the RF signal of the radio transmitters output is transmitted to the antenna via a feeder (cable) and radiated out by the antenna in the form of electromagnetic waves. When the electromagnetic wave arrives at the receiving location, it is received by the antenna (only a small fraction of the power is received) and sent to the radio receiver via the feeder. Therefore, it is very important to calculate the transmitting power of the transmitting device and the radiation ability of the antenna in the project of wireless network.

Tx is the abbreviation for emission (transmits). The transmitting power of radio waves refers to the energy in the range of a given frequency band, and there are usually two measures or measurement criteria:

1, Power (W): relative to the linear level of 1 watts (Watts). For example, WiFi wireless card transmission power is usually 0.036W, or said 36mW.

2, Gain (DBM): proportional to 1 MW (Milliwatt). For example, the WiFi wireless card has a transmit gain of 15.56dBm.

Conversion of power units MW and dBm:

1, dBm = ten x Log [power MW]

2, MW = 10 [gain DBM/10 dBm]

In wireless systems, antennas are used to convert current waves into electromagnetic waves, and in the process of transmitting and receiving signals can be "put The metric of this energy amplification becomes "gain (Gain)". The **unit of measure for antenna gain is "dBi"**. Since the electromagnetic wave energy in the wireless system is generated by the transmitting energy of the transmitting device and the amplification and superposition of the antenna, it is best to measure the emission energy of the same metric-gain (DB), for example, the power of the transmitting device is 100mW, or 20dBm; the gain of the antenna is 10dBi, then:

Total transmit energy = transmit power (dBm) + antenna gain (dBi) = 20dBm + 10dBi = 30dBm

Or: = 1000mW = 1W

Each db is important in a "small power" system, such as a wireless local area network device, especially when you remember the "3 DB rule." **each additional** **Adding or lowering 3 DB means increasing the power by one or less half** :

-3 DB = 1/2 Power

-6 DB = 1/4 Power

+3 DB = 2x Power

+6 DB = 4x power

For example,

10W Wireless transmission power of 40dBm

6.4W Wireless transmission power of 38dBm

3.2W Wireless Transmit power of + dBm

1.6W Wireless transmit power of up to DBm

the wireless transmitting power of the MW is

26dBm Wireless transmission power of up to MW

23dBm Wireless transmission power of up to MW

20dBm Wireless transmission power of up to MW

Wireless transmitting power of up to 17dBm

Wireless transmit power of 14dBm for MW

12.5 MW Wireless transmit power of 11dBm

Wireless transmit power of 10.34 MW is ten dBm

8.26 MW Wireless transmit power of 9 dBm

6.25 MW Wireless transmit power of 8 dBm

3.125 MW Wireless transmit power of 5 dBm

1mW Wireless Transmit power of 0 dBm

Power/Level (DBM): The output capability of the amplifier, in general units W, MW, dbm. **DBm is to take 1mW as the reference value to** **The absolute power level expressed in decibels** .

Conversion formula:

level (DBm) =10LGW

5W→10lg5000=37dBm

10W→10lg10000=40dBm

104→10lg20000=43dBm

power is increased by one times and the level value increases by 3dBm

Why do units in dbm? There are several reasons: 1, for the attenuation of wireless signal, is not linear, but the logarithmic relationship attenuation. This relationship can be more reflected in decibels. 2, with the decibel unit than with the tile is easier to describe, often in the transmitter out of the power of dozens of hundred Watts, to the receiving end has been a slight tile to calculate. 3, the calculation is convenient, the attenuation calculation formula uses the decibel to calculate only to use the addition and subtraction to be possible.

A 1mW-based DB algorithm, 0dbm=1mw,dbm=10*log (POWER/1MW).

Transmit Power dbm-path loss db= receive signal strength dbm

Minimum communication power dbm-path loss db≥ receive sensitivity lower bound dbm

Minimum communication power dbm≥ path loss db+ receive sensitivity lower bound dbm

1, DB **db is a representation of the value of the relative value, the pure ratio, only two of the relative size of the relationship, no unit** , when considering a power compared to B power or a small number of DB, according to the following formula: 10log (a power/b power), if the use of the two voltage ratio calculation, To use 20log (a voltage/b voltage).

[Example] a power greater than B power one times, then 10LG (a power/b power) =10lg2=3db. In other words, the power of A is 3 DB larger than that of B. Conversely, if the power of a is half the power of B, then the power of a is 3 DB smaller than the power of B.

2, DBI and dbd **dbi and DBD are the amount of antenna power gain** , both are a relative value, but the reference datum is not the same. The **reference datum of DBI is omni-directional antenna, and the reference datum of dbd is dipole** , so the two are slightly different. **It is generally believed that the same gain, expressed in dbi, is 2.15 larger than that of DBD. **

[Example] for an antenna with a gain of 16dBd, the gain is converted to a unit of DBI, then 18.15dBi (generally ignores the decimal place, is 18dBi).

[Example] 0dbd=2.15dbi.

3. DBC **DBC is also a unit that represents the relative value of the power, exactly the same as the DB calculation method** . In general,**DBc is relative to the carrier (Carrier) power** , in many cases, to measure the relative value of the carrier power, such as to measure interference (same-frequency interference, intermodulation interference, intermodulation interference, out-of-band interference, etc.) as well as the relative magnitude of the coupling, spurious, etc. In principle, db substitution can be used where DBC is used.

4, DBm **dbm is a value that represents absolute power (or a ratio that is based on 1mW Power) and is calculated as: 10log (Power value/1MW)**.

[Example] if the power P is 1mw, converted to dbm after 0dBm.

[Example] for 40W power, the value after the conversion in dbm units should be:

10log (40W/1MW) =10log (40000) =10log4+10log10000=46dbm.

5, DBw **and dbm, DBw is a unit that represents the absolute value of the power (also can be considered as a ratio of 1W power), the formula is: 10log (power value/1w)**. The conversion relationship between DBw and DBm is: 0 dBw = 10log1 W = 10log1000 mw =-dbm.

[Example] if the power P is 1w, converted to DBW after 0dBw.

In summary, the DB,DBI,DBD,DBC is a ratio of two, representing the relative size of two, while dbm and DBW are values that represent the absolute size of a power supply. In DB,DBM,DBW calculations, note the basic concept that **when you subtract another dbm (dBw) with one dbm (or dBw), the result is a DB**, such as: 30DBM-0DBM = 30dB.

in general, there is no multiplication between dbm (or DBW) and dbm (or DBW) in the project. And the most used is subtraction: dbm minus dbm is actually two power division, signal power and noise power divide is the Snr (SNR). DBM Plus dbm is actually two power multiplication.

Conversion of wireless power MW and DBm