5G NR Beam mobility procedure with its Management & Scheduling... all you should know about beam-forming...

Beam-forming.. Beam management or say Beam Scheduling in #5G ?? How will it work ?? Why is it needed ??

Read it for very crisp and clear information...

Based on current status it seems that most of the deployment of 5G network would be in very high frequency (millimeter wave) and this high frequency deployment would be one of the most important characteristics of 5G (NR).

What is Beamforming or Why Beamforming is needed ??

Beam-forming is a traffic-signaling system for cellular base stations that identifies the most efficient data-delivery route to a particular user, and it reduces interference for nearby users in the process. Beamforming is used to detect and estimate the signal of interest at the output of a sensor array by means of optimal spatial filtering and interference rejection.

Beamforming or sometimes said as beamshaping will also improve the overall radio environment of a cell by limiting interference to small fractions of the entire space around a transmitter and likewise limiting the impact of interference on a receiver to infrequent stochastic events.

Beam-forming can help massive MIMO arrays, which are base stations arrayed with dozens or hundreds of individual antennas, to make more efficient use of the spectrum around them. The primary challenge for m-MIMO is to reduce interference while transmitting more information from many more antennas at once. Hence, beam-forming in m-MIMO base stations does signal-processing algorithms to plot the best transmission route through the air to each user which helps the efficiency & helps to utilized all spectrum deployed into it. Then they can send individual data packets in many different directions, bouncing them off buildings and other objects in a precisely coordinated pattern. By choreographing the packets’ movements and arrival time, beamforming allows many users and antennas on a massive MIMO array to exchange much more information at once. For millimeter waves [traditional 5G as of now], which are high-frequency waves expected to play a key role in 5G networks, beamforming is primarily used to address a different set of problems: Cellular signals are easily blocked by objects and tend to weaken over long distances. In this case, beamforming can help by focusing a signal in a concentrated beam that points only in the direction of a user, rather than broadcasting in many directions at once. This approach can strengthen the signal’s chances of arriving intact and reduce interference for everyone else. [Shortly Say.. Narrow beam - Directional - Hence by nature will have extended coverage gain & Low interference].

Remember :: "Strong concentrated signals in one particular direction and enables mmWave frequencies to travel far with less interference from other signals."

More the antenna elements => sharper the beam shape => concentrated energy => Less Interfered => Great Performance

Depending on the situation and the technology, there are several ways to implement Beamforming in 5G networks [which is mention below].

A significant challenge observed in beam paring between the base stations and UEs, especially for fast moving UEs.. solution for it also mention below...

5G NR beams are formed by analog beam-forming technique, but for the data transmissions 5G system dynamically uses analog or digital or the combination of analog and digital beam-forming called Hybrid beam-forming technique. As the cell coverage is beams based, a mobile terminal [UE] in the 5G cell will sync, attach and report from a single beam only and has no relation with multi beam -- As stated by 3GPP in Rel.15 "The mobile terminal will only connect to a single beam, multi-beams connection is not supported"

Beam Management

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

The beam management is nothing but a procedure with set of phases which are as mention below ::

Note : TRxP stands for Transmission Reception Point.

Note : Beam Management comes into picture during the UE State is connected, for idle mode or for idle states its all about beam scanning or beam updating just to be ready when there is any connected transaction's.

(1) Beam sweeping - In short words its covering a spatial area [antenna beam-width/serving area of anteena] with a set of beams transmitted and received according to pre-specified intervals and directions. These Tx & Rx is done in a burst [bursting of signals] in a regular interval of time frame. Exhaustive search based on SS blocks received by UE. Each lob in below Image is SS blocks and arrow lines conveys that block by block searching is going on w.r.t UE's.

(2) Beam measurements - As the name says, its just the measurement or say evaluation of the quality of the received signal at the gNB [in Uplink] or at the UE end [in Downlink]. Different metrics could be used such as SS - RSRP, SS - RSRQ and SS - SINR or SNR for measurement purpose. At the end a UE selects that SS which has best RSRP & RSRQ and gNB selects one of the best beam pair [UE pairs] for communication between itself and UE , these process is said as Beam determination or selection.

Note difference between LTE & 5G. For 4G => The UE measurements are done for RSRP, RSRQ and SNR associated which are associated with CRS (Cell Specific Reference Signal). In 5G NR we uses SS (Synchronization Signal) and CSI (Channel State Information) instead of CRS, so as per that 3GPP had put new definitions for UE measurements in 5G i.e SS-RSRP/RSRQ/SINR etc. or say CSI-RSRP/RSRQ/SINR etc. Were as RSSI will be measured and will be at NR level so terminology will be NR-RSSI

Just to make it simple and easy to understand for Nokia sub-system : 32 beams in Nokia gNB will transmit 32 SS blocks in different predefined directions (beams) in regular interval [same as we see sun rays bifurcation i.e a circle with 360 lines made with a set pattern & at regular intervals], the set of directions covered by the SS blocks may or may not cover the entire set of predefined directions available or required. The maximum number of predefined directions (beams / SS blocks) in the SS burst set is frequency dependent, like up to 3 GHz its “4”, from 3 GHz to 6 GHz its “8”, and from 6 GHz to 52.6 GHz its “64”. WoW !!

(3) Beam determination or selection - (As mention in above para..) + In IDLE mode the measurement is based on SS (Synchronization Signal) measured values, were-as in the connected mode it’s based on CSI-RS in DL and SRS in UL.

(4) Beam reporting - The procedure used by the UE to send beam quality and beam decision information to the Radio Access Network (RAN) i.e message floating from UE to gNB or say any RAN which is like RACH and seen in Uplink.

(5) Beam failure recovery [Maybe or May not be -- depending upon the Tx/Rx] - When the UE is suffering from poor channel condition/ Radio condition, it will get it as a beam failure indication from lower layers. The UE will request for a recovery by indicating a new SS block or CSI-RS, this will be done by starting a RACH procedure. The gNB will transmit a DL assignment or UL grant on the PDCCH to end the beam failure recovery.

Below is the Summary chart for Beam management phase vs NSA/SA used in DL/UL

Beam Management Implementation in 5G NR

Beam management implementation happens both in downlink (from gNB to UE) and uplink (from UE to gNB) of the 5G NR system. The only point is that system uses different reference signals for beam management based on mode.

IDLE Mode : It uses PSS/SSS/PBCH DMRS (i.e. SSB). After the UE selected a SS block (beam), for that SS block there is a predefined one or more RACH opportunities with certain time and frequency offset and direction (special to this SS block only), so that the UE knows in which transmit (UL) beam to transmit the RACH preamble.

CONNECTED Mode : It uses CSI-RS (in the downlink) and SRS (in the uplink). The UE will provide feedback using control channel, in case of link failure and no directions can be recovered using CSI-RS, the mobile terminal will try to recover the link using the SS bursts.

Downlink Beam Management : "SS blocks are transmitted towards UEs at regular intervals based on periodicity set (i.e. 5/10/20/40/80/160 ms)". We need to understand that 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 dependent. There can be about 64 blocks per burst at frequencies above 6 GHz (i.e. mmwave frequencies).

As said above DMRS (Demodulation Reference Signal) is only associated with PBCH channel, which is used to estimate RSRP which is calculated from the received SS blocks at the UE.

There is always periodic transmission of SS burst (carrying SS blocks) from gNB to UE at every 20 ms interval. As per below diagram - 4 Slot [OFDM symbols in ms which is periodically] * 5 SS blocks [in ms which is periodically] = 20ms interval [periodically].

Note = Many SS blocks = 1 SS Burst & Many SS Burst = 1 SS Burt Set

Uplink Beam Management : "Sounding Reference Signals (i.e.SRSs) are transmitted by UEs and received by gNB". These signals are used to monitor uplink channel quality / RF strength. 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.

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