移动无线系统传播环境建模的现代处理

mick

IEEE COMSOC. SURVEY vol.3, no.3
Modern Approaches in Modeling of Mobile Radio Systems Propagation Environmen
t
Aleksandar Neskovic, Natasa Neskovic, and George Paunovic,  University of Be
lgrade
移动无线系统传播环境建模的现代处理
Abstract
In this article a review of popular propagation models for wireless communic
ation channels is given. Macrocell, microcell, and indoor prediction methods
are considered separately. Advantages and disadvantages of these models are
discussed. Also, some practical improvements of the existing models as well
as some new models are given.
摘要：
本文对流行的无线通信信道模型进行了综述。对宏蜂窝、微蜂窝以及室内预测模型分别
进行了讨论。讨论了这些模型的优缺点。此外，给出了对这些已有模型的一些实际改进
和一些新的模型。
Over the few past decades, radio communication systems underwent extensive d
evelopment. The demands that a radio system must fulfill are greater by the
day. Having in mind good quality and cost effective solutions, a new radio s
ystem must be designed carefully from the very beginning. The first step in
the process of a new radio system design is to determine base station arrang
ement and a frequency plan, both of which are chiefly dependent on environme
ntal characteristics.
在过去几十年里，无线通信系统得到了巨大的发展。无线系统所受到的制约也与日俱增
。为了寻求高质量、低成本的解决方案，一个新的无线系统必须从刚开始就加以精心设
计。对于一个新的无线系统设计而言，处理的第一个步骤就是确定基站位置以及频率规
划，这两件问题的解决主要取决于环境特性。
One of the most important characteristics of the propagation environment is
the path (propagation) loss. An accurate estimation of the propagation losse
s provides a good basis for a proper selection of base station locations and
a proper determination of the frequency plan. By knowing propagation losses
, one can efficiently determine the field signal strength, signal-to-noise r
atio (SNR), carrier-to-interference (C/I) ratio, etc.
传播环境最重要的特性之一是路径（传播）损耗。传播损耗的精确估计，可以为适当选
择基站位置和适当确定频率规划方案提供很好的基础。通过了解传播损耗，人们可以有
效地确定信号场强、信噪比（SNR）、载干（C/I）比等。
An accurate prediction of the field strength level is a very complex and dif
ficult task. To date, various field strength prediction methods have been pr
oposed in the literature. This article presents an overview of popular predi
ction models and describes some useful algorithms that are based on the auth
ors' experience, to improve their accuracy. It should be noted that in most
cases the models presented predict a local average value (median, mean, slow
fading) which is of particular interest for those system engineers who are
putting radio-systems in operation. Otherwise, models of time dispersion par
ameters, which are very important for device designers, are not considered.
对场强的精确估计是一件很复杂而又困难的任务。到目前为止，文献中提出了多种不同
的场强预测方法。本文对常用的预测模型进行归纳，基于本文作者的经验，介绍了一些
提高其精确度的有用的算法。应当注意在大多数情形下，所提出的模型预测的是本地的
平均值（中值、平均、慢衰落），这些对进行无线系统设计的系统工程师而言更有价值
。另一方面，时域弥散的模型则对设备生产商更有价值，这里不作介绍。
The main propagation mechanisms of the radio signal are described. Definitio
ns and some basic characteristics of the propagation models are given. Macro
cell, microcell, and indoor propagation models are considered in detail in t
he later sections. In these sections, an overview of popular prediction mode
ls is given. Their advantages and disadvantages are discussed, and in additi
on, some practical improvements of these models, along with some new models,
are described.
本文介绍了电波传播的主要传播机理。给出了传播模型的定义及一些基本的特性。在即
后的几节里详细讨论了宏蜂窝、微蜂窝以及室内传播模型。为此，对主要的预测模型进
行了综述，讨论了其优点及不足。此外，介绍了对这些模型的一些实用化改进以及一些
新模型。
Propagation Phenomena
传播现象
Propagation mechanisms are very complex and diverse. First, because of the s
eparation between the receiver and the transmitter, attenuation of the signa
l strength occurs. In addition, the signal propagates by means of diffractio
n, scattering, reflection, transmission, refraction, etc.
传播机制复杂而多变。首先，源于收发信机的分离，信号的强度出现了传播衰减。此外
，信号通过绕射、散射、反射、透射、折射等方式进行传播。
Diffraction occurs when the direct line-of-sight (LoS) propagation between t
he transmitter and the receiver is obstructed by an opaque obstacle whose di
mensions are considerably larger than the signal wavelength. The diffraction
occurs at the obstacle edges where the radio waves are scattered, and as a
result, they are additionally attenuated. The diffraction mechanism allows t
he reception of radio signals when the LoS conditions are not satisfied (NLo
S case), whether in urban or rural environments.
当尺寸远大于信号波长的（对电磁波）不透明的物体遮挡了收发信机之间的直达射线路
径时，会出现绕射现象。绕射出现在物体的边缘，电磁波被散射开来，其结果导致额外
的信号衰减。不管是在城市或农村环境中，绕射机制使得在不满足直达射线的条件下（
无直达射线情形）也可以接收无线信号。
Scattering occurs when the propagation path contains the obstacles whose dim
ensions are comparable to the wavelength. The nature of this phenomenon is s
imilar to the diffraction, except that the radio waves are scattered in a gr
eater number of directions. Of all the mentioned effects, scattering is the
most difficult to be predicted.
当传播路径上有其尺寸可以和信号波长相比拟的（若干）物体时，出现散射现象。这一
现象的特性类似于绕射，只是电磁波向更多的方向上散开。在前所提及的所有现象中，
散射是最难预测的。
Reflection occurs when the radio wave impinges the obstacle whose dimensions
are considerably larger than the wavelength of the incident wave. A reflect
ed wave can either decrease or increase the signal level at the reception po
int. In cases where many reflected waves exist, the received signal level te
nds to be very unstable. This phenomenon is commonly referred to as multipat
h fading, and the signal is often Rayleigh distributed.
当无线电波入射到其尺寸远大于入射波波长的物体表面时，出现反射现象。在接收点处
反射波既可能加强，也可能减弱信号电平。如果存在很多反射波时，接收信号就会变得
不稳定。这一现象通常称为多径衰落，这时的信号服从瑞利（Rayleigh）分布。
Transmission occurs when the radio wave encounters an obstacle that is to so
me extent transparent for the radio waves. This mechanism allows the recepti
on of radio signals inside buildings in cases where the actual transmitter l
ocations are either outdoors or indoors.
当无线电波遇上的物体在某种程度上对无线电波透明时，会出现所谓透射现象。无论实
际的发射机位置在室内还是室外，这一机理使得建筑物内可以接收无线信号。
Refraction is very important in macrocell radio system design. Due to an inc
onstant refractive index of the atmosphere, the radio waves do not propagate
along a straight line, but rather along a curved one. Therefore, the covera
ge area of an actual transmitter is usually larger. However, as a result of
the fluctuations of the atmosphere parameters, the received signal strength
level is fluctuating as well.
折射在宏蜂窝无线系统设计中非常重要。由于大气具有变化的折射率，无线电波不是沿
着直线传播，而是形成一条弯曲的传播路径。因此，实际发射机的覆盖范围通常较大。
另一方面，作为大气参数变化的结果，接收信号电平通常也会起伏不定。
Since there is frequently no LoS between the transmitter and the receiver, t
he received signal is a sum of components that often stem from several previ
ously described phenomena. Therefore, the received signal level is quite var
iable with respect to time and especially with respect to the receiver or tr
ansmitter displacement. Even a displacement of just a fraction of the wavele
ngth can cause the signal level to change by more than 30dB. These fluctuati
ons are known as short-term (or multi-path) fading. On the other hand, the l
ocal average of the signal varies slowly with the displacement. These slow f
luctuations depend mostly on environmental characteristics, and they are kno
wn as long-term fading. Both slow and fast fading are illustrated in Fig. 1.

由于通常在收发信机之间没有直达路径，接收信号通常是若干分量的合成结果，而这些
分量则是源于前所述及的现象中的若干种。因而，接收信号电平通常随时间，特别是随
接收或发射机的位移而快速变化。即便位移量只是若干分之一波长，这样的位移也会导
致接收信号电平30分贝以上的变化。这一波动即所谓的short-term（多径）衰落。另一
方面，信号变化的平均值随位置的变化而缓慢变化。这一慢速的变化主要取决于环境特
性，又称为long-term衰落（衰减）。图1中说明了两类衰落现象。
Since the short-term fading of the received signal is almost impossible to p
redict, all propagation models estimate either the average or median values.
When averaging is performed, the width of the averaging window should be ch
osen carefully. A window that is too narrow results in uncertain averages, w
hile a window that is too wide can wash out the detailed signal changes depe
nding on the local environment. Typically, a suitable window width for appli
cations in the bigger areas is ±20λ[1], while in smaller areas the window
must be narrower.
由于接收信号的多径衰落通常不能加以预测，因此所有传播模型都旨在估计平均值或中
值。当进行平均时，平均窗口的宽度应当加以仔细选择。过窄的窗口会使得平均结果不
可靠，而过宽的窗口则会掩盖本地信号变化的一些细节。典型情况下，在较大的场地(a
rea)下适当的窗口宽度应当在±20λ左右[1]，而在较小的场地内窗口宽度应当变窄。
Propagation Models
传播模型
A propagation model is a set of mathematical expressions, diagrams, and algo
rithms used to represent the radio characteristics of a given environment. G
enerally, the prediction models can be either empirical (also called statist
ical) or theoretical (also called deterministic), or a combination of these
two. While the empirical models are based on measurements, the theoretical m
odels deal with the fundamental principles of radio wave propagation phenome
na.
一个传播模型，是一组用于表示给定环境中无线电传播特性的数学表达式、图表或算法
。一般说来，预测模型可以是经验模型（又称统计模型），也可以是理论模型（也称确
定模型），或是二者的组合。经验模型主要基于测量的结果，而理论模型则处理电波传
播现象的基本原理。
In the empirical models, all environmental influences are implicitly taken i
nto account regardless of whether they can be separately recognized. This is
the main advantage of these models. On the other hand, the accuracy of thes
e models depends not only on the accuracy of the measurements, but also on t
he similarities between the environment to be analyzed and the environment w
here the measurements are carried out. The computational efficiency of these
models is usually satisfying.
在经验模型中，隐含地考虑了所有环境因素的影响，不管它们彼此之间是互相独立的或
是互相影响的。这也是这类模型的主要优点。然而，这些模型的精确程度不仅取决于测
量的精确程度，而且还和所分析的环境与所测试的环境二者间的相似程度有关。这类模
型的计算效率通常能满足要求。
The deterministic models are based on the principles of physics and, due to
that, they can be applied to different environments without affecting the ac
curacy. In practice, their implementation usually requires a huge database o
f environmental characteristics, which is sometimes either impractical or im
possible to obtain. The algorithms used by deterministic models are usually
very complex and lack computational efficiency. For that reason, the impleme
ntation of the deterministic models is commonly restricted to smaller areas
of microcell or indoor environments. Nevertheless, if the deterministic mode
ls are implemented correctly, greater accuracy of the prediction can be expe
cted than in the case of the empirical models.
确定模型基于物理原理，因此，在用于不同环境中是不影响其精确程度。实际上，其实
现通常需要一个很大的环境特性数据库，后者有时是不切实际的或是不可能的。确定模
型所使用的算法通常很复杂且计算效率不高。因为这一原因，确定模型的应用通常局限
于微蜂窝或室内环境等较小的场地范围。不过，一旦确定性模型得以正确应用，可望得
到较之经验模型更为准确的预测结果。
On the basis of the radio environment, the prediction models can be classifi
ed into two main categories, outdoor and indoor propagation models. Further,
in respect of the size of the coverage area, the outdoor propagation models
can be subdivided into two additional classes, macrocell and microcell pred
iction models.
基于无线环境的不同，预测模型可以分为室外和室内传播模型两种情形。而且，根据覆
盖范围大小之别，室外传播模型还可以进一步细分为宏蜂窝和微蜂窝预测模型两种子情
形。
Macrocell Propagation Models
宏蜂窝传播模型
Macrocell design philosophy is based on the assumptions of high radiation ce
nterlines, usually placed above the surroundings; transmitter powers on the
order of several tens of Watts; and large cells whose dimensions are on the
order of several tens of kilometers. Under these assumptions, LoS conditions
are usually not satisfied and the signal from the transmitter to the receiv
er propagates by means of the diffraction and the reflection. Also, for larg
e cells the effects of refraction are very important. All of these factors m
ake the problem of field strength prediction very difficult. For years, a la
rge number of researchers have been struggling with this problem. As a resul
t a large number of models have been proposed. The list includes, but is not
limited to: the Bullington model [2], the model of Okumura et al. [3], the
ITU (CCIR) model [4], the Hata model [5], the "Clearance angle" method [6],
the Polish Administration method [7], the Longley-Rice method [8], the Lee m
odel [9], the EPM-73 method [10], the Deutche Bundest Post (DBP) method [11]
, the Ibrahim-Parsons model [12], the Atefi-Parsons model [13], the Joint Ra
dio Committee (JRC) model [14], the TIREM model [10], the Walfish-Bertoni mo
del [10, 15], the Ikegami method [16, 17], the IRT method [18], the ETF-ANN
Macrocell Model [19], the Ericsson model 9999 [20], etc. In the following te
xt, only a few very popular models are discussed.
宏蜂窝的设计原理基于高辐射中心线的假设，发射机通常高出周围的建筑，其功率在数
十瓦量级，形成其半径可达数十公里量级的大蜂窝。在这样的假设下，通常不能满足直
达射线条件，且从发射机到接收机的信号传播借助于绕射及反射机制。此外，对大的小
区而言折射也很重要。所有这些因素使得场强预测问题变得很困难。在几年的时间里，
大量的研究人员致力于解决这一问题，其结果是出现了大量模型。如果列出一个清单，
则包括Bullington模型[2]、奥村等人的模型[3]、ITU（CCIR）模型[4]、Hata模型[5]、
"透过角"模型[6]、Polish Administration模型[7]、Longley-Rice模型[8]、李氏模型
[9]、EPM-73模型[10]、Deutche Bundest Post模型[11]、Ibrahim-Parsons模型[12]、
Atefi-Parsons模型[13]、Joint Radio Committee (JRC)模型[14]、TIREM模型[10]、W
alfish-Bertoni模型[10,15]、Ikegami模型[16,17]、IRT模型[18]、ETF-ANN宏蜂窝模型
[19]、爱立信模型9999[20]等等，有些甚至还未列出。在以下的内容里，我们只讨论其
中一些最常用的模型。
Model of Okumura et al.
奥村（Okumura）等人的模型
The Okumura et al. method [3] is based on empirical data collected in detail
ed propagation tests over various situations of an irregular terrain and env
ironmental clutter. The results are analyzed statistically and compiled into
diagrams. The basic prediction of the median field strength is obtained for
the quasi-smooth terrain in the urban area. The correction factor for eithe
r an open area or a suburban area should be taken into account. The addition
al correction factors, such as for a rolling hilly terrain, the isolated mou
ntain, mixed land-sea paths, street direction, general slope of the terrain
etc., make the final prediction closer to the actual field strength values.
奥村等人的模型[3]基于经验数据，这些数据源于在各种不规则地形和环境分布下进行的
详细的传播测试。这些结果以统计方法进行分析并合成为图表。在城区准光滑地形下可
以得到中值场强的基本预测结果。在开阔地带或郊区都有可供使用的修正因子。其他的
一些修正因子包括起伏的丘陵地貌、孤立的山峰、混合的陆地海面路径、街道走向、一
般的斜坡地貌等等，这使得最终的预测结果接近于实际的场强值。
In the present engineering practice, the Okumura et al. method is widely use
d. This is a method originally intended for VHF and UHF land-mobile radio sy
stems and involves neither complex computations nor an elaborate theory. Muc
h of its experimental data have been incorporated in the ITU (CCIR) referenc
e curves as well as in other popular models.
在当前的工程实践中，奥村等人的方法得到了广泛的使用。这一方法最初只是拟用于VH
F 和UHF频段的陆地移动无线系统，没有复杂的计算或精细的理论。其中的大多数经验数
据与ITU（CCIR）的参考曲线即其他一些常用模型一致。
However, many authors [9, 21, 23] show certain reserve toward the applicatio
n of the Okumura model. They note that extensive data regarding its performa
nce must be obtained before its use may be advocated. In addition, more care
ful interpretation of the definitions of various parameters needs to be made
. When assessing the values of the model's parameters, the influence of the
subjective factors is not easy to avoid, thus yielding different results for
the same problem.
然而，很多作者[9, 21, 23]表明其对奥村模型的应用持某些保留意见。他们指出在提倡
使用这一模型之前还需要得到更多的关系到其性能的数据。此外，还需要对各种参数进
行更仔细的定义。在评估模型参数的量值时，不可避免各种主观因素的影响，因此对同
一问题会产生不同的结果。