Abstract This paper addresses the congestion problem in cellular networks through the assistance of parcel delivery drones. Drones have the primary task of parcel delivery and a secondary task of acting as relay nodes (hop) to add extra capacity in a cellular network. Base transceiver stations in the cells can admit ground user equipment to communicate directly with the drone (device-to-device communications) through in-band signalling between them. This paper proposes a novel algorithm, called CARLO, which optimizes trajectory and altitude of the drones in order to deliver physical parcels, maximize network capacity and reduce network interference. Our simulation studies show that our solution improves the network throughput while the energy inefficiency of taking detour path to assist congested cells is kept to a minimum. Moreover, it is guaranteed that parcels are delivered before deadline.