By now you may have heard about the reception problems with iPhone 4. Apple has mentioned that the real problem has been with the way they mapped signal strength to the famous AT&T 5 bars icon (where 5 bars denotes excellent reception and no bars means very poor reception). Recently I had tried to solve the same problem so here is the mapping that I used. I wonder what Apple is now using.
But first a few words on signal strength and its unit. The cellular signal strength is represented in -dBm: the power ratio in decibels of the measured power referenced to one milliwatt. The useful range is somewhere between -50dBm to -110dBm. The smaller the number the worse the signal. So -50dBm is much better than -110dBm. A 3 dB increase represents doubling the power. I’ve not researched if it is possible to get the the actual signal strength using the iPhone SDK. But I’ve been using the Tetlit cellular modems and there are a couple of AT commands that provide this information:
CQS. The AT command
MONI returns the received signal strength in dBM. Based on testing, I’ve seen MONI returning signal strength anywhere from -55dbM to -110dBm depending on location. At lower dbms, e.g., -109 or above, you can definitely still send SMS but the quality of reception is lower (more bit rate errors) and I’m not sure if you can hold a conversation at these dBm levels.
The AT command
+CQS maps the signal strength into a signal quality number between 0 to 99 as shown in the table below:
|Signal Quality||dbM||Power (milliwatt)|
|0||-113 or less||0.0000000000050119 or less|
|2 to 30||-109 to -53||0.0000000000125893 to 0.0000050118723363 (2 dBm per step)|
|31||-51 or greater||0.0000079432823472 or greater|
|99||not known or not detectable|
AT&T converts the reception signal strength to a number between 0 to 5. The famous bars icons are used to denote this: 5 bars means excellent reception and 0 bars means almost no reception.
According to the CSQ command mapping -113 dBm maps to 0 and -51 is the best you can get. The difference between these two is about 62 dBm and if you divide 62 dBm range by 6 you get 10.3 dBm per step which must be more or less the dbM interval that AT&T uses to map the signal strength to bars. Based on this approach here is the rounded dBm table mapped to a number between 0 to 5.
|-60 or greater||5|
|-61 to -73||4|
|-74 to -85||3|
|-86 to -98||2|
|-99 to -110||1|
|-111 or less||0|
But if you use this it is very rare to get 5 bars, so it is possible that most cell phones will reduce the dBm interval mapping to bars and increase the lower range so that -75dBm or greater maps to 5 bars and end up with something like this:
|-75 or greater||5|
|-83 to -74||4|
|-95 to -82||3|
|-105 to -94||2|
|-111 or less||0|