Frequency offset compensation for crystal-free 802.15.4 communication

The IEEE 802.15.4 standard is widely used in low power wireless sensor networks. Providing 250kbps raw data band-width, its physical layer (PHY) mandates tight constraints on the RF carrier frequency, calling for an absolute accuracy of ±40 ppm. This specification necessitates the use of a crystal timing reference, thus requiring design of a multi-component PCB to implement any 802.15.4 compliant solution. However, in this work, a proposed frequency compensation algorithm can be added to the 802.15.4 PHY layer to relax the requirements on a timing reference to up to ±1000 ppm relative frequency error between a transmitter and receiver, allowing for the elimination of the crystal and paving the way for a single-chip integrated 802.15.4 wireless node. A wide bandwidth channel-select filter allows for offset in the received signal carrier frequency, while an additional demodulator output estimates that frequency offset. An adaptive feedback loop can then adjust the receiver clock rate to center the received signal in the channel, following which a narrowband filter can be applied to restore noise performance. Such a system has been simulated, and the results presented in this work demonstrate the feasibility of standards-compliant wireless communication using inaccurate timing references.