  In this paper, we investigate the potential benefits of the massive
multiple-input multiple-output (MIMO) enabled heterogeneous cloud radio access
network (C-RAN) in terms of the secrecy and energy efficiency (EE). In this
network, both remote radio heads (RRHs) and massive MIMO macrocell base
stations (BSs) are deployed and soft fractional frequency reuse (S-FFR) is
adopted to mitigate the inter-tier interference. We first examine the physical
layer security by deriving the area ergodic secrecy rate and secrecy outage
probability. Our results reveal that the use of massive MIMO and C-RAN can
greatly improve the secrecy performance. For C-RAN, a large number of RRHs
achieves high area ergodic secrecy rate and low secrecy outage probability, due
to its powerful interference management. We find that for massive MIMO aided
macrocells, having more antennas and serving more users improves secrecy
performance. Then we derive the EE of the heterogeneous C-RAN, illustrating
that increasing the number of RRHs significantly enhances the network EE.
Furthermore, it is indicated that allocating more radio resources to the RRHs
can linearly increase the EE of RRH tier and improve the network EE without
affecting the EE of the macrocells.
