A power transmitter for a wireless power transfer system, the power transmitter comprising: at least one transmit inductor arranged to generate a wireless inductive power signal for providing power to a plurality of power receivers (105, 109) powered by the power transmitter; a receiver for receiving data messages on a load modulation channel for load modulation of the wireless inductive power signal by the plurality of power receivers, the load modulation channel being divided into time slot frames with each time slot frame comprising a set of time slots; a time slot processor arranged to allocate time slots of the set of time slots to the plurality of power receivers (105, 109), the time slot allocator being arranged to allocate each time slot of a first set of time slots as dedicated time slots for power receivers of the plurality of power receivers (105, 109), and time slots of a second set of time slots as common time slots; characterized in the common time slots being available for load modulation by any power receiver of the plurality of power receivers (105, 109), and in the power transmitter further comprising: an identity controller for linking a temporary identity to each of the plurality of power receivers, the temporary identity being different for each power receiver of the plurality of power receivers (105, 109); and a message processor arranged to determine a source power receiver for messages from the plurality of power receivers in response to temporary identity information in the messages; wherein the identity controller is arranged to assign a first temporary identity to a first power receiver of the plurality of power receivers (105, 109), the first power receiver being allocated a first dedicated time slot; and the message processor is arranged to determine the source power receiver for a first message received in a common time slot as the first power receiver in response to the temporary identity information in the first message being indicative of the first temporary identity. at least one transmit inductor arranged to generate a wireless inductive power signal for providing power to a plurality of power receivers (105, 109) powered by the power transmitter; a receiver for receiving data messages on a load modulation channel for load modulation of the wireless inductive power signal by the plurality of power receivers, the load modulation channel being divided into time slot frames with each time slot frame comprising a set of time slots; a time slot processor arranged to allocate time slots of the set of time slots to the plurality of power receivers (105, 109), the time slot allocator being arranged to allocate each time slot of a first set of time slots as dedicated time slots for power receivers of the plurality of power receivers (105, 109), and time slots of a second set of time slots as common time slots; an identity controller for linking a temporary identity to each of the plurality of power receivers, the temporary identity being different for each power receiver of the plurality of power receivers (105, 109); and a message processor arranged to determine a source power receiver for messages from the plurality of power receivers in response to temporary identity information in the messages; wherein the identity controller is arranged to assign a first temporary identity to a first power receiver of the plurality of power receivers (105, 109), the first power receiver being allocated a first dedicated time slot; and the message processor is arranged to determine the source power receiver for a first message received in a common time slot as the first power receiver in response to the temporary identity information in the first message being indicative of the first temporary identity.