Transmit Beamforming (TxBF) January 4, 2010
Posted by Devin Akin in : Uncategorized , trackbackFor those engineers working in product management at Wi-Fi manufacturers, TxBF is a well-understood technology. For normal people (i.e. the rest of us), it can be a confusing thing. There are multiple technologies that get lumped together under the TxBF moniker. There are several types of TxBF, and I’d like to briefly (because this is a blog, not a whitepaper) describe each type for the purpose of clarification.
The purpose of TxBF is to raise the SNR at receiver for the purpose increased data rates and decreased retransmissions. This can be done in a variety of ways.
There are two general types of TxBF: open loop and closed loop. Open loop means that the transmitter has limited feedback to work with and is therefore essentially forced to estimate where the receiver is located. Closed loop means that accuracy of transmissions is improved by opening a feedback channel between the transmitter and receiver so that the receiver can provide direct and specific feedback on how well it is receiving signals.
Standard-based (802.11n) Beamforming (Explicit Feedback) - The 802.11n amendment actually specifies 3 sub-types of Explicit beamforming: compressed, uncompressed, and CSI (channel state information). CSI likely won’t be implemented. In compressed and uncompressed explicit feedback, the receiver computes a steering matrix and sends it to the transmitter, which uses it to configure the phases of the transmit chains on per-frame basis. This requires that both the transmitter and receiver understand and operate using the same TxBF protocol.
Standard-based (802.11n) Beamforming (Implicit Feedback) – The transmitter assumes that the channel is reciprocal (the same in both directions) and creates the steering matrix by tracking incoming training symbols. Implicit TxBF’s primary advantages over explicit TxBF methods are that it places only a small load on the receiver and imposes minimal transmission overhead. The primary disadvantage of implicit TxBF is that the transmitter needs to calibrate the differences between the transmit and receive chains, and the calibration process is completed using feedback from the receiver.
An advantage of 802.11n TxBF is that it’s standards-based, meaning that TxBF capable equipment will work with any other interoperability-certified (read: Wi-Fi Alliance) TxBF capable equipment. Another advantage is that it allows the transmitters to continue using an omni-directional RF pattern, which prevents hidden node problems. The disadvantage is that there won’t be an 802.11n TxBF-capable, Wi-Fi Alliance certified piece of equipment on the market for quite some time to come. Several chipset vendors will soon release chipsets that support 802.11n TxBF, but even after these chipsets are released into the market, it will still be a significant amount of time before that translates into interoperability-certified devices available for sale.
Discrete Beamforming – Composed of an antenna array that is capable of a static number of pre-defined beam patterns and a CPU that intelligently selects from the table of possible beam patterns for each transmitted frame (e.g. Ruckus Wireless).
Discrete beamforming is markedly different than 802.11n beamforming, which is considered a type of “Linear” beamforming because it can form an almost infinite number of phase differentials between its transmit chains in order to aim the transmission at a client that is located in almost any point in space around the transmitter.
An advantage of Discrete Beamforming is that it focuses the AP’s transmit beam in a specific direction, which has the desirable effects of:
1) high gain in the intended direction
2) interference mitigation in the unintended directions
3) minimal client participation (nothing beyond the 802.11 standard) is required
4) fast-moving clients will generally stay within the beam pattern.
Ruckus’s array in particular, on indoor AP models, yields the approximate equivalent of a 6dBi antenna in 2.4 GHz, which is quite nice. The disadvantage of this approach is that clients that are already predisposed to being sticky become very sticky and this approach to beamforming can produce significant hidden node issues, depending on how it is deployed and the client density of a cell or deployment as a whole.
An array of directional antennas - a cohesive set of directional antennas forming an array with a circular or spherical pattern when viewed as a whole (e.g. Xirrus).
With an array of directional antennas, each radio device is serving a smaller physical area, and there may be several radios, so capacity of a single physical array is significantly higher than a single- or dual-radio AP. Disadvantages are that the array is big, expensive (in comparison to a single AP of course), and may be difficult to deploy in desirable locations (which may lead to sub-optimal coverage). Additionally, having radios at such close range may cause inter-radio interference, and when array goes down, a large coverage area experiences an outage.
At this point, there’s no perfect solution, and even when the standards-based solution arrives, it’ll be immature, requiring some amount of vetting and adjustment before living up to its hype. I’ve heard my good friend Joe Epstein say the approximate of, “Why would you need beamforming when you could just turn up the power to get the same effect?” In today’s systems, that may cause some link balance issues with clients unless you have some fairly high-gain antennas (for the purpose of receive sensitivity) on your AP, but I get what he’s saying. 
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In today’s systems, that may cause some link balance issues with clients unless you have some fairly high-gain antennas (for the purpose of receive sensitivity) on your AP, but I get what he’s saying.
Tiny technical nit. Increased power PLUS high-gain antennas won’t mitigate the unbalanced power effect, since the high-gain antenna will increase gain on both the TX and RX side. In order to have a high-power transmitter without the unbalanced power effect, you need a receiver with a higher receive sensitivity. This, to me, is the major advantage of beamforming arrays. If you simply go up to a higher-gain antenna, you focus the antenna’s coverage pattern, sometimes to the point where it won’t work for your purposes. Oh, and the antennas also get physically cumbersome. Beamforming is the only way to get 1) increased coverage, 2) without creating the unbalanced power effect, and 3) while maintaining a wide, basically omnidirectional, field of coverage.
Hi Joshua,
Great to see you here on the blog! Long time, no talk-to. Thanks for pointing out these things. Much appreciated.
Devin