通信工程英文论文

英文原文

RESEARCH OF CELLULAR WIRELESS

COMMUNATION SYSTEM

Abstract

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 at 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 station, 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 Introduction

A wide variety of wireless communication systems have been developed to

provide access to the communications infrastructure for mobile or fixed users in a

myriad of operating environments. Most of today’s wireless systems are based on the

cellular radio concept。 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 at 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

station, 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） .

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 performance 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 analytical model, although several

models do provide analytical 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

analytical solutions for link performance when error control 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 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 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 within 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 performance， 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 multi—user effects upon any of 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

sensible in the design of error control codes, equalizers， and other components that