Adaptive Baseband Processing Techniques for Cognitive Radio Systems

Abstract

Cognitive Radio is a new paradigm in the wireless communications. It puts the intelligence and awareness dimension to the radio communication system. A Cognitive Radio system will be aware of the changing condition in its environment. The awareness in this thesis is related to the spectrum and the channel stipulation. By intelligent learning and understanding from the environment, a Cognitive Radio device will adapt its transmission parameters to the changing environment. The objective of a Cognitive Radio system is to have a reliable communication and efficient spectrum utilization. Although most of the spectral ranges are already licensed, studies and measurements have shown that most of the time the spectrum is not fully occupied, and even some bands are rarely occupied. The congestions happen because of the existing poor spectrum access technique. This condition raises the opportunity to rent or to access the spectrum while the licensed user is in idle condition or by having the Cognitive Radio side by side with the licensed users band occupying the spectrum holes. In this way, the spectrum is utilized in a more efficient way. The challenging problem is to settle a “win-win” co-existence between the Cognitive Radio, which is the rental system, with the legacy licensed system. Spectrum pooling has been proposed in the literature as a technique of sharing the spectrum with the licensed system by using the Orthogonal Frequency Division Multiplexing (OFDM) as modulation technique. Some of the OFDM carriers located in the licensed user’s band will be deactivated in order not to interfere the legacy system access. Due to this flexibility, the OFDM is considered as a proper modulation technique to be applied to a Cognitive Radio system. Deactivation of more carriers adjacent to the licensed user’s band, as well as windowing and several signal processing techniques could reduce the interference contribution of the OFDM based Cognitive Radio system to the licensed users signal. These techniques are studied and presented in this dissertation. The spectrum pooling technique could only work upon the reception of the accurate spectrum occupancy information. The spectrum occupancy information is derived from a spectrum sensing module. Spectrum sensing necessitates such a sophisticated module and requires a proper attention. This thesis does not focus on the spectrum sensing module, but rather it is assumed that the spectrum occupancy information is available and accurate. The work on spectrum sensing is conducted by out partner at the Twente University . Following the objective of Cognitive Radio to have reliable communications, the observation or emphasis of the technique is not only on the licensed users side but also on the rental user. Besides assuring the reliable communications of the licensed system, the target Quality of Service (QoS) of the Cognitive Radio system should be attained. The practical parameters to be observed are the Bit Error Rate (BER) and the bit rate of the system. The application of OFDM with spectrum shaping for the purpose of reducing the interference contribution to the licensed system can be achieved with the cost of self QoS degradation. Results from literature have shown that application of adaptive bit loading could enhance the BER of an OFDM system. By adaptive bit loading, the bits are allocated to each of the OFDM carrier intelligently according to the channel condition by setting the target BER and bit rate of the overall system. In this work, we propose to combine the adaptive bit loading with the spectrum shaping to preserve the Cognitive Radio system’s QoS. The impact of this combination on the OFDM Peak to Average Power Ratio (PAPR) growth is evaluated through simulations. There is a growing interest in replacing the Fourier transform in OFDM with wavelet basis functions. The technique is termed as the Wavelet Packet Multicarrier Modulation (WPMCM). Investigations reported in literature have presented the evaluation of WPMCM and compared the results with OFDM. Following the successful application of a frequency selective wavelet in the Ultra Wide Band system, in this dissertation we evaluate the suitability of the frequency selective wavelet in WPMCM combined with the spectrum pooling concept to be applied to the Cognitive Radio system. As the efficient spectrum utilization is one of the major objectives of Cognitive Radio, it is reasonable to include Multiple Input Multiple Output (MIMO) to the Cogntive Radio system. This subject is studied in this thesis and the performance of MIMO in the OFDM and WPMCM based Cogntive Radio system is evaluated. Channel estimation is the crucial module in every OFDM system. We propose an effective channel estimation scheme based on optimum pilot patterns of conventional OFDM using virtual pilots and apply it to Cognitive Radio systems. The virtual pilots are derived from the combination of the linear interpolation/extrapolation between two real pilots with the so called decision directed method. Without loss of generality we use the Wiener filter as the channel estimation technique due to its efficient and straightforward method in utilizing the channel correlation property according to the distance between pilots and data. First we adopt the Single Input Single Output (SISO) OFDM based Cognitive Radio system, and then we expand the scheme to the MIMO application. Beside OFDM and WPMCM based Cognitive Radio system, recently Transform Domain Communication System (TDCS) and Wavelet Domain Communication System (WDCS) have been introduced as promising modulation techniques for Cognitive Radio application. TDCS and WDCS have bit rate limitations. As an effort to enhance the bit rate of TDCS and WDCS, we propose to add an extra embedded symbol to TDCS and WDCS. We analyze the impact of the embedded symbol on the conventional TDCS and WDCS data detection. The impact of the embedded symbol on the data detection in a multi-user environment, which is inherently supported by the conventional TDCS and WDCS, is observed. In addition, we also evaluate the performance of the TDCS with an embedded symbol in the MIMO system. As a sort of verification platform for Cognitive Radio we proposed a practical Demonstrator that involved a spectrum scanning module and baseband processing transceiver module. The spectrum sensing is employed by the Universal Software Defined Radio Peripheral (USRP) while the baseband processing transceiver module is applied to an FPGA Development board. The current made Cognitive Radio verification platform is simple and still has a limited feature. While further developments are required in this field, details of this effort are also provided in this thesis.