Fabio Pisoni

Echo cancellation in DMT-receivers: circulant decomposition canceler

Asymmetric digital subscriber lines (ADSLs) employ discrete multitone modulation (DMT) as transmission format, where subcarriers are assigned to the up- and/or downstream transmission direction. To separate up- and downstream signals, the ADSL standard allows the use of echo cancellation resulting in improved bit rates, reach, and/or noise margins. In DMT-based modems, typically, the mixed time/frequency (MTF) domain echo canceling scheme, as proposed by Ho et al., is implemented. This technique estimates the echo filter in the frequency domain using the least mean square (LMS) algorithm with the transmitted echo symbols as update directions. Since not every tone of the transmitted echo signal will carry data, i.e., will be excited, the MTF adaptation process does not lead to a good estimate for the echo channel, unless extra power on unused echo tones is transmitted. However, transmitting extra power on such tones is often undesired. In this paper, we present an alternative echo canceling scheme referred to as the circulant decomposition canceler (CDC), which works without extra power requirements and with comparable complexity as the method of Ho et al. Similar to MTF echo canceling, the CDC scheme can easily be incorporated into a multirate environment with different transmit and receive rates and can also cheaply be combined with per-tone equalization and double talk cancellation to allow fast tracking and/or convergence in the presence of a far-end signal.

Correction of pseudorange errors in Galileo and GLONASS caused by biases in group delay

Both Galileo E1 OS and GLONASS L1 signals require a wider pre-correlation bandwidth than GPS C/A. The GLONASS channels are uniformly spaced on a frequency division grid, and occupy some 8MHz in total, whereas Galileo E1 OS has sidebands that are separated by about 2MHz. Being wideband is an advantage because of sharper correlation peak (Galileo) and better interference rejection; however it also exposes distant spectral components of the signal to larger variations of the front-end transfer function. Group delay differences are mostly caused by the analog components like the SAW filter or the integrated analog front-end whereas magnitude can be affected by signal conditioning done in the digital domain, for example by the insertion of recursive filters aimed at interference cancellation. These impairments must be estimated and compensated. The benefits for the consumer multi-constellation receiver are an improved precision and an enriched set of potential applications, including timing.

Echo Cancellation in DMT Modems With Frame-Asynchronous Operation

Echo cancellation in DMT modems has been long studied as a means to improve transmission performance. Most of the current echo-canceller structures make use of FIR filters that emulate the echo-path adaptively. In frame-asynchronous operation, the misalignment of the TX and RX DFT windows (combined with other modem and channel parameters like: the transmit tone-mask and the echo-path impulse response) determines the echo input statistics and can worsen the conditioning of the coefficient update formula. This effect is analyzed in the paper, within the context of two specific algorithms: the circulant decomposition canceller (CDC) and also the time-domain canceller (TD-EC). Self-orthogonalizing (S-O) update formulas, implemented through compensation matrices, and their respective low-rank approximations are proposed as a method to improve the convergence rate and, more generally, to render the echo-canceller performances less sensitive to the input statistics. The method introduced does not employ additional power on the unsent tones

Estimation of the GPS to Galileo time offset and its validation on a mass market receiver

The European GNSS, Galileo is currently in its In-Orbit Validation (IOV) phase where four satellites are available for computing the user position. Galileo El OS and GPS C/A represent a very effective constellation pair in a consumer grade receiver: the signals are conveyed on the same analog path and measurements can be combined in a single position, velocity and time (PVT) solution, provided that the Galileo to GPS Time Offset (GGTO) is available. Algorithms for GGTO estimation are presented in this paper, including a real-time implementation on a compact hardware receiver, based on the Teseo-II chipset. Experimental results are provided, showing the benefit in terms of accuracy and availability of the Galileo plus GPS multi-constellation solution respect to the GPS only case. Also analyzed are the effects on the GGTO estimate of different ionospheric models, namely: GPS Klobuchar and Galileo NeQuick. Both simulated scenarios and real, recorded Galileo IOV samples from a roof antenna are used in the validation of the proposed algorithms.

A multi-carrier echo canceller based on symmetric decomposition

Echo cancellation in DMT modems is a means to improve transmission performance. It consists in extending the downstream (DS) and upstream (UP) frequency bands so that they become adjacent or even overlap, and relaxing at the same time the existing filters. Modern cancellers make use of FIR filters that emulate the echo-path, and adaptive algorithms that estimate the correspondent coefficients. In this article, we start from the circulant decomposition canceller (CDC), an efficient algorithm recently developed for ADSL, and explore new kinds of decomposition, based on the diagonalization properties of the discrete cosine and sine transforms (DCT, DST).

A BeiDou hardware receiver based on the STA8088 chipset

The recent release, in December 2012, of the complete BeiDou B1I Interface Control Document has boosted new interest and R&D activities in the field. For a manufacturer of GNSS semiconductor devices, the first and logical step in the roadmap toward a full blown BeiDou receiver is the reuse of existing and proven silicon; the new product can blossom and reach the market more quickly, with good overall performance, though not yet optimized in every single aspect. Following this approach, the paper describes a BeiDou receiver implementation, based on the STA8088 “Teseo-II” consumer chip, which was originally designed for GPS, Galileo and GLONASS. Several technical solutions adopted in the receiver are disclosed, including aspects of the RF path, signal processing and higher level software algorithms like GEO velocity computation and ionosphere models. The main difficulty encountered was the generation of the B1I ranging codes, which was accomplished, with some performance trade-offs, by reusing the on-chip Galileo programmable code memory. The final result is a compact, consumer grade BeiDou hardware receiver, suitable for navigation and timing applications. Static positioning plots from real data, collected at a roof antenna are also reported.

Initial signal tracking tests of Galileo IOV and Compass/Beidou satellites

Initial results using Teseo-2 to track both Galileo IOV satellites and Compass MEO satellites are presented showing that the hardware is ready for the full constellations in both cases. Development software under test awaits satellite availability for full field testing. Future plans for other constellations and dual bands are discussed.

GPS-III L1C signal reception demonstrated on QZSS

Consumer Multiconstellation silicon started with GPS-Galileo around 2007 [1], but with Glonass satellites available before Galileo, was first seen publically Teseo-2, STA8088, in 2010/11 [2,3,4]. With QZSS, Compass, GPS-L1C, Glonass-CDMA all expected, the silicon manufacturer must continue the path towards the fully flexible multi constellation receiver, otherwise the number and rate of new required chips would be beyond design and test resource capability. The early availability of the L1C signal on the QZSS satellite helps greatly, and is studied in this paper.

Application of the chirp-z transform to fractional interpolation in DMT modems

Sampling clock synchronization in multi carrier systems, such as discrete multitone (DMT) digital subscriber line modems, can be done with a phase locked loop. This requires expensive voltage-controlled oscillators (VCO). Alternatively, clock-offset compensation can be done completely in the digital domain, replacing the VCO with a cheaper free-running oscillator. This solution requires the signal of interest to be fractionally interpolated in the digital domain. In DMT, the digital interpolator can be combined with the modulating DFT into one single fractional operator: the chirp-z transform. In this paper, we explore interpolation algorithms based on the Chirp-z transform and tailor them at clock-offset recovery for DMT modems.