4. Conclusion

Sensor networks are still in their infancy. Solutions do exist, but they are far from mature, and are therefore not applied in many situations. Improvements are needed at the physical layer level, and several solutions will undoubtedly have to coexist, depending on the requirements of the applications envisaged. It is essential, however, to combine physical layer solutions with higher layers, as well as with solutions for energy recovery and storage.

We have shown that the radio channel and interference are the essential limitations to link robustness. Hardware architectures offer numerous possibilities for ultra-low-power transmission. Ultra-broadband and millimetre-band technologies offer new solutions that should meet many challenges. But more work is needed to make these solutions commercially available. Current solutions already meet many specifications, and...