Malcolm Rutter

Design and realisation of adaptive lattice filters

This paper compares the relative advantages of discrete and integrated circuit transversal and lattice adaptive filter designs. It discusses the trade-offs between the traditional transversal filter and the gradient adaptive lattice (GAL), showing that the reliable rate of convergence of the lattice is offset by a greater complexity and algorithm noise. Two 16-stage hardware implementations are described. One is a TTL GAL equaliser based on a single 12-bit parallel multiplier. The second is a suite of 5 custom NMOS bit-serial arithmetic LSI chips, which together make a lattice prediction-error filter. Both implementations offer 12-bit precisions and bandwidths of over 8 kHz. This shows that the increased complexity of the lattice approach can easily be accommodated in custom VLSI circuit designs.

A new structure for adaptive echo cancellation

A structure is proposed for adaptive removal of echoes from communications signals. It consists of an FIR adaptive filter which models the echoing channel, and an IIR programmable filter which performs the echo removal. This technique allows the use of a recursive filter without incurring the stability problems normally associated with adaptive fIR filters. The adaptive filter programs the weights of the recursive filter via a simple transform, which may in fact be incorporated in the adaptive process. Results of computer simulations are presented, showing convergence on PRBS sequences corrupted by echoing channels.

A distributed optical fiber sensing system using elliptical core fiber

The novel technology of distributed optical fiber sensor (DOFS) is very attractive because it can measure the spatial distribution and temporal variation of a measurand field along the fiber simultaneously.’ Many arrangements of a DOFS system are based on the principle of Optical Time-Domain Reflectometry or Polarisation Optical Time-Domain Reflectometry. However, these backscatter DOFS systems have a main disadvantage: very low signal, which leads to a low sensitivity and low spatial resolution. A so-called forwardscatter method can avoid this problem and is being investigated with great interest.’” It is based on the nonlinear interaction between the two counterpropagating pump pulse light and probe light, and for some mechanism the emerging signal is dependent on the external measurand. There are several systems making use of the nonlinear effect between the high intensity pump pulse and the two linearly orthogonal-polarised eigenmodes of probe light in single-mode high birefringence fiber? which have complicated system structures. In this paper we present a DOFS system making use of forwardscatter method in elliptical-core two-mode fiber. The optical components and signal detection are relatively simple. The system arrangement is illustrated in Fig. 1. The elliptical core two-mode fiber can hold two orthogonal LPOl and LPll even modes, which are uniquely defined and have stable intensity lobe orientation. Both pump and probe light are operated at two-mode status. When the high intensity pump light pulse is injected into the fiber with equal power for its two modes in the same polarisation direction, the total optical intensity will vary periodically according to the beatlength and induce a refractive index change, which is considered as a periodic perturbation. Then by launching the Optical probe Microscope

An analysis of the linearly polarised modes in elliptical-core fiber

The linearly polarised (LP) modes in elliptical-core fibre have been analysed theoretically. It was found that the electromagnetic fields of the LP modes in elliptical-core fibre can be expressed as a combination of Mathieu and modified Mathieu functions. The modal characteristic equations and propagation constants are derived. The cutoff frequencies of several lower order modes have been calculated and their propagation characteristics plotted. The effects of core ellipticity on the modal cutoff values and mode transmissions are addressed

Theoretical analysis of the LP mode in e-core fiber

The LP mode in e-core fiber has been analyzed theoretically. It is found that the solution of the modal fields of e-core fiber is a combination of Mathieu and modified Mathieu functions. The modal eigenvalue equations are derived. The mode nondegeneracy and cutoff frequency are discussed. In addition to deriving a general expression for propagation constant, we have also obtained the propagation characteristics curves of several lower-order modes for different core ellipticities.