Multiuser diversity gain in cognitive networks with distributed spectrum access

Opportunistic allocation of resources to the best link in large multiuser networks offers considerable improvement in spectral efficiency, which is often referred to as multiuser diversity gain and can be cast as double logarithmic growth of the network throughput with the number of users. In this paper we consider large decentralized cognitive networks granted concurrent spectrum access with license-holding users. We assume that the primary network affords to accommodate one secondary user per any under-utilized spectrum band and seek allocating such spectrum bands to a subset of the existing secondary users. We first consider the optimal spectrum-secondary user pairing, which is supervised by a central entity fully aware of the instantaneous channel conditions, and show that the throughput of the cognitive network scales double logarithmically with the number of secondary users (N) and linearly with the number of available spectrum bands (M), i.e. M log logN. Next, we propose a distributed spectrum allocation scheme, which does not necessitate a central controller or any information exchange between different secondary users and obeys the optimal throughput scaling law. This scheme requires that some secondary transmitter-receiver pairs exchange logM information bits among themselves. We also show that the aggregate amount of information exchange between secondary transmitter-receiver secondary pairs is also asymptotically equal to M logM. Finally, we show that our distributed scheme, also guarantees fairness among the secondary users, meaning that they are equally likely to get access to an available spectrum band.