  Ultra-dense (UD) wireless networks and cloud radio access networks (CRAN) are
two promising network architectures for the emerging fifth-generation (5G)
wireless communication systems. By jointly employing them, a new appealing
network solution is proposed in this paper, termed UD-CRAN. In a UD-CRAN,
millimeter-wave (mmWave) wireless fronthaul is preferred for information
exchange between the central processor and the distributed remote radio heads
(RRHs), due to its lower cost and higher flexibility in deployment, compared to
fixed optical links. This motivates our study in this paper on the downlink
transmission in a mmWave fronthaul enabled, orthogonal frequency division
multiple access (OFDMA) based UD-CRAN. In particular, the fronthaul is shared
among the RRHs via time division multiple access (TDMA), while the RRHs jointly
transmit to the users on orthogonal frequency subchannels using OFDMA. The
joint resource allocation over the TDMA-based mmWave fronthaul and OFDMA-based
wireless transmission is investigated to maximize the weighted sum rate of all
users. Although the problem is non-convex, we propose a Lagrange duality based
solution, which can be efficiently computed with good accuracy. To further
reduce the complexity, we also propose a greedy search based heuristic, which
achieves close to optimal performance under practical setups. Finally, we show
the significant throughput gains of the proposed joint resource allocation
approach compared to other benchmark schemes by simulations.
