Introduction

Wirepas Massive includes multiple co-existence features for high interference tolerance and low interference generation. All the features are fully automatic and locally adaptive facilitating large scale installations where other wireless system work in different parts of the frequency band depending on the location.

Wirepas Massive provides high reliability and robustness, independent of scale and frequency band used, whilst having friendly operation towards other wireless systems.

The co-existence features introduced in this document are frequency band independent and implemented in all Wirepas Massive frequency band variants (i.e. PHY profiles). The numerical information provided in this document applies to the 2.4 GHz ISM frequency band profile.

 The content of this document is valid from Wirepas Massive release v4.0 onwards. 

Overview

Co-existence can be defined as the ability of two or more collocated wireless systems to perform their tasks. 

In one system point of view this means the ability to work under interference from other systems and the ability to interfere other systems as little as possible. 

Co-existence can be achieved by mitigating interferences in following domains;

• Time domain: Devices do not send at the same time.
• Frequency domain: Devices do not send in the same frequency.
• Spatial/power domain: Devices are far enough of each other and do not interfere even if sending at the same time and in same frequency. Note that the TX power of a device affects how long the interference range of a specific device is.

Wirepas Massive includes multiple features that mitigate interference in above domains. These features summarized in Figure 1 and Table 1.

Figure 1. Wirepas Massive co-existence features

Table 1. Wirepas Massive co-existence features explained

Adaptive Frequency Agility (AFA) with local channel blacklisting

Wirepas Massive is a multi-channel system using 40 frequency channels in the 2.4 GHz ISM frequency band. Wirepas Massive implements AFA where devices locally avoid frequencies used by other wireless system, such as Wi-Fi (IEEE 802.11). Devices continuously do background measurements, measure Packet Error Rate (PER), and measure the success rate of CCA operations to find out if other systems use the frequencies used by devices. This information is also locally exchanged by the devices to have a mutual view of the environment.

 Using the above information, Wirepas Massive devices locally whitelist and blacklist frequency channels. The whitelisted channels are used for communication among devices and blacklisted channels are avoided.

Local adaptation is essential in large scale systems, as the frequencies occupied by other systems (and other interferences) also vary depending on the location. E.g. adjacent Wi-Fi access points are configured to use non-overlapping frequencies. It means that in a large scale installations, without this local adaption Wirepas Massive has, blacklisting frequency channels globally would result in the whole network running out of usable channels.

Figure 2. Wirepas Massive avoids occupied frequency channels.

Narrow band TX

The occupied bandwidth of Wirepas Massive devices is small (~1 MHz in the 2.4 GHz ISM band). This minimizes collision probability with other wireless systems in frequency domain and leaves room for Wirepas Massive devices to blacklist channels and still have enough usable channels for communication.

Figure 3. Wirepas Massive minimizes collision probability with other wireless systems in frequency domain by using narrow band transmissions

Short TX time-on-air, low duty cycle, and CCA

Multi-hop mesh communication allows Wirepas Massive devices to use higher radio data rate (1 Mbit/s in the 2.4 GHz ISM frequency band) and still achieve good network coverage. Using a high radio data rate results in short TX time-on-air (< 1 ms per packet/frame).

Wirepas Massive uses synchronized communication to maintain the network. This results in efficient communication and very low duty cycle operation (< 0.1% for maintaining the network).
 In addition CCA is used to sense if there is other traffic ongoing in the area. If CCA senses other ongoing other TX then the current TX is backed off to a later time to avoid collisions.

Figure 4. Wirepas Massive minimizes collision probability with other systems in time domain

Low maximum TX power and Adaptive TX Power Control (APC)

Multi-hop mesh communication allows Wirepas Massive to use small TX powers and still achieve high network coverage. As an example even the maximum TX powers used by Wirepas Massive enabled devices are only 2.5% compared to Wi-Fi (IEEE 802.11). In addition, Wirepas Massive implements APC functionality that minimizes used TX powers per link. The APC functionality continuously senses the link reliability and automatically adjusts the used TX power to a minimum reliable one. 

Figure 5. Wirepas Massive minimizes collision probability with other systems in spatial domain

Appendix: Physical layer details of wireless technologies operating in the 2.4 GHz ISM frequency band

Figure 6. Physical layer parameters of different wireless technologies in the 2.4 GHz ISM frequency band

Figure 7. Frequency channel allocations of different wireless technologies in the 2.4 GHz ISM frequency band

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