Tobias Hidalgo Stitz

Channel equalization in filter bank based multicarrier modulation for wireless communications

Channel equalization in filter bank based multicarrier (FBMC) modulation is addressed. We utilize an efficient oversampled filter bank concept with 2x-oversampled subcarrier signals that can be equalized independently of each other. Due to Nyquist pulse shaping, consecutive symbol waveforms overlap in time, which calls for special means for equalization. Two alternative linear low-complexity subcarrier equalizer structures are developed together with straightforward channel estimation-based methods to calculate the equalizer coefficients using pointwise equalization within each subband (in a frequency-sampled manner). A novel structure, consisting of a linear-phase FIR amplitude equalizer and an allpass filter as phase equalizer, is found to provide enhanced robustness to timing estimation errors. This allows the receiver to be operated without time synchronization before the filter bank. The coded error-rate performance of FBMC with the studied equalization scheme is compared to a cyclic prefix OFDM reference in wireless mobile channel conditions, taking into account issues like spectral regrowth with practical nonlinear transmitters and sensitivity to frequency offsets. It is further emphasized that FBMC provides flexible means for high-quality frequency selective filtering in the receiver to suppress strong interfering spectral components within or close to the used frequency band.

Pilot-Based synchronization and equalization in filter bank multicarrier communications

This paper presents a detailed analysis of synchronization methods based on scattered pilots for filter bank based multicarrier (FBMC) communications, taking into account the interplay of the synchronization, channel estimation, and equalization methods. We show that by applying pilots designed specifically for filter banks, the carrier frequency offset (CFO), fractional time delay (FTD), and channel response can be accurately estimated. Further, a novel joint FTD and channel estimation scheme, based on iterative interference cancelation, permits extending the FTD estimation range well beyond the limit imposed by the pilot separation. The channel parameter estimation and compensation are successfully performed totally in the frequency domain, in a subchannel-wise fashion, which is appealing in spectrally agile and cognitive radio scenarios. The performance evaluation is done in a hypothetical WiMAX scenario in which an FBMC system would substitute OFDM maintaining as much physical layer compatibility as possible.

A block-Alamouti scheme for filter bank based multicarrier transmission

Filter bank based multicarrier (FBMC) is an interesting alternative to OFDM especially for spectrally agile communication waveform generation and for cognitive radio scenarios. For enhanced link performance and robustness, most of the common multi-antenna schemes can be combined with FBMC equally well as with OFDM. However, one significant shortcoming of FBMC is the difficulty of combining it with the famous transmit diversity scheme of Alamouti coding. In this paper, we present a block-wise Alamouti scheme for FBMC and test its performance.

Complex modulated critically sampled filter banks based on cosine and sine modulation

This paper explores subband processing of complex (I/Q) signals which finds various important applications especially in communications signal processing. Instead of using traditional DFT based systems, the filter bank based systems are very interesting choices when high selectivity in the subchannels is required. We present a novel efficient complex modulated critically sampled filter bank structure which is based on a combination of cosine and sine modulated filter banks. We show that in case of critically sampled analysis-synthesis filter bank system with complex input signal and highly selective channel filters, perfect reconstruction can be achieved only if the low-rate subchannel signals are real.

Practical issues in frequency domain synchronization for filter bank based multicarrier transmission

In this paper we address the practical problems when dealing with synchronization and channel estimation in a filter bank based multicarrier system. We present an approach for performing timing synchronization, carrier frequency offset estimation and correction, and channel estimation based on the use of the same training sequences for all the three tasks. We show that it is possible to perform them all after the analysis filter bank at the receiver, at low sampling rate, taking advantage of the high selectivity of the subband filters. The possibility to implement all the receiver baseband signal processing functionalities in frequency domain results in a very flexible overall receiver architecture, suitable for dynamic use of the frequency spectrum.

Efficient per-carrier channel equalizer for filter bank based multicarrier systems

Filter bank-based multicarrier (FBMC) systems have a number of advantages over the OFDM modulation method. However, efficient channel equalization techniques for FBMC systems are not mature yet. In this paper, a complex critically sampled filter bank, based on cosine- and sine-modulated filter banks, is used as a trans multiplexer and a low complexity equalizer called AP-ASCET is used to compensate the channel distortion. The use of an oversampled receiver filter bank makes it possible to equalize successfully each subchannel independently of the others. Furthermore, the subchannel amplitude and phase responses are equalized independently using low-order linear-phase FIR and allpass IIR filter sections, respectively. The system performance is studied using the ITU-R vehicular A channel model.

CFO estimation and correction in a WiMAX-like FBMC system

In this contribution we evaluate pilot-based carrier frequency offset (CFO) estimation and its correction in a filter bank based multicarrier (FBMC) system. We show that by applying certain pilots designed specifically for filter banks, the CFO can be accurately estimated. The performance evaluation is done in a hypothetical WiMAX scenario in which an FBMC system would substitute OFDM maintaining as much physical layer compatibility as possible.

Filter-bank-based narrowband interference detection and suppression in spread spectrum systems

A filter-bank-based narrowband interference detection and suppression method is developed and its performance is studied in a spread spectrum system. The use of an efficient, complex, critically decimated perfect reconstruction filter bank with a highly selective subband filter prototype, in combination with a newly developed excision algorithm, offers a solution with efficient implementation and performance close to the theoretical limit derived as a function of the filter bank stopband attenuation. Also methods to cope with the transient effects in case of frequency hopping interference are developed and the resulting performance shows only minor degradation in comparison to the stationary case.

Generation of Filter Bank-Based Multicarrier Waveform Using Partial Synthesis and Time Domain Interpolation

Recently, multirate filter banks have been recognized as high performance signal processing tools that could significantly enhance the efficiency and flexibility of future wireless communications. Filter bank-based multicarrier (FBMC) modulation allows increasing the spectral efficiency of high data rate transmission, both through the lack of time domain guard periods and the reduced frequency domain guard bands. Further, the inherent frequency selectivity is regarded as a vital characteristic, which enables high quality radio scene analysis, necessary for successful deployment of dynamic spectrum access in the context of cognitive radio technology. When applied to multicarrier communications, the filter banks are operated in the so-called transmultiplexer configuration where typically equal-size synthesis and analysis filter banks are utilized at the transmitter and receiver, respectively. This paper presents a new approach to generate an FBMC signal waveform. We focus on contiguous narrowband subcarrier allocation, which is often encountered in uplink transmission. Instead of using a full-size M-subchannel synthesis filter bank, the proposed scheme relies on a cascade of a P-subchannel synthesis bank (P ≪ M), time domain interpolation, and a user-specific frequency shift. This method allows constructing a transmitter-receiver pair using synthesis and analysis filter banks of different sizes. The presented approach is shown to provide notable computational complexity savings over a wide range of practical user bandwidth allocations, when compared to the conventional implementation consisting of equally sized filter banks. Therefore, this novel scheme provides flexible and low-complexity synthesis of spectrally well-localized FBMC uplink waveforms, which show strong potential for future broadband mobile communications.

Analysis of Clipping-Based PAPR-Reduction in Multicarrier Systems

This paper presents comparatory studies of two different peak-to-average power-ratio (PAPR) reduction methods when applied to orthogonal frequency division multiplexing (OFDM) and filter bank based multicarrier (FBMC) communication signals. The studied methods are based on clipping with windowing, and they were originally devised for OFDM transmitters. The performance comparisons are done by evaluating bit error rate and spectral regrowth behavior. The study also investigates the near-far effect for adjacent signals considering both the synchronized and unsynchronized cases. The results show that the methods designed for OFDM can be fully applied to FBMC with similar performance in the synchronized case. Further, the well-known advantage of high spectral selectivity of FBMC in unsynchronized scenarios is also clearly visible when applying the studied techniques.