  We consider spectrum sharing between a limited set of operators having
similar rights for accessing spectrum. A coordination protocol acting on the
level of the Radio Access Network (RAN) is designed. The protocol is
non-cooperative, but assumes an agreement to a set of negotiation rules. The
signaling overhead is low, and knowledge of competitor's channel state
information is not assumed. No monetary transactions are involved; instead,
spectrum sharing is based on a RAN-internal virtual currency. The protocol is
applicable in a scenario of mutual renting and when the operators form a
spectrum pool. The protocol is reactive to variations in interference and load
of the operators, and shows gains in a simulated small cell scenario compared
to not using any coordination protocol.
