Signal strength (RSSI) knowledge sorting

Why wireless signals （RSSI） It's negative
answer ： In fact, in the final analysis, why the received wireless signal is negative , Is it much easier to understand . Because most wireless signals are mW Level , So it's polarized , Turn into dBm nothing more , It doesn't mean the signal is negative .1mW Namely 0dBm, Less than 1mW It's negative dBm Count .

Just clarify the definition of signal strength ：
RSSI（ Received signal strength ）Received Signal Strength Indicator
Rss=10logP,
Just put the received signal power P The received signal strength is the received signal strength （ sensitivity ）.
[ example 1] If the transmit power P by 1mw, Converted to dBm after 0dBm.
[ example 2] about 40W Power , Press dBm The converted value of unit shall be ：
10lg（40W/1mw)=10lg（40000）=10lg4+10lg10+10lg1000=46dBm.

Why is it measured dbm Values are all negative ？
answer ： The first thing we need to know is the wireless signal dbm It's all negative , The biggest is 0. So the measured dbm The values must be negative . because dbm The value is only in one case 0, That is the result of experimental measurement in the ideal state , We generally think that dbm by 0 Is its maximum value , It means that the receiver has received all the wireless signals transmitted by the transmitter , That is, how much power the wireless router transmits , How much power does the received wireless network card get . Of course, this is measured in an ideal state , In practice, even if the wireless network card is next to the transmitting antenna of the wireless router, it will not achieve dbm by 0 The effect of . So the measured dbm Values are all negative , Don't blindly think that a negative number is a bad signal .

Power unit
dBm
dBm It is a value that considers the absolute value of power , The formula is ：10lgP（ Power value /1mw）.
[ example 1] If the transmit power P by 1mw, Converted to dBm after 0dBm.
[ example 2] about 40W Power , Press dBm The converted value of unit shall be ：
10lg（40W/1mw)=10lg（40000）=10lg4+10lg10+10lg1000=46dBm.

dB
dB It's a value that represents a relative value , When considering the power of a is larger or smaller than that of B dB when , Calculate according to the following formula ：10lg（ A power / B power ）
[ example 6] A has twice as much power as B , that 10lg（ A power / B power ）=10lg2=3dB. in other words , The power of a is greater than that of B 3 dB.
[ example 7] 7/8 Inch GSM900 Feeder 100 The transmission loss per meter is about 3.9dB.
[ example 8] If the power of a is 46dBm, The power of B is 40dBm, Then we can say , A is bigger than B 6 dB.
[ example 9] If antenna a is 12dBd, B antenna is 14dBd, It can be said that a is smaller than B 2 dB.

How to go from dbm The value shows the received power
dbm Is a unit of absolute power , His formula is 10lg Power value /1mw. For example, if the received power is 1mw, according to dbm The value after unit conversion should be 10lg 1mw/1mw=0dbm. Of course, in the actual transmission process, it is difficult for the receiver to achieve the receiving power 1mw Of . Therefore, through this formula, we can start from dbm Value reverses the power value received by the receiver .

Mistake 1 ：dbm The smaller the value, the better
Now that I mentioned dbm Values are all negative , So many people think dbm The smaller the value, the better . In fact, this cognition is wrong . As mentioned earlier dbm The maximum value is 0, And it's an ideal state . The closer to the ideal state dbm value , The more it shows that the power transmitted by the wireless router is received by the wireless network card . therefore dbm The higher the value, the better ,-50dbm It indicates that the received wireless signal is better than -70dbm.

Mistake 2 ：dbm The higher the value, the better
Many friends think that since dbm The value is 0 It shows that the effect of receiving and transmitting signals is the best , Then we should let the enterprise wireless network everywhere dbm Value as large as possible . In fact, this view is also wrong , although dbm The higher the value, the better the effect of sending and receiving signals , But at the same time, we also need to install enough wireless signal relay equipment for the internal wireless network of the enterprise , This is not small compared to the cost . Experiments show that in XP The wireless signal scanning component of the system is displayed as “ very nice ” The status can meet the requirements of network transmission , There are no problems with speed and stability , And this “ very nice ” The state corresponds to dbm The value is 0 To -50dbm. Therefore, we only need to ensure that all parts of the wireless network within the enterprise dbm The value is not greater than -50dbm that will do , The wireless network established in this way is a high-speed and stable network . The cost performance of our investment in wireless network will be the highest .

tip
Of course, sometimes due to financial considerations, we can't guarantee the security of every place in the enterprise's internal network dbm Values are 0 To -50dbm Between , Then when measuring, it should also be ensured that 0 To -70dbm Between . Because when XP The wireless signal received by the system is less than -70dbm Transmission instability will occur , The phenomenon of slow speed , Then our wireless network will not work properly .

Mistake 3 ： The reception power is low because the transmission is disturbed
As we all know, the transmission power of wireless routers is generally 100mw, And higher . So why do we receive so little power ？ It's because there is a lot of interference in the transmission process ？ Let's take the received signal as -50dbm namely 0.01μW For example , If the transmission power of the wireless router is 100mw, And only received 0.01μW, The difference between the two is 10000000 times .
In fact, this is a normal transmission , It's as if only one billionth of the energy emitted by the sun is received by the earth . The receiving power must be much less than the transmitting power . So network administrators need to worry more when measuring , As long as your signal strength is greater than -50dbm You can transmit data wirelessly without any problems , Even if it's one step back -70dbm Wireless can also guarantee speed 54M transmitted .

dB,dBi, dBd, dBc,dBm,dBw
1、dB
　　dB It's a value that represents a relative value , Pure ratio , It only represents the relative size of two quantities , No unit , When considering the power of a is larger or smaller than that of B dB when , Calculate according to the following formula ：10log（ A power / B power ）, If the voltage ratio of the two is used for calculation , Use 20log（ A voltage / B voltage ）.

[ example ] A has twice as much power as B , that 10lg（ A power / B power ）=10lg2=3dB. in other words , The power of a is greater than that of B 3 dB. conversely , If a's power is half of B's power , Then the power of a is smaller than that of B 3 dB.

2、dBi and dBd
　　dBi and dBd Is the amount of antenna power gain , Both are relative values , But the reference frame is different .dBi The reference of the antenna is omnidirectional antenna ,dBd The reference standard of the system is dipole , So the two are slightly different . It is generally believed , Represents the same gain , use dBi It's better to express it than to use dBd It should be big 2.15.

[ example ] For one side, the gain is 16dBd Antenna of , Its gain is expressed in dBi when , Then for 18.15dBi（ Generally, the decimal places are ignored , by 18dBi）.

[ example ] 0dBd=2.15dBi.

3、dBc
　　dBc It is also a unit representing the relative value of power , And dB The calculation method is exactly the same . Generally speaking ,dBc Is relative to the carrier （Carrier） In terms of power , in many instances , Used to measure the relative value of carrier power , If used to measure interference （ Co channel interference 、 Intermodulation interference 、 Intermodulation interference 、 Out of band interference, etc ） And coupling 、 Relative magnitude of spurious, etc . When using dBc The place of , In principle, you can also use dB replace .

4、dBm
　　dBm Is a value representing the absolute value of power （ It can also be considered as 1mW A ratio based on power ）, The formula is ：10log（ Power value /1mw）.

[ example ] If power P by 1mw, Converted to dBm after 0dBm.

[ example ] about 40W Power , Press dBm The converted value of unit shall be ：
　　10log（40W/1mw）=10log（40000）=10log4+10log10000=46dBm.

5、dBw
　　 And dBm equally ,dBw Is a unit of absolute power （ It can also be considered as 1W A ratio based on power ）, The formula is ：10log（ Power value /1w）.dBw And dBm The conversion relationship between is ：0 dBw = 10log1 W = 10log1000 mw = 30 dBm.

[ example ] If power P by 1w, Converted to dBw after 0dBw.
　　 All in all ,dB,dBi, dBd, dBc Is the ratio between two quantities , Indicates the relative size between two quantities , and dBm、dBw Is the value representing the absolute power . stay dB,dBm,dBw In calculation , Pay attention to the basic concepts , Use one dBm（ or dBw） Minus another dBm（dBw） when , And what you get is dB, Such as ：30dBm - 0dBm = 30dB.
　　 In general , In the engineering ,dBm（ or dBw） and dBm（ or dBw） There is only addition and subtraction , No multiplication and division . The most used is subtraction ：dBm reduce dBm It's actually the division of two powers , The signal-to-noise ratio is the division of signal power and noise power （SNR）.dBm Add dBm It's actually a multiplication of two powers

Why is the unit of antenna gain sometimes used dBi Express , And sometimes dBd, What is the difference between the two difference ？
answer ：dBi and dBd Are units of antenna gain , We are used to taking the ideal power radiator as a reference
To specify the gain of omnidirectional antenna ; There is an ideal half wave symmetrical oscillator as a reference to consider the directional antenna
The gain of .dBi and dBd Can be converted to each other ,G(dBd)=G(dBi)-2.15