Massimiliano Siti

A Novel Soft-Output Layered Orthogonal Lattice Detector for Multiple Antenna Communications

This paper presents a soft-output lattice detector algorithm for multiple-input multiple-output (MIMO) communications called layered orthogonal lattice detector (LORD). LORD adopts a new lattice formulation and relies on a channel orthogonalization process. Building on the optimality of LORD for two transmit antennas this paper is a generalization of the algorithm to any number of transmit antennas. The most interesting aspect of LORD is that for two transmit antennas max-log bit soft-output information can be simply generated and for greater than two antennas approximate max-log detection is achieved with reasonable complexity. LORD can be implemented in a parallel fashion, as desirable for VLSI. Extensive simulation results in different scenarios of interest for next generation WLANs (IEEE 802.11n) are reported. The simulations show that LORD is able to achieve very high signal-to-noise ratio (SNR) gains compared to current practical soft-output MIMO detectors.

Efficient OFDM Channel Estimation via an Information Criterion

In this paper, we consider joint estimation of the channel length and of the impulse response for OFDM systems, exploiting information criteria to find the best trade-off, in terms of Kullback-Leibler divergence, between noise rejection and channel description accuracy. So far, information criteria have not been used for practical channel length estimation methods, due to their prohibitive complexity. We show how to make them affordable, performing channel estimation in a recursive way that allows to establish the optimal channel length with a moderate incremental cost. With reference to IEEE 802.11 OFDM-based standards for WLAN, we investigate several cases, applying the joint channel length and impulse response estimation to many scenarios, ranging from the simplest pilot-aided channel estimation based on training sequences, to the most challenging data-aided channel tracking, driven either by detected or by decoded symbols. In all cases, the achieved performance and robustness are very good, with a very small increase in complexity w.r.t. estimation methods that assume fixed channel length.

Turbo-LORD: A MAP-Approaching Soft-Input Soft-Output Detector for Iterative MIMO Receivers

In this paper a novel Soft-Input Soft-Output detector, namely Turbo-LORD, is proposed for iterative MIMO receivers. This is an improved version, capable of managing a priori information, of the Layered ORthogonal lattice Detector recently presented. The implementation is straight and efficient when there are only two transmitting antennas. However, problems arising with more than two antennas are also discussed, along with possible solutions. It is shown that notwithstanding the suboptimal low-complexity implementation, this iterative receiver misses the performance of the turbo MAP detector by only few tenths of dB in various configurations, with very high spectral efficiency.

On Layer Ordering Techniques for Near-optimal MIMO Detectors

Recently a soft-output multiple-input multiple-output (MIMO) detector has been proposed, namely LORD (layered orthogonal lattice detector), able to achieve maximum-likelihood (ML) performance (in the max-log sense) for two transmit antennas, and near-optimal for greater than two, in MIMO-OFDM bit interleaved coded modulation (BICM) systems. This paper presents a near-optimal layer ordering technique applicable to the hard-output version of the algorithm, essential to achieve near-ML performance up to high-dimensional MIMO systems. The Euclidean distance metrics are minimized over a constant number of transmit sequences, a property desirable to maintain constant throughput and not guaranteed by the most known sphere decoder. Differently from previous art works, attention is also paid to the optimization of the implementation complexity. Finally, it is also shown how the above mentioned ordering technique should be modified for the soft-output version of the algorithm, significantly improving the performance of MIMO-OFDM BICM systems. Simulations in the realistic context of next generation WLANs (802.11n) confirm this statement.

Low Complexity, Quasi-Optimal MIMO Detectors for Iterative Receivers

We propose a novel family of Soft-Input Soft-Output detectors for iterative, point-to-point, MIMO receivers. Compared to the optimal Maximum A Posteriori receiver, low complexity is achieved restricting the detector search to small subsets of the entire QAM hyper-symbol constellation, through simple criteria. These criteria are applied to an improved version of the non-iterative Layered ORthogonal lattice Detector. We show that, notwithstanding the suboptimal low-complexity implementation, this detector approaches the EXtrinsic Information Transfer of the MAP detector. Therefore, when included in an iterative receiver it delivers the same performance. Furthermore, the deterministic complexity and highly parallelizable structure of the proposed detector are well suited for HDL and ASIC implementation. To focus on a specific setting, we consider the indoor MIMO wireless LAN 802.11n standard, taking into account errors in Channel Estimation and a frequency selective, spatially correlated channel model.

Low Complexity Decision-Directed Channel Estimation Based on a Reliable-Symbol Selection Strategy for OFDM Systems

This paper presents a low-complexity/low-latency algorithm for estimating and tracking time-varying fading channels in Orthogonal Frequency Division Multiplexing (OFDM) systems. More specifically, it is described a decision directed (DD) channel estimation (CE) method, consisting of two main steps: 1) Data detection of the current received OFDM symbol is performed by using the channel coefficients estimated in correspondence to the previous OFDM symbol; 2) The channel corresponding to the current OFDM symbol is estimated by using a subset of the newly detected data symbols. The data subset, used to perform successive refinement on the channel estimation, is found by choosing the OFDM subcarriers with the highest expected signal-to-noise ratio (SNR). Such a subcarrier selection strategy aims to obtain robustness against the introduction of incorrect data symbols in the channel estimation process. The extension of the presented technique to multiple receive antennas is also discussed. The proposed estimation method is potentially useful for IEEE 802.11p compliant vehicular communication systems, whose basic modulation parameters are herein used for computer simulation of the information error rate.

Generalized Soft-Output Layered Orthogonal Lattice Detector for Golden Code

The authors develop a generalized layered orthogonal lattice detector (G-LORD) for the golden code. G-LORD includes the previously developed LORD algorithm and exhaustive search ML detection as special cases, and is proved to achieve the perfect balance between error performance and complexity. Meanwhile, G-LORD is suitable for parallel implementations, and has a deterministic complexity as opposed to the sphere decoder approach. Thanks to the special structure of the golden code, G-LORD can implement the optimal ordering with very low complexity. Most importantly, G-LORD can efficiently generate bit soft-output metric when golden code is concatenated with outer channel code. The development of G-LORD boosts the practical application of the golden code.

Performance of concatenated Reed-Solomon and turbo codes with non ideal interleaving

The performance of a Reed-Solomon (RS) code has an analytical expression if the errors at the input of the decoder are independent. In concatenated schemes, this condition is often obtained through an interleaving device disrupting the correlation between erroneous symbols. Sometimes the ideal depth of such interleaver is too large to implement, and the RS decoder must operate in suboptimal conditions, for which no analytical formulas are available. In this paper, we present a statistical model that allows analytical evaluation of the performances of a concatenated scheme with an inner turbo code and an outer RS code, in the case of under-dimensioned interleavers. The model requires a statistical analysis of the erroneous symbols at the output of the inner decoder.

A Hardware Oriented, Low-Complexity LORD MIMO Detector

In this paper we introduce an innovative version of the recently proposed Layered ORthogonal lattice Detector (LORD). LORD is an attractive MIMO detection algorithm, which aims to approach the optimal Maximum-Likelihood (ML) detection performance with a reasonable complexity, quadratic in the number of transmitting antennas rather than exponential. LORD is also well suited to a hardware (e.g. ASIC or FPGA) implementation because of its regularity, deterministic latency and parallelism. Nevertheless, its complexity is still high in case of high cardinality constellations, such as the 64-QAM foreseen by the 802.11n standard. We show that, when only global latency constraints exist, e.g. a fixed time to detect the whole OFDM symbol, the LORD complexity can be remarkably reduced (up to 60%), still approaching the ML performance.

Doppler Spread Estimation and Channel Update Period Tuning in OFDM-Based Vehicular Communication Systems

Decision directed channel estimation (DD-CE) techniques turn useful for transmissions in time-varying wireless vehicular channels. CE has to be updated several times during the transmission of long data frames. Related works assume that, for IEEE802.11p-compliant physical layer systems, DDCE is performed for every input OFDM symbol, hence independently of the experienced Doppler effect. The purpose of this paper is twofold. First, it is shown how the estimation of the Doppler spread can be reliably performed in vehicular scenarios, and, moreover, an improved yet simplified version of the hybrid autocorrelation-based estimator is proposed. Second, it is discussed how the resulting vehicle relative speed estimate can be used to tune the CE update refresh period accordingly (and thus reduce the power consumption and processing time at the receiver for low to medium speed ranges). In order to reach this goal, it is introduced a novel method that relates the CE update rate to a maximum pre-established threshold value of the signal-to-noise ratio degradation due to CE update delay.