If the subcarrier spacing (SCS) of a low-frequency cell Is 30 kHz and the bandwidth of each RB Is 360 kHz, theoretically, what value should the noise (dBm) over the air interface be?

The theoretical noise power is calculated based on thermal noise (kTB) and the receiver's Noise Figure (NF). The thermal noise power is given by -174 dBm/Hz at room temperature. The bandwidth of one Resource Block (RB) with 12 subcarriers and 30 kHz Subcarrier Spacing (SCS) is 12 30 kHz = 360 kHz. Thermal Noise = -174 dBm/Hz + 10log10(360,000 Hz) ≈ -118.4 dBm. A typical gNodeB receiver has a Noise Figure of approximately 2-3 dB. Total Noise = Thermal Noise + NF ≈ -118.4 dBm + 2.5 dB = -115.9 dBm. This value is closest to -116 dBm.

A. -105 dBm: This noise level is too high for the specified bandwidth and would imply a very poor Noise Figure or significant interference.
C. -120 dBm: This value is below the calculated thermal noise floor of -118.4 dBm, which is physically impossible for a passive system at room temperature.
D. -97 dBm: This represents an extremely high noise level, indicative of strong interference, not the theoretical noise floor of the receiver.

1. 3GPP TS 38.104, "NR; Base Station (BS) radio transmission and reception," V17.5.0 (2022-06), Section 7.3.1, "Reference sensitivity level." This standard provides the basis for receiver sensitivity calculations, which depend on the noise floor.

2. Rappaport, T. S. (2002). Wireless Communications: Principles and Practice. Prentice Hall. Chapter 5, Section 5.8, "Link Budget Design," provides the fundamental formulas for calculating thermal noise (kTB) and its conversion to dBm.