Basic radio physics. Sebastian Büttrich, NSRC/ITU/wire.less.dk edit: June

Transcription

1 Basic radio physics Sebastian Büttrich, NSRC/ITU/wire.less.dk edit: June

2 Electromagnetic Fields Electromagnetic forces act between electric charges and electric currents. For every point in space, an electromagnetic field (the force at that very point) can be defined and measured. E Electric Field, H Magnetic Field Electromagnetic waves are fields travelling in space

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4 Electromagnetic Waves Examples of electromagnetic waves are: Light, Xrays, Microwaves, FM Radio waves. Electromagnetic waves need no medium they are the dance without a dancer (compare: sound waves)

5 Electromagnetic Waves c=λ*ν 8 c is the speed of light (3 10 m/s) λ ν Lambda is the wavelength [m] Nu is the frequency [1/s = Hz] Light needs 1.3 seconds from the moon to earth, and 8 minutes from the sun and how long for 100 km?...

6 A wave

7 Reminder: powers of ten CentiMilliKiloMegaGiga /100 1/1000 1,000 1,000,000 1,000,000,000

8 Electromagnetic Waves: Polarization Direction of the electric field vector Linear, elliptic, circular polarization

9 Example: Dipole

10 Electromagnetic spectrum Examples

11 Use of electromagnetic spectrum

12 Use of electromagnetic spectrum

13 Frequencies in wireless networking Focus on the ISM (license exempt) bands at 2.4 Ghz b/g 12 cm 5.x Ghz a cm Other bands interesting to us: 915 Mhz 3.5 GHz and more

14 Propagation of Radio waves Wavefronts Huygens principle: at any point, spherical waves start Therefore, radio waves (like light) do not propagate as a straight line.. not even light does that Behavior scales with wavelength

15 Huygens principle

16 Radio waves Absorption Reflection Diffraction Interference

17 Radio waves: Absorption Two main enemies :) Metal Water (rain, fog, waterpipes, ) discuss: Stones, bricks, concrete Wood, trees People: well... we are water... Power decreases exponentially in material: linear decrease in db

18 Radio waves: Reflection e.g. on Metal angle in = angle out plane parabole

19 Radio waves: Diffraction Diffraction is the apparent bending and spreading of waves when they meet an obstruction. Scales roughly with wavelength.

20 Radio waves: Interference Interference is one of the most misunderstood topics in wireless networking Interference often gets the blame when we are too lazy to find the real problem (or when a regulator wants to shut down someone for business reasons)

21 Interference: Reasons for misunderstanding There s really two meanings to the word the physicists' view... behavior of waves the engineering/telecoms view... any noise that gets in the way Both are relevant in wireless networking! Just in different ways...

22 Interference the physicists view Waves can annihilate each other, so that 1 +1=0 In order for that to happen, the interfering waves need to have fixed frequency and phase relation So in this sense of the wordfrequencies outside your band do not interfere microwave ovens do not interfere...

23 Interference a little thought experiment Imagine you have two torches (or lasers) one green, one red If you cross the beams will the one change the other? If you then point them in the same direction, will the one change the other? If you gave signals with them, both in the same direction, would you be able to read them? Now repeat all these questions for two torches/lasers of the same color what are your answers? What other measures can be taken to read two beams of the same color?

24 Interference the physicists view

25 Interference the physicists view

26 Interference the physicists view

27 Interference: MIMO, beam shaping Interference is used in beamshaping, smart antennas, MUMIMO, etc Modern MIMO techniques use interference to optimize antennas, allow for full multiplexing on same frequency

28 MU-MIMO, dynamic beam shaping In multi-antenna arrays, possibilities are virtually unlimited it s all in the dynamic processing power

29 Interference the engineering/telecoms view... any noise that gets in the way Looking at signal to noise ratio In this sense, microwave ovens, bluetooth, any source of radiation within the range of your device they all interfere!

30 Radio waves: Frequency dependence of behaviour Rule of thumb: the longer the wavelength, the further it goes Rule of thumb: the longer the wavelength, the better it goes through and around things Rule of thumb: the shorter the wavelength, the more data it can transport

31 Radio propagation in free space Free space loss Fresnel zones Line of Sight

32 Free Space Loss Proportional to the square of the distance and also proportional to the square of the radio frequency FSL [db]= C + 20 * Log(D) + 20 * Log(F) D distance, and F frequency [MHz]. The constant C is 36.6 if D is in miles, and 32.5 if D is in kilometers.

33 Fresnel zones

34 Line of sight In general, you need to have a free line of sight (LOS) for a radio link

35 Example: A full transmission path Image: radio cable antenna free space antenna cable - radio

36 The db Definition: 10 * Log (P1 / P0) 3 db = double power 10 db = order of magnitude = x 10 Calculating in dbs Relative dbs: dbm = relative to 1 mw dbi = relative to ideal isotropic antenna

37 The db: Examples 1 mw 100 mw 1W = = = 0 dbm 20 dbm 30 dbm An omni antenna with 6 dbi gain A cable (RG213) with 0.5 db/m loss

38 Transmit Power Example from a a/b card: Output Power: b: 18 dbm (65 mw) peak power a: 20 dbm (100 mw) peak power

39 Receive sensitivity Example from a Senao b card Receive Sensitivity: 1 Mbps: -95 dbm; 2 Mbps: -93 dbm; 5.5 Mbps: -91 dbm 11 Mbps: -89 dbm

40 Where physics matter Always!... and especially... (some real life examples based on trainer's experience) when an access point is placed under a desk when winter turns to springtime... when it is rush hour in the city... when doing very long distance links (speed of light!) when you need to tell marketing talk from truth

41 Examples: Office network Offices typically have massive multipath conditions Problem objects: People :), metal infrastructure (computers, radiators, desks, even CDs!) Choice of locations and antennas essential

42 Examples: when winter turns to spring... Regardless of your climate zone, factors like vegetation, humidity, rain etc change with the seasons! Dry trees might be transparent, green trees are not!

43 Examples: when it's rush hour in the city In urban environments, conditions change with the hour... people, vans, cars, electromagnetic interference,... Verify on a monday what you measure on a sunday :)

44 Examples: when speed of light comes into play Standard implementations of _ standards set time-out windows: PCF, DIFS, SIFS,... For long distance links, the travel time of the signal might lead to timeout and performance losses We have to hack the MAC layer to go long distance see e.g. TIER group, Berkeley

45 Examples: looking through marketing talk, e.g.... One antenna or radio device NEVER has a reach or distance... that is one hand clapping Even with WIMAX' NLOS (None line of sight), waves still do not go through absorbing walls Even $$$$ enterprise wireless fails, if you forget to check polarization