【正文】 types of channel models are macro cell type has cell sizes from 500 m up to 5000 m and a carrier frequency of 900 MHz or micro cell type is defined for cell sizes of about 300 m and a carrier frequency of GHz or 5 pico cell type represents an indoor channel model with cell sizes smaller than 100 m in industrial buildings and in the order of 10 m in an carrier frequency is GHz or 24 273: The COST 273 action additionally takes multiantenna channel models into account, which are not covered by the previous COST [7]: These channel models define typical outdoor and indoor propagation scenarios for macro, micro, and pico fading characteristics of the various propagation environments are specified by the parameters of the Nakagamim environment is defined in terms of a number of scatterers which can take on values up to channel models consider also the angular distribution of the have been developed for the investigation of 3G system cell channel type models have been developed for carrier frequencies around 900 MHz with 7 MHz micro and pico cell channel type models have been developed for carrier frequencies between GHz and 2 bandwidths of the measurements are in the range of 10–100 MHz for macro cells and around 100 MHz for pico [28]: The JTC channel models define indoor and outdoor scenarios by specifying 3 to 10 discrete taps per channel models are designed to be applicable for wideband digital mobile radio systems anticipated as candidates for the PCS(Personal Communications Systems)mon air interface at carrier frequencies of about 2 [18][44]: Test propagation scenarios have been defined for UMTS and UTRA system proposals which are developed for frequencies around 2 modeling of the multipath propagation corresponds to that used by the COST 207 channel [33]: Five typical indoor propagation scenarios for wireless LANs in the 5 GHz frequency band have been scenario is described by 18discrete taps of the delay power density time variance of the channel(Doppler spread)is modeled by a classical Jake’s spectrum with a maximum terminal speed of 3 m/ channel models exist which are, for instance, given in [16]. Channel Modeling Multicarrier systems can either be simulated in the time domain or, more putationally efficient, in the frequency for the frequency domain implementation are the absence of ISI and ICI, the frequency nonselective fading per subcarrier, and the timeinvariance during one OFDM proper system design approximately fulfills these discrete channel transfer function。chemical or electrochemical polishing where weak chemical reagents are used(sometimes with remote electric assist)for polishing or deburring and chemical jet machining where a single chemically active jet is schematic of chemical machining process is shown in the etchant attacks the material in both vertical and horizontal directions, undercuts may develop(as shown by the areas under the edges of the maskant in ).Typically, tolerances of 177。 types of nontraditional machining processes have been developed to meet extra required machining these processes are employed properly, they offer many advantages over traditional machining mon nontraditional machining processes are described in the following Discharge Machining(EDM)Electrical discharge machining(EDM)sometimes is colloquially referred to as spark machining, spark eroding, burning, die sinking or wire is one of the most widely used nontraditional machining main attraction of EDM over traditional machining processes such as metal cutting using different tools and grinding is that this technique utilizes thermoelectric process to erode undesired materials from the workpiece by a series of rapidly recurring discrete electrical sparks between workpiece and traditional machining processes rely on harder tool or abrasive material to remove softer material whereas nontraditional machining processes such as EDM uses electrical spark or thermal energy to erode unwanted material in order to create desired , the hardness of the material is no longer a dominating factor for EDM removes material by discharging an electrical current, normally stored in a capacitor bank, across a small gap between the tool(cathode)and the workpiece(anode)typically in the order of 50 volts/ shown in , at the beginning of EDM operation, a high voltage is applied across the narrow gap between the electrode and the high voltage induces an electric field in the insulating dielectric that is present in narrow gap between electrode and causes conducting particles suspended in the dielectric to concentrate at the points of strongest electrical the potential difference between the electrode and the workpiece is sufficiently high, the dielectric breaks down and a transient spark discharges through the dielectric fluid, removing small amount of material from the workpiece volume of the material removed per spark discharge is typically in the range of 105 to 106 gap is only a few thousandths of an inch, which is maintained at a constant value by the servomechanism that actuates and controls the tool Machining(CM)Chemical machining(CM)is a well known nontraditional machining process in which metal is removed from a workpiece by immersing it into a chemical process is the oldest of the nontraditional processes and has been used to produce pockets and contours and to remove materials from parts having a high strengthtoweight , the chemical machining method is widely used to produce microponents for various industrial applications such as microelectromechanical systems(MEMS)and semiconductor CM material is removed from selected areas of workpiece by immersing it in a chemical reagents or etchants, such as acids and alkaline is removed by microscopic electrochemical cell action which occurs in corrosion or chemical dissolution of a coatings called maskants protect areas from which the metal is not to be controlled chemical dissolution will simultaneously