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A How-to Guide for a Radio Frequency Identification Site Survey

Written By: Dylan Persaud
Published On: September 7 2007

A best practice approach to a radio frequency identification (RFID) implementation suggests that, as a preparatory task, a site survey be completed beforehand. A site survey will identify issues and hurdles to an RFID implementation, and suggest helpful, industry-proven techniques to solve these problems. The equipment, connection procedures, and methodology, as well as a list of tips for completing a site survey, are explored in this second part of our series on RFID. To learn more about the specifics of RFID, please see part one of this series, Are You Tuned into Radio Frequency Identification?.

Negative Consequences of an Incomplete Site Survey

Not completing a site survey before an RFID implementation carries several risks. For example, an ambient electromagnetic noise (AEN) survey can identify possible roadblocks to reader placement, as frequencies emitted by equipment and other sources may be the same as the reader, or stray backscatter may interfere with reader placement and function. Other companies that inadvertently interfere with the frequency of tags and readers may also cause interference on equipment. In addition, a site survey will expose radio frequency (RF) communications already present. Items that can cause such interference include bar code devices, long-range radios, cordless phones, alarm systems, and other wireless gadgets.

A site survey will also expose issues that may interfere with the operation of the RFID network and consequently delay the project timeline. Interference from other RF devices and forms of electricity can affect RFID accuracy. Wavelengths may cancel each other out where a particular frequency is already being used.

A site survey answers many questions that may arise on the IT side of an RFID implementation. It also provides a road map for reader placement, backscatter, and power requirements for new network equipment, both electrically and network architecture–wise. Interference from other radio equipment (AEN) and RF coverage of signal density is measured to aid in the RFID implementation.

Network infrastructure upgrades, if required, may impact the timeline for implementation, as some network hardware lead times and installation of additional power requirements can be lengthy. Any power requirements that need to be added will also further extend the timeline for an RFID implementation. One way to minimize these delays is to mitigate risk by building into the project plan enough time to execute and test such equipment before the RFID hardware is ordered.

The checklist in Table 1 provides criteria with which to address the site survey in preparation of the four phases of the implementation. These criteria refer to conditions that should be tested during the site survey.

Areas to Be Addressed by the Site Survey Complete Incomplete
1. Refer to the workflow and business process map created, and test the location of each reader. Some readers will require an additional power source with Ethernet, while some may be wireless.    
2. Look for two things: the strength of the signal that propagates through the interrogation zones, and the frequency that those waves are broadcast over. The stronger the waves in a particular frequency band, the more difficult implementing an RFID system will become.    
3. When evaluating AEN, examine the entire cycle for a period of 24 to 48 hours. This will provide a true picture of where fluctuations may occur.    
4. When conducting the AEN survey, it is recommended that you start with the outer most points of the warehouse and work inward toward the center.    

Table 1. Conducting a site survey.

Equipment Required for a Site Survey

A site survey requires the following equipment:

  • A spectrum analyzer—a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, the spectrum analyzer measures the power spectrum. (http://en.wikipedia.org/wiki/Spectrum_analyzer)

  • A signal generator—a device that produces RF signals at preset frequencies, strengths, and durations. This device is hooked up to a ¼ wave dipole antenna via a coaxial cable, which will transmit the generated RF field.

  • A circularly polarized, ultra high frequency (UHF) antenna—an antenna used by any UHF reader that is attached to a spectrum analyzer to measure the RF field received from the signal generator.

  • A ¼ wave dipole, 915 MHz–antenna and a ground plane plate—an antenna that will radiate an RF field of 360 degrees.

  • Two tripod stands—one stand to mount the antenna, the other to support the UHF antenna outside the interrogation zone.

  • A laptop computer.

Procedure

Step 1. Test where you would like to position the readers, and test for compatibility. AEN from other electrical equipment may interfere with the location where like frequencies are found. Check that power requirements are available for new equipment at the proposed area. If power requirements are not dealt with, the task is incomplete. Ensure that power is accounted for wherever the reader is to be placed.

Step 2. Check for the signal strength of the frequencies and the actual frequency emitted within the area. Record both signal strength and frequency, and compare these to existing noise, if available. AEN may already be present within that area. If AEN is found, then the location is not acceptable for the reader. No AEN in a zone qualifies as a passing mark for a possible reader location.

Step 3. Monitor the area for signal strengths and frequencies for a minimum of 24 hours to verify that no occasional, errant frequencies occur that can affect the reader's operation. If a like frequency is detected, a new location should be found for the reader prior to rollout.

Step 4. Place the reader once the results of the criteria are collected and validated. Certain spots may be more frequency-rich than others; this will give a more accurate picture of the frequency map found within the warehouse. Once the criteria are executed and validated, the reader placement should then fall into place based on the results.

Connection Procedure

The following tasks provide the road map for equipment placement and identify the obstacles that might affect RFID equipment. A proper site survey is required, as such issues as bandwidth sharing, ambient noise, etc. may impact the reader and portal placement, which may not suit the operations. If any of these scenarios exist, RFID reader placement will be difficult, and read rates will be affected. This procedure identifies the properties that will alter RFID information. The next section outlines the equipment required and the connectivity of that equipment.

A best practice approach to the connection of the equipment used to conduct a site survey will aid in the completion of the site survey. This section defines the tasks to install the core equipment.

Step 1. Place the antenna on the tripod in the middle of the interrogation zone. This placement should define the optimum position to place the portal in relation to the business process occurring on the flow of the warehouse. A common mistake, and one best to avoid, is to hang the coaxial cable from the antenna; doing this may impede the signal. Best practices dictate that the antenna can be mounted by a piece of rope hanging from above, as close to the middle of the interrogation zone as possible.

Step 2. Connect the spectrum analyzer to the antenna by attaching the coaxial cable to the input of the spectrum analyzer. Be sure to check that both units are off, as this is good practice and will protect the equipment from shorting out. This operation will evaluate the presence of any ambient noise that may adversely affect the RFID equipment.

Step 3. Use an RS-232 cable or an Ethernet cable to connect the laptop to the spectrum analyzer, and power it up. If there is no laptop, take a picture of the frequency every two hours. This connection allows the tracking to be traced over a period of time and should reveal any inconsistent AEN that may be cyclical.

Step 4. Check for existing AEN and frequencies. This task exposes any frequencies and other AEN within the range of 902–928 MHz, which is the effective North American frequency range in which RFID specifications have been set. AEN can also affect signal capabilities. Ambient noise from machinery and other RFID installations can affect bandwidth.

Set up the spectrum analyzer to the following specifications:

  • Set the span to 60 MHz.
  • Set the resolution bandwidth to 100 kHz.
  • Set the video bandwidth to 30 kHz.
  • Set the amplitude attenuation to 0 dB.
  • Turn off the maximum hold to capture the energy received from the radiating antenna.

The above steps describe the set-up of the equipment; the following will dissect the actual steps to perform the test.

The ¼ dipole antenna that is attached to the signal generator should be placed in the center of the proposed interrogation zone:

  • Connect the UHF directional antenna to the scope, and mount it at the same height as the ¼ wave dipole antenna. Place both antennae parallel to the dock doors at 0 degrees.

  • Tune the signal generator to 902 MHz and record the results.

  • Keep the UHF antenna in the same location, and set the signal generator for 915 MHz.

  • Repeat the process at 928 MHz.

  • Relocate the directional antenna to each of the 8 positions in increments of 0, 45, 90, 135, etc. degrees in a clockwise manner, and record the signal strength at each frequency with relation to 45-degree increments.

These results will indicate where the strongest signal strength is propagated through, and will determine where the best placement for the antenna is.


Figure 1. Antenna placement.

Once the above steps are completed, the suggestions below will provide guidance for completion of the site survey. The purpose of measuring AEN is to ensure the RFID equipment will function effectively.

Below are tips that will aid the preparation and site survey itself:

  • Identify all AEN within the facility.
  • Log data over a 24-hour cycle.
  • Measure each interrogation zone to assure reader operability.
  • Walk around every corner of the warehouse to assess AEN.
  • Triangulate any sources of interference and seek them out.
  • Run all machines that emit noise, and check if any affect the signal for RFID.
  • Map out interrogation zones with computer-aided design (CAD) for efficient reader placement.

Common Problems and Quick Fixes

Bar code equipment emits a conflicting frequency.

There a few work-arounds if interference is found. If bar code equipment is emitting a conflicting frequency, an effective solution would be to upgrade the band of the RF devices to 2.4 GHz or 5.8 GHz. The use of fixed RFID portals, or even deployment of handheld RFID units that require less power, may solve this problem. The portable handheld units are more controlled and are slightly less affected by AEN.

Protocol capability is limited.

Check for interrogators that have code hopping capabilities. Such capabilities reduce the possibility of signal interference. With additional frequency, the organization can move to another band if one is consumed with current equipment.

Proper documentation is lacking (for risk mitigation).

After the survey is complete, a written document should be produced by the surveying company for the organization's records and for use as a guide for hardware quantities. This responsibility should rest upon the chosen business partner. The document should contain the results from the test, the recommended hardware, and most importantly, equipment placement. Further, documentation will eliminate any issues for hardware placement if the vendor and the consumer are not “on the same page” (thinking alike and in agreement). Signatures and dates should be added after each change is made, as this implies sign off and responsibility.

Conclusion

The site survey is a preliminary yet critical step to a complete RFID implementation solution. However, the importance of performing a site survey is usually overlooked. A site survey provides the organization with pertinent information on reader placement. To not conduct a site survey often results in delays to the entire project because of incomplete project planning and possible costing issues. It is at this point that the cost for the software and hardware needed for the RFID project becomes apparent, which should have been budgeted for within the project plan accordingly.

Stay tuned for the next installment of Are You Tuned into Radio Frequency Identification?, which will discuss the first phase of an RFID implementation in detail.

For more information and to start your own custom solution comparison, please visit

TEC's Radio Frequency Identification (RFID) Evaluation Center.

 
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