Lecture Series in Mobile Telecommunications and Networks (1583KB)

1024, we transmit 10 bits, so to speak: we don’t transmit the joke, but we just transmit the number of the joke: we
receive a notice of the joke and then we can decode it. Then we have the gain factor and the filter coefficients.
We update the filter 50 times per second, and we update the excitation codebook 200 times per second. That is going
on in the mobile.
Adaptive multi-rate narrowband codec (AMR-NB)
Let’s discuss the idea of adaptive multirate coding. There are two modes, the full-rate mode, and there is the half-rate
mode. In the full-rate mode we have 22.8 kbit/s, in the full-rate channel and, for the AMR codec, we have a dynamic
allocation of the bit rates to speech coding and error protection, depending on the instantaneous quality of the
channel. Every 40 milliseconds, we can change the code and, if the channel gets bad, we should add more error
protection while, if the channel is clear, we need less error protection.
Let’s study the clear channel. We take the best version of the codec that we have, the 12.2 kbit/s version, the good
channel, and we need some error protection. If a mobile is inside a building, the channel gets worse and we have
additional attenuation – perhaps 20 dB attenuation, so we have a lot of errors. When we have a weak received signal,
we add at the transmit site a lot of error protection and we reduce the bit-rate for speech coding. That changes
dynamically.
The objective of the AMR narrowband codec is to increase the robustness – I am saying here at the expense of the
speech bit rate. What does that mean? If the bit rate is 12.2, then the speech quality at the transmitter is the best we
can have: if the bit rate is only 4.75, however, then you really recognise some coding artefacts. In the end, the user is not
interested in the quality at the transmitter but he would like to have the best possible quality at the receiving end.
If you have a lot of noise on the channel and bit errors, then it is better to reduce the basic quality that is the bit-rate for
speech coding, and to increase the bit-rate for error protection.
This is presently being introduced in the network and it is a very good solution because it is much cheaper for the
network operator than, let me say, improving the radio coverage inside buildings, because you would need additional
base stations and so on. That improves the quality in terms of the basic quality and robustness against error you can
still use the phone in places where you could not use it before, but the bandwidth is still 3.4 kHz.
Audio example 1: AMR narrowband and wideband
There is the wideband version which we have had since 2000 – roughly eight years. There is the standard and the AMR
wideband codec has nine different modes between 6.6 and 24 kbits/s, and there is one mode at 12.5 kbits/s which we
can use in GSM. Everywhere in the network, where we have a grey area, we have GSM coverage and we could use
wideband coding with more or less the same bit rate as with AMR narrowband coding.
Let’s listen to the narrowband quality and the wideband quality. [Examples of recorded speech played] I think you
recognised the difference: the intelligibility is much better than in the narrowband sound case, but it is the codec which
was designed for speech.
Audio example 2: AMR codecs music and speech
Now let’s listen if we apply music to the AMR narrowband codec, and then apply music to the AMR wideband coding –
but we carefully check the bit-rate, and we now allowed the bit-rate to be somewhat higher. We could not transport
that quality within GSM because in GSM the bit-rate should be at 12.65 but, if you have a UMTS network, we are allowed
to increase the bit-rate a little. Let’s listen to some music. [Examples of recordings of music played] The 7 kHz is better,
but it is not good, so we can do more with super-wideband.
There is a version called AMR wideband-plus, with a bandwidth of 14 kHz and, interestingly, there is a mode with almost
the same bit-rate, that is the progress that we have made in coding technology which means that we have to increase
complexity, but we can also increase the quality. Let’s listen once more to the music. [Music played] That seems to be
better but the phone is made for speech communication and the question is, why should we increase to AMR
wideband plus for speech?
Let’s listen once more to a different speech sample, in narrowband, wideband and super-wideband. [Examples of
recorded speech played] Amazingly, even for speech, super-wideband gives some clear improvement.
50 The Royal Academy of Engineering