  Currently, mobile network operators are allocated spectrum bands on an
exclusive basis. While this approach facilitates interference control, it may
also result in low spectrum utilization efficiency. Inter-operator spectrum
sharing is a potential method to enhance spectrum utilization. In order to
realize it, a protocol to coordinate the actions of operators is needed. We
propose a spectrum sharing protocol which is distributed in nature, it does not
require operator-specific information exchange and it incurs minimal
communication overhead between the operators. Operators are still free to
decide whether they share spectrum or not as the protocol is based on the book
keeping of spectrum usage favors, asked and received by the operators. We show
that operators can enhance their QoS in comparison with traditional orthogonal
spectrum allocation while also maintaining reciprocity i.e. no operator
benefits over the other in the long run. We demonstrate the usability of the
proposed protocol in an indoor deployment scenario with frequent network load
variations as expected to have in small cell deployments.
