5G mobile network testing using a passive network scanner (Part 3)

5G Synchronization Signal Blocks (SSB)

5G NR technology uses orthogonal frequency-division multiplexing (OFDM) [1], [2] for the downlink transmission. A band dependent table in [3], [4] defines whether time division duplex (TDD) or frequency division duplex (FDD) is used to divide downlink and uplink. A 5G NR cell broadcasts SSBs (synchronization signal/physical broadcast channel blocks) in order to enable cell search and initial access.

Figure 1 shows a 5G NR SSB where one SSB is mapped to 4 OFDM symbols and 240 subcarriers (SC). The SSB consists of one PBCH block, a primary, and a secondary synchronization signal (PSS and SSS). For more details, please refer to the previous blog post: Part 2.

How to analyze the 5G NR SSBs?

Accessing or measuring a 5G NR carrier starts with discovering the center frequencies of the SSBs. In LTE, the PSS/SSS and PBCH signals are always transmitted around the center frequency of the carrier with fixed periodicity. It is possible to manually detect them in the power spectral density, measured with a receiver. In 5G NR, however, the transmission characteristics of the SSBs are more flexible, creating many new challenges when it comes to configuring a 5G NR receiver/scanner.

The most challenging problem is the flexibility regarding the SSB center frequency (SSRef). The 3GPP standard defines a frequency raster for the appearance of SSBs [3], [4], but the raster is very narrow and there are hundreds of possibilities within a 5G NR carrier. Furthermore, a single SSB only appears for a short time. Therefore, it is difficult to detect the SSB with a traditional swept-tuned spectrum analyzer. In addition, the SSB-periodicity is flexible with the frame starting point and its corresponding period: 5ms, 10ms, 20ms, 40ms, 80ms or 160ms [6].

An example illustrating a period of 5ms is illustrated in Figure 2 where the SSB-index determines the starting time within the window and each index corresponds to a different beam from the basestation.

To avoid time-consuming spectrum scans, wrong scanner configuration, or simply guessing the correct SSB center frequency, the proposed receiver/scanner solution is to detect the SSRef using an algorithm called Automatic Channel Detection (ACD), as illustrated in Figure 3.

The ACD is able to search quickly through large frequency ranges where the ACD runs an internal spectrum scan and searches for SSBs [7]. The ACD delivers the correct SSRefs within seconds, allowing the scanner to analyze both in-band and out-of-band (competitor networks) where the cellular network parameters are completely unknown.

Algorithm for SSB Analysis

The ACD algorithm first provides a list of the center frequencies of the SSBs. Second, the scanner starts the SSB measurement algorithm (SSBmeas) for these frequencies. The ACD is able to run in the background of the baseband processing, continuously searching for new SSB center frequencies. As soon the scanner receives the signal of a basestation cell strong enough for the ACD sensitivity during a measurement, the corresponding SSRef will be scanned with the more sensitive SSBmeas for the remaining portion of the measurement.

The ACD expects a 3GPP compliant 5G NR SSB transmitted on at least four frames with an increasing system frame number. Internally, the ACD uses the 3GPP band table to select the SSB transmission case for the selected frequency bands. This accelerates the ACD and avoids checking the algorithm for unnecessary cases (e.g. Case E for band n1).

Figure 4 shows the steps for making accurate 5G SSB measurements using an TSME6 RF Scanner.

From the SSB-measurement, many network features can be obtained to assess the network quality, such as Synchronization Signal Signal-to-Interference-plus-Noise Ratio (SS-SINR) and Synchronization Signal Reference Signal Received Power (SS-RSRP) [8].

In addition, it is possible to extract the cell identity (PCI), the SSB index, the channel impulse response, and the arrival time for all detected cells and beams. The SSB detection includes a decoding of the PBCH, thus preventing ghost codes, as the PBCH-payload contains a cyclic redundancy check (CRC) for error detection.

SSB Analysis Results

Figure 5 illustrates the measured arrival times of peaks of the channel impulse response for SSB-indices 0 and 2 of two cells (PCI 75 and PCI 362) at SSRef 3574.56MHz (Case C).

The measurement is primarily interested in the arrival time difference to the first measured peak to measure the delay spread. The measurement confirms the expectation [5] that the SSB with index 2 is transmitted 500µs later than SSB with index 0. From Figure 5, the distribution of the peaks indicates how much reflection and scattering occurred with higher delay spread for index 0. Moreover, we can observe that the signal of the cell with PCI 362 arrives approximately 2µs later than the one of the other cells with PCI 75 which is mainly caused by the larger cell-to-receiver-distance for PCI 362.

Using the results shown in Figure 5, we will use these to solve the challenge of network synchronization in an upcoming blog post, so stay tuned for more scanning fun.

Conclusion

This set of blog posts is describing the mobile network testing approach for 5G NR with a passive scanner which measures synchronization signal (SS)/ physical broadcast channel (PBCH) blocks (or SSBs) broadcasted from 5G NR base stations. In the next blog post, we will address the challenge of measurement of basestation and network synchronization for TDD 5G networks—this is particularly important to eliminate inter-basestation interference due to timing misalignment.

Learn more about 5G site acceptance scenarios, KPIs, test techniques, and field measurement results and watch the on-demand Webinar “5G Site Testing including latest real world challenges”.

References

[1] S. Ahmadi, 5G NR: Architecture, Technology, Implementation, and Operation of 3GPP New Radio Standards, Academic Press, 2019
[2] M. Kottkamp, A. Pandey, D. Raddino, A. Roessler and R. Stuhlfauth, 5G New Radio: Fundamentals, procedures, testing aspects, Rohde & Schwarz, 2019
[3] 3GPP technical specification TS 38.101-1, NR; User Equipment (UE) radio transmission and reception; Part 1: Range 1 Standalone, V16.8.0, June 2021
[4] 3GPP technical specification TS 38.101-2, NR; User Equipment (UE) radio transmission and reception; Part 2: Range 2 Standalone, V16.8.0, June 2021
[5] 3GPP technical specification TS 38.213, NR; Physical layer procedures for control, V16.6.0, June 2021
[6] 3GPP technical specification TS 38.331, NR; Radio Resource Control (RRC) protocol specification, V16.3.1, Jan 2021
[7] O. Wanierke, Method and System for Channel Detection, U.S. Patent 10,833,800, Nov. 10, 2020.
[8] 3GPP technical specification TS 38.215, NR; Physical layer measurements, V16.4.0, Dec 2020

Read more about "5G mobile network testing using a passive scanner" part 1 - 4 of this series:

5G mobile network testing using a passive scanner (part 1)
5G mobile network testing using a passive scanner (part 2)
5G mobile network testing using a passive scanner (part 4)