Measuring for AEN during Normal Operations (and Beyond)

If you are prepping for the CompTIA exam, you are likely looking to go into RFID, or want to be credible with potential clients if you are already in RFID. Deciphering the existing RF situation and making sense of the data are critical for your success. With the spectrum analyzer active, you are now collecting the first part of that critical data. If you are using a laptop to log the data, you should set your virtual monitor to record information every hour and actively log it. If you are using a spectrum analyzer without a laptop, you should come back and take a digital picture every two hours over the course of the normal business cycle. After taking the picture, clear the video display by resetting the video screen and begin collecting data again.

Playing in the Frequency Band

You may hear people say they are setting up a UHF RFID system at 915 MHz frequency. This statement is only partially true. Because the FCC allows many unlicensed devices to operate at that frequency, they require that no one device broadcast for more than a certain amount of time. If only one frequency, or channel, was available and a device couldn't broadcast for more than a split second, it wouldn't be possible to put many devices on that channel. That is why devices that use unlicensed frequency spectrums (for example, 902–928 in the United States) are designed to broadcast across a range of channels. The process of moving through each channel is called frequency hopping (to learn more about this, see Chapters 2, 4, and 9) , and most devices in this band stay on one channel for only a few hundred milliseconds or so. Therefore, to catch all the broadcasts, you need to measure not just the 915 MHz channel but the whole ISM range that goes from 902 MHz to 928 MHz. (That's 13 MHz on either side of the center frequency, or a 26 MHz span.) The reason we measure for more than twice that (a 60 MHz span) is that other licensed frequencies may be operating close enough to our ISM band and with enough power to interrupt our communications.

This initial measurement process is effective but represents only one data point in a facility. How large the warehouse is and how strong interfering signals are will determine what you pick up from that one location. So if you have a large warehouse, you are going to have to set up the same test procedure at several locations within the warehouse to increase the accuracy. The areas you want to make sure you are close to are the potential RFID reader interrogation zones (usually the dock doors, conveyor or sort stations, and the like). One drawback to this static testing methodology is that it is difficult to find the location of any interference.

The next step toward increasing the accuracy of your full Faraday cycle analysis is to take a roving data capture of AEN. To take a roving capture, you will need the following:

• A portable uninterruptible power supply (UPS) or similar battery backup device that should be able to power the spectrum analyzer for 20–30 minutes (available from American Power Conversion for under $200 at most computer stores)

• A golf cart, shopping cart, or similar means of wheeling your equipment around a warehouse safely

• A willing friend to act as a human tripod and carry the antenna around next to the golf cart

The spectrum analyzer is set up in the exact same manner as it is for the static capture, except the power source and the antenna are both mobile. For this test you need to pay close attention to the display as you are moving about the facility and watch for anything that shows up as noise in the ISM band. This test is particularly effective if you are sharing a facility with other tenants who may have systems running that are only separated by sheetrock walls.

If you do find a source of interference, make sure you note on your CAD drawing the exact location and the strength in decibels. The next step is to begin moving away from that original spot in 10- to 15-foot increments on four sides. In other words, move north 10 feet, go back to the original spot, move south 10 feet, go back, and so on. This will give you four read points equidistant around the original interference location to compare relative strength. If any one of the four points is stronger than the first in terms of signal strength, that is the direction the interference is coming from. The same process should be carried out to triangulate the source of the interference.

Warning 

Many people incorporating RFID systems in a warehouse may find that there are wireless barcode systems that operate over the UHF unlicensed band as well. If this is the case, you can pretty much plan on calling your barcode vendor and asking them for an upgrade to 2.45 GHz.

The full Faraday cycle analysis is the foundation for building an RFID network; the goal is to make it a perfect foundation for the structures being put on top of it. You have tested the environment and now understand what you have to deal with. After the site assessment is done, it's like starting with a fresh canvas for your electromagnetic work of art:

• You have identified all of the AEN within the facility.

• You have logged data over the course of a full business cycle (all the shifts) and understand any changes that happen at different times of the day.

• You drilled down to specific interrogation zone sites, as well as roved around the facility to make sure you looked under every nook and cranny for rogue AEN.

• You have triangulated any sources of interference while roving about the facility.

• You took all the possible machinery and equipment that is likely to make its own electronic noise and ran it in the interrogation zone while recording the data to make sure that no potential source was overlooked.

• You addressed the potential interference found by either eliminating it or knowing that you will have to come up with some sort of creative work-around.

• You have mapped out the interrogation zones on CAD drawings or blueprints and have made sure that those areas are noise-free.

Now you have the basic starting point to begin designing the RFID network. The last step in the site assessment is figuring out how a perfect radio wave will behave in your real-world environment.