DC Carrier in NR

-
Home               LTE              NB-IoT             5G(NR-NSA)

 In this article, we will learn the basics of DC Carrier and why it is needed.

Why in NR, DC subcarrier is not reserved like LTE?

With legacy LTE (4G) it was decided that all devices (UE) should be capable of the maximum carrier bandwidth of 20MHz (UE can also support 10/15/5/3 MHz).

This was a reasonable assumption at the time given the relatively modest bandwidth compared to NR, Whereas NR is designed to support very wide bandwidths up to 400MHz (for FR2) and 100Mhz (for FR1) for a single carrier. As well as NR supports multiple numerologies on the same transmitted carrier frequency. It is not necessary that all devices can support up to 400 MHz Bandwidth, hence in NR BWP concept is introduced. (To learn more about BWP Click here).

Note: in the above and below NR case figure, we assume that for each and every BWP have configured some txDirectCurrentLocation value.

    

 As shown in the above figure DC subcarrier exists in NR but it is not reserved unlike LTE, and that DC subcarrier can also be used for DATA transmission.

 With the first release of mandating all devices to handle such wide carriers is not reasonable from a cost perspective. Thus, an NR device may see only a part of the carrier (BWP) and, for efficient utilization of the carrier, the part of the carrier received by the device may not be centered on the carrier frequency.



The handling of the "DC subcarrier" is different in LTE as compared to NR. In LTE, a DC subcarrier was reserved and not used for any data transmission on the DL, also a 7.5 kHz shift was introduced on the UL to avoid having DC completely aligned with a single subcarrier.

In LTE, the DC subcarrier is not used as it may be subject to high interference due to for instance local oscillator leakage. Since all LTE UEs can receives the full carrier bandwidth (UE can also support 10/15/5/3 MHz) and are centered on the carrier frequency. this was straight forward.

For NR (NSA and SA Case) the DL and UL are symmetric with no explicit DC subcarrier reserved. The transmit DC subcarrier is modulated (at either the gNB or UTE), and not rate-matched or punctured. The receiver may know if the transmit DC subcarrier is present or not within the receiver BW (As per Configure BWP), and if present the location. At the receive side, no special handling of DC subcarrier is specified in RAN l.

NR UEs (devices), on the other hand may not be centered on the carrier frequency and each NR device may have its DC located at different locations in the carrier bandwidth (As per Configure BWP), unlike LTE where all the devices typically have the DC coinciding with the center of the carrier. Thus, having special handling of the DC subcarrier would be inconvenient in NR and instead it was decided to exploit also the DC subcarrier for data, accepting that the quality of this subcarrier may deteriorate in some situations.

4G uses a single frequency raster for the channel bandwidth, the Synchronization Signals and the PBCH. This is possible because the Synchronization Signals and PBCH arc always positioned at the center of the channel bandwidth. There is a one-to-one relationship between the position of the channel bandwidth and the position of the Synchronization Signals and PBCH.

In the case of NR, separate frequency rasters have been specified for the channel bandwidth and the Synchronization Signals or Physical Broadcast Channel (SS/PBCH) Block. The channel raster has relatively high resolution to allow flexibility when configuring the position of the channel within the allocated spectrum. The SS/PBCH Block raster known as the synchronization raster has relatively low resolution to reduce the number of positions which a UE must check when completing a band scan, i.e. the band scan procedure becomes faster and more efficient.

One reason for a difference in handling of transmitter and receiver side DC in NR than from LTE is that there may be different minimum bandwidths (As network can configure different common/dedicated BWP to respective devices)  for different UE, with UE receive bandwidth not necessarily aligned in the center of the gNB carrier bandwidth. One important issue with NR is coexistence with LTE.

In case NR and LTE are both deployed in the same band (DSS Case) it is desirable to align the UL subcarriers to minimize the inter-subcarrier interference. Therefore NR (5G) has introduced the possibility to shift the uplink raster by 7.5 kHz. In this case, the uplink NR subcarriers and LTE will be aligned. 
Stay tune for more Update…………………

Pinal Dobariya…  

References:
TS 38.211, clause 4.4.2

TS 38.331


Comments

  1. Anonymous7 June 2020 at 23:44

    Thanks for this very informative article. Can I assume that one can still recover data even if part of the data is transmitted via the deteriorated DC sub-carrier? If so, is it due to the error correction nature of channel coding?

    ReplyDelete
  2. Unknown22 September 2020 at 12:24

    Thanks. Great Article.

    ReplyDelete

Post a Comment

Popular posts from this blog

5G Deployment Option-3/3a/3x

-
Image
Home                 LTE                NB-IoT            5G(NR-NSA)   Hi Guys today we are learning about 5G deployment options, if you want to know more about other 5G deployment option please  click here Option 3: EN-DC (EUTRA-NR Dual Connectivity) Option 3 represents a network having both LTE and NR radio access, but using only the EPC core of LTE to route the Control signals. In this option, LTE is used as the control plane anchor for NR, and both LTE and NR are used for user data traffic.(user plane). It could also be called as Non-Standalone (NSA) NR in Evolved Packet System. The only difference between Options 3 and 3A is based on whether the user plane data is sent to NR directly or via the LTE RAN. This standardization was concluded in 3GPP Rel-15. This is a possible first step in the migration towards 5G-SA. This option will be most attractive for operators which have early deployments of 5G NR access systems in areas where legacy eNB and EPC a
Read more

5G Deployment Option-7/7a/7x

-
Image
Home                 LTE                NB-IoT            5G(NR-NSA) Hi Guys today we are learning about 5G deployment options, if you want to know more about other 5G deployment option please  click here Option 7: Deployment Option 7 represents a deployment scenario in which the Next Generation Core (NGC) will be used with a mixture of LTE and NR radio.  Next Generation signaling will be used but it will be routed via the LTE RAN because the LTE network is more fully built out and thus more reliable for handling signaling.  Options 7 and 7a differ on whether the user plane data is sent to NR directly or via the LTE RAN. 7/7A/7X require an eLTE upgrade of the existing LTE network at the start. (Difference is UP path Non Standalone ( Dual Connectivity 4G/5G ) Option 7/7A: NSA NR in 5GS. This option is particularly attractive for areas in which legacy LTE eNBs and the EPCs are ready to be upgraded to ngeNBs and the 5GCs.  From the point of
Read more

Soft Handover vs Hard Handover

-
Home                 LTE                NB-IoT            5G(NR-NSA) Now before we move to other things like measurements in handover, handover decision, handover procedure, handover interruption time, and mobility robustness optimization, let us take a look into difference between soft handover and hard handover. Soft Handover Hard Handover The handover in which radio links are added and removed in a way that MS always keeps at least one radio link to the UTRAN is known as soft handover. The handover in which all the old radio links in MS are removed before the new radio links are established is known as hard handover. This can also be simplified as make before break This can also be simplified as break before make As call drop rate is lower this handover is used to lower the rate of call drop. In this case higher rates of call drops are found Why soft handover is not included
Read more

Authentication in LTE

-
Home                 LTE                NB-IoT            5G(NR-NSA)   What is Authentication and what type of Authentication is used in LTE? Authentication is a process by which both the UE and Network check whether the other party is authorized to communicate with them. In LTE and WCDMA, a mutual authentication process is used i.e. the Network checks whether the UE is authorized to camp on it and the UE also checks whether it is trying to camp on is the correct Network. The UE becomes authorized to camp on a network by subscribing to the network. In LTE, EPS AKA i.e. Authentication Key Agreement procedure is used for Mutual Authentication. Why is Authentication required from both sides in LTE? The UE needs to be authenticated by the NW to check whether it has subscribed to the NW or not. Only UEs which have a valid subscription should be allowed to camp on the NW. The NW should also be authenticated by the UE because if the UE does not check its
Read more

MAC-Contention Resolution Timer

-
Home                 LTE                NB-IoT            5G(NR-NSA)   What is MAC-ContentionResolutionTimer? The timer which states the number of consecutive subframe/ subframe s during which MAC entity will monitor the PDCCH after the Msg3 is transmitted. (3GPP TS 36.321 version 13.3.0 Release 13)  The UE will start mac-Contention resolution timer from the immediate subframe after transmitting Msg3.  As UE is waiting for the contention resolution which is through PDCCH reception on C-RNTI (in connected mode), the UE should be monitoring PDCCH. So that the time duration in which mac-Contention resolution timer is running, is also considered as Active Time. Which timer is started after msg3 is transmitted? Following two timers are started Once,  msg3 is transmitted: 1. T300 : Transmission of RRCconnectionRequest 2. Contention Resolution Timer: broadcasted in SIB2. If the UE doesn't receive msg4 (Contention Resolution message) within
Read more

PRACH Basic-3

-
Image
Home                 LTE                NB-IoT               5G(NR-NSA) In this section we will learn about PRACH Preamble Format and its basics as well as interview questions. Random access with NR supports different PRACH formats and numerologies to cope up with different deployments. The random access sequences themselves are based on the preamble format. Preamble Format: (Qus: How many new preamble format are introduce by 3gpp in NSA or SA case?) In 5G NSA or SA cases, there are 13 types of preamble format for FR1/FR2 and they are known as Format 0/1/2/3, Format A1/A2/A3, Format B1/B2/B3/B4, Format C0/C2. These 13 types of preamble format can be categorized into two categories: • Sequence length of L = 139, with subcarrier spacing of 15, 30, 60, and 120 kHz. These can be solutions for different coverage situations. • Sequence length of L = 839, with subcarrier spacing of 1.25 kHz and 5 kHz. This sequence can be used for very large cells (up to 100 km). (Qu
Read more