[32,33]: where PTX and PRX denote the transmitted and the received signal power, GTX and GRX denote the transmitting and receiving antenna gains, d is the distance between the transmit- ter and the receiver, and l is the wavelength of the RF signal. In mmWave bands, the smal- ler wavelength translates into smaller captured energy at the receive antenna. For example, in the frequency range 3-30 GHz, an additional 20 dB path loss is added due to effective Chapter 3 mmWave effective aperture Microwave effective aperture Transmitter antenna (isotropic radiator) 4t Grx Transmitter antenna Receiver antenna Receive spectral Effective receiver density aperture No =kT.B = Microwave noise bandwidth mm) Wave noise bandwidth Figure 3.2 Definition of gain and aperture in mmWave [32]. aperture reduction. On the other hand, larger bandwidths in mmWave mean higher noise power and lower signal-to-noise ratios (SNRs), for example, from 50 to 500 MHz band- width, the noise power is increased by 10 dB (see Fig. 3.2). Free-space conditions require a direct line of sight (LoS) between the two antennas involved. Consequently, no obstacles must exist in the path between the antennas at both ends. Furthermore, in order to avoid the majority of effects caused by superposition of direct and reflected signals, it is necessary that the first Fresnel zone 1 is completely free of obstacles. The first Fresnel ellipsoid is defined as a rotational ellipsoid with the two anten- nas at its focal points. Within this ellipsoid, the phase difference between two potential paths is less than half a wavelength. The radius b at the center of the ellipsoid can be calcu- lated as b = 8.66 /d/f where b is the radius in meters, d is the distance between the receiver and the transmitter in kilometers, and f is the frequency in GHz [26,27,32]. A Fresnel zone is one of a series of confocal prolate ellipsoidal regions of space between and around a trans- mitting antenna and a receiving antenna system. The regions are used to understand and compute the strength of