Re: 802.11n與 beamforming問題
Re: 802.11n與 beamforming問題
06-24-2014 11:44 PM
Beamforming - is a general signal processing technique to control the direction of the signals transmitted in the direction of a receiver. Alternatively, the receiver can be calibrated in a way that it receives signals predominantly from a given direction. It is applied in various fields, like audio signals, cellular, WiFi, WiMAX communications, etc.
When applied to WLAN communication, beamforming offers several advantages. It helps focus RF transmissions in the direction of the receiver, which leads to:
- Improving the signal quality at the receiver: A better signal quality reduces errors and retransmissions, and also enables the two stations to use higher data rates for communication. This helps improve the overall system throughput
- Reduced interference: Since the beams are directed at the receiver, and not scattered in all directions, it helps in reducing interference with neighbouring cells. This is especially useful in dense deployments where the APs are placed close to each other for serving a large number of clients. An example is a classroom or an airplane.
- Increased range. Since beamforming focuses energy in a direction, like a high gain directional antenna, the beams can reach a larger area.
- Better coverage. Since beamforming makes use of multipath to it’s advantage, it is possible to direct signals in hard to reach areas like behind a pillar. It thus helps in reducing coverage holes.
There are some concerns about the effectiveness of beamforming in some scenarios:
- Moving clients: Is it possible to effectively focus the beams on moving clients. Since WLAN communication is primarily meant for mobile users, not able to cater to this segment may limit it’s usefulness, which is probably the main reason why most of the major vendors have stayed away from it.
- Legacy devices: Some of the methods may need both the stations to support the beamforming feature, like 802.11n transmit beamforming and will not work with legacy clients.
- Compromise on spatial multiplexing benefits: When using beamforming, as proposed in the 802.11n draft, in the current evolution of technology, we cannot use other MIMO feature like spatial multiplexing since the same signal is sent from multiple antennas and currently most manufacturers support only two antennas for transmitting. This will result in some sacrifice of throughput. So, the devices should have the intelligence to decide which method to use, based on the RF environment.
- For devices which are not in the direction of the beam the signals may be weak so they may unknowingly transmit causing interference.
- Broadcast packets are sent to all the stations which are associated to the access point. So, the benefits of beamforming cannot be realized for broadcast packets.
- Beamforming is not helpful in case of interference from other sources like bluetooth, microwave ovens and cordless phones.
Recently, beamforming has derived a lot of interest from different quarters. We’ll discuss some of the methods:
802.11n transmit beamforming:
Transmit beamforming (TxBF) is an optional part of the 802.11n standard and can be used to optimize the communication between the 802.11n stations by making the best use of multipath effect. Both sides should be 802.11n capable to use this feature.
We know that the effect of multipath is additive at some points in space such that the RF signal quality is enhanced at those points, while at other points, the multiple signals differ in phase such that they cancel each other, thus degrading the signal quality. 802.11n transmit beamforming uses this property in a way that the multiple signals received at the client are additive and result in a better signal quality at the client. This is done by sending copies of signals from the multiple antennas with different RF characteristics (eg. phase and amplitude). The signals traverse different paths and when they reach the receiver, they add up in a way that the RF signal is enhanced. There are 2 components contributing towards this gain:
- More RF Power resulting from transmission from multiple antennas.
- Making the best use of the Multipath effect so that the signals are focused at the receiver.
The 802.11n draft mentions 2 options for transmit beamforming:
Implicit feedback beamforming
In this method, the beamformer (transmitter) analyzes the RF signals (special training symbols) it receives from the beamformee (receiver) to make an estimate of the MIMO channel between them. The beamformer uses the channel estimate for controlling transmissions in a way that focuses RF signals at the receiver. This estimate will be valid if the channel is reciprocal such that the multipath effect observed by the frames transmitted to the beamformee will be similar to that encountered by the frames received from the beamformee. But it needs client feedback to confirm reciprocity or make calibrations to it’s estimate of the channel.
Explicit feedback beamforming
In this method, the onus of determining the channel estimate is on the beamformee (receiver). The beamformee will derive the MIMO channel estimate from the RF signals (training symbols) sent by the beamformer. It will send the channel estimate back to the beamformer which uses it for beamformed transmissions to the receiver. Since the channel calculations are done at the receiver, and is a function of the actual RF path from the transmitter to the receiver, it is expected to be more accurate than implicit beamforming.
The 802.11n transmit beamforming will help improve the signal quality at the receiver and overall system throughput.
But it cannot be used with legacy devices. It’s not clear if it will also help in reducing interference since the beam is not focused in a direction, but at various points in space. When beamforming is used, there is some performance sacrifice which otherwise could have been accrued from spatial multiplexing since copies of signals are sent from multiple antennas.
Beamforming and Aruba Networks
While a couple of vendors have introduced beamforming for improving WiFi communication, and 802.11n and mobile WiMAX standard have suggested it’s use, the technology hasn’t yet matured enough to be used in enterprise communication. While it offers some improvement in certain scenarios, it may actually impact performance negatively in others. Aruba Networks will continue to work with chipset vendors to evaluate and bring to the market a robust and proven beamforming technology to support all the types of users in the Enterprise Customer deployments.