MIMO Spatial Stream Title

Understanding MIMO and Spatial Streams (Translated Publication)

Whether you are choosing a new AP for your home or your company, you may come across two peculiar numbers like “2×2” or “3×3” MIMO on the spec sheet. Have you ever wonder what these numbers are? What is MIMO? And how do you choose which one to buy? This post will introduce you to the concept of MIMO and Spatial Streams, and it will help you choose the right AP for your needs.

Radio Chains

So a quick answer to the above question–those were number of antennas that make up the “Radio Chains.” The first number shows how many antennas can transmit (Tx) and the second is the number of antennas capable of receiving (Rx). Therefore, a 4×4 AP can use 4 antennas to transmit and 4 to receive (it used each physical antenna for both transmitting and receiving).

Now, let’s examine the physical appearance of an AP.

This first one, I am sure most Cisco fans are familiar with–an Aironet 3802e. This enterprise-grade 802.11ac wave 2 AP features 4 external antennas is a very popular model among large organizations and enterprises.

Cisco 3202e
Source: Cisco Website

But don’t let the number of antennas fools you. Physically, it may have 4 antennas, but the radio chain can be 4×4, 3×4, or 4×3. The best way to check your AP radio chain is to read its spec sheet.

Source: Cisco Website

The next AP is Unifi UAP-AC-M from Ubiquiti, a leading Wi-Fi manufacturer for the SME vertical. This is a 2×2 802.11ac with 2 physical antennas. From the spec sheet, this is a 2×2 AP.

Ubiquiti UAP-AC-M
Source: Ubiquiti Website

Let’s have a look at an AP with internal antennas.

The image below shows the inside of a Ruckus R710–a 802.11ac wave 2 flagship AP. Ruckus is known for its BeamFlex+ antenna technology. This is a 4×4 AP and you can see 4 antennas inside.

Ruckus R710
Source: Ruckus Wireless / Commscope Website

Now we’ve covered radio chain, let’s have a look at RF transmission technologies.

Radio Communication

The most simple form of radio communication is SISO (Single-Input Single-Output) that uses a single antenna to transmit and one antenna to receive. With a single receiving antenna, there is no antenna diversity, meaning there is only one incoming source. The biggest issue of processing a single source is multipath interference, a RF propagation that results in signal reaching the receiving antenna at different time and/or polarization. It could cause destructive interference and results in signal fading.

With this limitation in mind, the next technology update is SIMO or Single-Input Multiple-Output. With multiple receiving antennas, receive diversity can be achieved. This mitigate multipath interference but wouldn’t it be better if we can transmit with multiple antennas? This should help improve the speed, shouldn’t it?

SIMO

MIMO (Multiple Input Multiple Output)

MIMO takes advantage of multiple antennas for both transmitting and receiving data simultaneously. Not only it resolves issue with multipath, it enables data transfer at a higher speed. By using multiple transmitting antennas, more data can be transmitted, hence increases the data rate.

MIMO
MIMO

On September 11, 2009, IEEE formally ratified 802.11n and that officially marked the birth of MIMO in Wi-Fi. MIMO APs started to proliferate–2×2 3×3 4×4 and today’s 802.11ax (Wi-Fi 6) theoretically support up to 8×8.

We can categorize MIMO into Single-User MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO). The difference is SU-MIMO can only transmit to only a single device, whereas MU-MIMO can transmit to different devices simultaneously.

Spatial Streams

Spatial streaming or spatial multiplexing is a MIMO technique that is used to transmit independent streams to single or multiple receivers simultaneously. The keywords are independent streams and simultaneously. So if a device is capable of receiving 2 streams, by sending 2 streams at the same time effectively doubles the data sent, therefore double the speed.

Remember the number of streams are limited by the number of Tx antennas, therefore, a 2×3 AP can only transmit 2 streams.

Below is a high-level illustration of how MU-MIMO and Spatial Streams works. This AP is sending 3 streams from 3 antennas to 3 different clients simultaneously.

3 Spatial Streams Communication Example 1

The second example below shows that a 2×2 client can receive 2 incoming streams, hence doubled the data received; but the 1×1 mobile device can only take 1 stream at a time.

3 Spatial Streams Communication Example 2

According to the MCS Index, if we configure the BSSID to use 40MHz channel, the maximum data rate of a 1×1 device is 200Mbps; however, if that device supports 2×2, the data rate effectively doubles to 400Mbps.

You can find out the number of spatial streams from the AP spec sheet. One common confusion is many vendors use the format AxB:C instead of AxB; again, A is the number of transmitting antennas and B is for receiving, and C is the number of spatial streams. So you can have an AP with 4 transmitting antennas but only 3 spatial streams; but you can’t have a 2×2 AP with 3 spatial streams because you can’t have more spatial streams than the AP Tx antennas. For example, Cisco Aironet 1830 is a 3×3:2, so with 2 spatial streams, the maximum data rate is equivalent to a 2×2:2. The AP uses the other antenna for Transmitting Beamforming (TxBF), a technique that helps increases the SNR and achieve higher data rate.

So when you decide to buy an AP, don’t go with the one with the highest number of antennas. You have to consider client capabilities, bandwidth requirements, and client density. If you use low bandwidth applications at home and your laptop is a 2×2, buying a 4×4 AP is probably an overkill.

Most laptops and smartphones in the market today are 2×2 except for the MacBook Pro that supports 3×3. Most IoT devices today’s like personal health tracker and sensors still use 1×1 because it’s cheaper to manufacture, and it conserves battery.

If you want to find out your device capabilities, have a look at https://clients.mikealbano.com/. The author built a list of most common Wi-Fi client capabilities, for example, iPad, iPhone, Samsung by using a feature in WLANPi. The way it works was he enabled WLANPi as a Wi-Fi hotspot, and once a target client connects, it will advertise its capabilities in the Association Request frame.

Wireshark Association Request Frame
Wireshark Association Request Frame

As mentioned above, you don’t always need to buy AP with the highest number of antennas. You could buy a flagship 8×8 for your IoT but that’s just a waste of money. However, if you plan to put a Wi-Fi service in a large meeting room or lecture hall with hundreds of participants, an 8×8 AP (if it exists) is a wise choice. But personally I’d still go with two 4×4 APs or three 3×3 instead of an 8×8 to better manage the channel utilization, but that’s a topic for another post.

Leave a Reply

Your email address will not be published. Required fields are marked *