Imagine you’ve spent months designing a new base station with the best-of-breed test instruments at your disposal in your lab. It should work correctly in the real world, shouldn’t it? Not so fast. Wireless networks have evolved drastically over the past two decades, and 5G elevates network complexity to a whole new level with beamforming, massive multiple input multiple output (mMIMO), millimeter-wave (mmWave) frequencies, and a new flexible air interface. Performing network testing is critical for successful 5G commercial deployments. Listen up engineers­ — don’t let drive testing derail your company’s path to market leadership in 5G.

5G network trials have started and will increase in 2019. Service providers across the world are eager to capture the 5G opportunity and fuel their growth and profitability in the face of a declining average revenue per user (ARPU). However, their leadership in 5G services depends on the rollout of effective networks. The onus is on network equipment manufacturers (NEMs), who must install these networks and prove to operators that they meet their criteria despite severe technical challenges. Field testing is a must to validate network coverage. With 5G, engineers must also understand network specificities that generate new considerations to reap the maximum benefits from mobile network testing. 

Beam-based coverage measurements
The shift to beam-based coverage measurement and reference signals galore require high-performance test equipment. Using different forms of MIMO and beamsteering to improve performance, 5G new radio (NR) lacks the cell-level reference channel that existed in long term evolution (LTE). It is no longer possible to measure the coverage of a cell. Engineers must carry out beam-based coverage measurements instead. Therefore, the methodology for key performance indicator (KPI) calculations must also change.

Moreover, there are several synchronization signal blocks (SSB) per cell, the maximum number of beams ranging from 4 to 64 depending on the frequency. Since SSB beams transmit at different times, there is no intracell interference. As a result, the number of reference signals in a 5G NR network increases exponentially.

SSB beams are static or semi-static. They always point in the same direction and form a grid of beams covering the whole cell area. User equipment (UEs) search for and measure the beams. They maintain a set of candidate beams that may be from multiple cells. A scanner or test UE that would detect reference signals from six cells in a place of poor coverage in an LTE network, could now see six beams for each cell in a 5G NR network.

The slot structure of SSBs mapped to a grid of static or semi-static SSB beams.

While these factors boost the need for field testing, engineers also need to reconsider their scanners and test UE setups to ensure that they catch the needed KPIs. The key metrics collected on each beam are reference signal received power (SS-RSRP), reference signal received quality (SS-RSRQ), and signal-to-interference-plus-noise ratio (SS-SINR).

With mMIMO having a tremendous impact on system capacity, operators will scrutinize this aspect of network equipment during the selection, commissioning, and acceptance process. Thorough verification of field performance of mMIMO implementations is needed for capacity gain.

Many variables impact the real gain provided by mMIMO, most notably the spatial distribution of UEs. mMIMO capacity gain occurs when multiple UEs generate downlink traffic simultaneously. If all users are in the same location, it becomes impossible to isolate the users to different non-overlapping beams. Minimum acceptable horizontal and vertical spatial separation between UEs may differ depending on the number of physical antenna elements in the gNodeB (gNB) antenna panel in the horizontal and vertical dimensions. The signal-to-noise ratio (SNR) for each user and the multipath propagation profile impact the achievable performance. The gNB makes scheduling decisions and decides the use of multi-user MIMO (MU-MIMO) every 1 ms.

Therefore, engineers should distribute the various test UEs across the cell area when testing for capacity gain and perform active bulk data transfer testing against a test server, simultaneously. Don’t forget to check if the radio interface is the bandwidth bottleneck in the test by also checking the core network and backend server bandwidth. Engineers can remove the negative effects of transmission control protocol (TCP) flow control by using multi-threaded data downloads in the tests.

[Continue reading on EDN US: Scanners and UEs in 5G field testing]

Jessy Cavazos is part of Keysight's Industry Solutions Marketing team.