How to Select a 5G Repeater

Cellular repeaters have been in existence since the very early days of telecommunication. Fundamentally they deliver cellular service to an underserved region where the base station signal does not reach an area where connectivity to handsets is necessary. Initial use cases for repeaters served public safety and 2-way radios. As commercial cellular radio became prevalent, repeaters were often the solution of choice to provide cellular availability for in-building use where the building materials or other obstructions left cellphone signals extremely weak. The function of the repeater was to amplify the external signal so that it could be detected by devices inside the building, and so that the signal from the in-building devices was seen by the radio tower. These early repeaters were also commonly known as Bi-Directional Amplifiers, or BDA for short. They were completely analog in nature and required manual alignment of the outdoor antenna toward the radio tower and redistribution of the signal inside the building with one or more indoor antennas. Repeaters are commonly called signal boosters or extenders. These are all terms for the same type of device.

Repeaters are fundamentally simple in nature, consisting of the outdoor antenna which runs into a filter to isolate the target band, then to an amplifier, with the resulting signal feeding the RF coax to indoor antennas. If multiple carrier bands are supported, splitters and combiners are used so the amplifier chains can use the same antenna ports.

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These BDA repeater systems worked well in the early days of cellular when cell sites were far apart and easy for the installer to spot on the roof.  In many cases, the repeaters were installed by specific carriers to provide in-building coverage exclusively for their service. The repeaters, being very simple in nature, were typically unmanaged, meaning that they were easily installed and forgotten. Over time, this created a set of problems for the carriers:

1. When these systems failed, it was typically many years after they were initially installed and often, no one knew they were present. If the failure was in a benign mode, there was usually little consequence. However, they would often fail and start to transmit a signal that became interference, and instead of helping in-building coverage, it made whatever coverage was present worse.  
2. As carriers started building out their networks and densifying, additional cell towers were added nearby to buildings with BDAs. These new cell sites were often powerful enough that their transmissions were seen inside those buildings. The resulting indoor coverage of two cells resulted in a rise in uplink noise for the farther cell site as it was now getting all the in-building transmissions that were destined for the new, closer site. This often led to poor performance for the farther cell site and frequent dropped calls for devices inside the building as devices ping-ponged between both cells depending on where they were in the building.

While the initial concept was good, the evolution and densification of the cellular networks made BDAs less necessary and only useful in limited scenarios. Not surprisingly, finding and removing most of these early repeaters was a complex task.

Modern repeaters, those servicing 5G, have evolved past these initial BDAs, and in many respects try to avoid using that term.  Features that modern repeaters may have are:

• Management interface: Most modern repeaters have both an on-device interface and a remote interface. They generally provide the same information, related to operational status, gain, power out, and location. This can be very important for businesses or enterprises trying to diagnose a potential issue without having to visit every site. The interface also provides alarm information much more conveniently than looking at LEDs mounted on the unit. This management provides an ultimate control over a repeater – one can turn it ON or OFF remotely. 
• Detection of cell site parameters: Some repeaters will decode the cell RSRP, RSRQ, SINR, and PCI. This is very important as it verifies that the repeater is aligned with the expected cell site and that the source signal is good. 
• Signal processing: Some repeaters will offer dynamic gain control to adjust the power of the signal in the received area and toward the base station. TDD synchronization is also important if there is a TDD signal in the repeated spectrum. Some systems also include echo cancellation.
• Electronic alignment / Beamforming: Installation is always a factor with any antenna system. Aligning the external antenna to the base station can be challenging, especially in areas where the direct view of the base station antenna is blocked by foliage or doesn’t have Line of Sight. Some repeater systems may offer an alignment or beamforming solution that allows the system to find the best orientation to the target network tower.

Repeaters have traditionally been installed such that the donor antenna is typically on the rooftop, and the service antennas are indoors so the repeater overcomes signal impairments due to the construction of the building. As frequencies used by carriers have increased to the mmWave range, repeaters are now also available to provide outdoor to outdoor coverage, where the mmWave signal is blocked by foliage or other outdoor obstructions.

There are several key parameters that need to be considered when selecting a repeater for a 5G system.

• System Gain: This is the end-to-end gain of the signal received at the donor antenna to the transmit signal from the service antenna. A repeater with a high system gain will be able to repeat a weaker signal from the donor site.
• Antenna Beamwidth: The beamwidth is a characteristic of how focused the RF signal is when it leaves the antenna. It is typical for the beamwidth of the donor antenna to be narrow so that repeater signals don’t cause excess interference in the network. The beamwidth of the service antenna is likely to be wide, and even omni-directional, to help spread the signal into the target area.    
• Service unit EIRP:  The EIRP is a measure of the radiated energy from the service antenna. In most cases, the EIRP will need to be calculated as the manufacturer antenna gain (in dBi) and transmit power from the repeater (in dBm), less the loss of any connecting RF cable (in dB). In cases where the repeater is handling multiple bands, power and antenna gain should be provided for each band. The higher the EIRP, the farther the signal will extend from the service antenna. Some repeater systems will have an integrated amplifier with the antennas; this will most typically occur with mmWave repeaters. In those cases, the manufacturer should specify the EIRP.
• MIMO support: 4G and 5G networks use a technique known as MIMO (Multiple-Input Multiple-Output) to improve performance. MIMO is characterized by having multiple antennas with different/orthogonal polarity. A repeater supporting MIMO should provide better performance than one that does not, and MIMO support will not affect coverage. Some 4G and 5G bands will support 4x4 MIMO, but a 4x4 MIMO repeater is unlikely to add any additional performance boost over a 2x2 MIMO repeater.
• Band support: 5G differs from 4G in that there are two configurations known as Standalone (SA) or Non-Standalone (NSA). When 5G was initially adopted, all networks were in an NSA configuration, which uses a 4G channel for control and the 5G channel(s) for data. To take advantage of the benefits of a 5G network, it is necessary to make sure that the control channel, commonly called the anchor, has sufficient coverage in the service area. The control channels are typically at lower bands, which will penetrate buildings better, but often benefit from a multi-band repeater where the carrier’s anchor band and 5G data band are also amplified. Eventually carrier networks will transition to SA where calls can be started on the 5G channel. However, the mass adoption of devices that support the SA configuration will lag network support.

Before 5G, all repeaters operated in what is called sub-7 GHz spectrum, and in fact most cellular frequencies were below 3 GHz for 4G. With 5G, new spectrum has been allocated in the range of 3GHz to 6 GHz, and the repeaters for networks supporting these bands are technically similar to 4G repeaters in that the antennas are passive. Most passive antennas in multi-band repeater systems will have elements that are specifically tuned for the different supported cellular bands. All the RF signals for these combined bands are carried over a single cable between the amplifier and the antenna(s).

In order to provide access to greater amounts of RF spectrum, 5G introduced the first use of mmWave spectrum for cellular usage. The mmWave spectrum is at 24 GHz and above and we expect the use of higher band spectrum in 6G and beyond. Due to the higher frequencies used, it is not practical to use the traditional repeater design of remote antennas and a centralized amplifier for mmWave band communications. Active antenna arrays must be used.

A repeater with active antennas has a different architecture, which effectively moves the power amplifiers from a central unit to the antenna module itself.

As all 5G mmWave bands support 2x2 MIMO for both uplink and downlink, the repeater system must also support that. An additional result of the move to an active antenna is that 5G mmWave antenna systems will be band-specific and will require dedicated Donor and Service antennas for each band that is being repeated.

In summary, repeaters that are currently serving the industry for 4G and 5G networks are conceptually very similar to those present during the early days of cellular communications, but there have been significant technology advancements in operation and maintenance. Moving further into mid-band spectrum support, mmWave, and higher frequencies, repeaters will see a greater role in outdoor as well as indoor deployments to cost-effectively bring cellular signals to shaded and indoor locations and improve the ability to provide ubiquitous cellular coverage.