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Bang for the buck with explicit beam forming in 802.11ac

October 16th, 2013

 

Bang for the buck with explicit beam forming in 802.11ac

802.11ac has brought with it MIMO alphabet soup … spatial streams, space-time streams, explicit beam forming, CSD, MU-MIMO. Alphabet soup triggers questions to which curious mind seeks answers. This post is an attempt to explore some questions surrounding explicit beam forming (E-BF) that is available in Wave-1 of 802.11ac. E-BF is a mechanism to manipulate transmissions on multiple antennas to facilitate SNR boosting at the target client.

How is E-BF related to spatial streams?

E-BF is a technique different from spatial streams. E-BF can be used whenever there are multiple antennas on the transmitter, irrespective of the number of spatial streams used for transmission.

In the transmission using multiple spatial streams, distinct data streams are modulated on signals transmitted from distinct antennas. The signals from different antennas get mixed up in the wireless medium after they are transmitted from the antennas. The receiver uses signal processing techniques to segregate distinct data streams from the mixture. The ability to separate these distinct data streams from the mixture is dependent on the channel conditions between the transmitter and the receiver (to be able to isolate `S’ streams at the receiver, the channel matrix needs to have rank of `S’ or more). There is no channel dependent processing of signal at the transmitter. Receiver performance is channel dependent. Some key points regarding multiple spatial streams (spatial multiplexing) are:

  • To support `S’-stream transmission, both AP and client must have at least`S’ antennas
  • Rich scattering environment (e.g., indoor) is conducive to give high ranked channel matrix
  • There is no need to send channel feedback from the receiver to the transmitter.

In the transmission using E-BF, the spatial streams are pre-processed (and pre-mixed) to match them to the channel characteristics from the transmitter to the receiver and the output of the pre-processor is transmitted from different antennas. Feedback from the receiver about the  channel characteristics is used in pre-processing. For practical implementation called ZF (Zero Forcing) receiver, E-BF causes SNR boosting for the spatial streams at the receiver. Some key points regarding E-BF are:

  • Feasible with multiple antennas on the AP, irrespective of the number of spatial streams
  • Affects SNR of spatial streams at the receiver
  • Requires channel dependent pre-processing of signal at the transmitter
  • Requires feedback on channel characteristics from the receiver to the transmitter
  • Does not require multiple antennas at the receiver.
When is E-BF truly beneficial?

In general, E-BF is truly beneficial when the number of spatial streams in use is less than the number of antennas on the AP. In Wi-Fi, this most commonly happens when the client has less number of antennas than the AP. For example, most smartphones and tablets have only 1 antenna on them.

Stream vs beam tradeoff:

For the example of 3-stream AP and 3-stream client, adding E-BF on top of 3-stream transmission may not give significant benefit. This is because, with E-BF different spatial streams typically experience unequal boost in SNR. The SNR can be significantly boosted for some spatial streams with E-BF. On the flip side, there will usually also be some spatial streams for which the SNR boost is not significant. Or, it could even be degradation of SNR for some spatial streams compared to the case when E-BF is not used. To be precise, the SNR boost for each spatial stream is dictated by the corresponding singular value of the channel matrix and the singular values of the channel matrix range from high to low for practical channels (E-BF is based on technique called Singular Value Decomposition or SVD). Couple this with the fact that practical implementations use the same MCS on all spatial streams. So, this means either using the MCS supportable by the weakest SNR spatial stream for all spatial streams or using high MCS for the strong streams and dropping the weak streams. There is excellent explanation of this tradeoff  in the book by Perahia and Stacey in Chapter 13 (if you are up for reading some math!).

However, if 3-stream AP can support only 1-stream transmission to the client, E-BF can give significant gain. This is commonly the case with smart devices which have only 1 antenna on them. In this case, the single stream will most likely get boosted in SNR and there isn’t another stream to counter the SNR boost.

How much overhead does E-BF feedback cause on wireless bandwidth?

E-BF requires feedback from the receiver to the transmitter about the channel characteristics. In order to trigger this feedback, the transmitter sends sounding packet to the client. Client performs channel measurements on the sounding packet and responds to the AP with the channel feedback. A question that often comes up in E-BF is how much of a wireless link overhead the E-BF feedback report would cause. To answer that question, take a look at this spreadsheet. From this spreadsheet, it appears that the feedback overhead is relatively small (only about 0.1% of airtime), particularly for the case where E-BF is going to be most beneficial, e.g., 3 or 4-antenna AP talking to 1-antenna client.

All factors considered, E-BF appears to provide benefits for smartphones and tablets, which typically have only 1 antenna and hence cannot support multiple spatial streams when connected to the 3 or 4-stream AP. On the other hand, when there are multiple antennas on both sides of the link (such as a 3 or 4-stream laptop connected to the 3 or 4-stream AP), spatial multiplexing without E-BF can be as good as one with E-BF. These are the inferences drawn from MIMO principles and it would be interesting to see if they match up with measurements from the practical Wave-1 environments.

Earlier Posts in 802.11 Network Engineering Series:

 

Hemant Chaskar

Hemant Chaskar is Vice President for Technology and Innovation at AirTight. He oversees R&D and product strategy for AirTight Wi-Fi and WIPS; and also performs roles in technical marketing, business development, and customer facing activities. Hemant has worked for more than 14 years in the networking, wireless and security industry, with 9 years in Wi-Fi. He holds several patents in these technology areas and has Ph.D. in Electrical Engineering from the University of Illinois at Urbana-Champaign. Follow on Twitter @CHemantC.

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Comments

  1. Jianpo Han
    January 20th, 2014 at 05:21 | #1

    Beam forming will bring additional antenna gain. Considering regulationary restrict, we have to lower tx power….

  2. Chris D’Amore
    December 17th, 2013 at 11:12 | #2

    is transmit power stepped down when E-BF is used?

    Very nice post BTW…

    • January 8th, 2014 at 11:00 | #3

      In general, total Tx power of radio is divided among different radio streams when multiple streams are used – either for multiple spatial streams Tx or for E-BF Tx.

  3. David Goldstein
    November 4th, 2013 at 14:42 | #4

    how is explicit beamforming in 802.11ac different from Cisco’s ClientLink?

    • November 4th, 2013 at 17:06 | #5

      All beamforming techniques require knowledge of channel state from Tx to Rx. Explicit beamforming is standards based technique which relies on feedback from Rx to Tx for channel state information. Cisco’s ClientLink is proprietary beamforming (called implicit beamforming) in which instead of Rx sending feedback to Tx about channel state, Tx measures channel from Rx to Tx based on received frames and assumes that it looks the same from Tx to Rx. Though reciprocity argument of radio waves is often presented to back this up, jury is still out if the channel is truly reciprocal when end to end factors such as radio hardware and DSP are accounted for. That is why, implicit beamforming was not included in 802.11ac standard.

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