1）和低频段相反，高频段毫米波更多使用的是beam forming而不是空分复用，原因是
Higher frequencies is typically more often power-limited than bandwidth-limited, making beam forming more important than, for example, high-order spatial multiplexing. The opposite is often true for lower frequency bands where the spectrum is a more sparse resource with less possibility for wide transmission bandwidths.
2）Beam forming应用通常使用模拟beam forming，原因是
The main drawback of digital processing is the implementation complexity, especially the need for one digital-to-analog converter per antenna element. In the case of operation at higher frequencies with a large number of closely spaced antenna elements, analog multi-antenna processing will therefore be the most common case
3）频段越高要求天线尺寸越小，这样接收到的功率就越小，但是如果保持天线尺寸（孔径）不变，即增加天线数量，接收功率不会变小，但是天线的指向性会增加（指向性 正比与 天线面积/波长平方），这就要求收发天线需要指向对方
If the receiver antenna size would instead be kept unchanged as the carrier frequency increases, the reduction in captured energy could be avoided. However, this would imply that the antenna size would increase relative to the wave length, something that inherently increases the directivity of the antenna(The directivity of an antenna is roughly proportional to the physical antenna area normalized with the square of the wave length). The gain with the larger antenna size can thus only be realized if the receive antenna is well directed towards the target signal.
High frequency (mm Wave) signal in 5G means that the size of single antenna will be very small and the aperture (the area for receiving energy) will be very small. To overcome this small aperture on reciever side at high frequency, we need to use a large number of transmission antenna, which, increases the directivity of the antenna.
