| Passive intermodulation (PIM) is defined as the distortion generated by the nonlinear characteristics of passive radio frequency components.The nonlinearities mainly include materials nonlinearity and contact nonlinearity. Traditionally, these passive nonlinearities are negligible when the input is very small.However, frequency bands are becoming more and more densely populated recently. Besides, multiple carriers with high transimitting power and hundreds of cascaded components are used in wireless networks to satisfy the requirement of high-speed data service. As more new operators emerge and more new systems are put into use, multiple different systems are more frequently located at the same site. Thus,passive weakly nonlinearities can not be ignored again and PIM has become one of the most important spurious interferences. Severe PIM distortion will not only limit the sensitivity of the receiver, but also degrade the system performance, especially for those frequency-division duplexing (FDD) systems, in which (close-by) transmit and receive bands are simultaneously utilized.In this thesis, the generation mechanism, behavioral models,superposition effect, and avoidance methods are studied. The behavioral characteristics of PIM in UHF band are analyzed. Two types of analytical models to evaluate the superpositon effect for systems operating in multiple carrires and series connections are established. The threshhold level of low PIM is suggested. PIM combinations in scene of the coexistence of different wireless communication systems are provided.The typical analytical models of PIM characteristics are described and the performance of the polynomial model is evaluated based on the test data. Behavioral charicteristics of PIM affected by various factors such as the carrier power, PIM order, time, temperature and VSWR, are studied with the analytical models and measurement results. Additionally,the conclusion that the bandwith of the PIM product is multiple of the fundamentals is derived with the method of convolution theory.The phenomenon of group inter-modulation is investigated based on numerical calculation and Monte-Carlo simulation. The definaton of the group inter-modulation factor (GIF) based on the concept of "effective hit” is provided and the evaluation mechanism of GIF is given. The relationship between GIF and the number of the carrirer is simulated. The results show that, compared with the CS scheme, the strength of PIM product with the MS scheme does not increases so greatly with the growth of the total number of the carriers.A modified point-source model with lossy components is proposed to evaluate the superposition effect of the forward and reflected PIM in case of series connection. The index of the series connection factor (SCF)is employed for systems that incorporate multiple PIM sources to predict the important characteristics of the cumulative PIM, such as the maximum and minimum values and the limit form. Furthermore, some initial experimental data are given and discussed.A simple model is developed for predicting the power rise of the passive intermodulation in the 1st zone generated by broadband signals compared with 2 tone test, followed by an experimental validation. The calculated results agree well with the measurements, having a mean difference of 0.5 dBFinally, the threshhold level of low PIM is suggested based on interference analysis and link budget. Since it is observed that inteference with traffic variations are usually associtated with PIM, a new method of locating the PIM sources with the network performance data is proposed.Furthermore, PIM interferences are more complex for co-site, shared or co-existence networks. All the PIM interferences faced by typical co-existence systems are analyzed, as far as the 3rd order, 5th order and 7th order of PIM are concerned.The main contributions of this dissertation are concluded as follows:Firstly, the evaluaton mechanism of superposition effect of PIM for systems operating in multiple carriers is proposed. Then the superposition PIM effects for the two power amplifier schemes (CS and MS) are simulated and the results are given.Secondly, a modified point-source model based on transaction function for series connection is provided, in which the loss of the path is considered and thus the model is more practical. Then the cumulative PIM level is measured by the constructed index of SCF. The relationship between the forward and reflected PIM level is studied and the maximum value of the forward PIM is achieved by deducing the limit form of the function of the reflected PIM.Thirdly, a simple model to explain the relationship between the power rise level of PIMP of broadband signals and that of CW signals has been derived. The calculated results correspond well with the measurements with a mean difference of 0.5dB. This knowledge allows us to predict the impact of PIM interference more accurately in the broadband wireless systems.Fourthly, PIM products results are given for coexistence combinatios of diferent wireless systems. PIM characteristics of Point of Interface(POI) systems, which are usually used in multi-operator environments,are also sdudied. These results provide valuable reference for the wireless network planning and optimization. |
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