Marc Moeneclaey

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. >

Log-domain decoding of LDPC codes over GF(q)

This paper introduces a log-domain decoding scheme for LDPC codes over GF(q). While this scheme is mathematically equivalent to the conventional sum-product decoder, log-domain decoding has advantages in terms of implementation, computational complexity and numerical stability. Further, a suboptimal variant of the log-domain decoding algorithm is proposed, yielding a lower computational complexity. The proposed algorithms and the sum-product algorithm are compared both in terms of simulated BER performance and computational complexity.

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.

Analysis and optimization of the performance of OFDM on frequency-selective time-selective fading channels

In mobile radio communication, the fading channels generally exhibit both time-selectivity and frequency-selectivity. Orthogonal frequency-division multiplexing has been proposed to combat the frequency-selectivity, but its performance is also affected by the time-selectivity. We investigate how various parameters, such as the number of carriers, the guard time length, and the sampling offset between receiver and transmitter, affect the system performance. Further, we determine the optimum values of the above parameters, which minimize the degradation of the signal to-noise ratio at the input of the decision device.

On the true and the modified Cramer-Rao bounds for the estimation of a scalar parameter in the presence of nuisance parameters

We consider the Cramer-Rao bound (CRB) for the estimation of a scalar parameter in the presence of nuisance parameters. Whereas this true CRB is hard to evaluate in general, we present a simple analytical expression for its high SNR asymptote, i.e., the asymptotic CRB (ACRB). We show that this ACRB is related to the CRB for the joint estimation of the scalar parameter and the nuisance parameters. Previously, a modified CRB (MCRB) for the estimation of a scalar parameter in the presence of nuisance parameters has been derived. This MCRB is also simple to evaluate and is related to the CRB for the estimation of the scalar parameter assuming that the value of the nuisance parameters is a priori known. We show that the MCRB can be quite loose at high SNR, when the scalar parameter is coupled with the nuisance parameters. In the case of synchronization parameter estimation, we find that the ACRB equals the MCRB, whether or not (some of) the nuisance parameters are a priori known.

Turbo synchronization: an EM algorithm interpretation

This paper is devoted to turbo synchronization, that is to say the use of soft information to estimate parameters like carrier phase, frequency offset or timing within a turbo receiver. It is shown how maximum-likelihood estimation of those synchronization parameters can be implemented by means of the iterative expectation-maximization (EM) algorithm [A.P. Dempster, et al., 1977]. Then we show that the EM algorithm iterations can be combined with those of a turbo receiver. This leads to a general theoretical framework for turbo synchronization. The soft decision-directed ad-hoc algorithm proposed in V. Lottici and M. Luise, [2002] for carrier phase recovery turns out to be a particular instance of this implementation. The proposed mathematical framework is illustrated by simulations reported for the particular case of carrier phase estimation combined with iterative demodulation and decoding [S. ten Brink, et al., 1998].

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.

On the Throughput Performance of Some Continuous ARQ Strategies with Repeated Transmissions

The paper considers a class of continuous ARQ strategies, whereby multiple copies of each data block are sent contiguously (instead of one single copy), and whereby the data blocks are delivered at the receiver side in their order of arrival at the transmitter. In order to achieve the best possible throughput efficiency-within this class-an optimum value for the number of copies to be sent is determined, in terms of the block error probability and the propagation delay. It turns out that, depending on these parameters, the optimum scheme is one of the two following alternatives: 1) sending each data block repeatedly until a positive acknowledgment is received for it; 2) sending each block a constant (optimum) number of times. A throughput comparison of the optimum scheme with other related ARQ strategies is given.

Code-Aided Turbo Synchronization

The introduction of turbo and low-density parity-check (LDPC) codes with iterative decoding that almost attain Shannon capacity challenges the synchronization subsystems of a data modem. Fast and accurate signal synchronization has to be performed at a much lower value of signal-to-noise ratio (SNR) than in previous less efficiently coded systems. The solution to this issue is developing specific synchronization techniques that take advantage of the presence of the channel code and of the iterative nature of decoding: the so-called turbo-synchronization algorithms. The aim of this paper within this special issue devoted to the turbo principle is twofold: on the one hand, it shows how the many turbo-synchronization algorithms that have already appeared in the literature can be cast into a simple and rigorous theoretical framework. On the other hand, it shows the application of such techniques in a few simple cases, and evaluates improvement that can be obtained from them, especially in the low-SNR regime.

Extending the capacity of multiple access channels

Multiple access techniques which allow a communication medium to be shared between different users represent one of the most challenging topics in digital communications. In terms of the number of users that can be accommodated on a given channel, there are two distinct classes of multiple access techniques. The first class includes the well-known FDMA, TDMA, and OCDMA. On a channel whose bandwidth is N times the bandwidth of the individual user signals, these techniques can accommodate N users without any mutual interference, but not a single additional user can be supported beyond this limiting number. The second class includes CDMA with pseudo-noise spreading sequences (which we refer to as PN-CDMA) and some other related schemes. PN-CDMA does not have a hard limit on the number of users that can be accommodated, but is subject to multi-user interference which grows linearly with the number of users. In this article, after reviewing the capacity limits of existing multiple access techniques, we describe some newly introduced concepts which allow us to accommodate N users without any interference while also accommodating additional users at the expense of some SNR penalty.