What is multi-user MIMO (MU-MIMO) in 5G?

My friend: Hi Soumya, I have a question for you.

Me: Go ahead, please.

My friend: During the trail of MU-MIMO currently running at one site, we didn’t notice an increase of user peak throughput, why?

Me: Actually, we aren't expecting increase of peak user throughput rather than increase of cell throughput which already was higher as simply MU-MIMO means assigning same resource blocks to different users, so spectral efficiency increases and cell capacity increases also. For example, spectral efficiency can be doubled if same resource blocks are allocated to 2 UEs and spectral efficiency can be 4 times if same resource blocks are allocated by 4 UEs and so on.

My friend: But, how can we allocate same resource blocks to different users? They will interfere each other, right?

Me: Yes, you are right, that’s why those users should be distributed across the cell area and thanks to beamforming, so each user should be served by separate beam and so beamforming can generate significant performance gains from MU-MIMO because the highly directional beams are able to increase the spatial separation between UEs.

My friend: Thanks a lot.

Me: You are most welcomed.

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What is massive MIMO (mMIMO) in 5G?

My friend: Hi Soumya, I have a question for you.

Me: Go ahead.

My friend: What is meant exactly by mMIMO?

Me: Actually, mMIMO is a MIMO system with high number of antennas (dozens or more than 100 elements) at BTS. So, simply the basic principle behind mMIMO is to get all the benefits of conventional MIMO, but on a much greater scale.

My friend: So, is it a new technology used in 5G?

Me: No, it is already available before in LTE. But for 5G, it is an essential key to achieve the target performance.

My friend: What are the benefits of mMIMO?

Me: Many benefits, 1st one is improved coverage as using many antenna elements can allow for efficient beamforming so, users can expect high data rates everywhere, even we call it 3D beamforming as beams are controlled horizontally and vertically and so adjusts the coverage to suit user location, even in locations that have relatively weak coverage. 2nd one is higher capacity by serving more users with MU-MIMO which we already discussed (please check the below link). And due to the above two reasons, user experience is enhanced.

My friend: Thanks a lot.

Me: You are most welcomed.

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ARQ vs HARQ in 5G?

My friend: Hi Soumya, I have a question.

Me: Go ahead.

My friend: What is the difference between ARQ and HARQ in 5G?

Me: It is the same as in 4G. Automatic Repeat Request (ARQ) is a re-transmission protocol in which the receiver checks for errors within the received data and if an error is detected then the receiver discards the data and requests a re-transmission from the sender. ARQ is implemented in RLC layer and is slower than HARQ.

My Friend: So, what about HARQ?

Me: Hybrid ARQ (HARQ) is a re-transmission protocol in which the receiver checks for errors in the received data and if an error is detected then the receiver buffers the data and requests a re-transmission from the sender. A HARQ receiver is then able to combine the buffered data with the re-transmitted data for better decision. It is a combination of Forward Error Correction (FEC) and ARQ. HARQ is controlled by MAC.

My Friend: But, why do we have two protocols with same function?

Me: As HARQ depends on one bit for ACK/NACK, some errors can happen, so ARQ is fixing these errors to ensure more reliability in our system, actually they are complementing each other.

My friend: Thanks a lot.

Me: You are welcome.

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How does the UE know the SCS of the SSB before cell search in 5G?

My friend: Hi Soumya, I have a question.

Me: Go ahead.

My friend: I was searching for the parameter that tells the UE the SCS of the SSB before cell search, so UE can use it in cell search but I didn’t find it, do you know it?

Me: Actually, you’ll never find it.

My Friend: Why do you say that?

Me: Because, simply there is no parameter to tell the UE what is the SCS of the SSB to use it in cell search.

My friend: But, how will the UE know the SCS before cell search and before reading the MIB?

Me: As per 3GPP, the SCS for SSB is already defined either in FR1 which is 15/30 KHz and for FR2 which is 120/240 KHz. Some bands are using only one choice like n1, n2 and n3 which is 15 KHz, while others like n5 is using both choices which will make cell search process takes longer time as UE will try both choices till it finds the right one but will allow operators to have greater flexibility. This is for SA while for NSA, simply it is sent to UE via eNB during EN-DC establishment. So, there is no parameter as SCS is already well-known for all UEs as it is standardized form the 3GPP.

My friend: Thanks a lot.

Me: You are welcome.

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SDAP in 5G

What is SDAP in 5G?

My friend: Hi Soumya, I have a question.

Me: Go ahead.

My friend: What is SDAP in 5G?

Me: It is the highest layer in the protocol stack of 5G user plane located at both gNB and UE. It is a newly introduced protocol in 5G that was not available before in 4G.

My friend: So, what is the function of SDAP?

Me: The primary task of the SDAP protocol is to map each QoS flow into a specific Data Radio Bearer (DRB). Multiple QoS flows can be mapped into a single DRB or a single QoS flow can be mapped into a single DRB based on the QoS needed for each QoS flow. So, assuming we are in the DL direction, the SDAP layer at the gNB side will map all QoS flows coming from UPF to DRBs and then these DRBs are transmitted via air interface to UE. While if we are in the UL direction, so the SDAP layer at the UE side will map all the QoS flows coming from the higher layers of the UE into DRBs to be transmitted at the air interface towards the gNB. Also, SDAP layer adds header info to the DL data packets when reflective QoS is enabled to let the UE use the DL QoS in UL as well.

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Me: You are welcome.

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What is meant by Access‐agnostic in 5G?

My friend: Hi Soumya, I have a question for you.

Me: Please, go ahead.

My friend: While reading about the 5G core features, I found that 5G core is Access‐agnostic, so what does it mean?

Me: It means your device can be connected to the 5G core network via different access networks whether it is 3GPP access (LTE and 5G) or non-3GPP access (WIFI and fixed landlines) or even both of them together based on the decision from the 5G core network according to the service type you are using, that’s why one of the main goals of 5G core network is to be access-agnostic and so you can access the 5G network regardless of the access method you are using with unified registration, authentication, session, mobility and policy management for all access types.

My friend: So, what is the gain here?

Me: Simply, increase of data rate if you are sending on the two access networks simultaneously, also seamless mobility will be achieved.

My friend: Thanks a lot. You made it clear.

Me: You are most welcomed.

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What is the difference between network slicing and QoS in 5G?

My friend: Hi Soumya, I have a question for you.

Me: Please, go ahead.

My friend: I read about network slicing and feel it is the same concept as QoS if it is End-to-End, is there any difference between both?

Me: Actually, network slicing provides a complete end-to-end virtual network for a given source. No existing QoS-based solution can offer anything like that.

My friend: Can you give me some examples?

Me: Of course, for example QoS can discriminate VoIP traffic from other types of traffic such as HD video and web browsing. However, QoS cannot discriminate and differentially treat the same type of traffic (e.g. VoIP traffic) coming from different sources, while if same type of traffic is assigned to different slices, it can be discriminated and treated differently. Also, QoS does not have the ability to perform traffic isolation at all. For example, IoT traffic from a health monitoring network (e.g. connecting hospitals and outpatients) typically has strict privacy and security requirements which can be achieved if it is assigned to a specific slice.

My friend: Thanks a lot. You made it clear.

Me: You are most welcomed.

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V2X in 5G

What is the V2X in 5G?

My friend: Hi Soumya, I have a question for you.

Me: Please, go ahead.

My friend: What is V2X?

Me: V2X means vehicle to everything and as per 3GPP, it has 4 types in 5G which are Vehicle to Vehicle (V2V), Vehicle to Network (V2N), Vehicle to Infrastructure (V2I) and Vehicle to Pedestrian (V2P).

My friend: Can you give me some examples?

Me: Of course, V2V means the transfer of data directly between vehicles, so vehicles can exchange information to avoid collision. V2I involves the transfer of data between vehicles and Road Side Units (RSU) and RSU are application servers positioned along the road with built-in Base Station functionality so they can communicate directly with passing vehicles. The RSU is aware of the timing used by nearby traffic signals and this timing is provided to vehicles approaching the signals so they are able to react appropriately. V2N involves the transfer of data between vehicles and the mobile network, e.g. providing Internet connectivity. V2P involves the transfer of data between vehicles and pedestrians for example for pedestrians to provide warnings to drivers.

My friend: Thanks a lot. You made it clear.

Me: You are most welcomed.

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