  We introduce a family of PCNs (Payment Channel Networks) characterized by a
semi-hierarchical topology and a custom set of channel rebalancing strategies.
This family exhibits two interesting benefits, if used as a platform for
large-scale, instant, retail payment systems, such as CBDCs: Technically, the
solution offers state-of-the-art guarantees of fault-tolerance and integrity,
while providing a latency and throughput comparable to centralized systems;
from a business perspective, the solution perfectly suits the 3-tier
architecture of the current banking ecosystem (central banks / commercial banks
/ retail users), assigning a pivotal and peculiar role to the members of each
tier. Furthermore, the cryptographic privacy of payments for retail users --
typical of PCNs such as the public Lightning Network -- is largely (possibly
fully) retained. We study the system by simulating a scaled-down version of a
hypothetical European CBDC, exploring the trade-offs among liquidity locked by
market operators, payment success rate, throughput, latency, and load on the
underpinning blockchain.
