François Rottenberg

Single-Tap Precoders and Decoders for Multiuser MIMO FBMC-OQAM Under Strong Channel Frequency Selectivity

The design of linear precoders or decoders for multiuser multiple-input multiple-output filterbank multicarrier (FBMC) modulations in the case of a strong channel frequency selectivity is presented. The users and the base station (BS) communicate using space division multiple access. The low complexity proposed solution is based on a single tap per-subcarrier precoding/decoding matrix at the BS in the downlink/uplink. As opposed to classical approaches that assume flat channel frequency selectivity at the subcarrier level, the BS does not make this assumption and takes into account the distortion caused by channel frequency selectivity. The expression of the FBMC asymptotic mean squared error (MSE) in the case of strong channel selectivity derived in earlier works is developed and extended. The linear precoders and decoders are found by optimizing the MSE formula under two design criteria, namely zero forcing or minimum MSE. Finally, simulation results demonstrate the performance of the optimized design. As long as the number of BS antennas is larger than the number of users, it is shown that those extra degrees of freedom can be used to compensate for the channel frequency selectivity.

Preamble-based channel estimation in asynchronous FBMC-OQAM distributed MIMO systems

This paper investigates downlink channel estimation in a distributed MIMO context employing Filter Bank-based Multi-Carrier Offset QAM (FBMC-OQAM) modulation. Training preambles are constructed based on two different subcarriers assignment schemes (SAS), with the aim of freeing the estimation procedure from the effects of the multi-stream interference (MSI). The Linear Minimum Mean Squared Error (LMMSE) estimator is considered for computing estimates of the channel frequency responses over the entire frequency band and shown to be robust to ill-conditioning associated with the SAS. The application of the different SAS to FBMC-OQAM is evaluated via simulations in an unsynchronized scenario. The performances are compared with the corresponding fully synchronized CP-OFDM scenario. The results demonstrate the robustness of FBMC-OQAM to asynchronism and reveal the advantages of each SAS in various situations.

Generalized optimal pilot allocation for channel estimation in multicarrier systems

This paper addresses the design of MSE-optimal preambles for multicarrier channel estimation under a maximum likelihood or minimum mean squared error criterion. The derived optimality condition gives insight on how to allocate the pilots that compose the preamble. While many papers show that equispaced and equipowered allocation is optimal, the generalized condition demonstrates that there exist many different configurations that offer the same optimal performance. Furthermore, the condition applies not only to maximum likelihood but also to minimum mean squared error channel estimation. An application of the generalized condition in the presence of inactive subcarriers (virtual subcarriers problem) is shown such that a non equispaced allocation can achieve the same optimal performance as if an equispaced one could be used.

Optimal zero forcing precoder and decoder design for multi-user MIMO FBMC under strong channel selectivity

This paper investigates the optimal design of precoders or decoders under a channel inversion criterion for multi-user (MU) MIMO filterbank multicarrier (FBMC) modulations. The base station (BS) is assumed to use a single tap precoding/decoding matrix at each subcarrier in the downlink/uplink, resulting in a low complexity of implementation. The expression of the asymptotic mean squared error (MSE) for this precoding/decoding design in the case of strong channel selectivity is recalled and simplified. Optimizing the MSE under a channel inversion constraint, the expression of the optimal precoding/decoding matrix is found. It is shown that as long as the number of BS antennas is larger than the number of users, the optimized precoder and decoder can compensate for the channel frequency selectivity and even restore the system orthogonality for a large enough number of BS antennas.

Parallel Equalization Structure for MIMO FBMC-OQAM Systems Under Strong Time and Frequency Selectivity

Offset-QAM-based filterbank multicarrier (FBMC-OQAM) has been shown to be a promising alternative to cyclic prefix-orthogonal frequency division multiplexing for the future generation of wireless communication systems. Unfortunately, as the channel gets more selective in time and frequency, the FBMC-OQAM orthogonality is progressively destroyed and distortion appears after the demodulation process at the receiver. While channel frequency selectivity has been very widely studied in the FBMC literature, the impact of time selectivity of the channel has not received that much attention. In this paper, the effect of the two types of selectivity on a multiple-input multiple-output (MIMO) FBMC system is characterized and a parallel equalization structure that can compensate for the doubly dispersive nature of the channel is proposed. This design uses multiple analysis filterbanks and extends previous approaches that were dealing only with channel frequency selectivity. A theoretical approximation of the remaining distortion after equalization is given. The study is performed for a general MIMO system but can also be particularized to single-input single-output systems. Simulation results demonstrate the high efficiency of the proposed receiver structure and the accuracy of the theoretical approximations is verified.

ML and MAP phase noise estimators for optical fiber FBMC-OQAM systems

This paper addresses the carrier phase recovery problem in Offset-QAM-based filterbank multicarrier (FBMC-OQAM) systems. The combined phase noise coming from the transmit and receive lasers, is known to induce a phase rotation of the demodulated symbols at the receiver. Several approaches have been proposed to recover the phase in FBMC-OQAM communication systems on optical fiber. Most of them have a significant complexity and do not make use of all information at disposal. In this paper, we propose two new estimators, obtained by minimizing a maximum likelihood (ML) and a maximum a posteriori (MAP) criteria. They use an error model formulation which allows to easily use priors on the phase noise statistics. By linearization of the error, an analytical solution is found for the phase error, which avoids the need for multiple phase tests. Simulation results demonstrate the better performance of the proposed estimators with respect to state of the art solutions in the low signal-to-noise (SNR) regime and for a small number of subcarriers.

Efficient Chromatic Dispersion Compensation and Carrier Phase Tracking for Optical Fiber FBMC/OQAM Systems

Offset quadratic-amplitude modulation (QAM) based filterbank multicarrier (FBMC/OQAM) is an attractive candidate to improve the spectral containment of optical fiber communication systems, especially when considering a sufficiently high number of subcarriers. As for other multicarrier modulations, the chromatic dispersion (CD) compensation is simplified in FBMC/OQAM systems since it is performed in the frequency domain. Unfortunately, FBMC/OQAM systems are sensitive to the laser phase noise (PN). The PN becomes difficult to mitigate when the number of subcarriers increases due to the increased symbol period. It results in intercarrier interference and intersymbol interference due to the loss of OQAM orthogonality. In this paper, we consider the use of moderate numbers of subcarriers to allow for simpler PN tracking. Consequently, more advanced CD compensation methods are required and a trade-off between CD and PN compensations needs to be studied. In this paper, the frequency sampling equalizer is used for the CD compensation, whereas an innovative adaptive maximum likelihood estimator is used for the PN compensation. A methodology is then presented to analyze this performance trade-off between CD and PN compensations, and design the desirable system parameters such as the number of subcarriers and the equalizer length. This is illustrated in the case of a terrestrial long-haul FBMC/OQAM transmission system, with 400-kHz laser linewidth and a 1000-km optical link.

Linear receivers for massive MIMO FBMC/OQAM under strong channel frequency selectivity

Filterbank Multicarrier (FBMC) modulations based on OQAM (FBMC/OQAM) have become a promising alternative to conventional OFDM because of their higher spectral efficiency and their improved selectivity in the frequency domain. Unfortunately, the orthogonality of these modulations is lost when the channel presents strong frequency selectivity, meaning that the channel response cannot be approximated as frequency flat within each subcarrier bandwidth. In this paper, this effect is analyzed in the massive MIMO setting, whereby the number of transmit and receive antennas is asymptotically large (but not as large as the number of subcarriers). It is formally shown that, under these asymptotic conditions, the output mean squared error (MSE) at each subcarrier converges to a constant independent of the subcarrier index. This was previously referred to as “self-equalization” principle in the FBMC/OQAM literature. It is demonstrated here that this phenomenon is a direct consequence of channel hardening effect in large scale MIMO configurations.

Single-tap equalizer for MIMO FBMC systems under doubly selective channels

Offset-QAM-based filterbank multicarrier (FBMC-OQAM) modulations are known to progressively loose their orthogonality as the channel gets more selective in time and frequency. The effect of channel frequency selectivity on FBMC-OQAM systems has been extensively studied in the literature. Many compensations methods have been proposed to combat it. However, most of them have a significant implementation complexity and do not take into account the time selective nature of the channel. In this paper, we propose a MIMO equalizing structure for doubly selective channel based on a simple single-tap per-subcarrier decoding matrix. The decoding matrices are designed to minimize the mean squared error of the symbol estimate. This decoder exploits the degrees of freedom offered by the extra antennas at the receiver to compensate for the distortion induced by time and frequency selectivity. Simulation results demonstrate the performance gain of the proposed design with respect to classical designs.

FBMC Distributed and Cooperative Systems

Distributed and cooperative systems exploit the benefits of Multiple-Input Multiple-Output (MIMO) in a flexible manner and are regarded as key enabling technologies for efficient spectrum utilization. Perfect synchronization, though stringently required, proves to be difficult to guarantee in such sophisticated communication scenarios. One intriguing advantage of FilterBank MultiCarrier (FBMC) in general and of its Offset-Quadrature Amplitude Modulation (OQAM) version in particular, is its strong resilience to synchronization errors. This makes it a promising multicarrier modulation scheme for distributed and cooperative systems, which is the focus of this chapter. First, we present FBMC-based Coordinated Multi-Point (CoMP) schemes. Partial coordination of adjacent cells is enabled to assist cell edge users to combat the intracell interference, the intercell interference, and the greater path loss compared to cell interior users. This is achieved via an Intrinsic Interference Mitigating Coordinated Beamforming (IIM-CBF) scheme, which jointly and iteratively computes the precoding and the decoding matrices for each subcarrier. Second, channel state information acquisition, which is a requirement in order to implement the schemes presented in the first part, is addressed for FBMC-based distributed systems. Preamble-based techniques are analyzed, and a study is performed of the influence of the assignment of pilot subcarriers to different base stations. Then, a method of tracking channel estimates for the distributed MIMO downlink is also investigated that does not require any pilot and only relies on the feedback of Signal-to-Noise Ratio (SNR) measurements.