Title of Invention	A METHOD FOR EMITTING A USER-DATA STREAM WHEN CHANGING BETWEEN A FIRST AND A SECOND TRANSMISSION PATH,AND A COMMUNICATION TERMINAL
Abstract	The invention relates to a method for emitting a user-data stream when changing between a first and a second transmission path, wherein a reception pause for the user-data stream occurs at a receiver during the change between the end of the reception of a first partial user-data stream (A) with first data packets of the first transmission path and the beginning of the reception of a second partial user-data stream (B) with second data packets of the second transmission path, and wherein the user-data stream (A, B) is output at the receiver end, wherein, during the output of the user-data stream (A, B), the reception pause is filled in with additional user data that are generated from the first partial user-data stream (A) previously received over the first transmission path, the additional user data are generated by time-stretching at least part of the first partial user-data stream (A), with a plurality of new data packets, each consisting of user data of the first data packets and of other user-data values generated from the user data of the first data packets, being generated for the output.

Title of Invention

A METHOD FOR EMITTING A USER-DATA STREAM WHEN CHANGING BETWEEN A FIRST AND A SECOND TRANSMISSION PATH,AND A COMMUNICATION TERMINAL

Abstract

The invention relates to a method for emitting a user-data stream when changing between a first and a second transmission path, wherein a reception pause for the user-data stream occurs at a receiver during the change between the end of the reception of a first partial user-data stream (A) with first data packets of the first transmission path and the beginning of the reception of a second partial user-data stream (B) with second data packets of the second transmission path, and wherein the user-data stream (A, B) is output at the receiver end, wherein, during the output of the user-data stream (A, B), the reception pause is filled in with additional user data that are generated from the first partial user-data stream (A) previously received over the first transmission path, the additional user data are generated by time-stretching at least part of the first partial user-data stream (A), with a plurality of new data packets, each consisting of user data of the first data packets and of other user-data values generated from the user data of the first data packets, being generated for the output.

Full Text	Description Method for outputting a useful data stream and communications terminal for receiving and for outputting a useful data stream The invention relates to a method for outputting a useful data stream at the changeover between a first transmission path and a second transmission path according to the preamble of patent claim 1, and to a communications terminal for receiving and for outputting a useful data stream according to the preamble of patent claim 10. For mobile communication, communications terminals which are connected to a line-bound communication network by means of a radio link are frequently used. Examples of such arrangements are the known mobile radio networks, for example the GSM network, private telephone networks (DECT arrangements), WLAN arrangements etc. All these arrangements have in common the fact that bidirectional data streams in which useful data (audio data, video data etc.) is transmitted are used for the communication between one or more base stations (access point) and the mobile communications terminals (mobile telephones, cordless telephones, PDAs, multimedia PCs etc.). In many cases it is not possible to cover a desired geographic area completely by means of a single base station (access point). For this reason, the mobile communication arrangements are generally embodied in a multi-cellular fashion so that a plurality of overlapping radio zones are used in order to bring about area coverage. A particular requirement in multi-cellular communication arrangements based on radio is the free mobility of the mobile communications terminals. Such a communications terminal not only has to be accessible within the entire arrangement but also a changeover of the radio cell should also be possible during the ongoing operation without interrupting the communication link and without interrupting the call. For this purpose, the individual radio cells are arranged in an overlapping fashion so that at the changeover of the mobile communications terminal from one radio cell to an adjacent radio cell a communication link can change the radio cell used while present in an overlapping radio zone. In the arrangements with mobile communications terminals it has proven disadvantageous that a period of time in which the communication link is briefly interrupted and during which no useful data can be transmitted to the communications terminal or from the communications terminal is present between the release of the communication link in a first radio cell and the setting up or the continuation of the communication link in a second radio cell. This brief interruption is usually felt to be disruptive. The disadvantages described apply equally to the transition between different networks, for example for the handover between a private (W)LAN and a public GSM network. These disadvantages are often overcome in the prior art by virtue of the fact that the individual radio cells of communication arrangements are operated in synchronism with one another. For example, the useful data which is transmitted from the fixed network to the mobile communications terminal is transmitted simultaneously "in parallel" by all the radio cells at least at the time of the switchover so that at the switchover no breaks in transmission occur as a result of the release or the setting up of partial communication links - the term useful data streams is frequently also used. A disadvantage with this procedure is, however, the increased network load and the problem that a plurality of radio base stations (access points) have to be supplied simultaneously with the same useful data (duplicated useful data stream) . This can usually be implemented only in arrangements with a small number of radio cells which are preferably controlled by the same network node, and is therefore generally done in this way only in the case of relatively small, private "cordless" arrangements. In the meantime, nonsynchronous radio networks, for example radio-based "WLAN" data networks according to IEEE 802.11 are being increasingly used. While brief breaks in transmission can usually be tolerated during a changeover of the radio cell for non-real-time communication, that is to say for the classic data transmission mode, the interruptions have an adverse effect on the convenience of use for real-time communication, that is to say for example for the transmission of voice data (VoIP = Voice-over-Internet-Protocol). This problem is minimized by keeping the interruption at the changeover of a radio cell as short as possible. For this purpose, work is currently also being carried out on new standards, for example IEEE 802.11R "fast roaming"/"fast handoff". By accelerating the "handover" process (changeover of the radio cell) it is possible here to reduce the adverse effect on the communication link but not eliminate it completely. This applies in particular at the multiple changeover of the radio cells, the propagation time delays either cumulating or fragments of the voice data stream having to be rejected in order to avoid exceeding an appropriate overall delay time. It is therefore an object of the present invention to minimize the adverse effects at the changeover of a connecting path during a communication link. The object is achieved for the method by means of the features of independent patent claim 1, and for the communications terminal by means of the features of independent patent claim 10. The solution provides a method for outputting a useful data stream at the changeover between a first transmission path and a second transmission path, wherein a break in reception occurs for the useful data stream at a receiver during the changeover between an end of reception of a first partial useful data stream of the first transmission path and a start of reception of a second partial useful data stream of the second transmission path, and wherein the useful data stream is output at the receiver end. Here, during the output of the useful data stream the break in reception is filled with additional useful data, wherein the additional useful data is generated from the first partial useful data stream which has previously been received over the first transmission path. The application of this method makes the changeover from the first transmission path, for example first radio cell, to the second transmission path, for example second radio cell, virtually or completely disruption-free for the receiver. Disruptive gaps (artifacts) are filled in by the method so that a useful signal can be output for the receiver continuously at any time. The solution of the object further provides a communications terminal for receiving and for outputting a useful data stream, wherein the useful data stream can be switched over from a first link to a second link. In this context, the communications terminal is equipped with a generating device for generating additional useful data, and the communications terminal is provided with a mixing device for the weighted addition of the additional useful data to the useful data stream which is received over the second link. The use of such a communications terminal fills in a "gap" in the useful data stream at the changeover of a communication link, for example between two radio cells. It is irrelevant here whether the "gap" arises due to a delay or because parts of the useful data stream (data packets) are lost. The weighted addition of the additional useful data with the received useful data stream ensures a "gentle" transition in which specifically the weighting can be used to maintain a constant signal energy level. Avoiding a fluctuating average signal energy level prevents a receiver being irritated. The method is advantageously developed by means of the features of dependent patent claims 2 to 9. The features and advantages described here apply appropriately also to the communications terminal according to the invention. The communications terminal is advantageously developed by means of the features of dependent patent claims 11 to 14 whose features and advantages are also to be appropriately applied to the method according to the invention. Irritation at a receiver or user is reduced or avoided if the additional useful data is generated by chronologically stretching at least part of the first partial useful data stream. As a result, the chronological gap at the changeover is filled by useful data which originates to a large extent from the first partial useful data stream and therefore ideally has a real information content, in contrast to the alternative, which is however easy to implement, of filling the chronological gap, that is to say the break in reception, with a noise signal or the like. If, during the outputting after the break in reception, a data stream which is composed of the additional useful data and the second useful data stream is output for a transition time period, a "gentle" transition occurs which is frequently unremarkable and which a receiver therefore subjectively feels is nondisruptive. It is advantageous here if, for the generation of the composite useful data stream, the additional useful data is weighted with a first variable factor and the second useful data stream is weighted with a second variable factor, in which case the sum of the squared first variable factor and the squared second variable factor is at least virtually constant in each case. As a result, a largely constant signal energy level is ensured, with this signal energy level which is constant for the transition advantageously corresponding to the average signal energy level of the first partial useful data stream. Whereas phase shifts, for which the human ear is not very sensitive, can therefore occur at the changeover to a limited degree, signal energy fluctuations, that is to say consequently amplitude fluctuations, to which the human ear is sensitive, are partially or completely avoided. As a result, a subjectively pleasant, often unnoticeable changeover occurs between the transmission paths. The "filling in" of the break in reception results in a delay (phase shift) for the second partial useful data stream. This delay can be compensated for by chronologically compressing the second partial useful data stream after the complete changeover of the outputting onto the second partial useful data stream until the delay which has occurred owing to the break in reception is compensated for. This is particularly advantageous after a multiple changeover of the transmission paths because the breaks in reception (delays) which occur here in each case otherwise add to one another (cumulate), thus leading to a progressive worsening in quality. A particularly frequent application case occurs if a digital audio signal of a voice link (VoIP link) is used as the useful data stream. In these cases, cellular communication networks which are not optimized for voice data transmission (for example LAN/WLAN arrangements) can also be used for real-time communication links. In this case the mobility of a receiver is advantageously given by the fact that a radio link is used for at least one of the transmission paths. In this context, the method can also advantageously be used for nonhomogenous structures in which line-bound and radio-based transmission paths are used alternately in a mixed fashion. Another important application occurs if a first radio cell of a multi-cellular radio network is used as the first transmission path and a second radio cell of a multi-cellular radio network is used as the second transmission path, in which case the changeover is used for transferring a radio link from the first radio cell to the second radio cell. In this context, in particular the disadvantages which nonsynchronous radio cell arrangements have for real-time communication are reduced or even entirely avoided. The communications terminal according to the invention is advantageously used to carry out the method described above, with said terminal having a generating device for implementing a changeover process which is as unnoticeable as possible, said generating device being configured to stretch the useful data stream which is received over the first link. As a result, the break in reception is filled with "meaningful" useful data. The entire delay which occurs here can be compensated for again by the communications terminal if the latter has a device for chronologically compressing the useful data stream which is received over the second link, said device chronologically compressing the second partial useful data stream until the delay which has occurred owing to the break in reception has been compensated for again. The adverse effect during the outputting of the useful data stream is reduced further if the communications terminal has a sensing device for sensing breaks in speech in the useful data stream which is received over the first link and/or in a useful data stream which is to be transmitted, with the sensing device being configured to control the time for the switchover by reference to a detected break in speech. As a result, the changeover can be "shifted" into one of the breaks in speech which frequently occur in any .case so that the probability of a perceptible adverse effect is reduced further. In the case of useful data streams which do not have a break in speech or other signal breaks for a relatively long period of time, it is also possible to generate such a break "artificially" by, for example, "gentle" brief attenuation of the useful data stream, in which case the changeover of the transmission path is performed during this brief attenuation. The communications terminal also advantageously comprises a control device for defining parameters which, during the addition, determine the weighting of the additional useful data and/or of the useful data stream which is received over the second link. This ensures that a useful data stream with a largely constant signal energy level is output at the receiver, with irritation at the receiver as a result of a fluctuating signal energy level ("volume") being avoided. Exemplary embodiments of the method according to the invention are explained below with reference to the drawings which serve at the same time to explain a communications terminal according to the invention. e In said drawings:* figure 1 is a schematic illustration of a communication arrangement in which a mobile communications terminal changes over between two radio cells, figure 2 is a schematic illustration of the useful data packets of two partial useful data streams along a time axis, and figure 3 is a schematic illustration of a communications terminal which is configured to change over between the two partial useful data streams. Figure 1 is a schematic illustration of a communication arrangement in which a mobile communications terminal EG is connected to a stationary party CP to the communication. Here, the party CP to the communication is connected via a communication node (not illustrated) to a public communication network NW, while the mobile communications terminal EG is connected via radio base stations API, AP2 (access points) to the communication network NW. The radio base stations API, AP2 each supply a radio zone FBI, FB2 (radio cell), the radio zones FBI, FB2 having a common overlapping area. The radio base stations API, AP2 are part of an asynchronous multi-cellular radio network, in this case what is referred to as a wireless LAN (WLAN). The useful data for the communication link is transmitted between the party CP to the communication and the communications terminal EG at least on the partial links between the radio base stations API, AP2 and the communications terminal EG, in each case packet-switched according to the Internet protocol (TCP-IP, UDP-IP). Within the scope of this exemplary embodiment it is assumed that the communications terminal EG is intended to change over from the radio zone FBI into the radio zone FB2 while the communication link is in existence, which is symbolized in figure 1 by an arrow. The communication link between the party CP to the communication and the mobile communications terminal EG comprises not only signaling messages (not considered here) but also a bidirectional useful data stream, and within the scope of this exemplary embodiment only the useful data stream from the party CP to the communication in the direction of the mobile communications terminal EG is to be considered. In this context, the communications terminal EG is firstly supplied in the radio zone FBI by a first partial useful data stream A from the radio base station API in a first link, and after the changeover in the radio zone FB2 by a second partial useful data stream B by the radio base station AP2 in a second link. The changeover is carried out while the communications terminal EG is located in the overlapping area of the radio zones FBI, FB2. The radio technology used in the radio zones FBI, FB2 is optimized for non-real-time communication, i.e. that at a changeover from one radio cell into an adjacent radio cell a chronological "gap" (delay) can occur but in this context no useful data or useful data packets are lost. Instead of the WLAN radio network considered here, it is however also alternatively possible to use a transmission method which is optimized for real-time communication links. In such a method, no reception acknowledgement for useful data packets is often performed for reasons of optimizing propagation time (for example UDP protocol) . In this context it is possible, in particular at a changeover of the radio cell, for useful data packets either to be lost or else to arrive delayed at the receiver in such a way that they can no longer be taken into account (passed on) . In this context it is also possible for there to be chronological gaps which can be due both to delays and to data losses. Figure 2 is a schematic illustration of the partial useful data streams A, B plotted against a time axis T, with the partial useful data streams A, B having data packets n, n+1, n+3, n+4 which are illustrated as squares and have a digitized audio signal (audio data) which are transmitted from the party CP to the communication to the mobile communications terminal EG. At the time Tl, the changeover from the first partial useful data stream A to the second partial useful data stream B is initiated, with no further data packets being transmitted to the communications terminal EG by means of the partial useful data stream A after the time Tl. Since the next data packets n+2, n+3, n+4 which are illustrated as squares do not arrive at the communications terminal EG until after the time T2, a break in reception occurs which is filled in with additional useful data. In the present case, therefore, the last data packets n, n+1, which arrive by means of the partial useful data stream A and which are present in a reception data buffer, are chronologically stretched in such a way that new data packets are produced which themselves consist partially of the contents of the originally transmitted data packets n, n+1, and partially of artificially generated, additional information which consists, for example, of echo signal components and of interpolated useful data values. Methods for "stretching" digitized audio signals are adequately known in the prior art, for example also in the German patent application with the official application file number 103 270 57.4. A simple alternative is to output a "colored" noise signal which should correspond in its frequency spectrum to the average frequency spectrum of the previously received useful data. The additional data packets are characterized in figure 2 by means of triangles with the designations n+2, n+3. Starting from the time T2, "real" useful data packets n+2, n+3, n+4 are received by means of the partial useful data stream B, these useful data packets also being buffered in the reception data buffer and then being output. Therefore, at the time T2, when packets are passed on, switching over is performed between the additional data packets n+2, n+3 (represented by means of triangles) and the newly received data packets n+2, n+3, n+4 (represented by squares). In order to make the transition "gentler", a mixed signal is output in the transition time between T2 and T3, with the data packets of the first partial useful data stream A being weighted (multiplied) with a variable factor Ci, and the data packets of the partial useful data stream B being weighted (multiplied) with a variable factor C2. In this context, the signal profiles for the factors Ci, C2 are strictly monotonously declining or rising. The sum of the squared factors Ci, C2 should at least approximately yield a constant value (in standardized systems this is the value 1) at any time so that a jump in the signal energy level of the last output signal passed on is avoided. The previously described process is advantageously performed in a break in speech. In this context, use is made of the fact that a relatively long time window is available for the changeover, and in this time window each of the two radio zones FBI, FB2 can reliably supply the communications terminal EG. Figure 3 is a schematic illustration of the most important functional blocks of the communications terminal EG. Here, the voice data which is generated (the microphone M is illustrated here symbolically as a generating element) is directed either over a first transmission path A or over a second transmission path B depending on a switch position 0.1. On the other hand, the communications terminal EG is also configured to receive partial useful data streams A, B. A control device "Control" contains a sensing device VAD ("voice activity detection") for monitoring the voice activity in a useful data stream and a logic device "handover" for controlling the changeover process. As a result, the control device "Control" determines the time S of the switching over (that is to say of the changeover) and the value of the factors (parameters) Ci, C2 at any time. Furthermore, the control device "Control" generates control signals x, h (not illustrated here) for controlling a signal stretching device TSh and a signal compression device TSj.. While the incoming partial useful data stream A firstly runs through the signal stretching device TSh as a generating device and is then weighted with the factor Ci, the useful data of the partial useful data stream B is weighted with the factor C2. Both resulting signals are added in a mixing device "+", are fed to the signal compression device TSi which is controlled by the control device Control, and said signals are then output via a loudspeaker L. It is to be noted here that the control device "Control" is not necessarily part of the terminal EG but can also be arranged in another entity (component) of the communication arrangement, even in a "distributed" form. WE CLAIM 1. A method for emitting a user-data stream when changing between a first and a second transmission path, wherein a reception pause for the user- data stream occurs at a receiver during the change between the end of the reception of a first partial user-data stream (A) with first data packets of the first transmission path and the beginning of the reception of a second partial user-data stream (B) with second data packets of the second transmission path, and wherein the user-data stream (A, B) is output at the receiver end, wherein, during the output of the user-data stream (A, B), the reception pause is filled in with additional user data that are generated from the first partial user-data stream (A) previously received over the first transmission path, characterised in that the additional user data are generated by time-stretching at least part of the first partial user-data stream (A), with a plurality of new data packets, each consisting of user data of the first data packets and of other user- data values generated from the user data of the first data packets, being generated for the output. 2. A method as claimed in claim 1, wherein during the output after the reception pause, a user-data stream made up of the additional user data and the second user-data stream (B) is output for a transitional period. 3. A method as claimed in claim 2, wherein for the generation of the composite user-data stream the additional user data are weighted with a first variable factor (C1) and the second user-data stream (B) with a second variable factor (C2), whereby the sum of the squared first and the squared second variable factor (C1, C2) is constant. 4. A method as claimed in any one of the preceding claims, wherein after the complete change of the output to the second partial user-data stream (B), the second partial user-data stream (B) is time-compressed until the delay caused by the reception pause is equalised. 5. A method as claimed in any one of the preceding claims, wherein a digitised audio signal of a voice connection is used as the user-data stream (A, B). 6. A method as claimed in claim 5, wherein this audio signal is monitored for speech pauses and the change is carried out in a speech pause. 7. A method as claimed in any one of the preceding claims, wherein a radio link is used for at least one of the transmission paths. 8. A method as claimed in claim 7, wherein a first radio cell is used as the first transmission path and a second radio cell of a multi-cell radio network is used as the second transmission path, with the change being used for a transfer of a radio link from the first to the second radio cell. 9. A communication terminal (EG) for receiving and emitting a user-data stream (A, B), wherein the user-data stream (A, B) can be switched over from a first connection to a second connection, the communication terminal (EG) is equipped with a generating device (TSh) for generating additional user data, in that the communication terminal (EG) is provided with a mixing device (+) for the weighted addition of the additional user data with the user-data stream (B) received over the second connection, and in that the generating device (TSh) is configured for the time- stretching of at least part of the user-data stream (A) received over the first connection, whereby a plurality of new data packets, each consisting of user data of the first data packets and of other user-data values generated from the user data of the first data packets, can be generated for the output. 10. A communication terminal (EG) as claimed in claim 9, comprising a device (TSi) for the time-compression of the user-data stream (B) received over the second connection. 11.A communication terminal (EG) as claimed in any one of the preceding claims, comprising a detection device (VAD) for detecting speech pauses in the user-data stream received over the first connection (A) and/or in a user-data stream that is to be sent, with the detection device (VAD) being configured to control the time for the switchover by reference to a detected speech pause. 12.A communication terminal (EG) as claimed in any one of the preceding claims, comprising a control device (Control) for defining parameters (C1, C2) which determine the weighting of the additional user data and/or of the user-data stream (B) received over the second connection during the addition. ABSTRACT TITLE : "A METHOD FOR EMITTING A USER-DATA STREAM WHEN CHANGING BETWEEN A FIRST AND A SECOND TRANSMISSION PATH. AND A COMMUNICATION TERMINAL" The invention relates to a method for emitting a user-data stream when changing between a first and a second transmission path, wherein a reception pause for the user-data stream occurs at a receiver during the change between the end of the reception of a first partial user-data stream (A) with first data packets of the first transmission path and the beginning of the reception of a second partial user-data stream (B) with second data packets of the second transmission path, and wherein the user-data stream (A, B) is output at the receiver end, wherein, during the output of the user-data stream (A, B), the reception pause is filled in with additional user data that are generated from the first partial user-data stream (A) previously received over the first transmission path, the additional user data are generated by time-stretching at least part of the first partial user-data stream (A), with a plurality of new data packets, each consisting of user data of the first data packets and of other user-data values generated from the user data of the first data packets, being generated for the output.

Full Text

Description
Method for outputting a useful data stream and communications
terminal for receiving and for outputting a useful data stream
The invention relates to a method for outputting a useful data
stream at the changeover between a first transmission path and
a second transmission path according to the preamble of patent
claim 1, and to a communications terminal for receiving and for
outputting a useful data stream according to the preamble of
patent claim 10.
For mobile communication, communications terminals which are
connected to a line-bound communication network by means of a
radio link are frequently used. Examples of such arrangements
are the known mobile radio networks, for example the GSM
network, private telephone networks (DECT arrangements), WLAN
arrangements etc. All these arrangements have in common the
fact that bidirectional data streams in which useful data
(audio data, video data etc.) is transmitted are used for the
communication between one or more base stations (access point)
and the mobile communications terminals (mobile telephones,
cordless telephones, PDAs, multimedia PCs etc.).
In many cases it is not possible to cover a desired geographic
area completely by means of a single base station (access
point). For this reason, the mobile communication arrangements
are generally embodied in a multi-cellular fashion so that a
plurality of overlapping radio zones are used in order to bring
about area coverage.
A particular requirement in multi-cellular communication
arrangements based on radio is the free mobility of the

mobile communications terminals. Such a communications terminal
not only has to be accessible within the entire arrangement but
also a changeover of the radio cell should also be possible
during the ongoing operation without interrupting the
communication link and without interrupting the call. For this
purpose, the individual radio cells are arranged in an
overlapping fashion so that at the changeover of the mobile
communications terminal from one radio cell to an adjacent
radio cell a communication link can change the radio cell used
while present in an overlapping radio zone.
In the arrangements with mobile communications terminals it has
proven disadvantageous that a period of time in which the
communication link is briefly interrupted and during which no
useful data can be transmitted to the communications terminal
or from the communications terminal is present between the
release of the communication link in a first radio cell and the
setting up or the continuation of the communication link in a
second radio cell. This brief interruption is usually felt to
be disruptive. The disadvantages described apply equally to the
transition between different networks, for example for the
handover between a private (W)LAN and a public GSM network.
These disadvantages are often overcome in the prior art by
virtue of the fact that the individual radio cells of
communication arrangements are operated in synchronism with one
another. For example, the useful data which is transmitted from
the fixed network to the mobile communications terminal is
transmitted simultaneously "in parallel" by all the radio cells
at least at the time of the switchover so that at the
switchover no breaks in transmission occur as a result of the
release or the setting up of partial communication links - the
term useful data streams is frequently also used. A
disadvantage with this procedure is, however, the increased
network load and the problem that

a plurality of radio base stations (access points) have to be
supplied simultaneously with the same useful data (duplicated
useful data stream) . This can usually be implemented only in
arrangements with a small number of radio cells which are
preferably controlled by the same network node, and is
therefore generally done in this way only in the case of
relatively small, private "cordless" arrangements.
In the meantime, nonsynchronous radio networks, for example
radio-based "WLAN" data networks according to IEEE 802.11 are
being increasingly used. While brief breaks in transmission can
usually be tolerated during a changeover of the radio cell for
non-real-time communication, that is to say for the classic
data transmission mode, the interruptions have an adverse
effect on the convenience of use for real-time communication,
that is to say for example for the transmission of voice data
(VoIP = Voice-over-Internet-Protocol). This problem is
minimized by keeping the interruption at the changeover of a
radio cell as short as possible. For this purpose, work is
currently also being carried out on new standards, for example
IEEE 802.11R "fast roaming"/"fast handoff". By accelerating the
"handover" process (changeover of the radio cell) it is
possible here to reduce the adverse effect on the communication
link but not eliminate it completely. This applies in
particular at the multiple changeover of the radio cells, the
propagation time delays either cumulating or fragments of the
voice data stream having to be rejected in order to avoid
exceeding an appropriate overall delay time.
It is therefore an object of the present invention to minimize
the adverse effects at the changeover of a connecting path
during a communication link.
The object is achieved for the method by means of the features
of independent patent claim 1, and for the communications
terminal by means of the features of independent patent claim 10.

The solution provides a method for outputting a useful data
stream at the changeover between a first transmission path and
a second transmission path, wherein a break in reception occurs
for the useful data stream at a receiver during the changeover
between an end of reception of a first partial useful data
stream of the first transmission path and a start of reception
of a second partial useful data stream of the second
transmission path, and wherein the useful data stream is output
at the receiver end. Here, during the output of the useful data
stream the break in reception is filled with additional useful
data, wherein the additional useful data is generated from the
first partial useful data stream which has previously been
received over the first transmission path. The application of
this method makes the changeover from the first transmission
path, for example first radio cell, to the second transmission
path, for example second radio cell, virtually or completely
disruption-free for the receiver. Disruptive gaps (artifacts)
are filled in by the method so that a useful signal can be
output for the receiver continuously at any time.
The solution of the object further provides a communications
terminal for receiving and for outputting a useful data stream,
wherein the useful data stream can be switched over from a
first link to a second link. In this context, the
communications terminal is equipped with a generating device
for generating additional useful data, and the communications
terminal is provided with a mixing device for the weighted
addition of the additional useful data to the useful data
stream which is received over the second link. The use of such
a communications terminal fills in a "gap" in the useful data
stream at the changeover of a communication link, for example
between two radio cells. It is irrelevant here whether the
"gap" arises due to a delay or because parts of the useful data
stream (data packets) are lost. The weighted addition of the
additional useful data with the received useful data stream
ensures a "gentle" transition in which specifically the

weighting can be used to

maintain a constant signal energy level. Avoiding a fluctuating
average signal energy level prevents a receiver being
irritated.
The method is advantageously developed by means of the features
of dependent patent claims 2 to 9. The features and advantages
described here apply appropriately also to the communications
terminal according to the invention. The communications
terminal is advantageously developed by means of the features
of dependent patent claims 11 to 14 whose features and
advantages are also to be appropriately applied to the method
according to the invention.
Irritation at a receiver or user is reduced or avoided if the
additional useful data is generated by chronologically
stretching at least part of the first partial useful data
stream. As a result, the chronological gap at the changeover is
filled by useful data which originates to a large extent from
the first partial useful data stream and therefore ideally has
a real information content, in contrast to the alternative,
which is however easy to implement, of filling the
chronological gap, that is to say the break in reception, with
a noise signal or the like.
If, during the outputting after the break in reception, a data
stream which is composed of the additional useful data and the
second useful data stream is output for a transition time
period, a "gentle" transition occurs which is frequently
unremarkable and which a receiver therefore subjectively feels
is nondisruptive. It is advantageous here if, for the
generation of the composite useful data stream, the additional
useful data is weighted with a first variable factor and the
second useful data stream is weighted with a second variable
factor, in which case the sum of the squared first variable
factor and the squared second variable factor is at least
virtually constant in each case. As a result,

a largely constant signal energy level is ensured, with this
signal energy level which is constant for the transition
advantageously corresponding to the average signal energy level
of the first partial useful data stream. Whereas phase shifts,
for which the human ear is not very sensitive, can therefore
occur at the changeover to a limited degree, signal energy
fluctuations, that is to say consequently amplitude
fluctuations, to which the human ear is sensitive, are
partially or completely avoided. As a result, a subjectively
pleasant, often unnoticeable changeover occurs between the
transmission paths.
The "filling in" of the break in reception results in a delay
(phase shift) for the second partial useful data stream. This
delay can be compensated for by chronologically compressing the
second partial useful data stream after the complete changeover
of the outputting onto the second partial useful data stream
until the delay which has occurred owing to the break in
reception is compensated for. This is particularly advantageous
after a multiple changeover of the transmission paths because
the breaks in reception (delays) which occur here in each case
otherwise add to one another (cumulate), thus leading to a
progressive worsening in quality.
A particularly frequent application case occurs if a digital
audio signal of a voice link (VoIP link) is used as the useful
data stream. In these cases, cellular communication networks
which are not optimized for voice data transmission (for
example LAN/WLAN arrangements) can also be used for real-time
communication links. In this case the mobility of a receiver is
advantageously given by the fact that a radio link is used for
at least one of the transmission paths. In this context, the
method can also advantageously be used for nonhomogenous
structures in which line-bound and radio-based transmission
paths are used alternately in a mixed fashion. Another
important application

occurs if a first radio cell of a multi-cellular radio network
is used as the first transmission path and a second radio cell
of a multi-cellular radio network is used as the second
transmission path, in which case the changeover is used for
transferring a radio link from the first radio cell to the
second radio cell. In this context, in particular the
disadvantages which nonsynchronous radio cell arrangements have
for real-time communication are reduced or even entirely
avoided.
The communications terminal according to the invention is
advantageously used to carry out the method described above,
with said terminal having a generating device for implementing
a changeover process which is as unnoticeable as possible, said
generating device being configured to stretch the useful data
stream which is received over the first link. As a result, the
break in reception is filled with "meaningful" useful data. The
entire delay which occurs here can be compensated for again by
the communications terminal if the latter has a device for
chronologically compressing the useful data stream which is
received over the second link, said device chronologically
compressing the second partial useful data stream until the
delay which has occurred owing to the break in reception has
been compensated for again.
The adverse effect during the outputting of the useful data
stream is reduced further if the communications terminal has a
sensing device for sensing breaks in speech in the useful data
stream which is received over the first link and/or in a useful
data stream which is to be transmitted, with the sensing device
being configured to control the time for the switchover by
reference to a detected break in speech. As a result, the
changeover can be "shifted" into one of the breaks in speech
which frequently occur in any .case so that the probability of a
perceptible adverse effect is reduced further. In the case of

useful data streams which do not have a break in speech or
other signal breaks for a relatively long period of time, it is
also possible to generate such a break "artificially" by, for
example,

"gentle" brief attenuation of the useful data stream, in which
case the changeover of the transmission path is performed
during this brief attenuation.
The communications terminal also advantageously comprises a
control device for defining parameters which, during the
addition, determine the weighting of the additional useful data
and/or of the useful data stream which is received over the
second link. This ensures that a useful data stream with a
largely constant signal energy level is output at the receiver,
with irritation at the receiver as a result of a fluctuating
signal energy level ("volume") being avoided.
Exemplary embodiments of the method according to the invention
are explained below with reference to the drawings which serve
at the same time to explain a communications terminal according
to the invention.
e
In said drawings:*
figure 1 is a schematic illustration of a communication
arrangement in which a mobile communications terminal
changes over between two radio cells,
figure 2 is a schematic illustration of the useful data
packets of two partial useful data streams along a
time axis, and
figure 3 is a schematic illustration of a communications
terminal which is configured to change over between
the two partial useful data streams.
Figure 1 is a schematic illustration of a communication
arrangement in which a mobile communications terminal EG is
connected to a stationary party CP to the communication.

Here, the party CP to the communication is connected via a
communication node (not

illustrated) to a public communication network NW, while the
mobile communications terminal EG is connected via radio base
stations API, AP2 (access points) to the communication network
NW. The radio base stations API, AP2 each supply a radio zone
FBI, FB2 (radio cell), the radio zones FBI, FB2 having a common
overlapping area.
The radio base stations API, AP2 are part of an asynchronous
multi-cellular radio network, in this case what is referred to
as a wireless LAN (WLAN). The useful data for the communication
link is transmitted between the party CP to the communication
and the communications terminal EG at least on the partial
links between the radio base stations API, AP2 and the
communications terminal EG, in each case packet-switched
according to the Internet protocol (TCP-IP, UDP-IP).
Within the scope of this exemplary embodiment it is assumed
that the communications terminal EG is intended to change over
from the radio zone FBI into the radio zone FB2 while the
communication link is in existence, which is symbolized in
figure 1 by an arrow.
The communication link between the party CP to the
communication and the mobile communications terminal EG
comprises not only signaling messages (not considered here) but
also a bidirectional useful data stream, and within the scope
of this exemplary embodiment only the useful data stream from
the party CP to the communication in the direction of the
mobile communications terminal EG is to be considered. In this
context, the communications terminal EG is firstly supplied in
the radio zone FBI by a first partial useful data stream A from
the radio base station API in a first link, and after the
changeover in the radio zone FB2 by a second partial useful
data stream B by the radio base station AP2 in a second link.
The changeover is carried out while the communications terminal

EG is located in the overlapping area of the radio zones FBI,
FB2.
The radio technology used in the radio zones FBI, FB2 is
optimized for non-real-time communication, i.e. that at a
changeover from one radio cell into an adjacent radio cell a
chronological "gap" (delay) can occur but in this context no
useful data or useful data packets are lost. Instead of the
WLAN radio network considered here, it is however also
alternatively possible to use a transmission method which is
optimized for real-time communication links. In such a method,
no reception acknowledgement for useful data packets is often
performed for reasons of optimizing propagation time (for
example UDP protocol) . In this context it is possible, in
particular at a changeover of the radio cell, for useful data
packets either to be lost or else to arrive delayed at the
receiver in such a way that they can no longer be taken into
account (passed on) . In this context it is also possible for
there to be chronological gaps which can be due both to delays
and to data losses.
Figure 2 is a schematic illustration of the partial useful data
streams A, B plotted against a time axis T, with the partial
useful data streams A, B having data packets n, n+1, n+3, n+4
which are illustrated as squares and have a digitized audio
signal (audio data) which are transmitted from the party CP to
the communication to the mobile communications terminal EG. At
the time Tl, the changeover from the first partial useful data
stream A to the second partial useful data stream B is
initiated, with no further data packets being transmitted to
the communications terminal EG by means of the partial useful
data stream A after the time Tl. Since the next data packets
n+2, n+3, n+4 which are illustrated as squares do not arrive at
the communications terminal EG until after the time T2, a break
in reception occurs which is filled in with additional useful
data. In the present case, therefore, the last data packets n,

n+1, which arrive by means of the partial useful data stream A
and

which are present in a reception data buffer, are
chronologically stretched in such a way that new data packets
are produced which themselves consist partially of the contents
of the originally transmitted data packets n, n+1, and
partially of artificially generated, additional information
which consists, for example, of echo signal components and of
interpolated useful data values. Methods for "stretching"
digitized audio signals are adequately known in the prior art,
for example also in the German patent application with the
official application file number 103 270 57.4. A simple
alternative is to output a "colored" noise signal which should
correspond in its frequency spectrum to the average frequency
spectrum of the previously received useful data.
The additional data packets are characterized in figure 2 by
means of triangles with the designations n+2, n+3. Starting
from the time T2, "real" useful data packets n+2, n+3, n+4 are
received by means of the partial useful data stream B, these
useful data packets also being buffered in the reception data
buffer and then being output. Therefore, at the time T2, when
packets are passed on, switching over is performed between the
additional data packets n+2, n+3 (represented by means of
triangles) and the newly received data packets n+2, n+3, n+4
(represented by squares).
In order to make the transition "gentler", a mixed signal is
output in the transition time between T2 and T3, with the data
packets of the first partial useful data stream A being
weighted (multiplied) with a variable factor Ci, and the data
packets of the partial useful data stream B being weighted
(multiplied) with a variable factor C2. In this context, the
signal profiles for the factors Ci, C2 are strictly
monotonously declining or rising. The sum of the squared
factors Ci, C2 should at least approximately yield a constant
value (in standardized systems this is the value 1) at any time
so that

a jump in the signal energy level of the last output signal
passed on is avoided.
The previously described process is advantageously performed in
a break in speech. In this context, use is made of the fact
that a relatively long time window is available for the
changeover, and in this time window each of the two radio zones
FBI, FB2 can reliably supply the communications terminal EG.
Figure 3 is a schematic illustration of the most important
functional blocks of the communications terminal EG. Here, the
voice data which is generated (the microphone M is illustrated
here symbolically as a generating element) is directed either
over a first transmission path A or over a second transmission
path B depending on a switch position 0.1. On the other hand,
the communications terminal EG is also configured to receive
partial useful data streams A, B. A control device "Control"
contains a sensing device VAD ("voice activity detection") for
monitoring the voice activity in a useful data stream and a
logic device "handover" for controlling the changeover process.
As a result, the control device "Control" determines the time S
of the switching over (that is to say of the changeover) and
the value of the factors (parameters) Ci, C2 at any time.
Furthermore, the control device "Control" generates control
signals x, h (not illustrated here) for controlling a signal
stretching device TSh and a signal compression device TSj..
While the incoming partial useful data stream A firstly runs
through the signal stretching device TSh as a generating device
and is then weighted with the factor Ci, the useful data of the
partial useful data stream B is weighted with the factor C2.
Both resulting signals are added in a mixing device "+", are
fed to the signal compression device TSi which is controlled by
the control device Control, and said signals are then output
via a loudspeaker L. It is to be noted here that the control
device "Control" is not necessarily

part of the terminal EG but can also be arranged in another
entity (component) of the communication arrangement, even in a
"distributed" form.

WE CLAIM
1. A method for emitting a user-data stream when changing between a first
and a second transmission path, wherein a reception pause for the user-
data stream occurs at a receiver during the change between the end of
the reception of a first partial user-data stream (A) with first data packets
of the first transmission path and the beginning of the reception of a
second partial user-data stream (B) with second data packets of the
second transmission path, and
wherein the user-data stream (A, B) is output at the receiver end,
wherein, during the output of the user-data stream (A, B), the reception
pause is filled in with additional user data that are generated from the first
partial user-data stream (A) previously received over the first transmission
path,
characterised in that
the additional user data are generated by time-stretching at least part of
the first partial user-data stream (A), with a plurality of new data packets,
each consisting of user data of the first data packets and of other user-
data values generated from the user data of the first data packets, being
generated for the output.
2. A method as claimed in claim 1, wherein during the output after the
reception pause, a user-data stream made up of the additional user data
and the second user-data stream (B) is output for a transitional period.

3. A method as claimed in claim 2, wherein for the generation of the
composite user-data stream the additional user data are weighted with a
first variable factor (C1) and the second user-data stream (B) with a
second variable factor (C2), whereby the sum of the squared first and the
squared second variable factor (C1, C2) is constant.
4. A method as claimed in any one of the preceding claims, wherein after the
complete change of the output to the second partial user-data stream (B),
the second partial user-data stream (B) is time-compressed until the delay
caused by the reception pause is equalised.
5. A method as claimed in any one of the preceding claims, wherein a
digitised audio signal of a voice connection is used as the user-data
stream (A, B).
6. A method as claimed in claim 5, wherein this audio signal is monitored for
speech pauses and the change is carried out in a speech pause.
7. A method as claimed in any one of the preceding claims, wherein a radio
link is used for at least one of the transmission paths.
8. A method as claimed in claim 7, wherein a first radio cell is used as the
first transmission path and a second radio cell of a multi-cell radio network
is used as the second transmission path, with the change being used for a
transfer of a radio link from the first to the second radio cell.

9. A communication terminal (EG) for receiving and emitting a user-data
stream (A, B), wherein the user-data stream (A, B) can be switched over
from a first connection to a second connection, the communication
terminal (EG) is equipped with a generating device (TSh) for generating
additional user data, in that the communication terminal (EG) is provided
with a mixing device (+) for the weighted addition of the additional user
data with the user-data stream (B) received over the second connection,
and in that the generating device (TSh) is configured for the time-
stretching of at least part of the user-data stream (A) received over the
first connection, whereby a plurality of new data packets, each consisting
of user data of the first data packets and of other user-data values
generated from the user data of the first data packets, can be generated
for the output.
10. A communication terminal (EG) as claimed in claim 9, comprising a device
(TSi) for the time-compression of the user-data stream (B) received over
the second connection.
11.A communication terminal (EG) as claimed in any one of the preceding
claims, comprising a detection device (VAD) for detecting speech pauses
in the user-data stream received over the first connection (A) and/or in a
user-data stream that is to be sent, with the detection device (VAD) being
configured to control the time for the switchover by reference to a
detected speech pause.

12.A communication terminal (EG) as claimed in any one of the preceding
claims, comprising a control device (Control) for defining parameters (C1,
C2) which determine the weighting of the additional user data and/or of
the user-data stream (B) received over the second connection during the
addition.

ABSTRACT

TITLE : "A METHOD FOR EMITTING A USER-DATA STREAM WHEN
CHANGING BETWEEN A FIRST AND A SECOND TRANSMISSION PATH.
AND A COMMUNICATION TERMINAL"
The invention relates to a method for emitting a user-data stream when
changing between a first and a second transmission path, wherein a reception
pause for the user-data stream occurs at a receiver during the change between
the end of the reception of a first partial user-data stream (A) with first data
packets of the first transmission path and the beginning of the reception of a
second partial user-data stream (B) with second data packets of the second
transmission path, and wherein the user-data stream (A, B) is output at the
receiver end, wherein, during the output of the user-data stream (A, B), the
reception pause is filled in with additional user data that are generated from the
first partial user-data stream (A) previously received over the first transmission
path, the additional user data are generated by time-stretching at least part of
the first partial user-data stream (A), with a plurality of new data packets, each
consisting of user data of the first data packets and of other user-data values
generated from the user data of the first data packets, being generated for the
output.

A METHOD FOR EMITTING A USER-DATA STREAM WHEN CHANGING BETWEEN A FIRST AND A SECOND TRANSMISSION PATH,AND A COMMUNICATION TERMINAL

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