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Smart Repeaters for 5G Networks

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There is much excitement around 5G private networks and the use cases the technology can enable for many enterprises. Such networks promise to deliver the best of all worlds—purpose-built networks with unprecedented data rates, low latency, secure connectivity and reliable indoor and outdoor coverage, as well as security and privacy of data.

This is a welcome development, because for years, industrial enterprises have struggled with Wi-Fi as a means to deliver mission-critical connectivity, partly because Wi-Fi was never designed for mobile, indoor-outdoor, reliable, deterministic and low-latency connectivity.

However, a challenge that remains is achieving good coverage at a cost acceptable to industrial enterprises. One way to achieve lower cost is to substitute high-cost radio network elements like base stations with lower-cost ones like repeaters where feasible. Smart interference-cancellation repeaters (ICRs) that have higher gain further help optimize cost, as fewer repeaters are needed.

Early implementations of self-interference cancellation (SLIC) technology include pilot programs initiated by the U.S. Department of Defense (DoD), as part of the National Spectrum Consortium’s private 5G program.

In this case, Kumu Networks demonstrated its KR9 5G ICRs for a DoD test bed at Hill Air Force Base in Utah. This ICR is now being evaluated by several cellular network operators, offering the possibility for high-range, infrastructure-grade repeaters to be rapidly deployed on a single pole or mast.

In the most popular spectrum bands available for private networks—the CBRS 3.5-GHz band in the United States is a good example—coverage challenges are greater than in lower bands like the 1.8-GHz band used by LTE or the 2.4-GHz ISM band used by Wi-Fi, because higher-frequency bands attenuate faster with distance and don’t penetrate through walls, as compared with lower-frequency bands.

In a traditional approach, this results in the use of a large number of base stations (gNBs) to achieve coverage. Mobile operators’ public networks address these 5G coverage challenges using high-transmit–power macro base stations, including massive MIMO to reduce the number of sites that need to be provisioned to achieve coverage. But in most private network scenarios, this approach is not a viable option. This is because installing large macro base stations on tall towers and provisioning backhaul and kilowatts of power to them is not always possible. Furthermore, in many private network use cases, part of the objective is to provide service for indoor locations where an alternative architecture is desired in any case.

The net result is that most 5G private networks largely plan to use small-cell technology for the radio network. What this entails then is the need to provision high-quality backhaul to each of these many small-cell base-station sites.

At many sites, this backhaul is not cost-effective to provision, requires a long time to set up or may not be feasible at all. In such situations, a 5G repeater provides even more value and can be a great solution to achieve private network coverage in a fast and cost-effective manner. In many use cases, this can make the difference between a viable private network deployment versus one that is not viable for cost or time-to-market reasons.

Repeaters, however, historically suffer from their own limitations.

In particular, the gain available from a standard repeater is limited, as boosting the signal too much could cause the repeater to become unstable and start oscillating, which makes it unusable. This issue occurs when the gain of the repeater exceeds the isolation between the donor and service antenna of the repeater.

For donor and service antennas that are mounted in proximity (for example, on the same pole or near a window) in sub-6-GHz bands, isolations of greater than ~50 dB are rarely achievable even with a few feet of separation, thus limiting repeater gains to less than about 50 dB. This severely limits the coverage improvement the repeater can provide.

Repeater for 5G private network block diagram.
Figure 1: Repeater for 5G private network block diagram

Smart ICRs using Kumu’s SLIC technology provide more than 1,000× gain than standard repeaters.

Figure 2: Kumu 5G SLIC ICR

Kumu’s SLIC technology generally works by taking a sample of the transmit signal just before the transmit antenna and generating a 180˚ amplitude inverted, phase-shifted and delayed “antidote” that is then injected into the receiver such that the noise from the transmitter (self-interference) into the receiver is minimized.

In the case of a TDD ICR, when the service unit is transmitting a boosted signal to end devices, Kumu’s canceler takes a sample of that transmit, generates an “antidote” and injects it into the donor unit’s receiver so that the receive sensitivity of the donor unit is not degraded by the self-interference caused by the service unit. Kumu’s cancellation technology includes real-time tuning algorithms that constantly adjust the “antidote” as the real-world environment around the repeater changes due to the changing RF environment, channel variations and other changes so that the cancellation performance is maintained at all times.

Measured ICR gain with SLIC turned on and off.
Figure 3: Measured ICR gain with SLIC turned on and off

The yellow line in Figure 3 shows that the ICR delivers 80-dB gain when SLIC is on. The magenta line shows ~55-dB gain with SLIC turned off. In this instance, we see that the ICR with SLIC on shows ~562× gain improvement compared with when SLIC is off.

Standard analog repeaters have not been manageable network nodes in the past due to lack of a control channel for management. Smart repeaters provide a way for the network to configure, control and manage repeater nodes. In Kumu’s smart ICR, remote management of the repeater node can be done using a side-band LTE control channel. These repeaters include a GUI that provides real-time visibility into key repeater metrics and performance monitoring while allowing for configurability and management of the repeater. This includes identification, authorization, transmit power control, beamforming control, timing and TDD configuration control and on/off functions via the control channel.

One particular scenario in which manageability is particularly useful is when there is a need to adjust the gain of the repeater to maximize overall network performance and throughput in case network environment has changed, for example, due to deployment of additional cell sites in the area. In the case of private networks, this manageability is even more important, as it ensures network performance visibility at all times to ensure reliable connectivity.

Real-time visibility and adjustment of 5G repeater performance.
Figure 4: Real-time visibility and adjustment of 5G repeater performance

Smart ICRs can contribute significantly to lower total cost of ownership in private network deployments. Consider the case of a private network that requires multiple small-cell gNBs to achieve coverage. Each of these gNBs require a fiber or Gigabit Ethernet backhaul (assuming such a service is available). For a repeater, the “backhaul” cost is non-existent for both opex and capex. In such situations, a mix of gNBs and smart ICRs can help achieve coverage without excessive backhaul costs.

In a recent U.S. DoD private 5G demonstration, Kumu Networks vividly illustrated these benefits. This demonstration used Kumu’s KR9 SLIC repeater shown in Figure 5.

KR9 SLIC outdoor repeater including antennas.
Figure 5: KR9 SLIC outdoor repeater including antennas

Coverage extension of more than three to four blocks was provided due to the high gain of the ICR as shown in Figure 6.

Example of coverage extension using SLIC ICR.
Figure 6: Example of coverage extension using SLIC ICR

Smart ICRs enable private 5G deployments to achieve coverage even in environments where backhaul is not always easy to provision in some of the locations where base stations need to be installed. By using ICRs as one of the network elements in private 5G deployments, enterprises have another tool in the toolbox to easily and cost-effectively achieve reliable coverage while retaining full network management capabilities.

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