dback, and the channel of the victim UE is known at the gNodeB based on the uplink SRS, which can then be used to zero-force toward that UE, that is, null the interference. This description assumes zero-forcing toward a victim UE in the same cell (MU-MIMO). To zero-force toward a victim UE in a neighbor cell, the cross-channel to the neighbor cell victim UE needs to be estimated by the gNodeB, and we would need also some coordination between the schedulers. The zero forcing algorithm can bring substantial improvements to the performance. Figure 10.40 illustrates the spectral 10 5G Performance Spectral efficiency bps/Hz/cell 750 m 1500 m 3000 m No Zero Forcing With Zero Forcing Figure 10.40 Downlink spectral efficiency with 32TRX with and without the zero forcing algorithm. efficiency with and without zero forcing for the intra-cell coordination. The gain from zero forcing is 30-50% depending on the cell size. 10.7.2.3 Hybrid Beamforming The number of TRXs in the beamforming antenna can be lower than the number of antenna elements. Part of the beamforming is done by digital processing in the baseband, and part of the beamforming by analog RF beamformers. This solution is called hybrid beamforming. Fully digital beamforming requires each antenna element to have a dedicated RF chain. The motivation for hybrid beamforming is to minimize the number of RF chains, which may lower the cost and complexity of the beamforming antenna. Hybrid beamforming is illustrated in Figure 10.41. This section illustrates the system performance of hybrid beamforming compared to fully digital beamforming at the 3.5 GHz band in urban microcells. Hybrid beamforming simulations are run with 20 MHz bandwidth and with 10 UEs per cell, full buffer traffic, and single-user wideband scheduling. The user data rates would naturally be much higher with 100 MHz and fewer UEs with bursty traffic. The antenna is assumed to have 128 elements with eight rows, eight columns, and cross-polarization. The antenna element spacing is half wavelength. Four cas