Lecture Series in Mobile Telecommunications and Networks (1583KB)

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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
Bandwidth extension with hidden side information Time domain BWE with 600 bit/s of side information But there is another solution, which is to use artificial wideband extension but, this time, we allow some side information but we do not have a channel for transmitting it. The idea is to use watermarking techniques to hide the enhancement bits for artificial wideband extension at the receiver in the bit-stream, such that we have wideband terminals which produce a certain amount of additional information which is hidden in the bit-stream. The bit-stream is still compatible with the narrowband terminal, you can use the old terminal as it is and you should not notice any difference. However, the more sophisticated new terminal is capable of extracting the side information. The nice thing with this approach is that you don’t need to modify the network at all. As long as the terminals are in the same network, you just need to sell the customer two new phones and say, ‘this is the wideband phone’. This seems to be a very attractive solution. The missing frequencies are reproduced here such that at the transmitter we have some wideband processing and we extract parameters to re-synthesise at the receiver the higher frequencies. We receive the side information due to the watermarking process and we do some decoding. There, we exploit the insensitivity of the human ear at the higher frequencies and, in the simplest cases, we have just the noise generator and digital filter which is controlled, and we produce the frequencies between 3.4 and 7 kHz. That works quite well with speech. In this case we used 600 bit/s for wideband extension with this simplified module we contributed recently to one of the latest ITU coding standards for voice-over IT. There is bandwidth extension with side information and we use this model. Wideband speech with hidden side information We have played with it a little and the solution looks like this. You have the enhanced wideband encoder running at 16 kHz, and you have the low power. You modify the coder such that you can hide there the parameter which has been extracted in the high-pass band. The normal decoder does not know about it and is not irritated – you don’t recognise the degradation in the quality and the enhanced decoder extracts the parameters, does the synthesis, adds it to the decoded signal, and you get wideband. The secret behind that is simple to explain but difficult to understand. We have code books there and their address base is huge – therefore, at the transmitter, where we are looking for the number of the joke, we cannot check all the jokes – we have a limited set of addresses that are allowed. We therefore allow different code books at the transmitter and the sophisticated receiver finds out which code book is used and the standard decoder can decode it because it is compatible – as it uses only allowed addresses in the vector space. Example 4: enhanced codec We applied it to several codecs. We have enhanced narrowband codec with hidden information and I would like to play one example of this. This is the AMR codec – and with the red cross, I don’t say ‘AMR plus’, but I say, ‘AMR – Red Cross’. As you know the Red Cross gives help in situations where you need to improve something. This is the enhanced fullrate codec 12.2 kbits, with a hidden data stream of 600 bits/s, but you can increase it for that codec to up to 2 kbits/s. You can hide 2 kbits – you can do anything with the 2 kbits, and we are using it here for wideband extension. Let us compare it to dedicated wideband standard at the same bit rate. We start with the narrowband first. The quality measured, by the way, is the PESQ measure here, where low numbers are bad and higher numbers are good. Here, it seems to be comparable in terms of the objective quality which reflects with perceptual quality. [Examples of speech played] For speech, it is almost as good – and that could be a solution if only parts of the network 52 The Royal Academy of Engineering
Page 1 and 2: Lecture Series in Mobile Telecommun
Page 3 and 4: Foreword This compilation brings to
Page 5 and 6: From wireless networks to sensor ne
Page 7 and 8: From wireless networks to sensor ne
Page 9 and 10: As far as the routing of packets is
Page 11 and 12: easons. In the internet, one never
Page 13 and 14: Implementation with laser pointers,
Page 15 and 16: here of laptops, and all these lapt
Page 17 and 18: adds a great deal of discrete compl
Page 19 and 20: Questions & Answers Mike Walker: Th
Page 21 and 22: Human beings will also evolve and w
Page 23 and 24: Welcome and introduction Professor
Page 25 and 26: ComReg Collection - Start 16/Apr/20
Page 27 and 28: evidence that people give in favour
Page 29 and 30: chunks of spectrum where you are in
Page 31 and 32: As an aside, I was reading a few of
Page 33 and 34: Of course, it is not just one mask,
Page 35 and 36: I apologise for hopping between top
Page 37 and 38: Questions & Answers Michael Walker:
Page 39 and 40: Nobody - believe you me, it came ou
Page 41 and 42: The second aspect of my comment is
Page 43 and 44: want to call it - that sits there a
Page 45 and 46: From plain old Telephony to flawles
Page 47 and 48: From plain old Telephony to flawles
Page 49 and 50: You can see that there is much room
Page 51: 1024, we transmit 10 bits, so to sp
Page 55 and 56: 4 Vision: ambience audio communicat
Page 57 and 58: Questions & Answers Michael Walker:
Page 59 and 60: I said five years ago and what I he
Page 61 and 62: them and perhaps leave out some of

Lecture Series in Mobile Telecommunications and Networks (1583KB)

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