Among other things, a communication system comprising remote units and a controller is described. The remote units wirelessly exchanges radio frequency (RF) signals with mobile devices. Each RF signal comprises information destined for, or originating from, at least one of the mobile devices. The remote units and the controller communicate baseband data corresponding to the information across an intermediate network. The remote units each implement at least some physical layer processing for an air interface used to wirelessly communicate with the mobile devices. Two or more of the remote units transmit a downlink reference signal to a mobile device of the mobile devices.