Marco Ferrari

A Low Complexity Turbo MMSE Receiver for W-LAN MIMO Systems

In this paper we consider an iterative detection and decoding scheme for Space-Frequency-Bit-Interleaved Coded Modulation (SF-BICM) MIMO-OFDM systems as a candidate receiver architecture for Next Generation Wireless LANs. This work is focused on the implementation complexity reduction of the overall turbo MIMO scheme through the simplification of the three main blocks: the Soft-Interference-Canceller, the MMSE-MIMO detector and the MIMO Soft-Symbol demapper which uses extrinsic soft information, produced by a Soft-Output-Viterbi-Algorithm (SOVA), to perform the LLRs calculation of the coded bits. A new receiver architecture is proposed, its computational complexity is estimated and compared with a more classical turbo MMSE receiver, both for 16-QAM and 64-QAM constellations.

Prunable S-random interleavers

We propose a variation of the algorithm for the construction of spread or S-random interleavers. This new version guarantees good spread properties that are also maintained under pruning. This makes this kind of interleaver attractive for those applications which require flexible size interleavers, such as flexible block size turbo codes, and flexible rate serially concatenated turbo codes.

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.

Importance sampling simulation of turbo product codes

An importance sampling technique, tuned for the simulation of ideal or suboptimal MAP bit-per-bit decoding, is applied to some examples of turbo product codes, i.e. to turbo decoding of the product of block codes. The simulation results show the remarkable performances of the suboptimal algorithms. The presence of the so called knee in the performance curves, predicted by the maximum likelihood asymptotic bound, is also confirmed.

On the selection of semi-orthogonal users for zero-forcing beamforming

We reconsider the role of user selection in multiuser MIMO broadcast channels (downlink), in the relevant regime where the number of users K is linear in the number of transmitter (base station) antennas M. User selection is known to achieve mutually quasi-orthogonal user channels and, at the same time, a multiuser diversity effect in terms of receiver SNR. These goals are achieved in the regime of fixed number of transmit antennas, and very large number of users. In contrast, we show that when K = O(M) these effects cannot be achieved, and the role of user selection is marginal. In terms of system design, our results suggest that only a small number K ≈ M of users should feedback their channel state information at each point in time. This greatly alleviates the burden of the channel state information feedback, while achieving essentially optimal performance.

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 the Structure of Cyclotomic Fourier Transforms and Their Applications to Reed-Solomon Codes

This paper is focused on cyclotomic Fourier transforms in GF(2m), and on their applications to algebraic decoding of Reed-Solomon codes, like the evaluation of syndromes and of error locator (or evaluator) polynomials. Cyclotomic transforms are much more efficient than straightforward evaluation. In particular, the number of multiplications is quite small. In this paper it is shown that also the number of additions can be considerably reduced with respect to previous analyses. A simple interpretation of the cyclotomic Fourier transform best suited for the evaluation of syndromes allows to assemble the required matrix easily and quickly, even in large fields. Fast construction of such matrices is important to obtain the best results, since as many matrices as possible must be generated and compared. It is shown that both the structure of the matrix and of bilinear convolutions need to be exploited, to reduce the complexity of the costly part of cyclotomic Fourier transforms, which is a matrix-vector product. Heuristic algorithms for matrix-vector product are to be run as many times as possible to obtain the best transform. It is shown with several examples that very good results can be obtained even with very simple algorithms.

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.

Rate Variable, Multi-Binary Turbo Codes with Controlled Error-Floor

In this letter we propose rate variable turbo codes based on the parallel concatenation of tailbiting recursive systematic multibinary (m-ary) convolutional codes. Rate variability is not achieved by puncturing, which can have adverse effects on the minimum distance of the code. Using a variable number of input lines of the encoder, we obtain several different overall rates ranging from 1/2 to 7/8. The most suitable soft-in- soft-out decoding algorithm for these turbo codes is based on the dual reciprocal code, which is very efficient for high rate codes. A particular interleaver design, namely the backbone interleaver, guarantees a high hamming weight in codewords with information weight 2 and 3, as well as good minimum distances and fairly low multiplicities for higher information weights. Therefore, these codes have very low error floors.

On the Reduction of Additive Complexity of Cyclotomic FFTs

We investigate a property that we have found in many efficient bilinear cyclic convolutions in GF (2m). We show that this property can reduce the additive complexity of cyclotomic FFTs. We explain how it arises, and why the most common constructions of cyclic convolutions yield this beneficial feature.