es are Analog RF Antenna Figure 10.41 Hybrid beamforming beamformers elements concept. RF chain Baseband digital beam- forming RF chain 10.7 Massive MIMO and Beamforming Algorithms User throughput Fixed beams Hybrid (4RF) Hybrid (8RF) Digital Mbps 5.0 Cell edge Figure 10.42 Downlink user throughputs with different beamforming solutions. studied: fixed analog beams, fully digital beamforming with 128 RF chains, and hybrid solutions with four and eight RF chains. The results are illustrated in Figure 10.42. The simulations show that digital beamforming gives the best performance, while fixed beams have the lowest performance. The hybrid beamforming performance lies between the two other options. The difference between fully digital and hybrid beamforming is larger in the cell edge performance than in the average data rate. The average data rate with hybrid beamforming is 1-5% lower than with digital beamforming. The difference in the cell edge data rate is up to 10%. The gap between hybrid beamforming and full digital beamforming would become larger with frequency domain multiplexing of different UEs, and multi-user MIMO, given that the analog beamforming weights need to be applied jointly for all the scheduled UEs in the frequency and spatial domains. However, having twice the number of RF chains as the number of multiplexed streams in the spatial and frequency domains is expected to be enough to provide close-to-optimal performance [10]. Fixed beamforming provides a clearly lower data rate than digital beamforming: the difference is 20% in the average data rates and 35% in the cell edge data rates. The results show that it is feasible to consider hybrid beamforming from the performance point of view to simplify the practical implementation of the beamforming antenna. 10.7.3 Radio Network Architecture and Functionality Split There are different options for splitting the baseband functionality in cloud RAN; see Figure 10.43. If more processing is located in the Radio Unit (RU) close to the antenna, the performance 