3. Hybrid Precoding and Combining for Massive MIMO Wireless Backhauling

In this chapter, we explore a joint design of two-timescale hybrid precoding and combining for massive multiple-input multiple-output (MIMO) wireless backhaul communications. With the objective of effective capacity maximization, the statistical channel state information (CSI)-based RF and the instantaneous effective CSI-based baseband solutions are jointly derived for the transmitter and receiver. Previous work such as El Ayach et al. (IEEE Trans Wireless Commun 13(3):1499–1513, 2014), Yu et al. (IEEE J Sel Topics Signal Process 10(3):485–500, 2016), where perfect CSI-based RF and baseband designs were simultaneously obtained as a reconstruction of the optimal counterpart, is no longer applicable. Under the assumption of jointly correlated channels (Weichselberger et al. IEEE Trans Wireless Commun 5(1):90–100, 2006, Tulino et al. IEEE Trans Wireless Commun 5(3):662–671, 2006, Gao et al. IEEE Trans Inf Theory 55(8):3735–3750, 2009), the objective function does not have a closed form. Considering traditional MIMO processing at baseband, we address the RF design with nonuniform- and constant-modulus elements.