第三章 无线电波传播--大尺度衰落(1)

Small-scale Propagation Models小尺度传播模型


Propagation models that characterize the rapid fluctuations of the received signal strength over very short travel distances (a few wavelengths) or short time durations (on the order of seconds). (该传播模型主要用 来表征接收信号在非常小的距离/时间内的快速变化) The instantaneous received signal power may vary by as much as three or four orders of magnitude (30 or 40 dB) when the receiver is moved by only a fraction of a wavelength.(当接收机移动不到一个波长时, 瞬时接收 信号功率也会发生3-4个数量级的变化)
Example： Foliage植物, street signs, and lamp posts. Rich scattering is very important to MIMO.

Propagation Models


Propagation Models: predicting the average received signal strength at a given distance from the transmitter, as well as the variability of the signal in close spatial proximity to a particular location. Two class models: large-scale propagation models大尺度传播模型 small-scale or fading models.小尺度传播模型(衰落)
Mobile Radio Propagation Overview (6)


Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in channel.

Mobile Radio Propagation Overview (5)


Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle. Example： Mountain
Friis free space equation （1）
Pr (d )

4
PG t t Gr
2 2
2
d L
Where Pt is the transmitted power, Pr(d) is the received power, Gt is the transmitter antenna gain, Gr is the receiver antenna gain, d is the T-R separation distance in meters, L is the system loss factor not related to propagation (L≥1), and is the wavelength in meters.


The Path Loss (PL), which presents signal attenuation as a positive measured in dB is defined as the difference (in dB) between the effective transmitter power and the received power, and may or may not include the effect of the antenna gains. 路径损耗表示信号衰减,单位为dB的正值,定义为有效发射功 率和接收功率之间的差值,可以包括也可以不包括天线增益.
Friis free space equation （2）
c 2 c f c
PG t t Gr c Pr (d ) 2 2 2 2 2 4 d f L 4d L
PG t t Gr c
2
2

Where c 3.0 108 m / s is the speed of light given in meters, f is the carrier frequency in Hertz, is the carrier frequency in radians per second .
Friis free space equation Antenna Gain


The gain of an antenna (天线增益G) is related to its effective aperture (有效截面) Ae. The effective aperture Ae is related to physical size of the antenna.
Friis free space equation Far field , Fraunhofer region

Friis free space equation （3）

ቤተ መጻሕፍቲ ባይዱ
L is the miscellaneous losses (L≥1) which are usually due to transmission line attenuation, filter losses, and antenna losses in the communication system. (L为综合 损耗,包括传输线\滤波器\天线等损耗,即硬件损耗) L=1 indicates no loss in the system hardware. The received power decays with distance at the rate of 20dB/decade.(接收机功率随T-R距离的平方而衰减,即 接收功率与距离的关系为20dB/十倍程).
Small-scale and large-scale fading
Figure 4.1 Small-scale and large-scale fading.
Free space propagation Model自由空间传播模型


Predict received signal strength when the transmitter and receiver have a clear, unobstructed line-of-sight path between.(预测处于视距范围内的接收信号强度) Example: Satellite communication system Microwave line-of-sight radio links
Large-scale Propagation Models


Propagation models that predict the mean signal strength for an arbitrary transmitter-receiver (T-R) separation distance.(传播模型主要用来预测收发信机之间的平 均信号强度) Estimating the signal strength over large T-R separation distances (several hundreds or thousands of meters) in the radio coverage area of a transmitter. (估计在电波覆 盖范围内的接收机接收信号的强度)
Friis free space equation Path Loss
包括天线增益时:
不考虑天线增益时:
G G 2 P PL(dB) 10 log t 10 log t r2 2 Pr 4 d
2 Pt PL(dB) 10 log 10 log 2 2 Pr 4 d
Chapter 3 Mobile Radio Propagation: Large-Scale Path Loss (1)
Mobile Radio Propagation Overview (1)

Electromagnetic wave propagation mechanisms: Direct line-of-sight path (LOS)视距传播 Reflection反射 Diffraction衍射 Scattering散射
G
4 Ae

2
Friis free space equation EIRP

EIRP: Effective isotropic radiated power (有效全向天线发射功 率)is defined as
EIRP PG t t


EIRP represents the maximum radiated power available from a transmitter in the direction of maximum antenna gain, as compared to an isotropic radiator. Isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions, and is often used to reference antenna gains in wireless systems.
Mobile Radio Propagation Overview (2)
Mobile Radio Propagation Overview (3)
Mobile Radio Propagation Overview (4)
Reflection occurs when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagation wave.(当电磁波在传 播过程中碰到比其波长尺寸更大的物体时就会产 生反射现象) Example: the surface of earth (lake, river) buildings and walls