A cellular communications system includes a base station and a plurality of user devices, wherein each user device is operable to communicate with the base station over a communication channel having a plurality of frequency resources, wherein the base station is operable: i) to provide each user device with a respective initial allocation of the frequency resources; and ii) to provide at least one user device with a periodic communications opportunity in which the user device can communicate with the base station, wherein each user device is operable to apply a frequency shift to its initially allocated frequency resource in accordance with a frequency hopping sequence, wherein the user devices use the same frequency hopping sequence and are synchronized with each other so that, at any point in time, a common frequency shift is applied by the user devices.