 not expect in case of large antennas. These extra degrees of freedom allow to increase the number of M. D. Migliore spatial subchannels at the PL level. The effect of "extra" degrees of freedom can be significant in the case of small electrical antennas [73, 74]. In order to obtain an estimation of the maximum STPNDF of a communication channel, let us consider an observation surface covering the entire sphere surround- ing the source . In a very narrow bandwidth df, the ST NDF (without considering polarization) is 2 2Tdf2W(f)E(f) wherein T is the observation interval, W(f) is the spatial bandwidth, and E(f) is the extension of the observation domain. Note that W and E, and hence the STNDF, depend on the frequency. In particular, the SNDF is proportional to the electric area of the surface of the electromagnetic source, i.e.,~ A/(22/4)[22]. Consequently, an upper bound for the STPNDF in case of an electromagnetic field carrying passband modulated signals (fi < f < fu) with bandwidth B = fu - fi is [19]: (2.15) wherein fc = (fi + fu)/2 is the central frequency, the factor 2 at the beginning of the integrand is the Polarization Number of Degrees of Freedom, and C is the speed of light in the vacuum. The STPNDF is a fundamental quantity that limits the maximum performance of the system. For example, supposing that the bits transmissible per degree of freedom is log2(1+ SNR)/2, SNR being the Signal to Noise Ratio at the receiver, the capacity at the DPL in case of No Channel State Information (NCSI), i.e., if the receiver has no feedback information on the channel by the transmitter, turns out to be3[19]: (2.16) It is useful to note that any mathematical processing performed at the higher layer of the communication system cannot increase the amount of information set at the lower layer [75]. Accordingly, the maximum amount of information that can be reliably transmit- ted using the electromagnetic field is fixed in the Deep Physical Layer. Formula (2.15) shows that in case of a radiating surface the SNDF grows accord