Original: http://www.cnblogs.com/lele/articles/2832885.html
why Wireless Signal (RSSI) is negativeAnswer: In fact, why the wireless signal received is negative, this is not easy to understand more.because the wireless signal is more than the MW level, so it is polarized and converted to dbm, which does not mean that the signal is negative. 1mW is 0dBm, and less than 1mW is the number of dbm with negative numbers.
Figure out the definition of the signal strength:RSSI (Receive signal strength) Received Signal strength Indicator
Rss=10logp,
Simply substituting the received signal power P is the received signal strength (sensitivity).
[Example 1] if the transmit power P is 1mw, converted to dbm after 0dBm.
[Example 2] for 40W power, the value of the conversion in dbm units should be:
10LG (40W/1MW) =10LG (40000) =10lg4+10lg10+10lg1000=46dbm.
Why are the measured dbm values negative?
A: The first thing we need to know is that the wireless signal dbm is negative, the maximum is 0. So the measured dbm value is definitely negative. Because the dbm value is only 0 in one case, that is the result of an experimental measurement in the ideal state, we generally think that dbm is 0 is its maximum value, which means that the receiver will transmit all the wireless signal is received, that is, how much power the wireless router emits, how much power the wireless network card received. Of course, this is measured in the ideal state, in practice, even if the wireless card next to the wireless router's transmitting antenna will not reach the effect of dbm 0. So the measured dbm value is negative, do not blindly think negative is bad signal.
Power Unit
dBm
dbm is an absolute value of the test power, calculated as: 10lgP (Power value/1MW).
[Example 1] if the transmit power P is 1mw, converted to dbm after 0dBm.
[Example 2] for 40W power, the value of the conversion in dbm units should be:
10LG (40W/1MW) =10LG (40000) =10lg4+10lg10+10lg1000=46dbm.
DB
A DB is a value that characterizes a relative value,When considering the power of a a compared to the power of a large or small number of db, according to the following formula: 10LG (a power/b power)
[Example 6] 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.
[Example 7] 7/8 inch GSM900 feeder 100 m transmission loss is about 3.9dB.
[Example 8] If a power of 46dBm, B power of 40dBm, it can be said that a greater than B 6 DB.
[Example 9] If a antenna is 12dBd, b antenna is 14dBd, you can say a smaller than B 2 DB.
How to see the received power from the DBM value
DBM is a unit that represents the absolute value of power, and his formula is 10LG power/1MW. For example, if the received power is 1MW, the value after the conversion in dbm units should be 10LG 1mw/1mw=0dbm. Of course, in the actual transmission process receiver is difficult to reach the receiving power of 1MW. So we can use this formula to reverse the value of the received power from the DBM value.
Myth: The smaller the dbm value the better
Since we mentioned that dbm values are negative, many people think that the smaller the dbm value, the better. In fact, this cognition is wrong. As previously mentioned, the maximum value of dbm is 0, and it is an ideal state. Then the closer to the desired dbm value, the more the wireless router transmit power is received by the wireless network card. Therefore, the greater the dbm value, the better, the -50DBM indicates that the received wireless signal is better than -70dbm.
Myth: The larger the dbm value the better
And a lot of friends think that since the dbm value is 0 to indicate the best way to receive the signal, then we should make the dbm value as large as possible in each place of the enterprise wireless network. In fact, this view is also wrong, although the higher the DBM value, the better it will be, but at the same time we need to install enough wireless signal relay equipment for the internal wireless network of the enterprise, which is not a small cost. The experiment shows that the wireless signal scanning component of XP system can meet the requirements of network transmission when it is displayed in the "very good" state, and the speed and stability have no problem, and the dbm value of this "very good" state corresponds to 0 to -50dbm. Therefore, we only need to ensure that the internal wireless network in each place of the dbm value is not more than -50dbm, so the establishment of the wireless network is a high-speed stable network. We have the highest price/performance ratio for wireless network inputs.
Tips:Of course, sometimes we can't guarantee that the dbm value of each place in the intranet is between 0 and -50dbm for the sake of funding, so be sure to measure between 0 and -70dbm. Because when the XP system receives the wireless signal less than -70DBM, there will be transmission instability, slow speed, so our wireless network will not be able to use the normal.
Myth: The reception power is small because the transmission is disturbed
It is well known that wireless router transmission power is generally 100mw, there is a higher. So why is the power we receive so small? Is it because of the interference in the transmission process is relatively large? Below we take the received signal for -50DBM that is 0.01μw as an example, if the wireless router transmitting power of 100MW, and received only for 0.01μw, the difference is 10 million times times. This is actually a normal transmission, just as the sun emits only One-one hundred millonth of the energy it receives from Earth. The receiving power must be much smaller than the transmitting power. So the network administrator in the measurement than need to worry, as long as your signal strength is greater than -50dbm can not have any problem of wireless transmission data, and then step back even to the -70DBM can also guarantee the wireless speed of 54M transmission.
db,dbi, dBd, Dbc,dbm,dbw
1, DB
DB is a representation of the relative value of the value, the pure ratio, only represents 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
The DBI and DBD are the amounts that represent the power gain of the antenna, both of which 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 as the DB is calculated. 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
Like DBM, the DBW is a unit that represents the absolute value of the power (or a ratio that is based on the 1W Power) and is calculated as: 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 short, db,dbi, DBd, DBC is the ratio between two quantities, representing the relative size between two measurements, while dbm, DBW is the absolute size of the power value. 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 a two power multiplier
Why is the unit of the antenna gain sometimes expressed in dbi, and at times with DBD, what is the difference between the two?
A: Both DBI and DBD are the units of antenna gain, and we are accustomed to using the ideal power emitter as a reference
To specify the gain of the omni-directional antenna, with the ideal half-wave symmetric oscillator as a reference to consider the antenna
The gain. DBI and DBD can be converted to each other, G (dBd) =g (DBI)-2.15
Android Developer--Bluetooth Development Example Four why is the wireless signal (RSSI) negative?
