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Presentation transcript:

Training materials for wireless trainers dB math This talk is about decibels and how to use them, compared to mW. Duration: 30 minutes Version 1.8 by Carlo, @2009-11-25 Version 1.11 by Rob, @2010-03-03 Version 1.12 by Rob, @2010-03-12 Training materials for wireless trainers

Goals Electromagnetic waves carry power measured in milliwatts. Decibels (dB) use a relative logarithmic relationship to reduce multiplication to simple addition. You can simplify common radio calculations by using dBm instead of mW, and dB to represent variations of power. It is simpler to solve radio calculations in your head by using dB. We are going to solve some simple exercises and convert between dB and mW. 2

Power Any electromagnetic wave carries energy - we can feel that when we enjoy (or suffer from) the warmth of the sun. The amount of energy received in a certain amount of time is called power. The electric field is measured in V/m (volts per meter), the power contained within it is proportional to the square of the electric field: The power P is of key importance for making wireless links work: you need a certain minimum power in order for a receiver to make sense of the signal. In practice, we measure the power by means of some form of receiver, e.g. an antenna and a voltmeter, power meter, spectrum analyzer, or even a radio card and laptop. P ~ E2 The unit of power is the watt (W). For wireless work, the milliwatt (mW) is usually a more convenient unit. 3

Gain & Loss If the amplitude of an electromagnetic wave increases, its power increases. This increase in power is called a gain. If the amplitude decreases, its power decreases. This decrease in power is called a loss. When designing communication links, you try to maximize the gains while minimizing any losses. You “gain” signal at the transmitter, using an amplifier, and when using a properly aligned antenna. You “lose” signal at connectors, transmission lines, and naturally as waves propagate through the environment. 4

Intro to dB Decibels are a relative measurement unit unlike the absolute measurement of milliwatts. The decibel (dB) is 10 times the decimal logarithm of the ratio between two values of a variable. The calculation of decibels uses a logarithm to allow very large or very small relations to be represented with a conveniently small number. On the logarithmic scale, the reference cannot be zero because the log of zero does not exist! The human ear responds logarithmically to the power of sound. That is why decibels are also used for measuring sound. 5

Why do we use dB? Power does not fade in a linear manner, but inversely as the square of the distance. You move by x and the signal decreases by 1/x2; hence, the “inverse square law.” Why we do use dB? There is a physical motivation for that: the inverse square law of distance. It will be explained in the next slide. 1 meter away → some amount of power 2 meters away → 1/4 power at one meter 4 meters away → 1/16 power at one meter 8 meters away → 1/64 power at one meter The fact that exponential relationships are involved in signal strength measurement is one reason why we use a logarithmic scale. 6

Inverse square law The inverse square law is explained by simple geometry. The radiated energy expands as a function of the distance from the transmitter. This law says that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity. The inverse-square law generally applies when some force, energy, or other conserved quantity is radiated outward radially from a source. Since the surface area of a sphere (which is 4πr2) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it must spread out over an area that is proportional to the square of the distance from the source. Hence, the radiation passing through any unit area is inversely proportional to the square of the distance from the source. ( from http://en.wikipedia.org/wiki/Inverse_square ) Figure from http://en.wikipedia.org/wiki/Inverse_square

A quick review of logarithms log(0)=undefined log(10)=1 The logarithm of a number in base 10 is the exponent to which ten must be raised in order to produce the number. For more details see: http://en.wikipedia.org/wiki/Logarithm Question: at what point will the line cross 2? ( Answer: at one hundred. 100=102, so log 10(100)=2 ) Keep in mid that there are also other bases for the logarithms, like the number e which is used in neperian logarithms (information theory). If x=10y, then y=log10(x) Logarithms reduce multiplication to simple addition, because log(a×b)=log(a)+log(b) it is called the logarithm in base 10 of x 8

Definition of dB The definition of the decibel uses a logarithm to allow very large or very small relations to be represented with a conveniently small number. Let assume we are interested in the ratio between two values a and b. ratio= a/b In dB the ratio is defined as:ratio[dB]= 10 log10 (a/b) It is a dimensionless, relative measure (a relative to b) For more details see: http://en.wikipedia.org/wiki/Decibel continue in the next slide 9

Definition of dB ratio = 10 log10(a/b) What if we now use a value of a that is 10 times bigger? newratio = 10 log10(10a/b) = 10 [log10(10) + log10(a/b)] = 10 log10(10) + 10 log10(a/b) = 10 + ratio The new value (in dB) is simply 10 plus the old value, so the multiplication by ten is now expressed by a simple addition of 10 units. Note that log10(10) = 1 Remember log(a×b)=log(a)+log(b) 10

Using dB Commonly used (and easy to remember) dB values: For example: +10 dB = 10 times the power -10 dB = one tenth power +3 dB = double power -3 dB = half the power Memorize these values! You will use them very often. For example: some power + 10 dB = 10 times the power some power - 10 dB = one tenth power some power + 3 dB = double power some power - 3 dB = half the power 11

dBm and mW What if we want to measure an absolute power with dB? We have to define a reference. The reference point that relates the logarithmic dB scale to the linear watt scale may be for example this: What if I want to use a logarithmic scale to express a power? To do so, I have to use a reference value of power. Let’s assume it’s 1 mW. So I define the dBm. Using dBm, the reference is 1 milliwatt. 0 dBm is therefore equal to 1 mW. Note that it is impossible to represent 0 mW (zero power, i.e. no power at all) using dBm! 1 mW → 0 dBm The new m in dBm refers to the fact that the reference is one mW, and therefore a dBm measurement is a measurement of absolute power with reference to 1 mW. 12

dBm and mW To convert power in mW to dBm: PdBm = 10 log10 PmW 10 times the logarithm in base 10 of the “Power in mW” PdBm = 10 log10 PmW These formulas are the definition of dBm and can be used to convert between dBm and mW, but there is a simpler way to do it that does not require a calculator, and we will see it very soon! To convert power in dBm to mW: PmW = 10 PdBm/10 10 to the power of ( “Power in dBm” divided by 10 ) 13

Signal measurement: 17dBm dBm and mW Example: mW to dBm Radio power: 100mW PdBm = 10 log10(100) 100mW → 20dBm Example: imagine a radio with an output power of 100mW Example: imagine a signal measurement of 17 dBm To find the log or the power, in these formulas, you will probably need a calculator. But there are much simpler ways to do these calculations in your mind, we are going to see them in a few minutes! Example: dBm to mW Signal measurement: 17dBm PmW = 10 17/10 17dBm → 50 mW 14

Using dB When using dB, gains and losses are additive. Remember our previous example: some power + 10 dB = 10 times the power some power - 10 dB = one tenth power some power + 3 dB = double power some power - 3 dB = half the power Using dB makes the math much simpler, specially when you have to deal with gains and losses. Explain all the examples step by step, using the relations above and the fact that 100mW = 20dBm (seen before). You can give even more examples now, or ask the students to do the calculation for more examples. You can now imagine situations in which: 10 mW + 10 dB of gain = 100 mW = 20 dBm 10 dBm = 10 mW = one tenth of 100mW 20 dBm - 10 dB of loss = 10 dBm = 10mW 50 mW + 3 dB = 100 mW = 20 dBm 17 dBm + 3 dB = 20 dBm = 100 mW 100mW - 3 dB = 50 mW = 17 dBm 15

Using dB

dB and milliwatts It is easy to use dB to simplify the addition of gains and losses, then convert back to milliwatts when you need to refer to the absolute power. 1 mW = 0 dBm 2 mW = 3 dBm 4 mW = 6 dBm 8 mW = 9 dBm 10 mW = 10 dBm 20 mW = 13 dBm 50 mW = 17 dBm 100 mW = 20 dBm 200 mW = 23 dBm 500 mW = 27 dBm 1000 mW (1W) = 30 dBm Students don’t need to memorize this table, they only have to remember the meaning of 3 and 10 dB and then can easily calculate all other cases, you can show some examples now! 17

Simple dB math How much power is 43 dBm? +43 dBm is 43 dB relative to 1 mW 43 dB = 10 dB + 10 dB + 10 dB + 10 dB + 3 dB If you have a radio and the specification says that the output power is 43dBm, how much is it in Watts? You proceed step by step... 1 mW x 10 = 10 mW x 10 = 100 mW x 10 = 1000 mW x 10 = 10 000 mW x 2 = 20 000 mW = 20 W Therefore, +43 dBm = 20 W 18

What about negative values? Negative doesn’t mean bad. ;-) How much power is -26 dBm? -26 dBm is 1mW (0dBm) “minus” 26 dB -26 dB = -10 dB - 10 dB - 3 dB - 3 dB If you have a received signal measurement of -26 dBm, how much is it in Watts? You proceed step by step... An analogy with altitude may help in explaining negative dBm. Suppose that in the plans for a building we choose the street level as our reference. Every floor will have a positive height but the basement or cellar will have negative height. Also keep in mind that using the most common reference for the altitude, meters above the sea level, there are some places (depressions) on earth that have a negative altitude, like Death Valley in U.S. It is suggested that the instructor makes an example with a building known to the students. 1 mW / 10 = 100 µW / 10 = 10 µW / 2 = 5 µW / 2 = 2.5 µW (2.5*10-6 W) Therefore, -26 dBm = 2.5 µW 19

Difference between dB, dBm and dBi? dB: dB is a RELATIVE measure of two different POWER levels. 3dB is twice (or half) as much, 6dB is four times, 10dB is ten times, and so on. The formula for calculating gain or loss in dB is: 10log P1/P2. It's used for stating the gain or loss of one device (P1) IN RELATION to another (P2). dBm:- 1 mW = 0 dBm The m in dBm refers simply to the fact that the reference is 1 milliwatt (1 mW) and therefore a dBm measurement is a measurement of absolute power. dBi: The unit of measurement dBi refers only to the gain of an antenna. The “i” stands for “isotropic”, which means that the change in power is referenced against an isotropic radiation. An isotropic radiator is a theoretical ideal transmitter that produces useful electromagnetic field output in all directions with equal intensity, and at 100-percent efficiency, in three-dimensional space. One example of an isotropic radiator is the sun. If you have a received signal measurement of -26 dBm, how much is it in Watts? You proceed step by step... An analogy with altitude may help in explaining negative dBm. Suppose that in the plans for a building we choose the street level as our reference. Every floor will have a positive height but the basement or cellar will have negative height. Also keep in mind that using the most common reference for the altitude, meters above the sea level, there are some places (depressions) on earth that have a negative altitude, like Death Valley in U.S. It is suggested that the instructor makes an example with a building known to the students. 20

Example using mW Using mW It is difficult to do the math using absolute amounts (mW). Using mW Radio card power Loss in pigtail Power leaving Access point Loss of transmission line Power entering antenna Gain of antenna Power leaving antenna 100 mW loose half 16 times the power 100 mW / 2 50 mW 50 mW / 2 25 mW 25 mW x 16 400 mW 21

Example using dB Using dB Calculation is easier using dB Radio card power Loss in pigtail Power leaving Access point Loss of transmission line Power entering antenna Gain of antenna Power leaving antenna 20 dBm -3 dB +12 dBi 17 dBm - 3 dB 14 dBm + 12 dBi 26 dBm (400mW) 22

Conclusions Using decibels (dB) provides an easier way to make calculations on wireless links. The main advantage of using dB is that gains and losses are additive. It is simple to solve radio calculations in your head by using dB instead of using milliwatts.

Thank you for your attention For more details about the topics presented in this lecture, please see the book Wireless Networking in the Developing World, available as free download in many languages at: http://wndw.net/ See WNDW pages 24-26 for relevant material. 24