dB should be the most basic and habitual concept in wireless communication. We often say that "the propagation loss is xx dB", "the transmit power is xx dBm", "the antenna gain is xx dBi"...
Sometimes, these dBx that look like them can get confused and even cause miscalculations. What exactly is the difference between them?
This has to start with dB.
And when it comes to dB, the most common is 3 dB!
3 dB is often seen in power plots or bit error rate plots. In fact, there is nothing mysterious, a drop of 3 dB refers to a power drop of half, and a 3 dB point refers to a half power point.
+3 dB means a double increase, and -3 dB means a decrease of 1/2. How did this come about?
In fact, it is very simple, let's look at the calculation formula of dB together:
dB represents the magnitude relationship of power P relative to reference power P. If P is 2 times P, then:
If P is half of P, then:
Regarding the basic concepts and properties of logarithm, you can review the next high school mathematics...
The logarithmic function is a function in which the power (true number) is the independent variable, the exponent is the dependent variable, and the base is the constant. The logarithmic function is one of the 6 classes of basic elementary functions. Definition of logarithm:
If a = N ( a >0 , and a ≠1 ) , then the number x is called the logarithm with a base N , denoted as x = logN, read as the logarithm with a base N , where a is called the base of the logarithm and N is called the true number.
In general, the function y=logX(a>0, and a≠1) is called a logarithmic function, that is, a function with power (true number) as the independent variable, exponent as the dependent variable, and base as a constant, called a logarithmic function.
where x is the argument and the defining field of the function is (0,+∞), i.e. x>0. It is actually the inverse of the exponential function and can be expressed as x=a. Therefore, the provisions for a in exponential functions also apply to logarithmic functions.
"log" is an abbreviation for the Latin logarithm (logarithm).
Now the debut questions test your level of understanding:
【Q】The power increase is 10 times, with ? dB is indicated
Click the empty space below to see the answer
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Here's a mantra to keep in mind. With this mantra in mind, you can basically walk sideways.
+3 dB indicates a 2x increase in power, and +10 dB indicates a 10x increase in power.
-3 dB indicates a power reduction of 1/2; -10 dB indicates a power reduction of 1/10.
It can be seen that dB is a relative value, and its mission is to express a large or small number in a short form.
This can greatly facilitate our calculations and descriptions. Especially when drawing the table, you can make up for it yourself, before it is converted to dB, so many 0s, the coordinate axis has to be pulled to outer space...
Understand dB, you can only walk sideways, understand the other members of the dB family, you can lie down and win.
Let's also talk about the most commonly used dBm, dBw.
dBm and dBw are to replace the reference power P0 in the dB formula with 1 mW and 1 W, respectively:
1 mW, 1 W are all definite values, so both dBm and dBw can represent absolute values of power.
Directly on the power conversion table for your reference.
Here, we need to remember:
1 W = 30 dBm。
The simplified recipe is "30 is the benchmark, equal to 1 W whole".
Remember this, combined with the previous "add 3 times 2, add 10 times 10; subtract 3 by 2, subtract 10 by 10", you can do a lot of calculations.
Hurry up and test the debut question.
【问】44 dBm=？W
Are you right?
Here we need to note that except for 30 dBm on the right side of the equation, the rest of the split items are represented in dB. That is, when one dBx is subtracted from another, the result obtained is expressed in dB.
[Example] If A has a power of 46 dBm and B has a power of 40 dBm, it can be said that A is 6 dB larger than B.
[Example] If the A antenna is 12 dBd and the B antenna is 14 dBd, it can be said that A is 2 dB smaller than B.
For example, 46 dB means that P1 is 40,000 times that of P0, and 46 dBm means that the value of P1 is 40 W. The difference of only one m in the symbol can represent a completely different meaning.
Also common in the dB family are dBi, dBd, and dBc. They are calculated in exactly the same way as the dB, representing the relative value of power.
The difference is that their reference references are different, i.e. the reference power P0 on the denominator represents a different meaning.
It is generally believed that the same gain is represented by dBi than in dBd. This difference is caused by the different directionality of the two antennas, and we will not expand on it here.
In addition, the dB family can not only represent the gain and loss of power, but also represent voltage, current, audio, etc., and we must apply specific scenarios.
It should be noted that for the gain of power, we use 10lg (Po/Pi), for the gain of voltage and current, we use 20lg (Vo/Vi), 20lg (Io/Ii).
How did this 2 times more come about?
This 2 is derived from the square of the electrical power conversion formula. The nth power in the logarithm corresponds to n times after calculation.
Regarding the conversion relationship between power and voltage and current, you can review the junior high school physics by yourself...
Finally, Xiaobian sorted out some of the main dB family members for your reference.
Relative value:
absolute value:
Finally, let's come up with two questions to test everyone's results.
1. The power of 30 dBm is ()
A. 1 W
B. 10 W
C. 1 mW
D. 10 mW
「A」
This is a send-off question ah ~ remember the recipe: 30 is the benchmark, equal to 1 W whole.
2. Assuming that the total power of the cell output is 46 dBm, at 2 antennas, the single antenna power is ( )
A. 46 dBm
B. 43 dBm
C. 23 dBm
D. 40 dBm
「B」
Remember the mantra "minus 3 divided by 2", two antennas are 46 dBm, a single antenna is to reduce the power by half, that is, to subtract 3 dB Oh: 46 dBm -3 dB = 43 dBm.
Alternatively, you can calculate 46 dBm corresponding to 40 W, then the single antenna power is 20 W, that is, 10 lg (20 W/1 mW) = 43 dBm.
Well, that's all for today's article. Tell the editor, have you insisted on it for 100 minutes?
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: ZTE Documentation
Edit: Herding fish