Zsolt Kollár

PAPR Reduction of FBMC by Clipping and Its Iterative Compensation

Physical layers of communication systems using Filter Bank Multicarrier (FBMC) as a modulation scheme provide low out-of-band leakage but suffer from the large Peak-to-Average Power Ratio (PAPR) of the transmitted signal. Two special FBMC schemes are investigated in this paper: the Orthogonal Frequency Division Multiplexing (OFDM) and the Staggered Multitone (SMT). To reduce the PAPR of the signal, time domain clipping is applied in both schemes. If the clipping is not compensated, the system performance is severely affected. To avoid this degradation, an iterative noise cancelation technique, Bussgang Noise Cancelation (BNC), is applied in the receiver. It is shown that clipping can be a good means for reducing the PAPR, especially for the SMT scheme. A novel modified BNC receiver is presented for SMT. It is shown how this technique can be implemented in real-life applications where special requirements must be met regarding the spectral characteristics of the transmitted signal.

Evaluation of Clipping Based Iterative PAPR Reduction Techniques for FBMC Systems

This paper investigates filter bankmulticarrier (FBMC), a multicarrier modulation technique exhibiting an extremely low adjacent channel leakage ratio (ACLR) compared to conventional orthogonal frequency division multiplexing (OFDM) technique. The low ACLR of the transmitted FBMC signal makes it especially favorable in cognitive radio applications, where strict requirements are posed on out-of-band radiation. Large dynamic range resulting in high peak-to-average power ratio (PAPR) is characteristic of all sorts of multicarrier signals. The advantageous spectral properties of the high-PAPR FBMC signal are significantly degraded if nonlinearities are present in the transceiver chain. Spectral regrowth may appear, causing harmful interference in the neighboring frequency bands. This paper presents novel clipping based PAPR reduction techniques, evaluated and compared by simulations and measurements, with an emphasis on spectral aspects. The paper gives an overall comparison of PAPR reduction techniques, focusing on the reduction of the dynamic range of FBMC signals without increasing out-of-band radiation. An overview is presented on transmitter oriented techniques employing baseband clipping, which can maintain the system performance with a desired bit error rate (BER).

Physical Layer Considerations for Cognitive Radio: Modulation Techniques

Cognitive radio (CR) is one of the promising future possibilities for wireless broadcasting and communication. One of the key issues is the physical layer, where the modulation waveform best suitable for the CRs has to be chosen. In this paper we present the most promising modulation schemes with their advantages and disadvantages. Orthogonal frequency division multiplexing (OFDM) is the most favored modulation nowadays for high speed digital communications, but many other modulation techniques show advantages over it. Notably DFT-spread OFDM (DFTS-OFDM), Constant Envelope OFDM (CE-OFDM) and Filter Bank Multicarrier (FBMC).We give an overview of these schemes to see which is the most suitable with a given set of parameters or in a specific scenario. We compare these methods via spectral characteristics, signal metrics and system performance in the presence of non-linear distortions.

Comparison of signal processing methods for calculating point-by-point Discrete Fourier Transforms

This paper presents and compares signal processing architectures for calculating point-by-point discrete signal spectra. Besides the well known Discrete Fourier Transform (DFT) and its well known fast implementation, the FFT, the Sliding Discrete Fourier Transform (S-DFT) and the less known Recursive Discrete Fourier Transform (R-DFT) are discussed. These algorithms are compared by calculation complexity for FPGA implementation. Furthermore, their sensitivity to quantization effects are also examined, based on simulations.

Software implementation of the Recursive Discrete Fourier Transform

The Discrete Fourier Transform (DFT) is one of the fundamental operations in digital signal processing. This paper presents a software based implementation of a less known observer based Recursive DFT on a PC ×86 architecture and on a Microchip PIC30 microcontroller. The results are compared with an efficient/optimized Fast Fourier Transform (FFT) implementation. Both algorithms require complex operations with real valued adders and multipliers, thus a complex result has to be calculated separately for the real and imaginary parts. During the implementation floating-point and fixed-point number representation is applied. The comparison is performed based on the required resources and the computational time.

Iterative signal reconstruction of deliberately clipped SMT signals

Staggered MultiTone (SMT) is a modulation technique showing significantly reduced Adjacent Channel Leakage Ratio (ACLR) resulting in a more compact Power Spectrum Density (PSD) for the transmitted signal, than the well-known and already widely adopted Orthogonal Frequency Division Multiple Access (OFDMA) scheme. However, the unique spectral properties of an SMT signal could be degraded by a non-linear element (e.g. a Power Amplifier (PA)) in the transmitter. Deliberate baseband clipping can be applied to the transmitted signal, reducing the notable high Peak-to-Average Power Ratio (PAPR). The objective of this paper is to give a brief introduction to the SMT scheme, with a special emphasis on deliberate clipping effects and their compensation. The paper introduces two receiver-oriented iterative methods aiming at the restoration of the baseband Bit Error Rate (BER) performance of a non-clipped signal. The methods are evaluated and compared based on numerical simulations. The paper concludes with the selection of a possible candidate for use in systems applying deliberately clipped SMT signals.

Physical Layer Considerations for Cognitive Radio: Synchronization Point of View

In this paper, we briefly describe four modulation schemes (Orthogonal Frequency Division Multiplexing (OFDM), DFT-spread OFDM, Constant Envelope OFDM (CEOFDM)and Filter Bank Multicarrier (FBMC)) for Cognitive Radios (CR) in white space. In the first part, we give a short overview of the main signal processing blocks and signal properties. Then, we compare them by means of sensitivity to various synchronization errors such as timing error, frequency mismatch, phase noise and IQ-imbalance. These errors can be caused by either non-ideal analog components, movement of the transceiver or imperfect synchronization. Each synchronization error is examined separately; the performance is verified by means of bit error rates. Each modulation has a different sensitivity for a given type of synchronization error, which has to be taken into account when choosing the most suitable modulation scheme for a given application.

Effects of quantization on Golay sequence based channel estimation

In this paper we investigate a widely adapted complementary Golay sequences which are used for channel estimation in digital data transmission systems. The performance of the Golay correlator based and the fast Fourier transform based channel estimation methods are evaluated. The effects of quantization on the estimation of the channel impulse response is compared through simulations.

Peak-to-average power ratio reduction for OFDM based on dynamic range compression

We present a peak-to-average power ratio (PAPR) reduction method for orthogonal frequency-division multiplexing (OFDM) or similar modulation schemes based on dynamic range compression and decompression. Initially, the decompressor was developed for compressed audio signals. With regard to OFDM, the greatest benefit of the method is that it can be easily adjusted to the system requirements and a tradeoff can be found between the PAPR gain and signal distortion. Practically, it requires no additional side information at the receiver. In a pilot experiment, we evaluate the method using four different metrics and give a brief interpretation of the obtained results.