Cisco Cisco Aironet 3700i Access Point 白皮書
![Cisco](https://files.manualsbrain.com/attachments/7380d0050044647c30f5c24bbbf5d0c0b6d9bb84/common/fit/150/50/faa183d287233c52228cfea3dbc2a127fe780f60564fcb0955d9c3d1cd23/brand_logo.png)
© 2014 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information.
Page 4 of 10
Diversity Gain
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