Networking

802.11n: MIMO really needs smart antennas

I remain very optimistic about 802.11n being a disruptive technology that will alter everyone’s perception of data networks. If smart antenna systems and beam forming are included in the mix, it's a for sure thing.

As Wi-Fi standards go, 802.11n has a lot to live up to, especially after hearing how 802.11n's advertised throughput, security, and reliability will allow Wi-Fi to replace existing wired networks. This means 802.11n's RF technology needs to be rock-solid, just like Ethernet cables, while facing ever-changing environmental conditions.

Initially I felt it was entirely possible. 802.11n's new RF technology was certainly enough to take on all real-world demands, but I'm not so sure now. I'd like to explain, but before doing so I feel it's important to really understand what challenges 802.11n technology must overcome in order to become rock-solid. To begin with, Ethernet bits flow nicely through solid amorphous materials like copper, whereas Wi-Fi bits travel through a variety of media and environments, which can affect the following parameters:

  • Received signal strength is dependent on the distance between the transmitter and receiver. Physical obstructions along the link path that absorb or disperse the RF signal also affect signal strength. Ultimately, received signal strength must exceed the receiver's noise floor by a certain amount; otherwise, the signal cannot be processed.
  • In-band RF interference comes in two flavors. The first flavor is non-802.11 RF capable devices like cordless phones or microwaves, which happen to share the same frequency band as Wi-Fi networks. The second flavor pertains to co-channel and or adjacent channel interference from other Wi-Fi networks. Both types of interference if strong enough will create sufficient RF noise to make it difficult or impossible for the receiver to distinguish between the interference and real traffic.
  • Out of band RF interference is something most people don't think about. This interference emanates from devices that are not normally considered RF transmitters. Any electromagnetic (fluorescent light) or thermal (lightning) radiation has the potential to disrupt the RF link between two Wi-Fi devices.
  • Multipath interference or fading occurs when a RF signal encounters objects on its way to the receiving antenna. These objects could reflect or refract the original RF signal, creating variations that have different timing and phase characteristics. When the original RF signal and variations reach the destination antenna, that receiver usually has a difficult time trying to sort out what's what. I went into more detail about this subject in an article named "Multipath environments and how they affect Wi-Fi propagation."

Many people will argue that the previously mentioned types of interference exist in both wired and wireless networks. I agree, with the exception of multipath interference or fading, which is unique to RF propagation. The TRSFC crew may disagree and bring up the topic of electron or photon barrier activity in a captive medium, but that's another topic. The simple reality is Wi-Fi networks are much more susceptible to interference than wired networks.

The fallout from poor signal quality is the retransmission of digital traffic to meet TCP/IP requirements of error-free data transmission. With sufficient errors, the connected 802.11 devices will renegotiate the transmission rate incrementally until the error count is below a set level, which dominos into lower data throughput and decreased network efficiency. The following chart graphically shows the extent of signal reduction caused by interference. I'd like to thank Ruckus Wireless for use of the chart.

signal-strength-versus-noise-rev.jpg

Pre-802.11n solutions

Prior to 802.11n there were various methods to reduce the affects created by interference. Most helped to a limited extent, and I wrote an article "How to make the best of 802.11 multipath environments" that looks at the different solutions.

Now that we are on the same page as to what a RF signal has to contend with on its way to the receiving antenna, let's proceed to the next topic. 802.11n uses RF technology based on MIMO, antenna diversity, and spatial multiplexing to help deal with the above-mentioned challenges. I'd like to take a few moments to explain the inner-workings of MIMO as a prelude to pointing out why MIMO in and of itself is not the definitive answer.

MIMO: Antenna diversity

Antenna diversity isn't new to Wi-Fi technology. It's just becoming official as part of the 802.11n standard. Wikipedia does a great job of explaining antenna diversity:

"Antenna diversity is especially effective at mitigating multipath situations. This is because multiple antennas afford a receiver several observations of the same signal. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal. Collectively such a system can provide a robust link. While this is primarily seen in receiving systems (diversity reception), the analog has also proven valuable for transmitting systems (transmit diversity) as well.

Antenna diversity can be simple as "receive selection combining." Where a multi-antenna device transmits using the same antenna from which it just successfully received digital traffic. Or as complicated as equipment using "maximum ratio combining," which allows multiple RF signals to be sent simultaneously between two proprietary devices. The following graphs from Ruckus Wireless show the difference in signal gain between the two different approaches.

receive-diversity.jpg

MIMO: Spatial multiplexing

If you remember earlier in the article I mentioned that RF signals will be altered as they traverse multipath environments. Well, spatial multiplexing is counting on that, as it's the only way a receiving 802.11n device will be able to distinguish between the different RF signals. The Ruckus Wireless chart below depicting spatial multiplexing helps explain the process. As you can see in the first graph, the signals are similar enough to make it difficult to distinguish the two, whereas the second graph depicts two uncorrelated signals.

spatial-multiplexing.jpg

If everything is working correctly, one 802.11n device using spatial multiplexing will transmit a unique data stream using N (number of antennas) antennas. The receiving 802.11n device with at least N antennas will then receive N unique data streams. Therefore, the link's total throughput capacity is equal to the individual data throughput multiplied by N antennas. If you're interested, I went into more detail about this in the article "802.11n, MIMO, and multipath environments."

MIMO: kind of hit or miss

Now it's easy to see how antenna diversity and spatial multiplexing theoretically improve throughput and the reliability of Wi-Fi networks. My concern is what happens when dealing with real-world environments that are constantly changing. For example, if there isn't enough alteration to a RF signal, the receiver using spatial multiplexing will not be able to distinguish it from the rest. Another example pertains to antenna diversity. What if it's a bad assumption to transmit using the same antenna that worked the best for receiving? Seems to me that too much is left to chance. 802.11n networks need to be more self-determining and less reliant on the RF environment if they are going to compete with wired networks.

Smart antennas and beamforming

It took awhile but with all that background information, we can now tackle smart antenna technology. The term smart antenna in reality is a misnomer as all of the intelligent signal conditioning takes place before the RF signal gets to the appropriate set of antennas. Beamforming is the technology that does all the hard work. The following definition is from a University of Washington website. It's the best explanation of beamforming I've come across. The site even has interactive models to help explain the technology.

"Beamforming is a general signal processing technique used to control the directionality of the reception or transmission of a signal on a transducer array.

Using beamforming you can direct the majority of signal energy you transmit from a group of transducers (like audio speakers or radio antennae) in a chosen angular direction. Or you can calibrate your group of transducers when receiving signals such that you predominantly receive from a chosen angular direction."

Beamforming isn't new, being a key component of both radar and sonar systems for many years. Recently, telco and Wi-Fi researchers have become interested in beamforming and the ability to steer signals to where they do the most good. Ruckus Wireless is one such company and has a great deal of research expertise in beamforming. Ruckus Wireless also has been instrumental in introducing products into the Wi-Fi market that have beamforming capabilities. BeamFlex is their interpretation of beamforming, and the following description comes from one of their technical articles:

"Central to BeamFlex is an agile antenna system with multiple antenna elements that can be combined in real time to offer an exponential increase in diversity order. With N number of high-gain, directional antenna elements, a BeamFlex antenna array provides 2N-1 unique radiating patterns to maxi­mize range and coverage in a home.

A Diversity Combiner composed of low cost, software-controlled circuitry allows the BeamFlex software to manage antenna combining in real time. The core of the BeamFlex software is an expert system that constantly learns the environment - the RF conditions, communicating devices, network performance and application flows.

A Path Control module selects optimum antenna combinations on a per packet basis to ensure a quality signal path to each receiving device.

The Transmission Control module sets the transmission policies including data rate and queuing strategy based on application and station knowledge. The BeamFlex software interfaces to the 802.11 MAC layer and is compatible with standard 802.11 chipsets. Residing in the host processor, it adds minimal incremental CPU load and memory utilization."

In my research on smart antenna systems and beam forming, the Ruckus Wireless approach has surfaced as a very elegant design. It has the potential to alleviate my concerns about the inability of MIMO and spatial multiplexing to be reliable enough. The individual advantages are as follows:

  • BleamFlex antenna arrays can rapidly present many different antenna configurations. Which translates into significantly different RF signal patterns that will afford spatial multiplexing technology the best opportunity of success.
  • BeamFlex antenna arrays use both horizontal and vertical polarized antenna elements, once again, to create RF signal patterns with increased diversity and ensure recognition by the 802.11n receiver using spatial multiplexing.
  • BeamFlex architecture uses application-level performance parameters when making decisions on how to optimize the signal quality rather than information from the PHY and MAC layer that doesn't take into account QoS or application networking requirements.

The following diagram depicts current equipment from Ruckus Wireless, which include all of the above-mentioned features.

beamflex1.jpg

I'm more interested in a symbiotic relationship between the BeamFlex antenna and 802.11n technology so as to have the best of both worlds. Ruckus is continuing work on this front as shown in the following diagram.

beamflex2.jpg

Final thoughts

I remain very optimistic about 802.11n being a disruptive technology that will alter everyone's perception of data networks. 802.11n's antenna diversity and spatial multiplexing are vast improvements over what's been available in previous standards. I'm just concerned that the required reliability will not be there until additional RF signal conditioning like that offered by Ruckus Wireless is used to combat environmental variables.

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