DC Carrier in NR

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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. 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?

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