  The operating range of a single millimeter wave (mmWave) access point (AP) is
small due to the high path loss and blockage issues of the frequency band. To
achieve the coverage similar to conventional sub-6GHz networks, the ultra-dense
deployments of APs are required by the mmWave network. In general, the mmWave
APs can be categorized into backhaul-connected APs and relay APs. Though the
spatial distribution of backhaul-connected APs can be captured by the Poison
point process (PPP), the desired locations of relay APs depend on the
transmission protocol operated in the mmWave network. In this paper, we
consider modeling the topology of mmWave AP network by incorporating the
multihop protocol. We first derive the topology of AP network with the spatial
multiplexing disabled for each transmission hop. Then we analyze the topology
when the spatial multiplexing is enabled at the mmWave APs. To derive the
network throughput, we first quantify the improvement in latency and the
degradation of coverage probability with the increase of spatial multiplexing
gain at mmWave APs. Then we show the impact of spatial multiplexing on the
throughput for the ultra-dense mmWave AP network.
