Introduction to wireless communication systems(无线通信技术)
Part 1: RF(Radiofrequency)/microwave electronics
Decibels, gain, and insertion loss
Digital communication environment: Guided propagation , “Free space” propagation(including electromagnetic waves which we will learn).
Flowgraph of a communication system:
[picure:flowgraph_of_system.jpg]
{bi} -> digital communication transmitter -> radio frequency transmitter ~~~> radio frequency receiver -> digital commnication receiver -> {b’i}
Examples of RF/microwave systems
[picture:block_digram_wireless_radio_system.jpg]
FDMA,TDMA,SDMA:Frequency,Time,Space division multiple address
Standard components:
- Power dividers and combiners/directional couplers
- Filters
- Oscillators
- Amplifiers
- Mixers
- Antennas
Decibels
Difinition: dimensionless number that express the radio of two values of a physical quantity, often power or voltage.
Conversion from linear to dB values:
- Power ratio in dB = $10log_{10}(\frac{P_1}{P_2})$
- Voltage ratio in dB = $20log_{10}(\frac{V_1}{V_2})$
Conversion from dB to linear values:
- Power radio = $10^{(\frac{dB}{10})}$
- Voltage radio = $10^{(\frac{dB}{20})}$
也就意味着，同一个dB对应了不同的功率比率和电压比率。
Gain, insertion loss and return loss
Transmission coefficient in dB:
- if $P_{out} \geq P_{in}$, gain [增加量] G = $10log_{10}(\frac{P_out}{P_in})$
- if $P_{out} \leq P_{in}$, insertion loss [减少量] IL = $10log_{10}(\frac{P_in}{P_out})$
Total gain $G_T = (G_1 + G_2 + …) - (IL_1 + IL_2 + …)$,so $P_{out} = G_T + P_{in}$
Filter -> IL
Ampligier -> G
Attenuator -> IL
Notice: Overall gain in linear : $G_T in linear = \frac{G in linear}{IL in linear}, P_{out} in linear = P_{in} in linear * G_T in linear$
Decibels as absolute units
P in dBW = $10log_{10}(P in W)$
P in dBm = $10log_{10}(P in mW)$
So, value of dBm = value of dBW + 30 [另外，每当真实值减小一半的时候，dB值减少约3dB]
2W = 3.010dBW = 2000mW = 33.010dBm
1W = 0dBW = 1000mW = 30 dBm
0.001W = -30dBW = 1mW = 0 dBm
关于Insertion Loss,如果IL= 0,那么损失值为0，也就是说，功率转换率为100%；而如果IL = 3.01,那么转换系数为0.71，功率转换率为50%。
RF/microwave circuits
Power dividers/combiners and directional couplers
- Poweer dividers/combiners
Main characteristics(in its frequency band):
- Good input and output impedance matching
- Low additional insertion loss
- Good isolation between outputs
- Constant coupling factors(amplitude and phase)
- Directional couplers
input port ; direct port ; coupled port ; isolated or uncoupled port.
Important parameters:
- Coupling factor
- Directivity
- Isolation
- Insertion loss
Main characteristics(in its frequency band):
- Good input and output impedance matching
- Low additional insertion loss
- High isolation and directivity 高隔离度和方向性
- Constant coupling(amplitude and phase)
- Hybrids
Principle: 3dB directional couplers [定向耦合器] with specific phase difference between the outputs of the through and coupled arms(isolated port terminated with a matched load).
在直通和耦合的两个方向之间有特定相位差。
Filters
Purpose:four types of passive devices to control the frequency content of RF/microwave signals.
- Low-pass
- High-pass
- Bandpass
- Bandstop
Main characteristics:
- Good input and output impedance matching in the passbands
- Low insertion loss in the passbands
- High rejection(attenuation or insertion loss) everwhere else.
switches, attenuators[衰减器], phase shifters
purpose: passive or active devices to probide electronic control of the phase and amplitude of RF/microwave signals.
Main characteristics(in its frequency band):
- Good input and output impedance matching
- Lowadditional insertion losss and hiah isolation for switches
- Large number of values for the attenuation or phase shift
- Fast switching speed
Oscillators [振荡器]
Purpose: acive devices to provide RF/microwave sources in transmitters and local oscillators in upconverters and downconverters.
有源器件，可在发射机中提供射频/微波源，并在上变频器和下变频器中提供本地振荡器。
Main characteristics:
Amplifiers
Purpose: active devices to provide power gain to the input RF/microwave signal.
Different topologies:
- High Power Amplifiers(HPA) for transmitters: very high output power but noisy
- Low Noise Anplifiers(LNA) for receivers:low output power but less noisy
Typical response:
[picture:response_amplifiers.jpg]
Main characteristics(in its frequency band):
- Good input and output impedance matching
- High dynamic
- High output power for HPA and low noise figure for LNA
- Good linearity
- High Power Added Efficiency(PAE): $PAE = \frac{P_{out}-P_{in}}{P_{DC} * 100\%}$
Mixers
Purpose: Nonlinear devices to achieve frequency conversion
Basic principle:
- Up-conversion(transmitter): mix an intermediate-frequency signal with a local oscillator signal to obtain a high-grequency signal= IF + LO = RF
- Down-conversion(receiver):RF + LO = IF
Main characteristics:
- Good …
- stable LO power
- Low conversion loss
- Low noise figure
- Good isolation between any two of the RF, IF and LO ports
- Good dynamic range
RF/microwave antennas
Purpose: passive device to radiate and receive RF/microwave signals(transition between guided and free-space waves)
Time-domain Maxwell equations;
Far-field radiation produced by an antenna:
properties and average power density.
- Fundamental parameters
- Antenna efficiency e: $e = \frac{P_rad}{P_{in}} = e_re_{rad} \leq 1$
- $e_r$ Reflection(mismatch) efficiency[反射（不匹配）效率]
- $e_{rad}$ Radiation(conduction-dielectric) efficiency[辐射（传导介电）效率]
- Field regions
- Usual subdivision of the space surrounding an antenna:
- Rayleigh or reactive near-field region $R_1 = 0.62 \sqrt{\frac{D^3}{\lambda}}$
- Fresnel or radiating near-field region $R_2 = \frac{2D^2}{\lambda}$
- Fraunhofer of far-field region
- Radiation patterns[辐射图]
- Major(main)lobe
- Side lobe
- Side lobe level(SLL)
- Mian radiation patterns:
- isotropic antenna
- directional antenna
- omnidirectional antenna
- Radiated power $P_{rad}$: integration of the average power density(the far-field region)
- Directivity $D(\theta,\phi)$
- dimensionless and independent of r(in the far-field region)
- Often expressed in decibels
- Gain $G(\theta,\phi)$: take into account the efficiency of the antenna
高增益天线意味着高方向性的天线。 - Antenna effective area $A_e(\theta,\phi)$
- Gain $G(\theta,\phi)$: take into account the efficiency of the antenna
- the aperture efficiency
- to obtain a large gain we need a large antenna relative to the wavelength
- Antenna noise temperature
- Background noise temperature TB
- Brightness temperature Tb
- Antenna noise temperature
link budget
Friis equation in free-space
- Power density radiated by:
- an isotropic antenna
- a directive antenna
- EIRP: Equivalent Isotropic Radiated Power $EIRP = P_{in} G_{tmax}$
- SNR at receiving antenna terminals
- Frequency
- Distance
- Characteristics of the transmitter(EIRP) and receiver (G/T)
System noise
- Noise in RF/microwave receivers:
- external noise
- internal noise
- Thermal noise power
-
Equivalent noise temperature
-
Noise figure
-
Special case
- Noise for a cascaded system
Part 2 - Principles of digital communications
Digital communications: transmitting elements from a discrete alphabet through a continuous environment.
带通信号，实际上就是一个实窄带高频信号，因为是real，所以傅里叶变换后的频谱共轭对称； 因此分为正频谱和负频谱，我们可以认为正频谱作为分析（解析）信号或者预包络，因为正频谱已经携带了所有该信号的信息。 载频信号往往等于带通信号的信号中心f0，但是也可能不等于；我们在分析带通信号时，可以不考虑载频信号，因为载频信号与数字信息内容无关。 因此，把带通信号去掉载频信号，然后再变成等效低通信号表示，更有利于我们的分析。
我们定义这个低通信号，也就是基带信号，是两倍的正频谱信号，然后把他移到频谱中心位置（向左移f0到低频）；由于我们只是单纯平移，并没有要求该信号在频谱上共轭对称，因此需要对该信号用复数形式表示，也就是有两个正交分量， 分别叫做in-phase signal 和quadrature signal。
带通信号是真实存在的实信号，基带信号是复信号。
我们需要建立两者之间的表达式。
首先，对带宽信号进行解释：信号的带宽，是整个非零频谱的一半，所以带通信号的带宽是2B，基带信号的带宽是B。 其次，公式推导（纸质版），可以获得带通信号的三种表示方式。
Transmission over a carrier frequency
Real transmission system
- Baseband transmission
- Prqctical IQ transceiver
Complex equivalent system
- real bandpass signal
- analytic signal
- complex envelope signal
等效低通信号，也称复包络。其频谱等于两倍的向左移f0的解析信号的频谱。进行傅里叶反变换，可以得到其时域信号。
linear modulations[线性调制]
Transmission system model
Amplitude-shift keying(ASK)
调制解调与希尔伯特变换
调制解调，我们是根据载波信号进行调制。也就是说，等效低通信号对载波频率进行调制再取实部获得带通信号，解调的时候，我们对预包络信号进行解调，得到等效低通信号。
Enjoy Reading This Article?
Here are some more articles you might like to read next: