Reference signals Part-1

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In this section we will learn about reference signal and its basics as well as interview questions.

(Why UE requires Reference signals?)

Reference signals are needed by the UE in order to search cell, channel estimation and decoding and mobility measurements.
In the release 8 that is first release of LTE, channel knowledge for the downlink transmission direction was solely acquired with the help of device measurements on the so called CRS (cell specific reference signals). In LTE CRS are transmitted over the entire carrier bandwidth within every LTE subframe of length one ms, and also can be assumed to be transmitted over the entire cell area. Therefore, the device accessing an LTE network can assume that CRS are always present and can be measured on. CRS are transmitted four times per millisecond by the LTE base station.

(For How many layers CRS was designed for channel estimation?)

In Rel.8 CRS was designed for used in channel estimation for up to 4 layer spatial multiplexing with separate CRS sequences for each antenna port (0,1,2,3).

(What changes were comes in release 10?)

Afterwards in LTE release 10 the CRS were complemented by another reference signal called CSI- RS. In contradiction to CRS, the LTE CSI-RS are not necessarily transmitted continuously. Rather, an LTE device is explicitly configured to measure on set of CSI-RS and do not make any assumptions with reference to the presence of a CSI-RS unless it is explicitly configured for the device.

(What is CSI-RS?)

It is the channel state information reference signal and is used by DUT to estimate the channel and report channel quality information to the BS(Base Station).

(For How many layers CSI-RS was designed for channel estimation in Rel.10?)

Rel.10 come with need for 8 layers channel estimation however, extending CRS to 8 layers would add more signaling overhead than was desired so CSI-RS was added.

(Is CSI-RS and CRS are transmitted on same antenna ports?)

No, CSI-RS is transmitted on different antenna port (15-20) then CRS antenna port (0-3). And instead of using only time and frequency orthogonality like CRS,CSIRS uses code domain orthogonality as well.
Here the sparse placement of CSIRS means that CQI will be reported over longer time intervals than CRS CQI.

(Do 5g reference signals transmitted all the time as LTE RS?)

Even if there are no connected UEs in the cell, LTE reference signals must be transmitted all the time. Whereas in 5G system design there are no CRSs at all.
Instead, 5G has user specific RS, where the reference signals are transmitted together with data. If no user data transmission takes place, there is no RS transmission either.

Following are the benefits in flexible reference signal transmission:  
1) First benefit is lower base station power consumption. The NR base station can utilize power saving mode during low load cases as there are no need to keep up the frequent reference signal transmission. Here the ultimate target is zero power consumption for zero users.
2)  Secondly, more efficient beam forming. For user specific beam forming CRSs are not of any use, as it requires user specific RSs. As beam forming is an essential part of NR systems, the structure of reference signal needs to be designed accordingly.
3) Third is less interference from reference, thus minimizing inter cell interference and improving network capacity. LTE cell RS make of the total base station power during maximum loading, but the share of RS is significantly higher even of the interference during low load.
The transmission of LTE RS is 20 times in 5 ms period; that is four times every millisecond. NR does not have any CRS. Only broad cast and synchronization channels are transmitted with a typical frequency of 20 Ins. When the subcarrier spacing increases the relative share of NR common becomes lower. Broad cast and synchronization channels uses 4 symbols that is, 1.4% of all symbols in 20ms with 15 kHz and 0.7% of all symbols with 30 kHz.

Difference between LTE reference signal and NR reference signals:

Similar to LTE many types of reference signals are used in NR and most of them plays the similar role as in LTE. However some important differences between LTE reference signals and NR reference signal are –
  • In NR there is no cell specific reference signal (CRS)
  • New reference signals i.e. phase tracking reference signal, PBCH reference signal, Time or frequency tracking reference signal are introduced in NR.
  • DMRS has been introduced in NR for both uplink and downlink channels.
  • In LTE there was constant exchange of reference signals to manage link whereas in NR, reference signals are transmitted only when it is necessary.

In the next section, we will learn about different NR reference signals. So keep reading……. And Stay tune……………
Pinal Dobariya………..

3GPP TS 38.211,
3GPP TS 38.212,
3GPP TS 38.213


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