Multipath in Location Mode

Overview

When implementing location mode, you may encounter a challenge called multipath. Multipath is a phenomenon where RF signals reach the receiving antenna on the xArray by two or more paths because of RF signals bouncing off reflective surfaces, such as metal or glass. As a result, multipath decreases the accuracy of x and y locations. This article provides steps that help you minimize multipath and improve the accuracy of location mode.

This article requires knowledge of xArray location parameters. For more information about these location parameters, consult the xArray/xSpan Deployment Guide.

Recommendations

There are a number of recommended approaches for reducing multipath. Keep in mind that experimentation is always required to determine what works best.

Disable Beams that Reflect Off Walls or Surfaces

Beams reflecting off metal surface, such as those shown in the figure below, should be disabled.

See beam mapping when choosing which beams to disable. ItemTest, Octane SDK, and ItemSense all support disabling beams in location mode. A beam map shows the number and orientation of xArray antennas for selective disabling.  

Use a Longer Compute Window

A longer compute window will increase the number of tag read samples, thereby increasing x, y location accuracy effectively averaging out the effects of multipath reflections. As always, the larger the tag population, the longer the compute window required to ensure that each tag is read often during the compute window's time frame.

Use Foam to Reduce Detuning

When tagging metal items, place ¾ to 1-inch foam spacers under each tag. This greatly reduces the detuning effect of the metal. Alternatively, use on-metal tags.

Space xArrays Closer Together

Space the xArray with a separation equal to or less than 2 times the height to increase accuracy. For example, if 2 xArrays are 3.5 meters height above the floor, space them no more than 7 meters apart. Tags more directly under the xArray will be read with greater location accuracy, and tags read by multiple xArrays will benefit from weighted average calculations. For information, contact Impinj Support.

Use Reader Modes with High Read Counts

Higher read counts are essential for obtaining location accuracy. We recommend that you use 1003: Autoset Static Fast or Mode 4: Max Miller (not available in ETSI region).

By contrast, do NOT use mode 1002: Autoset Dense Reader Deep Scan. 1002 has a much slower read rate resulting much lower read count than modes 1003 and 4. In addition, the powerful readability of mode 1002 results in more reflective multipath reads.

Place Tags Facing Upwards

If possible, place tags facing upwards toward the xArray. Vertically hanging tags pick up reflective reads more easily than tags facing up, especially with sensitive Monza R6 based tags.

Install Version 5.12.1 or Greater

Last but not least, install firmware version 5.12.1 or later which implements valuable location accuracy calculation improvements.