Cognitive channel models

Depending on the nature of side information available at the cognitive users, and on the interference constraints on the primary users, we can identify three different models for a cognitive network: the underlay, overlay and interweave paradigm.

In the underlay model, the cognitive users assume knowledge of the interference caused by their activity to the primary receivers. Under this assumption, the cognitive users are allowed to transmit, as long as an interference constraint on the primary users is not violated. This is accomplished, e.g., by the secondary users knowing the channel state of the primary links, as well as of the interfering links (secondary to primary). The secondary users can thus adapt their transmission power, so as to meet the minimum SINR requirement at the primary receivers.

In the overlay model, the cognitive transmitters have knowledge either of the primary users' codebooks or their messages. This knowledge can be exploited to mitigate the interference of the primary users' transmissions to the secondary receivers, via techniques such as dirty paper coding; alternatively, the cognitive users may sacrifice part of their power to assist the primary users, so as to offset the decrease in SINR due to interference from the secondary users to the primary users, with an increase in SINR due to secondary users' assistance. This latter technique allows the cognitive users to achieve a non-trivial information flow, while not disrupting the primary users' communication.

In the interweave model, on the other hand, the cognitive users are required to not interfere with the primary users. This is accomplished by leveraging the so called spectrum-holes, i.e., the space-time-frequency slots which are not used by the primary users, as a consequence of their intermittent activity. This requires the cognitive users to collect side information about the primary users' activity, which is accomplished by sensing the radio channel.