Wireless mesh hidden node problem

通过Zigbee和Thread标准组的市场采用，IEEE 802.15.4标准已成为低功耗无线通信的基石，无论是消费者还是商业。Zigbee和Thread协议都支持无线网状网络，Zigbee是一个专有网络，Thread基于互联网协议版本6 （IPv6）。IEEE 802.15.4标准是专门为与干扰的互操作性而设计的，无论是通过检测另一个IEEE 802.15.4载波的存在，还是来自另一个带内协议（如Wi-Fi）。抗干扰机制采用CSMA/CA （Carrier Sense Multiple Access with Collision Avoidance）信道接入机制。利用这种机制，发射设备在尝试发射之前首先检查信道是否没有干扰。如果它观察到信道是畅通的，它将开始发射。如果没有，它将在可编程限制内的随机时间间隔后重试整个过程。当所有节点都能彼此“听到”时，这种方法工作得很好，这在大型网络中很少见。如果一个节点无法发现在其接收范围之外发生的事情，那么它的传输可能导致隐藏节点碰撞。这种现象被称为隐藏节点（或隐藏终端）问题。增加密集网络中节点的传输功率并不一定能解决问题，因为它增加了在网状网络的另一部分引起隐藏节点碰撞的可能性。此外，IEEE 802.15.4协议的发射功率在北美以外的世界大部分地区被限制在仅10mW。从产品制造商的实际部署中可以明显看出，由于隐藏的干扰，IEEE 802.15.4协议难以应对存在的干扰。以前解决这个问题的尝试包括改变干扰检测的模式，增加重试次数，以及在传输后没有收到确认后移动到额外的重试延迟。该项目旨在测量提高接收器灵敏度作为减轻隐藏节点问题的一种手段的效果。

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.