RF MEMS for 5G Small Cell Applications

San Jose, CA  - April 19, 2019

Cavendish Kinetics RF MEMS for 5G Small Cell Applications

To achieve the performance goals of 5G, network infrastructure will need to be considerably denser than the 4G LTE network.  Proximity to a network node will have a large effect on the data rates realized by the user.  As such, carriers plan to use a large number of small cells with phased array antennas connected to a local processing resource. The number of small cells required to cover a given area will depend on a number of factors including terrain, buildings, foliage, and population density. It will also depend on the frequency the carrier is using in that region.

The Friis Transmission Formula (shown below) reveals the underlying physics driving the need for small cells.

As seen from this equation, the received power (PR) is directly related to the transmitted power (PT) and gain of both the transmit and receive antennas (GT, GR), and inversely related to the square of separation distance R, and frequency of operation f.

There are several new frequency bands being considered for 5G network deployment. Most of these frequencies are above the existing 4G LTE bands.

According to the Friis Transmission Equation, the received power will drop as the frequency goes up with all other terms held fixed. Doubling the frequency will cause the received power to drop by 6 dB. This is the same effect as cutting the range in half. Depending on regulatory restrictions it may not be possible to compensate by arbitrarily increase the transmitter power. An effective option is to shorten the distance between the cell site and the user (hence small cells) to recover the losses incurred from higher frequency of operation.

Another effective option to improve the received power is to increase the antenna gain on both the small cell and user equipment.   Antenna gain is defined as follows:

G = ED, where G = gain, D = directivity and E= efficiency

This equation shows a direct relationship between antenna gain and directivity. As the gain of an antenna goes up it becomes more directive and the beam width becomes narrow.

To effectively improve network performance, an electronically steerable phased array can be used to make sure the antenna pattern is always pointed in the right direction to maximize the received power. Phased arrays can be used on both the small cell and CPE. This feature becomes important at higher frequencies since line of sight between transmitter and receiver is not always possible. Beam forming is used to optimize the link by redirecting the antenna beam peak toward the best signal even if that is a reflected path rather than a direct path.

The following illustration shows an array of antenna elements where the peak of the antenna pattern is pointed in different directions by adjusting the phase relationship between the radiating elements.

In addition to beam forming, the figure below shows how phased arrays are used to develop multiple beams that can track several users simultaneously. Individually tracking several users improves the link and reduces the interference between those users.

Cavendish Kinetics' (CK) RF MEMS technology can provide significant value when used in the RF front end as part of a phased array. CK MEMS tuners have been selected by 5G infrastructure vendors for use in RF phase shifter circuits attached to each element in the array. The critical parameters for the tuner are ESR (equivalent series resistance) and capacitor step value.  ESR impacts the insertion loss through the phase shifter.  Capacitor step value impacts the size of the phase increments between settings. Small capacitor step value leads to finer phase increment.

The plot below shows a comparison of insertion loss between CK RF MEMS and SOI technology between 0.5 and 6 GHz. This illustrates the benefit of MEMS which delivers half the loss compared to SOI for sub-6 GHz 5G frequencies.

Many wireless network providers will implement 5G at higher frequencies than current 4G LTE networks. To compensate for the additional path loss at higher frequencies, network base stations will need to be more numerous and closer together (small cells). Phased arrays with high gain, electronically steerable beams will also be used to improve the link between the small cell and user. CK MEMS tuners will be used to create phase shifters with low loss and high resolution, enabling better system performance.