Research On Interference Control In Mobile Communication Systems

Mobile communication is one of the rapidly developing parts in communication areas. With the large base number and high mobile requirement, mobile communication is progressing sufficiently on its theory and application. As mobile communication system has its particularity on transmitting, channel transferring and receiving compared to wire communication, interference in communication procedures is much more obvious than those of other systems. Therefore, interference control has become a significant research point, which provides special effect on ensuring normal operation and raising quality of service. This paper discusses three types of interference:spurious emission, intermodulation and block distortion. It also innovates fractional frequency reuse from physical layer and data link layer. The content of this paper is divided into six parts.The first part studies basic theories of interference in mobile communication system, including models of wireless channel, sorts of interference and analysis techniques which are distinct from cable channel. This part is the theoretical and technical basis of following researches. There are several ways to classify communication interferences, and this part introduces some types different from that in following chapters. Meanwhile, the core types of interferences in this paper are gradually referred.The second part studies a significant interference, which is called spurious emission interference, in mobile communications. As long as the reject band is not equal to zero in transmitting or receiving terminals, signals will produce adjacent frequency leakage. Then the spurious emission interference takes place if such leakage locates in passband of receiver. At present the available resource of frequency band (especially between 800 to 2500 MHz) is very rare, where adjacent bands of existed systems have little gaps, which leads a high probability of spurious emission interference. This part analyzes its theoretical basis and spurious emission between distinct systems, calculated practical minimum coupling loss by simulation, and raises schemes to restrain such interference. Reducing spurious emission interference is an essential method to upgrade system performance.The third part focuses on intermodulation interference in mobile communications. Just like spurious emission caused by nonideal filtering, nonideal transfer function leads intermodulation. The ideal transfer function is linear (only first order), while the nonideal one contains second or higher orders, which affect the linear character of transferring. If more carriers or frequency bands are activated, intermodulation interference is more likely to occur. This part detailedly analyzes and deduces the elements of intermodulation, and demonstrates the quantitative relationship between intermodulation elements and the linearity of transfer function. After that the methods to mitigate intermodulation interference are put forward. Generally speaking, the methods include both hardware and software, that is, broaden linear input region and channel arrangement.The fourth part investigates block interference. If input power does not match the object device (usually the power exceeds the upper bound), the block interference comes out. It is quite familiar as high-power transmitter is widely used. Block brings an obvious compression on amplitude compared to expectation, which makes distortion in output signal. Block interference takes place when the compression falls out of the permitted region of system. This part is still based on linear character of transferring. The relationship between compression ratio and transfer function is detaily described, after which the techniques to avoid block interference are introduced. Block elements are dependent on several aspects such as intermodulation, additive noise and other types of interference.After analyzing those three types of interference, we integrate them by a simulation platform in the fifth part. There are two reasons on the feasibility of such integration:One reason is their common requirement on practical system data for input parameters; platform users could set these data before calculation for any one sort of interference. The other reason is users'interest on simulation result instead of the complex medium calculating process. The platform separates such process into background and just provides input and output interfaces for a concise man-machine interaction. The simulation platform in programmed by Graphical User Interface (GUI) code in MATLAB environment. Users input data by parameter boxes and multiple choices, and control the programme execution by certain buttons in the input window.An effective scheme for interference control, fractional frequency reuse, is studied in the sixth part from the view of data link layer. In cellular wireless communication netwok, fractional frequency reuse depresses interference between adjacent cells (or sectors), while margin users obtain a higher quality of service. Traditional fractional frequency reuse has two modes:time-divided and frequency-divided. This part raises a new mode, time-frequency mode, based on the previous two modes. This mode has a more flexible adjustment on full reuse and partial reuse by a newly constructed frame structure. Time-frequency mode has a management with better fairness, in which a more subtle granularity leads a performance closer to the ideal adaptive reuse mode. By setting a suitable full-partial reuse ratio in frames, time-frequency mode can improve fairness in cell management, where margin users are able to enjoy more transmission resources.