The ant and the elephant: ambient RF harvesting from the uplink

This work investigates the available ambient radio frequency (RF) power density in dynamic, outdoor environments with a specific focus in the uplink. A spectrum survey was carried out around Bristol, UK between 500 MHz and 3 GHz using a handheld spectrum analyser and an omnidirectional broadband discone antenna. Measurements were performed while walking, travelling in a car and on a train. The results are compared with baseline indoors measurements, and as expected, ambient RF power densities in the outdoor environments were significantly higher. Interestingly, in some cases the power contained in the uplink of cellular communication networks is shown to be a better energy source than the downlink. It was found that in a train during rush hour, there is 17 times the mean power density and 45 times the peak power density in the uplink compared with the downlink. This shows that there is scope for ambient energy harvesting in environments with a large density of user equipment. Finally, by accounting for the rectifier efficiency it is estimated that during the train commute between Bristol and Bath in the UK a total of 27.2 mJ of energy could be collected.