  Coordinated multi-point (CoMP) communication is attractive for heterogeneous
cellular networks (HCNs) for interference reduction. However, previous
approaches to CoMP face two major hurdles in HCNs. First, they usually ignore
the inter-cell overhead messaging delay, although it results in an irreducible
performance bound. Second, they consider the grid or Wyner model for base
station locations, which is not appropriate for HCN BS locations which are
numerous and haphazard. Even for conventional macrocell networks without
overlaid small cells, SINR results are not tractable in the grid model nor
accurate in the Wyner model. To overcome these hurdles, we develop a novel
analytical framework which includes the impact of overhead delay for CoMP
evaluation in HCNs. This framework can be used for a class of CoMP schemes
without user data sharing. As an example, we apply it to downlink CoMP
zero-forcing beamforming (ZFBF), and see significant divergence from previous
work. For example, we show that CoMP ZFBF does not increase throughput when the
overhead channel delay is larger than 60% of the channel coherence time. We
also find that, in most cases, coordinating with only one other cell is nearly
optimum for downlink CoMP ZFBF.
