Marius Caus

Transmitter-Receiver Designs for Highly Frequency Selective Channels in MIMO FBMC Systems

This paper studies the MIMO applicability to filter bank based multicarrier (FBMC) modulations for low coherence bandwidth channels. Under these conditions the channel frequency response cannot be modeled flat at a subcarrier level. This implies that the techniques originally devised for OFDM do not restore the orthogonality between subcarriers when they are directly applied to FBMC. Aiming at circumventing this problem we propose the design of two MIMO FBMC schemes, which are based on a new subband processing. The figures of merit that govern the design of the first and second scheme are the signal to leakage plus noise ratio (SLNR) and the signal to interference plus noise ratio (SINR), respectively. However, we do not restrict the analysis to the design of a new subband processing but we also carry out an asymptotic analysis of the complexity as well as tackle the problem of estimating the channel. Simulation-based results have demonstrated that the addressed solutions clearly outperform previous MIMO FBMC schemes in terms of BER. In comparison to OFDM, the devised techniques are able to remain competitive even if the knowledge of the channel state information is not perfect. In those scenarios where the cyclic prefix (CP) length is not sufficiently large to avoid inter block interference, the proposed designs are able to reduce the BER. However, the price that should be paid is the increase of the complexity.

Multi-Stream Transmission for Highly Frequency Selective Channels in MIMO-FBMC/OQAM Systems

This paper addresses the joint design of MIMO precoding and decoding matrices for filter bank multicarrier (FBMC) systems based on OQAM, known as FBMC/OQAM. Existing solutions that support multi-stream transmission only give satisfactory performance in scenarios with high coherence bandwidth channels. To make progress towards the application of FBMC/OQAM to MIMO channels, we study the design of novel solutions that provide robustness against the channel frequency selectivity and support multi-stream transmission. To this end, two techniques have been devised under the criterion of minimizing the sum mean square error. The non-circular nature of the OQAM symbols has not been ignored, making evident the convenience of performing a widely linear processing. The first technique keeps the complexity at a reasonable level but in exchange the original problem is relaxed yielding a suboptimal solution. With the objective of performing closer to the optimum solution, the second option iteratively computes precoders and equalizers by resorting to an alternating optimization method, which is much more complex. We have demonstrated via simulations that the first technique nearly achieves the same results as the iterative design. Simulation results show that the proposed low-complexity solution outperforms existing MIMO-FBMC/OQAM schemes in terms of bit error rate. As for the comparison with OFDM, the numerical results highlight that FBMC/OQAM remains competitive, with and without perfect channel state information, while it provides spectral efficiency gains. Under highly frequency selective channels the proposed technique significantly outperforms OFDM.

MIMO Signal Processing in Offset-QAM Based Filter Bank Multicarrier Systems

Next-generation communication systems have to comply with very strict requirements for increased flexibility in heterogeneous environments, high spectral efficiency, and agility of carrier aggregation. This fact motivates research in advanced multicarrier modulation (MCM) schemes, such as filter bank-based multicarrier (FBMC) modulation. This paper focuses on the offset quadrature amplitude modulation (OQAM)-based FBMC variant, known as FBMC/OQAM, which presents outstanding spectral efficiency and confinement in a number of channels and applications. Its special nature, however, generates a number of new signal processing challenges that are not present in other MCM schemes, notably, in orthogonal-frequency-division multiplexing (OFDM). In multiple-input multiple-output (MIMO) architectures, which are expected to play a primary role in future communication systems, these challenges are intensified, creating new interesting research problems and calling for new ideas and methods that are adapted to the particularities of the MIMO-FBMC/OQAM system. The goal of this paper is to focus on these signal processing problems and provide a concise yet comprehensive overview of the recent advances in this area. Open problems and associated directions for future research are also discussed.

Multi-stream transmission in MIMO-FBMC systems

This paper addresses the design of MIMO precoding and decoding techniques for the filter bank multicarrier (FBMC) modulation. Existing solutions give satisfactory performance in scenarios with high coherence bandwidth channels. With the aim of increasing the robustness against the channel frequency selectivity, we have rethought the problem, which results in a new subband processing. Simulation-based results show that the proposed solution can achieve similar bit error rates as the OFDMsolution, while the spectral efficiency is increased. These results are theoretically justified. As a conclusion, FBMC becomes attractive in frequency selective channels not only because it relaxes the frame synchronization with respect to OFDM, but also because it presents spatial multiplexing competitive results.

SDMA for FBMC with block diagonalization

This paper deals with the design of MIMO pre-coding and decoding matrices for the filter bank multicarrier (FBMC) modulation. With the aim of increasing the system throughput we focus on the downlink of a multi-user communication system, where users share the same time slots and frequency resources. In this sense, we revisit the block diagonalization (BD) technique since it provides a good trade-off between complexity and performance. Based on the BD concept, we have formulated a new set of conditions that achieve interference-free data multiplexing in the FBMC context. In addition, we have carried out a theoretical analysis that reveals under what conditions MIMO-FBMC shows superior performance than MIMO-OFDM systems. Simulations results demonstrate that in the cases of study, the proposed solution slightly outperforms the classical BD technique, which can be combined with either OFDM or FBMC systems, in terms of bit error rate.

SDMA for filterbank with Tomlinson Harashima precoding

Several studies have revealed that OFDM is outperformed in several areas by the filter bank based multicarrier (FBMC) modulation thanks to pulse shaping techniques. Nevertheless, the combination of FBMC with multi-antenna architectures is nontrivial. Aiming at making progress towards this direction we study MIMO precoding techniques for the MISO broadcast channel (MISO-BC). Due to its superior performance over linear precoding techniques we have focused on the spatial Tomlinson Harashima precoder (STHP). The analysis carried out in this paper concludes that the conventional STHP performs poorly when it is combined with FBMC systems since it is not able to cope with ISI and ICI. To combat the interferences we propose a novel subband processing based on the STHP concept. The numerical results show that FBMC and OFDM almost give the same performance in terms of BER. However, FBMC is able to achieve spectral efficiency values that are 31% higher in comparison to the ones obtained in OFDM when 20% of the symbol duration corresponds to the cyclic prefix.

Space-time receiver for filterbank based multicarrier systems

By including a cyclic prefix (CP), systems based on orthogonal frequency division multiplexing (OFDM) are capable of mitigating multipath with a single tap equalizer. Aiming at increasing the spectral efficiency, filter banks based multi carrier systems (FBMC) along with an offset quadrature amplitude modulation (OQAM) can be seen as a good alternative, although single tap equalizers may not provide enough robustness. In this regard we adapt to FBMC systems the structure consisting of a space-time processor together with a maximum likelihood sequence estimator (MLSE). Simulations show that in presence of large delay spreads the proposed scheme outperforms orthogonal frequency division multiplexing (OFDM) systems in terms of bit error rate (BER).

An innovative interference mitigation approach for high throughput satellite systems

This paper investigates how to boost the forward link capacity in multibeam satellite systems. Since co-channel interference is the main inhibitor to achieve the full potential of frequency reuse, we propose to mitigate the negative impact of interference through cooperative transmission schemes. When desired information is shared among two beams, rates can be assigned to each user independently. This is crucial to benefit from the use of adaptive coding and modulation. In addition, the receiver can resort to multi-user detection, which is proven to give satisfactory performance when several signals are simultaneously received. Numerical results reveal that the proposed cooperative strategy brings substantial improvements, in terms of spectral efficiency, with respect to multibeam satellite systems, where receivers treat the interference as noise and data content is plainly delivered through the corresponding beam.

Transmit and receive filters for MISO FBMC systems subjected to power constraints

Build upon the OFDM/OQAM scheme this paper addresses the design of linear transmit and receive filters. Aiming at enhancing the robustness against multipath fading, two novel techniques are proposed. The first one jointly designs transmit and receive filters under the MMSE criterion. The second scheme follows a ZF approach. In this case transmit and receive filters are independently devised. Pre-equalizers obtained by both strategies are subjected to power constraints. Simulation-based results show that the two proposed techniques benefit from having perfect channel knowledge at both ends of the link to outperform the Alamouti scheme. The best performance in terms of BER is given when transmit and receive filters are jointly designed.

Optimal MISO pre-equalization for filter bank based multicarrier systems

One of the phenomena that degrades the performance of multicarrier modulation (MCM) systems is the spectral nulls that multipath channel may have. As a result, the affected subcarrier signals may be severely degraded resulting in a loss of symbols. In this paper we have studied how to overcome this problem by taking advantage of spatial diversity along with the knowledge of channel state information at both ends of the link. To that end we devise broadband beamformers that work on a per-subcarrier basis for filter bank based multicarrier (FBMC) systems. In particular we have focused on the orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) scheme. The criterion that has been followed to design the transmit filters consists of minimizing the transmitted power while the quality of service (QoS) constraints are satisfied. Simulation-based results demonstrate the efficacy of the proposed optimal and suboptimal designs to reduce the transmitted power when compared to the Alamouti scheme at the same BER targets. Although the optimal design is more demanding in terms of complexity it yields 1dB of savings in the transmitted power in comparison to the sub-optimal alternative.