However, to make use of the increased antenna gain the transmit (Tx) and receive (Rx) beams must be aligned. If the antenna area is assumed to be constant at both ends of the link, the free-space transmission loss will actually decrease with frequency since the antenna gain increases with frequency if the antenna area is constant. If instead the antenna area is assumed to be constant at one end of the link, the free-space transmission loss is frequency independent. This is an effect of the fact that the antenna effective area is proportional to the wavelength squared, as explained in Chapter 3. In free-space propagation, the transmission loss increases with frequency if the antenna gain is assumed to be constant at both ends of the link see Chapter 3. To make use of this directivity, dynamic and user-specific beam-forming is needed, since the directions to the users in a wireless access network are not known a priori and are dynamically changing. This may, depending on the directional properties of the channel, compensate for the increase of transmission loss with frequency and even be turned into a gain, as explained in Chapter 3. On the other hand, increasing the carrier frequency also means that, for a given physical size of an antenna, it becomes more directive.
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Compensating this with higher transmission power might not be possible due to limitations in millimeter-wave hardware design and to current regulations on transmitted power being stricter at frequencies above 6 GHz. A large bandwidth can exacerbate this further due to increased thermal noise power in the receivers. Instead, obtaining coverage is the main challenge as substantially higher transmission losses may occur due to smaller antenna apertures and in some cases also higher attenuations, as explained in Chapter 3.
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Xiaoming Chen, in 5G Physical Layer, 2018 7.1.2 High FrequenciesĪt high frequencies, the spectral efficiency is less crucial since there is plenty of spectrum available.