Wireless mesh hidden node problem

The IEEE 802.15.4 standard has become a cornerstone of low power wireless communications, both consumer and commercial, through market adoption by the Zigbee and the Thread standards groups. Both the Zigbee and Thread protocols support wireless mesh networking, with Zigbee being a proprietary network and Thread being based on the internet-protocol version 6 (IPv6).

The IEEE 802.15.4 standard has been designed specifically for interoperability with interferers, both through the detection of the presence of another IEEE 802.15.4 carrier, or from another in-band protocol such as Wi-Fi. The interference-avoidance mechanism uses the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) channel access mechanism. With this mechanism, a transmitting device first checks whether the channel is clear of interferers, before attempting transmission. If it observes that the channel is clear, it will start transmitting. If not, it will retry the entire procedure after a random time interval within a programmable limit.

This approach works well when all nodes can "hear" each other, which is a rare case in large networks. If a node is not capable of discovering what is happening beyond its receiver range, then its transmission can lead to hidden-node collision. This phenomena is known as the hidden node (or hidden terminal), problem.

Increasing the transmit power of nodes within a dense network does not necessarily solve the problem, as it increases the probability of causing hidden-node collisions in another part of a mesh network. Moreover, the transmit power of the IEEE 802.15.4 protocol is limited to just 10mW in most of the world outside of North America.

It has become evident through real-world deployments from product manufacturers that the IEEE 802.15.4 protocol struggles to cope in the presence of interferes, due to the hidden interferers.

Previous attempts to solve the problem include changing the modes of interferer detection, increasing the number of retries and moving to an additional retry delay after not having received an acknowledgement after transmission. This project aims to measure the effect of increasing receiver sensitivity as a means of mitigating against the hidden node problem.