Supporting a narrow serving beam in a hierarchical beam configuration

An apparatus, comprising:

• at least one processor;
• and
• at least one memory including computer program code, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to:
• receive information of at least one antenna configuration of at least one tracking reference signal for at least one synchronization signal block sub-beam, wherein the at least one tracking reference signal is self-contained, aperiodic, and semi-persistent, and wherein at least one source for the at least one tracking reference signal comprises at least one channel state information reference signal without repetition of the at least one channel state information reference signal on a same sub-beam;
• determine that the at least one tracking reference signal is not associated, or sharing a same antenna configuration, with a periodic tracking reference signals based on the at least one tracking reference signal;
• receive information of at least one other antenna configuration via at least one transmission configuration indicator state configuration for the at least one synchronization signal block sub-beam, wherein the at least one antenna configuration or the at least one other antenna configuration includes at least one quasi co-location configuration;
• and
• use one or more of the at least one tracking reference signal as at least one quasi co-location source for receiving at least one channel associated with the at least one synchronization signal block sub-beam based on determining that:
• the at least one tracking reference signal and the at least one channel state information reference signal cannot be detected,
• wherein the at least one channel includes at least one physical downlink control channel and at least one physical downlink shared channel.