Turning a +CESQ Index into dBm

Turning a +CESQ Index into dBm

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

Turning a +CESQ Index into dBm

During modem bring-up, the +CESQ command reports signal as an index, not a power. The chapter says its RSRP field runs 0–97 and maps linearly to roughly −141 dBm up to −44 dBm, with 255 reserved to mean unknown. The same read-the-number discipline governs the power-saving timers, whose eDRX maxima reach 43.7 minutes for LTE-M and 2.9 hours for NB-IoT. This audit pins down the exact index-to-dBm conversion the word “linearly” hides.

Companion to the chapter Lab: Cellular Modem Bring-Up — every number here comes from that chapter.

See the relationship before changing it

The figure reads from left to right. The blue card is cesq rsrp index. The middle card applies the page rule. The green card is rsrp. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

Walk the arrows. Each index step is exactly one decibel inside the valid range.

Derive the baseline in four named moves

1. 1

   Name the input. The chapter baseline is 40.

2. 2

   Name the relationship. RSRP = index - 141

3. 3

   Substitute with units. 40 - 141 = -101 dBm

4. 4

   Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change cesq rsrp index

Try Predict the direction of RSRP = index - 141. Test another cesq rsrp index, then compare rsrp.

CESQ RSRP index

40

Chapter baseline

RSRP

Observe Each index step is exactly one decibel inside the valid range. Reset cesq rsrp index to 40 and compare rsrp.

Explain Each index step is exactly one decibel inside the valid range.

Check yourself

What should you do before trusting a moved-control result?

Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.

What does this small model leave out?

Answer: Only cesq rsrp index moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only cesq rsrp index. It is not a field prediction.

TryRecompute The reported range covers -44 - (-141) = 97 dB, spread over the 97 index steps from 0 to 97, so each step is exactly 97 / 97 = 1 dB.

ObserveTrack the linear RSRP rule: index 0 maps to -141 dBm, index 97 maps to -44 dBm, and reserved value 255 remains unknown.

ExplainExplain So a logged index of 40 is not vague - it is precisely 40 - 141 = -101 dBm.

Ready: use the stated baseline inputs, then compare each displayed result.

Ada: The chapter says the +CESQ RSRP index runs 0-97 and maps “linearly to roughly -141 dBm up to -44 dBm.” Linear over an integer index hides an exact conversion - let me pin it down.

• The reported range covers -44 - (-141) = 97 dB, spread over the 97 index steps from 0 to 97, so each step is exactly 97 / 97 = 1 dB.
• That gives the clean rule RSRP(dBm) = index - 141: index 0 is 0 - 141 = -141 dBm (the floor) and index 97 is 97 - 141 = -44 dBm (the ceiling), matching both endpoints. The reserved value 255 still means “unknown.”
• So a logged index of 40 is not vague - it is precisely 40 - 141 = -101 dBm.

The power-saving ceilings show the same “read the number, do not trust the label” habit: the chapter’s eDRX maxima of 43.7 min (LTE-M) and 2.9 h (NB-IoT) differ by (2.9 x 60) / 43.7 = 174 / 43.7 ~= 4x, so NB-IoT can sleep about four times longer between pages. That is the reachability-versus-battery knob the lab must record from the modem’s negotiated timers, not assume from the request.

Every number above is taken from the chapter’s own material and re-derived step by step.

Technical boundaries. This conversion deliberately does not simulate modem calibration, network reselection, fading, or antenna loss. It maps the stated +CESQ index to dBm using the chapter's fixed linear conversion and validity range.