Although all three beamforming technologies provide SNR gain, they are quite different. In addition, all types of beamforming can exacerbate the following problems:

Transmit beamforming (TxBF) is a method of transmitting two or [...]]]> 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 802.11 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 (source: Cisco Systems)

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.

Static beamforming provides a fixed radiation pattern by using a directional antenna. Virtually [...]]]> This post continues my discussion on beamforming which 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.  In this post, we discuss static beamforming.

Static beamforming provides a fixed radiation pattern by using a directional antenna. Virtually every vendor provides APs with removable antennas so it is easy to swap an OMNI antenna for a directional antenna. Some vendors, such as Xirrus, use an array of static beamforming antennas to create densely packed multiple channels in much the same way that a cellular tower uses directional antennas to create cellular sectors. By assigning a different channel to each beam, many non-overlapping channels can be densely packed together within a single array (see Figure).

Figure 1: Static beamforming (source: Xirrus Networks)

Static beamforming can provide an signal-to-noise (SNR) benefit to both legacy wirless protocols like 802.11a/b/g and newer 802.11n stations.  Since the antenna propagation pattern is static, the AP cannot adjust the radiation pattern on a frame-by-frame basis in order to track a station as it moves through the enterprise. Therefore, unlike transmit beamforming and dynamic beamforming, it does not take advantage of knowledge of the WLAN channel between the AP and the station in order to further optimize signal propagation. An array of directional antennas, such as the Xirrus Array, can improve SNR in 360 degrees.

Next time we’ll look at Transmit beamforming.

Femtocells are all the rage.   These tiny cellular radios promise to improve in-building cellular coverage and are primarily targeted at the home user.  However, some vendors are now building enterprise femtocells.  Should you consider femtocell deployment in order to improve your cellular coverage?  This article identifies the key questions you should ask before you buy [...]]]> Introduction

Femtocells are all the rage.   These tiny cellular radios promise to improve in-building cellular coverage and are primarily targeted at the home user.  However, some vendors are now building enterprise femtocells.  Should you consider femtocell deployment in order to improve your cellular coverage?  This article identifies the key questions you should ask before you buy a femtocell.

Value proposition

Network operators now offer femtocells such as the AT&T 3G MicroCell, Sprint AIRAVE, and Verizon Wireless Network Extender. The devices provide in-building cellular coverage and look like a Wi-Fi access point (See Figures).

The devices communicate with the cellular phone and convert voice calls into Voice over IP (VoIP) packets.  The packets are then transmitted over your Ethernet network, through your firewall, and over your broadband connection to the network operator’s servers.

At first glance, the femtocell value proposition seems like a win-win for the customer and the network operator.  The customer receives better in-building coverage. The network operator not only avoids the cost and hassle of building a macrocell, they also use the customer’s power, internal network, and broadband connection, further saving themselves money.  What’s not to love?   Well, let me count the ways.

Key questions

Before you add femtocells to your telecommunication strategy, you should consider the following questions.

• Which cellular operator’s signal needs improvement? If you are like most enterprises, your employees probably use cellular service from several network operators.   But a femtocell only supports one operator (e.g., AT&T or Verizon Wireless).  So you may need to install femtocells from many operators throughout your enterprise.

• Where do you need coverage? Oftentimes cellular signals from a macrocell can’t penetrate into the innermost regions of a building (e.g., the basement). Unfortunately, femtocells must be installed near a window so that the attached GPS antenna has a line-of-sight view to orbiting satellites. For instance, AT&T recommends that their microcell be located within three feet of a window and not in the basement or a closet.  So a femtocell might not be able to provide coverage where you need it.

• What type of phone is supported? The AT&T 3G Microcell supports 3G phones ONLY (sorry, no EDGE support).  The Verizon Wireless Network Extender supports 2G phones but no 3G phones (yup, no EV-DO support).  So it is very likely that many employees will not reap the femtocell benefits because their phone is not supported.

• How will a femtocell affect battery life? A femtocell could save battery life because less power is required to transmit a signal over the short distance to the femtocell rather than over the long distance to a macrocell. On the other hand, lots of handoffs from femtocell to femtocell may drain the battery because the phone needs to continually scan for a new femtocell.  So be sure to test phone battery life in your environment.

• How will you identify rogue femtocells? A rogue femtocell is an unauthorized femtocell connected to your network.  In the case of Wi-Fi, vendors provide rogue access point detection software to help detect the rogue AP.  However, what if an intruder configured a rogue femtocell with a valid AT&T account ID (but not the enterprise account ID) and then connected the femtocell to the enterprise network? How would you detect this rogue femtocell?

• How will you isolate femtocell traffic? Femtocell traffic is backhauled across the enterprise network.  Enterprises may want to isolate the femtocell traffic from all other enterprise traffic by using a virtual LAN (VLAN) connection or a separate physical Ethernet connection.

• Is the femtocell solution scalable? Most femtocells limit the number of simultaneous calls to three or four.  Does this provide sufficient capacity for your enterprise?

• How will you block access to non-employees? A femtocell signal may spill into an adjacent building or floor.  Most femtocells provide a “white list” or “black list” mechanism to explicitly include/exclude phone numbers.  This is akin to configuring a Wi-Fi AP with MAC addresses for allowed/disallowed laptop connections.  Is this mechanism a manageable solution?

• How will you manage interference? As with any radio, femtocells are subject to interference.  Unlike Wi-Fi, which uses unlicensed spectrum, femtocells use licensed spectrum that is controlled by the network operator.  So if your femtocell is subjected to interference from another femtocell or macrocell, how will you alleviate the interference?   Note that your ability to move the femtocell may be restricted by the need to maintain the GPS connection.

Conclusion

Femtocells promise to dramatically improve in-building cellular coverage but they raise many questions.  This article identified the key questions you should answer before investing in a femtocell solution.

This blog post was originally written by me and published on www.searchmobilecomputing.com. It is posted here with permission from TechTaget.

As Steve Jobs said in his 2005 Stanford University commencement speech: Stay Hungry, Stay Foolish [...]]]> Thanks to all of you that have been reading and commenting on my blog.  I will continue to focus on providing in-depth enterprise wireless and mobility analysis.  Year 2 will feature more frequent postings and I may experiment with video blogging.

As Steve Jobs said in his 2005 Stanford University commencement speech: Stay Hungry, Stay Foolish (SHSF)!