December 1, 2020
The rollout of 5G New Radio (5G NR) introduces many new design and test considerations for mobile operators, as well as engineers and field technicians responsible for the deployment and installation of the new network. To accommodate the use cases outlined in 3GPP Release 16 and their associated bandwidth, speed, and latency specifications, millimeter wave (mmWave) spectrum is being accessed. The higher frequencies, among other factors, place greater importance on test instruments that can conduct real-time spectrum analysis and support wide frequency coverage.
Enhanced Mobile Broadband (eMBB), Massive Machine Type Communications (mMTC) and Ultra Reliable/Low Latency Communications (UR/LLC) require high bandwidth that can be best found in the mmWave spectrum. While the high frequencies help deliver on the promise of up to 20 Gbps transmission, 1 ms latency and other 5G network key performance indicators (KPIs), there are inherent transmission issues associated with commercial environments. Examples include propagation loss and network timing. Beam forming and Time Division Duplex (TDD) technologies are being utilized in mmWave 5G to help overcome those challenges.
Beam forming improves propagation loss inherent with high-frequency mmWave signals. It significantly increases transmission power by concentrating RF energy into a directional beam toward the UE.
To determine the UE location for directing the data beam, synchronization signal beams (SSBs) are transmitted in the 5G radio sector. All cell information is contained in the SSB and sets up the connection with the UE. This includes primary and secondary synching and physical broadcast channel (PBCH). SSB is always positioned before the data and is necessary for timing and reference. It is a key element of the TDD technology.
5G NR utilizes Frequency Range 1 (FR1), which incorporates sub-6 GHz spectrum, and Frequency Range 2 (FR2), which covers the mmWave frequencies. All FR2 and most FR1 transmissions will utilize TDD.
TDD creates dramatic changes in signal transmission and associated testing. As noted above, TDD introduces time slots into the signal, which are split among the SSB, uplink, and downlink. Figure 1 shows a typical configuration where the yellow and orange represent the SSB and the blue and green represent the downlink and uplink, respectively.
The split of uplink and downlink in the subframes can be configured to one of 62 different options. Figure 2 shows an example of the first ten defined formats. Red indicates download (DL) slots, blue references upload (UL) slots and the X is designated “flexible.” While not relevant in the initial 5G rollout, flexible slots will be important as 5G evolves, as it allows for more dynamic applications.
Eventually, UE will be adaptive and communicate with the base station to determine the number of frames allocated to DL and UL based upon application. For example, there will be many more UL slots in stadiums and arenas during an event while there will be primarily DL slots at airports as travelers download music, movies, et al as they board flights.
Need for Real-Time Spectrum Analysis
Because 5G NR transmissions are TDD signals, the radio will be constantly switching between SSB, downlink, and uplink. The necessary switches occur rapidly, as the 5G NR frame is only 10 ms, meaning the test instrument used must be able to capture signals with no gaps. It is one reason why a real-time spectrum analyzer (RTSA) is best suited for 5G network installation.
A traditional swept spectrum analyzer conducts signal sweeps; it captures energy then processes it for a plot that is displayed on the instrument screen. An RTSA acquires the entire signal chunk and processes it simultaneously, eliminating coverage gaps associated with swept instruments that result in the possibility that the SSB or data will be missed.
Much of 5G testing will be done over the air (OTA), especially power measurements. The main reason is that the multi element antennas required to support beamforming do not have a single RF test port. Power and modulation cannot be measured in the near field, but must be done in the far field, several meters from the transmitter (note: far field distance depends on transmitter frequency).
An RTSA provides the best view of the overall 5G spectrum. Without an RTSA, a swept tuned analyzer must be used in a MAX HOLD mode to build an image of the TDD signals. This is a relatively slow process and misses detection of intermittent interferers.
For detailed analysis of the 5G signal using post processing software, an IQ capture capability is required. A capture bandwidth of over 100 MHz is necessary for most 5G spectrum allocations, especially in the mm wave bands. In addition to the bandwidth, the capture time must ensure at least one full 10 ms 5G frame is captured. Post processing software can then be used to view the individual resource elements of the framed and the SSB position.
For accurate OTA measurements, field technicians need to understand four aspects:
- The RTSA with an antenna needs to be in far field to get fully formed beams for power measurements
- The RTSA with an antenna needs to be in the boresight (highest power zone) of the beam being measured
- Total radiated power of beamforming antennas must be measured with the Equivalent Isotropic Radiated Power (EIRP)
- To accurately measure EIRP, distance to antenna and all cable and antenna gains/losses must be considered
An RTSA needs wide frequency coverage for 5G network test to handle current and future FR1 and FR2 needs. The Field Master Pro™ MS2090A (figure 3) has integrated and continuous frequency coverage from 9 kHz to 54 GHz, which allows users to view the RF spectrum and measure all transmissions. In addition to the frequency coverage, a spectrogram display is beneficial, as it helps locate interference quickly and accurately. Spectrograms are views of how the signal amplitude changes with time, making them especially useful when monitoring for intermittent or interfering signals.
Key 5G Measurements
Table 1 provides a list of key measurements to be made on 5G networks to ensure KPIs are met.
|Adjacent Channel Power (ACPR)||Modulation Quality|
|Cell/sector ID Verification||Occupied Bandwidth|
|Equivalent Isotropic Radiated Power (EIRP)||SSB Power and Position|
|Frequency Error||Time Offset|
EIRP – In 5G networks, EIRP needs to be measured directly from a base station. The analyzer used by the field technicians must have sufficient bandwidth to make accurate measurements on signals occupying 100 MHz or more, as well as enough sensitivity and low noise floor to record EIRP at realistic distances from an active base station. The ability to gate sweeps on the downlink or SSB portions of the 5G frame is also critical to making a stable measurement.
SSB – Decoding the SSB can reveal important cell characteristics, such as cell ID, frequency error, and beam power. SSB measurements allow transmitter testing on a live gNB. To properly decode the signal, the field technician must know center frequency, bandwidth, and subcarrier spacing of the signal under test. This can be entered manually or by using a 3GPP-defined band and Absolute Radio-Frequency Channel Number (ARFCN).
It is also critical to know the frequency position of the SSB relative to the center frequency of the signal. This can also be entered manually as an offset from center or by entering the Global Synchronization Channel Number (GSCN). In cases where the SSB location is unknown, an analyzer with an auto SSB detect feature can automatically search the 3GPP-defined raster of potential SSB positions to locate it.
When making measurements on the 5G base station, the analyzer is typically only receiving the SSB and downlink power, so the trace will rise and fall rapidly as the radio switches between transmitting and receiving over the 5G frame. This makes it very difficult to conduct accurate power or bandwidth measurements on the signal because of the drastic power fluctuations. To stabilize these measurements, time gating can be used. It allows users to focus the spectrum analyzer results on a specific section of time within the frame.