>>"So, again, what capacity are you referring to?"
Seemed obvious to me Rajala was saying 1) the voice capacity, 2) the erlangs, and 3) the number of voice users per 200 kHz bandwidth are all doubled when Half Rate (HR) is used instead of Full Rate (FR).
>>"The late Claude Shannon captured the entire problem in a single equation relating data rate, bandwidth and noise. Modern bandlimited systems compress the source material before modulation. This leads to an apparent increase in capacity."
Nobody is talking about changing the bandwidth, the power level, the data rate, or anything else that would make Shannon's law applicable to this discussion.
>>"Voice quality is, in general, inversly related to compression ratio. Higher ratios of compression lead to lower voice quality."
True .. everything else being equal. But a higher compression ratio, higher complexity algorithm, can lead to voice quality approximately equal to that if a less complex algorithm.
>>"So, AMR is a new compression scheme .. "
It's been around quite a while. Maybe you will recognize it as ACELP (Algebraic-Code-Excited Linear-Prediction). It is used for the Enhanced Full Rate (EFR) vocoders of GSM, TDMA, and PDC at the data rates of 12.2, 7.4, and 6.7 kbps, respectively.
>>>"Ahh wait, from the documents it appears what will happen is the users will signal to reduce their timeslots."
Is this an indirect reference to TDMA (IS-54/136, GSM, and PDC) HR operation? HR operation has been in the specs from the beginning.
GSM full-rate --> 8 time-slots --> 8 mobiles per 200 kHz
GSM half-rate --> 16 time-slots --> 16 mobiles per 200 kHz
The tradeoff for the increased capacity is, of course, reduced voice quality.
As your post shows, GSM EFR is 12.2 kbps. But do you know that it is "bit exact" to 12.2 kbps AMR. The kbps rate for HR AMR has not been published anywhere that I can find. I just know that it must be one of the 8 rates in the AMR spec, and it must be approx half the GSM EFR rate. (Why didn't they just call this 4th GSM vocoder enhanced half-rate (EHR) .. similar to EFR?) I have seen no evidence to indicate the AMR vocoder rates change during a call.
So can EHR compare to EFR in quality? No way. But it may be comparable to GSM full-rate, 13 kbps RPE-LPC (Regular Pulse Excited Linear Predictive Coder) .. which may be in many of the GSM phones in use today. But there seem to be no Mean Opinion Score (MOS) rankings of the various vocoders. As to whether most phones today are FR or EFR, I will try to find when EFR became common.
>>"plans to introduce Adaptive Multirate Codec technology in its GSM base stations and handsets, which is expected to quadruple GSM voice capacity"
That is a curious quote from www.uwcc.org. Obviously, a change from full-rate to half-rate cannot quadruple voice capacity. Some other improvement is 'in play' here also. Do you know what it might be?
Ron
P.S. Incidentally, I have no idea how, or even if, full-rate and half-rate mobiles can co-exist on the same GSM channel. |
