What is RSSI

RSSI (Received Signal Strength Indicator)

What does the value say and what is it important for?

The RSSI value is, so to speak, a pure overview measurement. When determining this value, the entire power that falls into the frequency channel used is measured and displayed without any evaluation. There is also no differentiation according to the own useful signal or the external service. A selection of the signals is also not provided. Due to this very simple determination, all interference signals that also fall into the frequency channel used are of course also measured. For example, the transmission signals from another LTE base station from the same provider or adjacent channel interference from radio-based alarm systems and / or video transmission systems.

Definition according to 3GPP:

The RSSI is the broadband power received in the frequency channel, including thermal noise and noise generated in the receiver itself. The power components from all other sources, for example nearby neighboring cells and adjacent channel interference, also flow into the measurement.

The measurement is made within the bandwidth defined by the receiver pulse shaping filter. The reference point for the measurement must be the antenna connection of the UE (User E.quipment). E.g. your LTE router. If the receiver of the UE works in diversity mode, the determined value must not be lower than the corresponding RSSI values ​​of the individual reception branches.

The picture shows the frequency-power diagram of an LTE transmission. The RSSI value measured by the UE is the sum of the areas 1 ... 3. The area up to marking 1 (gray) is the so-called noise floor. For physical reasons, this is always available, even your own terminal device and the base station used generate a portion of it. By using high quality equipment, it can be minimized somewhat, but never completely eliminated. Whereby the effort is getting bigger and the benefit is getting smaller (decreasing marginal utility).

The critical part is the area between marks 1 and 2 (red). This share of performance can vary greatly as it can come from different sources. Both a neighboring eNodeB and interferers from your own environment can make themselves felt here. If this area becomes larger, however, area 3 (light blue) is not shifted upwards, but narrower.

Which brings us to the real problem: The area from 0 to marker 3 is the power share of its own subcarriers, i.e. the actual useful signal. If the proportion of interferers now increases, the power proportion of the subcarriers does not increase, but rather the interferers superimpose their own useful signal. This reduces the signal-to-noise ratio. It means nothing more if the light blue area becomes narrower. Then the eNodeB is forced to assign a different modulation to the individual subcarriers (or resource units). Instead of 64QAM (6 bits per carrier), data transmission would then, for example (depending on the deterioration), only take place with 16 QAM (4 bits per carrier) or QPSK (2 bits per carrier). This inevitably results in a significant reduction in the maximum possible data rate.

However, the measure also leads to a significantly more stable transmission without packet losses and transmission errors, since the less modulated signals still function reliably even with a lower signal-to-noise ratio. The base station operates according to the motto: It's better to have slow data transmission than none at all.

More on the subject:

»Advisor" Improve reception with LTE "
»Significance of the RSRQ
»Importance of the RSRP

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