Cisco Cisco Aironet 1140 Access Point 백서

White Paper

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To understand the improvement brought by MIMO technology, it is important to understand some basic radio

operating principles. In a radio system, the amount of information that can be carried by a signal depends on the

signal-to-noise ratio, or SNR, typically expressed in decibels (dB). The greater the SNR the more information is

carried on the signal and recovered by the receiver.

In a typical indoor WLAN deployment at an office, hospital, or warehouse the radio signal rarely takes the direct,

shortest path from transmitter to receiver because walls, doors, or other structures obscure the line of sight. Luckily,

most of these environments are full of surfaces that reflect a radio signal similarly to how a mirror reflects light. When

a signal travels over different paths to a receiver, the signal traveling the shortest path arrives first, followed by

copies or echoes of the signal slightly delayed by each of the longer paths. This situation is called multipath

propagation (see Figure 1). Multipath conditions are constantly changing as clients, people, and objects move

throughout the network of access points.

Figure 1. Multipath Propagation

802.11n systems take advantage of multipath by sending multiple radio signals at the same time. Each of these

signals, called a spatial stream, is sent from its own antenna using its own transmitter. Because there is some

space between these antennas, each signal follows a slightly different path to the receiver, a situation called spatial

diversity. The receiver has multiple antennas as well, each with its own radio that independently decodes the

arriving signals, and each signal is combined with the signals from the other receive radios. The result is that

multiple data streams are received at the same time. This enables much higher throughput than previous 802.11a/g

systems, but requires an 802.11n capable client to decipher the signal.

802.11n also specifies how MIMO technology can be used to improve SNR at the receiver by using transmit

beamforming. With this technique, it is possible to coordinate the signal sent from each antenna so that the signal

at the receiver is improved. In other words, in addition to sending multiple streams, multiple transmitters can also be

used to achieve a higher SNR (and therefore data rate) on each stream. But there are two things to note: 1) The

transmit beamforming specified in the 802.11n standard requires cooperation from the receiver to feed back training

and/or channel information about the received signal to the transmitter—so both AP and client need to support this

capability. 2) Due to the complexity issues, in the first generation of mainstream 802.11n chipsets, neither the AP

nor client chipsets implemented 802.11n transmit beamforming. So, 802.11n standard transmit beamforming will

come eventually, but not until the next generation of chipsets take hold in the market. Cisco intends to lead in this

area going forward.