posted by: Paul DeBeasi
In this post, we continue my discussion of beamforming by focusing on transmit beamforming. As previously mentioned, beamforming is a method of concentrating radio frequency (RF) energy in order to improve the signal to noise ratio (SNR) at the receiver, thereby improving network performance and predictability.
Transmit beamforming (TxBF) is a method of transmitting two or more phase-shifted signals so that they will be in-phase at particular points in space where the transmitter believes the receiver to be, thereby increasing SNR. Two forms of TxBF are optional components of the IEEE 802.11n draft amendment to the 802.11 standard. Explicit and implicit TxBF require feedback from 802.11n stations (STA) and thus will not operate with legacy stations at all. Enterprise WLAN vendors do not support implicit and explicit TxBF at this time because the 802.11n chipsets do not currently support either form of TxBF.
Cisco has introduced a proprietary form of TxBF called ClientLink. ClientLink can work with 802.11b/g/a STAs because it requires no modifications in the STA. ClientLink is designed to improve the SNR for legacy STAs in the downlink (AP-to-STA) direction only. A boost in SNR can improve the STA’s “rate over range” performance because the modulation rate for 802.11 STAs will increase as the SNR increases. Improving rate over range performance is particularly important for legacy STAs because they can consume considerably more airtime than 802.11n STAs, and therefore can reduce the achievable throughput of 802.11n STAs. Alternatively, Air Time Fairness (ATF) mechanisms can also regulate legacy STA airtime consumption.
TxBF changes the phase of the original signals in relation to each other and transmits the phase-shifted signals using two or more antennas to the STA (see figure). As the signals propagate through the air, they additively combine at various points in space. The figure shows an example of two out-of-phase signals propagating from an AP to a legacy STA. The green dots represent the points in space where the two out-of-phase signals combine to form a signal with an SNR that is up to 3 dB higher than (i.e., twice as high as) the original signal. In a multipath-rich environment, even higher levels of gain are theoretically possible. Cisco is the first enterprise WLAN vendor to implement TxBF, and it claims it can achieve 4 to 6.5 dB of SNR gain in a multipath-rich environment. A 6.5 dB gain is an increase of approximately 4.5 times the original signal.
Figure 1: Transmit beamforming
The challenge with TxBF is figuring out how to modify the transmit signal phases for the greatest possible gain. In the ClientLink implementation, the AP uses frames received from the STA in the uplink direction to determine how to modify the phase in the downlink direction. TxBF assumes that the uplink channel characteristics and the downlink channel characteristics are “reciprocal” (i.e., the same) in both directions. In reality, the uplink and downlink channels may not be reciprocal, especially when the STA is moving. So, in practice, TxBF performance gains will vary from moment to moment and STA to STA.
Unlike static and dynamic beamforming, TxBF does not change the antenna radiation pattern. Cisco’s TxBF implementation uses omnidirectional antennas that cause the signals to radiate in a doughnut-shaped pattern. Therefore, referring to TxBF as “beamforming” is somewhat misleading because it does not actually form a directed beam. Cisco’s TxBF achieves an SNR gain at “points in space” (i.e., the green dots in the figure). So the receiving STA must be located at the right point in space in order to achieve the maximum SNR gain. In contrast, static and dynamic beamforming achieve SNR gain throughout the radiated coverage area because both techniques focus the radiated energy.
TxBF and spatial multiplexing are mutually exclusive. This is because spatial multiplexing transmits different signals on each antenna, whereas TxBF transmits the same (phase-shifted) signals on each antenna. So, it is impossible to use TxBF and spatial multiplexing at the same time. That is why enterprises should use TxBF to provide SNR gain for legacy STAs only. In contrast, static and dynamic beamforming can operate in conjunction with spatial multiplexing.
Next time, we will look at dynamic beamforming.
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