New factor graph based multiuser detector for spectrally efficient CPM

This paper presents a new iterative multiuser detection algorithm for asynchronous spectrally-efficient M-ary continuous-phase modulation in additive white Gaussian noise. This detection algorithm is closely related to another algorithm that was recently proposed by the same authors, but it follows from applying the sum-product algorithm to a different factor graph of the same multiuser detection problem. This, in turn, results in a different way to approximate the marginal bit a-posteriori probabilities that are used to perform minimum bit error rate multiuser detection. The girth of the factor graph considered in this contribution is twice as large, which is known to be potentially beneficial for the accuracy of the a-posteriori probabilities. The size of the largest factor graph variable alphabets also multiplies with M, rendering the straightforward application of the sum-product algorithm more complex. Through approximating a suitable set of sum-product messages by a Gaussian distribution, this complexity is significantly reduced. For a set of system parameters yielding high spectral efficiency, the resulting algorithm significantly outperforms the previously proposed solution

Synchronization in digital communication systems: performance bounds and practical algorithms

Communication channels often transfer signals from different transmitters. To avoid interference the available frequency spectrum is divided into non-overlapping frequency bands (bandpass channels) and each transmitter is assigned to a different bandpass channel. The transmission of a signal over a bandpass channel requires a shift of its frequency-content to a frequency range that is compatible with the designated frequency band (modulation). At the receiver, the modulated signal is demodulated (frequency shifted back to the original frequency band) in order to recover the original signal. The modulation/demodulation process requires the presence of a locally generated sinusoidal signal at both the transmitter and the receiver. To enable a reliable information transfer, it is imperative that these two sinusoids are accurately synchronized. Recently, several powerful channel codes have been developed which enable reliable communication at a very low signal-to-noise ratio (SNR). A by-product of these developments is that synchronization must now be performed at a SNR that is lower than ever before. Of course, this imposes high requirements on the synchronizer design. This doctoral thesis investigates to what extent (performance bounds) and in what way (practical algorithms) the structure that the channel code enforces upon the transmitted signal can be exploited to improve the synchronization accuracy at low SNR

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

Frame synchronization for pulsed jammed satellite telecommand links

A new issue of the satellite telecommand synchronization and channel coding sublayer protocol 1 includes LDPC coded communication link transmission units (CLTU) that contain a 64-bit start sequence. The novel data structures allow operation at lower signal-to-noise ratios than before, and offer improved protection against jamming attacks. This paper considers the corresponding CLTU frame synchronization process. We derive practical algorithms to locate the start sequence in the presence of high noise levels and pulsed jamming. The different algorithms are compared in terms of implementation complexity and performance under various jamming conditions. It is shown that among the considered frame synchronizers, those involving a full search over the entire observation window provide the desired accuracy, i.e., they guarantee a frame synchronization error probability that is significantly smaller than the codeword error rate, for codeword error rates near a target value of 10−4 . Among these synchronizers, the full-search hard-decision-directed correlation-based algorithm has the lowest complexity

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

Pilot based single user frequency offset estimation in spectrally-overlapping FDMA CPM systems

The spectral efficiency of a frequency division multiple access system can be increased by allowing some spectral overlap of adjacent user signals, at the expense of higher interuser interference. We derive the linearized mean square error of pilot based single user maximum likelihood frequency offset estimation in such a system, assuming continuous phase modulation. We consider synchronous as well as asynchronous reception of the pilot signals from the various users. Moreover, the pilot signals are assumed to be either constant and equal to 1, or pseudo-random and independent for all users. In spite of the presence of interuser interference, we obtain relatively simple closed-form expressions, from which the effect of the modulation parameters, the pilot signal structure and the number of users is easily derived

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]