RF coaxial connectors are still alive and kicking, even with the promise of massive MIMO (mMIMO) antenna systems threatening to eliminate RF jumper cables and RF connectors on these cables, the remote radio units (RRU) and base station antennas.
Figure 1 Connectors for a Huawei Technologies 16-port antenna. Source: EJL Wireless Research LLC.
Historically, the infrastructure market has transitioned from N-type to 7/16 DIN to 4.3/10 DIN (or 4.3-10) to NEX10 and 2.2/5 (or 2.2-5) coaxial connectors. The need for smaller RF coaxial connectors has been driven by the physical size limitations of the base station antenna faceplate (see Figure 1). This photo shows a 16-port base station antenna using 4.3/10 DIN connectors. Current generation antennas may require up to 30 or more RF connector ports, which is possible but not feasible, using current 4.3/10 DIN connectors. The example with 16 ports is near the maximum RF connector port density that can be achieved with the standard width of a base station antenna.
IS SMALLER BETTER?
Beyond the current 2.2/5 and NEX10 shrink lies a new solution from Telegartner, the 1.5/3.5 (or 1.5-3.5) connector, which is 75 percent smaller than the current 4.3/10 DIN connectors (see Figure 2). This is the newest connector in the Telegartner Cell IQ family. The footprint is based on the SMA form factor of a 12.7 mm square footprint for a flange mount. These connectors are targeted for small cell applications supporting up to 1/4 in. corrugated coaxial cables. While smaller is better, outdoor small cells currently support up to four transmit and four receive (4T4R) configurations and are not space limited using 4.3/10 DIN connectors, although some equipment manufacturers migrating to NEX10 or 2.2/5 connectors.
Figure 2 1.5-3.5 DIN connector. Source: Telegärtner Karl Gärtner GmbH.
RF cable losses are still critical for outdoor applications, with jumper cables typically a minimum of 3/8 in. diameter and 1/2 in. diameter preferred. Indoor small cells typically have integrated antennas that use SMT antenna components rather than flange mount antenna ports. So it is somewhat perplexing which small cell applications would require a 1.5/3.5 connector. Additionally, creating a four-port cluster connector with the 1.5/3.5 for macrocell applications is not feasible, due to potential limitations using 1/4 in. coaxial cables with their RF losses. Perhaps the market for the 1.5/3.5 is test and measurement equipment, where front panel space is limited, or for mMIMO switch matrices supporting the test equipment.
ONE FOR ALL AND FOUR (OR FIVE) FOR ONE
Figure 3 Connectors for a Huawei 26-port FDD/TDD antenna. Source: EJL Wireless Research LLC.
Figure 4 Huawei MQ4/MQ5 cluster connector. Source: Huawei Technologies.
As previously noted, a standard width antenna face plate can typically support up to 16 single RF ports, maybe 18 or 20 if the area reserved for the AISG remote electrical tilt interface is reduced. Beyond this port density, a cluster connector solution is the only viable approach to support the growing number of ports for passive antennas.
As 5G mMIMO solutions emerge, why are the port counts continuing to increase for passive antennas? The answer is that 4G services will be around for many years, with typically five frequency bands for the U.S. market (600, 700, 800, 1900 and 2100 MHz) and seven frequency bands for mobile networks in Europe and the rest of the world (700, 800, 900, 1500, 1800, 2100 and 2700 MHz), each having a separate RRU for each frequency band. The port count for the RRUs in each frequency band has been increasing as the MIMO order has increased, resulting in the current 4T4R RRU configuration for most mid-band (1800 to 2700 MHz) RRUs globally. A fully loaded, seven band, passive antenna array supporting 4×4 MIMO RRUs requires 28 antenna ports.
The advent of 5G is forcing mobile operators to consolidate all legacy 2G/3G/4G services onto a single antenna to make room for the new 5G NR antennas for each sector at a macro site. In many countries, TDD frequency bands are available in addition to FDD frequency bands for LTE services. Many mobile operators are choosing to deploy a multi-mode FDD/TDD service, which requires an FDD/TDD antenna supporting all of the operator’s frequency bands. The typical TDD configuration supports 8T8R RRUs, which requires nine RF ports. This leads to cluster connectors in 4- and 5-port configurations, where two clusters can support all nine ports. The FDD/TDD antenna example shown in Figure 3 has 10 single-port connectors along the perimeter of the antenna faceplate and four multiport clusters in the center. Each pair of the multiport cluster connectors is a 4-port + 5-port combination supporting a single 8T8R RRU, resulting in a total of 26 RF ports and two calibration ports for the TDD bands.
