Wave 2: Antennas in the spotlight

By Eric Johnson, Director of Product Management
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Wow. 802.11ac Wave 2. What is that all about?

Well, more than ever it is all about the antennas. You have got to get that right.

The primary functional enhancements/differentiators to 11ac are:

  • Wider channels
  • More streams
  • 802.11ac Explicit Beamforming

Let's take a look at this. Wider channels are self explanatory.

Bigger is better. Right? (Well maybe, but that is the topic for another blog)

The other 3 items are intimately tied to the transceiver and antenna sub-system in the access point.

This article is the first of many which will take you from the simplest antenna construct with electrons flowing up and down a wire right through to MU MIMO, which is the ultimate blending of the awesome compute power that is available in today's radio chipset and refined antenna design.

You'll notice that I did not say complicated antenna design. It is quite the opposite in fact. 11ac beamforming and 11ac MU MIMO drive radio system design to use high quality but VERY simple antenna designs, usually omnis.

Here are some concepts to start your brain:

1. 11ac Beamforming is actually an inadequate label for what this capability provides.

  • Beamforming implies that the function is somehow limited to the AP and its local antenna subsystem
  • A far better description of this would be to refer to it as 11ac channel optimization. This is because the scope of this capability includes EVERYTHING that is in the path between the digital portion of the AP and the digital portion of the client. All of the analog impairments in the transmitters and receivers, and all of the interactions of the RF waves and the objects around the AP are included in the calculation. The capability, in a single exchange of a channel sounding packet, establishes the optimum excitation of the antennas on the APs to deliver the highest possible signal strength to the client AND maximizes MIMO operation at the same time. This is repeated many time per second to track the user as they moveBy contrast "smart antenna" steered beam solutions attempt to maximize signal strength, they take a while to build up their location estimate, and they can actually impair the capability of the client/AP system to support multiple streams.

2. Supporting more streams requires more degrees of freedom

Q: What the heck does that mean?
A: In the simplest terms this relates to the number of antennas and the number of paths (bounces) excited.

  • A closed space like a meeting room, an enclosed courtyard, and so on will have MANY MANY reflection paths. In those situations even 2 or 3 bounce paths may have a lot of energy on them. As a result these spaces are characterized as having many degrees of freedom. If the degrees of freedom exceeds the number of antenna then the full number of streams may be supported with single user MIMO and Multi user MIMO
  • An open field with no reflection surfaces or with reflectors that are far away (meaning low signal strength on the reflections) has no or very restricted degrees of freedom and so will struggle to support more than 1 stream for MIMO operation
  • An open field environment MAY be able to send signals to more than one use in MU MIMO operation but only with very specific conditions in place.
  • The number of streams cannot exceed the minimum of the number of antennas at the transmitter or the receiver

Q: What does that mean for antenna design?
A: let's think this through:

  • Let's look at the indoor case. We know that many/most indoor environments give many degrees of freedom. To maximize the reflection paths and serve users that are widely spaced in angle simple quality omnis elements are the best solution. Polarization diversity is generally not required is these cases. Note also that the process of reflection randomizes polarization as well. The use of fixed or steered directional solutions will reduce the degrees of freedom as the number of paths will be greatly reduced do to the antenna directionality. Thus, "smart antennas" are NOT consistent with either quality multi-stream single user MIMO or multi user MIMO.
  • In the outdoor case for single user MIMO we can introduce a 2nd degree of freedom by using antenna polarization. Polarization is the direction of the electric field. Horizontally and vertically polarized waves are physically and mathematically distinct and so can be used to send 2 streams of independent data. Thus, polarization can be used to create a 2nd degree of freedom in this environment. NOTE: there are only two polarizations that are separate. Anything else is a combination of the two independent states.
  • In the outdoor case for multi user MIMO there are certain instances where you can use an array of antenna elements to create beam patterns that are distinct. In general for N antennas you can create N-1 nulls. The tradeoff is that all of the elements in the array have to be the same polarization AND in general they need to be matched omni patterns. The simplest case I can give you is if a MU MIMO radio has to send to one user in front and one behind the basic radiating element needs to be able to see both clients at the same time. This has significant implications for providers of "smart antenna" solutions that steer directional antenna patterns. Simply put those solutions are completely inconsistent with MU MIMO operation in outdoor situations.

So, those a couple of high level snapshots. As indicated more articles will follow and explain these in detail (with pictures) along with a deeper dive in how beamforming actually works, how antenna placement on an access point affects that, and ultimately how that maps to MU MIMO.

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