True Cramer-Rao bounds for carrier and symbol synchronization

This contribution considers the Cramer-Rao bound (CRB) related to estimating the synchronization parameters (carrier phase, carrier frequency and time delay) of a noisy linearly modulated signal with random data symbols. We explore various scenarios, involving the estimation of a subset of the parameters while the other parameters are either considered as nuisance parameters or a priori known to the receiver. In addition, some results related to the CRB for coded transmission will be presented

BER analysis of high-speed OFDM systems in the presence of time-interleaved analog-to-digital converter's offset mismatch

Time-interleaved analog-to-digital converters (TI-ADCs) are widely used for multi-Gigabit orthogonal frequency division multiplexing (OFDM) systems because of their attractive high sampling rate and high resolution. However, mismatch between the parallel sub-ADCs can severely degrade the system performance. Several types of mismatch can be distinguished, one particular kind of mismatch is offset mismatch, which originates from the different DC offsets in the different sub-ADCs. Although some authors have studied the effect of offset mismatch on the bit error rate (BER) performance, exact close-form BER expressions in the presence of offset mismatch have not been derived yet. In this poster, we derive such BER expressions. Gray-coded PAM or QAM signaling over an additive white Gaussian noise channel is considered. Our numerical results show that the obtained theoretical BER expressions are in excellent agreement with the simulated BER performance. We also investigate simplified expressions for the error floor occurring at large SNR and large offset mismatch. Our finding shows that this error floor is essentially independent of the modulation order and the type of modulation

BER of high-speed OFDM systems in the presence of offset mismatch of TI-ADCs

Time-interleaved analog-to-digital converters (TI-ADCs) are widely used for multi-Gigabit orthogonal frequency division multiplexing (OFDM) systems because of their attractive high sampling rate and high resolution. However, in practice, offset mismatch, one of the major mismatches of TI-ADCs, can occur between the parallel sub-ADCs. In this poster session, we theoretically analyze the BER performance of high-speed OFDM systems using TI-ADCs with offset mismatch. Gray-coded PAM or QAM signaling over an additive white Gaussian noise channel is considered. Our numerical results show that the obtained theoretical BER expressions are in excellent agreement with the simulated BER performance

The true Cramer-Rao bound for estimating the carrier phase of a convolutionally encoded PSK signal

This contribution considers the true Cramer-Rao bound (CRB) related to estimating the carrier phase of a noisy linearly modulated signal in the presence of encoded data symbols. Timing delay and frequency offset are assumed to be known. A generall expression and computational method is derived to evaluate the CRB in the presence of codes for which a trellis diagram can be drawn (block codes, trellis codes, convolutional codes,...). Results are obtained for several minimum free distance non-recursive convolutional (NRC) codes, and are compared with the CRB obtained with random (uncoded) data [1] and with the modified Cramer-Rao bound (MCRB) from [2]. We find that for small signal-to-noise ratio (SNR) the CRB is considerably smaller for coded transmission than for uncoded transmission. We show that the SNR at which the CRB is close to the MCRB decreases as the coding gain increases, and corresponds to a bit error rate (BER) of about 0.001. We also compare the new CRBs with the simulated performance of (i) the (code-independent) Viterbi & Viterbi phase estimator [3] and (ii) the recently developed turbo synchronizer [4,5]

Pilot allocation based on simulated annealing for sparse channel estimation in UWB OFDM systems

In ultra wideband (UWB) orthogonal frequency-division multiplexing (OFDM) systems, compressive sensing (CS) is often employed to produce a pilot-assisted estimate of the sparse channel. The corresponding estimation performance depends to a large extent on the considered pilot allocation (PA) method, i.e., the way to select which OFDM subcarriers are best used to transmit the pilot symbols. The development of good practical PA methods has recently received a lot of attention in the scientific literature. The main challenge is to provide an attractive trade-off between the complexity of the PA method and the achieved channel estimation performance (and by extension the achieved bit error rate). In this paper, we propose a novel PA method based on simulated annealing (SA). Simulations are conducted to confirm the validity of our approach. Compared to the state-of-the-art method, the proposed PA method is shown to achieve better performance with a lower complexity

Accurate and efficient BER evaluation for high-speed OFDM systems impaired by TI-ADC circuit’s gain mismatch

This paper presents an efficient procedure to numerically evaluate the exact bit error rate of a rectangular quadrature amplitude modulated-orthogonal frequency division multiplexing (OFDM) system that is impaired by the gain mismatch of a time-interleaved analog-to-digital converter. As opposed to previous contributions, no approximations are involved in this procedure. The obtained results allow to accurately analyze the effect of this type of mismatch on the performance of practical high-speed OFDM systems. The accuracy and efficiency of the proposed technique are demonstrated by comparing analytical results with brute-force Monte-Carlo simulations

Offset mismatch calibration for TI-ADCs in high-speed OFDM systems

Time-interleaved analog-to-digital converters (TIADCs) are widely used for multi-Gigabit orthogonal frequency division multiplexing (OFDM) based systems because of their attractive high sampling rate and high resolution. However, when not perfectly calibrated, mismatches such as offset mismatch, gain mismatch and timing mismatch between parallel sub-ADCs can significantly degrade the system performance. In this paper, we focus on offset mismatch. We analyze two calibration techniques for the offset mismatch, based on the least-squares (LS) and linear minimum mean-squared error (LMMSE) algorithms assuming an AWGN channel. The simulation results show that our method is capable of improving the BER performance. As expected, the LMMSE estimator outperforms the LS estimator. However, at large offset mismatch levels or low noise level, both estimators converge. In this paper, we derive the condition on the mismatch level for convergence between the two estimators

The CXCL12/CXCR4 chemokine ligand/receptor axis in cardiovascular disease

The chemokine receptor CXCR4 and its ligand CXCL12 play an important homeostatic function by mediating the homing of progenitor cells in the bone marrow and regulating their mobilization into peripheral tissues upon injury or stress. Although the CXCL12/CXCR4 interaction has long been regarded as a monogamous relation, the identification of the pro-inflammatory chemokine macrophage migration inhibitory factor (MIF) as an important second ligand for CXCR4, and of CXCR7 as an alternative receptor for CXCL12, has undermined this interpretation and has considerably complicated the understanding of CXCL12/CXCR4 signaling and associated biological functions. This review aims to provide insight into the current concept of the CXCL12/CXCR4 axis in myocardial infarction (MI) and its underlying pathologies such as atherosclerosis and injury-induced vascular restenosis. It will discuss main findings from in vitro studies, animal experiments and large-scale genome-wide association studies. The importance of the CXCL12/CXCR4 axis in progenitor cell homing and mobilization will be addressed, as will be the function of CXCR4 in different cell types involved in atherosclerosis. Finally, a potential translation of current knowledge on CXCR4 into future therapeutical application will be discussed