Lecture Series in Mobile Telecommunications and Networks (1583KB)

From plain old Telephony to flawless mobile audio communication
The introduction of wideband speech in GSM
What is the present situation? I asked two leading operators in Germany last week, with the following result. Vodafone
is presently studying – as are other operators in Europe – the topic of wideband speech but the problem is that the
terminals are not yet commercially available but there are some under test. If these terminals become available in
sufficient quantity, then implementation is an options. The introduction of super-wideband might become an option if
we have UMTS LTE, so that sounds quite optimistic.
The introduction of wideband speech in GSM
The answer from T-Mobile, the second largest operator in Germany, was that they have equipped some parts of the
network with the AMR wideband codec and they plan to offer this service in some parts of the network in the third
quarter of this year, if the terminals become commercially available – and they say that they are waiting for the
terminals. That sounds very optimistic, something might happen there this year.
The compatibility and complexity problem
If we start in some parts of the network – if we start at all – then you can clearly see that we would have to rely first on
GSM, and only to a certain part on UMTS. That requires new terminals on both sides and it requires modification in the
network, we have two worlds – the narrowband world and the wideband world – why should a customer buy a
wideband phone if does not know anyone who has one? That is the same situation as applied with fax machines many
years ago, and the solution is costly. In any case, there will be a very long transition period until – if ever – the whole
telephone network will have been converted into a wideband service. However, there is a chance of competition with
the fixed network, with the voice-over IP phones, if we succeed in implementing that in the wireless world.
In the transition phase: artificial bandwidth extension (BWE) for supporting user acceptance
There is not a solution but a helper in the transition phase,
and that helper is called artificial bandwidth extension (BWE),
which could increase user acceptance if, at the receiving end,
we succeed in artificially increasing the bandwidth. As a
sound engineer, you would say that that is impossible: we
learned from Shannon that, if you have thrown the
frequencies away, you don’t reconstruct them at the receiver –
but it is voice, and the receiver is the ear. You can do a lot of
tricks and there is still a great deal of redundancy in the signal,
such that you can extract from the narrow band signal at the
receiving end some information to produce the higher
frequencies artificially, where the ear is not that sensitive as at
the lower frequency. It is a mixture of speech recognition, of
estimation and speech synthesis and there is some theory but
I can tell you that most listeners prefer artificial wideband
extension in comparison to the narrowband transmission. Foreign guests, who speak and understand German a little,
say that it is much easier to understand.
BWE: the theory
There is some theory behind that. I will not go into too much detail but we can have a discussion after the talk on the
individual equations.
Audio example 3: telephone speech without and with BWE
I would like to play you an example of artificial bandwidth extension at the receiving site. Here, we have narrowband
speech at 3.4 kHz, more or less. The lower limit is 200 or 300 Hz and we artificially increase the bandwidth at the
receiving end but we have no additional side information – we do not transmit side information. [Example of speech
played] You might argue whether that is better or not. It gives some improvement in quality and probably also of
intelligibility – it is a compromise, but it might help to introduce the wideband service.
The Royal Academy of Engineering 51