  We consider the uplink of a cellular massive MIMO network. Acquiring channel
state information at the base stations (BSs) requires uplink pilot signaling.
Since the number of orthogonal pilot sequences is limited by the channel
coherence, pilot reuse across cells is necessary to achieve high spectral
efficiency. However, finding efficient pilot reuse patterns is non-trivial
especially in practical asymmetric BS deployments. We approach this problem
using coalitional game theory. Each BS has a few unique pilots and can form
coalitions with other BSs to gain access to more pilots. The BSs in a coalition
thus benefit from serving more users in their cells, at the expense of higher
pilot contamination and interference. Given that a cell's average spectral
efficiency depends on the overall pilot reuse pattern, the suitable coalitional
game model is in partition form. We develop a low-complexity distributed
coalition formation based on individual stability. By incorporating a base
station intercommunication budget constraint, we are able to control the
overhead in message exchange between the base stations and ensure the
algorithm's convergence to a solution of the game called individually stable
coalition structure. Simulation results reveal fast algorithmic convergence and
substantial performance gains over the baseline schemes with no pilot reuse,
full pilot reuse, or random pilot reuse pattern.
