Aspects of the present disclosure are directed to communicating data for authentication and location determination, such as for authenticating a key FOB and locating the key FOB within a defined distance. As may be implemented in accordance with one or more embodiments, a plurality of headerless packets, respectively including scrambled timestamp sequence (STS) packets but not including encoding and/or synchronization headers, are communicated between respective communication circuits. A time of flight (TOF) value indicative of time elapsed between transmission and reception of the plurality of headerless packets is assessed based on the STS packets. A distance between the communication circuits is determined based on the assessed time of flight.