Research On Directional Modulation In Multipath Channels

With the rapid development of communications technology, wireless communi-cation has been widely used in various fields. At the same time, the security problem has become increasingly important. In traditional communications technologies, smart antenna is used to enhance system capacity. Here we will apply it to information se-curity area to improve the information security during transmission. Researches of this thesis show that this technique still has a very high security performance even in multipath fading wireless communications environments. The main works of this the-sis are as follows.Firstly, thesis introduces the Directional Modulation (DM) based on phased array technology. We compared its security performance with that of conventional beam-forming technology. Based on the existing algorithms, an improved counterpart with high power efficiency and low bit error rate (BER) is proposed. Then beamforming technique based on array redundancy is introduced. Similarly, a DM scheme based on random distortion is proposed, which is called the Random Distortion DM. Compared with the traditional fixed distortion DM, we get a narrower null at the desired direc-tion and a more stable BER at undesired directions.Secondly, the thesis proposes a counter-multipath DM method. Considering the problem of finite numbers of plane-wave interferences, the proposed DM method with adaptive signal processing techniques can well cancel these known and unknown in-terferences. Simulation results show that the algorithm can effectively solve the re-flective interferences in multipath channels.Thirdly, the thesis analyses the statistical characteristics of receiving signal in case of local scattering around a receiver. The relationship between the DM security performance and scattering factors is also analyzed.Lastly, other than uniform linear arrays, we study the DM techniques in non-uniform arrays and sparse arrays. Due to having a narrower beamwidth for a sparse array, we develop an appropriate DM algorithm to overcome the shortcomings of grating lobes of the sparse array. For a Minimum Redundancy Linear Array (MRLA) with few antennas, the corresponding DM algorithm with low implementa-tion complexity is discussed, which has an improved BER performance compared with a uniform linear array(ULA) which has the same elements number and compara- ble performance compared with a ULA with the same aperture.