Antenna Developments for WiFi Phase Applications Diversity MIMO

Phase Applications for Antennas Antenna Diversity Multiple streams, multiple antennas

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

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?

Multipath Effect What is the distance to the right antenna? The difference between the left and right distances is merely 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

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

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

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

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

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)

MIMO Principles Main Idea

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.

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 plus ¼ of a wave One wave AB D

Introduction to MIMO I will make these waves green to differentiate this scenario One wave AB D One wave plus ¼ of a wave

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. D C B One wave plus ¼ of a wave One wave

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

Receiver at Point B ƛ A ƛ B + ¼ ƛ B Receiver at point B gets ƛ A and ƛ B + ¼ ƛ B at the same time.

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 A is a negative sine B B is a negative cosine

Receiver at Point D ƛ B ƛ B + ¼ ƛ B Receiver at point D gets ƛ B and ƛ B + ¼ ƛ B at the same time.

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 A is a negative cosine B B is a negative sine

Two signal transmitted, Two signal received Take the signals at B with the additional delay and add them to the signals at D; result: Cancels outAdds up

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

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!

EXAMPLE MIMO Principles

MIMO Frequency = 300 MHz Lambda = 1 m Distance D = 100 metres Hypotenuse = 100 metres plus ¼ lambda What is height h 2 ?

MIMO Frequency = 300 MHz Lambda = 1 m Distance D = 100 metres Hypotenuse = 100 metres plus ¼ lambda What is height h 2 ?

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

Pythagoras Distance D = 100 metres Hypotenuse = metres What is height h? Height is 7.07 metres

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. TX A TX B Receiver RX A gets the signals from both TX A and TX B. Receiver RX B gets the same signals than RX A

Two TXs, Two RXs Let’s call: TX A ƛ A – The signal transmitted from TX A lambda A (ƛ A ) and... TX B ƛ B – The signal transmitted from TX B lambda B (ƛ B ) Both Receivers RX A and RX B get the signals transmitted from SITE A.

Two TXs, Two RXs All together: ƛ A ƛ B delayed ¼ ƛ B Receiver RX A gets ƛ A and ƛ B delayed ¼ ƛ B at the same time. ƛ B ƛ A delayed ¼ ƛ A Receiver RX B gets ƛ B and ƛ A delayed ¼ ƛ A at the same time.

RX A ƛ A ƛ B delayed ¼ ƛ B Receiver RX A gets ƛ A and ƛ B delayed ¼ ƛ B at the same time. ƛ B delayed ¼ ƛ B ƛAƛAƛAƛA ¼ wave

RX B ƛ B ƛ A delayed ¼ ƛ A Receiver RX B gets ƛ B and ƛ A delayed ¼ ƛ A at the same time. ƛBƛBƛBƛB ƛ A delayed ¼ ƛ A ¼ wave

Two TXs, Two RXs Let’s take the received signals at Receiver RX A ƛ A ƛ B delayed by ¼ ƛ B That is ƛ A and ƛ B delayed by ¼ ƛ B combined. ƛ B delayed ¼ ƛ B ƛAƛAƛAƛA ¼ wave

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

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

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

All together Now, let’s add the output of the delay device to the signals present in RX B What is the result?

Adding the Signals What is the result? This is the result of received signals at RX A after being delayed again by ¼ wavelength These are the signals as they are received at RX B 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 - sine - cosine + sine - cosine This is exactly the desired effect. The antenna RX A receives the two signals but only one is obtained at the end of the process. Even better, the signal is doubled in amplitude.

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

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.

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.

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

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

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

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

Spatial Multiplexing A processor extracts the desired signals by performing math operations (S 1 +S 2 )+(S 1 -S 2 )=2S 1 (S 1 +S 2 )-(S 1 -S 2 )=2S 2

Spatial Multiplexing n 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