Cisco Cisco Aironet 3700i Access Point 白皮書

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In the end, the fourth transceiver on the access point gives extra decibels of link margin, which translates to better

performance. As this paper will explain, in the uplink direction (client to access point), the extra receiver allows for

MIMO equalization gain. This means that 1300 Mbps can be achieved at twice the distance of existing solutions

(30 feet instead of 15 feet).

In the downlink direction, the extra transmitter allows for beamforming to the client. The Cisco implementation of

beamforming, known as ClientLink 3.0, was designed to work with all 802.11ac clients-both those that support

standards-based beamforming and those th

at don’t. This includes support for beamforming to clients that support

three, two, or even one spatial stream.

So, while a 3x3:3 architecture offers only a nice peak-data story, the 4x4:3 Cisco Aironet 3700 Series offers both a

high peak-data rate and real-world utility. Of course, building a 4x4:3 solution requires a custom chipset and

additional engineering, so it takes a customer-responsive vendor such as Cisco to deliver it.

For real-world benefit, three-spatial-stream devices must work with usable range for a typical enterprise use. To get

this, you really need the Cisco Aironet 3700 Series. Competitive designs deliver data rates of 1300 Mbps only at a

range of 15 feet or less. And since access points are usually ceiling mounted, a range of 15 feet does not cover 15

feet of floor space. Because the signal must also travel from the ceiling to the typical height of a device, the area

covered at the maximum rate by current designs is closer to 12 feet from the access point. With the Aironet 3700

Series, the usable range for three spatial streams at 1300 Mbps goes to 30 feet. This doubling in range actually

results in a 500 percent increase in the coverage area that can achieve 1300 Mbps.

The features in the Cisco Aironet 3700 Series also yield excellent benefits with older 802.11n clients and with

802.11ac devices that support fewer than three spatial streams. These benefits are very important, because it will

take a long time for all clients to support three spatial streams, and many phones will likely stay at one spatial

stream (even though they are 802.11ac and can support 256-QAM).

The Gory Details: Uplink

Although three MIMO receivers are theoretically sufficient to handle three spatial streams, the problem is that this

configuration provides no redundancy against channel fading or any of the inevitable hardware impairments: its

operation is disappointingly short range or erratic in practice. For this reason, in the Cisco Aironet 3600 Series,

Cisco leapfrogged the products that depend on pure spatial multiplexing and delivered hybrid spatial multiplexing

and diversity with the addition of a fourth receiver. Cisco continues this uncompromising approach in the Aironet

3700 Series.

Cisco’s solution is akin to GPS positioning: to determine the latitude and longitude of a GPS receiver, three GPS

satellite signals are necessary. More than three GPS satellites are required for superior location accuracy, and

even entry-level GPS products can track 12 satellites or more.

The benefits of adding the fourth receiver to support three spatial data streams come from redundancy gain and

diversity gain.

Redundancy Gain

The signal captured by the extra antenna provides some redundancy gain. As in linear algebra, the fourth receiver

provides four equations while there are only three unknowns. The extra equation provides an additional dimension

of freedom when resolving the transmitted signals. On its own, redundancy gain increases range by about 10

percent.