Need clarification for the sub carrier spacing of 15KHz and 3.75KHz in NB-IoT mode

Question

Regarding nRF9160 modem 
 Average current consumption, radio resource control (RRC) mode, Cat-NB1 : page no 67 in nRF9160 product specification v2.1 
 Both sub carrier frequencies(15 kHz and 3.75 kHz) have different average current consumption, These two sub carrier frequencies are user configurable or it is automatically configure according to the availability of network ? please confirm. 
 Thanks, 
 Paresh Patel

Michal · Answer

Hello again, 
 Here is the information that I got: 
 Subcarrier spacing is determined by the network. Current consumption differs as the uplink transmission time and available data rate depend on subcarrier configuration. Here are a couple of helpful links/references: 
 https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_application/application_notes/1ma266/1MA266_0e_NB_IoT.pdf 
 
 3.3.1 Slot Structure 
 In the UL, Single Carrier Frequency Division Multiple Access (SC-FDMA) is applied, either with a 3.75 kHz or 15 kHz subcarrier spacing. eNB decides which one to use 
 
 3GPP TS 36.201 V13.3.0 (2017-03) 
 
 4.2.1 Multiple access 
 For Narrowband Internet of Things (NB-IoT) operation, a UE operates […] in the uplink using a single sub-carrier with a sub-carrier bandwidth of either 3.75kHz or 15kHz or alternatively 3, 6 or 12 sub-carriers with a sub-carrier bandwidth of 15kHz . 
 
 3GPP TS 36.211 V13.13.0 (2019-12) 
 
 10.1.2.3 Resource unit

3GPP TS 36.213 V13.16.0 (2020-03) 
 
 16.5.1.1 Resource allocation 
 The subcarrier spacing [Delta-f] of NPUSCH transmission is determined by the uplink subcarrier spacing field in the Narrowband Random Access Response Grant according to subclause 16.3.3. 
 16.3.3 Narrowband random access response grant 
 The higher layers indicate the Nr-bit UL Grant to the physical layer, as defined in 3GPP TS 36.321 [8]. This is referred to as the Narrowband Random Access Response Grant in the physical layer. 
 
 Best regards, 
 Michal