Anything higher than -70dBm is considered good signal power, but what would be the theoretical “maximum” value that you could achieve/have seen “in the wild” on regular consumer hardware (e.g. smartphones) and an average cell tower, without any data transfer issues? A measurement that would be read out if you were to hold a phone out standing very near an average LTE cell tower? 0 dBm? Probably too high. -30?

  • NetDork@alien.topBEnglish
    1·
    11 months ago

    The signal level is 0 in the radio itself. As soon as you go out a cable to an antenna you have loss. That’s why the value is always negative.

    I’ve seen RSSI values as good as -26 using a good LTE router with a high end outdoor antenna very close to a tower.

    • eNB256@alien.topBEnglish
      1·
      11 months ago

      not necessarily though, if the signal level is 0 and the radio’s transmit power were to be doubled, the signal level would be 3.

      each time the signal is doubled, the value increases by 3, each time the signal is halved, the value decreases by 3

      the signal is significantly weaker after going through the air etc, so it is negative

  • eNB256@alien.topBEnglish
    1·
    11 months ago

    It’s worth noting that the RSSI counts all of the signal in the air at a certain channel and width - think of channel like wifi channel, and think of width like 20MHz/40MHz/80MHz/160MHz in wifi, but with LTE instead.

    This means utilization (load) and useless signal (e.g. interference) are also counted.

    The tower sends ‘test signals’ in a known sequence. The (type of) average strength of one ‘test signal’ is the RSRP.

    So, the RSRP may correlate more with the distance from the tower.

    e.g. let’s take RSRP:-44, at one point the highest value supported in measurement reports.

    There are grids that are 0.180MHz wide that can vary in interference, utilization, etc. For example, the load and interference / RSRQ may be -14. That’s 14 to add to the RSSI. There could be more.

    LTE can normally have up to 100 of these grids. That’s 100 more = 20dBm more, so there’s 20 to add to the RSSI, with the above load and interference on average.

    (-44) + 14 + 20 ≈ -10

    So, for the RSSI to be high,

    • The distance from the tower should indeed not be excessive. The frequency and obstructions also affect the RSRP.
    • The tower should have a high enough referenceSignalPower.
    • The LTE should be 20MHz (actually 18MHz) wide.
    • Sector antennas on towers focus signal in a specific pattern, so, the user equipment should be where the signal is focused at. Sector antennas are normally not aimed straight down so the RSRP might not be the highest while close to the tower. On the other hand, the device might receive more interference from other sector antennas on the tower.
    • The down utilization should be 100%.
    • etc

    However,

    • The maximum value of RSSI that can be displayed is likely device-specific.
    • do not microwave the user equipment or yourself, and do not cause interference aside from what occurs from normal use.

    Overall, the RSSI is weird. -70dBm is not necessarily good, it is ambiguous. Use SNR if available (it’s simple, higher is better) and RSRP.