With the opening up of white spaces, efficient use of the fragmented spectrum - TV white space in particular - has become an extremely important focus of research. Apart from efficient spectrum usage, special care needs to be taken to maintain low out-of-band radiation to avoid harmful interference to incumbent services like TV signals and wireless microphones. For this reason, a flexible digital radio with multicarrier modulation scheme is the only solution. In this paper, we show the performance of two innovative multicarrier systems, Filter Bank Multi Carrier (FBMC) and Generalized Frequency Division Multiplexing (GFDM). A simple interference cancellation technique called serial inter-carrier interference cancellation scheme has been used to improve performance of the GFDM system. Channel equalization techniques have been implemented for FBMC.

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.

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.

In contrast to a plane-wave situation electromagnetic wave polarization is a 3-dimensional phenomenon in multipath scatter environments. Recently there is much research work done in order to utilize this fact in MIMO antennas. E.g. it is shown that with appropriate antenna arrangement increase in capacity can be as much as 4-fold or so in terrestrial rich scattering environments. In this paper satellite systems are investigated from this point of view. It is shown that it is hard to achieve an increase of capacity. However, diversity gain (or order of diversity) can be increased and in principle diversity gain of 6 or more is possible while this gain is at most 4 in the 2D (i.e. plane wave) case.