Research On Modulation Methods In Digital Communications Based On Non-orthogonal Ideas

Modulation method is the fundamental part of a digital communication system. It plays a critical role in system's performance. Nowadays, the communication technology develops very rapidly, while the development of modulation methods becomes relatively pale by comparison. This thesis focuses on the orthogonality restriction (OR) in traditional modulation methods. Based on the ideas of relaxing the OR, modulation methods are investigated and developed. The main work and its contribution are given as follows.First of all, a uniform model of traditional modulation methods is given in signal space representation. Based on this model, the positive and negative effects of OR are analyzed. A framework of modulation methods which can cover orthogonal and non-orthogonal modulation methods is introduced. To mitigate the negative effect of OR, the ideas of relaxing the OR is proposed to develop modulation methods, called non-orthogonal ideas.The pulse shaping of orthogonal frequency division multiplexing (OFDM) system is strictly limited by the OR, which increases its sensitivity to frequency dispersion. In this thesis, a new OFDM scheme called GOFDM is proposed, in which the real and imaginary parts of complex symbols are allowed to use different pulse shaping. That means the pulse shaping design in GOFDM is subject to less restriction and obtains more flexibility than conventional OFDM. As a result, the robustness against frequency dispersion is improved, and there is no degradation in noise immunity or robustness against time dispersion. This feature is unavailable in existing related techniques. The relationship between the performance against the frequency dispersion and pulse shaping design in is elaborated, and the distribution of interference in constellations caused by frequency dispersion is examined. One and two dimensional constellations to fight against the frequency dispersion are optimally designed, respectively. The numerical experiments show that the above-mentioned designs outperform conventional OFDM in frequency-dispersive channel.The OR involved in Nyquist-I criterion makes it difficult to design pulse shaping with high spectrum efficiency. In order to solve this problem, a method is proposed to orthogonalize the non-orthogonal basis composed by non-orthogonal pulses. Applying this method in modulation, a time-frequency joint interpolation filtering scheme called ORTH is obtained. The performances of ORTH in multi-carrier modulation are analyzed, including implementation complexity, bandwidth efficiency and equalizer design. Its design in band-limited applications is also discussed. The experiment results show that ORTH is more efficient than conventional OFDM in approaching Nyquist rate. The application of ORTH in baseband modulation is investigated, and a new parametric Nyquist pulse design method is proposed. Compared to classical raised cosine pulse, the pulse generated by the proposed method has better orthogonality and spectral efficiency.The symbol rate of traditional modulation method is limited by Nyquist rate because of OR. In this thesis, the existing attempt in breaking through the limit is reviewed, and further research is made. Occupied bandwidth (OB) and necessary bandwidth (NB) are discussed, and it is emphasized that the research on "breaking through Nyquist rate" must be done in the sense of NB. It can be proven that, in the sense of NB, any system relying on linear inverse mapping to demodulate symbols cannot exceed Nyquist rate, unless it gives up orthogonality and adopts maximum-likelihood (ML) demodulation. The concept of equivalent NB is introduced based on ML demodulation. With this concept, a comparison of transmission efficiency between the interested system and traditional orthogonal system can be drawn. A non-orthogonal modulation technique free of OR, called NMT, is proposed. First, its symbol error rate formula is derived, and, in the sense of OB, its bandwidth efficiency is investigated theoretically and experimentally. The results prove its superiority to traditional orthogonal methods. Second, based on the performance in the sense of OB, it is proven that NMT can break through Nyquist rate. In order to reduce the complexity of ML, an idea of combination ML demodulation with solving linear equations is proposed, and two practical methods are presented.Finally, computer simulations of NMT in xDSL loop are performed, and spectrum compatibility evaluation are carried out. The result also proves its practicability.