sion over the air. Scrambling. Scrambling makes the transmitted bits less prone to interference by making them into a pseudo-random sequence through an XOR operation with pseudo-noise gold sequence generator. Modulation. Converts the scrambled signals to IQ values that are transmitted over OFDM subcarriers. Modulation options are QPSK, 16QAM, 64QAM, and 256QAM. Layer mapping maps the IQ samples onto layers, antenna ports, and virtual and PRBs. These steps convert the bit sequence to logical layers as needed for spatial multiplexing 76 Open Radio Access Network (O-RAN) Systems Architecture and Design that are mapped to antenna ports which are unique entities for every base station client communication and have unique reference signals. MIMO is a key technology in 5G (and 4G). It introduces the concept of the antenna which (as opposed to representing a physical antenna) is a logical entity. It is associated with a ref- erence signal that can be used by the UE to estimate the channel from that antenna port to it. Mapping to the physical channel can be done by 1: 1 or 1: many physical antennas. Each antenna port is associated with its own resource grid in the time and frequency domain and associated reference signals that are used for determining the channel state information associated with the port. If an antenna port is mapped to more than a single physical antenna, beamforming is (typically) used to focus energy to the user where the sig- nal from multiple physical antennas-all carrying the same reference signal-to the UE. This beamforming is transparent to UE. Given that the reference signals are unique to the antenna port rather than the physical antenna count, the reference signals define the maxi- mum number of concurrent transmissions. Separation of antenna port from physical antenna also explains the concept of the Type A and Type B RUs where the location of the port physical antenna mapping is likely to be done in the RU if the physical antenna port count is higher than the (logical) antenna port count. Ra