Technology Blog

Wave 2: MU-MIMO and Directional Antennas

by Moderator on ‎05-17-2015 09:47 PM - last edited on ‎05-18-2015 08:00 AM by

This article has lots of pictures. We will be visually discussing how antenna choices indoors can affect the operation of MIMO and it more advanced cousins with 11ac which are Tx BF and MU MIMO.

 

If you recall from the Antennas in the Spotlight posting we talked a lot about degrees of freedom. I will show you how the use of directional antennas and steered directional antennas can have a direct negative impact on the operation of the features listed above when operating with more than 1 stream with a focus on MU-MIMO.

 

First: The channel

 

To reiterate. The channel is everything that happens between the AP and the client. All of the walls, the ceiling, the floor and all of the obstacles in the room

 

Second: Degrees of freedom

 

As discussed in the earlier blog the degrees of freedom in a space are determined by the number and power level of the reflection paths in the channel. Weak paths do not contribute to the degrees of freedom.

 

The number of streams that are well supported in a channel has to be the same or less than the degrees of freedom

 

Using directional antennas, steered or fixed, reduces the number of reflection paths illuminated which goes directly to the degrees of freedom and the number of supported streams.

 

While it seems intuitive that a directional antenna can increase the signal strength towards a single client, with the advent of multi-stream handsets and the continued proliferation of multi-stream tablets and PCs, the antenna choices that are available with a product can significantly impact delivered performance.

 

MU-MIMO further complicates this because not the targets of a transmission can be in three totally different directions. For MU MIMO to work properly all of the antenna elements have to be able to “see” all of the clients. The antenna subsystem, in concert with the chipset has to be able to peak a particular stream at a given client and NULL out the others that are sent simultaneously. The nulls cannot be created at the AP. The AP has to use the channel to create the nulls and those nulls coincide directly at the antenna on the client.

 

To achieve this nulling in a particular place all of the antenna elements have to send, adjusted for amplitude and phase a copy, of all streams. If you have multiple beams steered in multiple direction this is completely broken.

 

3: The analysis and the pictures

 

The Space

 

For the purposes of this blog we will define a space which is 20m x 20m x 7m

 

We will place the AP at 5m to the right and 5 m down from the center of the room and we will look at clients on a line 5 m up from the center line of the room

MU MIMO 1.jpg 

 

We are placing the clients on a line to simplify presentation of the data. The approach here is fully generalized.

 

 

We’ll look at the electric field characteristics with 1, 5, 10, and 25 dB loss on each bounce. Then we’ll focus on drywall construction where reflections from walls are typically 6 dB down. That means after 3 bounces the energy is reduced by 18 dB.

 

The Antennas

 

Case 1: 4 omni elements in a square spaced ½ wavelength apart

Case 2: 4 directional elements in a line with 90 degree beamwidths

            All directionals are pointing in the same direction which gives the best result.

 

 

Impact of reflections

 

The ideal case for MIMO and the related features is a reflection rich environment maximizes the degrees of freedom in the channel.

 

Each of the plots shows a cross section of about 2.1m which is ~40 wavelengths at 5.5 GHz

 

This is what the fields look like with varying losses on each bounce and the AP antenna elements are in a line with a half wavelength between each.

 

As can be seen in the plots as the loss on the bounce increases the variations of the field decreases. This makes it more difficult for MIMO, Tx BF, and MU MIMO to work for multi stream operation.

MU MIMO 2.png

Fields from each of the AP antennas with 1 dB loss on wall bounces

MU MIMO 3.png

Fields from each of the AP antennas with 5 dB loss on wall bounces

MU MIMO 4.png

Fields from each of the AP antennas with 10 dB loss on wall bounces

MU MIMO 5.png

Fields from each of the AP antennas with 25 dB loss on wall bounces

 

Impact of reflections and directional   antennas

 

The ideal case for MIMO and the related features is a reflection rich environment maximizes the degrees of freedom in the channel.

 

Each of the plots shows a cross section of about 2.1m which is ~40 wavelengths at 5.5 GHz

 

This is what the fields look like when the varying losses on each bounce and the AP antenna elements are in a line with a half wavelength between each and an antenna pattern with 90 degree vertical and horizontal beamwidths:

MU MIMO 6.png

Antenna element pattern used for directional simulations. 3 dB per division.

MU MIMO 7.png

Fields from each of the AP directional antennas with 1 dB loss on wall bounces

MU MIMO 8.png

Fields from each of the AP directional antennas with 5 dB loss on wall bounces

MU MIMO 9.png

Fields from each of the AP directional antennas with 10 dB loss on wall bounces

MU MIMO 10.png

Fields from each of the AP directional antennas with 25 dB loss on wall bounces

 

From the above we can see that the directional antennas substantially reduce the amplitude and frequency of the ripples for each of the cases simulated. Therefore, as expected the use of directional antennas, the apparent reduction will negatively impact the support of MIMO, Tx BF, and MU MIMO

 

Direct Comparison in Room with Drywall

 

Case 1: 4 Omnis in a Square

 

So let’s look at a particular case now. Standard drywall give reflections at ~6 dB down.

 

We will now look in more detail in a drywalled space.

 

First with omnis on the AP arranged in a square with sides about ½ wavelength across. Here is the plot showing the fields in the area of the clients. MU MIMO 11.png

  

After MIMO processing here is the field plot of stream 1 at the same location. The blue dots show the location of 3 single stream clients. Note that the MU-MIMO processing has left nulls under clients 2 and 3 and the plot has been normalized to 1 where client 1 is. A well behaved system has relatively even amplitudes on the peaks

MU MIMO 12.png

Here are the patterns for each stream

 

There are noticeable differences in the patterns and this in turn shows good separation of the streams mathematically. This indicates a much higher probability in proper decoding of the three streams for MU MIMO.

MU MIMO 13.png

It is interesting to note that the optimum result actually moves the peak gain away from the client locations which in this case is straight up. This makes sense as the system has to cancel unwanted streams so it reduces the peak towards the client and reinforces the reflections so that the amplitudes are similar.

 

Case 1: 4 Directionals in a line

 

Now let’s do this for the directional case with the same reflection conditions

 

As before, here are the fields from the AP before MIMO processing. Aside from the antenna change all else is held constant.

MU MIMO 14.png

Here are the fields for stream 1 after MU MIMO processing.

MU MIMO 15.png

This is actually showing that the MU processing is having a difficult time converging. Client 1, as before, is located at the -10 wavelength position and the amplitude there should be normalized to 1. The math is trying to reach down to that point but since it cannot this indicates a failure of the math to converge.

 

And here are the patterns

 MU MIMO 16.png

Unlike the arrangement of the 4 omnis all three patterns are very similar. This implies that the ability to decode the 3 streams is going to be much more difficult and any errors will be amplified. MU MIMO processing will have a more difficult time to support multiple streams/clients

 

Summary:

 

As discussed in previous articles the choice of antenna element can significantly impact the operation of MIMO, Tx BF and MU MIMO. Even using relative wide directional elements has a deleterious impact on the operation of these advanced features when supporting more than 1 stream.

 

In short, directional antennas, whether fixed or steered should not be used for systems that expects MU MIMO to operate.

 

@ej_wireless

 

Previous Article:

http://community.arubanetworks.com/t5/Technology-Blog/Wave-2-MIMO-and-Polarization/ba-p/234087

 

Comments

Brilliant explanation!

For sure the Ruckus guys will not like your conclusion on directional antenna and MU-MIMO.

 

Without stealing your post, Veriwave has an excellent FAQ to explain the channel matrice http://www.veriwave.com/gurus/faq_802.11n_mimo.asp#50 in the context of cable testing, which amazingly confuse lots of testers to understand the magic of multistream signal.

 

Cheers.

Chi-Thanh

Slightly different topic, but can Aruba APs take advantage of MU-MIMO for mesh links? I.e. is the mesh link treated like a client connection for MU-MIMO purposes?

Moderator

The operation of mesh links is no different than a client connection from the point of view of MU MIMO. For P2P with very directional antennas you still have the same issues. The lack of side reflections is the primary restriction. For that reason all of the external panels that Aruba makes available for the AP-274 for long shot are fixed 30 x 30 x pol antennas. the x pol will give 2 streams of operation.

If you had a large enough angular seperation between the end point and use all vertically polarized elements it is possible to get MU working but only 1 stream per end point. 2 end points max for a 3x3 AP. That is not any different that sending 2 streams to each end point and alternating.

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