is used to capture the image. While the product moves across the sensor area, the X-ray angle changes during the pass. As a countermeasure, adjusting the sensor shift speed on a single focal plane of the product can be selected. However, the changing angle result in a blurred image in dependance to the thickness of the product. This so-called ''laminographic effect'' can be compensated individually for one plane by inverse filtering. As the plane of contamination is unknown, the blurred image will be inversely filtered for different planes, but only one of these images shows the correctly focussed object. If the correct image can be found, the plane containing the contamination is identified. In this contribution we demonstrate how the correctly focussed images can be found by analyzing the images of all planes. Different characteristics for correctly and incorrectly focussed planes like sharpness, number of objects and edges are investigated by using image processing algorithms

A relation between algebraic and transform-based reconstruction technique in computed tomography

In this contribution a coherent relation between the algebraic and the transform-based reconstruction technique for computed tomography is introduced using the mathematical means of two-dimensional signal processing. There are two advantages arising from that approach. First, the algebraic reconstruction technique can now be used efficiently regarding memory usage without considerations concerning the handling of large sparse matrices. Second, the relation grants a more intuitive understanding as to the convergence characteristics of the iterative method. Besides the gain in theoretical insight these advantages offer new possibilities for application-specific fine tuning of reconstruction techniques

Performance of combined constellation shaping and bit interleaved coded modulation with iterative decoding (BICM-ID)

The increasing demand of achieving high data rates in modern communication systems requires highly efficient bandwidth utilization. For this purpose, multilevel modulation schemes are used in association with forward error correction (FEC) codes in order to approach the channel capacity. However, there is a gap between the capacity of a uniform signal constellation and the Shannon unconstrained capacity. This gap can be reduced by applying constellation shaping. The task of shaping is to modify a uniform distributed signal constellation towards a Gaussian like distribution. In this paper, we investigate different approaches to combine the constellation shaping with a bit interleaved coded modulation with iterative decoding (BICM-ID) system. Simulation results show that this combination can offer a shaping gain up to 0.6 dB

Inverse filtering for time, delay and integration X-ray imaging

In food industry, most finished products are scanned by X-ray for contaminations. These X-ray machines continuously scan the product passing through. To minimize the required X-ray power, a Time, Delay and Integration (TDI) CCD sensor is used to capture the image. While the product moves across the sensor area, the angle of the X-rays changes during the pass. This can be compensated for by adjusting the sensor shift speed to focus on a single plane of the product. If the product has a significant thickness, the image will show artifacts due to the laminographic effect. In this contribution we demonstrate that by the use of inverse filtering images which are focused on planes of different height can be generated out of a single X-ray image

All-Electronic High-Resolution Terahertz Thickness Measurements

Broadband laser based terahertz systems become currently established for inline multilayer paint inspection in the automotive industry. This technology has also proven to be suitable for inspections of certain multilayer plastic structures with up to a few millimeters of thickness. We present a complementary technique for the measurement of dielectric multilayer structures with thicknesses of sub millimeter to several centimeters, using frequency-modulated continuous-wave electronic transceivers. In order to resolve layers below the inherent resolution limit by the modulation bandwidth, we take advantage of model-based signal processing techniques

Analyzing the Precision of Frequency Modulated Continuous Wave Distance and Thickness Measurements

Using millimeter and terahertz waves, novel solutions to nondestructive testing can be realized such as contactless distance and thickness measurements of multilayered dielectrics. Recently, we have presented a signal model based approach for the highly accurate inspection of multilayers which benefits both from frequency and phase information. In this contribution, we derive the theoretical minima of the distance and thickness variances directly considering the dependency of frequency and phase. These limits are compared with simulation and measurement results, applying different signal processing techniques. The highest precision is achieved by our signal model based correlation approach

High-resolution FMCW millimeter-wave and terahertz thickness measurements

We have adapted the FMCW radar technique to perform high-resolution thickness measurements within the millimeter-wave and terahertz frequency domain. High signal modulation bandwidths of several 10 GHz conform to millimeter resolution limits as well as micrometer accuracies. However, for our target application - the thickness measurement of single- and multi-layer plastics such as tube walls - the adapted approach for FMCW radar distance measurements is insufficient. Thick layers restrict the penetration depth of high frequency signals. Therefore, operation frequencies in the millimeter-wave or lower terahertz regime are required, which provide reduced modulation bandwidths and hence limit the resolution in the order of approximately one to several millimeters. Simultaneously, fine layers have to be separated. In this contribution, we present a correlation approach to overcome the Rayleigh resolution limit including first promising results for single and multi-layer structures

Turbo Equalization Of Nonlinear ISI-channels Using High Rate FEC Codes

International audience(CJCE 7 mai 1998, Clean Car Autoservice, déjà cité

A transfer matrix modification for accurate terahertz FMCW thickness measurements

Millimeter and terahertz waves offer novel solutions for nondestructive testing such as imaging and layer thickness measurements within dielectrics. For the thickness measurement of plastics, we use the frequency modulation continuous wave technique within fully electronic terahertz systems. The available modulation bandwidth inherently restricts the resolution to several millimeters. Our correlation approach, which compares the acquired measurement data with a signal model, overcomes this limit for predefined measurement conditions. However, to obtain high measurement accuracies - especially in the case of compact multilayer structures and dielectric coatings on conductive substrates - beam propagation aspects such as multiple reflections between the boundary surfaces of the layers have to be considered. Therefore, we adapt the Transfer Matrix method to our measurement scheme with optimizations with regard to computation complexity