ACCC 碳纤维复合芯导线 (Aluminum Conductor Composite Core) 复合材料合成芯导线。
Antennas and Propagation ( 天线与传播 ) Chapter 5. Introduction An antenna is an electrical...
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Transcript of Antennas and Propagation ( 天线与传播 ) Chapter 5. Introduction An antenna is an electrical...
Antennas and Propagation( 天线与传播 )
Chapter 5
Introduction An antenna is an electrical conductor (电子导
线) or system of conductors (导线系统) 发送 - radiates electromagnetic energy into space 接收 - collects electromagnetic energy from space
In two-way communication, the same antenna can be used for transmission and reception
假定:两个方向上使用同一频率
Radiation Patterns (辐射模式) Radiation pattern
Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section
Beam width (or half-power beam width) (光束宽度或称半功率光束宽度)
Measure of directivity of antenna (计算天线方向性的计量值)半功率光束宽度表示一个角度,在这个角度内,天线辐射功率至少是该
天线在最佳方向上辐射功率的一半 Reception pattern
Receiving antenna’s equivalent to radiation pattern
Types of Antennas Isotropic antenna (idealized) (各向同性天线)
Radiates power equally in all directions 辐射模式:以天线为中心的一个球体 通常表示为三维模式的一个二维剖面
Dipole antennas (偶极天线) Half-wave dipole antenna (or Hertz antenna) (半波偶极天线或称赫兹天线) Quarter-wave vertical antenna (or Marconi antenna) ( 1/4 波垂直天线或称马可尼天线)
Parabolic Reflective Antenna (抛物反射天线)
半波偶极 ¼ 波垂直天线
Antenna Gain (天线增益) Antenna gain
Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)
天线增益不是为了似的输出功率大于输入功率,而主要是为了方向性
Effective area Related to physical size and shape of antenna 针对不同天线的经验计算公式见表 5.2, 如:抛物线,
抛物面面积为 A ,则其有效面积为 0.56A.
Antenna Gain Relationship between antenna gain and effective
area
G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength
2
2
2
44
c
AfAG ee
Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation
Ground Wave Propagation
Ground Wave Propagation Follows contour of the earth ( 沿地表传播 ) Can Propagate considerable distances Frequencies up to 2 MHz…>1km Example
AM radio ( 调幅无线电 )
Sky Wave Propagation
Sky Wave Propagation Signal reflected from ionized layer of atmosphere
back down to earth Signal can travel a number of hops, back and forth
between ionosphere and earth’s surface Reflection effect caused by refraction ( 折
射 ) 。。。尽管实际效果看起来象反射 Examples
Amateur radio CB radio (民用波段无线电) 国际广播 ( BBC, VOA)
Line-of-Sight Propagation
Line-of-Sight Propagation Transmitting and receiving antennas must be within line of
sight Satellite communication – signal above 30 MHz not reflected by
ionosphere Ground communication – antennas within effective line of sight
( 有效的直线范围 ) due to refraction 。。因为微波会被大气弯曲或折射,大多数微波弯曲程度与地球曲率相同,因而比直线广播传输的远些
Refraction – bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the
medium When wave changes medium, speed changes Wave bends at the boundary between mediums
光波地平线和无线电地平线
Line-of-Sight Equations Optical line of sight
Effective, or radio, line of sight
d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction,
rule of thumb ( 经验法 ) K = 4/3
hd 57.3
hd 57.3
Line-of-Sight Equations Maximum distance between two antennas
for LOS propagation:
h1 = height of antenna one
h2 = height of antenna two
2157.3 hh
天线高度计算 例 5.2: 假定天线高度 100m, 另一天线在地平线上,则
两天线直线传播的最大距离是
假设接收天线 10 m ,试问为获得同样传播距离,发送天线高度几何?
h1=7.842/1.33=46.2 m
kmKhd 4113357.357.3
)3.13(57.341 1 Kh
84.73.1357.3
411 Kh
LOS Wireless Transmission Impairments ( 传输损伤 ) Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise
Attenuation Strength of signal falls off with distance over
transmission medium Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion
跨频带的衰减均等化
Free Space Loss Free space loss, ideal isotropic antenna
Pt = signal power at transmitting antenna
Pr = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (3 10 8 m/s)
where d, , and c are in the same units (e.g., meters)
2
2
2
2 44
c
fdd
P
P
r
t
Free Space Loss Free space loss equation can be recast:
d
P
PL
r
tdB
4log20log10
dB 98.21log20log20 d
dB 56.147log20log204
log20
df
c
fd
Free Space Loss Free space loss accounting for gain of other
antennas
Gt = gain of transmitting antenna
Gr = gain of receiving antenna
At = effective area of transmitting antenna
Ar = effective area of receiving antenna
trtrtrr
t
AAf
cd
AA
d
GG
d
P
P2
22
2
224
Free Space Loss Free space loss accounting for gain of other
antennas can be recast as
rtdB AAdL log10log20log20
dB54.169log10log20log20 rt AAdf
表 5.2: 天线增益与等效面积
Categories of Noise Thermal Noise Intermodulation noise ( 互调噪声 ) Crosstalk ( 串扰 ) Impulse Noise
Thermal Noise Thermal noise due to agitation of electrons (电
子的热搅动) Present in all electronic devices and transmission
media Cannot be eliminated Function of temperature Particularly significant for satellite communication
因为卫星地面站收到的卫星信号较弱
Thermal Noise Amount of thermal noise to be found in a
bandwidth of 1Hz in any device or conductor is:
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzmann's constant = 1.3803 10-23 J/K T = temperature, in kelvins (absolute temperature)
W/Hz k0 TN
Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz
(in watts):
or, in decibel-watts
TBN k
BTN log10 log 10k log10 BT log10 log 10dBW 6.228
Noise Terminology Intermodulation noise – occurs if signals with different
frequencies share the same medium Interference caused by a signal produced at a frequency that is the
sum or difference of original frequencies Crosstalk – unwanted coupling between signal paths (不希望的信号路径之间的偶合) Impulse noise – irregular pulses or noise spikes
Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and
flaws in the communications system (外部电磁干扰或通信系统的故障或缺陷)脉冲噪音是数字数据误码的一个主要来源
如: 0.01 秒的脉冲在 56kbps 的传输中,会破坏 560bits 数据
Expression Eb/N0
Ratio of signal energy per bit to noise power density per Hertz
R: 数据比特率 ; S 信号功率 The bit error rate for digital data is a (decreasing) function
of Eb/N0 Given a value for Eb/N0 to achieve a desired error rate, parameters
of this formula can be selected As bit rate R increases, transmitted signal power must increase to
maintain required Eb/N0
TR
S
N
RS
N
Eb
k
/
00
Other Impairments Atmospheric absorption – water vapor
(水蒸气) and oxygen contribute to attenuation
Multipath – obstacles reflect signals so that multiple copies with varying delays are received
Refraction – bending of radio waves as they propagate through the atmosphere
Multipath Propagation
反射 (R), 散射 (S), 衍射 (D)
Multipath Propagation Reflection ( 反射 ) - occurs when signal
encounters a surface that is large relative to the wavelength of the signal
Diffraction( 衍射 ) - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave
Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less
The Effects of Multipath Propagation Multiple copies of a signal may arrive at different phases
If phases add destructively, the signal level relative to noise declines, making detection more difficult
Intersymbol interference (ISI) One or more delayed copies of
a pulse may arrive at the
same
time as the primary pulse
for a subsequent bit
Types of Fading Fast fading Slow fading Flat fading
( 平面衰落 ) Selective fading Rayleigh fading
( 瑞利衰落 ) Rician fading
( 瑞森衰落 )
Error Compensation Mechanisms( 差错补偿机制 )
Forward error correction Adaptive equalization Diversity techniques
Forward Error Correction Transmitter adds error-correcting code to data
block Code is a function of the data bits
Receiver calculates error-correcting code from incoming data bits If calculated code matches incoming code, no error
occurred If error-correcting codes don’t match, receiver attempts
to determine bits in error and correct
Adaptive Equalization(自适应均衡) Can be applied to transmissions that carry analog
or digital information Analog voice or video Digital data, digitized voice or video
Used to combat intersymbol interference Involves gathering dispersed symbol energy back
into its original time interval Techniques
Lumped analog circuits (集总模拟电路) Sophisticated digital signal processing algorithms
Diversity Techniques (分集技术) Diversity is based on the fact that individual channels
experience independent fading events Space diversity – techniques involving physical
transmission path Frequency diversity – techniques where the signal is
spread out over a larger frequency bandwidth or carried on multiple frequency carriers
如:扩频 Time diversity – techniques aimed at spreading the data
out over time 交织 (interleaving) 代价是时延。。交织度越大,接收端重构原序列的时延越大
作业1. 命名并简要定义四类噪声2. 抛物反射天线的优点是什么 ?3. 假设发送器产生 50W 的功率
(1) 以 dBm 和 dBW 为单位表示传输功率 (2) 如果上述发送器的功率输入到一个增益为 1 、载波频率为
900MHz 的发送天线上 ,则在距离 100m 远的自由空间处 , 接收功率是多少 dBm? (假定接收天线增益也为 1 )
(3) (2) 中 ,如果距离为 10km, 距离如何 ? (4) (3) 中 ,假设接收天线有 2 的增益 , 结果又如何 ?
4. 确定信号必须能够达到 80km 远的用户处的电视台天线的高度
下周六停一次课!