Note: Descriptions are shown in the official language in which they were submitted.
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Transcoder with prevention of tandem coding
of speech
Field of the Invention
The present invention relates to a transcoder
comprising means for preventing tandem coding of speech in
a mobile to mobile call within a mobile communication
system which employs a speech coding method reducing
transmission rate on the radio path, a speech coder for
encoding the speech signal to be transmitted to a mobile
station into speech parameters, and decoding the speech
parameters received from the mobile station into a speech
signal according to said speech coding method, and a PCM
coder for transmitting an uplink speech signal to and for
receiving a downlink speech signal from a PCM interface in
the form of PCM speech samples.
Background of the Invention
In recent years, digital mobile communication
systems for fully digital speech and data transmission
have been introduced. As far as the mobile communication
network is concerned, the most limited resource is the
radio path between the mobile stations and the base
stations. To reduce the bandwidth requirements of a radio
connection on the radio path, speech transmission utilizes
speech encoding providing a lower transmission rate, for
example 16 or 8 kbit/s instead of the 64 kbit/s,
transmission rate typically used in telephone networks.
For speech encoding, both the mobile station and the fixed
network end must have a speech encoder and a speech
decoder. On the network side, the speech coding functions
may be located in several alternative places, for example
~ at the base station or in the mobile services switching
center. The speech encoder and decoder are often located
~ remote from the base station in so-called remote
transcoder units. In the latter case, speech encoding
parameters are sent between the base station and the
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transcoder unit in specific frames.
In each mobile originating or mobile terminating
speech call, a transcoder is connected to the speech
connection on the network side. The transcoder decodes the
mobile originating speech signal (uplink direction), and
encodes the mobile terminating speech signal (downlink
direction). This type of an arrangement causes no problems
as long as only one of the speech participants is a mobile
station and the other, for example, a subscriber of a
public switched telephone network (PSTN).
If the call takes place between two mobile stations
(Mobile to Mobile Call, MMC), the operation of the mobile
communication network involves a transcoder on the
connection between the calling mobile station and the
mobile services switching center, and, correspondingly, a
second transcoder between the called mobile subscriber and
the (same or another) mobile services switching center.
These transcoders are then inter-connected by the mobile
services switching center (centers) as a result of normal
call switching. In other words, for each MMC call there
are two transcoder units in a series connection, and
speech encoding and decoding are performed twice for the
call. This is known as tandem coding. Tandem coding
presents a problem in mobile communication networks as it
degrades the speech quality due to the extra speech
encoding and decoding. So far, tandem coding has not been
a major problem because relatively few calls are MMC
calls. However, as the number of mobile stations
increases, the number of MMC calls will also be higher and
higher.
Disclosure of the Invention .
It is an object of the present invention to prevent
tandem coding and consequently to improve speech quality
in mobile to mobile calls.
This object is achieved by a transcoder which,
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according to the invention, is characterized by the
transcoder comprising means for transmitting and receiving
said speech parameters in a subchannel formed by one or
more least significant bits of said PCM speech samples
simultaneously with said PCM speech samples.
The invention also relates to a method for preventing
tandem coding of speech in a mobile to mobile call, which
method comprises the steps of : encoding a speech signal by
a speech coding method which reduces transmission rate and
provides speech parameters, transmitting the speech
parameters over the radio interface to a first transcoder
in the mobile communication network, decoding the speech
parameters by said speech coding method so as to restore
said speech signal, transmitting said speech signal from
the first transcoder to a second transcoder as PCM speech
samples. The method of the invention is further
characterized by transmitting said speech parameters,
received over the radio interface, from the first
transcoder to the second transcoder simultaneously with
said PCM speech samples in a subchannel formed by one or
more least significant bits of said PCM speech samples.
According to a further broad aspect of the present
invention there is provided a transducer for a mobile
communication system which employs a speech coding method
which is arranged to reduce transmission rate of the speech
on a radio path. The transcoder comprises a speech coder
for encoding the speech signal to be transmitted to a
mobile station into speech parameters, and decoding the
speech parameters received from the mobile station into a
speech signal according to the speech coding method. A PCM
coder is provided for transmitting an uplink speech signal
to and for receiving a downlink speech signal from a PCM
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interface in the form of PCM speech samples. Means is also
provided for transmitting and receiving the speech
parameters in a subchannel formed by one or more least
significant bits of the PCM speech samples simultaneously
with the PCM speech samples, in order to prevent tandem
coding of speech in a mobile to mobile call.
According to a still further broad aspect of the
present invention there is provided a method for preventing
tandem coding of speech in a mobile to mobile call. The
method comprises the steps of encoding a speech signal by a
speech coding method which reduces transmission rate and
provides speech parameters. The speech parameters are
transmitted over a radio interface to a first transcoder in
the mobile communication network. The speech parameters
are decoded by the speech coding method so as to restore
the speech signal. The speech signal is transmitted from
the first transcoder to a second transcoder as PCM speech
samples. The speech parameters, received over the radio
interface, are transmitted from the first transcoder to the
second transcoder simultaneously with the PCM speech
samples in a subchannel formed by one or more least
significant bits of the PCM speech samples.
According to a still further broad aspect of the
present invention there is provided an arrangement for
preventing tandem coding of speech in a mobile
communication system wherein the mobile stations and the
mobile communication network comprise speech coders for
transmitting a speech signal over a radio path in the form
of speech coding parameters at a speech coding rate. A
mobile to mobile call comprises in the mobile communication
network a tandem connection of two speech coders, there
being a normal PCM interface between the speech coder in
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the tandem connection. There is also a speech coder
arranged to provide a subchannel in one or more least
significant bits of PCM speech samples in the PCM interface
for forwarding the speech coding parameters provided by the
speech coder in the mobile station through the mobile
communication network without decoding or encoding being
carried out in the speech coders of the mobile
communication network.
The invention further relates to an arrangement for
preventing tandem coding of speech in a mobile
communication system in which the mobile stations and the
mobile communication network comprise speech coders for
transmitting a speech signal over the radio path in the
form of speech coding parameters at a speech coding rate,
and in which a mobile to mobile call comprises in the
mobile communication network a tandem connection of two
speech coders, there being a normal PCM interface between
the speech coders in said tandem connection, characterized
by a subchannel formed by one or more least significant
bits of PCM speech samples in the PCM interface for
forwarding the speech coding parameters provided by the
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speech coder in the mobile station through the mobile
communication network without decoding or encoding being
carried out in the speech coders of the mobile
communication network.
In the preferred embodiment of the present
invention, an MMC call can be established according to the
normal procedures of the mobile communication network so
that the connection has two transcoders in a tandem
configuration. The speech transferred between a transcoder
and a mobile station is coded by a speech coding method
which reduces the transmission rate. Both the transcoders
carry out the normal transcoding operations for the speech
by decoding it in one transcoder into normal digital pulse
code modulated (PCM) speech samples which are forwarded to
the second transcoder and encoded therein by said speech
coding method. In the subchannel formed by one or two of
the least significant bits of the PCM speech samples,
there is simultaneously transferred speech information
which is in accordance with said speech coding method and
received from the mobile station, i.e. speech parameters
for which no transcoding operations (encoding and
decoding) are carried out in either of the tandem
connected transcoders. The receiving transcoder primarily
chooses the speech information that is in accordance with
this speech encoding method to be transmitted over the
radio interface to the receiving mobile station.
Consequently, speech coding is carried out primarily in
the mobile stations only, and the coded speech
information, i.e. speech parameters, are passed through
the mobile communication network without tandem coding,
which improves speech quality. When the receiving
transcoder cannot find the coded speech information in the
least significant bits of the PCM speech samples, the
speech information to be transmitted over the radio
interface is encoded in the normal manner from the PCM
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speech samples. The use of one or two least significant
bits of the PCM speech samples as a subchannel according
to the invention only marginally deteriorates the speech
quality of the PCM connection, and the effect is even
5 alleviated by the inevitable deterioration of speech
quality due to the speech coding at low bit rate.
The solution in which the only aim is to evade
tandem coding requires signalling outside the traffic
channel, which, apart from the modifications in various
network elements, also causes many other problems as a
result of e.g. various kinds of supplementary services.
Such supplementary services include call transfer, call
hold in which, for example, music can be provided by the
mobile services switching center to the calling party,
services for a multitude of subscribers (conference
calls), etc. For example, announcements or music from the
center would not reach the subscribers if only speech
encoding information were transferred between the
transcoders. As there exists a normal PCM interface
between the tandem connected transcoders of the invention,
it is possible to maintain all the standard signalling
associated with in this interface as well as the
supplementary services, and to avoid the problems
disclosed above. In the implementatior~ of the invention,
only the transcoder need be modified. The modifications do
not have to be standardized either, but they can be
implemented manufacturer specifically without any
compatibility problems being created. The invention can be
applied to all MMC calls regardless of whether the mobile
stations are within the service area of the same mobile
services switching center or not. The only prerequisite is
a digital end-to-end connection between the transcoders;
however, the lack of such a connection does not create any
additional problems, but merely breaks the "subchannel"
according to the invention, which corresponds with a
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situation of a normal tandem coded call.
In the mobile communication system, a transcoder
may be located in several alternative locations, for
example at the base station or separate from the base
station. In the latter case, the transcoder is referred to
as a remote transcoder, and speech encoding information is
in the mobile communication network transferred between
the base station and the remote transcoder in specific
frames, which also contain synchronization and control
information. By doing so, tandem coding can be avoided in
the MMC calls by forwarding, with minor modifications, the
frames received from a base station to another base
station through a "subchannel" between two tandem
connected transcoders without the transcoders carrying out
any speech encoding or decoding. The receiving transcoder
continuously searches for synchronization in the one or
two least significant bits of the PCM speech samples, and
simultaneously encodes PCM samples. If synchronization is
found, the receiving transcoder transfers the frames
received from the subchannel to the base station. If there
exists no synchronization, or it is lost, the receiving
transcoder transmits speech encoding information which is
encoded from normal PCM samples and packed into frames to
the base station. Thus, the transcoders need not know that
the call is an MMC call, and that a tandem prevention mode
is required. Such information may, however, be given to
the transcoder by providing the frames sent from the base
stations to the transcoders (uplink frames) with
information that the frames are associated with an MMC
call. Thus, the decision on the handling of each frame is
based on the information in the particular frame.
Furthermore, the frame handling may vary according to
control information contained in them. As examples of
special handling, bad frame handling and discontinuous
transmission (DTX) are mentioned.
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If the transcoder is located at the base station,
one solution is to convert the speech coding information
received from the radio interface into frames which are
transferred to a second base station through a
"subchannel" of the invention, for example by following
the same principle as in the case of remote transcoders.
Brief Description of the Drawings
In the following, the invention will be described
by means of the preferred embodiments with reference to
the accompanying drawings, in which
Figure 1 illustrates a mobile communication system
according to the invention,
Figure 2 illustrates a TRAU speech frame in
accordance with the GSM recommendation 8.60, and
Figures 3 and 4 are block diagram illustrations of
TXDTX and RXDTX handlers.
Preferred Embodiments of the Invention
The present invention can be applied to any mobile
communication system which utilizes digital speech
transmission and speech encoding techniques reducing the
transmission rate.
One example is the European digital cellular mobile
communication system GSM (Global System for Mobile
Communications) which is becoming a world-wide standard
for mobile communication systems. The basic elements of
the GSM system are described in the GSM Recommendations.
For a closer description of the GSM system, the GSM
recommendations and "The GSM System for Mobile
Communications", by M. Mouly & M. Pautet, Palaiseau,
France, 1992, ISBN: 2-9507190-0-7 is referred to.
The GSM and a modification thereof, DCS1800
(Digital Communication System) operating at the 1800 Mhz
frequency range, are the primary targets for the
invention, but it is not intended that the invention be
restricted to these radio systems.
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Figure 1 very briefly describes the basic elements
of the GSM system. A mobile services switching centre MSC
handles the connecting of incoming and outgoing calls. It
performs functions similar to those of an exchange of a
public switched telephone network (PSTN) . In addition to
these, it also performs functions characteristic of.mobile
communications only, such as subscriber location
management. The mobile stations MS are connected to the
center MSC by base station systems. The base station
system consists of a base station controller BSC and base
stations BTS. One base station controller BSC is used to
control several base stations BTS.
The GSM system is entirely digital, and speech and
data transmission are also carried out entirely digitally,
which results in a uniform quality of speech. In speech
transmission, the current speech encoding method in use is
the RPE-LTP (Regular Pulse Excitation - Long Term
Prediction) which utilizes both short and long term
prediction. The coding produces LAR, RPE and LTP
parameters which are sent instead of the actual speech.
Speech transmission is dealt with in the GSM
recommendations in chapter 06, and speech encoding
particularly in the recommendation 06.10. In the near
future, other coding methods will be employed, such as
half rate methods, with which the present invention can be
used as such. As the actual invention does not relate to
the speech encoding method, and as it is independent of
it, any speech coding method will not be described in any
further detail herein.
Naturally, the mobile station must have a speech
coder and decoder for speech coding. As the implementation -
of the mobile station is neither essential to the present
invention nor unusual, it is not dealt with here in any
closer detail.
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1. Transcoder and TRAU Frames
On the network side, various functions relating to
speech coding and rate adaptation are concentrated in a
transcoder unit TRCU (Transcoder/Rate Adaptor Unit). The
TRCU may be located in several alternative locations in
the system according to choices made by the manufacturer.
The interfaces of the transcoder unit include 64 kbit/s
PCM (Pulse Code Modulation) interface towards the mobile
services switching center MSC (A interface), and a 16 or 8
kbit/s GSM interface towards the base station BTS.
Relating to said interfaces, the terms uplink and downlink
are also used in the GSM recommendations, the uplink being
the direction from the base station BTS to the mobile
services switching center MSC, while the downlink is the
reverse direction.
In cases the TRCU is placed remote from a base
station BTS, information is sent between the base station
and the transcoder/rate adaptor unit TRCU in so-called
TRAU frames. The TRAU frame includes 320 bits according to
the recommendation 08.60 and 160 bits according to the
recommendation 08.61. There are presently four different
frame types defined according to information content in
them. These are speech, operations/maintenance, data, and
the so-called idle speech frame.
Typically, the transcoder unit TRCU is located at
the mobile services switching center MSC, but it may also
be a part of a base station controller BSC or a base
station BTS. A transcoder unit placed remote from the base
station BTS must receive information on the radio
interface for efficient decoding. For such control and
synchronization of the transcoder, a special kind of in-
band signalling is used on the 16 kbit/s channel between
the base station and the transcoder unit. This channel is
also used for speech and data transmissions. Such remote
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control of a transcoder unit is described in the GSM
recommendation 08.60 and 08.61. In the following, only the
system in accordance with the recommendation 08.60 will be
dealt with, but the methods described are easily
5 applicable to a system in accordance with the
recommendation 08.61 as well.
To carry out synchronization, the first two octets
of each frame comprise 16 synchronization bits. In
addition, the first bit of the 16-bit words (2 octets)
10 that constitute a frame is a synchronization check bit. In
addition to the bits containing the actual speech, data or
operation/maintenance information, each frame comprises
control bits in which information of the frame type and a
varying amount of other frame-type-specific information is
conveyed. Furthermore, the last four bits T1-T4 of, for
example, speech and idle frames are assigned for the
aforementioned time alignment.
Figure 2 illustrates a TRAU speech frame having 21
control bits C1-C21 and, in addition, the last 4 bits T1
T4 of the frame are assigned for time alignment. The
actual speech information bits are in the octets 4-38. In
practice, the speech information consists of LAR, RPE and
LTP parameters of the RPE-LTP (Regular Pulse - Long Term
Prediction) speech encoding method. The idle speech frame
is similar to the speech frame illustrated in Figure 2,
except that all the traffic bits of the frame are in logic
state "1".
The GSM recommendation 8.60 defines control bits as
follows. The bits C1-C4 determine the frame type, i.e.
C1C2C3C4=1110=downlink speech frame and
C1C2C3C4=0001=uplink speech frame. Bit C5 determines the
channel type, i.e. C5=0=full rate channel and C5=1=half
rate channel. Bits C6-C11 are control bits for time
alignment. Bits C12-C15 are frame indicators for uplink
direction, and C16 is the frame indicator for downlink
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direction, primarily relating to discontinuous
transmission. The coding and use of the frame indicators
are described in the GSM recommendations 08.60 and 06.31.
C12 is a Bad Frame Indicator BFI, which is also used in
continuous transmission, i.e. BFI=0=good frame and
BFI=1=bad frame. C13-C14 establish the SID code (Silence
Descriptor). C15 is a time alignment bit TAF. C17 is a
downlink DTX bit indicating whether the discontinuous
transmission DTX is in use in the downlink (DTX=1) or not
(DTX=0). Bits C18-C21 are spare bits in the uplink
direction. C16 is a SP bit indicating in the downlink
direction whether said frame comprises speech. In the
downlink direction, the other control bits are spare bits.
Along with the recommendation 08.60, a newer GSM
recommendation 08.61 now exists, in which several TRAU
frame types are also defined, but in which the control
bits used differ from the ones of the GSM recommendation
08.60. All the basic solutions relating to the invention
of this application can, however, easily be implemented in
a mobile communication system according to the GSM
recommendation 08.61.
2. Discontinuous Transmission DTX
Discontinuous transmission DTX refers to a method
by which a transmission on the radio path may be
interrupted for the duration of pauses in speech. The aim
is to decrease transmitter power consumption, a prime
concern for mobile stations, and general noise level on
the radio path, which effects the system capacity. DTX
causes certain distinctive features in the tandem
prevention operation of the present invention, and thus
the following at first examines an ordinary DTX by using
the GSM system as an example.
The discontinuous transmission is carried out by
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means of three major elements. On the transmitting side, a
Voice Activity Detection VAD is required, which is used
for checking whether a signal under examination contains
speech or solely background noise. The VAD function is
determined in the GSM recommendation 6.32 and it is
basically based on analysis of signal energy and spectral
changes. In addition, a function for calculating the
background noise parameters is required on the
transmitting side. On the basis of the noise parameters
obtained from the transmitting side, so-called comfort
noise is generated on the receiving side in order not to
subject the listener to unpleasant switching between
speech with background noise and total silence. All the
elements of discontinuous transmission are largely based
on a speech codec implementing RPE-LTP coding, and its
internal parameters.
2.1 Functions of Transmittincr Side (Transcoder
downlink DTX)
The function which handles the discontinuous
transmission on the transmitting side, i.e the TXDTX
handler (Transmit DTX), is illustrated in Figure 3. I
sends speech frames to the communication systetr:
continuously. The speech frames are marked with an SF
(Speech) flag in the control bits, indicating whether said
frame includes speech or whether it is a so-called SIL
frame (Silence Descriptor) which contains information o.~.
the background noise for comfort noise generation on the
receiving end. The SP flag is determined on the basis of a
VAD flag obtained from a voice activity detection unit. As
the VAD flag goes to zero, indicating that no speech is
detected in the signal, a transition of the SP flag into ,
zero will also occur after the number of frames reauired
for calculating the background noise parameters have
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passed. The transmitting unit of the radio system further
sends this frame, which is marked by zero SP flag and
includes the noise parameters, after which transmission on
the radio path is discontinued. The TXDTX handler does,
however, continue to send frames containing noise
information to the radio system which, at predetermined
time intervals, sends one of them to the radio path in
order to update the noise parameters of the receiving
side. When speech is again detected in the signal, the SP
flag is set to 1 and continuous transmission is
recommenced.
In discontinuous transmission, a function is thus
required on the transmitting side for calculating the
background noise parameters. The aforementioned encoder at
the transmitting side generates the parameters that
represent background noise. Out of the normal parameters,
those parameters that provide information on the level and
spectre of the noise are selected to represent background
noise, i.e. block maxims and reflection coefficients,
which have been converted into T-AR coefficients. Over a
time period of four speech frames, average values are
further calculated for the selected parameters. One common
value of four block maxims is calculated over the time
period of four speech blocks. These parameters are
transmitted over the radio path as described above. Thus,
only part of the speech parameters are sent, and some of
the parameters are substituted for a SID code word which
consists of 95 zeros. The rest of the parameters that are
not used are coded to 0.
2.2 Receivina Side Functions (Transcoder Uplink
DTX
The discontinuous transmission of the receiving
side is correspondingly handled by a RXDTX (Receive DTX?
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handler whose structure is illustrated in Figure 4. It
receives frames from the radio system, and handles them on
the basis of the three flags obtained in the control bits.
The BFI flag (Bad Frame Indicator) indicates
whether the frame in question contains sensible
information. I.e. if the frame has been corrupted, for
example, on the radio path in such extent that it cannot
be reconstructed in the radio section of the base station,
the BFI flag is used for marking said frame defective. As
such a bad frame is received, i.e. a frame with a BFI flag
value 1, the speech parameters of said frame are
substituted for the speech parameters of the previous
frame before decoding. If several frames that are marked
by a BFI flag are received, muting operations according to
the GSM recommendations are performed. The only exception
to the aforementioned handling of a BFI flag is an uplink-
DTX situation, i.e. a valid SID updating frame has been
received, but no normal speech frame with a BFI value 0
has been received after that. In such an uplink DTX
situation, a frame with a BFI flag only means that comfort
noise generation should be continued. Alternatively, this
can be carried out by sending idle frames.
The SID flag consists of two bits and it is used
for classifying the SID frame sent by the radio system
based on errors in the specifically formed code word in
the frame. A decision is made on the basis of this
classification on how the frame will later be used. If the
SID has the value 2 and the BFI has the value 0, it is a
valid SID frame that can be used for updating the noise
parameters.
A TAF flag (Time Alignment Flag) is used for
indicating whether the frame in question has been used in
signalling outside of this sub-system, i.e. its purpose is
mainly to indicate when the next SID updating is expected.
The operations relating to comfort noise
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generation, such as muting, are carried out in accordance
with the GSM recommendations 06.11 or 06.21 and 06.21 or
06.22 and 06.31 or 06.41 depending on the combinations of
the aforementioned three flags, but generally speaking it
5 can be said that the comfort noise generation is started
or comfort noise is updated upon receiving a new valid SID
f rame .
The comfort noise generation correspondingly
employs the decoder described above for generating
10 background noise. The averaged parameters received from
the transmitting side are used in the normal manner, and
they are maintained unchanged until the next updating. The
other parameter values are set so that the location of a
decimation grid and 13 RPE samples are frame-specifically
15 allotted integers uniformly distr~_buted within the range
0-3 and 1-6, respectively. The delay parameter a of the
long term residual signal is set in the sub-frames to
values 40, 120, 40, 120, in that order, i.e. the minimum
and maximum values in turn, and the gain parameter of the
long term residual signal is set to zero in all the sub-
frames .
3. Prior art MMC Call
When a mobile station MS originating call is made
in the prior art mobile communication system, the
associated signalling is forwarded from a base station BTS
to a mobile services switching center MSC which in turn
establishes connection, for example, between the PSTN line
and the line of the aforementioned A interface. At the
same time, the transcoder unit TRCU is allocated and
connected to the A interface line. The mobile services
switching center MSC further commands the base station
controller BSC to connect the base station BTS, which the
calling mobile station MS communicates with, to the A
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interface line allocated. The base station controller BSC
establishes connection between the A interface line and
the base station BTS which the calling MS communicates
with. The base station BTS independently handles call
establishment on the radio path. Thereby, a connection is
established having in series a mobile station MS, a base
station BTS, a base station controller BSC, a transcoder
unit TRCU and a mobile services switching center MSC.
Thus, this connection is used for sending coded speech
between MS-TRCU, and TRAU frames between BTS-TRCU.
If the prior art mobile communication system
handles an MMC (Mobile to Mobile Call) between two mobile
stations MS, the call connecting proceeds, as far as the
calling mobile station is concerned, similarly to above,
but now the mobile services switching center establishes
the connection between the A interface line allocated for
the calling MS and the A interface line allocated for the
called MS. The A interface line of the called MS is
connected to a second transcoder unit. From said second
transcoder unit, a connection will be established to the
base station of the called mobile station MS. In other
words, for each MMC call two transcoder units are
connected in series, and the call is encoded and decoded
twice. This is known as tandem coding, which deteriorates
speech quality due to the extra encoding and decoding.
4. MMC Call According to the Invention
In the present invention, an MMC call can be
connected according to normal procedures of the mobile
communication network so that the connection comprises two
transcoders TRCU in a tandem configuration. Tandem coding
can be avoided in MMC calls by sending the frames received
from a base station BTS with minor changes through said
two tandem coded transcoders TRCU to a second base station
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BTS without the transcoders carrying out any speech
_ encoding or decoding. As a result, speech coding is only
carried out in the mobile station MS and the speech
parameters are simply forwarded through the mobile
communication network, which considerably improves speech
quality compared to the conventional tandem coding.
The transcoders can implement several different
coding types, such as full rate and half rate, and contain
a tandem prevention mode according to the invention for
each coding type. Alternatively, transcoders TRCU
representing different types may be grouped in pools from
which an appropriate transcoder may be selected on a call
by call basis.
An Abis interface may be maintained the same with
the exception of the additions to the signalling
procedures described here. In the TRAU frames of the Abis
interface, the only additions in the primary embodiment of
the invention will in the uplink be indication of the
tandem prevention mode, and in the downlink informing of
lack of synchronization or synchronization errors in the A
interface, as will be described below.
The following will describe an arrangement
according to the preferred embodiment of the invention for
preventing tandem coding in an MMC call within one mobile
services switching center MSC. For reasons of clarity, a
three-part description will be given: uplink transfer BTS-
TRCU, transfer between transcoders TRCU-TRCU, and downlink
transfer TRCU-BTS.
At first, in Figure 1 it is assumed that there is
within a base station BTS1 coverage area a mobile statio:,
MS1 which initiates an MMC call set up to a second mobile
station MS2 which is located within a coverage area of a
base station BTS3. In such a case, the normal call
establishment for a mobile originating call MOC is carried
out according to the GSM recommendations. The call
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establishment involves signalling between the MS1 and the
base station controller BSCl, as well as signalling
between mobile services switching center MSC1 and the -
visitor location register VLR (not shown) for the purpose
of subscriber authentication and exchange of encryption
keys. The MSC1 receives from the MS1 the directory number
of the B subscriber, and as it notices that the B
subscriber is another mobile subscriber it performs an
interrogation according to the GSM recommendations to the
home location register HLR (not shown) of the B
subscriber. Due to the fact that the B subscriber is
within the MSC1 area, the HLR responds by giving the MSCl
as the routing address . Consequently, the MSC1 performs,
according to the GSM recommendations, call establishment
of a mobile terminating call MTC, which call establishment
involves a database enquiry to a visitor location register
VLR, paging of the mobile station MS2, authentication,
exchange of encryption keys, etc.
The MSCl reserves a dedicated PCM line of the A
interface for both mobile stations MS1 and MS2. In
addition, the MSC1 reserves the transcoder unit TRCU1 for
the MSl, and for the MS2 the transcoder TRCU2 which is
connected to the corresponding PCM lines of the A
interface. The MSC1 establishes the connections MS1-TRCU1
and MS2-TRCU2 as well as a connection between the A
interface line which is allocated for the MS1 and the A
interface line which is allocated for the MS2. Thus,
between the MS1 and the MS2 there is a speech connection
having two transcoders TRCU1 and TRCU2 connected in
series. Between the transcoders, there exists the A
interface, i.e. a digital PCM connection. -
In the call establishment, GSM recommendations are
strictly followed. However, call set-up may be changed so .
that when the MSC1 detects an MMC call and a need for
preventing tandem coding, it signals information.
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concerning it to the base stations BTS1 and BTS3. This
piece of information may be included in the existing
messages.
4.1. Uplink Transfer BTS-TRCU
In the following, only the uplink transfer BTS1-
TRCU1 is described. The uplink transfer BTS3-TRCU2 is
carried out by the very same principle. As the BTS1
channel codec unit CCU, determined in the GSM
recommendation 08.60, receives information on the tandem
prevention mode, it provides the uplink TRAU frames with
information that indicates to the transcoder TRCU1 that
the frames are associated with an MMC call and no speech
decoding is to be carried out to them. This information
can be transferred in one of the free control bits C18-C21
of the uplink TRAU frame, or a combination of them. For
the tandem prevention operation, it is also possible to
determine a new frame type, of which control bits C1-C4
notify. In the example below, tandem prevention mode is
indicated by the control bit C21, i.e. C21=0=tandem
prevention mode and C21=1=normal mode.
It must, however, be noted that the implementation
of the invention is possible without the TRAU frames
transferring any kind of information on tandem prevention.
In other words, the mobile services switching center MSC
does not signal to the base station that the call is an
MMC call, and neither is the control bit of the TRAU
frame, e.g. C21, in the base station uplink direction used
for informing of tandem prevention. Neither does the TRCU1
in such a case check the control bit C21 but continuously
operates, as will be described below, with the control bit
value C21=0, i.e. in tandem prevention mode. The example,
- however, describes an embodiment employing signalling and
the C21 control bit, because it is the more complicated
one out of these two.
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Figure 5 shows a flow chart illustrating the
transcoder TRCUl uplink direction functions according to
the invention. Upon receiving a TRAU speech frame from the
base station BTSl, the transcoder TRCUl carries out all
5 the procedures (block 51) for the received TRAU frame that
are determined in the GSM recommendations, except speech
decoding. The TRAU frames handled in the block 51 are fed
to speech decoding 52 and supplementary processing 53.
In addition, the block 51 checks the state of the
10 control bit C21 in the frame. If the control bit C21 is in
state 1, the TRCU1 operates in the normal mode of
operation and transfers the TRAU frame to speech decoding,
but not to supplementary processing 53. In such a case,
the operation of TRCU1 is entirely in accordance with the
15 GSM recommendations.
Speech coding 52 is in accordance with the GSM
recommendations, and produces from the speech coding
parameters a digital speech signal which is applied to a
pulse code modulation (PCM) block 54, which, by means of
20 pulse code- modulation (PCM) in accordance with, for
example, CCITT recommendations 6.711-6.716, converts the
digital speech signal to a bit rate of 64 kbit/s. The
pulse code modulation (PCM) at the rate of 64 kbit/s
functions so that the speech signal is sampled every 125
microseconds, i.e. the rate of sampling is 8 kHz, and the
amplitude of each sample is quantized to an 8 bit code by
using A-law or u-law coding. The block 54 sends the PCM
speech samples to TRCU2 through the A interface.
If, however, the block 51 detects that the BTS1
has, despite all, inserted the TRAU frame control bit
C21=0, the TRCU1 shifts to tandem prevention mode. In
tandem prevention mode, the block 51 sends the TRAU frame
to both decoding 52 and supplementary processing 53. The
speech signal decoded in block 52 is supplied to PCM block
54 in which it is coded l.Ilto PCM speech samples as in
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normal mode of operation.
The supplementary processing block 53 produces a
TR.AU frame in accordance with the GSM recommendation 08.60
(or 08.61) to be forwarded to a second transcoder through
the PCM block 54 and the A interface. As decoding is not
performed, the TRAU frames that are forwarded to the A
interface comprise essentially the same speech parameters
and control data as the frames received from the base
station BTSl. The block 53 does, however, check the
control bits of the received TRAU frame and may depending
on their contents carry out supplementary functions which
may change the contents of the TRAU frames that are sent
to the A interface.
Upon receiving a bad uplink TRAU frame, in other
words a TRAU frame marked with a BFI flag value l, the
block 53 substitutes the speech parameters of said uplink
TRAU frame with the speech parameters of the previous
uplink TRAU frame and sets the value of the BFI flag to 0
prior to sending said TRAU frame forward to the A
interface and TRCU2. If several uplink TRAU frames that
are marked with a BFI flag are received, the block 53
carries out muting procedures according to the GSM
recommendations to the values of the speech parameters,
and sets the BFI flag values to 0 in the bad TRAU frames
prior to their sending to the A interface, but decoding is
not performed in this case either.
The only exception is a situation of an uplink TDX
whereby an idle speech frame or a TRAU frame marked with a
BFI flag value 1 only mean that the block 53 should
continue generating comfort noise.
When the block 53 receives a valid SID updating, it
functions the way it does in a normal uplink DTX
situation, i.e. comfort noise generation is activated or
comfort noise is updated to the parameters, but decoding
is yet again not performed.
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In a nutshell, it can be said that all the
operations determined by the GSM recommendation 06.11 or
06.21 are performed with the exception of decoding, and
the BFI flag is set to 0 and the uplink DTX is handled as
follows. Upon reception of SID frames according to the GSM
recommendation 06.31 or 06.41, comfort noise generation
according to the GSM recommendation 06.12 or 06.22 is
performed, but again in such a manner that decoding is not
carried out, but the modified parameters are packed back
to the frames. Control bits are not changed at all in
these flags that will be sent forward to the A interface.
In the TRAU frames according to the GSM
recommendation 08.61, the control bits further comprise a
UFI flag and a check sum for cyclic redundancy check
(CRC), which may cause changes in the speech parameters of
the received TRAU frame prior to sending the TRAU frames
forward to the A interface. For the TRAU frames in
accordance with the GSM recommendation 08.61, the
transcoder TRCUl must, among other things, calculate a new
CRC check sum in case it has been necessary to change the
parameters associated with it.
The block 53 transfers the TRAU frames handled in
tandem prevention mode to the PCM block, which
incorporates them in the normal PCM speech samples by
2S inserting the TRAU frames in a "subchannel" formed by the
least significant bit (e.g. 8 kbit/s speech coding) or two
least significant bits (e.g. 16 kbit/s speech coding). It
is possible to employ even a higher number of least
significant bits, but that would result in a more
noticeable deterioration in speech quality.
Figure 6 illustrates the insertion of a TRAU frame _
according to Figure 2 into 160 successive 8 bit PCM
samples. Into each PCM sample, two bits of a TRAU frame ,
have been inserted in place of two least significant bits
of the PCM speech sample. The PCM samples 1-8 contain
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synchronization zeros, the PCM samples 9-18 control bits
Cl-C15, the PCM samples 19-155 data bits, and the PCM
samples 156-160 control bits C16-C21 and T1-T4. The six
most significant bits of the PCM samples are original bits
of the PCM speech sample (marked with the symbol x).
In the preferred embodiment of the invention, if
TRCU1 is in normal state (C21=1), the PCM block 54 does
not insert the bits of the TRAU frame into the two least
significant bits of the PCM speech sample, in other words
all the bits of the sample sent to the A interface are
original bits of the PCM sample. This is how unnecessary
deterioration of speech quality can be avoided due to
tandem prevention not being employed in calls having,
e.g., a PSTN subscriber as one party.
It is, however, possible that the TRCU1 is
continuously in tandem prevention mode, i.e. it always
carries out the insertion of TRAU frames in the PCM
samples. It such a case, the receiving end makes the
decision whether to use the PCM samples or the TRAU
frames. An advantage here is that the mobile services
switching center MSC and the base stations BTS1 do not
have to know that the call is an MMC call as all the
modifications required by the invention are concentrated
at the transcoder. In such a situation, the control bit
C21 is not used the way described above, either. A
disadvantage that results is a slightly poorer speech
quality in normal calls to the PSTN. If frames in
accordance with the recommendation 08.61 and a subchannel
formed of one least significant bit are utilized, the
deterioration cannot be noticed in practice.
The transcoder TRCU2 also contains a transmit unit
according to Figure 5 for traffic in the uplink direction.
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4.2.Transfer between transcoders TRCU1-TRCU2
The TRCU2 handles the uplink TRAU frames it
receives from the. base station BTS3 the same manner as
described above in connection with the transcoder TRCU1,
and goes to tandem prevention mode upon detecting that
C21=0, or is in the prevention mode permanently.
Similarly, the TRCUl handles the TRAU frames and the PCM
samples received from the A interface as will be described
below in connection with TRCU2.
The TRAU frames in accordance with the GSM
recommendations 08.60 or 08.61 are transferred between the
transcoders TRCU1 and TRCU2 through the A interface in a
subchannel formed by one or more least significant bits of
the PCM speech samples. In the transfer between the
transcoders TRCU1 and TRCU2, it is furthermore possible to
use the same synchronization methods according to the GSM
recommendations 08.60 and 08.61 that are used between the
base station BTSl and the transcoder TRCU1.
4.3. Downlink Transfer TRCU2-BTS3
In the following, only the reception at the
transcoder TRCU2 from the A interface, and the downlink
transfer TRCU2-BTS3 are described. The very same principle
can be applied to the reception from the A interface at
the transcoder TRCUl and the downlink transfer TRCU1-BTS3.
Figure 7 shows a flow chart illustrating the
transcoder TRCU2 functions in the downlink direction
according to the invention. In accordance with the GSM
recommendations 08.60 or 08.61, the synchronization block
71 of the receiving transcoder TRCU 2 continuously tries
to find synchronization in the subchannel in the PCM
samples received from the A interface, i.e. in the one or
two least significant bits. The synchronization with the
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TRAU frames takes place by means of the synchronization
zeros and ones in the frames. At the beginning of the
call, when synchronization has not yet been found, or
during the call, when synchronization has been lost, a
5 sufficient number of TRAU frames is awaited to be received
in order to assure that a 8 or 16 kbit/s subchannel
containing TRAU frames has been found, and not a random
synchronization pattern in the least significant bits of
normal PCM samples. Synchronization with the frames is
l0 carried out continuously, and the time period used for
analysis is altered in accordance with possible timing
variations.
A separation block 72 separates the PCM speech
samples to an encoding block 73 and the TRAU frames to a
15 supplementary processing block 74.
Entirely, in accordance with the GSM
recommendations, the encoding block 73 carries out an
encoding for the PCM speech samples into speech encoding
parameters of the lower rate speech encoding method. The
20 encoding of the PCM samples takes place continuously
regardless of whether synchronization with TRAU frames has
been achieved or not.
If synchronization with TRAU frames has not taken
place, or verification of synchronization is awaited, the
25 speech coding parameters that have been encoded from the
PCM speech samples are forwarded from the encoding block
73 to a processing block 75. The processing block 75
inserts the speech coding parameters in the TRAU frames
and carries out all the procedures for them that are
determined for the transcoder TRCU2 in the GSM
recommendations, prior to transmitting the frames in
accordance with the GSM recommendation 08.60 or 08.61 to
the base stations BTS3.
If synchronization with the TRAU frames has taken
place, the speech coding parameters are not forwarded from
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the encoding block 73 to the processing block 75. Instead,
the processing block 75 is provided with the TRAU frames
received from the A interface, which TRAU frames have been
processed in the -supplementary processing block 74. For
these TRAU frames, the processing block 75 performs all
the procedures determined for the transcoder in the GSM
recommendations, and produces a TRAU frame in accordance
the GSM recommendation 08.60 (or 08.61) to be forwarded
through the Abis interface to the BTS3. As the speech
encoding is not carried out, the TRAU frames to be
forwarded contain essentially the same speech parameters
and control data as the TRAU frames received through the A
interface. The supplementary processing block 74 does,
however, check the control bits of the TRAU frame received
from the A interface and may depending on their contents
carry out supplementary functions which may change the
contents of the TRAU frames that are sent to the BTS3.
In the following, the supplementary features of the
supplementary processing block 74 are described.
If the TRCU2 by means of the uplink TRAU frames
sent by the BTS3, or by some other means, is set in
downlink DTX off mode, the block 74 forwards the TRAU
frames received from the A interface by only changing the
type of the frames to that of normal downlink speech
frames. In other words, in the frames according to the GSM
recommendation 08.60, the supplementary processing block
74 sets the control bits Cl-C4 to indicate that the TRAU
frame is a downlink frame, i.e. C1C2C3C4=1110, and the SP
bit and the spare bits always to 1.
If the downlink DTX is set on, the supplementary
processing block 74 sends the TRAU frames forward by
changing the type of the TRAU frames received from the A
interface to that of normal downlink speech frames, if
they are normal speech frames as to their control
information. In all the TRAU frames received from the A
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interface that can, on the basis of control bits, be
interpreted as comfort noise generation frames, i.e. in
frames that the TRCUl would interpret as belonging to
comfort noise generation frames of the uplink DTX, the SP
flag is kept 0 and a SID code word is set to unnecessary
speech parameters, and the rest of the unnecessary
parameters are also set to 0. As a normal speech frame is
received from the A interface, the SP is again set to 1 in
these downlink frames.
The transcoder TRCU1, too, contains a receiving
unit according to Figure 7 for processing speech
information in the downlink direction.
It is also possible to transfer the functions of
the transcoders TRCUl and TRCU 2 described above so that
the TRCU2 downlink supplementary processing block 74
carries out all the modifications to the frames received
from the BTS1 (i.e. also the functions determined above
for the TRCU1 uplink supplementary processing block 53),
and the TRCUl downlink supplementary processing block 74
carries out all the modifications to the TRAU frames
received from the BTS2 (i.e. also all the functions
determined for the TRCU2 uplink supplementary processing
block 53) whereby a configuration slightly different
regarding the location of the functions is obtained. In
such a case the TRCU1 forwards the uplink frames as such
to the TRCU2, and the TRCU2 correspondingly to the TRCU1.
The example above describes an MMC call within the
area of one mobile services switching center MSC1. The
invention can also be applied to an MMC call with mobile
stations MS located in different mobile services switching
centers.
Let us assume that in the system according to
Figure 1 a MS1 located within the area of a base station
BTS1 and a mobile services switching center MSC1 places a
call to a mobile station MS3 located within the area of a
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base station BTS4 and mobile services switching center
MSC2. The beginning of the MMC call proceeds as in the
previous example, but now the home location register HLR
returns the address of the mobile services switching
center MSC2 to the mobile services switching center MSCl.
The MSC1 routes the call to the MSC2 and includes
information in the signalling that the call is an MMC
call. The MSC1 performs the allocation of the transcoder
TRCU1 and the call establishment to the mobile station MS1
direction, as in the previous example. The MSC2, in turn,
allocates the transcoder TRCU3 and carries out the call
establishment to the mobile station MS3 the same way that
the MSC1 performed the allocation of the transcoder TRCU2
and the call establishment to the mobile station MS2 in
the previous example. A connection is established between
the mobile services switching centers MSC1 and MSC2, and
the transcoders TRCUl and the TRCU3 are connected in
series. Following this, the TRAU frame uplink transfer
between BTS-TRCU, the downlink transfer between TRCU-BTS
and the transfer between the transcoders are performed, as
in the previous example.
The figures and their description are only intended
to illustrate the present invention. It should, however,
be understood that changes can be made to the embodiments
of the invention without departing from the scope and
spirit of the attached claims.
35
