1、RESEARCH OF CELLULAR WIRELESS COMMUNATION SYSTEMAbstract Cellular communication systems allow a large number of mobile users to seamlessly and simultaneously communicate to wireless modems at fixed base stations using a limited amount of radio frequency （RF） spectrum. The RF transmissions received a
2、t the base stations from each mobile are translated to baseband, or to a wideband microwave link, and relayed to mobile switching centers （MSC）, which connect the mobile transmissions with the Public Switched Telephone Network （PSTN）. Similarly, communications from the PSTN are sent to the base stat
3、ion, where they are transmitted to the mobile. Cellular systems employ either frequency division multiple access （FDMA）, time division multiple access （TDMA）, code division multiple access （CDMA）, or spatial division multiple access （SDMA）.1 IntroductionA wide variety of wireless communication syste
4、ms have been developed to provide access to the communications infrastructure for mobile or fixed users in a myriad of operating environments. Most of todays wireless systems are based on the cellular radio concept. Cellular communication systems allow a large number of mobile users to seamlessly an
5、d simultaneously communicate to wireless modems at fixed base stations using a limited amount of radio frequency （RF） spectrum. The RF transmissions received at the base stations from each mobile are translated to baseband, or to a wideband microwave link, and relayed to mobile switching centers （MS
6、C）, which connect the mobile transmissions with the Public Switched Telephone Network （PSTN）. Similarly, communications from the PSTN are sent to the base station, where they are transmitted to the mobile. Cellular systems employ either frequency division multiple access （FDMA）, time division multip
7、le access （TDMA）, code division multiple access （CDMA）, or spatial division multiple access （SDMA） .Wireless communication links experience hostile physical channel characteristics, such as time-varying multipath and shadowing due to large objects in the propagation path. In addition, the performanc
8、e of wireless cellular systems tends to be limited by interference from other users, and for that reason, it is important to have accurate techniques for modeling interference. These complex channel conditions are difficult to describe with a simple analytic-al model, although several models do prov
9、ide analytic-al tractability with reasonable agreement to measured channel data . However, even when the channel is modeled in an analytically elegant manner, in the vast majority of situations it is still difficult or impossible to construct analytic-al solutions for link performance when error con
10、trol coding, equalization, diversity, and network models are factored into the link model. Simulation approaches, therefore, are usually required when analyzing the performance of cellular communication links.Like wireless links, the system performance of a cellular radio system is most effectively
11、modeled using simulation, due to the difficulty in modeling a large number of random events over time and space. These random events, such as the location of users, the number of simultaneous users in the system, the propagation conditions, interference and power level settings of each user, and the
12、 traffic demands of each user,combine together to impact the overall performance seen by a typical user in the cellular system. The aforementioned variables are just a small sampling of the many key physical mechanisms that dictate the instantaneous performance of a particular user at any time withi
13、n the system. The term cellular radio system,therefore, refers to the entire population of mobile users and base stations throughout the geographic service area, as opposed to a single link that connects a single mobile user to a single base station. To design for a particular system-level performan
14、ce, such as the likelihood of a particular user having acceptable service throughout the system, it is necessary to consider the complexity of multiple users that are simultaneously using the system throughout the coverage area. Thus, simulation is needed to consider the muti-user effects upon any o
15、f the individual links between the mobile and the base station.The link performance is a small-scale phenomenon, which deals with the instantaneous changes in the channel over a small local area, or small time duration, over which the average received power is assumed constant . Such assumptions are
16、 sensible in the design of error control codes, equalizers, and other components that serve to mitigate the transient effects created by the channel. However, in order to determine the overall system performance of a large number of users spread over a wide geographic area, it is necessary to incorp
17、orate large-scale effects such as the statistical behavior of interference and signal levels experienced by individual users over large distances, while ignoring the transient channel characteristics. One may think of link-level simulation as being a vernier adjustment on the performance of a commun
18、ication system, and the system-level simulation as being a coarse, yet important, approximation of the overall level of quality that any user could expect at any time.Cellular systems achieve high capacity （e.g., serve a large number of users） by allowing the mobile stations to share, or reuse a com
19、munication channel in different regions of the geographic service area. Channel reuse leads to co-channel interference among users sharing the same channel, which is recognized as one of the major limiting factors of performance and capacity of a cellular system. An appropriate understanding of the
20、effects of co-channel interference on the capacity and performance is therefore required when deploying cellular systems, or when analyzing and designing system methodologies that mitigate the undesired effects of co-channel interference. These effects are strongly dependent on system aspects of the
21、 communication system, such as the number of users sharing the channel and their locations. Other aspects, more related to the propagation channel, such as path loss, shadow fading （or shadowing）, and antenna radiation patterns are also important in the context of system performance, since these eff
22、ects also vary with the locations of particular users. In this chapter, we will discuss the application of system-level simulation in the analysis of the performance of a cellular communication system under the effects of co-channel interference. We will analyze a simple multiple-user cellular syste
23、m, including the antenna and propagation effects of a typical system. Despite the simplicity of the example system considered in this chapter, the analysis presented can easily be extended to include other features of a cellular system.2 Cellular Radio SystemSystem-Level Description：Cellular systems
24、 provide wireless coverage over a geographic service area by dividing the geographic area into segments called cells as shown in Figure 2-1. The available frequency spectrum is also divided into a number of channels with a group of channels assigned to each cell. Base stations located in each cell a
25、re equipped with wireless modems that can communicate with mobile users. Radio frequency channels used in the transmission direction from the base station to the mobile are referred to as forward channels, while channels used in the direction from the mobile to the base station are referred to as re
26、verse channels. The forward and reverse channels together identify a duplex cellular channel. When frequency division duplex （FDD） is used, the forward and reverse channels are split in frequency. Alternatively, when time division duplex （TDD） is used, the forward and reverse channels are on the sam
27、e frequency, but use different time slots for transmission.Figure 2-1 Basic architecture of a cellular communications system High-capacity cellular systems employ frequency reuse among cells. This requires that co-channel cells （cells sharing the same frequency） are sufficiently far apart from each
28、other to mitigate co-channel interference. Channel reuse is implemented by covering the geographic service area with clusters of N cells, as shown in Figure 2-2, where N is known as the cluster size.Figure 2-2 Cell clustering:Depiction of a three-cell reuse patternThe RF spectrum available for the g
29、eographic service area is assigned to each cluster, such that cells within a cluster do not share any channel . If M channels make up the entire spectrum available for the service area, and if the distribution of users is uniform over the service area, then each cell is assigned M/N channels. As the
30、 clusters are replicated over the service area, the reuse of channels leads to tiers of co-channel cells, and co-channel interference will result from the propagation of RF energy between co-channel base stations and mobile users. Co-channel interference in a cellular system occurs when, for example
31、, a mobile simultaneously receives signals from the base station in its own cell, as well as from co-channel base stations in nearby cells from adjacent tiers. In this instance, one co-channel forward link （base station to mobile transmission） is the desired signal, and the other co-channel signals
32、received by the mobile form the total co-channel interference at the receiver. The power level of the co-channel interference is closely related to the separation distances among co-channel cells. If we model the cells with a hexagonal shape, as in Figure 2-2, the minimum distance between the center
33、 of two co-channel cells, called the reuse distance, is （2-1）where R is the maximum radius of the cell （the hexagon is inscribed within the radius）. Therefore, we can immediately see from Figure 2-2 that a small cluster size （small reuse distance）, leads to high interference among co-channel cells.The level of co-channel inter
