Thierry Pollet

BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise

In this contribution the transmission of M-PSK and M-QAM modulated orthogonal frequency division multiplexed (OFDM) signals over an additive white Gaussian noise (AWGN) channel is considered. The degradation of the bit error rate (BER), caused by the presence of carrier frequency offset and carrier phase noise is analytically evaluated. It is shown that for a given BER degradation, the values of the frequency offset and the linewidth of the carrier generator that are allowed for OFDM are orders of magnitude smaller than for single carrier systems carrying the same bit rate. >

Per tone equalization for DMT-based systems

An alternative receiver structure is presented for discrete multitone-based systems. The usual structure consisting of a (real) time-domain equalizer in combination with a (complex) 1-tap frequency-domain equalizer (FEQ) per tone, is modified into a structure with a (complex) multitap FEQ per tone. By solving a minimum mean-square-error problem, the signal-to-noise ratio is maximized for each individual tone. The result is a larger bit rate while complexity during data transmission is kept at the same level. Moreover, the per tone equalization is shown to have a reduced sensitivity to the synchronization delay.

The BER performance of OFDM systems using non-synchronized sampling

In fully digital receivers, carrier and timing information is derived from samples of the (anti-aliasing-filtered) received continuous-time signal. In case of synchronized sampling, this information is used to align the sampling clock of the receiver with the remote transmit clock. In nonsynchronized sampling systems, the sampling at the receiver is performed by means of a fixed free-running clock, and additional post-processing is necessary to perform timing correction in the digital domain. We investigate the effect of non-synchronized sampling on the BER performance of OFDM systems. We calculate the BER degradation caused by a given frequency offset between receiver and transmitter clock, as compared with the case of ideal sampling. The obtained results are compared with the performance of synchronized sampling systems.

Synchronizability of OFDM signals

This paper deals with symbol timing and carrier phase estimation for orthogonal frequency division multiplexed signaling (OFDM) in satellite/terrestrial systems. For transmission over frequency selective channels, a synchronizer at the receiver estimates the phase rotation of each individual carrier; the synchronizer can not distinguish between the phase shift introduced by the channel and by a symbol timing delay. It is shown that for transmission over a nondispersive channel (e.g. fixed satellite link) this algorithm performs suboptimal as compared to a synchronizer which separately estimates the unknown phase and timing. A lower bound on the timing error variance and phase error variance for both synchronizers is calculated. The BER degradation due to imperfect estimation of the timing and phase is given.

Synchronization with DMT modulation

In a digital transmission system, synchronization is an essential receiver function. Accurate timing information must be known to the demodulator in order to produce reliable estimates of the transmitted data sequence. In this article, synchronization for discrete multitone transmission (DMT) is examined. The effect of imperfect timing on the receiver performance is investigated. An overview of timing estimation and correction circuits based on data-aided and non-data-aided algorithms derived from maximum-likelihood estimation theory is given.

Estimation of the transfer function of a subscriber loop by means of a one-port scattering parameter measurement at the central office

In order to qualify a subscriber loop for xDSL transmission, the channel capacity has to be estimated, which depends on the transfer function of the network. A method is provided to estimate the transfer function of the subscriber loop only measuring the one-port scattering parameter at the central office. We consider three types of networks according to their topology: a single line, a homogeneous network with a bridged tap, and a cascade of two line sections. For each type of network a parametric model is derived of its one-port scattering parameter and transfer function based on the physical line model VUB0. The model for the scattering parameter is used to identify the network based on the corresponding measurements by means of a maximum-likelihood estimator. The estimated parameters are substituted in the transfer function model, which is needed for the capacity estimation. The proposed models and estimators are validated by measurements and simulations. For the measurements, which were performed with a network analyzer, three types of twisted-pair cables were used: British Telecom (BT), Deutsch Telekom (FT), and Belgacom.

Equalization for DMT based broadband modems

Faced with fierce competition from cable operators, telephone companies are now aggressively investing in digital subscriber line technology with the objective of providing an economic broadband access alternative. However, high-bit-rate transmissions over the metallic access network is far from trivial. The subscriber loop predominantly consists of interconnection of twisted pair copper sections. This causes severe attenuation of the high-frequency components of the transmitted signal. Equalization refers to the signal processing in the transceiver to combat the effects of the dispersive channel on the transmitted signal. This article overviews equalization methods for discrete multitone modulation (DMT). The interference structure created by a non-ideally equalized channel is analyzed. Time modulation as well as frequency domain equalization methods are addressed.

RLS-based initialization for per-tone equalizers in DMT receivers

Per tone equalization has been proposed in [6] [7] as an alternative to time domain equalization for DMT-based systems. In this paper, an iterative initialization scheme based on so-called RLS with inverse updating is presented for these equalizers. Simulation results show convergence with an acceptably small number of training symbols. Complexity calculations are made for per tone equalization and for the case where tones are grouped. It is demonstrated with an example that in the latter case, initialization complexity becomes sufficiently low and comparable to complexity during data transmission.

Combined RLS-LMS initialization for per tone equalizers in DMT-receivers

In classical discrete multi tone receivers, the equalizer structure consists of a (real) time domain equalizer (TEQ) combined with (complex) 1-taps frequency domain equalizers. Recently, an alternative receiver was introduced based on a separate per tone equalization, where the SNR on each tone and hence the bitrate is maximized. In this paper, a new adaptive initialization scheme for this per tone equalizer (PTEQ) is introduced, based on a combination of Least Mean Squares (LMS) and Recursive Least Squares (RLS) with inverse updating. It will be shown that the proposed method exhibits only slightly slower convergence compared to full RLS with inverse updating, while complexity as well as memory cost are reduced considerably.

The effect of carrier frequency offset on the performance of band limited single carrier and OFDM signals

In this contribution, the bit error rate (BER) degradation caused by a carrier frequency offset between the transmitter and receiver carrier oscillator is investigated for single carrier (SC) and multi carrier (OFDM) signaling. For both modulation formats a carrier frequency offset results in a rotation and an attenuation of the transmitted symbols. In addition, this impairment gives rise to inter-symbol interference (ISI) and destroys the orthogonality between the carriers of the multi carrier signal which yields inter-carrier interference (ICI). The analysis shows that for SC transmission the BER degradation is dominated by the reduction of the useful signal component, provided that the carrier frequency offset is small with respect to the SC symbol rate. For OFDM transmission, a comparison of the different interferences reveals that the BER degradation is determined by the ICI.