June 17, 2020
5G radio access networks (RANs) are about to get a major upgrade that can extend coverage and boost performance for end users. The deployment of 5G New Radio (NR) in low-band FDD spectrum, in combination with mid- and/or high-band TDD spectrum, promises to be a game changer for 5G network operators.
Today's 5G networks mostly operate in mid-band (e.g., 3.5GHz) and high-band (e.g., 28GHz) TDD spectrum. These bands offer fantastic capacity, but inferior propagation to current mobile frequencies. Mid- and high-band systems are also, relatively speaking, more uplink (UL) limited. The result is that 5G coverage does not match 4G coverage when overlaid on the same site grid.
To address this, operators can deploy low-band 5G in FDD spectrum (e.g., in the 600MHz, 700MHz or 800MHz bands) to rapidly extend coverage without requiring new sites. However, due to the narrow channel widths (e.g., 2x10MHz) in low-band spectrum allocations, capacity is limited and the service is not strongly differentiated from LTE. In fact, many operators will use dynamic spectrum sharing (DSS) to support 4G and 5G in the same low-band radio carrier.
One way to mitigate these trade-offs is to use NR carrier aggregation to combine mid-/high-band TDD with low-band FDD spectrum to create a higher bandwidth connection to the mobile device and simultaneously extend the coverage of mid-/high-bands. Critically, in this scenario, UL control plane and user plane data are moved to the FDD UL channel. Then, because they are no longer limited by poor UL channel conditions, mid-/high-band TDD base stations can be optimized for downlink (DL) throughput and operators can take full advantage of capabilities such as massive MIMO and beamforming to extend the cell edge. This has the effect of increasing the DL range of mid- and high-band 5G, which is positive for network economics and the user experience.
How does NR carrier aggregation extend 5G coverage?
To illustrate how NR carrier aggregation with coverage extension works and why it can have such a positive impact, consider the mid-band 5G scenario in the figure below. In this scenario, a 5G base station is deployed on an existing 4G site.
Figure 1: Coverage extension with 5G carrier aggregation (Source: Ericsson)
In step one (leftmost part of the diagram), with 5G NR deployed in mid-band without low-band support, 45% of users can access 5G service. Because both UL and DL are supported in mid-band, there is no 5G coverage limitation for these users. However, the majority of users in the cell — 55% — cannot reliably access 5G due to UL limitations. In step two, NR is deployed using dual connectivity with LTE low-band (the most common deployment today, known as non-standalone, or NSA). In this step, UL user plane data is moved to LTE, which improves DL coverage on the 5G mid-band by up to 9dB. This increases the percentage of users served by 5G TDD spectrum to 67%. In step three, NR is also deployed on low-band spectrum (using DSS), which now carries nearly all the UL/DL control signaling. In this step, the DL low-band is aggregated with the mid-band TDD spectrum. This improves DL performance by up to 7dB and increases the percentage of users in the cell that can access 5G using the mid-band TDD spectrum to 84%. The combined effect of steps two and three is to increase the coverage area of the mid-band TDD system by a factor of 8, which translates to almost double the number of users the cell site can serve with mid-band 5G. This is what makes NR carrier aggregation with coverage extension a game changer for 5G operators. Although the gains from carrier aggregation vary according to spectrum bands and the deployment environment, the same basic principles hold across many scenarios. For example, when mmWave 5G is aggregated with low-band FDD, the gain in the mmWave DL can be up to 10dB, increasing the high-band cell coverage area more than threefold. Flexible deployment using advanced RAN coordination
Operators also need deployment flexibility to maximize their investment in 5G carrier aggregation. In the scenario discussed above, radio transmissions must be tightly coordinated, ideally at the millisecond level based in the 1ms LTE timeslot used in LTE (and therefore used in a DSS radio carrier). This is a challenging design constraint. Using advanced RAN coordination techniques (where supported by the RAN vendor), operators can reliably achieve tight 1ms level coordination between primary and secondary cells across different basebands. And in cases where basebands and radios are physically distributed, operators can use low latency inter-site coordination to design networks optimized at the site cluster level, providing greater deployment flexibility, the ability to target investment at the most in-demand locations, and opportunities to improve the spectral efficiency of mid-band TDD systems. What's the timeline?
When will these capabilities be available and deployed to end users? The answer depends on network readiness and device/chipset compatibility. Right now, it looks like the first two-carrier NR aggregation with coverage extension capabilities will go live in commercial networks in 4Q20. Then, as device compatibility allows, operators will be able to launch three-carrier NR aggregation from 1Q21 onwards. — Gabriel Brown, Principal Analyst, Mobile Networks & 5G, Heavy Reading This blog is sponsored by Ericsson.
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