Monday, March 1, 2010

Omnidirectional antennas, sectorized antennas, location tracking and RRM

Recently there has been some debate between two wireless vendors regarding which type of antenna is better than another.  Words were exchanged, and the one side made the statement that omnidirectional antennas suck.

I started to think on this a little more and tried to see where they were going with this train of thought.  There are instances where you need a patch, yagi or otherwise sectorized antenna - usually when getting the longest range is the goal of your wifi deployment.  This being said, I'd hardly agree with the statement that omnidirectional antennas suck.

If you're deploying a wireless infrastructure to support location tracking - omnidirectional antennas are your best friend.  Since the horizontal lobe of an omnidirectional antenna spreads out in a 360 degree fashion - this RF spread pattern aides the access point in reporting client location data back to a Cisco Mobility Services Engine (MSE) or an AeroScout Engine (for non Cisco shops).

The Cisco Wi-FI Location Based Services 4.1 Design Guide has a good graphic that explains mathematically how the location of a client device is calculated based on tri-lateration (using the combined client RSSI data from at least three or more APs).
  • ...the concept of ToA tri-lateration. The amount of time required for a message transmitted from station X to arrive at receiving sensors A, B, and C is precisely measured as tA, tB, and tC. Given a known propagation velocity (stated as c), the mobile device distance from each of these three receiving sensors can then be calculated as DA, DB, and DC, respectively. Each calculated distance value is used to construct a circular plot around the respective receiving sensor. From the individual perspective of each receiver, station X is believed to reside somewhere along this plot. The intersection of the three circular plots resolves the location of station X as illustrated in Figure 2-3. In some cases, there may be more than one possible solution for the location of mobile device station X, even when using three remote sensors to perform tri-lateration. In these cases, four or more receiving sensors are employed to perform ToA multi-lateration

From ccie(w)

OK - now that we know how a group of APs send the client RSSI information to an MSE or AeroScout Engine to calculate the approximate location of a client device - how would an access point that uses sectorized antennas calculate a client location using this same type of RF location calculation?  I would think that the access point using sectorized antennas would have to have a central "brain" in the array to combine all the client RSSI information that could be "heard" by the separate arrays.  Every sectorized antenna has some sidelobe to its RF beam, and surely antenna arrays arranged close next to one another would have some overlap, and therefore neighboring array antennas could potentially "hear" the RSSI of a given client device.

colorized array RF footprint to show channelization of the arrays' antenna orientation

From ccie(w)

My point is - from the information at hand regarding how client locations are triangulated, I believe that using patch, yagi or sectorized antennas would adversely affect the accuracy of location tracking.  The RF footprint has been changed to such a degree by the use of a sectorized antenna that the mathematical calculations used to determine client locations are no longer valid.

The same could be said of using sectorized antennas in a deployment utilizing Radio Resource Management (RRM).  RRM calculations are based upon each AP "hearing" a given number of neighboring APs at a given RSSI signal strength (usually -75dBm).  If antennas that have non omnidirectional lobes (patch/directional) the access points nearer the smaller back lobe of a patch/directional antenna would most likely sense this neighbor with a patch antenna at a lower RSSI and attempt to adjust RRM to power up in order to compensate for their "less heard" neighbor.

Point being - if you're using RRM and patch antennas - you should be very aware of how your access points are deployed and perhaps statically assign power levels to APs that are RF neighbors of the AP using sectorized antennas.

-- I invite additional information about how an array type access deployment handles calculating clients' RSSI and turning that into an X,Y location.. I'd like to understand how that works.


  1. Jennifer,
    You are spot on. A very large customer of mine has several directional antennas (1231 AP). After everything was deployed, calibrations completed what was notice is that client tracking around the cell boundary of a directionals were sadly skewed.

    The arguement was made to replace the 1231 with 1131. The goal here was to have all the same APs with the same antenna and DB gains on the floor. This fixed the issue on client tracking.

    Great article. BTW I am schedule for September's exam. I tried to get in earlier but waited too long.

    Good luck....


  2. Good topic Jennifer. The Omni antenna is a very good antenna for 360° coverage. It also is necessary if you wish to do any sort of tracking by signal location in a tri-lateration system to see the receiver reliably. With a single Sector Panel antenna you need to be in the antenna beam, which is by definition narrow and directional. If you are not in the beam, you will not be seen nor be able to connect. Sector Panel antennas can be assembled into arrays and cover an area which is 360° (usually with 3-120° panels or 4-90° panels). Attention to down tilt is important to be sure you have the beam width covering the areas of interest to the system (a similar concern with Omni antennas need also be considered). Tri-lateration could certainly be accomplished using the array of sector panel antennas in this fashion. As for differences, the two types of antennas should perform equally well in a tri-lateration application. Antennas like these can be found at if anyone is interested.

  3. It's interesting; I think I definitely have to agree with you initially, Jennifer. However, I'm curious how location tracking changes to adapt to transmit beam forming, as executed by some one like Ruckus wireless or even Xirrus, since they use arrays of what essentially amount to directional antennas in a circular pattern.

  4. jcorcoran - I agree that tri-lateration could be accomplished by the array of sector panel antennas - *if* there were a central "brain" to put together the disparate location data received by all of the sector panel antennas and achieve a true sense of the location of an 802.11 client.