There are technology challenges in all the building blocks (Figure 6-10 ) to implement millimeter wave transceivers. First there is the implementation of high-gain adaptive antennas with a small form factor and real-time beam-switching to overcome the propagation loss at millimeter wave bands; these antennas are also needed to address blockage, even in the case of mobility, to ensure continuous connectivity. Further essentials include radio-frequency front ends with up to 2 GHz operational bandwidth, phase shifting capabilities, and power amplifiers that support MIMO with high linearity and viable power efficiency. Finally, a high-performance digital baseband capable of processing several giga-samples per second with power consumption useful for mobile devices is needed to run the 5G protocols.
Figure 6-10. Millimeter wave transceiver building blocks Figure 6-9. Human body blockage and shadowing
Ecosystem stakeholders are working toward new air interfaces as well as new antenna designs based on emerging massive MIMO technology that will become available in 2020. So, for example, LTE-A Pro as well as IEEE 802.11ax will provide more than one Gbps in addition to a new radio access technology based on millimeter wave technology and will be integrated with legacy radios into a transceiver in 2020 (Figure 6-11 ).
Moving in this direction, Intel rolled out an IEEE 802.11ad product for wireless docking stations [ 16 ] . The Intel Tri-Band Wireless-AC 17265 client brings 60 GHz 802.11ad wireless docking connectivity for mobile client devices. Combined with the dedicated Intel Wireless Gigabit Antenna-M 10041R antenna module and the Intel Wireless Gigabit Sink-M 13100 wireless dock, it supports a wire-equivalent user experience for wireless docking, access to peripherals, and D2D communications.
Antennas
In designing millimeter-wave communication systems, high-gain antennas are a must-have building block to compensate for the high path loss (inversely proportional to the squared wavelength) and the oxygen absorption coefficient (around 15 dB/km at 60 GHz), both of have stronger effect on data transmission over greater distances.
All of these path losses might be offset by higher antenna gain and transmit power.
Antennas for millimeter wave frequencies can be built smaller than those for lower bands, an advantage that is very significant in wireless systems, where small antennas are indispensable. But the quest for high antenna gain requires a large antenna aperture that scales proportionally to the wavelength square and creates a boundary condition.
Reducing the physical size of the millimeter wave antennas raises the issues of heat dissipation and losses in thin feeding lines.
The high antenna gain required to improve the link budget creates very narrow beams, which means the antennas of the transmitter and receiver have to be perfectly adjusted; this is done by beam steering and beam tracking in milliseconds in case of AP and/or UE mobility. Among many options to build antenna arrays there are two major ones to build a millimeter wave phased antenna array (PAA), which will be discussed further shortly. One is example of the PAA is the modular antenna array Figure 6-11. Wireless transceiver in 2020
(MAA), and another is the lens array antenna (LAA); both are verified to be practicable as implementation options for highly-directional steerable millimeter wave antennas for the 5G roll out.
PAAs in millimeter wave bands consist of multiple antenna elements each of which has its own phase shifter. Phase shifters allow us to control the antenna pattern by adjusting phases of the signal on each antenna element. The gain of the PAA depends on the number of antenna elements, but the antenna pattern is also defined by the antenna element configuration. A block diagram for the transceiver using the PAA is shown in Figure 6-12 . The transmitter (TX) includes digital baseband (BB), digital-to-analog converter (DAC) , and a radio-frequency front-end (RFE), which consists of phase shifters and power amplifiers (PA) for each antenna and the antenna array itself.
The benefit of PAAs is that the antenna array gain is proportional to the number of antenna elements. Their properties work well for interference rejection, they have good receive sensitivity, and they support fast multi-beam scanning. But the model requires a large number of antenna elements, which must be connected with phase shifters, amplifiers, and antenna elements. These connections could lead to losses and impedance mismatch, which could distort phase, and finally to a deterioration of the antenna performance.
LAA , the other implementation option for millimeter wave antennas, contains a discrete lens and a feeding source placed in the focal plane—usually a directional antenna (Figure 6-13 ).
Figure 6-13. LAA overview Figure 6-12. PAA overview
PAA and LAA are currently being improved further in particular for backhaul and front-haul millimeter wave applications, since the antenna requirements for access regarding transmit power, antenna gain, beam steering capability, physical size and cost are by far more challenging. To this end Intel has developed a modular antenna array (MAA) [ 29 ] such as shown in Figure 6-14 , which consists of multiple independent antenna subarrays, where each subarray has its own phase-shifting circuitry and RFE.
This basic antenna array module (Figure 6-15 ) has dimensions of around
9 mm × 25 mm and could be populated with 2 × 10 antenna elements, for example [ 31 ] . In the middle of the array (shown in yellow) 2 × 8 elements are used to generate a radiation pattern required for directive transmission (beamforming). One element (blue) performs an omnidirectional transmission, and three other elements (white) are there for further subarray extensions.
Figure 6-14. MAA overview
Figure 6-15. Intel basic antenna array module