5G NR: Beam-Based Coverage Measurements in 5G NR

The basic concepts of network coverage measurements are different in 5G NR compared to LTE. In 5G NR, the coverage is beam-based, not cell-based. Also, there is no cell-level reference channel from where the coverage of the cell could be measured. Instead, each cell has one or multiple synchronization signal block (SSB) beams (see Figure 4). The maximum number of SSB beams per cell is between 4 and 64, depending on the frequency range. SSB beams are static, or semi-static, always pointing to same direction. They form a grid of beams covering the whole cell area. The UE searches for and measures the beams, maintaining a set of candidate beams. The candidate set of beams may contain beams from multiple cells. The metrics measured are SS-RSRP, SS-RSRQ, and SS-SINR for each beam. Physical cell ID (PCI) and beam ID are the identifications separating beams from each other. In field measurements, these metrics can be collected both with scanning receivers and test UEs. Hence, SSB beams show up as a kind of new layer of mini-cells inside each cell in field measurements. As can be seen from Figure 4, the different SSB beams of a cell are transmitted at different times. Therefore, there is no intra-cell interference among the SSB beams, and at least scanning receivers should be able to detect also extremely weak SSB beams, even in the presence of a dominant, strong beam from the same cell. In general, the amount of reference signals in the air will increase. As an example, let us imagine a place of poor dominance in an LTE network, where a scanner or a test UE detects reference signals from six different cells. If it were a 5G NR network, the device could see, for example, six beams of each six cells, in total 36 reference signals. Provided of course that the scanner or test UE is fast enough to catch all these signals. The performance of UEs as well as scanners is yet to be seen both in the spec sheets and in practice.

Scanner-Based vs UE-Based Field Measurements

Figure 8 illustrates the steps of a UE accessing the network in 5G NR. SSB beams (PSS, SSS, and PBCH) are the only signals common to the cell and always in the air in 5G NR. CSI-RS is a UE-specific reference signal, and PDSCH is the traffic channel for downlink. Both CSI-RS and PDSCH are beamformed. When a UE is moving in the cell, the UE-specific beams are adjusted to follow the UE based on the CSI feedback collected from the radio channel. SSB beams, on the other hand, remain static, and the UE performs beam switching between the SSB beams, similarly to handovers between Find us at www.keysight.com Page 11 cells performed in legacy technologies. It should be noted that initial implementations will not necessarily utilize all the beamforming features as defined here. For example, the UE-specific traffic channels may be initially transmitted using the same beam (precoding) as the reference beams. Going back on what can be measured with a scanner and with a UE, a scanner can only see the SSB beams (cell-wide part of Figure 8) whereas all the channels, signals, and beams of Figure 8 are visible for the test UE. Figure 8. UE access process, reference beams and UE-specific beams in FDD, one CSI-RS mode. Channel state information (CSI) measurements can be performed in different ways depending on the network configuration and TDD/FDD mode, as illustrated in Figure 9. CSI information includes channel quality indicator (CQI), rank indicator (RI), codebook index (precoding weights as suggested by UE), and CRI, the ID of the strongest CSI-RS beam as seen by the UE in case of multiple CSI-RS beams. The FDD cases provide more visibility from the field measurement perspective as the CSI information is measured by the UE from CSI-RS and this information will then be available also in the diagnostics data of the test UE.

Figure 8. UE access process, reference beams and UE-specific beams in FDD, one CSI-RS mode.

There is one SS/PBCH block per beam used for the cell search procedure. The number of beams is referred to as LMAXLMAX in the specifications with common cases of LMAX=4LMAX=4 and LMAX=8LMAX=8 although there can be more cell search beams in the higher subcarrier spacings using higher frequencies (120 kHz and 240 kHz subcarrier spacings with carrier frequency above 6 GHz).

PSS and SSS

PSS and SSS are transmitted in SS blocks together with PBCH. The blocks are transmitted per slot at fixed locations. During initial search, UE correlates received signal and synchronisation signal sequences by means of matched filters and performs the following steps [15]:

1. Find PSS and obtain symbol and 5ms frame timing.

 2. Find SSS and detect CP length and FDD or TDD duplexing method and obtain exact frame timing from matched filter results, from PSS and SSS and obtain cell identity from reference signal sequence index.

3. Decode PBCH and obtain basic system information.

4. The UE reads PBCH providing the basic cell configuration and finds the downlink control channel (which schedules the shared channel).

5. UE reads minimum system information, providing scheduling information for all other system information blocks. 
6. UE reads other required system information.

7. UE requests on-demand system information, eg. System information that is only relevant to a specific UE. Fig

                                                        12: Call flow procedure

6.1 Initial Access Procedure

Initial access consists of downlink synchronisation and RACH procedure (uplink synchronisation). 1. Downlink synchronisation: UE detects the radio frame and OFDM symbol boundaries, by analysing the SS Block.

SSB indexing:

Each SS Block set (group of SS Blocks) is transmitted after every 20ms, with each SS Block set occupying an interval of 5ms.Each SS Block is given a unique number starting from 0 and incrementing by 1, which is informed to UE via two different parts within SSBlock. This is known as SSB indexing. 

a. One part is carried by PBCH DMRS (Minimum System Information). It consists of a parameter c_init (initial value), which is composed of physical cell ID, half frame number and SSB index. By decoding the DMRS, UE is able to figure out SSB index and half frame number. 

b. Other part is carried by PBCH Payload (Remaining Minimum System Information). 

2. Uplink synchronization (initial access): It is through RACH that uplink synchronization can be achieved between UE and gNB. It also helps to obtain the resources for RRC connection request. In NR, synchronisation on the downlink side is achieved by periodic transmission of SSBlock after a certain interval. However, in uplink, it is not efficient as it may cause high interference to other UEs and energy wastage, if such periodic broadcasting mechanism is adopted. The major difference encountered here is just before when RACH preamble is transmitted. Unlike LTE, beamforming mode is used by UE to detect and select the best beam for RACH process.

Random access after acquisition of Broadcast System Information: After broadcast system information is acquired, mobile terminals use the four-step process for random access, as used in LTE. However, the fundamental differences with respect to LTE are: 1. For PRACH transmitted as Msg.1, in addition to some formats using same sequence length and OFDM subcarrier spacing as LTE PRACH formats, PRACH formats using wide OFDM subcarrier spacing and shorter sequence length are used for high frequency band. 2. In case transmission beamforming is applied to SS Block for additional cell coverage, it is vital to apply equivalent reception beamforming at base station for receiving the PRACH preamble from mobile terminals. Rest of the steps are similar to as of LTE.

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https://literature.cdn.keysight.com/litweb/pdf/5992-3299EN.pdf?id=3013192