New technologies in 802.11n

Michael Kassner TechRepublic

Published: 24 Jun 2008

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Go to section: Firmware enhancements MIMO and multipath environments MIMO really needs smart antennas MIMO really needs smart antennas (cont.)

802.11n: MIMO really needs smart antennas
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 that 802.11n's RF technology needs to be rock-solid — just like Ethernet cables — while facing ever-changing environmental conditions.

Initially it seemed entirely possible: 802.11n's new RF technology was certainly enough to take on all real-world demands, but doubts have crept in. Before explaining why, it's important to understand the 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 flavours. The first flavour 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 flavour 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.

Many people will argue that the previously mentioned types of interference exist in both wired and wireless networks. That's true, with the exception of multipath interference or fading, which is unique to RF propagation. The simple reality is that 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 (courtesy of Ruckus Wireless) graphically shows the extent of signal reduction caused by interference.

Pre-802.11n solutions
Prior to 802.11n there were various methods to reduce the effects of interference. Most helped to a limited extent. 802.11n uses RF technology based on MIMO, antenna diversity and spatial multiplexing to help deal with the above-mentioned challenges. Let's 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.

MIMO: spatial multiplexing
Earlier in the article we mentioned that RF signals will be altered as they traverse multipath environments. 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.

If everything is working correctly, one 802.11n device using spatial multiplexing will transmit a unique data stream using N 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.

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. The 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? Too much may be 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.

 

Go to section: Firmware enhancements MIMO and multipath environments MIMO really needs smart antennas MIMO really needs smart antennas (cont.)

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