5G/NR - Beam Management                                           Home : www.sharetechnote.com





Even though 3GPP would not preclude the use of Sub 6 Ghz deployment of 5G(NR), at least based on the current status it seems that most of the deployment would be in very high frequency (millimeter wave) and this high frequency deployment would be one of the most important characteristics of 5G (NR).




Why we need Beam ?


Mostly by Nature of the wave (by Physics), when we use low and mid range of frequency, we can transmit a signal in all direction (as in (A)) or relatively wide angles (as in (B)). However, when we use very high frequency, we would not have much choice except using a huge antenna array. As a result of using this kind of huge antenna array, the resulting radiation would be a beam as in (C). Refer to Why Massive MIMO page for the details of this background.





Why Beam Management / Beam Control ?


I don't think trasmitting signal in Beam in high frequency deployment would be the matter of choice. It is a kind of 'MUST' implementation. In case of low / mid frequency region without using massive antenna array (as in (A) / (B)), a single transmission would cover a lot of UEs simultaneously. However, when the radiation become beam-shaped as (C), it is very difficult to cover multiple UEs in single transmission unless those multiple UEs are located in very close proximity. To handle this problem, we need a very sophisticated idea of managing/controlling the beam to cover the multiple devices scattered in all directions and the management/control mechanism should be different depending on the situtations. All of these collection of idea would fall into the title of "Beam Management" in the specification.



Beam Management/Control for each specific situation will be described in separate pages with relavent situation (like Beam Management during Synchronization, Beam Management during Initial Attach, Beam Management in connected status etc). In thise page, I would describe on general idea.




Beam Management/Control when a transmitter has no information on the location of the reciever


Now let's look into a more specific cases where the Beam Management/Control become crucial. As an example, let's think of following case. There is a Base Station with Massive MIMO operating at the very high frequency. There is a UE around the Base Station and you are just about to turning on the UE. Once the UE is turned on, it would start Synchronization process. For this step, the Base Station would transmit the special signal called Synchronization Signal and the signal should be able to reach to every UEs around the base station. However, here comes a serious problem with the base station sending signal in Beam. It is the fact that the signal beam can point to a very narrow area and it cannot cover a very wide area at the same time. Simply put, now you have the following question.



What would be the answer for this ? If everything works as you draw in power point, you may draw a solution as follows. You may want to transmit a lot of beams in all direction simultaneously. Looks good ? Looks like a flower :).



Would the solution above be feasible, reasonable and effective ? The simple answer is NO (I would not explain why. You may easily guess why).


Then what can be another idea (possible soultion) for the problem ? There can be multiple ideas and proposals, but the most popular proposal as of now seems to be that the base station transmit the beam to a specific direction at a specific time and then change the direction a little bit in a next time frame and so on until it can scan all the area it should cover.



Then, the next question would how to reflect / implement this concept in the radio frame design. I would not go too much detail on this until this is explicitely determined in 3GPP TS (Technical Specification) document, but you can get the general ideas on various options /proposals from TDocs listed in Reference Section.




Beam Management/Control when the connection is already established


Now let's talk about more serious case of Beam Management. In terms of 3GPP TDocs, Beam Management handles mostly with this topic (Beam Management during the connected states) and the one mentioned in previous section are described as a part of the topic Cell Search / Initial Access.


Once UE gets into a connection states with a Network, at least one beam (or multiple beam) is properly in connection between UE and the network. Theretically there can be so many different ways in which UE and Network beam is connected, but we can reduce it down to roughly four differences case as shown below.


In case 1, UE and Network is connected through a single TRP (Tx/Rx Point) and a single beam.

In case 2, UE and Network is connected through multiple TRP (Tx/Rx Point) and a single beam for each TRP.

In case 3, UE and Network is connected through a single TRP (Tx/Rx Point) and multiple beams

In case 4, UE and Network is connected through multipe TRP (Tx/Rx Point) and multiple beams for each TRP


You may think of many other cases and ask "How about this case ? How about that case ?". Whatever you think of and whatever you are asking, I think all of those would be valid thinking and valid question until 3GPP reach a explicit conclusion. So keep asking and try to find your own answers until you see the explicit 3GPP specification.




Now the important and tough task is to maintain the connection. For this, I would not write much details until 3GPP specifies it in detail. Sorry for skipping too much with the execuse of 3GPP specification unavailability :). But I don't want to write many things now and rewrite too much after 3GPP Technical Specification come out. For now, my purpose is to give you very broad and general idea, and let you know what you may need to study in further details if you are really interested in technical details. For this purpose, I will list up most of TDocs about each topics in Reference section, so that you can get more detailed idea proposed by many companies / organizations in the industry.


The general idea of the beam management during the connected states would be

    i) Network transmit a specific reference signal for beam management

    ii) UE detect the signal and perform some measurement and send feedback to Network

As you may notice, the general idea would be very similar to CSI report mechanism that are currently used in current LTE. However, a lot of details are yet to be determined.  For example,

    i) Baseband Signal (Symbol) generation formula

    ii) Resource Allocation mapping (How to allocate these reference symbols to which specific resource element)

    iii) How often UE need to perform these measurement

    iv) How UE report the measurement result ? (via RRC messages ? or via MAC / PHY layer transactions ?)




NR Beam Management in a Nutshell


If you ask me to explain about NR Beam Management in a few seconds, I would summarize the whole process in an illustration as shown below. As you see here, Beam Management plays important role in two period - During RACH procedure and After the call connection.








[1]3GPP R1-166089. 3GPP TSG RAN WG1 Meeting #86 - Beam Management Procedure for NR MIMO

[2] 3GPP R1-166214. 3GPP TSG RAN WG1 Meeting #86 - Discussion on the beam management for the NR

[3] 3GPP R1-166389. 3GPP TSG RAN WG1 Meeting #86 - Beam Management in Millimeter Wave Systems

[4] 3GPP R1-166565. 3GPP TSG RAN WG1 Meeting #86 - Beam management without prior beam information

[5] 3GPP R1-166657. 3GPP TSG RAN WG1 Meeting #86 - Views on beam management for NR

[6]3GPP R1-166785. 3GPP TSG RAN WG1 Meeting #86 - Discussion on TRP beamforming and beam management

[7]3GPP R1-167466. 3GPP TSG RAN WG1 Meeting #86 - Key principles for beam management

[8] 3GPP R1-167467. 3GPP TSG RAN WG1 Meeting #86 - Reference signals and reports to support beam management

[9] 3GPP R1-167543. 3GPP TSG RAN WG1 Meeting #86 - Beam Management Considerations for above 6 GHz NR

[10] 3GPP R1-1712221. 3GPP TSG RAN WG1 Meeting #90 - DL Beam Management Framework

[11] 3GPP R1-1610243. 3GPP TSG-RAN WG1 #86-BIS : On procedures for beam selection and feedback signaling

[12] 3GPP 38.300 NR;Overall description;Stage-2 - 9.2.4 Measurements

[13] 5G NR Beam Management and Beam Scheduling (everything about the beams)

[14] 5G NR Beam Managament SA, NSA | Beam Management in 5G NR