A millimeter-wave wireless multiple antenna system (80) and method (100) are provided in which a UE (120) uses a multi-antenna subsystem (81) to identify a plurality of m strongest transmit beams (122) from the base station (110) based on power measurements of a plurality of synchronization signal blocks (SSBs) transmitted on a corresponding plurality of transmit beams by the base station (110), and to generate multiple uplink random access channel (RACH) preambles (123) that is sent (124) to the base station (110) to identify the plurality of m strongest transmit beams and relative weights for each of the plurality of m strongest transmit beams which are used by the base station (112) to generate an optimal downlink transmit beam for use in sending a RACH response to the UE (120).