To understand the improvement brought by MIMO technology, it is important to understand some of the basics that determine how well a traditional radio operates. In a traditional, single-input single-output radio, the amount of information that can be carried by a received radio signal depends on the amount by which the received signal strength exceeds the noise at the receiver, called the signal-to-noise ratio, or SNR. SNR is typically expressed in decibels (dB). The greater the SNR, the more information that can be carried on the signal and be recovered by the receiver.
To understand this situation, imagine the analogy of your eye as the receiver. Is your eye able to detect whether a table lamp is on or off in the house next door? In this analogy, ambient light is the noise. At night, detecting that the lamp is on or off is quite easy. However, in full daylight, it is much more difficult to make the same determination, because the ambient light is much brighter, and the tiny amount of additional light from the lamp can be undetectable.
Light, like a radio wave, disperses uniformly from its source. The farther the receiver is from the source, the less power is received from the source. In fact, the amount of power received decreases more rapidly than the square of the distance from the source. Noise, unfortunately, is often constant in the environment, due to both natural and man-made causes.
So, returning to the table lamp example, when it is too bright to determine if the lamp next door is on or off, it might be possible to make that determination from just outside the neighbor's window. Alternatively, it might be possible to make the determination if the neighbor changed the 40 watt bulb for a 150 watt bulb. In both cases, the SNR increases-in the first case, because the distance to the source is reduced, and in the second case, because the power of the transmitter is increased.
Once the minimum SNR is achieved to allow information to be exchanged at the desired rate, any additional SNR is like money in the bank. That additional SNR can be spent on increasing the information rate, increasing the distance, or a little bit of both. However, you can't spend the same dB more than once, just as you can't spend the same dollar more than once (at least not without encountering some unpleasant consequences).
All this is background to understand the improvements that MIMO technology brings to 802.11.
MIMO Technology: Beamforming
MIMO technology takes advantage of other techniques to improve the SNR at the receiver. One technique is transmit beamforming. When there is more than one transmit antenna, it is possible to coordinate the signal sent from each antenna so that the signal at the receiver is dramatically improved. This technique is generally used when the receiver has only a single antenna and when there are few obstructions or radio-reflective surfaces-for example, open storage yards.
To understand transmit beamforming, consider a radio signal as a wave shape, with a wave length that is specific to the frequency of the signal. When two radio signals are sent from different antennae, these signals are added together at the receiver's antenna (see Figure 1). Depending on the distance that each radio signal travels, they are very likely to arrive at the receiver out of phase with each other. This difference in phase at the receiver affects the overall signal strength of the received signal. By carefully adjusting the phase of the radio signals at the transmitter, the received signal can be maximized at the receiver, increasing SNR. This is what transmit beamforming does-it effectively focuses the transmitters on a single receiver, as shown in Figure 2.
Figure 1. Destructive Interference
Figure 2. Transmit Beanforming (Constructive Interference)
Transmit beamforming is not something that can easily be done at the transmitter without information from the receiver about the received signal. This feedback is available only from 802.11n devices, not from 802.11a, b, or g devices. To maximize the signal at the receiver, feedback from the receiver must be sent to the transmitter so that the transmitter can tune each signal it sends. This feedback is not immediate and is only valid for a short time. Any physical movement by the transmitter, receiver, or elements in the environment will quickly invalidate the parameters used for beamforming. The wave length for a 2.4-GHz radio is only 120mm, and only 55mm for 5-GHz radio. So, a normal walking pace of 1 meter per second will rapidly move the receiver out of the spot where the transmitter's beamforming efforts are most effective.
Transmit beamforming is useful only when transmitting to a single receiver. It is not possible to optimize the phase of the transmitted signals when sending broadcast or multicast transmissions. For this reason, in general networking applications, the utility of transmit beamforming is somewhat limited, providing improved SNR at the receiver for only those transmissions that are sent to that receiver alone. Transmit beamforming can increase the data rate available at greater distances from the AP. But, it does not increase the coverage area of an access point, since that is determined, in large part, by the ability to receive the beacons from the access point. Beacons are a broadcast transmission that does not benefit from transmit beamforming.
MIMO Technology: Multipath or Spatial Diversity
In typical indoor WLAN deployments-for example, offices, hospitals, and warehouses-the radio signal rarely takes the direct, shortest path from the transmitter to the receiver. This is because there is rarely "line of sight" between the transmitter and the receiver. Often there is a cube wall, door, or other structure that obscures the line of sight. All of these obstructions reduce the strength of the radio signal as it passes through them. Luckily, most of these environments are full of surfaces that reflect a radio signal as well as a mirror reflects light.
Imagine that all of the metallic surfaces, large and small, that are in an environment were actually mirrors. Nails and screws, door frames, ceiling suspension grids, and structural beams are all reflectors of radio signals. It would be possible to see the same WLAN access point in many of these mirrors simultaneously. Some of the images of the access point would be a direct reflection through a single mirror. Some images would be a reflection of a reflection. Still others would involve an even greater number of reflections. This phenomenon is called multipath (see Figure 3).
Figure 3. Multipath
When a signal travels over different paths to a single receiver, the time that the signal arrives at the receiver depends on the length of the path it traveled. The signal traveling the shortest path will arrive first, followed by copies or echoes of the signal slightly delayed by each of the longer paths that the copies traveled. When traveling at the speed of light, as radio signals do, the delays between the first signal to arrive and its copies is very small, only nanoseconds. (A rule of thumb for the distance covered at the speed of light is roughly one foot per one nanosecond.) This delay is enough to be able to cause significant degradation of the signal at a single antenna because all the copies interfere with the first signal to arrive.
A MIMO radio sends multiple radio signals at the same time and takes advantage of multipath. Each of these signals is called a spatial stream. Each spatial stream is sent from its own antenna, using its own transmitter. Because there is some space between each of these antennae, each signal follows a slightly different path to the receiver. This is called spatial diversity. Each radio can also send a different data stream from the other radios. The receiver has multiple antennas as well, each with its own radio. Each of the receive radios independently decode the arriving signals (see Figure 4.) Then, each radio's received signal is combined with the signals from the other receive radios. With a lot of complex math, the result is a much better receive signal than can be achieved with either a single antenna or even with transmit beamforming. One of the two significant benefits of MIMO is that it dramatically improves the SNR, providing more flexibility for the WLAN system designer.
Figure 4. Spatial Multiplexing
MIMO systems are described using the number of transmitters and receivers in the system-for example, 2x1 is "two by one," meaning two transmitters and one receiver. 802.11n defines a number of different combinations for the number of transmitters and the number of receivers, from 2x1, equivalent to transmit beamforming, to 4x4. Each additional transmitter or receiver in the system increases the SNR. However, the incremental gains from each additional transmitter or receiver diminish rapidly. The gain in SNR is large for each step from 2x1 to 2x2 and to 3x2, but the improvement with 3x3 and beyond is relatively small. The use of multiple transmitters provides the second significant benefit of MIMO, the ability to use each spatial stream to carry its own information, providing dramatically increased data rates.