Microwave Journal
www.microwavejournal.com/articles/27955-base-station-market-reinventing-connectors

Base Station Market Reinventing Connectors

March 13, 2017

Figure 1

Figure 1 Typical configurations for a single sector 2 × 2 MIMO architecture: RRU at the top of the tower (a) RRU and tower mast amplifier (TMA) at the top of the tower (b) and TMA at the top and RRU at the bottom of the tower (c).

Traditional RF connectors for base station antennas, remote radio units (RRU) and jumper/feeder cables have relied upon legacy 7-16 DIN and N-type connector technologies. For legacy 2G/3G networks, these RF connectors performed adequately. However, the migration to 4G networks has prompted the industry to rethink its strategy for addressing one of the significant linearity issues of LTE technology: passive intermodulation— better known by its acronym PIM.

Poor PIM performance from any component in the transmit or receive path within an LTE system causes poor linearity, which ultimately degrades the modulation capability of the overall sector, site and network. Frequency division duplex (FDD) LTE networks are more susceptible to PIM than time division duplex (TDD) LTE networks. FDD-LTE networks have used 7-16 DIN connectors for everything in the transmission line system from the radio to the antenna. TDD-LTE networks have relied on N-type connectors for the transmission line system. The PIM performance of 7-16 DIN connectors is significantly better than that of N-type connectors, yet it is still not what the market would like to see.

To support a single frequency band, each sector of a macro cell site using 2×2 multiple-input-multiple-output (MIMO) requires two RF connectors for the tower-top RRU, four RF connectors for the jumper cables and another two RF connectors on the base station antenna—a total of eight connectors (see Figure 1a). The number of connectors increases by eight for each additional frequency band added to the sector; with 4x4 MIMO, the number doubles to 16 per frequency band per sector.

To meet the market’s appetite for ever-higher data rates, the wireless infrastructure will incorporate multiple changes over the next few years:

  • From 2×2 MIMO to 4×4/4×2 and 8×8 MIMO for FDD-LTE RRUs
  • From 6/8-port base station antennas to 12/14 and, in some cases, 24-port antennas
  • New LTE spectrum
  • RRUs deployed at the bottom of the tower to minimize tower loading
  • In-building distributed antenna systems (DAS) for LTE/UMTS coverage
  • Massive MIMO for 5G antenna systems.

These changes will increase the connector volume while demanding better connector performance. The following paragraphs discuss each of these drivers.

Higher-Order FDD-LTE MIMO

The migration to higher order MIMO for RRUs will double the port and RF connector count from two to four for the majority of RRUs shipping in the future. First-generation FDD-LTE MIMO is based on a 2T2R (T = transmit and R = receive) configuration. Second-generation FDD-LTE MIMO has been deployed with some mobile operators in the United States and Canada, along with adoption in Korea and Japan. European operators are lagging behind North America and Asian operators in migrating to 2T4R and 4T4R RRUs. One of the drivers for the 2T4R RRU configuration is to improve uplink (UL) performance as operators deploy voice over LTE (VoLTE). VoLTE reduces UL performance by up to 3 dB, resulting in significant cell shrinkage and a higher rate of dropped calls, as handoffs are not possible with coverage gaps between macro cell sites. Doubling the number of receivers in the RRU extends the macro cell coverage on the UL path, which can limit the required output power from the user equipment (UE)—also improving battery performance.

A potential driver for adopting 4T4R RRUs within an FDD-LTE network is the migration of UE video displays and streaming from 1K to 2K and >4K for iPhones and Android mobile phones and tablets. The Apple iPhone 7 is equipped with a 1K display, while the new iPhone 7+ has a 2K display. The new Samsung Galaxy 8 is reported to have a 4K display. Video services such as Netflix, Amazon Prime, Hulu and YouTube are already streaming 4K video with some level of compression. New mobile phones and tablets will drive the penetration of 2K and 4K devices through 2020. The requirements for increased downlink (DL) throughput to support 4K video can be achieved with LTE-Advanced by using a combination of MIMO and multi-band carrier aggregation (CA). The demand from mobile subscribers may force network upgrades to 4×4 MIMO if the bandwidth to support video streaming over mobile networks is not available with the use of CA.

Higher Port Antennas

Site restrictions from weight, wind load and aesthetics are major issues plaguing all mobile operators. Some countries have a strict regulatory environment that limits a macro cell site to only one panel antenna per sector. To meet such requirements, mobile operators need to deploy multiple frequency bands and, potentially, higher-order MIMO on a single sectorized panel antenna. In North America, a mobile operator supporting 2×2 MIMO for LTE at 700, 1900 and 2100 MHz will require two RF connector ports for each spectrum band, or a total of six RF connector ports on a single base station antenna. If the MIMO technology is upgraded to 4×4, 12 connector ports will be needed for the same antenna. Adding 850 MHz spectrum to the mix will require 16 RF connector ports. This example is a simplified version of what really may be required, as it assumes the same electrical tilt can be used for multiple air interfaces (UMTS/LTE) within the same spectrum.

New LTE Spectrum

Each new spectrum auction adds more RRU deployments for the mobile operators within a country. Currently, the North American market is undergoing an RRU upgrade in the 2100 MHz spectrum, from band 4 to a wider band 66. The eventual conclusion of the FCC’s UHF reverse/forward auctions will lead to new RRUs deployed at 600 MHz for the winning mobile operators. Greenfield deployment of the Citizens Band Radio Service (CBRS) at 3.5 GHz will also increase the demand for RF connectors. It’s hard to see an end to new spectrum for mobile operators for 4G/4.5G and 5G technology.

Tower Loading

Most mobile operators deploy the RRU as close to the base station antenna as possible, to improve UL performance and reduce RF loss through the jumper cables. However, the typical weight of an RRU is approximately 25 kg, and each frequency band requires a different RRU. A three-sector macro site supporting four frequency bands results in a weight load of 300 kg, not including the weight of the sectorized panel antennas and ancillary tower equipment. Current towers were designed to hold base station antennas, and the tower designs did not account for the additional weight of RRUs. In some regions, the towers cannot accommodate the additional weight and wind load, so the RRUs must be deployed at the bottom of the tower. In such a traditional configuration, a jumper cable (two RF connectors) is required for each RRU port to connect to the main feeder cable (another two RF connectors), which is then connected via a jumper cable (two more RF connectors) to a tower mast amplifier (TMA) and then another jumper cable (two more RF connectors) to one RF connector at the base station antenna (see Figure 1c). The TMA and extra jumper cables are required to compensate for the UL losses from the antenna down through the jumper cable/feeder cable to the RRU. The feeder cable can be as long as 100 m in some cases, resulting in approximately 3 dB signal loss.

Figure 2

Figure 2 DAS RRUs (a) and head-end (b). Source: EJL Wireless Research LLC ©2016.

In-Building DAS

Typically, the majority of multi-operator active DAS deployments range from a few sectors to more than 50 for a sports stadium. The signaling source for the DAS head-end is an array of macro cell digital baseband units supporting a few to hundreds of macro cell RRUs, depending on how many frequency bands are required and how many operators are supported (see Figure 2). Each output of the RRU requires a jumper cable to a DAS attenuator tray and then another RF cable to the DAS head-end. The majority of DAS attenuator trays today still use the legacy 7-16 DIN connectors; however, next generation products will likely switch to the 4.3-10 DIN connectors. The cable between the DAS attenuator tray and the DAS head-end may use a DIN connector to a smaller QMA type RF connector. The DAS power amplifier nodes (usually one per sector) typically use legacy 7-16 DIN connectors and require one or more jumper cables to the wideband DAS antenna. Passive DAS solutions require even more RF connectors, from splitting the RF signal across multiple paths within a building and the greater use of coaxial cables.

5G Massive MIMO  Antenna Systems

The 5G era will result in a dramatic departure from traditional low-order MIMO (2/4/8) RRUs to higher order 32/64/128/256 MIMO technology. The actual radiated RF power output remains essentially the same, but the higher-order MIMO dramatically reduces the RF output power per port. For example, a 2×2 MIMO RRU providing 80 W of RF power per channel results in 160 W total to the antenna. To support the same 160 W EIRP to the antenna, a 64x64 MIMO RRU requires only 2.5 W RF output per channel.

With this architecture, the antenna is combined with the RRU and forms a complete integrated transceiver/antenna system, which the industry sometimes calls a massive MIMO active antenna array. Because the antenna is integrated within the RRU, there are not 64 RF connector ports on the outside of the RRU that need jumper cables to connect to a 64 port antenna. Current massive MIMO active antenna designs are based on many large printed circuit boards (PCB) for the antenna arrays and the RF transceiver subsystem. Each antenna array is connected to a corresponding RF transceiver module using small-scale board-to-board RF connectors. For example, a 64×64 massive MIMO active antenna system requires 64 connectors for the RF transceiver PCBs, 64 connectors for the antenna array PCBs and 64 board-to-board connectors—a total of 192 RF connectors per antenna system.

SMALLER, BETTER CONNECTORS

To address these changes in base station configuration and the requirement for better PIM performance, the higher power RF connectors for RRUs, antennas and jumper/feeder cables are in a transition, as the legacy 7-16 DIN connectors are replaced with higher performance and smaller 4.3-10 DIN and the newly developed NEX10™ connectors, as well as the smaller MCX/MMCX connectors for 5G massive MIMO antenna systems.

The 4.3-10 DIN has several advantages over the legacy 7-16 DIN connector, including a smaller footprint and better  PIM performance. Pre-LTE systems were more immune to PIM degradation, but the advent of LTE, LTE-Advanced and soon to be deployed LTE-Advanced Pro and 5G require significant linearity from the RF connectors.

The TDD-LTE market has relied on legacy N-type RF connectors which exhibit poor PIM performance; however, TDD-LTE is more immune to PIM than FDD-LTE systems. The massive TDD-LTE network for China Mobile Communications was built using N-type connectors because of their significantly lower cost than 7-16 DIN connectors. However, the FDD-LTE networks for China Unicom and China Telecom use 7-16 DIN connectors. In North America, Verizon Wireless chose to deploy 4.1-9.5 DIN technology instead of waiting for the 4.3-10 DIN connector to become available. They appear to be the only mobile operator in the world to pick the 4.1-9.5 DIN standard, which may create significant problems for network upgrades in the future.

Figure 3

Figure 3 NEX10 connectors, single (a) and multi-port (b).

The advantage of the NEX10 connector is that it offers high PIM performance in both single and multi-port versions and is 50 percent smaller than the 4.3-10 DIN (see Figure 3).

CONCLUSION

As operators move from LTE to LTE-Advanced, LTE-Advanced Pro and, ultimately, 5G massive MIMO, the connector interfaces will migrate from 7-16 DIN to 4.3-10 DIN, NEX10 and MCX/MMCX. The requirements and demand for the RF connectors used throughout the wireless infrastructure network will continue to increase, which makes for a bullish outlook for the global RF connector market over the next several years. n

Editor’s Note: In this same issue, read “Making Connections – Collaborating to Develop the NEX10 Interface,” which describes the development of the NEX10 connector, page 6.