## 4 Cognitive Solutions for HetNets and M2M Communications
### Short-Term Cognitive Approaches
Indeed, when a resource is declared as used by the primary system, it is considered busy by all secondary users and cannot even be used by a user located far away from the primary system, which could in fact operate without interference. The second method is completely cognitive, but presents more challenges in the sensing phase, not only in terms of accuracy of the results, but also in terms of latency between acquisition of the context awareness and its use and channel reciprocity between the UL and the DL. In some \begin{table}
\begin{tabular}{c c c} \hline \hline
**Cognitive approaches** & **Long-Term** & **Short-Term** \\
**features** & & \\ \hline Smart city & local and access layer & access layer \\ communication layer & (CM2M and CHetNets) & (CHetNets) \\ \hline Sensing period & several frames & scheduling \\  & & period \\ \hline Transmission & & Resource \\ opportunity & sub-bands & Units \\ \hline Technical & suitable trade off: & fast sensing \\ challenges & cost-accuracy & feedback information \\  & of sensing & joint resource allocation \\ \hline Spectrum efficiency & Low & High \\ \hline Secondary Network & & synchronization \\ Requirements & & and legacy terminal \\ \hline Distributed sensing & yes & no \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cognitive Approaches. CM2M: Cognitive machine-to-machine; CHetNet: Cognitive heterogeneous networks. cases, hybrid techniques can represent a viable solution whenever limited knowledge of the network is available, and therefore only partial coordination among the cells is possible. The method proposed in [55] first detects channel occupation by estimation of the energy in the UL sub-channels, and then allocates the sub-channels with the lowest interference signatures to the small cell users. The hypothesis here is that the same resource scheduling process is used in both UL and DL transmissions. However, it is more likely that the DL and the UL are characterized by asymmetric traffic, and thus adopt different resource allocation policies. Therefore, UL sensing cannot be used for DL allocation. An alternative approach is proposed in [43] by considering a hybrid sensing scheme in which the scheduled macrocell BS information is available at the small cell BS (SBS) in order to increase the spectrum sensing accuracy. In this way, the secondary system finds more spectrum opportunities by identifying nearby macrocell users. Inter-layer interference can also be limited or prevented using optimal power allocation and using underlay cognitive approaches, as in [56], where the SBS senses both the UL and DL of the macrocell to be aware of both the resource occupancy and nearby macrocell users. The algorithm then adapts the power on each resource element to maximize the achievable small cell throughput and fulfill the macrocell users' outage constraints. Exploitation of multi-antenna technologies is an additional opportunity to use information related to the primary and secondary users' position in order to allow co-channel frequency allocation between primary and secondary systems. A useful approach is to use beamforming at the small cell transmitter in order to maximize the secondary system performance while the interference on the primary system receiver is minimized [57]. This operation is named Cognitive Beamforming (CB) and requires complex numerical solutions and the knowledge of all propagation channels. This could be impracticable in actual scenarios, as the two systems operate in an independent mode that prevents each of them from detecting the presence of the other. Hence, it is necessary to resort to sub-optimal solutions that can work with a partial knowledge of the channel state information and with some information exchange between primary and secondary networks. A viable low complexity CB approach is based on the exploitation of the direction of arrival (DoA): the secondary system can transmit avoiding interference on the primary user by placing nulls in its direction.