5G NR Beam Management: SA and NSA Modes Explained

This page describes 5G NR Beam Management basics in IDLE and CONNECTED modes. It mentions Beam management operations viz. beam sweeping, beam measurement, beam determination and beam reporting. It mentions 5G NR beam management frameworks for NSA (Non StandAlone) and SA (StandAlone) modes.

Introduction

The 5G NR (New Radio) is the latest in the series of 3GPP standards which supports very high data rate with lower latency compared to its predecessor LTE (4G) technology. 5G NR supports FR1 and FR2 frequency bands. FR1 is sub-6 GHz, from 450 to 6000 MHz where as FR2 is mmwave band (from 24.25 GHz to 52.6 GHz).

As the mmwave band uses very high frequency, it leads to propagation loss and other losses. To compensate for the losses, directional communication is essential at such frequencies. Antenna arrays with large number of antenna elements make it possible due to smaller wavelengths. This concept provide beamforming gain to the RF link budget which helps in compensation of propagation loss.

Moreover, large antenna array helps to achieve higher data rate due to spatial multiplexing technique. These directional links require accurate alignment of transmitted and received beams. In order to achieve alignment of beam pair and to have required end to end performance with desired delay, beam management operations are introduced in the 5G NR.

Beam management operations are essential during Initial Access (i.e. IDLE mode) when UE is not in connection with gNB and during tracking (i.e. CONNECTED mode) when UE is exchanging data with the gNB (i.e. network).

Beam Management in 5G NR

Beam management procedure is used in 5G NR in order to acquire and maintain a set of TRxP (s) and/or UE beams which can be used for DL and UL transmission/reception. TRxP stands for Transmission Reception Point.

Following operations are performed in 5G NR beam management procedure. They are applied to both the modes viz. SA (StandAlone) and NSA (Non StandAlone).

• Beam Sweeping: It refers to covering a spatial area with a set of beams transmitted and received according to pre-specified intervals and directions.
• Beam measurement: It refers to evaluation of the quality of the received signal at the gNB or at the UE. Different metrics could be used such as RSRP, RSRQ and SINR or SNR for this purpose.

• Beam determination: It refers to the selection of the suitable beam or beams either at the gNB or at the UE, according to the measurements obtained with the beam measurement procedure.

• Beam reporting: It refers to the procedure used by the UE to send beam quality and beam decision information to the Radio Access Network (RAN).

Beam Management Implementation in 5G NR

• Let us understand beam management implementation in both downlink (from gNB to UE) and uplink (from UE to gNB) of the 5G NR system. The system uses different reference signals for beam management.
• It uses PSS/SSS/PBCH DMRS (i.e. SSB) during IDLE mode .
• It uses CSI-RS (in the downlink) and SRS (in the uplink) during CONNECTED mode .

Downlink Signal Measurements for Beam management

Using SS Blocks: In the 5G NR downlink frame, SS blocks are transmitted towards UEs at regular intervals based on periodicity set (i.e. 5/10/20/40/80/160 ms). Multiple SS blocks are carried in a SS burst. A single SS block spans 4 OFDM symbols on time axis and 240 subcarriers on frequency axis. SS block carry PSS (Primary Synchronization Signal), SSS (Secondary Synchronization Signal) and PBCH with DMRS. The SS blocks are grouped into first 5 ms of the SS burst. The maximum number (L) of SS blocks in single burst is frequency dependant. There can be about 64 blocks per burst at frequencies above 6 GHz (i.e. mmwave frequencies).

Here DMRS (Demodulation Reference Signal) associated with PBCH channel is used to estimate RSRP (Reference signal received power) is calculated from the received SS blocks at the UE.

The figure depicts periodic transmission of SS burst (carrying SS blocks) from gNB to UE at every 20 ms interval.

Using CSI-RS: In LTE, connected UEs estimate channel conditions regularly and report CQI (Channel Quality Information) to their serving eNBs. Like LTE, in 5G NR these signals are used for RRM measurements for mobility management purposes in CONNECTED mode. CSI-RS spans N = 1/2/4 OFDM symbols. There are two transmission modes of CSI-RS viz. periodic and semi-persistent/aperiodic.

Uplink Signal Measurements for Beam management

Using SRS: Sounding Reference Signals (i.e.SRSs) are transmitted by UEs and received by gNB. These signals are used to monitor uplink channel quality. UEs are configured with multiple SRSs for beam management purposes. They can span about 1 to 4 OFDM symbols and occupy portion of the bandwidth allocated to UE.

Beam Management Phase in SA and NSA modes

Following table mentions comparison of beam management phases in SA (DL) and NSA (DL/UL) modes.

References

• 3GPP Website
• An IEEE paper (Standalone and Non-Standalone Beam Management for 3GPP NR at mmWaves)
• An article from Keysight Technologies

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