5G New Radio (5G NR), release 15 reallocates some existing LTE bands and introduces new mmWave bands up to 40 GHz. While initial 5G devices will implement some type of point-to-point wireless link, smartphone manufacturers are already planning the introduction of their products that contain multiple radios. Adding yet another radio adds new coexistence challenges that designers must address.

5G NR mid-band (1 GHz to 6 GHz) and high-band (above 24 GHz) operate in the same or in adjacent spectrum to other wireless communications systems. With devices covering multiple bands, there is increased risk for sideband interference or new shared spectrum issues. 5G NR devices will need to operate adjacent to or even in the same spectrum as existing wireless communications systems without causing interference. Designers of 5G chipsets and components need to know the different types of coexistence interference issues, where coexistence interference is likely to occur, and how to test for coexistence interference.

Types of coexistence issues
There are many types of coexistence interference, but two primary issues require new coexistence testing. The first involves testing in-band and out-of-band emissions and testing the impact of the 5G NR emissions on other radio signals. These tests are important because you must ensure that a 5G radio doesn't cause interference with other radios in the device, with other radios signals in the channel, or with signals in an adjacent spectrum. Such testing is similar to 4G coexistence issues, but the increasing number of radios in a device and the increasing number operating bands where 5G NR will operate will compound the problem.

Second, because a goal of 5G is to improve data throughput, shared spectrum will be a key feature in 5G. To operate simultaneously in a shared environment, new procedures and protocols must be developed to ensure successful operation in the environment. At the highest level, these policies specify that devices must listen before they talk. Specifically, a device needs to detect coexistence traffic and allocate or reallocate spectrum dynamically based on what it hears. This presents potential quality of service issues (QoS) issues for device users caused by latencies while the radio switches channels. This will require special tests not previously done on cellular devices.

Potential coexistence interference
There will be multiple spectra where 5G NR designers may encounter coexistence issues. In frequency range 1 (FR1) up to 6 GHz, 4G LTE Advanced Pro offers LTE Unlicensed (LTE-U), licensed-assisted access (LAA), and MulteFire, all of which allow LTE operation in unlicensed spectrum. LAA uses the 4G network as an anchor and implements listen-before-talk process to ensure no other operation is taking place in the unlicensed spectrum. LAA requires careful coexistence design and test with many different permutations due to the many different protocols being used in the same frequency bands.

In addition to existing 4G operating bands, the new 5G mid-band frequencies -- 3.3 GHz to 4.2 GHz, 3.3 GHz to 3.8 GHz, and 4.4 GHz to 5 GHz -- need to operate without causing interference with adjacent IEEE 802.11ac and 802.11ax in the 2.4 GHz and 5 GHz Wi-Fi networks (Table 1). 5G NR introduces new operating bands in FR1 up to 6 GHz and FR2 up to 52.6 GHz. 5G NR bands are identified with the letter ‘n’ preceding the band number. There are many areas for new coexistence issues mainly in the 3.5 GHz to 6 GHz, 24 GHz to 27 GHz, and 37 GHz to 40 GHz spectrum:

Table 1 New frequency bands for 5G New Radio.
Frequency Range 1: 400 MHz to 6 GHz Frequency Range 2: 24.25 GHz to 52.6 GHz
Adds 1.5 GHz of new spectrum in frequency bands Adds 8.25 GHz of new spectrum in frequency bands
n77: 3.3 GHz–4.2 GHz
n78: 3.3 GHz–3.8 GHz
n79: 4.4 GHz–5 GHz
n257: 26.5 GHz-29.5 GHz
n258: 24.25 GHz-27.5 GHz
n260: 37 GHz–40 GHz

Without proper filtering for each band, emissions from intermodulation products can cause spurs that interfere into adjacent bands. There is also a proposal to use the unlicensed Industrial, Scientific and Medical (ISM) bands as a secondary channel, which creates a shared spectrum scenario that needs to be tested.

In new 5G mmWave spectrum, policy for sharing of licensed and unlicensed spectrum remains undefined. In July 2016, the FCC allocated 11 GHz of spectrum for wireless broadband for both fixed and cellular operators in the high-band spectrum, including 3.85 GHz of licensed spectrum and 7 GHz of unlicensed spectrum in the 27.5 GHz to 28.35 GHz, 37 GHz-38.6 GHz, and 38.6 to 40 GHz bands. Some of these overlap with newly specified 5G NR frequency Range 2 (FR2) operating bands and with Fixed-Satellite Services (FSS) earth station uplinks at 27.5 GHz to 29.5 GHz and FSS downlinks at 37.5 GHz to 40 GHz.

Who has priority? How is the spectrum allocation managed? What about incumbents in the new operating frequencies? These are among the many questions that need to be answered by standards committees and local government agencies. 3GPP will begin work on licensed and unlicensed spectrum sharing in 5G NR phase II, planned for release in 2019. Engineers designing devices today can extrapolate insights from other spectrum sharing implementations such as IEEE 802.11h Dynamic Frequency Selection for Wi-Fi networks, which implements RADAR detection and avoidance in the 5 GHz band.

Testing for coexistence
Sharing airwaves puts more burden on operators and equipment manufacturers to ensure 5G will coexist with the existing commercial wireless infrastructure, with non-military radar signals, and with military usage in agencies such as the US Department of Defense (DoD). Designers will need to evaluate device operation in controlled coexistence scenarios to mitigate the effects of interference and to ensure predictable and reliable device and network behavior.

[Continue reading on EDN US: Minimizing in-band and out-of-band emissions]

Sheri DeTomasi is the 5G New Radio Solutions Lead at Keysight Technologies.

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