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 be "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.

// Now that 3GPP Technical Specification on Beam Management has been released and I could write on this mechanism based on the formal specification. You may jump to NR Beam Management in a Nutshell section and read from there if you are interested in the formal specification.

 

 

 

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 ?)

NOTE : Now that the technical specification on all of the questions listed above has been released and I wrote a few separate pages regarding this topic. Refer to CSI RS signal page and CSI Report page for further details based on 3GPP technical specification.

 

 

 

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.

 

 

 

 

Where to point my beam ?

 

We can think of this question in terms of two different cases : transmitting case and receiving case.

 

When transmitting,

 

Now let's think of which direction a gNB or UE has to point its beam when they try to transmit the signal ?

The answer is simple.  They (gNB or UE) has to transmit the signal in the direction that can reach the reciever with the best signal quality'.

 

Then you would have another question. How can they(gNB or UE) figure out which direction is the one that can reach the reciever with the best signal quality ?

Now the answer would be a little bit trickier, but the big picture is as follows.

  • When gNB is transmitting, gNB figure out this direction by evaluating the quality of a specific reference signal of multiple beam from UE. gNB evaluate the quality of the reference signal from each of the multiple beam and chose the best one. The reference signal from UE is called SRS.
  • When UE is transmitting, UE figure out this direction by evaluating the quality of a specific reference signal of multiple beam from gNB. UE evaluate the quality of the reference signal from each of the multiple beam and chose the best one. The reference signal from gNB in this case can vary depending on situation. Sometimes it can be SSB and sometimes it can be CSI-RS. (NOTE : CSI-RS play many different roles in addition to beam management and very complex topic. Refer to CSI-RS signal generation and CSI report page for further details).
  •  

    NOTE : This kind of estimation of reference signal quality should be done sometime before they transmit signal.

3GPP TR 38.802 (V14.2.0)-6.1.6.1 describes on this situation as follows :

  • TRP is able to determine a TRP Tx beam for the downlink transmission based on TRPs uplink measurement on TRPs one or more Rx beams
  • UE is able to determine a UE Tx beam for the uplink transmission based on UEs downlink measurement on UEs one or more Rx beams
  •  

    NOTE : TRP is a transmission point of a gNB.

 

 

When recieving,

 

Now let's think of which direction a gNB or UE has to point its beam when they try to recieve signal ? (In this case, the word 'beam' may be a little misleading because the reciever does not form any real beam. So it would be better to change the phrase 'to point its beam' to 'to tune its reciever to a certain direction').

The answer is simple.  They (gNB or UE) has to tune their reciever in the direction in which they can receive the signal from the transmitter with best quality.

 

Then you would have another question. How can they(gNB or UE) figure out which direction is the one in which they can receive the signal from the transmitter with best quality ?

Overall logic is as follows :

  • When gNB receiving signal from UE, (before doing this) gNB is supposed to get the information of the best direction from UE in the form of CSI report.
  • When UE receiving signal from gNB, (before doing this) UE is supposed to get the information of the best direction from gNB (gNB has detected the best direction based on the measurement of SRS signal quality of multiple beams from UE and indicates the UE of the best direction).

 

3GPP TR 38.802 (V14.2.0)-6.1.6.1 describes on this situation as follows :

  • TRP is able to determine a TRP Rx beam for the uplink reception based on UEs downlink measurement on TRPs one or more Tx beams.
  • UE is able to determine a UE Rx beam for the downlink reception based on TRPs indication based on uplink measurement on UEs one or more Tx beams.

 

 

 

P1, P2, P3 - What is It ?

 

As illustrated in Beam Management in a Nutshell, P1/P2/P3 are a set of processes that are designed for beam management while in connected state.

 

I will talk about a high level view on these processes in this section. For further details, you would need to understand the very details on CSI report for Beam Management which is another huge topic and will be described in a separate page here.

 

 

According to 38.802-6.1.6.1, P1/P2/P3 are stated as follows.

 

P-1: is used to enable UE measurement on different TRP Tx beams to support selection of TRP Tx beams/UE Rx beam(s). For beamforming at TRP, it typically includes a intra/inter-TRP Tx beam sweep from a set of different beams. For beamforming at UE, it typically includes a UE Rx beam sweep from a set of different beams.

 

Does this make sense to you ? It may take time to get clear understanding on this. Following illustration is my understanding/interpretation of this statement.

 

 

 

P-2: is used to enable UE measurement on different TRP Tx beams to possibly change inter/intra-TRP Tx beam(s). From a possibly smaller set of beams for beam refinement than in P-1. Note that P-2 can be a special case of P-1.

 

Following is my understanding on this process in illustration.

 

 

 

P-3: is used to enable UE measurement on the same TRP Tx beam to change UE Rx beam in the case UE uses beamforming

 

Following is my understanding on this process in illustration.

 

 

 

 

Building Up Intuition

 

Usually when a designer/inventor postulate any technology or theory, they first come up with those idea in their brain in intuitive way and then formulate those idea in various formal documents like technical papers, thesis or industry document. These formal document would be easily understood by those who has similar level of experties as the original designer, but for those who does not have the same level of experties or experience, it is very hard to get the clear picture just by reading the formal documents. I think the best way to build up intuition is to try things with your own hands and see the result with your own eyes, but it is not always easy to have this kind of physical setups especially for Beam related issues. Then, what would be the alternative to the physical setup ? My alternative is to use software simulator like Matlab. Actually this kind of software simulator give some additional advantage.

  • You can try many things with easy and in very short time (almost no preparation / setup time).
  • You can visualize many things that cannot easily be done by physical setup (like visuallizing beams).

First level intuition that I want to build up is to understand 'what kind of antenna array can form what shape of beam'. What I have tried with Matlab Antenna toolbox for this are as follows.

 

Next level intuition is to associate the physical antenna array mechanism with high level beam management scenario explained in previous section.

 

Also it would be good to have some intuitive unerstanding on how Precoding and Equalization works : here

 

Even though it is not directly related, it would be helpful if you have some intuitive understanding on the concept of precoding and OTA(Over the Air) channel properties : here(for LTE), here

 

 

 

From Intuition to 3GPP

 

Now we have trick part.... that is, associating your intuition to 3GPP terminology and process. I don't think I can do this in single step. First, we need to understand the fundamental concept/terminology defined in 3GPP specification. Here goes the list of items that I suggest you to tackle first.

  • Undersand the concept of QCL : here
  • Understand how to define/allocation the physical resource of CSI-RS : here
  • Understand what is CSI-RS codebook and how it works : here for LTE, here for NR
  • Understand how CSI-RS antenna (virtual antenna) is defined and configured : here, here
  • Understand how all of these basic concept are put together in signaling message : here

 

 

 

Reference

 

[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   

[15] 3GPP TR 38.802 - 6.1.6.1 Beam Management