Research On HF Transmission Technologies Based On Non-continuous Spectrum

HF (high frequency) communication plays an important role in the military communications and civilian communications due to its good features such as low cost, high reliability, high flexibility and long communication distance. And it has also become a hot spot of communication research field. However, conventional HF bandwidth is relatively narrow. Windeband communication, which makes use of multicarrier modulation and the bandwidth extension to increase its transmission rate, is an important trend in the development of HF communication technologies. HF waves can only propagate at3-30MHz. The radio band is open to the public and has a large number of different users. How to improve the bandwidth efficiency for the limited HF band, use the idle bands rationally and avoid mutual interference between different users to keep the HF communication links robust, is still a practical and meaningful research topic.In this paper, we combine thecognitive radio technology with the HF communication, and research on the HF transmission technologies based on the non-continuous spectrum. Non-continuous OFDM (NC-OFDM) is chosen as the modulation scheme in the physical layer, owing to its outstanding spectrum adaption ability and its inherent robustness to multi-path fading. We will provide the solution suitable for HF communication to the mutual interference between HF cognitive users and their adjacent users. And we will also focus on the problem of high peak-to-average power ratio (PAPR) caused by the mutli-carrier scheme. The structure of this paper can be divided into four parts, as follows:First, the sidelobes of NC-OFDM signal decay slowly and may cause great interference on the adjacent users. N-continuous OFDM renders the connection between sequential OFDM symbols continuous to deal with this problem and effectively reduce the out-of-band leakage compared to the conventional OFDM signals. This paper provides projection precoding scheme for N-continuous OFDM, both in memory fashion and memoryless fashion. The expressions of the precoded signals are also derived. Theoretical analysis and simulations show that N-continuous OFDM will greatly reduce the interference to the adjacent users. This paper presents direct decoder and iterative decoder structure for the reception scheme. The former is simple to be implemented, but with poor bit error rates. The latter has satisfactory bit error rate performance which is close to the uncoded schemes, but with high complexity. A novel EVM-constraint projection precoder is subsequently proposed. Simulations show that it can suppress the out-of-band spectrum to a certain degree, yet the receiver performance degradation brought by the procession of projection precoding will be under control.Second, in order to design a precoding scheme without receiver performance degradation, this paper proposes orthogonal precoders for N-continuous OFDM. The orthogonal precoders is achieved at the cost of bandwidth efficiency, and designed in both memory and memoryless fashion. The data rate loss for the memoryless fashion is reduced to half compared to the memory-less scheme. It is proved that the orthogonal precoder for N-continuous OFDM can achieve the same power suppression performance as the projection precoder, but with better bit error rate performance. This paper also proposes a novel orthogonal precoder, which makes the generated spectrum edge steeper with the limited coding redundancy. At the receiver, three alternative decoding schemes, such as direct decoder, iterative decoder, orthogonal decoder, are provided to obtain tradeoff between the performance and complexity. Among these reception schemes, the iterative decoder has a best balance performance. For the HF channel condition with powerful adjacent users, the novel orthogonal precoder obtains more gains over the original one.Third, NC-OFDM system is struggling with a more severe PAPR problem than traditional OFDM. In this paper, we consider both optimizations for PAPR and sidelobe suppression. Firstly, the orthogonal precoding scheme combines the precoder for PAPR reduction and the precoder for N-continuous OFDM. Its data rate loss is rather small, but the receivers have to be changed for the predecoder procedure. Secondly, the tone reservation scheme utilizes those predefined subcarriers which do not bear any data information. The reserved subcarriers are used to cancel the peak value in the time domain and also generate N-continuous OFDM. Moreover, the tone reservation scheme offers more flexibility in the generated spectrum and PAPR distribution, by changing the relative weighting of the two optimization problem. Unlike the orthogonal precoding scheme, the receiver structure for tone reservation does not need to be changed, but the complexity at the transmitter is relatively high. Simulations show that both of the proposed schemes can significantly reduce the out-of-band power and PAPR. Besides, no extra side information transmission is needed, and the proposed scheme is quite suitable for further HF transmissions.Finally, in order to mitigate the interference introduced from the powerful adjacent users, we study the receiver window design for NC-OFDM. The window is imposed on the part of the cyclic prefix which is not disturbed by the multi-path, and relives the edges of the truncated signal to reduce the spectral leakage due to the rectangular window. Based on SINR maximization, we derive a novel window algorithm which takes advantage of the interference parameters. When the interference parameters are unknown, the window is designed based on minimization the sidelobe power in the specified range. From the simulation results, we show the superior performance of our algorithms in comparison to the conventional scheme. And the former window design is slightly better than the latter one.