Белая книга для Cisco Cisco Aironet 3700i Access Point

 

 

© 2014 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. 

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Diversity gain is more important than redundancy gain. Due to channel fading, one antenna may see a deep fade, 

resulting in a very low signal level received on that antenna. For three spatial data streams, one needs a minimum 

of three strong signals so that three data streams can be resolved. If just one of them experiences deep fading, the 

MIMO detection will fail. By comparison, with 4x4 MIMO, if one antenna experiences a deep fade, there are still 

three good signals from which three data streams can be resolved. In actual implementations, the signals from all 

four antennas are carefully weighted according to quality, ensuring that the signals with better quality are utilized. 

4x4 MIMO has markedly better performance in fading channels due to its balanced hybrid of spatial multiplexing for 

speed and diversity gain for robustness. 

The technical term for the way the fourth receiver gets used is MIMO equalization. MIMO equalization is the 

comprehensive means to make best use of the received signal, whether it is transmit beamformed, space-time 

block-coded, spatially expanded, or unimproved. For MIMO equalization, having more receive chains helps the 

most, with the biggest gain coming from one extra receive chain, and diminishing returns thereafter. Accordingly, a 

3x3 access point is a reasonable choice for receiving two spatial streams, but not for three spatial streams. 

The Gory Details: Downlink 

To get a parallel benefit in the downlink direction (from the fourth transmitter), Cisco created ClientLink 3.0, which 

is a scheme to fully exploit the four MIMO transmit chains. It combines beamforming with spatial multiplexing to 

improve the speed and reliability of downlink traffic. Even when clients transmit frames with fewer spatial streams 

than antennas, ClientLink 3.0 uses its advanced, patent-pending algorithms to provide full beamforming gain, 

whether clients help sound out the channel or not. Furthermore, Cisco access points are fully engineered to avoid 

any dependence on clients for calibration assistance. However, if such client assistance is available, ClientLink 3.0 

is ready to take advantage of it. (802.11ac defines a single, though optional, protocol for an 802.11ac device to 

assist an 802.11ac access point. The protocol selected closely follows the 802.11n Explicit Compressed 

Beamforming Feedback protocol. Se

e the section “Standards-Based Beamforming” for additional information.) 

Consider the case of a client limited to three transceivers and trying to receive three spatial streams. Detection is 

very complicated and sensitive to poor signal quality, so any help from the access point is very welcome. As Figure 

1 illustrates, using their four transmit chains, Cisco Aironet 3700 Series Access Points have the degrees of 

freedom to form three spatial beams (where each beam carries one stream of data) and direct them to each of the 

receive chains at the client. These beamformed signals add up in-phase at the receiver and thereby combat 

channel fading. 

Figure 1.    Beamformed Signal Projection with Fourth Transceiver