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Interference Mitigation in Xirrus Access Points

Bruce Miller
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microschemeOver the past few years, the Wi-Fi industry has undertaken a widespread transition to the latest 802.11ac technology standard. To support this transition, Xirrus Access Points (APs) use adaptable radios that are programmable for operation in either Wi-Fi band (2.4GHz or 5GHz), and in any Wi-Fi mode (802.11a/b/g/n/ac). This is in contrast to traditional competitive APs that are fixed with one radio operating only in the 2.4GHz band and one radio operating only in the 5GHz band. Xirrus AP radio programmability enables organizations to optimize their Xirrus Wi-Fi network to match the clients connecting to it.

The ability of Xirrus APs to contain and minimize RF interference between radios is a fundamental element of their design that enables the programmable operation. In particular, this capability allows both radios in a single AP to operate at 802.11ac (5GHz) simultaneously, creating the highest performance and most future-proof Wi-Fi networks on the market.

Other Wi-Fi vendors have disputed the ability of Xirrus APs to control this interference and enable simultaneous operation with multiple 11ac (5GHz) radios. In some cases, they have resorted to lab testing and white board theory sessions to help ‘prove’ their points. Every day, thousands of Xirrus customers operate their Wi-Fi networks running multiple 5GHz radios per AP, and to us this is proof enough of the capabilities behind our solutions. However, to dispel any false claims and inaccuracies, we will highlight here some of the specific technologies which minimize RF interference and enable software programmable radios.

For the purposes of this discussion, we will talk about two types of interference: 1) co-channel interference (CCI) which exists between radios operating on the same Wi-Fi channel, and 2) adjacent channel interference (ACI) which exists between radios operating on Wi-Fi channels close or adjacent to each other in the RF spectrum. While neither can be completely eliminated in any Wi-Fi system, what matters is how well the effects of the interference are controlled and minimized.

The effects of interference are relevant to system-wide AP deployment as well. To support the increasing densities of Wi-Fi clients seen on networks today, more APs must be installed in a given space. Wi-Fi vendors today commonly recommend deploying one AP per 2,000 square feet (186 square meters). In contrast, the recommendation 10 years ago was one AP per every 10,000 square feet, or even more. The more closely APs are placed to each other, the greater the impact of CCI and ACI within the overall system.

Xirrus APs (2 radios) and High Density APs (4+ radios) employ multiple techniques to minimize the effects of CCI and ACI, enabling multiple radios to operate simultaneously in the same band (2.4GHz or 5GHz) in the same AP. In some cases, the techniques differ by device type (AP and High Density AP) and where this is the case, they are discussed separately.

  1. Directional Antennas: Xirrus 2-radio APs utilize directional antenna elements to create radio isolation. Directionality focuses RF energy in a given direction, helping to isolate RF from components of the other radio. Each radio incorporates multiple antenna elements corresponding to the number of streams supported by the radio. For example, a 3×3 802.11ac radio has three antenna elements. The elements from one radio are interleaved with those from the other radio in a circular fashion around the AP. The interleaving provides physical separation and helps increase isolation. Xirrus High Density APs differ from 2-radio APs in that the radios and associated antennas are collocated together and physically isolated from other radios in the access point. The radios are placed in a circular fashion around the AP to provide full 360-degree coverage. A metal reflector is used with each radio to produce gain, reduce back lobes, and increase isolation from other radios. Greater directionality of the antennas in the High Density AP design enhances radio isolation as well.
  2. Channel Selection:  Maintaining proper Wi-Fi channel selection for each radio is important to minimizing interference. Maximum channel separation between radios minimizes adjacent channel interference. Furthermore, individual channels are never repeated within a given access point, so there is no co-channel interference within an AP. Xirrus access points use a sophisticated algorithm to auto-select channels used within an AP and between multiple APs. In addition, nulls in antenna patterns are leveraged to increase isolation in addition to antenna directionality.
  3. Power Settings: In most Wi-Fi deployments today, the network must be designed for capacity as opposed to coverage, meaning more APs are deployed in a given area. In these scenarios, the transmit power on Xirrus APs is typically reduced since there is plenty of signal being generated from the APs since they are placed relatively closely together. This in turn lowers the prevalence of ACI and CCI. In addition, Xirrus APs support an ultra–low transmit power mode that allows power to be tuned down even further.
  4. Distributed Controller: Many traditional APs rely on a central controller that manages RF settings and interference across multiple access points within a network. The APs themselves are dependent on this centralized decision-making before their settings can be changed, creating a single point of failure and adding a level of complexity to coordinating the process. In Xirrus APs, the controller function is integrated directly in the AP. RF control is cooperatively distributed between APs in a given area, providing on-the-spot intelligence to optimize the RF settings and control AP operation to ensure best performance.

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