Serdar Sezginer

Full-rate full-diversity 2 x 2 space-time codes of reduced decoder complexity

Multiple-input multiple-output (MIMO) techniques have become an essential part of broadband wireless communications systems. For example, the recently developed IEEE 802.16e specifications for broadband wireless access include three MIMO profiles employing 2x2 space-time codes (STCs) and two of those are mandatory on the downlink of Mobile WiMAX systems. Conventional approaches to STC design are based on performance criteria such as coding gain, diversity gain, multiplexing gain, and ignore the decoder complexity. In this paper, we take an alternative approach and present a full-rate full-diversity 2x2 STC design leading to substantially lower complexity of the optimum detector than existing schemes. This makes the implementation of high performance full-rate codes realistic in practical systems.

On High-Rate Full-Diversity 2 × 2 Space-Time Codes with Low-Complexity Optimum Detection

The 2times2 MIMO profiles included in mobile WiMAX specifications are Alamoutis space-time code (STC) for transmit diversity and spatial multiplexing (SM). The former has full diversity and the latter has full rate, but neither of them has both of these desired features. An alternative 2times2 STC, which is both full rate and full diversity, is the Golden code. It is the best known 2times2 STC, but it has a high decoding complexity. Recently, the attention was turned to the decoder complexity, this issue was included in the STC design criteria, and different STCs were proposed. In this paper, we first present a full-rate full-diversity 2times2 STC design leading to substantially lower complexity of the optimum detector compared to the Golden code with only a slight performance loss. We provide the general optimized form of this STC and show that this scheme achieves the diversity multiplexing frontier for square QAM signal constellations. Then, we present a variant of the proposed STC, which provides a further decrease in the detection complexity with a rate reduction of 25% and show that this provides an interesting trade-off between the Alamouti scheme and SM.

Metric-Based Symbol Predistortion Techniques for Peak Power Reduction in OFDM Systems

In this paper, we present a novel metric-based symbol predistortion algorithm and describe three variants for peak-to-average power ratio (PAPR) reduction in OFDM transmission. The algorithm consists of predistorting a set of input symbols per block using simple metrics, which measure how much each symbol contributes to the output signal samples of large magnitudes. The symbols to be predistorted in each block are selected as those with the largest positive-valued metrics. Predistortion of input symbols is performed either by scaling only the amplitude or by scaling separately the real and/or the imaginary parts of the selected symbols. The simple metric-based structure of the proposed algorithm gives high flexibility which enables various tradeoffs between performance and complexity. Another important feature is that the algorithm does not require transmitting any side information to the receiver and it does not involve any additional complexity for symbol detection at the receiver side. It is shown by simulations that a considerable improvement can be achieved with these simple techniques, which can be implemented as one-shot or iterative procedures.

OFDM peak power reduction with simple amplitude predistortion

In this paper, we propose a new peak-to-average power ratio (PAPR) reduction algorithm for OFDM transmission. The algorithm is based on simple amplitude predistortion of a subset of the input symbol blocks, the predistorted symbols being selected according to a metric which measures their contribution to the output signal samples of large. magnitude. This metric-based algorithm is simple, highly flexible, and can be implemented as a one-shot process, although its performance can be further improved by iterating the process one or more times

Enhanced Spatial Modulation With Multiple Signal Constellations

In this paper, we introduce a new spatial modulation (SM) technique using one or two active antennas and multiple signal constellations. The proposed technique, which we refer to as Enhanced SM or ESM, conveys information bits not only by the index(es) of the active antenna(s), but also by the constellations transmitted from each of them. The main feature of ESM is that it uses a primary signal constellation during the single active antenna periods and some other secondary constellations during the periods with two active transmit antennas. The secondary signal constellations are derived from the primary constellation by means of geometric interpolation in the signal space. We give design examples using two and four transmit antennas and QPSK, 16QAM, and 64QAM as primary modulations. The proposed technique is compared to conventional SM as well as to spatial multiplexing (SMX), and the results indicate that in most cases, ESM provides a substantial performance gain over conventional SM and SMX while reducing the maximum-likelihood (ML) decoder complexity.

Asymptotically Efficient Reduced Complexity Frequency Offset and Channel Estimators for Uplink MIMO-OFDMA Systems

In this paper, we address the joint data-aided estimation of frequency offsets and channel coefficients in uplink multiple-input multiple-output orthogonal frequency-division multiple access (MIMO-OFDMA) systems. As the maximum-likelihood (ML) estimator is impractical in this context, we introduce a family of suboptimal estimators with the aim of exhibiting an attractive tradeoff between performance and complexity. The estimators do not rely on a particular subcarrier assignment scheme and are, thus, valid for a large number of OFDMA systems. As far as complexity is concerned, the computational cost of the proposed estimators is shown to be significantly reduced compared to existing estimators based on ML. As far as performance is concerned, the proposed suboptimal estimators are shown to be asymptotically efficient, i.e., the covariance matrix of the estimation error achieves the Cramer-Rao bound when the total number of subcarriers increases. Simulation results sustain our claims.

Overview of ARQ and HARQ in beyond 3G systems

In this paper, we present a review of automatic repeat request (ARQ) and hybrid ARQ (HARQ) mechanisms implemented or proposed in beyond 3rd generation (B3G) wireless systems based on OFDMA. In particular, we will focus on part of the IEEE 802.16 standard family (IEEE 802.16–2005, IEEE 802.16m) and on 3GPP Long Term Evolution (LTE). In the second part of this overview, some performance curves show how HARQ can help in reducing performance degradation in mobility context.

A High-Rate Full-Diversity 2x2 Space-Time Code with Simple Maximum Likelihood Decoding

Multiple-input multiple-output (MIMO) techniques have become an essential part of broadband wireless communications systems. For example, the recently developed IEEE 802.16e specifications for broadband mobile wireless access include three MIMO profiles employing 2times2 space-time codes (STCs) and two of those, namely, Alamoutis STC for transmit diversity, and the 2times2 spatial multiplexing (SM) scheme, are mandatory on the downlink of Mobile WiMAX systems. The first of these has full diversity but it is only half rate, while the second is full rate but suffers from diversity loss. In this paper, we develop a full-diversity STC of size 2times2, which represents an interesting trade-off between these two schemes. More specifically, the presented code is of rate 3/4, it has full diversity, and it leads to simple maximum-likelihood (ML) detection whose complexity grows only linearly with the size of the signal constellation. This makes it a suitable candidate for future evolutions of broadband wireless systems toward higher data rates and performance.

A Full-Rate Full-Diversity 2×2 Space-Time Code for Mobile WiMAX Systems

The recently developed IEEE 802.16e specifications for broadband wireless access include three multiple-input multiple-output (MIMO) profiles employing 2×2 space-time codes (STCs) and two of those are mandatory on the downlink of mobile WiMAX systems. One of these is full rate, and the other is full diversity, but neither of them has both of the desired features. The third profile, which is not mandatory, is both a full-rate and a full-diversity code, but it has a high decoder complexity. In this paper, we develop a full-rate full-diversity STC of size 2×2 which can be optimally detected at a substantially lower complexity than conventional schemes. This makes it a suitable candidate for future evolutions of WiMAX systems toward higher data rates and performance.

Iterative compensated MMSE channel estimation in LTE systems

In this paper, an iterative channel estimation algorithm is considered in LTE systems. In order to take advantage of “null” sub-carriers (guard band) in LTE systems and make the algorithm simpler, an iterative compensated MMSE (IC-MMSE) channel estimation algorithm is proposed in frequency domain. Together with a simple “linear interpolation” in time domain, channel estimates are obtained in an iterative way with lower complexity. Theoretical analysis shows that the compensation process does not affect the accuracy of iterative MMSE channel estimation. Simulation results confirm that the proposed channel estimation has very robust performance and approaches the best achievable performance.