The 3G4G Blog

Sunday, 19 January 2020

2-step RACH Enhancement for 5G New Radio (NR)

5G Americas recently published a white paper titled, "The 5G Evolution: 3GPP Releases 16-17" highlighting new features in 5G that will define the next phase of 5G network deployments across the globe. It's available here. One of the sections in that details the 2-step RACH enhancement that is being discussed for a while in 3GPP. The 2-step process would supercede the 4-step process today and would reduce the lartency and optimise the signalling.

The 5G Evolution: 3GPP Releases 16-17 white paper from @5GAmericas highlights specific updates in Releases 16-17. Good whitepaper, worth reading - https://t.co/wVHBj7ZunE #Free5GTraining pic.twitter.com/lshYbzP5ne
— 5G Training (@5Gtraining) January 18, 2020

Here are the details from the 5G Americas whitepaper:

RACH stands for Random Access Channel, which is the first message from UE to eNB when it is powered on. In terms of Radio Access Network implementation, handling RACH design can be one of the most important / critical portions.

The contention-based random-access procedure from Release 15 is a four-step procedure, as shown in Figure 3.12. The UE transmits a contention-based PRACH preamble, also known as Msg1. After detecting the preamble, the gNB responds with a random-access response (RAR), also known as Msg2. The RAR includes the detected preamble ID, a time-advance command, a temporary C-RNTI (TC-RNTI), and an uplink grant for scheduling a PUSCH transmission from the UE known as Msg3. The UE transmits Msg3 in response to the RAR including an ID for contention resolution. Upon receiving Msg3, the network transmits the contention resolution message, also known as Msg4, with the contention resolution ID. The UE receives Msg4, and if it finds its contention-resolution ID it sends an acknowledgement on a PUCCH, which completes the 4-step random access procedure.

The four-step random-access procedure requires two round-trip cycles between the UE and the base station, which not only increases the latency but also incurs additional control-signaling overhead. The motivation of two-step RACH is to reduce latency and control-signaling overhead by having a single round trip cycle between the UE and the base station. This is achieved by combining the preamble (Msg1) and the scheduled PUSCH transmission (Msg3) into a single message (MsgA) from the UE, known as MsgA. Then by combining the random-access respond (Msg2) and the contention resolution message (Msg4) into a single message (MsgB) from the gNB to UE, see Figure 3.13. Furthermore, for unlicensed spectrum, reducing the number of messages transmitted from the UE and the gNB, reduces the number of LBT (Listen Before Talk) attempts.

Design targets for two-step RACH:

A common design for the three main uses of 5G, i.e. eMBB, URLLC and mMTC in licensed and unlicensed spectrum.
Operation in any cell size supported in Release 15, and with or without a valid uplink time alignment (TA).
Applicable to different RRC states, i.e. RRC_INACTIVE, RRC_CONNECTED and RRC_IDLE states.
All triggers for four-step RACH apply to two-step RACH including, Msg3-based SI request and contention-based beam failure recovery (CB BFR).

As described earlier, MsgA consists of a PRACH preamble and a PUSCH transmission, known as MsgA PRACH and MsgA PUSCH respectively. The MsgA PRACH preambles are separate from the four-step RACH preambles, but can be transmitted in the same PRACH Occasions (ROs) as the preambles of fourstep RACH, or in separate ROs. The PUSCH transmissions are organized into PUSCH Occasions (POs) which span multiple symbols and PRBs with optional guard periods and guard bands between consecutive POs. Each PO consists of multiple DMRS ports and DMRS sequences, with each DMRS port/DMRS sequence pair known as PUSCH resource unit (PRU). two-step RACH supports at least one-to-one and multiple-to-one mapping between the preambles and PRUs.

After the UE transmits MsgA, it waits for the MsgB response from the gNB. There are three possible outcomes:

gNB doesn’t detect the MsgA PRACH ➡ No response is sent back to the UE ➡ The UE retransmits MsgA or falls back to four-step RACH starting with a Msg1 transmission.
gNB detects MsgA preamble but fails to successful decode MsgA PUSCH ➡ gNB sends back a fallbackRAR to the UE with the RAPID (random-access preamble ID) and an uplink grant for the MsgA PUSCH retransmission ➡ The UE upon receiving the fallbackRAR, falls back to four-step RACH with a transmission of Msg3 (retransmission of the MsgA PUSCH).
gNB detects MsgA and successfully decodes MsgA PUSCH ➡ gNB sends back a successRAR to the UE with the contention resolution ID of MsgA ➡ The reception of the successRAR successfully completes the two-step RACH procedure.

As described earlier, MsgB consists of the random-access response and the contention-resolution message. The random-access response is sent when the gNB detects a preamble but cannot successfully decode the corresponding PUSCH transmission. The contention resolution message is sent after the gNB successfully decodes the PUSCH transmission. MsgB can contain backoff indication, fallbackRAR and/or successRAR. A single MsgB can contain the successRAR of one or more UEs. The fallbackRAR consists of the RAPID: an uplink grant to retransmit the MsgA PUSCH payload and time-advance command. The successRAR consists of at least the contention resolution ID, the C-RNTI and the TA command.

For more details on this feature, see 3GPP RP-190711, “2-step RACH for NR” (Work-item description)

Tuesday, 14 January 2020

EN-DC SRB3 Demystified

3GPP 37.340 says that it is up the secondary node to establish "SRB3", but what exactly does this mean and how is it done?

Simple answer: The establishment of a signaling radio bearer (SRB) 3 in EN-DC mode means that RRC Measurement Reports for NR quality can be sent directly to the SgNB. This enables the 5G node to make intra-SgNB handover decisions and start the handover execution without involving the master eNodeB of the connection.

To prevent confusion the figure below shows a simplified scenario in which the Complete/Acknowledgement messages are not mentioned although they will be seen in the message flow.

A prerequisite is the successful addition of 5G radio resources as described in an earlier blog post. After this is completed the UE in the example transmits user plane information over the NR cell with the physical cell ID (PCI) = 12. In the transport network this cell is identified by NR CGI = xxxx52 (where „xxxx“ stands for a valid PLMN-ID and gNodeB-ID).

In the figure below the SgNB sends a X2AP SgNB Modification Required message that carries an embedded NR RRC cG-Config message. This cG-Config message is transparently forwarded by the MeNB to the UE. When arriving at the UE it activates CSI reference signal measurements on the 5G frequency including the serving 5G cell as well as its neighbors. It shall be noticed that here the concept of the Special Cell (SpCell) applies as it was defined for LTE-A CoMP scenarios.

Instead of the X2AP SgNB Modification Required message the information for activating the CSI reference signal measurements can alternatively transported using the X2AP SgNB Addition Request Acknowledge or X2AP SgNB Change Required message.

In step 2 the UE sends a NR RRC (3GPP 38.331) Measurement Report that indicates a stronger 5G cell (the neighbor cell with PCI = 11) was measured. It might be a vendor-specific implementation to send this NR RRC Measurement Report simultaneously over uplink channels of the LTE radio link where it is carried by the LTE RRC Uplink Information Transfer MRDC (Multi-RAT Dual Connectivity) as well as over NR radio links where it is forwarded by the SgNB to the MeNB embedded in a X2AP RRC Transfer message.

Indeed, it is the SgNB that makes the handover decision, but since the MeNB is in charge of the signaling connection the handover command (here: another NR RRC cG-Config message that orders to switch the 5G radio link to the cell with PCI = 11) must be transmitted to the MeNB by using another X2AP SgNB Modification Required message.

After the UE received the NR CC cG-Config message sent by the SgNB the HO is executed and the 5G cell with PCI = 11 becomes the new primary secondary cell of the EN-DC connection.

Figure: Measurement Configuration, Reporting and Execution for intra-SgNB Handover

Sunday, 5 January 2020

Free 5G Training

Many readers of this blog would be aware that I run 5G 'An Advanced Introduction to 5G Technology' training course for Cambridge Wireless at regular intervals. The next one is on 10th March 2020, in Cambridge. In fact I am also running a 'Introduction to Private 4G & 5G Networks' workshop on 4th Feb 2020 in London.

Many people ask me if they I have an online course available. While I haven't, I have quite a few videos on 3G4G YouTube channel and also have a 3G4G training page. People (mainly students or newbies) still ask me what sequence of videos they should go through, etc. So to make everyone's life simple, I created 2 YouTube playlists. Both are embedded below for ease.

Part 1. This is for people who know basic telecommunications theory and don't know much about mobile technology in general

Part 2. This is for people who understand 2G/3G/4G basics and want to learn about 5G.

I will add/remove/update videos when we add new videos on our channel. Feel free to skip videos if you already know a topic. There is quite a lot of other information on the 3G4G YouTube channel if you want to explore more

Where do you go after you have watched these videos? Go to the 3GPP homepage and start looking at specifications, news, etc. You can also start looking at the specifications on 3G4G page here.

Hopefully this is a good starting point.

SEE ALSO: https://www.free5gtraining.com/

Monday, 23 December 2019

Monday, 16 December 2019

5G Integrated Access and Backhaul (IAB) Enhancements in Rel-17

It's been a while since I last wrote about IAB on this blog here. At that time 3GPP Release-16 was being discussed. Since then things have moved on. While Release-16 is being prepared for final release soon, Release-17 study and work items have just been agreed upon.

IAB is included as part of Rel-16 but there isn't a comprehensive document or presentation easily available to details all that it will contain. Similarly the enhancements for Release-17 are available only superficially. Qualcomm is well known for making some really excellent presentations available on 5G. One of their presentations from January (here) has some details on IAB (pg. 32 - 35). There was also an excellent presentation by Navid Abedini, Qualcomm from IEEE Sarnoff Symposium, 2019 which is embedded at the end.

In a 3GPP RAN#84 discussion document RP-191181, Samsung has provided a comprehensive summary of what is being done as part of Rel-16 and what did not make in that:

Rel-16 IAB aims at basic operations

Architecture and protocol design
IAB integration procedure
Routing, BAP and BH configuration
CP and UP data transmission via IAB
Topology support:

Spanning Tree (ST) and Directed acyclic graph (DAG)
Intra-Donor adaptation is prioritized

The following cannot be supported in Rel-16

Mobile IAB
Topology support: Mesh

Some functionalities in Rel-16 may not be completed due to time constrains e.g.

Topology adaptation between IAB donors
Mechanisms for efficient control signaling transmission

Ericsson also provides a good summary in RP-190971 regarding Release 16 IAB and Rel-17 enhancements:

IAB Rel-16 provide basic support for multi-hop and multi-path relaying.
The solution supports

QoS prioritization of traffic on the backhaul link
Flexible resource usage between access and backhaul
Topology adaptivity in case link failure

In Rel-17 it would be possible to further evolve the IAB solution targeting increased efficiency and support for new use cases

Meanwhile in the recently concluded RAN#86, AT&T provided a good detailed summary on what enhancements are required for IAB as part of Rel-17 in RP-192709

Duplexing enhancements

Multiplexing beyond TDM (FDM/SDM/multi-panel Tx/Rx) including multi-parent scenarios, case 6/7 timing alignment, power control/CLI optimizations

Topology enhancements

Mobile IAB: CP/UP split + Group mobility
Inter-CU topology adaptation
Mesh-connectivity between IAB nodes for local control/user plane routing

User plane enhancements

Multi-hop scheduling enhancement – exchange of benefit metric between IAB nodes to enable radio-aware multi-hop scheduling to improve throughput performance

Network Coding

Study benefits compared to duplication over redundant backhaul routes

We will have to wait and see what makes it into the enhancements and what don't. Meanwhile here is a video from Navid Abedini, Qualcomm from IEEE Sarnoff Symposium, 2019

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Monday, 9 December 2019

5G Evolution with Matthew Baker, Nokia

I wrote a summary of CW (Cambridge Wireless) TEC conference here a couple of months back. The last session was on "Getting ready for Beyond-5G Era". Matthew Baker, Head of Radio Physical Layer & Co-existence Standardization, Nokia Bell Labs was one of the speakers. His talk provided a summary of 3GPP Rel-15 and then gave a nice and short summary of all the interesting things coming in Rel-16 and being planned for Rel-17. The slides from his presentation is embedded below:

5G Evolution: Progressive Enhancement & N ew Features for New Markets from 3G4G

Nokia also created a short video where Matthew talks about these new features. It's embedded below:

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Wednesday, 4 December 2019

Challenges of 5G Inter-Node Handovers

In all mobile communication networks handovers are the most complex signaling procedures, because multiple network elements (or network functions) are involved. Thus, it is logical that dual connectivity with two different base stations contributing to the radio connection simultaneously are even more complicated. And in EN-DC these two base stations are often covering different footprints using different carrier frequencies.This leads to a situation where we have more options for performing a handover in detail compared with plain LTE handover scenarios before.

The two signaling scenarios presented below illustrate in which different ways a change of the LTE master eNodeB can be performed during an ongoing EN-DC radio connection by using the X2 interface. In a very similar way it is also possible to perform S1 handover from old to new MeNB.

The pros and cons of these options have been discussed already by Martin Sauter in his Wireless Moves blog.

Inter-MeNB Handover without 5G Inter-Site Anchor

Figure 1 shows the easiest way of handing over the signaling connection from one MeNB to another one. Here it is up to the new MeNB to decide if and how the 5G part of the radio connection is continued.

Figure 1: X2 Handoverof EN-DC connection without 5G inter-site anchor

The handover is triggered when the UE sends a RRC Measurement Report (step 1) indicating that a stronger 4G cell than the currently used primary cell was measured. From its neighbor list the current MeNB detects that this better cell belongs to a neighbor eNB.

To provide both, the the Master Cell Group (MCG) and Secondary Cell Group (SCG) parameters to this neighbor eNB the old MeNB queries the SCG configuration parameters from the old SgNB by performing the X2AP SgNB Modification procedure (step 2+3).

Then it sends the X2AP Handover Request message to the target MeNB (step 4) including all information necessary to continue the 5G radio link in case the target MeNB decides to go for this option.

However, what comes back from the target MeNB is a plain LTE handover command (LTE RRC Connection Reconfiguration message [step 6]) embedded in the X2AP Handover Request Acknowledge message (step 5).

Due to this the old MeNB releases all 5G resources and the UE context in the SgNB (steps 7 + 10).

After the UE successfully connected via radio interface with the target cell in the new MeNB the S1AP Path Switch procedure is executed to re-route the GTP/IP-Tunnels on S1-U (step 8) and releases the X2 UE context in the old MeNB (step 9)

The new MeNB then waits for a new inter-RAT measurement event B1 (step 11) before starting a new SgNB addition procedure (step 12). Once the SgNB addition is successfully completed including all necessary reconfigurations/modifications on RRC and S1 the payload transmission over 5G resources is continued.

Inter-MeNB Handover with 5G Inter-Site Anchor

Now figure 2 shows what happens when the new MeNB decides to keep the existing UE context in the SgNB while the RRC measurement results and parameters are identical with what was presented above.

Figure 2: X2 Handoverof EN-DC connection with 5G inter-site anchor

The difference in the call flow starts at step 5 when the new MeNB after receiving the X2AP Handover Request (step 4) starts the X2AP SgNB Addition procedure towards the SgNB (old = new!). The SgNB-UE-X2AP-ID earlier requested in step 2+3 acts as the reference number for the existing context that is going to be continued.

After adding the SgNB UE context successfully the new MeNB sends the X2AP Handover Request Acknowledge message including an UE Context Kept = "true" flag and the Handover Command (step 8).

After the UE successfully connected to the target cell of the new MeNB the S1AP Path Switch procedure is performed and the temporary X2 UE context between old and new MeNB is released (step 10).

The big advantage of handling the handover in this way: The duration of the interruption of the payload transmission over 5G radio resources is minimalized and subscriber experience is significantly better compared to the scenario in figure 1.

Monday, 2 December 2019

Guest Post: Exploring Network Convergence of Mobile, Broadband and Wi-Fi

This is a guest post by Ben Toner, Founder and Director, Numerous Networks

Are multiple networks better than one?

How many articles have you read with a title similar to "Which technology is better, 5G or Wi-Fi6?" If, like me, you regularly use Wi-Fi and cellular (I still use 4G though) then you might find it hard to take sides.

Enter Network Convergence - the concept of bringing multiple networks together to get the best of them all. Imagine, as an end user, not having to decide which network to use but instead feeling satisfied that your data was traversing the best combination of networks at that moment in time.

Imagine a business traveler being connected to Wi-Fi which is slow or busy while trying to take that all important conference call while sitting in an airport. Because you are roaming you want to use that Wi-Fi but you do not want to compromise the video call quality. If your network and device could work together to use just enough cellular data to supplement the slow Wi-Fi so that you stayed within your daily roaming quota but never lost a moment in the video call - then you would probably be very happy with that service. Better still, as you start walking off, if the call transitioned from Wi-Fi to cellular with no dropouts or hangup then you might be delighted!

Earlier I underlined best because that in itself is somewhat complicated. The example above is easy to desribe but quite hard for to achieve within a framework where all possible scenarios are handled that well, for every user. The common questions which need to be factored into any such choice are:

What do I as the end user want?
What performance can each network deliver.
How important is the transfer of content at that time and
How much am I willing to pay for it (how many MB of my data plan am I willing to use?).

This is one of the challenges that we cannot easily solve today, but technology is being developed to help in that process. The operators and device vendors are working within standardisation to develop technology which can provide such a converged service. However at this time there is still a rules mechanism behind it all which does not really describe how user input and preference is going to be captured.

In the last 10 years I have witnessed many battles within service providers when deciding what "one size fits all" service to offer everyone when deciding how to make service provider Wi-Fi available to their customers; all fuelled by my points above.

A lot of concepts are well designed and somewhat mature but deciding exactly what will be implemented in standards is currently ongoing.

In the following slides and video I introduce this whole concept of Network Convergence. The following content introduces the concept and then takes a detailed look at the ATSSS; technology being defined in 3GPP. I also have highlighted the technologoies you can get hold of today to try out network convergence.

I encourage you all to download the example technologies and try convergence for yourself. I'm eager to hear opinions of what technologies work best for each of you. And better still, what is not being provided which you think should be...

Looking forward to your feedback and answering your questions...

Network Convergence of Mobile, Broadband and Wi-Fi from 3G4G

Ben Toner
Founder and Director, Numerous Networks

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Wednesday, 27 November 2019

Private 4G / 5G Cellular Networks and Bring Your Own Spectrum

With 4G maturing, private cellular networks are finally getting the attention that they deserve and has been promised for quite a while. In a Industry Analyst event, Nokia announced that they are running 120+ private networks including transportation, Energy, Public sector, Smart cities, manufacturing and logistics, etc. (tweet below). The Enterprise Business division is now accounting for 5% of the revenue.

Interesting to see that Nokia is running 120+ private networks including:
* 24 in transportation
* 35 in Energy
* 32 in public sector and smart cities
* 11 in manufacturing and logistics#CWTechTrain
Nice tweet 👇 Details 👉 https://t.co/pU9gnuh5Wp https://t.co/FHGM1e2JxK
— Zahid Ghadialy (@zahidtg) November 19, 2019

Ray Le Maistre, Editor-in-Chief at Light Reading, in an opinion on Telecoms.com pointed out:

One of the more immediate revenue stream opportunities right now is wireless private networks, and the good news is that this opportunity doesn’t require 5G. Instead, the potential looks set to be enhanced by the availability of a full set of 5G standards (including the yet-to-be concluded core network specs) and the maturity of associated technology.

In the meantime, 4G/LTE has already been the cellular foundation for an increasingly thriving wireless private networks sector that, according to ABI Research, will be worth $16.3 billion by 2025

Another market sizing prediction, this time by SNS Telecom & IT, pitches annual spending on private 4G and 5G networks at $4.7 billion by the end of 2020 and almost $8 billion by 2023.

However this plays out, there’s clear anticipation of growing investment. What’s particularly interesting, though, is which organizations might pocket that investment. That’s because enterprises and/or organizations looking to benefit from having a private wireless network have a number of options once they decide to move ahead with a private network – here are three permutations that look most likely to me:

Build and run it themselves – technology vendors get some sales in this instance
Outsource the network planning, construction and possibly even the day-to-day. management of the network to a systems integrator (SI) – the SI and some vendors get the spoils. It’s possible here, of course, that the SI could be a technology vendor.
Outsource to a mobile network operator – the operator and some vendors will get some greenbacks.

For sure there will be other permutations, but it shows how many different parts of the ecosystem have some skin in the game, which is what makes this sector so interesting.

What’s also interesting, of course, is what the enterprises do with their private networks: Does it enhance operations? Help reduce costs? Create new business opportunities? All of the above?

Let’s not forget the role of the regulators in all of this. In the US the private wireless sector has been given a shot in the arm by the availability of CBRS (Citizens Broadband Radio Service) shared spectrum in the currently unlicensed 3.5 GHz band: This has given rise to numerous trials and deployments in locations such as sports stadiums, Times Square and even prisons.

In Germany, the regulator has set aside 100MHz of 5G spectrum for private, industrial networks has caused a storm and even led to accusations from the mobile operators that the move ramped up the cost of licenses in the spectrum auction held earlier this year.

In the UK, Ofcom is making spectrum available in four bands:

the 1800 MHz and 2300 MHz shared spectrum bands, which are currently used for mobile services;
the 3.8-4.2 GHz band, which supports 5G services, and
the 26 GHz band, which has also been identified as one of the main bands for 5G in the future.

Slide shared by Mansoor Hanif, CTO, Ofcom at TIP Summit 2019

The process to enable companies and organizations (Ofcom has identified manufacturers, business parks, holiday/theme parks and farms as potential users) in the UK to apply for spectrum will go live before the end of this year, with Ofcom believing that thousands of private networks could be up and running in the coming years.

Dean Bubley from Disruptive Analysis recently spoke about BYOSpectrum – Why private cellular is a game-changer at TAD Summit. The talk is embedded below and is definitely worth listening:

TelecomPaper reported:

The German Federal Ministry for Economic Affairs and Energy said that companies can start to apply to use 5G frequencies in the 3.7-3.8 GHz range on industrial campuses. Local frequencies enable firms to build their own private networks, rather than rely on telecommunications providers to build networks.

The Automotive Industry Association (VDA) and other industry associations including the VCI, VDMA and ZVEI have welcomed the allocation of frequencies for industrial campuses. According to VDA, several dozen companies have already registered their interest in such frequencies with the Federal Network Agency.

The firms believe that 5G can replace existing networks, including WLAN, provide improved coverage of entire company premises, enable full control over company data and reduce disruption to public mobile networks.

The spectrum licences will be allocated based on the applicant's geographic footprint and use of a certain area. Prices also take account the area covered by the network, as well as the amount of bandwidth used and duration of the licence.

The formula for the prices is very interesting as shown in the tweet below

License fee formula for local frequency use published by the German regulator yesterday

Formula: fee = 1000 + B * t * 5 * (6a1 + a2)

* 1000: base fee in €
* B: bandwidth in MHz (10-100MHz)
* t: allocation time in years
* a: area in km²https://t.co/NuAPyMAYuQ #5G #IIoT @UK_5G pic.twitter.com/a4ullfkKze
— Sylvia Lu (@SylviaLuUk) November 1, 2019

Unsurprising to see @BoschGlobal apply for local 3.7GHz spectrum for #Private5G #5G networks at key site in Germany

Initially it's for "the lead plant for Industry 4.0 in Stuttgart-Feuerbach and the research campus in Renningen"https://t.co/1OJ3tksse3
— Dean Bubley (@disruptivedean) November 22, 2019

In Japan, NTT Docomo is working in co-operation with industry partners to help them to create their own private 5G networks. More announcements on this are expected at MWC next year.

Finally, I am running an Introduction to Private 4G /5G Networks Workshop with Dean Bubley on 04 Feb 2020. If this is an area of interest, consider attending it.

I will be running an "Introduction to Private 4G & 5G Networks" workshop / training along with Dean Bubley (@disruptivedean) on 04 Feb 2020 in London. Agenda and other details here: https://t.co/x6xsCpd946 @cambwireless @UK_5G #CWTechTrain pic.twitter.com/iwYzXyZzua
— Zahid Ghadialy (@zahidtg) November 18, 2019

Related Posts:

Non-public networks (NPN) - Private Networks by another name

Friday, 22 November 2019

5G Call Drops in EN-DC: A Thread for Service Quality?

As explained in the post about EN-DC setup the addition of 5G NR radio resources to an ongoing LTE connection provides additional bandwidth for user plane data transmission. And it seems to be fair to say that at least in social media today 5G speed test results, especially throughput measurements, are treated as the benchmark for EN-DC service performance. Hence, it is also logical that a loss of the physical 5G radio link (5G drop) could have a serious impact on user experience.

I write "could", because as a matter of fact many 5G drops will not be recognized by subscribers using non-realtime services including HTTP streaming.

Due to the dual connectivity of LTE Master eNodeB (MeNB) and Secondary gNodeB (SgNB) the signaling trigger points indicating a 5G drop are also a bit more complex compared to what we know from LTE. Indeed, both network nodes are able to release 5G radio resources abnormally using three different X2AP message flow scenarios as shown in figure 1.

Figure 1: Three Basic Signaling Flows for Abnormal Release of 5G Radio Resources

Which of these individual message flows will be found in the trace data depends on which of the two base stations is the first one that detects a problem on the 5G radio link.

A particular case that is seen quite often in live networks is illustrated in figure 2.

Figure 2: 5G Drop due to SGC Failure in UE

Here the trigger is a LTE RRC SCG Failure Information NR message sent by the UE to the MeNB. Thus, the MeNB requests the release of 5G radio resources, which is acknowledged and executed by the SgNB.

In addition (not show in the figures) also the GTP/IP-Tunnel for user plane transport between S-GW and gNB is released by the MeNB after successful completion of the X2AP SgNB Release procedure.

For the UE the 5G drop is not as serious as a drop of the LTE radio connection would be. It is just a fallback on plain LTE, so to say. And after the switching the GTP/IP-Tunnel back to a downlink endpoint at the eNB 4G payload transmission continues.

The longer the overall duration of the radio connection the higher is the risk that the 5G radio resources are lost during an EN-DC call. One of my favorite cases is a subscriber with a radio connection that last a bit more than two and a half hours - see figure 3.

Figure 3: Location Session Record of a Single Subscriber indicating a total number 340 SgNB Drops over 2:33 Hours

Thanks to the smart algorithms of NETSCOUT's TrueCall geolocation engine there is high confidence that she or he sits in an indoor environment, but is served by an outdoor 5G cell. Thus, the penetration loss of the 5G signal is significant. Due to the higher frequency the path loss has also higher impact on the 5G than on the 4G radio signal. This seems to be the main reason why the 5G radio link drops as often as 340 times, which leads to an overall 5G (SgNB) Drop Rate of 83% for this connection.

However, the impact on the subscriber experience might not be a serious one as a different KPI, the 5G EN-DC Duration Rate indicates. According to the Duration Rate 99.99% of all the time 5G radio resources have been available for the subscriber. This is possible, because as also shown in figure 2 within a relatively short time new 5G radio resources are allocated again to this connection. Even if the subscriber is watching e.g. a Netflix video the buffering of already downloaded data on the end user device should be sufficient to conceal the short interruption of the data transfer over 5G resources.

With rising amount of EN-DC traffic it might be rather problematic for the network to handle the additional signaling load originating from the frequent 5G additions and releases. In extreme cases this may even lead to congestion due to CPU overload in RAN nodes or virtual network functions.

For realtime services like Voice over New Radio (VoNR) the entire situation changes. Here even short interruptions of the user plane radio transmission can be perceived by subscribers so that the above discussed 5G Duration Rate KPI will become insufficient to estimate the service quality. Hence, this will drive the demand for a fully integrated view of 5G RAN and Core KPIs covering both, signaling and application quality.

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