With the gradual formation of 5G technical standards, the pool of candidate technical solutions is becoming more and more clear. Due to the cyclical trough of the electronic component market in recent years, global manufacturers are betting on various technical solutions in the hope that 5G commercial use can catalyze another explosion of the electronic component market. One example is the gradual substitution of ceramic dielectric filters for traditional metal cavity filters. This paper will discuss this problem from the two perspectives of miniaturization and 5G equipment power consumption.

The need for miniaturization of components in 5G base station construction

The construction of 5G base station system is a dynamic process, which can be divided into Macro-cell Base Station (mBS) and Small-cell access point (sAP) according to different usage scenarios. mBS refers to an information tower with a special frame to provide large-capacity data transmission, and is equipped with a special machine room with a wide range of signal coverage and a large capacity to carry the number of users. sAP, on the other hand, has all its devices in a small case, which is like an all-in-one device. It is often installed in buildings or streets where the number of users is dense.

According to the rules of 3GPP, besides Acer station and micro base station, there are also Pico Site (also known as Pico station/enterprise small base station) and fly base station (Pico station/family small base station). Leather base station and flying base station are also collectively known as "leather flying station" in the industry; In addition to the Acer station, the three types of wireless base station are collectively referred to as "microbase station (also known as small base station)". Huawei has said that 5G network construction needs to be selected from a variety of factors, in the initial construction of 5G base station system will be 64T64R (note: Massive MIMO antenna application requires the number of RF channels to increase to 64 send and 64 receive, or 32T32R, 16T16R. The 4G era is 2T2R.) Base station equipment for the Acer station scheme. The 2019 Spring Festival Gala in Shenzhen was broadcasted live in HD using the above networking scheme.

The microbase station is mainly built in the area where the area of the Acer station is limited. The base station is located in high user density areas such as indoor buildings, stations, airports, shopping malls and large enterprises. Base stations are installed in homes or micro businesses.

The function of base station filter is to make the specific frequency components in the sent and received signal pass through, and greatly attenuate other frequency components, is the key element of RF unit. The filter at the base station side in the 4G era generally uses the metal coaxial cavity filter. By oscillating electromagnetic waves of different frequencies in the metal coaxial cavity, the electromagnetic waves conforming to the resonant frequency of the filter are retained and the electromagnetic waves of other frequencies are dissipated. The fixed resonator is an essential component. Nowadays, most of the miniaturization researches of cavity filters are trying to optimize the design of fixed resonators to reduce the filter volume.

The ceramic filter is sintered by dry pressing powder, two sides covered with silver electrode metallization, after DC high voltage polarization to produce piezoelectric effect. The two sides of the ceramic plate are deformed to generate electric fields and have series resonance characteristics. Because the above electromagnetic resonance occurs inside the pressure ceramic medium, no fixed resonator is needed, so the volume of the dielectric filter is smaller. On the other hand, compared with the metal cavity, the microwave ceramic powder has the material characteristics of high Q value, greatly reducing the plug loss, and has the characteristics of high out-of-band suppression, good temperature drift characteristics, wide temperature application range, various forms of packaging structure and output interface form. It meets the development trend of the base station filter miniaturization, and becomes suitable for the 5G technology framework. Due to the increase of frequency band, the number of equipment filters needs to increase, and the higher requirements for Q value and power capacity of high-frequency resonators (" Why Dachang chooses ceramic filter instead of BAW with higher performance Coming of 5G era ", engineering signal integrity).

The need for low power consumption of components in 5G base station construction

Another hot topic in the construction of 5G base stations is the visible growth of high power consumption. Base station equipment is composed of AAU, BBU, RRU and other unit modules. The function of AAU is to convert baseband digital signals into analog signals, and then to make high-frequency radio frequency signals, which are amplified by power amplifier unit and transmitted by antenna. BBU is responsible for baseband digital signal processing; RRU is responsible for the conversion of the IF signal to the RF signal by the transceiver module, and then the RF signal is transmitted through the antenna port through the power amplifier and filtering module. Taking Huawei's 5G base station as an example, the base station units include: 5G AAU+2/3/4G antenna + multi-frequency RRU+BBU+ power supply.

However, during the networking of the base station system, the speed increases and the energy consumption increases significantly. According to the industry conference last year, the report pointed out that: [1] the power consumption of 5G single station is 2.5~3.5 times that of 4G single station, and the increase of AAU power consumption is the main reason for the increase of 5G power consumption; [2] Currently, the full load power of a single station is nearly 3700W, so it is necessary to expand the power supply and supporting facilities of the live network in advance.

It is an indisputable fact that power consumption increases. According to the case experience of Acer Station built by China Tower, the original 4G base station uses 220V voltage power supply, while 5G base station needs to be expanded to 380V voltage power supply. However, transformer capacity of power companies has not been expanded, so China Tower needs to be charged extra during the construction stage. In the base station renovation project, three-phase four wires and a ground wire are needed for 380V voltage. The original 220V only has neutral line and live line, which means that the line needs to be re-drawn and the three-phase electricity meter needs to be hung. Not to mention the cost of electricity.

The increase in power consumption brings a direct problem to the equipment components, namely the increase in temperature, especially for the micro base station without machine room facilities. Taking the working condition specifications of Huawei AAU3940 outdoor microbase station as an example, without considering solar radiation, the operating temperature ranges from -40 ℃ to +50℃ when the transmitted power is 30W, and from -40 ℃ to +40℃ when the power is 40W. At present, ceramic filter has been supplied in the market, working temperature specifications in the range of -40~+85℃.

When the filter is applied in 5G equipment, compared with the cavity filter, the volume and shape of the resonant cavity of the latter will change with the temperature, thus causing the change of the resonant frequency. Therefore, the metal cavity filter needs to use the temperature compensation technology. In the temperature compensation method of coaxial resonator, it is common to limit the length of conductor in coaxial cavity. However, in this scheme, the external conductor changes and the resonant frequency is also affected.

In terms of material classification, quartz BAW (bulk acoustic wave) filter is as insensitive to temperature as ceramic medium. It is not easy to bend at higher temperature as SAW (surface acoustic wave) filter. Although FBAR (thin film cavity acoustic resonance) filter is superior to BAW and ceramic dielectric filter in high frequency range, Q value, insertion loss, size and integrability, the current process and capacity are still in the development stage. And domestic BAW filter, now concentrated in the Neville technology, Xinwei Communication, Sega technology, Wuhan Fan Valley and other a few enterprises.