The evolution and future of Wi-Fi (Part 2)

by [Published on 6 Oct. 2009 / Last Updated on 6 Oct. 2009]

Discussing the present state of Wi-Fi technology, 802.11n.

If you would like to be notified when Russell Hitchcock releases the next part of this article series please sign up to the WindowsNetworking.com Real time article update newsletter.

If you would like to read the first part of this article series please go to The evolution and future of Wi-Fi (Part 1).

Introduction

In my last article I began by introducing the Institute for Electrical and Electronics Engineers (IEEE) and went on to describe the evolution of the Wi-Fi technology. In this article I will discuss the Wi-Fi technology in the most recent form, commonly known as 802.11n.

MIMO

The big new innovation with 802.11n is the introduction of Multiple Input Multiple Output (MIMO) antennae into the the Wi-Fi standards. Previously Wi-Fi antennae configurations were only Single Input Single Output (SISO). As its name suggests, MIMO means that there are multiple antennae for input as well as multiple antennae for output. MIMO is one configuration of three general configurations possible for multiple antennae. These configurations, shown in Figure 1, are:

  1. Single Input Multiple Output (SIMO)
  2. Multiple Input Single Output (MISO)
  3.  Multiple Input Multiple Output (MIMO)


Figure 1: Various multiple antennae configurations, courtesy of The Computer Desktop Encyclopedia

The MIMO technology will be beneficial to users immediately. First MIMO is beneficial when there is more than one user accessing the same Wi-Fi source. For instance, in your office you may have a single Wi-Fi node in a lounge, and you and your coworkers can connect to this node when you are having your mid-morning coffee. Prior to 802.11n if there were multiple users accessing a single Wi-Fi node, performance would degrade significantly. Now, with 802.11n and MIMO a single antennae can be assigned to each user and all users (assuming that the number of users is less than or equal to the number of antennae) will not observe speed decreases.

Antennae Diversity 

MIMO is also beneficial when there is only a single user as well. Let us go back to the office lounge scenario for a minute. Now let's say that you are the only user accessing the Wi-Fi node. However, some of your coworkers are on their blackberries, there's a couple of microwaves running, and someone is even using a cordless phone. This is a classic problem for Wi-Fi. It's the exact situation in which one would want to use Wi-Fi and yet it is very difficult to receive a reliable signal in such a noisy (electro-magnetically noisy that is, although your coworkers may also be loud) environment. MIMO can counteract this interference by sending the same signal to the same user over multiple antennae. The user receiving these signals can then compare each of the signals to each other and then determine what the true signal (the signal before the interference) was.

This method of countering the signal interference is called antennae diversity. There are five general ways to achieve antennae diversity, which I will now describe.  

Spatial diversity

When an application employs spatial antennae diversity, the base station consists of multiple antennae each physically separated from the others. It is most common that these antennae all have the same characteristics. The spacing of these antennae can be anything. Usually the spacing is equivalent to the length of one wavelength of the signal to be transmitted. In other instances the antennae can be separated by miles (although in this case I do not think it is still accurate to refer to them as a single base station). This is the most common antennae diversity scheme you will see in Wi-Fi 802.11n base stations.

Pattern Diversity

Pattern diversity is used most often with directional antennae. In this antennae diversity scheme, multiple directional antennae are placed in close proximity with each antennae having a difference radiation pattern. This antennae diversity scheme can often produce better performance results when compared with schemes using a single omni-directional antennae.

Polarization Diversity

Polarization diversity consists of a pair (or multiple pairs) of antennae each having an opposite polarization. Because the signals transmitted from each of these antennae have opposite polarization, the interference seen by the signals will be different. It is therefore more likely that one of these signals will be understood by the receiver, or at least the receiver could use both signals to reconstruct the original transmission.

Adaptive Arrays

Adaptive arrays consist of a single array with multiple elements which can change their polarization patterns easily. These kind of antennae are very expensive and require a lot of control circuitry which add even greater expense. For this reason it is unlikely that this technology would ever be used Wi-Fi technologies.

Transmit/Receive Diversity

Transmit/receive diversity can occur when a base station has one antennae for transmitting and a separate antennae for receiving. There are no transmission benefits with this scheme, however, it does save on cost and the need for a duplexer is eliminated. There is nothing too exciting here, just know that when you see this, someone was trying to save a bit of money.

Future benefits

Previously I mentioned that MIMO will be immediately beneficial to users. While this is true, there is even more benefits to come. 

Dirty Paper Coding

One technology which I am very excited for is called Dirty Paper Coding (DPC). DPC is essentially a mathematical problem and involves complex pre-coding of the signals prior to transmission. Before I explain what DPC is, let me explain what problem it is going to solve. For that, let's go back to the office lounge scenario with you and some of your co-workers each accessing the base station (to read WindowsNetworking.com articles of course!). So, as I explained before MIMO allows each antennae to be assigned to a single user so that each user is using their own base station antennae. When this happens the signals (on top of seeing interference from microwaves, cordless phones, etc.) interfere with each other and thus reduce the range of transmission. DPC promises to solve this problem. Basically, the DPC theory says that if you know both signals which are being transmitted you therefore know the interference (and the effect of the interference) and you should be able to alter the signals so that the interference restores the signal so that the receiver receives the intended signal.

This sounds easy, but it's not. That's because if you change one of the signals then the interference seen also changes, which requires you to change the other signal, which again changes the interference. So with complicated signals being transmitted over a Wi-Fi base station it's very difficult to calculate the changes required for DPC. It is even harder to do it fast enough that the user does not notice a lag.

Multiple Source for Single User

Multiple Source for Single User (MSSU) is something I simply can not find any information about on the Internet; it is something I have thought about that I think should be available soon. In this scenario there is only one user connecting to a MIMO base station. Now, instead of each antennae transmitting a copy of the same data, the data could be split and each antennae could transmit part of the data which could then be reassembled by the receiver. It's kind of like your own personal torrent. In theory, the user could receive the same amount of data in half of the time. That would not be what is seen in practice though; for starters the wireless part of the total path the data travels would have to be a bottle neck in order to see any improvements at all. Of course the wireless path is usually not the bottle neck of the data path, which is probably why I have not been able to find any information about this. But, I think this is a possibility in the future and I will be waiting for it to come.

If you would like to be notified when Russell Hitchcock releases the next part of this article series please sign up to the WindowsNetworking.com Real time article update newsletter.

If you would like to read the first part of this article series please go to The evolution and future of Wi-Fi (Part 1).

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