DISCONTINUOUS TRANSMISSION CDMA SYSTEM

Abstract: A method of transmitting from a first node to a second node, said method comprising the steps of transmitting in a first mode at least one data frame from said first node to said second node; and determining if there is no data to be transmitted and if there is no data to be transmitted transmitting at least one empty frame in the first mode; and switching to a second mode in which no transmission is made in the absence of data to be transmitted at least some of the time.

Description: DISCONTINUOUS TRANSMISSION CDMA SYSTEM Field of the invention The present invention relates to a method of transmitting and in particular but not exclusively to a method of transmitting in a wireless telecommunications network. Background of the invention The use of code division multiple access (CDMA) is being proposed for the next generation of cellular telecommunication networks . Additionally, code division multiple access is also being used in the IS-95 Standard in the USA. CDMA is a direct sequence spread spectrum technique. In a wireless cellular network using CDMA, the mobile terminals in one cell associated with a first base station will use the same frequency as mobile stations in an adjacent cell associated with a second base station. The different mobile stations can be distinguished by the respective base stations as each mobile station will be using a different spreading code . In US-A-5101501 a CDMA system is described which uses "soft" handoff. With soft handoff, a mobile station is capable of communicating with more than one base station at the same time. This will typically occur when the mobile station is close to the boundary defined between two cells. The signals sent by the mobile stations will be received and processed by both of the base stations. Likewise, the mobile station will receive the same signal from the two base stations. The signals from the two base stations may be combined. The combined signal may provide better quality -than the any of the individual signals received by the mobile station. Thus, one mobile station can listen to two or more base stations simultaneously. It is therefore important that the information received by the mobile stations from the different base station be identical . If different information is received from the different base stations, the mobile station will have no way of determining which is the correct information and which information can be ignored. Additionally it will not be possible to combine the signals from the different base stations to provide a meaningful combined signal. Generally, different information will be received by the mobile station because an error in transmission has occurred and one of the expected transmissions from one or more of the base stations is not transmitted by the base station. The base station may not receive any information to be transmitted to the mobile station for two reasons. Firstly there has been an error in the information received by the base station. Secondly, there is in fact no information to be received. The lack of information may be due to, for example a pause in conversation or because the transmission is a discontinuous transmission. It is therefore necessary to determine whether the lack of received information is intentional or whether it is due to an error. It has been suggested by the inventor that empty frames of data can be sent to the base station in two different ways. These two ways do not constitute prior art. Firstly, the transmission is suspended. In other words, the base station will be sent nothing. This has the advantage that the processing resources at the base station are saved as well as in other network elements . The load at the interface between the base station and the network controller, which is sending the information to the base station, is reduced. However this method has the disadvantage that it is not possible to distinguish between a transmission error and the fact that there is no data. This leads to two problems. Firstly, failures in the interface between the base station and the network controller are not detected by the base station in the case of information being sent from the network controller to the base station or by the network controller in the case of information being sent from the base station to the network controller . Secondly, if a data frame is not received, due to an error, by a base station, that data frame cannot be transmitted by the base station to the mobile station. If the mobile station is in soft handoff, that is communicating with more than one base station at a time, the mobile station will receive different data streams from the base stations due to the error. This may lead to errors in the mobile station when it attempts to combine the different data streams. The second method for dealing with empty frames of data is for the base station to be sent empty frames of data. This makes it possible to distinguish between errors and the fact that there is no information. However, this method does have the disadvantage that additional processing capacity is required at the base station and other network elements. Additionally, the load at the interface between the base station and the network controller is increased. Similar problems can also occur in the uplink directions. Summary of the invention It is an aim of embodiments of the present invention to provide a method of dealing with the absence of information or data which addresses the disadvantages of the known methods . According to one aspect of the present invention, there is provided a method of transmitting from a first node to a second node, said method comprising the steps of transmitting in a first mode at least one data frame from said first node to said second node; and determining if there is no data to be transmitted and if there is no data to be transmitted transmitting at least one empty frame in the first mode; and switching to a second mode in which no transmission is made in the absence of data to be transmitted at least some of the time. The first and second nodes may be any suitable elements in a telecommunications network. According to a second aspect of the present invention there is provided a method of receiving transmissions from a first node at a second node, said transmission have a first mode in which if there is no data to be transmitted, empty frames are transmitted and a second mode in which if there is no data to be transmitted nothing is transmitted, said method comprising the steps of: receiving transmissions from said first node, wherein if the transmissions have a first mode and a frame is not received and error is determined to have occurred. According to a third aspect of the present invention there is provided a first node for transmitting data to a second node, wherein said first node comprises transmitting means for transmitting data to said second node, said transmitting means have a first mode of operation in which data is transmitted in data frames and if there is no data to be transmitted empty frames are transmitted and a second mode of operation in which if there is no data no frames are transmitted. According to a fourth aspect of the present invention, there is provided a second node for receiving transmissions from a first node at a second node, said transmissions have a first mode in which if there is no data to be transmitted, empty frames are transmitted and a second mode in which if there is no data to be transmitted nothing is transmitted, said second node comprising means for receiving transmissions from said first node, and determining means for determining that if the transmissions have a first mode and a frame is not received, an error has occurred Brief description of the drawings For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: Figure 1 shows a schematic diagram of part of a cellular telecommunications network incorporating base transceiver stations and mobile stations; Figure 2 shows part of the network of Figure 1 in more detail; and Figure 3 illustrates the normal and silent modes used by the base stations of Figure 1. Detailed description of embodiments of the invention Reference will first be made to Figure 1 in which three cells 2 of a cellular telecommunications network are shown. Each cell 2 is served by a respective base transceiver station (BTS) 4. Each base transceiver station 4 is arranged to transmit signals to and receive signals from the mobile stations 6 located in the cell associated with the given base transceiver station 4. Likewise, each mobile station 6 is able to transmit signals to and receive signals from the respective base transceiver station 4. The cellular telecommunications network shown in Figure 1 uses a code division multiple access technique. Accordingly, at least some of the mobile stations will be in communication with more than one base station at the same time. This, however, will be described in more detail hereinafter. Reference is now made to Figure 2. As can be seen from this figure, the base stations are connected to a SRNC (serving radio network controller) 20. Data blocks are transmitted between the SRNC 20 and the base stations 28 and 30 using a frame protocol FP. A data frame contains data which is to be transmitted in one interleaving period and a transport format indicator TFI . The interleaving period is the period over which data is interleaved. The transport format indicator TFI provides information as to the format of the data frame such as the size of the data block, the interleaving period and the like. Each UE (user equipment for example mobile stations or the like) has an independent transport connection which means that each frame contains the data and current transport format indicator TFI of one bearer only. Typically a call will have more than one bearer, for example a signalling bearer and a data or voice bearer. For downlink communications (from the SRNC 20 to the base station direction) , all the data blocks of users having the same user equipment are multiplexed onto a single radio link. The manner in which the multiplexing is done will depend on the transport format indicator TFI of each frame which indicates the amount of data in each frame for the bearers . A transport format combination information TFCI field of the radio frame provided in a dedicated physical control channel DPCCH will indicate the manner in which the multiplexing is carried out. The transport format combination information TFCI field will indicate the transport format indicator TFI for each of the bearers whose data is multiplexed in the radio frame or frames in the dedicated physical data channel DPDCH. In the case of soft handoff where a mobile station is in communication with more than one mobile station at a time, the dedicated physical data channels DPDCH from each of the bases stations communicating with the mobile station in question will be the same as will the rate information RI fields in all the radio links. The format described hereinbefore is part of the currently proposed UMTS (universal mobile telephone service) standard. It should be appreciated that any other standard can alternatively be used. In the following, it is assumed for simplicity that there is only one bearer per transport channel and the SRNC does not carry out any multiplexing. The transport format information can have a particular value which indicates that the frame does not contain any data. For simplicity it is assumed that the particular value is zero although the particular value can be any other value. The SRNC 20 shown in Figure 2 has first and second medium access controllers MAC 24 which receive the data from respective bearers. For simplicity, the medium access controllers 24 are each shown as receiving information from one bearer only. The medium access controllers can receive information from more than one bearer and carry out the multiplexing function described hereinbefore . The medium access controller 24 receives an input from a first bearer Bearer 1 and a second bearer, Bearer 2 and outputs a first transport channel 1 and a second transport channel 2. The input from Bearer 1 maps to the first transport channel 1 and the input from Bearer 2 maps to the second transport channel 2. The transport channels are input to respective macrodiversity combiners 26. The macrodiversity combiners 26 output the transport channels to each base station which is in communication with the mobile station for which the transport channel is intended. In the example shown in Figure 2, the first transport channel 1 is output to first and second base stations 28 and 30 as is the second transport channel 2. Thus the first and second base stations are each arranged to transmit the same channels to a single mobile station. The mode of operation for transmission of information between the SRNC 20 and base stations will now be described with reference to Figure 3. Two modes are defined. The first mode is the normal mode and the second mode is the silent mode. The data which is to be sent to the mobile station is interspersed with periods where there is no data. This may occur during pauses in a conversation between two parties. Alternatively, in the case of non speech data, the data may be discontinuous. In the normal mode A of operation, on the transmit side, the SRNC 20 transmits a frame F for every interleaving period. As shown in Figure 2, frames FI and F2 contain data. If no data is present, frames F3 , F4 and F5 which are empty frames E are sent. After N consecutively received empty frames E, the frame protocol connection is switched to the silent mode S. N can have any suitable value such as 1 , 2 or 3 or any other suitable value. In the silent mode S, on the transmit side, the SRNC 20 is prevented from transmitting any frame if there is no data in the frame . The SRNC may transmit K empty frames F6 at the beginning of the silent mode S to ensure that the base station has switched to the silent mode; also if up to K of the N empty frames previously sent have been lost during the transmission. After M consecutive frames F7 and F8 including data are transmitted, the frame protocol connection is switched to the normal mode A again. K may be 0 or 1 or any other suitable number. M can be 1 , 2, 3 or any other suitable number. In the normal mode A, on the receive side, if a frame is not received in an interleaving period, the base station generates an interface failure alarm and decides whether to transmit the radio frame without the data or to not transmit the frame at all . After the base station has received N empty frames, the frame protocol connection is switched to the silent mode S. In the silent mode S, on the receive side, if a frame is not received in one interleaving period, the base station considers the bearer to be in an inactive state and sets the transport format indicator TFI to zero. After M consecutive data frames with data have been received, the frame control layer connection is switched to the normal mode A. The parameters M, N and K may be: 1. predetermined, for example fixed in a standard used by the network; 2. Iub/Iur (Iub is the interface between the SRNC and the base station while Iur is the interface between two radio network controllers) interface protocol configurable parameters, that is the parameters can be set at each interface as required; and 3. signalled from the SRNC to base stations every time a new branch or bearer is set. The parameters may be reconfigurable as required. Control frames which do not contain data but instead contain control information such as power control and synchronisation information are sent in both the normal mode and the silent mode. Control frames may not be present in either or both of these modes . Reference will now be made to a second embodiment of the present invention which uses explicit signalling in order to switch from the normal mode to the silent mode. In this second embodiment, the base station will not automatically switch to the silent mode when a predetermined number of consecutive empty frames have been received. However, the transmission of a predetermined number of empty frames when in the normal mode can be used as an indication that the silent mode should be used. Of course, any other suitable measure can be used to determine that a switch should be made between the normal and the silent mode. Once a determination has been made that the base station should stop using the normal mode and start using the silent mode, the radio network controller will include in the next frame sent to the base station a mode bit indicative of this. This mode bit may have a given value for the silent mode and the other value for the normal mode. Alternatively, one value of the bit indicates that the mode should change either from the normal to the silent mode or from the silent mode to the normal mode. The other value of the bit would indicate that the mode is not to be changed. When the base station receives the frame transmitted by the radio network controller, it checks the mode bit. If this bit indicates that the mode is to change from the normal mode to the silent mode, the base station changes its mode and sends an acknowledgement confirming that the base station has understood that the mode is changed. Once the radio network controller has received the acknowledgement from the base station, the radio network controller will use the silent mode. Changes from the silent mode to the normal mode can be achieved in the same way as in the first embodiment. However, it is also possible to alter the mode from the silent to normal mode in the following. When it is determined that the base station is to receive using the normal mode, the radio network controller will alter the mode bit in the next frame sent to the base station, to indicate that the mode should change to the normal mode. If a data frame is being sent to the base station, the mode bit will be altered in that frame. If no data frame is being sent to the base station due to an absence of data, the radio network controller will send an empty frame with the mode bit set accordingly. The base station will send an acknowledgement indicating that it has received the frame and has noted that the mode is changed to the normal mode. When the radio network controller receives the acknowledgement, the radio network controller will then use the normal mode . It is possible to use this second method to notify the base station by sending in advance information that the mode will change after a predetermined number of frames or the like. The method used by the first embodiment has the advantage that the frame structure does not need to be altered. A second advantage of the first method is that two way communication does not have to be set up. if one party, for example, the radio network controller is sending data to the base station and the base station is not sending any data to the radio network controller. The second method is advantageous in that both parties are certain as to which mode of operation is being used. The acknowledgement sent by the base station to the radio network controller may use a mode bit in the frame, as described hereinbefore in relation to the frames sent from the radio network controller to the base station. Thus the acknowledgement may comprise the mode bit in the frame sent by the base station to the radio network changing to reflect the new mode. Either of the options for the mode bit described hereinbefore may be used. It should be appreciated that although the transmissions from the SRNC to the base station are described as using the normal and silent mode of operation, this mode of operation can alternatively or additionally be used for transmissions from the base station to the SRΝC. The values of the parameters used in the downlink and uplink directions may differ. Embodiments of the present invention may be applied to the communications between any two nodes of a telecommunications network, wired or wireless. It should be appreciated that although embodiments of the present invention are particularly advantageous when used in a soft handoff or similar context, that is when a mobile station is in communication with more than one base station at the same time, embodiments of the present invention can also advantageously be used where the mobile station is in communication with one base station at a time. In an alternative embodiment of the present invention, the normal and silent mode of operation may only be used when the mobile station is in communication with more than one base station at a time. A different mode of operation may be used when the mobile station is in communication with only one base station. The different mode of operation may be one which incorporates one of the previously known methods for dealing with empty frames. Whilst preferred embodiments have been described in the context of a code division multiple access system, embodiments of the present invention can be used with any other spread spectrum technique or any other suitable access technique such as time division multiple access, frequency division multiple and space division multiple access as well as hybrids thereof.

Claims: CLAIMS ; 1. A method of transmitting from a first node to a second node, said method comprising the steps of: transmitting in a first mode at least one data frame from said first node to said second node; and determining if there is no data to be transmitted and if there is no data to be transmitted transmitting at least one empty frame in the first mode; and switching to a second mode in which no transmission is made in the absence of data to be transmitted at least some of the time . 2. A method as claimed in claim 1, wherein said second mode is used if a predetermined number of empty frames are transmitted consecutively. 3. A method as claimed in claim 2, wherein the first station, if using the first mode, switches automatically to the second mode if the predetermined number of empty frames are transmitted consecutively. 4. A method as claimed in claim 1 or 2 , comprising the step of the first node sending information to the second node advising that the mode is to change from the first mode to the second mode. 5. A method as claimed in claim 4, wherein the second node is arranged to confirm receipt of said information and when the first node has received the confirmation from the second node, said first node is arranged to use said second mode. 6. A method as claimed in claim 4 or 5 , wherein said information is in a frame transmitted from the first node to the second node . 7. A method as claimed in any of claims 4 to 6 , wherein said information is sent in advance of the change of mode. 8. A method as claimed in any preceding claim, wherein in said second mode, data to be transmitted is transmitted in data frames and if a predetermined number of data frames are transmitted, the first mode is used. 9. A method as claimed in claim 8, wherein if the predetermined number of data frames is received in said second mode, the first mode is automatically used. 10. A method as claimed in any of claims 1 to 8 , wherein when the first node is using the second mode and the mode is to be switched to the first mode, the first node is arranged to transmit information that the mode is to be switched to said second mode . 11. A method as claimed in claim 10, wherein the second node is arranged to confirm receipt of said information that the mode is to be switched from said second mode to the first mode and the first node is arranged to use the first mode when the confirmation has been received by the first node. 12. A method as claimed in claim 2 or any claim appended thereto, wherein said predetermined number of data frames are consecutive data frames . 13. A method as claimed in any one of the preceding claims wherein if no data is to be transmitted at the beginning of the second mode, a predetermined number of empty frames are first transmitted and then no transmission is made in the absence of data to be transmitted. 14. A method as claimed in any one of the preceding claims, wherein said first node is arranged to transmit the same data to a plurality of second nodes, said plurality of second nodes transmitting the same data to a third node. 15. A method of receiving transmissions from a first node at a second node, said transmission have a first mode in which if there is no data to be transmitted, empty frames are transmitted and a second mode in which if there is no data to be transmitted nothing is transmitted, said method comprising the steps of: receiving transmissions from said first node, wherein if the transmissions have a first mode and a frame is not received and error is determined to have occurred. 16. A method as claimed in claim 15, wherein if an error is determined to have occurred, said second node transmits an empty frame to a third node. 17. A method as claimed in claim 15, wherein if an error is determined to have occurred, said second node transmits nothing to a third node. 18. A method as claimed in any of claims 15 to 17, wherein after a predetermined number of empty frames are received in said first mode, determining that the second mode is used by the first node. 19. A method as claimed in claim any of claims 6 to 9, wherein when a predetermined number of frames of data are received in said second mode, determining that the first mode is used by the first node. 20. A method as claimed in any of claims 15 to 20, wherein a change in operation of said first node is determined from information received by the second node from the first node. 21. A method as claimed in claim 20, wherein said information is in at least one frame transmitted from the first node to the second node. 22. A method as claimed in claim 16 or any claim appended thereto, wherein said third node is a mobile station. 23. A method as claimed in any preceding claim, wherein said first node is a base station or a radio network controller in a wireless cellular telecommunications network. 24. A method as claimed in any preceding claim, wherein said second node is a base station or a radio network controller in a wireless cellular telecommunications network. 25. A first node for transmitting data to a second node, wherein said first node comprises transmitting means for transmitting data to said second node, said transmitting means have a first mode of operation in which data is transmitted in data frames and if there is no data to be transmitted empty frames are transmitted and a second mode of operation in which if there is no data no frames are transmitted at least some of the time. 26. A second node for receiving transmissions from a first node at a second node, said transmissions have a first mode in which if there is no data to be transmitted, empty frames are transmitted and a second mode in which if there is no data to be transmitted nothing is transmitted, said second node comprising means for receiving transmissions from said first node, and determining means for determining that if the transmissions have a first mode and a frame is not received, an error has occurred.

Inventors: AHMAVAARA KALLE, LONGONI FABIO, AHMAVAARA, KALLE, LONGONI, FABIO
Applicants: AHMAVAARA KALLE, LONGONI FABIO, NOKIA NETWORKS OY, AHMAVAARA, KALLE, LONGONI, FABIO
Application year: 2000
Main classification: H04B7
IPC codes: H04B7