LTE-M on nRF9160 for bandwidth 5 MHz

[Carla31zv]

Hello,

I would like to test TX for ltem. I'm using nrf9160.
I would like to use:
- Band: 1
- Frequency: 1950 MHz
- Tx power: 23 dBm
- Bandwidth: 5 MHz
- Modulation: QPSK

I'm using AT command XRFTEST:

AT%%XRFTEST=1,1,1,19500,23,1,0,6,0,0,3,3,0

I have some questions:

1. I've read on nRF9160 documentation that its maximum transmission bandwidth is of 1.4 MHz (https://docs-be.nordicsemi.com/bundle/ps_nrf9160/page/nRF9160_PS_v2.2.pdf?_LANG=enus, page 428, 7.5.1 Key RF parameters for Cat-M1).

So in that case:

=> why on AT command we can choose a system bandwidth of 5 MHz (https://docs.nordicsemi.com/bundle/ref_at_commands/page/REF/at_commands/production_test/xrftest_tx_set.html) ?
=> When I set 5 MHz of bandwidth, at spectrum analyser I get a bandwidth of 1.4 MHz. This is normal ?
=> If maximum transmission bandwidth is of 1.4 MHz, why it supports band 1 ?

I've found this website https://www.sharetechnote.com/html/Handbook_LTE_BL_CE_Narrowband.html which explains the following:

"LTE-M1 System Bandwidth is always 1.4 Mhz, but legacy LTE System BW with which LTE-M1 operate can be wider than 1.4. When LTE-M1 operate in a wider legacy LTE band, it split the wider band into multiples of 1.4 Mhz band slot and use any one of those slots. These 1.4 slot for LTE-M1 is called 'Narrowband'."

So maybe this explains why nRF9160 supports band 1 and bandwidth of 5 MHz.

2. As you can see on my AT command, I've set: RB/tone_count=6, RB/tone_start_position=0 and Narrowband index=3. I've set these values following an example.
My questions are:
=> What does RB/tone_count mean ? It indicates the number of subcarriers ?
=> What does RB/tone_start_position mean ?
=> What is the effect of choosing another narrowband index, for example 0?

Thanks in advance !

[dejans]

Hi,

Documentation specifies maximum transmission bandwidth to be 1.4 MHz.
Apart from what is shown in the %XRFTEST set command, there is no additional information about specific parameters that are used in %XRFTEST command. I will check this internally and get back to you probably during next week.

Best regards,
Dejan

[dejans]

Hi,

why on AT command we can choose a system bandwidth of 5 MHz (https://docs.nordicsemi.com/bundle/ref_at_commands/page/REF/at_commands/production_test/xrftest_tx_set.html) ?

While the maximum system bandwidth for LTE-M is always 1.4 MHz, legacy LTE bands (such as band B1) are designed to operate with wider system bandwidths, such as 5 MHz, 10 MHz, or higher. Within these wider bands, LTE-M devices like the nRF9160 select narrow slices of 1.4 MHz, called "narrowbands," for communication.
This behavior allows LTE-M to coexist within the wider legacy LTE bandwidth while utilizing its narrower 1.4 MHz transmission bandwidth. The %XRFTEST command uses the wider system bandwidth to simulate the legacy LTE environment in which LTE-M operates while restricting the actual transmission to the 1.4 MHz narrowband as per standard LTE-M specifications.

When I set 5 MHz of bandwidth, at spectrum analyser I get a bandwidth of 1.4 MHz. This is normal ?

It is expected to get bandwidth of 1.4 MHz. Even when a wider system bandwidth (e.g., 5 MHz) is selected, the active transmission for LTE-M utilizes only a 1.4 MHz subset of the spectrum, as a single narrowband is used for transmission.

If maximum transmission bandwidth is of 1.4 MHz, why it supports band 1 ?

LTE band B1 (1920–1980 MHz uplink, 2110–2170 MHz downlink) is a defined frequency band used in legacy LTE systems with broader bandwidths. The nRF9160 can operate within this band while complying with LTE-M's narrowband constraints by carving out 1.4 MHz slices within the larger band. The protocol ensures compatibility with legacy LTE infrastructure while adhering to LTE-M standards.

What does RB/tone_count mean ? It indicates the number of subcarriers ?

Resource Block (RB) refers to LTE-M and tones refer to NB-IoT mode. The RB/tone count parameter refers to the number of contiguous resource blocks (RBs) or tones utilized for the transmission. A single LTE(-M) resource block corresponds to 12 subcarriers (each 15 kHz wide) in frequency. In NB-IoT, tones refer to the individual subcarriers within the 180 kHz narrowband.

For instance, setting RB = 6 configures the LTM-M transmission to use the full 1080 kHz (1.4M BW) bandwidth.

What does RB/tone_start_position mean ?

The RB/tone_start_position specifies the starting position of the resource block. In NB-IoT it is a group of tones within the available narrowband and it determines where the set of configured tones begins within the 1.4 MHz operating band. Adjusting this value effectively shifts the transmission within the narrowband frequency window.

What is the effect of choosing another narrowband index, for example 0?

The narrowband index defines which specific 1.4 MHz slice within the wider system bandwidth is used for transmission. Changing this value specifies a different 1.4 MHz narrowband to operate in. 

With Band B1 and a system bandwidth of 5 MHz, the 5 MHz spectrum is divided into several 1.4 MHz narrowband slots. When
narrowband index = 3, the transmission occurs in the 4th narrowband (starting from index 0). Changing this to narrowband index = 0 would shift the transmission to the first 1.4 MHz narrowband slot. This flexibility is particularly useful when testing interference, coexistence, or specific carrier frequency scenarios.

Best regards,
Dejan