Jürgen Lindner

Diversity comparison of spreading transforms for multicarrier spread spectrum transmission

Multicarrier spread spectrum transmission methods based on orthogonal frequency division multiplexing employ a linear transform to spread energy of transmitted symbols over statistically independent Rayleigh fading subcarriers, in order to enable a diversity gain at the receiver. We jointly treat the mapping from bits into transmit symbols and the spreading transform as a code in Euclidean space. We describe criteria to identify good and bad spreading transforms, in terms of the asymptotic bit-error probability (BEP) at high signal-to-noise ratio. Upper and lower bounds on the BEP are derived. It is shown that the BEP performance of the commonly used Hadamard transform is asymptotically bad. Alternative orthogonal transforms with better asymptotic performance are proposed. Simulation results are given for the example of 2-phase-shift keying, block length 8.

MC‐CDMA in the Context of General Multiuser/ Multisubchannel Transmission Methods

MC-CDMA is considered in the context of more general multiuser / multisubchannel transmission systems. Those systems understand the transmission task for all users as an integrated whole and they allow an easy understanding of the advantages and disadvantages of MC-CDMA in comparison with alternative methods. Multiple antennas are also included. An appropriate vector-valued transmission model is explained and then special the cases OFDM-OFDMA and MC-CDMA are considered. Examples are discussed which demonstrate the advantages of spreading techniques in the context of OFDM-OFDMA and the additional gain in power (or bandwidth) efficiency compared with pure OFDM-OFDMA. Simulation results for an indoor scenario with 8 users are presented. Variants of MC-CDMA, extensions and alternatives are also discussed.

Multiple Binary ZCZ Sequence Sets With Good Cross-Correlation Property Based on Complementary Sequence Sets

In this paper, two types of multiple binary zero correlation zone (ZCZ) sequence sets are constructed: 1. Each set is a binary <i>(2</i>ⁿ⁺¹<i>Z</i>_cz,2ⁿ, <i>Z</i>_cz)-ZCZ sequence set, which is one of the best cases in the known constructions. 2. Among all the sets, the sequences still possess good cross-correlation property within the zone of length <i>Z</i> ≤ <i>Z</i>_cz. In particular, the first type sequence sets have a common zero correlation zone of length <i>Z</i>, which solves the previous research problem. These multiple binary ZCZ sequence sets are suitable for multiuser environments.

Estimation of rapid time-variant channels for OFDM using Wiener filtering

The performance of orthogonal frequency division multiplexing (OFDM) deteriorates in a rapidly time-varying environment due to the loss of orthogonality, resulting in inter-channel interference (ICI). Moreover, the quality of model-based channel estimation may degrade if the estimator has been designed for a time-invariant scenario. In this paper, we consider OFDM and pilot-aided channel estimation via Wiener filtering and derive the filter for a time-variant WSSUS model. In addition, the analysis allows to compute the performance loss for an improper estimation filter which suffers from the problem of model mismatch.

An improved block equalization scheme for uncertain channel estimation

We consider the design of a block equalizer for an intersymbol interference channel, given that the channel impulse response is not perfectly known at the receiver. In contrast to other schemes, our receiver is designed for imperfect channel state information and incorporates the statistics of the channel estimation error. In particular, we suggest an error model for data transmission that takes the influence from the data symbols on the estimation noise into account. We derive the optimum detection rule for the considered error model according to the maximum likelihood criterion and verify that the covariance matrix of the estimation noise depends on the actual transmitted data symbols. Motivated by this result, we propose a realizable receiver structure adopting the turbo principle that exploits the data-dependency of the covariance matrix of the estimation noise. The proposed scheme outperforms conventional receivers that neglect the exact statistics of the estimation noise. The core of our receiver is a soft-input soft-output block equalizer based on constrained minimum variance filter design. We assess the performance of the proposed turbo equalization scheme for block Rayleigh fading channels, applying both one-shot training-based channel estimation and iterative data-aided channel estimation

How to Use A Priori Information of Data Symbols for SNR Estimation

We address the problem of how to use a priori information of data symbols to improve signal-to-noise ratio (SNR) estimation. Digital transmission with binary phase-shift keying (BPSK) over additive white Gaussian noise (AWGN) channels serves as a background. At the receive side, both pilot and data symbols are used to estimate the SNR. In addition, we assume that a priori information of the data symbols is available. Our proposed estimator is then derived as an approximate solution of the maximum-likelihood (ML) approach. A significant improvement in the low-SNR regime over the corresponding estimator without a priori information is shown. Hence, an estimator that uses a priori information of data symbols is suitable to be embedded in iterative decoding schemes like, e.g., turbo decoding or turbo equalization. In addition, the Cramer-Rao lower bound (CRLB) for SNR estimators using a priori information of data symbols is derived

Modeling and performance evaluation of multiuser MIMO systems using real-valued matrices

A discrete-time matrix transmission model is introduced for wideband multiuser multiple-input multiple-output (MIMO) transmission systems. The model is more general than existing ones in the sense that it incorporates correlated interference and noise by means of prewhitening. It allows a flexible treatment of users that may or may not be included in a joint detection process. On the basis of the matrix transmission model, conventional multiuser detection techniques are easily adapted for the multiuser MIMO case. It is shown that for certain types of interference, a linear matrix transmission model exists only for a real-valued notation. Moreover, the real-valued notation provides the possibility to include space-time block codes in the linear transmission model. In order to endorse the introduction of this particular matrix model, we show that the discrete-time transmission matrix immediately determines the capacity of the MIMO transmission system. Some simulation results based on optimum and suboptimum detection techniques indicate that the loss of performance due to the exclusion of users from joint detection is quite small.

Almost Quadriphase Sequence With Ideal Autocorrelation Property

In this letter, a new class of almost quadriphase sequences with a single zero element of period N, where N=1( mod 4) a prime, is presented. The new sequences have ideal autocorrelation property.

Robust Transmission Over Fast Fading Channels on the Basis of OFDM-MFSK

We present an OFDM-based transmission scheme which is suitable for robust transmission in fast fading environments, where a reliable channel estimate is impossible or very difficult to obtain. Our scheme is based on the combination of noncoherently detected M-ary frequency shift keying (MFSK) and orthogonal frequency division multiplexing (OFDM). Noncoherent detection of OFDM-MFSK allows an arbitrary phase choice for all subcarriers in the transmitter. One possibility to exploit this degree of freedom is to choose the subcarrier phases such, that the peak-to-average power ratio (PARR) is reduced. A second possibility is to use the subcarrier phases to transmit additional data. This can be done by differentially modulating the subcarriers that are occupied by the OFDM-MFSK scheme. Both possibilities do not affect the robustness of the underlying noncoherently detected OFDM-MFSK modulation.

Equalization and coding for extended MC-CDMA over time and frequency selective channels

In this work Multi Carrier Code Division Multiple Access (MCCDMA) — based on Orthogonal Frequency Division Multiplex (OFDM) combined with Hadamard spreading sequences — is extended by an additional spreading in time direction. Introducing the Kronecker Product the two transformations in time and frequency direction can be merged, resulting in one larger transformation.