23 Jun 2008 14:21
The proposed 802.11n standard incorporates some impressive improvements to the physical hardware and firmware. In this guide, we first look at the main enhancements, then discuss MIMO and multipath environments in more detail, and finally examine why MIMO may require an added layer of intelligence to cope with real-world environments.
802.11n firmware enhancements
The physical data rate selection algorithm has the tough job of determining what data transfer rate should be used based on the measured signal strength. Whereas 802.11a/g uses 12 steps from 1Mbps to 54Mbps, 802.11n has a total of 88 incremental data rate steps, which provides a more granular drop-off when the signal strength weakens.
Whereas 802.11a/g uses transmit diversity, which is useful and logical as the device transmits from the antenna that displayed the best reception characteristics during the last receive cycle, 802.11n uses spatial multiplexing. This technique divides the information to be transmitted into independent and separately encoded data signals called streams. Each data stream is then transmitted from an independent antenna. The 802.11n standard allows up to four transmit streams. This is an interesting concept that increases data transmission capacity by multiplexing or reusing the space dimension multiple times.
To be able to transmit multiple data streams, designers had to rework OFDM (Orthogonal Frequency Division Multiplexing), which is the digital multi-carrier modulation scheme used by 802.11a/g. MIMO-OFDM, used by 802.11n devices, is the result. This development is extraordinary, as evidenced by the fact that it took almost ten years and a great deal of innovative work to perfect. MIMO-OFDM is viewed by many as the single most significant development of 802.11n.
802.11a/g networks do not work well in multipath environments: having multiple (and slightly different in phase or timing due to the environment) copies of the same transmitted RF signal arrive at the receiving antenna drives the receiver nuts, to put it politely. Since the real world is mostly a multipath environment, 802.11n was developed to make use of the slight differences exhibited by the arriving RF signals to distinguish the different data streams being transmitted. We discuss this subject in the next section of this guide.
Channel size is one determinant of how much data can be passed over a wireless link. The 802.11a/g standard uses 20MHz channels and history has proven that amount of bandwidth to be a limiting factor. In the past few years equipment developers have tried to improve the physical transfer rate of data by using proprietary technology combining adjacent channels to support greater data rates. The 802.11n standard describes how to use the much wider 40MHz channels, which are easy to implement, cost effective and only require moderate increases in digital signal processing. If properly implemented, 40MHz channels can provide greater than two times the usable channel bandwidth.
By design, TCP/IP traffic requires error-free transmission of data, and one of the controls used to regulate traffic processing is the ACK bit. The ACK bit is sent by the receiver to acknowledge receipt of each frame, which is a significant management overhead just for receipt verification. One way to improve throughput is to devise a way to acknowledge the receipt more efficiently — and that's exactly what 802.11n does with the Block-ACK. By removing the need for one acknowledgment frame for every data frame, the amount of overhead required for the ACK frames, as well as preamble and framing, is reduced.
No stone was unturned when the developers were looking at ways to improve throughput and efficiency: even the lowly guard interval was tweaked. The guard interval is used to prevent data loss from propagation anomalies as well as interference created if the following transmission starts too soon. Can that interval be reduced? It would help throughput, even if just slightly. 802.11n specifies two guard intervals 400ns and 800ns. Under optimal conditions, the 802.11n device will drop down to the 400ns guard interval to reduce what's considered unnecessary idle time.
Other improvements
For the most part, these enhancements are already designed into the pre-release hardware. Still, 802.11n equipment developers are not satisfied with just these improvements. Smart antennas, multiple radios and mesh technology are some heavy-duty technologies that are being added to enterprise 802.11n appliances, which will allow 802.11n kit to approach wired network parameters.
802.11n, MIMO and multipath environments
One researcher and wireless pioneer, Dr Greg Raleigh, was especially instrumental in determining how to use multipath environments to an advantage. One of the better known developments resulting from the research is Multiple Input/Multiple Output (MIMO) smart antenna technology.
How MIMO works
Qualcomm, a wireless chipset developer that acquired Airgo, the company founded by Dr Raleigh, has the best definition of how MIMO works:
"MIMO systems divide a data stream into multiple unique streams, each of which is modulated and transmitted through a different radio-antenna chain at the same time in the same frequency channel. A revolutionary technique that reverses 100 years of thinking about how radio signals are transmitted, MIMO leverages environmental structures and takes advantage of multipath signal reflections to actually improve radio transmission performance.
Through the use of multipath, each MIMO receive antenna-radio chain is a linear combination of the multiple transmitted data streams. The data streams are separated at the receiver using MIMO algorithms that rely on estimates of all channels between each transmitter and each receiver. Each multipath route can then be treated as a separate channel creating multiple 'virtual wires' over which to transmit signals. MIMO employs multiple, spatially separated antennas to take advantage of these 'virtual wires' and transfer more data. In addition to multiplying throughput, range is increased because of an antenna diversity advantage, since each receive antenna has a measurement of each transmitted data stream. With MIMO, the maximum data rate per channel grows linearly with the number of different data streams that are transmitted in the same channel."
That, in a nutshell, describes the basic tenets behind MIMO antenna systems.
Two distinct environmental conditions
To avoid confusion when discussing MIMO and multipath propagation, it's important to define the two different yet related environmental conditions encountered by MIMO RF propagation: RF Line of Sight and RF Non-line of Sight.
RF Line of Sight (LoS): Under this condition, RF signal propagation — regardless if Single Input/Single Output (SISO) or MIMO technology — will not encounter any physical interference along the link path. This eliminates any multipath advantage gained by MIMO technology and any multipath fading disadvantage seen by SISO technology.
Even with a level playing field, MIMO technology still has a distinct advantage, because it uses a process called spatial multiplexing. In explanation, if a SISO system and a MIMO system are being supplied with an identical data stream, the MIMO system's data rate will be X times the data rate of the SISO system — where X is the number of receive/transmit antenna pairings. Even the minimal doubling of the data rate is quite significant when considering today's bandwidth-intensive applications.
RF Non-Line of Sight (NLoS) : This is a condition where the RF signal encounters significant physical interference along the link path and only altered RF signals reach the receiving antenna. These altered RF signals have the tendency to interfere with each other, often destructively which results in multipath fading — the bane of conventional radios using SISO technology.
Wireless pioneers like Dr Raleigh decided to take advantage of the specific multipath phenomena in which received signals from one transmitting antenna will have different phase, timing or signal-strength characteristics from received signals transmitted by a different antenna. This line of thinking brought about one of those all-too-seldom 'Ah-ha!' moments. Using multiple transmit/receive antenna pairings to overcome multipath fading also complements the concept of higher data rates being derived from having multiple RF streams.
The last piece of the puzzle, and where MIMO technology finally comes together, is the advent of new receiver technology. By using advanced digital signal processing hardware and very sophisticated algorithms that deal with space-time coding, it becomes possible to decipher the multipath-differentiated RF signals even though they are all on the same frequency.
Technology development timelines continue to shorten, the recent developments in 802.11n technology — especially MIMO — being one example. Even more important is the fact that these advances will effectively change how everyone accesses data networks and the internet.
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.
Smart antennas and beamforming
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 web site and is the best explanation of beamforming we'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 its interpretation of beamforming, and the following description comes from one of the company's 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 beamforming, the Ruckus Wireless approach has surfaced as a very elegant design. It has the potential to alleviate 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. This 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 polarised 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 optimise 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 includes all of the above-mentioned features.
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.
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.
Story URL: http://reviews.zdnet.co.uk/hardware/networking/0,1000000696,39437729,00.htm
Copyright © 1995-2009 CBS Interactive Limited. All rights reserved
ZDNET is a registered service mark of CBS Interactive Limited. ZDNET Logo is a service mark of CBS Interactive Limited.