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Understanding 802.11n MIMO
and Multiple Spatial Streams

Multiple-Input / Multiple-Output (MIMO) is the often misunderstood 802.11n capability that allows more than one antenna to transmit a separate data stream at the same time and on the same channel thereby multiplying the throughput accordingly

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The Path to Understanding 802.11n Multiple-Input / Multiple-Output (MIMO)
 

RF Signal Propagation and Bit Transmission

Key terms explained in this section that you need to understand to fully appreciate 802.11n MIMO:
sine wave, channel, coding, error-detection bits, carrier signal, modulation,
input, direct ray, multipath signals, output, demodulation, bit recovery

A circuit in a transmitter creates an electrical signal that constantly and regularly varies in voltage at a fixed frequency. This is a sine wave and the frequency is in the center of the allowable lower and upper boundaries of a frequency range called the channel. A stream of data bits will now be transmitted. The pattern of data bits is determined by the application that created them. The bits could represent data in a file, text characters in an email or web page, elements of an image, digitized voice or many other types of information. Some patterns of bits are more easily distorted when they're converted to RF signals for transmission. For example, a long, repetitive series of binary 1's or 0's can cause a receiver to fall out of synchronization with the sequence. The bit stream is mathematically adjusted to minimize potential distortions (a process called coding) and error-detection bits are added to help the receiver confirm that an uncorrupted stream of bits was received. The center-frequency sine wave is then altered so that the varying electrical signal represents the stream of data bits. The sine wave now carries the data stream and is referred to as the carrier signal. There are many ways to alter the sine wave, each method representing a greater number of individual bits in a shorter number of sine wave cycles. Each method results in a different bit rate. The process of altering the center-frequency sine wave to represent bits is called modulation.

The circuit that modulates the sine wave is ultimately connected to an antenna where the electrical signals become predominantly magnetic signals and an electromagnetic wave propagates outwards from the antenna and into the transmission medium (which is, in this case, the air). An antenna transmitting a signal is said to create an input to the medium.

The electromagnetic signal travels through space where it may reach a receiver antenna without bouncing off a reflective surface. This part of the electromagnetic signal is called a direct ray. The electromagnetic signal may also be reflected off numerous objects in the environment and some of those reflections may ultimately reach the receiver's antenna. Because each reflected signal travels a different path from the transmitter to the receiver they are referred to as multipath signals.

 
'Input' is when a signal is transmitted into the air and 'Output' is when a signal is received from the air.

The magnetic aspect of the received electromagnetic signal induces an electrical current into the receiver's antenna. The signal comes out of the medium (i.e. out of the air) and this aspect of the process is referred to as signal output. The original modulation (whereby the carrier was modulated to represent the bit stream) is reversed through a process called demodulation. The original bits are extracted from the demodulated carrier signal (a process called bit recovery) and the transmission is complete.

Signal Reflection and Multipath Distortion

Key terms explained in this section that you need to understand to fully appreciate 802.11n MIMO:
out-of-phase, amplitude, constructive interference, destructive interference, multipath distortion

     

As an electromagnetic signal propagates outwards from a transmitting antenna it may have a direct ray and many (actually an infinite number) of reflected multipath rays. Just like members of a marching band who march exactly in step to the music the direct and multipath rays remain exactly at the same frequency and the rise and fall of the modulated sine wave (the 'phase' of the signal) remains exactly the same.

Shown below is a diagram of two reflected rays depicted as two marching band members taking different paths across the football field. Just like electromagnetic signals are reflected by objects in the environment the diagram shows the paths of the two band members being 'reflected' as they reach the sidelines of the field. The arrows on the path each represent a ten foot long path. Both marchers traveled 110 feet. As they approach the 'X' in the lower left-hand corner it can be seen that while the red path has arrived at the 'X' the yellow path is not quite there. When the marcher on the yellow path reaches the 'X' he will have traveled a slightly longer distance than the one on the red path. Since the electromagnetic sine wave carrier remains constant this means that the two reflections will arrive at the 'X' slightly out of phase with each other. The crests and troughs of the two waves no longer line up because they're out-of-step.

 
Members of a band march in exact step to the music just like two signal paths maintain the exact same frequency and phase relationship

When the two reflected signals reach the 'X' the signal traveling across the yellow path
will have traveled a longer distance than the one on the red path. The two signals will
arrive slightly out of phase with each other.
 

When the crests of two waves are synchronized (i.e. they 'line up with each other' - referred to as being in phase) the two waves add together and the resulting waveform has a greater amplitude (the height of the wave crest representing electrical voltage level) than either single wave. When the two waves add together the result is referred to as constructive interference. If the two waves met and they were exactly in opposition (one wave's crest met the other wave's trough) they cancel each other out. They are 180 degrees out of phase and the result is destructive interference. Of course, there are an infinite number of possible phase relationships between fully synchronized, matching constructive interference and fully opposed, canceling destructive interference. The point is that when two waves meet the result is either constructive or destructive to some degree unless they're perfectly synchronized. Two or more signal reflections will not be perfectly in phase unless the path that they take is exactly the same length. Almost all reflections will arrive slightly (or completely) out of phase with each other.

 
CONSTRUCTIVE
INTERFERENCE
(Waves add together)

DESTRUCTIVE
INTERFERENCE
(Waves cancel each other out)
 

Signals are reflected from surfaces and objects in the environment. A drywall partition may reflect between 20% and 40% of the 2.4 GHz or 5.8 GHz signal that strikes it. As a result, a room or other coverage area is filled with reflections of varying strength. A particular building or outdoor area will have an ever-changing pattern of signals bouncing around in it.

To help visualize the reflective aspects of a coverage area the two animations below are presented. On the left, a single transmitter is propagating a signal into the room. On the right there are two signals to represent two reflected rays of a transmission. The right-hand animation provides a conceptual view of how two signals interact with each other.

 
 
Above we see a single RF signal being transmitted from an antenna and propagating outwards in all directions. The light areas represent the peaks of the signal envelope and the dark areas are the valleys between them.
 
Above, the propagated wave has hit an obstruction and we see two rays of resulting reflected signal bouncing back. The dark areas are the result of destructive interference and the light areas are the result of constructive interference.
 
At a location where signals interact destructively a wireless client device may not be able to connect or may have significantly reduced throughput. This is called "multipath distortion". Because a conventional Wi-Fi device is unable to separate the two (or more) reflected signals it can only try to recover bits from the resulting, corrupted signal. Multipath distortion, also referred to as "the multipath problem" is a serious concern for 802.11b, 802.11g and 802.11a radio equipment. With 802.11n the two (or more) signals can be differentiated (in some cases) and multipath reflections actually help improve throughput. This is the basis for "Multiple Input / Multiple Output" (MIMO) and the concept of multiple "spatial streams" in 802.11n.

Overcoming Multipath Distortion Using Antenna Diversity

Key terms explained in this section that you need to understand to fully appreciate 802.11n MIMO:
antenna diversity, signal wavelength, reflected rays

In the 802.11abg world a mechanism was needed to help minimize multipath distortion. This mechanism is called "antenna diversity" and it involves equipping the receiver with two antennas. The signal received by each antenna is sampled and the higher quality signal is used as the basis for recovering bits.

Consider two antennas that are separated by 1.5 times the signal wavelength. For a 2.4 GHz signal one full wavelength is is roughly 4.5" (and roughly 2" at 5.8 GHz). There is a chance that if significant destructive interference occurs at one of the antennas then it may not be occurring at the other (because of the spacing). The diagram below depicts this situation. A signal is transmitted from client device on the left. The diagram shows four rays which are reflected from different parts of the building. As it turns out in the example the black and gold rays happen to arrive at the left-hand antenna in phase (constructive interference) but the yellow and red rays happen to arrive at the right-hand antenna out of phase (destructive interference). The circuitry in the access point, acting as if it were an A-B switch, selects the right-hand antenna for reception. This access point has a pair of "diversity antennas" and the effects of multipath distortion (lost data, retransmissions, poor performance) are reduced.

 
The access point selects the antenna experiencing
the least amount of multipath distortion

Moving From Diversity to MIMO

Key terms explained in this section that you need to understand to fully appreciate 802.11n MIMO:
cluster, spatial stream, multiple input, multiple output, MIMO

In the example given by the diagram above, imagine what would happen if the client device moved to the right by 1/2 wavelength. It could be imagined that there would be a position that would result in the right-hand antenna now being the better choice. This is the situation depicted in the diagram below. Now the black and orange ("good") rays are converging at the right-hand antenna.

 
If the client device moves to the right it may be possible
for the right-hand access point antenna to be the better receiver
 
It was from this concept that the engineering community devised a remarkably clever scheme for doubling the capacity of a single wireless channel. If two antennas are available for simultaneous transmission and two antennas are available for reception then there may be some locations where the left-hand transmission antenna is the "good" signal for the left-hand receiver antenna and, at the same time, the right-hand transmission antenna is the "good" antenna for the right-hand receiver antenna. Remember that there is no guarantee that all locations for all client devices will actually be in a place where this left/right relationship will be possible - but some locations will have this dual relationship. The locations where two simultaneous transmissions on the same channel can be differentiated by the two receiver antennas is called a "cluster" location. The two transmissions are referred to as two "spatial streams" and the result is that multiple transmissions are made into the air ("multiple input") and two spatial streams are recovered from the air ("multiple output") - and this is 802.11n MIMO. The situation is depicted below.
 
With two antennas the client device can transmit two spatial streams in a MIMO system
   

Conclusions

Unlike using a 40 MHz channel instead of a 20 MHz channel to double the channel capacity, MIMO increases the capacity of the channel by transmitting multiple spatial streams. They are transmitted at the same time and at the same frequency within the same channel width. The receiver differentiates them because they take different reflective paths through the environment. There is no guarantee that a transmitter and receiver will be able to achieve more than one spatial stream. MIMO, therefore, is a statistical probability not an absolute guarantee. As it turns out, it's a very good statistical probability and the formation of MIMO cluster areas (where multiple spatial streams are achievable) is fairly typical.

The Draft 2.0 802.11n standards define up to 2 spatial streams which allows the channel capacity to double. The final 802.11n standard, completed in September 2009, defines three and four spatial stream mechanisms which allows 802.11n to achieve channel rates up to 600 Mbps (i.e. 72.5 Mbps in a 20 MHz channel doubled to 150 Mbps in a 40 MHz channel multiplied by the number of spatial streams achieved. 4 spatial streams = 600 Mbps)

 

Web References Relating to 802.11n Technology and Engineering

You'll find numerous Web references and reviews of the IEEE 802.11n standards and 802.11n equipment, 802.11n products and 802.11n services that are available. Below are some targeted discussions to provide you with additional perspective on how an 802.11n solution can be implemented to meet your requirements for a secure, easy-to-manage wireless LAN system.

 
IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO) and 40 MHz channels to the PHY (physical layer), and frame aggregation to the MAC layer.

MIMO is a technology which uses multiple antennas to coherently resolve more information than possible using a single antenna. One way it provides this is through Spatial Division Multiplexing (SDM). SDM spatially multiplexes multiple independent data streams, transferred simultaneously within one spectral channel of bandwidth. MIMO SDM can significantly increase data throughput as the number of resolved spatial data streams is increased. Each spatial stream requires a discrete antenna at both the transmitter and the receiver. In addition, MIMO technology requires a separate radio frequency chain and analog-to-digital converter for each MIMO antenna which translates to higher implementation costs compared to non-MIMO systems.

40 MHz channels is another feature incorporated into 802.11n which doubles the channel width from 20 MHz in previous 802.11 PHYs to transmit data. This allows for a doubling of the PHY data rate over a single 20 MHz channel. It can be enabled in the 5 GHz mode, or within the 2.4 GHz if there is knowledge that it will not interfere with any other 802.11 or non-802.11 (such as Bluetooth) system using those same frequencies.

Coupling MIMO architecture with wider bandwidth channels offers increased physical transfer rate over 802.11a (5 GHz) and 802.11g (2.4 GHz).

Multiple Input, Multiple Output (MIMO) technology is a wireless technology that uses multiple transmitters and receivers to transfer more data at the same time. MIMO technology takes advantage of a radio-wave phenomenon called multipath where transmitted information bounces off walls, ceilings, and other objects, reaching the receiving antenna multiple times via different angles and at slightly different times.

MIMO technology leverages multipath behavior by using multiple, “smart” transmitters and receivers with an added “spatial” dimension to dramatically increase performance and range. MIMO allows multiple antennas to send and receive multiple spatial streams at the same time. This allows antennas to transmit and receive simultaneously.

MIMO makes antennas work smarter by enabling them to combine data streams arriving from different paths and at different times to effectively increase receiver signal-capturing power. Smart antennas use spatial diversity technology, which puts surplus antennas to good use. If there are more antennas than spatial streams, as in a 2x3 (two transmitting, three receiving) antenna configuration, then the third antenna can add receiver diversity and increase range.

In order to implement MIMO, either the station (mobile device) or the access point (AP) need to support MIMO. Optimal performance and range can only be obtained when both the station and the AP support MIMO.

Legacy wireless devices can’t take advantage of multipath because they use a Single Input, Single Output (SISO) technology. Systems that use SISO can only send or receive a single spatial stream at one time.