Collision Avoidance and Extending Range and Capacity in ZigBee
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ZigBee technology is a low data rate, low power consumption, low cost, wireless networking protocol built on top of the IEEE 802.15.4 short range communications protocol . It is usually targeted towards automation and remote control applications. ZigBee is one of the most widely used communication protocols of the Internet of Things (IoT) for transmission of information in a wireless mesh network between sensors and/or actuators, and for network connectivity which enables devices to connect and exchange data with each other.
ZigBee applications require that the devices operate for long periods of time with small, non-rechargeable batteries which in turn mandates low power consumption which limits transmission distance to 10-100m line of sight. As ZigBee operates in the same radio frequency band as Wi-Fi which is 2.4 GHz, it sometimes experiences interference when a Wi-Fi user is trying to transmit at the same time, resulting in a collision. Similarly a ZigBee network also experiences collisions when two or more ZigBee devices are trying to transmit data at the same time. Even though ZigBee employs a Direct Sequence Spread Spectrum system and uses the ALOHA protocol to overcome these interferences and collisions, some messages are still lost during transmission.
The Spread Spectrum system reduces the effects of noise and increases the probability of a message being successfully received despite collision with a message from a Wi-Fi user, but if a message collides with another ZigBee message then both messages are still always lost. The reason for this loss is the fact that ZigBee employs only one particular set of Pseudorandom Noise (PN) codes to perform the spreading of data during modulation. When two or more messages transmitting at the same time are involved in a collision using the same PN sequence, the signal-to-interference ratio is too low for successful recovery of either message, resulting in either the loss of data or the need of retransmission of both messages involved in the collision. Retransmission requires additional time and uses additional power which results in energy wastage, slower transmission, and reduction of system throughput.
This thesis will propose and analyze a system where ZigBee transmitters are allowed to randomly choose a PN code from among a large set of possible PN codes. Performance improvement will be shown in terms of increased percentage of successfully transmitted messages, which produces improved accuracy, improved energy efficiency (enabling increased range and/or longer battery life) and increased system throughput.