Hybrid Beamforming for mmWave Massive MIMO
Recently due to the increasing of data traffic, millimeter wave (mm-Wave) band has been considered because of its license-free and wide bandwidth for the fifth generation (5G) mobile network. However large path loss is a critical problem when using mm-Wave. To compensate for the large pathloss, massive MIMO antenna as the base station antenna is a useful technology for the next generation cellar network. Massive MIMO antenna is equipped with large number of elements and so it is possible to form accurate beams and high antenna directivity. As a result, the burden at user terminal side can be reduced.
Figure 1. Hybrid beamforming structure
However, the technique has a problem of expensive design cost and large amount of signal processing due to the large number of antenna elements. And so, hybrid beamforming is a useful technology for the realization of massive MIMO. Hybrid beamforming is a technology that includes both analog beam forming at RF and MIMO processing (digital precoding) at baseband.
To reduce calculation cost of signal processing at baseband, analog beamforming at RF should consider the interferences between users. So we propose the method of analog weight decision that direct null beam to other users. Owing to the proposed analog beamforming scheme, it is possible to remove the user interferences and reduce the burden of signal processing at baseband.
As a comparison beamforming method, we select a conjugate beamforming (CBF). CBF is to direct main beam to each user without considering user interferences. For both proposed beamforming and CBF, the residual interference components from other users are removed at baseband digital precoding. We adopt zero-forcing (ZF) as the method of digital precoding. By using the proposed algorithm, system rate has increased more than 20 percent after hybrid beamforming as well as after only analog beamforming as shown in Fig. 2 and Fig. 3 respectively.
Figure 2. System rate after hybrid beamforming
Figure. 3. System rate after analog beamforming
- K. Sakaguchi, “Millimeter-wave small cells vs. below 6GHz Massive MIMO –which can give greater value?-,” IEEE Globecom 2016, Washington DC, Dec. 5, 2016. (Panelist)
- Y. Chang, Sayako Haze, G.K. Tran, K. Sakaguchi, K. Araki, Hikokazu Fukawa, “User Scheduling for Millimeter Wave MU-MIMO in Small Conference Room–Evaluation with 3-sector Access Point Model and Proportional Fairness–,” BCT Technical Committee, Jul. 2016.