A. Ozgur Yilmaz

A low-complexity graph-based LMMSE receiver designed for colored noise induced by FTN-signaling

We propose a low complexity graph-based linear minimum mean square error (LMMSE) equalizer which considers both the intersymbol interference (ISI) and the effect of non-white noise inherent in Faster-than-Nyquist (FTN) signaling. In order to incorporate the statistics of noise signal into the factor graph over which the LMMSE algorithm is implemented, we suggest a method that models it as an autoregressive (AR) process. Furthermore, we develop a new mechanism for exchange of information between the proposed equalizer and the channel decoder through turbo iterations. Based on these improvements, we show that the proposed low complexity receiver structure performs close to the optimal decoder operating in ISI-free ideal scenario without FTN signaling through simulations.

Practical Methods for Wireless Network Coding With Multiple Unicast Transmissions

We propose a simple yet effective wireless network coding and decoding technique for a multiple unicast network. It utilizes spatial diversity through cooperation between nodes which carry out distributed encoding operations dictated by generator matrices of linear block codes. In order to exemplify the technique, we make use of greedy codes over the binary field and show that the arbitrary diversity orders can be flexibly assigned to nodes. Furthermore, we present the optimal detection rule for the given model that accounts for intermediate node errors and suggest a low-complexity network decoder using the sum-product (SP) algorithm. The proposed SP detector exhibits near optimal performance. We also show asymptotic superiority of network coding over a method that utilizes the wireless channel in a repetitive manner without network coding (NC) and give related rate-diversity trade-off curves. Finally, we extend the given encoding method through selective encoding in order to obtain extra coding gains.

Low-complexity iterative channel estimation and tracking for time-varying multi-antenna systems

The iterative channel estimation and tracking problem is considered for time-varying frequency-flat fading multi-input multi-output (MIMO) systems. In order to jointly estimate and track the time-varying multi-antenna channel, the pilot symbol assisted modulation (PSAM) technique is generalized to the multi-antenna systems. By distributing the pilot symbols along the transmitted block, i.e., PSAM transmission, the proposed system gains the ability to track the channel variation with the same amount of pilots whereas the system with all the pilots are employed prior to the data block has no tracking capability. The error performance is further improved by iteratively estimating the channel which employs the optimal MMSE filtering and also makes use of the soft probabilities of the coded symbols provided by the decoder. In order to decrease the computational complexity due to the iterative usage of MMSE filtering, we introduce the low-complexity 2-way LMS algorithm based on the forward-backward operation of conventional forward only LMS algorithm. By simulations, 2-way LMS is shown to have a near optimal error performance and highly better channel tracking ability than the conventional LMS algorithm with no significant complexity increase.

Joint channel estimation and decoding with low-complexity iterative structures in time-varying fading channels

A low-complexity iterative channel estimation (ICE) algorithm is proposed with the promise of improved error performance. The new algorithm operates the LMS filter both in the forward and the backward directions along a block. The feedback from the decoder to the estimator is in the form of soft decisions. The pilot symbol assisted modulation (PSAM) is used as the transmission technique. The effect of code choice on various ICE algorithms is also explored by considering the blockwise concatenated codes initially offered for block-fading channels. The performance of the new estimation algorithm with the proposed coding is shown to outperform the conventional estimation algorithms over a fast time-varying Rayleigh fading channel beside its low complexity structure.

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.

Efficient scheduling and power allocation for multiuser decoding receivers in OFDMA networks with minimum rate requirements

Abstract In recent years, techniques for exploiting interference on the receiver side to improve the performance of future cellular networks have been of interest. Due to the dominance of data in network traffic, user rate demands are becoming crucial for resource allocation of cells in the network. We propose an efficient transmission scheme for the downlink of an OFDMA multicell multiuser system with multiuser decoding (successive interference cancellation (SIC), joint decoding (JD), single decoding (SD)) capable receivers. We define a marginal rate maximization problem taking into account minimum rate demands of users and develop a practical scheduling (subchannel assignment) and power allocation algorithm using Lagrangian dual decomposition and gradient (ellipsoid and projected subgradient) methods. In the subchannel assignment, we propose the ratio of direct-to-cross channel strengths and multiuser rate regions different than the studies in the literature. Through Lagrangian dual decomposition, we derive the optimal power allocation rule using multiuser decoding modes for a given subchannel assignment. We observe that the method proposed here has low computational complexity and can be easily integrated to next generation networks as well as achieving high performance in practical scenarios.

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.

Practical wireless network coding and decoding methods for multiple unicast transmissions

We propose a simple yet effective wireless network coding (NC) and decoding technique. It utilizes spatial diversity through cooperation between nodes which carry out distributed encoding operations dictated by generator matrices of linear block codes. For this purpose, we make use of greedy codes over the binary field and show that desired diversity orders can be flexibly assigned to nodes in a multiple unicast network, contrary to the previous findings in the literature. Furthermore, we present the optimal detection rule for the given model that accounts for intermediate node errors and suggest a network decoder using the sum-product algorithm. The proposed sum-product detector exhibits near optimal performance.

A new equivalent relay channel model for analyzing the error performance of a GF(4) network coded detect-and-forward relay

In this paper, a new equivalent relay channel model is developed for analyzing the error performance of detect-and-forward (DetF) relay networks and is compared to the minimum equivalent relay channel model typically used in DetF systems. The considered cooperative relay network comprises two users, single destination, 4-ary Galois field (GF(4)) network coded, QPSK modulated, DetF in a flat Rayleigh fading environment. The destination performs maximum likelihood (ML) decoding utilizing the error rates of the source-relay links. Pairwise error probability results show that the proposed equivalent channel model sucessfully approximates the relay channel.

Spectral Mask Compliance and Amplifier Nonlinearity in Single Carrier and OFDM Systems

In this paper we investige the comparative merits of two transmission techniques, OFDM and single carrier, with respect to spectral mask compliance when a nonlinear amplifier is utilized in transmission. Two different nonlinear power amplifier models are used for analyzing the behavior of the two techniques in terms of average transmitted power and distortion.