  Heterogeneous network (HetNet) is a key enabler to largely boost network
coverage and capacity in the forthcoming fifth-generation (5G) and beyond. To
support the explosively growing mobile data volumes, wireless communications
with millimeter-wave (mm-wave) radios have attracted massive attention, which
is widely considered as a promising candidate in 5G HetNets. In this article,
we give an overview on the end-to-end latency of HetNets with mm-wave
communications. In general, it is rather challenging for formulating and
optimizing the delay problem with buffers in mm-wave communications, since
conventional graph-based network optimization techniques are not applicable
when queues are considered. Toward this end, we develop an adaptive low-latency
strategy, which uses cooperative networking to reduce the end-to-end latency.
Then, we evaluate the performance of the introduced strategy. Results reveal
the importance of proper cooperative networking in reducing the end-to-end
latency. In addition, we have identified several challenges in future research
for low-latency mm-wave HetNets.
