There have been considerable work on mmWave propagation at the 60 GHz band [5], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. The free space propagation loss is proportional to the square of the carrier frequency. With a wavelength of about 5 mm, the free space propagation loss at 60 GHz is 28 decibels (dB) more than at 2.4 GHz [14].  Besides, the Oxygen absorption in the 60 GHz band has a peak, ranging from 15 to 30 dB/km [30]. The channel characterization in [31] shows that the non-line-of-sight (NLOS) channel suffers from higher attenuation than the line-of-sight (LOS) channel. The large scale fading F (d) can be modeled as follows.
F (d) = P L(d 0 ) + 10nlog 10 d d 0 − S σ ,(1)
where P L(d 0 ) is the path loss at reference distance d 0 , n is the path loss exponent, and S σ is the showing loss. σ is the standard deviation of S σ . In table I, we list the statistical parameters of the path loss model obtained in a corridor, a LOS hall, and a NLOS hall [31]. We can observe that the path loss exponent in the LOS hall is 2.17, while the path loss exponent in the NLOS hall is 3.01. To combat severe propagation loss, directional antennas are employed at both transmitter and receiver to achieve a high antenna gain. For the small-scale propagation effects in the 60 GHz band, it is found that the multipath effect is not obvious with directional antennas. By using circular polarization and receiving antennas of narrow beam width, multipath reflection can be suppressed [32], [33]. In the LOS channel model in the conference room environment proposed in IEEE 802.11ad [9], the direct path contains almost all the energy, and nearly no other multipath components exist. In this case, the channel can be regarded as the Additive White Gaussian Noise (AWGN) channel. In the NLOS channel, there is no direct path, and the number of paths with significant energy is small. To achieve high **data** rate and maximize the power efficiency [34], mmWave communications mainly rely on the LOS transmission.