Gökhan Muzaffer Güvensen

A General Framework for Optimum Iterative Blockwise Equalization of Single Carrier MIMO Systems and Asymptotic Performance Analysis

The paper proposes a general framework for both time-domain (TD) and frequency-domain (FD) iterative blockwise equalization in single carrier (SC) wideband multiple-input multiple-output (MIMO) channels. First, a novel turbo blockwise operating equalizer structure is proposed by jointly optimizing the feed-forward and feedback filters at each iteration based on the minimum mean squared error (MMSE) criterion. Optimization of the filter coefficients, utilized for feed-forward equalization and decision feedback, is performed in both time and frequency domains, and the equivalence between them is established by unifying the iterative block equalization with the feed-forward and feedback filters in various domains. The corresponding filters are obtained analytically in a closed form as a solution of the constrained Wiener-Hopf equation with the use of average reliability information. Second, asymptotic performance and diversity analysis of the proposed single carrier frequency domain equalizer (SC-FDE) is carried out that leads to insights on the structures of the filters in the high SNR regime. Furthermore, we elaborate on the rate and diversity tradeoff of the space-time coded SC-FDE based on the proposed equalizer structure and the space-frequency coded OFDM systems. Performance comparison between the proposed SC-FDE scheme and OFDM is made. The proposed SC-FDE scheme performs very close to the genie-aided performance bounds and better than OFDM based transmission.

Iterative Decision Feedback Equalization and Decoding for Rotated Multidimensional Constellations in Block Fading Channels

It is known that rotated multidimensional constellations can be used effectively to achieve full-rate and full-diversity transmission in block fading channels. However, optimal decoding complexity is exponential with the number of fading blocks (or degrees of freedom). In this paper, we propose a reduced-complexity iterative receiver structure operating on a block basis for coded modulation schemes with rotated constellations. The proposed detector is based on iterative forward and backward filtering followed by a channel decoder that uses a priori log- likelihood ratios (LLR) of coded symbols. Forward and feedback filters are jointly optimized according to the minimum mean square error (MMSE) criterion to minimize the spatial interference induced by rotation. It is observed that the proposed structure achieves full diversity and performance close to outage probability for rotated inputs even with simple Discrete Fourier Transform (DFT) rotations.

On generalized eigenvector space for target detection in reduced dimensions

The detection and estimation problems with large dimensional vectors frequently appear in the phased array radar systems equipped with, possibly, several hundreds of receiving elements. For such systems, a preprocessing stage reducing the large dimensional input to a manageable dimension is required. The present work shows that the subspace spanned by the generalized eigenvectors of signal and noise covariance matrices is the optimal subspace to this aim from several different viewpoints. Numerical results on the subspace selection for the radar target detection problem is provided to examine the performance of detectors with reduced dimensions.

A Reduced-State Ungerboeck Type MAP Receiver with Bidirectional Decision Feedback for M-ary Quasi Orthogonal Signaling

In this paper, we propose a generic receiver based on optimum maximum likelihood sequence estimation (MLSE) employing a generalized Ungerboeck metric with significantly reduced complexity for the general class of M-ary quasi orthogonal signaling (MOS) in severe multipath fading channels. First, by using the factor graph (FG) of the obtained generalized Ungerboeck metric for MOS and sum-product algorithm (SPA) framework, a highly efficient reduced state sequence estimation (RSSE) algorithm is proposed that operates on forward and backward directions and is directly applied to unwhitened channel matched filter (CMF) and code matched filter outputs at symbol rate. The proposed structure, generating the a posteriori probabilities (APP) by bidirectional RSSE recursions, are substantiated with symbol rate bidirectional decision feedback (BDF) based on surviving paths in order to eliminate the post- and pre-cursor inter-symbol interference (ISI) as well as multi code interference (MCI) due to the non-ideal properties of the signaling waveforms and multipath channel. It appears as the unification of Ungerboeck type reduced trellis equalization framework for MOS and achieves near-optimum performance especially for channels with large dispersion. Second, we identify a bias term through an error probability analysis which helps us improve the proposed receivers performance. Furthermore, a tight approximation for the bit error rate (BER) of the proposed receiver is derived. These analyses lead to significant insight on the selection of system parameters and clearly demonstrate the high performance and efficiency of the proposed scheme by a proper choice of the signaling parameters.

On the Carrier Frequency Offset Estimation for Frequency Hopping Burst Mode Mobile Radio

In this paper, we evaluate the Cramer-Rao bounds (CRB) for the estimation of the carrier frequency offset (CFO) for general QAM modulations with no knowledge of the transmitted sequence (blind operation) in frequency hopping (FH) based short burst mode mobile radios. We investigate the blind CFO estimation problem in FH based mobile system that uses multiple short narrowband bursts in different frequency channels for the transmission of one packet. Joint blind channel and CFO estimation over multiple frequency bursts is considered. In the first part of the paper, we determine the limiting performance by finding the CRB for blind estimation, and compare it to the CRB for known data (genie aided) case. The CRB relates the reliability of CFO estimation to the number of radio frequency bursts (RFB) used, constellation and SNR level, and is an important measure for system design without the need to simulate specific algorithms. It is shown that the linear combination (in MMSE sense) of multiple frequency estimates, each obtained from a single RFB independently, achieves the joint CRB over multiple bursts. In the second part of the paper, some practical blind estimation methods are proposed. They are shown to exhibit close performance to the true CRB.

An efficient ungerboeck type MAP receiver for multi-user channel with M-ary quasi orthogonal signaling

In this paper, a high performance MAP receiver with significantly reduced complexity is proposed for multiple-access channel with M-ary quasi orthogonal signaling. The proposed architecture operates on the channel and code matched filters (MF) belonging to each users signaling waveforms. The proposed MAP receiver is based on the factor graph (FG) obtained by using the equivalent channel model, and it operates on forward and backward directions along signaling intervals iteratively. Also, it is substantiated with bidirectional decision feedback (BDF) in order to limit the number of states used by each user. This Ungerboeck type MAP receiver, where Sum-Product Algorithm (SPA) with the support of RSSE, without need for whitening filter shows a very close performance to matched filter bound (MFB) at significantly reduced complexity for multipath channels with large delay spread and number of users.

Robust spread spectrum type communication with M-ary quasi orthogonal signaling for wireless fading channels

In this paper, a receiver architecture with reduced complexity is proposed for spread spectrum type communication with M-ary quasi orthogonal signaling. The proposed structure comprises of the channel and code matched filtering (MF) concatenated with a reduced state maximum likelihood sequence estimation (MLSE) type processing at symbol rate. MLSE, which takes the interference caused by nonideal cross- and autocorrelations of the spreading codes and multi-path channel into account, is directly applied to the unwhitenedmatched filter outputs. The hypothetical matched filter bound (MFB) is achieved by the proposed structure with the use of a reasonable number of states. By using the proposed reduced state Ungerboeck type MLSE based receiver, it is seen that M-ary quasi orthogonal signaling is an effective spread spectrum type modulation technique for multi-path channels with large delay spread at low power.

Diversity analysis of optimal SC-FDE MIMO systems and comparison with OFDM based transmission

In this paper, we propose a general framework for both time domain (TD) and frequency domain (FD) block iterative equalization schemes for wideband multiple-input multiple-output (MIMO) channels. The equalization systems are designed by jointly optimizing the forward and backward filters based on minimum mean square error (MMSE) criterion. The equivalence of block iterative TD and FD equalization with decision feedback is established by analytically showing that the same mean square error (MSE) is achieved by the two systems. Asymptotic performance of the studied block iterative FD equalization scheme is analyzed, and the rate-diversity tradeoffs of space time / frequency coded MIMO SC-FDE and MIMO OFDM are compared.

Cut-off rate based outage probability analysis of frequency hopping mobile radio under jamming conditions

This paper deals with the achievable spectral efficiency and outage analysis of short burst frequency hopping (FH) mobile radios under heavy jamming scenarios. With the use of outage probability analysis based on cut-off rate, the maximum spectral efficiency (bits / dimension) and the required number of radio frequency bursts (RFB) (or degrees of freedom) is determined in order to transmit a message reliably for a given target outage probability, number of information bits (message information length) and constellation. A Cut-off rate approach is exploited in order to take the influence of constellation into account, and outage metric is obtained depending on the jamming state, fading, number of RFB and constellation. On-off jamming with a given probability assignment is assumed for each burst. It is shown that the message length limits the achievable rate of the system. An upper bound on the code rate as a function of message length is derived for given outage probability, constellation, and channel statistics.

Probability of full-diversity for simple coded and rotated multidimensional constellation systems

The case of rotation, in use together with coding in a channel with B fading blocks, is analyzed in terms of diversity properties of the coded systems with multi-dimensional rotated constellations (coded-rotated system) output. First closed-form expression obtained is related to the diversity performance of unrotated generic coding structures with given minimum distance, d, block length, and number of fading blocks. Afterwards, another closed-form expression is derived for distinguishing the effect of rotation applied on the given coding structures. This latter expression yields a strict lower bound on the probability of attaining full diversity for any coded-rotated system and gives us a tool to evaluate the improvement with respect to systems involving only coding and also systems with only rotation. The results shown summarize the benefits of utilizing rotation together with coding, even in the cases of very simple non-full-diversity rotations like DFT rotations.