In this paper we propose a novel Irregular Variable Length Coding (IrVLC) scheme for near-capacity joint source and channel coding. We employ a number of component Variable Length Coding (VLC) codebooks having different coding rates for encoding particular fractions of the input source symbol stream. These fractions may be chosen with the aid of EXtrinsic Information Transfer (EXIT) charts in order to shape the inverted EXIT curve of the IrVLC codec for ensuring that it does not cross the EXIT curve of a serially concatenated channel codec. In this way, an open EXIT chart tunnel may be created even at low E b /N0 values that are close to the capacity bound of the channel. We propose iteratively decoded serially concatenated IrVLC designs amalgamated with Trellis Coded Modulation (TCM). These schemes are shown to be capable of operating within 0.6 dB of the uncorrelated narrowband Rayleigh fading channel's capacity bound using an average interleaver length of 217,500 bits and an effective bandwidth efficiency of 1.56 bit/s/Hz, assuming ideal Nyquist filtering. By contrast, the equivalentrate regular VLC-based bench-marker schemes were found to be capable of operating at a higher distance of 1.1 dB from the capacity bound, which is nearly twice that of the proposed IrVLC-TCM scheme. Additionally, an Irregular Convolutional Coding (IrCC) based bench-marker was found to be capable of operating at 0.8 dB from the capacity bound, owing to its slightly eroded performance when operating with the considered interleaver length.
In this article we present a study of a new class of optical fibers based on lead germanate glass. The maximum vibrational frequency of this glass is intermediate between silica and zirconium barium lanthanum aluminum fluoride glass, causing a beneficial change in nonradiative decay and therefore quantum efficiency for particular laser transitions. Fabrication of high-strength, low-loss fibers of this glass has been acheived by modification of the composition to produce optimal physical properties for fiber drawing, while retaining the useful vibrational properties of the original PbGeO2 glass. Measurements of both the thermal and optical properties are described. The fibers produced are ideal for many applications in fiber devices.
We report what we believe to be the first planar-technology waveguide laser in the 2-,um region. Laser operation of the 'H4 to 3 H 6 transition of Tm 3 + ions in a lead germanate glass host has been observed in an ion-implanted planar waveguide.The development of coherent light sources in the wavelength region around 2 ,um is of interest because of the presence of absorption bands of several important molecules. Lead germanate was shown to be a particularly suitable glass host for operation of the 2-Am Tm 3 , 3 H 4 to 3 H 6 transition in fiber samples" 2 (see Fig. 1). The maximum phonon energy is greater than in fluoride glasses, leading to efficient population of the upper laser level through nonradiative decay out of the 3 F 4 pump level. The maximum phonon energy is, however, less than in silicate glasses, leading to an increase in the lifetime of the upper laser level 3 H 4 . This favorable combination of decay rates has permitted us to demonstrate, using a fiber made of modified lead germanate (55GeO 2 -2OPbO-lOBaO-lOZnO-5K 2 0), thresholds for 790-nm pumping that are easily within reach of single-stripe diode lasers. ' Waveguides based on planar technology offer a number of potential advantages over fibers. For example, there is the possibility of diode-bar side pumping for high-power operation. 3 In addition, the active region is readily accessible for fabrication of feedback gratings, possibly through the photorefractive effect. Because high doping levels are possible with this glass, compact single-longitudinal-mode devices may be possible.Recently it was shown that waveguides could be fabricated in this modified lead germanate glass by use of He' ion implantation 4 and that, in addition, the losses of these guides were significantly lower than the -1-dB cm-' value that has been typical for ion implantation. 5 In fact, losses of 0.15 dB cm-' were obtained 4 (compared with 0.02 dB cm'I for the fiber), with index profiles suitable for guiding 2-/ttm radiation. The effect of the propagation loss can be reduced further by the use of shorter cavity lengths and correspondingly higher doping levels. The use of Tm 3 + is also attractive because there exists a cross-relaxation process that offers the possibility of 200% pumping quantum efficiency for highly doped samples. To assess the prospects for active waveguide devices in lead germanate glass, we have made an initial investigation of the 2-Am Tm 3 + laser in a planar waveguide.The waveguide used in these experiments was a planar guide (i.e., with guiding in one transverse dimension) created by implantation of the polished surface of the Tm-doped glass with 2.9-MeV 3 He' ions at liquid-nitrogen temperature at a dose of 4 X 1016 ions/cm 2 . This was followed by annealing at 200'C, which was found to be the annealing temperature at which propagation losses were minimized while the index increase in the guide region remained relatively large. 4 The dark mode pattern of the resultant waveguide was investigated, and from this the refractive-index profile shown ...
A joint video and channel coded system employing an iteratively decoded serial concatenation of a Vector Quantization (VQ) based video codec and a Trellis-Coded Modulation (TCM) scheme is proposed. The video codec imposes VQ-induced code constraints, which may be completely described by a trellis structure, which is employed as the basis for optimal minimum mean-squared-error VQ-encoding and -decoding. In the latter case, the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm is employed to facilitate the iterative exchange of soft information between the VQ and TCM decoder. An error-free video reconstruction quality is supported using 16-level Quadrature Amplitude Modulation (16QAM) based TCM for transmission over Rayleigh-fading channels at a Signal-to-Noise Ratio (SNR) per bit of 5.25 dB. This value is within 1.29 dB of the Rayleigh channel's capacity at our system's effective bandwidth-efficiency of 2 bits/s/Hz. Owing to its ability to exploit the VQ-induced code constraints during iterative decoding, the joint video and channel coding approach is found to consistently outperform the Shannonian source and channel separation philosophy. This is achieved at the cost of a 1.6 times higher computational complexity. Finally, the convergence of the iterative decoder is investigated with the aid of a novel so-called EXtrinsic Information Transfer (EXIT) chart.
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