Tero Ihalainen

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.

Channel Equalization for Multi-Antenna FBMC/OQAM Receivers

In this paper, the problem of channel equalization in filter bank multicarrier (FBMC) transmission based on the offset quadrature-amplitude modulation (OQAM) subcarrier modulation is addressed. Finite impulse response (FIR) per-subchannel equalizers are derived based on the frequency sampling (FS) approach, both for the single-input multiple-output (SIMO) receive diversity and the multiple-input multiple-output (MIMO) spatially multiplexed FBMC/OQAM systems. The FS design consists of computing the equalizer in the frequency domain at a number of frequency points within a subchannel bandwidth, and based on this, the coefficients of subcarrier-wise equalizers are derived. We evaluate the error rate performance and computational complexity of the proposed scheme for both antenna configurations and compare them with the SIMO/MIMO OFDM equalizers. The results obtained confirm the effectiveness of the proposed technique with channels that exhibit significant frequency selectivity at the subchannel level and show a performance comparable with the optimum minimum mean-square-error equalizer, despite a significantly lower computational complexity. The possibility of tolerating significant subchannel frequency selectivity gives more freedom in the multicarrier system parameterization. For example, it is possible to use significantly wider subcarrier spacing than what is feasible in OFDM, thus relieving various critical design constraints.

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.

Frequency-domain equalization in single-carrier transmission: filter bank approach

This paper investigates the use of complex-modulated oversampled filter banks (FBs) for frequency-domain equalization (FDE) in single-carrier systems. The key aspect is mildly frequency-selective subband processing instead of a simple complex gain factor per subband. Two alternative low-complexity linear equalizer structures with MSE criterion are considered for subband-wise equalization: a complex FIR filter structure and a cascade of a linear-phase FIR filter and an allpass filter. The simulation results indicate that in a broadband wireless channel the performance of the studied FB-FDE structures, with modest number of subbands, reaches or exceeds the performance of the widely used FFT-FDE system with cyclic prefix. Furthermore, FB-FDE can perform a significant part of the baseband channel selection filtering. It is thus observed that fractionally spaced processing provides significant performance benefit, with a similar complexity to the symbol-rate system, when the baseband filtering is included. In addition, FB-FDE effectively suppresses narrowband interference present in the signal band.

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.

OFDM and FBMC transmission techniques: a compatible high performance proposal for broadband power line communications

A filter bank multicarrier (FBMC) system having a high level of compatibility with the IEEE P1901 OFDM scheme is proposed. In order to reach the level of robustness, selectivity and performance required by the broadband power line, the approach is based on near perfect reconstruction (NPR) filters combined with OQAM modulation. A key feature of the approach is the fractionally-spaced sub-channel equalizer, which is able to compensate the channel distortions and cope with residual timing offsets. The system initialization procedure and the results of OFDM are exploited by FBMC and an efficient and accurate technique is described for the derivation of the sub-channel equalizer coefficients from the OFDM frequency domain equalizer coefficients. Then, the impact of the filter impulse response on the efficiency in packet transmission is minimized. Finally, the impact of the sub-channel spacing is investigated and, in a comparison on similar basis, it appears that the proposed FBMC system can reach 228 Mbit/s in maximum bit rate, versus 197 Mbit/s for OFDM, while providing a higher level of tone protection and robustness to jammers.

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.