Performance Research On Hybrid Carrier System Based On Fractional Fourier Transform

In the current wireless communication system, the electromagnetic environment isbecoming worse, and the interference, such as channel fading, multipath interfercence,multiple access interference, has serious impact on the communication quality. Thereforeinterference is a evasive problem. And how to resit the interference is a research hot topic.The traditional Fourier transform can be seen as a signal processing method, whichbuilds a relationship between the time domain and frequency domain. However, it islimited to observe the signal in the time or frequency domain independently. While thejoint time-frequency methods are more effective to suppress the interference, comparingto the traditional Fourier transform. And fractional Fourier transform (FRFT) is one ofthese methods. FRFT is a generalization of Fourier transform by eigenvalues fraction-alization, and it mainly contains Chirp-type fractional Fourier transform (CFRFT) withChirp signal as orthogonal basis function and weighted-type fractional Fourier transform(WFRFT). CFRFT has been adopted for interference suppression in the communicationsystems. While WFRFT is still mainly in the stage of theoretical research and the initialapplication stage.The thesis focuses on the influences of multipath interference, multiple access inter-ference, single frequency interference and other interferences in the wireless communi-cation system, and adopts FRFT, frequency domain equalization, code sequence designto enhance the system anti-interference ability. In these approaches, WFRFT can be un-derstood as a hybrid carrier (HC) scheme combining single carrier (SC) and multi-carrier(MC) schemes. In the HC system signal energy is distributed more uniformly and evenlyin the time-frequency plane. The transmission schemes combing hybrid carrier and codedivision multiple access (CDMA) are proposed in this thesis. Spreading is applied in acertain fractional Fourier transform domain, and the HC characteristic of SC and MC in-tegration is achieved by the WFRFT. Then the data are transformed to the time domainby the WFRFT. By spreading in the fractional domain, the data symbol is distributed inthe time-frequency domain simultaneously. Meanwhile, the interference is averaged tomore chips to achieve the resistance against single frequency and multipath interferences.However, the orthogonality of spreading sequence is destroyed by channel frequency se- lectivity, resulting in multi-user interference. In order to resist this interference, takinginto account the HC characteristic, HC-DS-CDMA scheme using time domain spreadingis proposed.Convergence of the existing techniques in a common platform will be a main featurefor future wireless systems. The proposed systems can not only provide the compatibilitywith single carrier and multi-carrier systems, but also achieve a smooth and seamless tran-sition between the two. The proposed system can switch to single carrier or multi-carrierscheme under the specific conditions. At the same time the combination of HC-CDMAapproach with other physical layer techniques are further discussed. And HC-CDMA isadopted in the existing multi-antenna and Device-to-Device (D2D) communication sys-tems. In the former system, HC scheme obtains the outstanding performance in the fre-quency selective channel. And in the latter system, HC-CDMA transmission scheme stillexhibits great anti-interference ability in the face of more real and complex interferenceenvironment.Multi-user interference is a common interference in the wireless communication sys-tems, especially in the CDMA system, and the spreading sequence selection is a decisivefactor on system performance. A flexible multi-level sequence design method is pro-posed in order to resist multi-user interference. The multi-level sequence design methodis applied in the traditional time domain synchronous and asynchronous systems. Thenit is extended to the HC system based on the WFRFT, considering channel conditionand equalization technique, and the method is more suitable to the real communicationscene. Peak-to-average ratio (PAPR) directly influences the transmitter power amplifierefficiency and service life. In the MC system it is an important restricting factor. HC sys-tem is compatible with MC system, and its PAPR value is smaller. However, the PAPRin HC system can be further reduced. The PAPR problem is analyzed in HC-CDMAtransmission scheme, and a multi-level sequence design method is adopted to reduce it.Chirp signal is a typical broadband signal, and it is also used as carrier signal adoptedin IEEE802.15.4. Chirp is the orthogonal basis function of Chirp-type fractional Fouriertransform, and CFRFT can be considered as an angle rotation in the time-frequency plane.In a specific order fractional Fourier transform domain Chirp signal has the property ofenergy concentration, and it forms a peak. Based on this property, CFRFT is thought tohave the potential superiority to resist Chirp interference. The symmetrical property of time-limited Chirp signal in the fractional Fourier transform domain is analyzed in thethesis, and a interference suppression method is proposed. In the method the spectrumin the interference existing band is displaced and corrected. By the correction, the phaseinformation is utilized effectively, and compared to the simple narrow band filtering inthe fractional domain, the distortion of the carrier signal is smaller, resulting in bettersuppression effect and bit error rate (BER) performance. Meanwhile Chirp interferencesuppression is also provides a guarantee at the compatibility of existing system and othersystem using Chirp signals.