Antenna Developments for WiFi

1. Antenna Developments for WiFi Phase Applications Diversity MIMO

2. Phase Applications for Antennas • Antenna Diversity • Multiple streams, multiple antennas

3. Multipath Effect • Wireless station STA transmits RF waves which are received by the wireless access point WAP • Both antennas Left and Right pick up the signals

4. Multipath Effect • The antennas are separated by certain distance • Typically, the wavelength size • Suppose that the STA is separated from the left antenna by a distance of 1.2 metres and that the wavelength is 0.12 metres (for 2.4 GHz Band) • What is the distance to the right antenna?

5. Multipath Effect • What is the distance to the right antenna? • The difference between the left and right distances is merely 0.0059 metres (5.9 mm) • However, the WAP is designed to perceive such slight difference by detecting the phase difference • The WAP can detect which of the two path is the shortest • Consequently, the WAP can choose the strongest and most direct signal path

6. Multipath Effect • The WAP can detect the position of the client depending of the combination of phases that appear in the two antennas

7. Antenna Diversity • An internal controller contrast the signals received by both antennas. • The reflected path signal will be out of phase and also attenuated by the longer distance and the reflection off the surface. • The AP selects which antenna to use for listening

8. Configuration ap(config-if)#antenna ? gain Configure Resultant Antenna Gain receive receive antenna setting transmit transmit antenna setting ap(config-if)#antenna receive ? diversity antenna diversity left antenna left/secondary right antenna right/primary

9. Extending this Concept • Multipath has been a traditional problem for wireless communications • However, by implementing a very clever disposition of antennas, and signal processors, multipath can become and ally

10. Spatial Multiplexing • Spatial Multiplexing is a wireless technology based on the separation of the main data stream into several, separate, streams • Each separate mini-stream is transmitted by a separated antenna • However, all the separated mini-stream are modulated in the same frequency channel • In the receiving side, all the mini-streams are reassembled together using a special signal processing technology (MIMO)

11. MIMO Principles Main Idea

12. MIMO • The idea of MIMO is actually very old. • It was difficult to implement very it requires very precise phase locking. • Nowadays, this is easier to implement with modern electronics digital processors and phase locking chips.

13. Introduction to MIMO • Imagine that one EM wave front is transmitted from point A. • One full wave fits in the shortest path between two points A and B. • The longer path has been set up in such way that one wave and a quarter fits between point A and D. One wave A B One wave plus ¼ of a wave D

14. Introduction to MIMO • I will make these waves green to differentiate this scenario One wave A B One wave plus ¼ of a wave D

15. Introduction to MIMO • Another wave front is transmitted from point C. • It is received in both positions B and D. • In the path from C to D, one full wave fits. • In the path between C and B, a wave an one quarter fits. B One wave plus ¼ of a wave C D One wave

16. Introduction to MIMO • The two scenarios are placed together now. • Point B receives ƛAand ƛB + ¼ ƛB at the same time. • Point D receives ƛB and ƛA+ ¼ ƛA at the same time. A B D C

17. Receiver at Point B • Receiver at point B gets ƛAand ƛB + ¼ ƛB at the same time.

18. Two signal transmitted, Two signal received • Let’s place a device, in series, that further displaces the signals by ¼ wavelength. Additional displacement of ¼ wave • The signals are now: • A is a negative sine • B is a negative cosine

19. Receiver at Point D • Receiver at point D gets ƛBand ƛB + ¼ ƛBat the same time.

20. Two signal transmitted, Two signal received • Let’s place a device, in series, that further displaces the signals by ¼ wavelength. Additional displacement of ¼ wave • The signals are now: • A is a negative cosine • B is a negative sine

21. Two signal transmitted, Two signal received • Take the signals at B with the additional delay and add them to the signals at D; result: Adds up Cancels out

22. Two signal transmitted, Two signal received • Take the signals at D with the additional delay and add them to the signals at B; result: Cancels out Adds Up

23. Summary So Far • Two signals with modulated information are transmitted simultaneously. • These two signals have the same carrier frequency or wavelength. • But there are two different flows of information • By placing the receivers in a very specific position, the signals are received with certain amount of out of phase. • By adding specific delays and then combining the signals, the two flows of information can be separated. • Very clever!

24. MIMO Principles Example

25. MIMO • Frequency = 300 MHz • Lambda = 1 m • Distance D = 100 metres • Hypotenuse = 100 metres plus ¼ lambda • What is height h2?

26. MIMO • Frequency = 300 MHz • Lambda = 1 m • Distance D = 100 metres • Hypotenuse = 100 metres plus ¼ lambda • What is height h2?

27. Pythagoras • Frequency = 300 MHz • Lambda = 1 m • Distance D = 100 metres • Hypotenuse = 100 metres plus ¼ lambda • What is height h?

28. Pythagoras • Distance D = 100 metres • Hypotenuse = 100.25 metres • What is height h? • Height is 7.07 metres

29. Two TXs, Two RXs • Now, let’s add a second transmitter B with the same separation of 7 metres and the same wavelength as before. • Receiver RXA gets the signals from both TXA and TXB . • Receiver RXB gets the same signals than RXA

30. Two TXs, Two RXs • Let’s call: • The signal transmitted from TXA lambda A (ƛA) and... • The signal transmitted from TXB lambda B (ƛB) • Both Receivers RXA and RXB get the signals transmitted from SITE A.

31. Two TXs, Two RXs • All together: • Receiver RXA gets ƛA and ƛB delayed ¼ ƛB at the same time. • Receiver RXB gets ƛB and ƛA delayed ¼ ƛA at the same time.

32. RXA ƛA ƛB delayed ¼ ƛB • Receiver RXA gets ƛA and ƛB delayed ¼ ƛB at the same time. ¼ wave

33. RXB • Receiver RXB gets ƛBand ƛA delayed ¼ ƛA at the same time. ƛA delayed ¼ ƛA ƛB ¼ wave

34. Two TXs, Two RXs ƛA ƛB delayed ¼ ƛB • Let’s take the received signals at Receiver RXA • That is ƛA and ƛB delayed by ¼ ƛB combined. ¼ wave

35. Two TXs, Two RXs • Let’s place a device, in series, that further delays the signals by ¼ wavelength.

36. The Effect of a new delay • Let’s place a device that delays the signals by another ¼ wavelength. • This happens: Reference Line ƛB delayed ¼ ƛB ƛA Input Signals After the added delay (see how the signals have shifted to the right with respect the input signals above) ½ wave

37. Delay Effect • The signals are delayed ¼ of a wave again causing: • A to become a negative cosine and • B to become a negative sine Additional Delay of ¼ wave

38. All together • Now, let’s add the output of the delay device to the signals present in RXB • What is the result?

39. Adding the Signals • This is the result of received signals at RXA after being delayed again by ¼ wavelength • What is the result? - cosine + sine - sine • These are the signals as they are received at RXB - cosine • This is exactly the desired effect. • The antenna RXA receives the • two signals but only one is • obtained at the end of the • process. • Even better, the signal is doubled • in amplitude. • This is the result of adding the previous signals: • The + sine cancels out with the – sine. • The two – cosine signals add up to have a stronger signal

40. The Complete Scheme • What is the point of all this complication?

41. The Complete Scheme • What is the point of all this complication? • The point is that both “sources of information A and B” can be transmitted using the same licensed carrier frequency. • Just one carrier frequency for two different radio links. • The signals interfere with each other in the air, but it does not matter, because they are recovered at the destination by this ingenuous system.

42. The Complete Scheme • This is the fundamental principle of “spatial multiplexing” • A type of spatial mux are MIMO radios or Multiple Inputs Multiple Outputs radios. • It is a more efficient way to use the radio spectrum. • However, do not see this technology as a “license to print money” There are practical boundaries like in any other technology.

43. Spatial Multiplexing • The main data bit stream is separated into several streams • Each separate mini-stream is transmitted by a separated antenna • All the separated mini-streams are modulated in the same frequency channel

44. Spatial Multiplexing • The problem is: how do the receiving-end differentiate and detect the several streams and put the data back together?

45. Spatial Multiplexing • Multipath signals are actually used to recover the data stream

46. Spatial Multiplexing • A MIMO (multiple input multiple output) digital signal processor recovers and reassembles the data

47. Spatial Multiplexing • A processor extracts the desired signals by performing math operations • (S1+S2)+(S1-S2)=2S1 • (S1+S2)-(S1-S2)=2S2

48. Spatial Multiplexing • 802.11n defines Multiple Input Multiple Output in configurations of MxN:S • M is the number of transmitters • N is the number of antennas • S is the number of spatial streams • For example, 3x3:2 means 3 transmitters, 3 antennas and 2 spatial streams