Dear sir

You can deploy your network in Adhoc manner, which the case of lot  of CRN.

Please, read this article:

http://www.academia.edu/1062014/Efficient_Spectrum_Utilization_A_Cognitive_Approach

Best regards

you can use the cognitive radio, spatially the sensing techniques (energy detection, matched filter and cyclostationary)   .

Please see some of our D2D communication papers ... Spatial reuse is the main thing! Reduce the cell size and increase reuse.

There are various efficient proposed schemes that aim at the maximization of spectrum usage in the context of CRNs. The following link provides a brief and vivid description of all the existing such techniques:

http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5639025

Thank you all for your contributions.

There are so many papers dealing with cognitive radio networks and suggest new architectures for deployment.

But there are some economical and technical problems associated with small cells (femto and pico cells), their coverage, the operating frequeny range, the interferences. Other problems are linked to the limitations in the Rf-FE  and antenna designs which are submitted to laws of physics.

A cooperation between operators is a must for a better use of radio spectrum, but are they willing to do so?

Exactly. In addition to everything mentioned above: smaller cells, CR principles, D2D, spectrum sharing could contribute towards greater spectrum utilisation efficiency.

Frameworks such as licensed shared access, generalized authorized access, unlicensed access (CR) are being investigated to achieve that. I (optimistically) believe that operators should be interested in spectrum sharing if all parties involved benefit from it. We just need to keep working on schemes and frameworks that could achieve that reliably (with some QoS/QoE guarantees to everyone).

I totally understand your concerns. I have talked about many such issues on my website: http://www.cognitive-radio-networks.blogspot.com.

Operators does not like sensing mechanisms due to the risk of interference, even short, with primary traffics and prefer central databases for free bands assignment. In addition, many operators already possess full rights on some bands and it is very hard for them to accept spectrum sharing except with real primary traffic protection and QoS guarantees. However, cognitive radio is gaining high user acceptance during the last years and operators are conducting many trials around the world to test the performance of cognitive radio technology. At the meantime, sensing does make sense because it is very difficult to monitor and coordinate some traffics like wireless microphones, remote control, tele-health, ....

We can increase the spectral-efficiency by using techniques like super-dense small cells with D2D communication. Although there are plenty of other legacy spectrum-efficiency techniques like spectral-efficient modulation and coding techniques, but these days the D2D communication with greater reuse of the same band within a given space is possible by deploying Super Small Cells (SCM). 

Answer to your question is wide. One way is D2D communications. Also, underlaying CRN is another solution. Using MIMO methods, Smart antenna and so on are another solutions.

we can increase spectral efficiency by different multiple access schemes like NOMA,SCMA etc. As well as MIMO-NOMA is a mere technique to improve spectral efficiency..