unction of both downlink and uplink channel conditions [76]. We defined digital precoding as adaptive or non-adaptive weighting of the spatial streams prior to transmission from each antenna port (in a multi-antenna configuration) using a pre- coding matrix for the purpose of improving the reception or separation of the spatial streams at the receiver. Both feed-back and feed-forward precoder matrix selection schemes can be used in order to select the optimal weights. Feed-back precoding matrix selection techniques do not rely on channel reciprocity, rather they use feedback channels provided that the feed- back latency is less than the channel coherence time. In feed-forward approaches, the neces- sary CSI can be theoretically obtained through direct feedback, where the CSI is explicitly signaled to the transmitter by the receiver or estimated using the SRS. The direct channel feedback methods preclude the channel reciprocity requirement, whereas channel sounding methods rely on channel reciprocity. Therefore, explicit control signaling is required for the PMI-based (feedback) schemes. However, in reciprocity-based schemes, the sounding sig- nals in the uplink and precoded pilots in the downlink are used to assist the transmitter and receiver to appropriately select the precoding matrix. The reciprocity-based schemes have the additional advantage of not being constrained to a finite set of codebooks. Beamforming relies on long-term statistics of the radio channel and, unlike reciprocity-based techniques, does not require short-term correlation between the uplink and downlink in order to prop- erly function. 4.1.9.3.3 Hybrid Beamforming Hybrid beamforming has been proposed as a possible solution that is able to combine the advantages of both analog and digital beamforming architectures. The idea of hybrid beam- forming is based on the concept of phased array antennas commonly used in radar applica- tions. Due to the reduced power consumption, it is also seen as a possible solution for mmWave mobile broadband comm