Introduction

VergeSense wireless sensors (L302) use a mesh protocol (Wirepas) that operates in the 2.4GHz spectrum. Several communication technologies operate in the 2.4GHz ISM spectrum and coexistence with these technologies is required. The following article intends to provide insight into the coexistence techniques employed by Wirepas to avoid interference.

Channel Selection

Of the 40 channels available in the 2.4Ghz spectrum, WiFi typically occupies a 22MHz wide range of channels, and the center channel in each range is used to identify the group of channels. Typically channels 1, 6, and 11 are assigned to Access Points when a Wireless LAN is built to avoid duplicate channels on adjacent Access Points.

Wirepas uses 1MHz wide channels, which allows Wirepas to select the 2.4GHz channels that are in between the channels used by WiFi, or at the edge of the WiFi channel selection which reduces the likelihood of any interference.

The following diagram illustrates how narrow the Wirepas channel usage is across the 2.400GHz to 2.425GHz channel range that is occupied by a single WiFi channel.

In addition to the narrow channel usage, Wirepas also employs an adaptive frequency agility mechanism where it evaluates whether there is any activity on a channel prior to using the channel for communication. Each cluster of wireless sensors negotiates the channel to be used for their local communication, having first carried out an adaptive channel evaluation and selecting a channel where there is no WiFi activity present.

This channel adaptation technique ensures that Wirepas sensors skirt around the channels that are being used by WiFi.

Transmission Power

WiFi clients and Access Points are designed to provide as large a coverage area as possible to ensure persistent connections between clients and APs. The AP will typically transmit its WiFi signals at Tx powers of up to 20dBm, and all the clients within the coverage area of the Access Point need to have sufficient TX power to transmit back to the AP and will have Tx power up to around 14dBm. VergeSense wireless mesh sensors are not trying to maintain a connection with an Access Point at the center of the coverage area. The wireless mesh sensor is battery-powered and tries to use as little power as possible to maintain its adjacent neighbor, as a result, the wireless mesh sensor uses a transmit power of 4dBm-8dBm depending on how far away the adjacent neighbor is.

This adaptive low TX power mechanism between adjacent wireless mesh sensors restricts the range of the transmission to small cells between adjacent sensors.

Duty Cycle

WiFi clients and Access Points need to maintain a persistent connection after authenticating and associating with Access Points. The overhead of authentication and potential delay in authentication for WiFi clients can be problematic to real-time communication applications (such as voice or video calls), and many fast roaming techniques are employed by WiFi clients.

VergeSense wireless mesh sensors do not need to maintain persistent connections between each other. The wireless mesh sensors do not need to authenticate to an AP, they use a common authentication key to associate to each other, and only create an association when there is telemetry to transmit.

A WiFi client could be continuously transmitting or receiving data at a 10Mb, 54Mb, or 600Mb per second data rate, while a wireless mesh sensor will be transmitting or receiving data for less than 1 second at a 1Mb per second data rate. Prior to transmission the wireless mesh sensor will have checked that there is no active WiFi transmission on the selected channel prior to its own small transmission.

Summary

With the combination of narrow channel usage, adaptive channel selection, low transmit power, low data rate, and minimal duty cycle, the Wirepas mesh protocol has not displayed any measurable impact on WiFi performance. WiFi Access Points and controllers do not recognize Wirepas as a “rogue” access point, or a “rogue” client, the Wirepas transmissions are seen as low-level background noise.

Updated August 27, 2024 23:13