DISTRIBUTED TRANSMITTING APPARATUS AND METHOD WITH RETRANSMISSION FUNCTION

Abstract: The present invention discloses a distributed transmitting apparatus and method in a communications system in which retransmission of data frames can be requested by a receiver MS. According to the present invention, a transmitting unit CCU in the distributed transmitting apparatus transmits data frames to a receiver MS and stores the transmitted data frames. The apparatus further comprises a control unit PCU which transmits data frames to the transmitting unit CCU for transmission to the receiver MS, and further transmits control data to the transmitting unit CCU for controlling retransmission of data frames, wherein the control unit PCU controls the transmitting unit CCU to retransmit data frames stored in the transmitting unit CCU to the receiver MS.

Description: TITLE OF THE INVENTION Distributed transmitting apparatus and method with retransmission function. FIELD OF THE INVENTION The present invention relates to a distributed transmitting apparatus and method in a communications system in which retransmission of data frames can be requested. BACKGROUND OF THE INVENTION In communications where retransmission (s) can be requested when a data frame is received erroneously, an Automatic Repeat reQuest (ARQ) protocol for controlling the retransmission (s) may be used. In a communications system employing the ARQ protocol, a transmitter transmits data frames to a receiver and receives acknowledgment frames from the receiver. In the following, the communications channel between the transmitter and the receiver is referred to as primary link. The transmitter is divided in two parts: a Protocol Control Unit (PCU) and a Channel Coding Unit (CCU) . The PCU contains an implementation of the ARQ protocol used over the primary link. The CCU includes lower layer functions such as channel coding and modulation for the primary link. The services of the CCU are used by the PCU. The PCU and the CCU may be placed in different locations. In the following, the communications channel between the PCU and the CCU is referred to as the secondary link. This kind of situation may for example exist in a cellular GPRS (General Packet Radio Service) network in the communication between a Mobile Station (MS) and a Base Transceiver Station (BTS) which is controlled by a Base Station Controller (BSC) as it is shown in Fig. 1. The functions of the CCU which is a Channel Codec Unit according to GPRS are included in the BTS and the functions of the PCU which is a Packet Control Unit according to GPRS are included in the BSC. Alternatively, in some implementations the PCU is not located in the BSC, but may be located in the BTS or the GSN (GPRS support node) . In the following, it is assumed that the PCU is located in the BSC. The ARQ protocol is used in a radio interface which forms the primary link between the mobile station MS and the GPRS network via the BTS and BSC. The secondary link is formed by an Abis interface between the BSC and the BTS. According to a conventional communications method using ARQ, the PCU stores all the data frames transmitted from the GPRS network until they are no longer needed. The exact time when transmitted data frames are no longer needed depends on the acknowledgment frames received from the mobile station MS for the transmitted data frames and on the ARQ protocol used. When a data frame is to be transmitted, the PCU sends it to the CCU through the Abis interface, and the CCU transmits the data frame to the mobile station MS through the radio interface. Retransmissions have to be done in exactly the same way. Thus, the same data frame may have to be sent over the Abis interface multiple times, thereby consuming communications recources. SUMMARY OF THE INVENTION It is therefore an object of the present invention to improve retransmission of data frames in a distributed transmitting apparatus which is a transmitting apparatus consisting of separate CCU and PCU units. According to an aspect of the present invention, this object is achieved by a distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver MS, the apparatus comprising: a transmitting unit CCU for transmitting data frames to a receiver MS and for storing the transmitted data frames; and a control unit PCU for transmitting data frames to the transmitting unit CCU for transmission to the receiver MS, and for transmitting control data to the transmitting unit CCU for controlling retransmission of data frames, wherein the control unit PCU controls the transmitting unit CCU to retransmit data frames stored in the transmitting unit CCU to the receiver MS. According to another aspect of the present invention, the above-mentioned object is achieved by a transmitting method in a distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver MS, the method comprising the steps of: transmitting data frames from a control unit PCU to a transmitting unit CCU for transmission to a receiver MS; transmitting data frames from the transmitting unit CCU to the receiver MS; storing the transmitted data frames in the transmitting unit CCU; transmitting control data from the control unit PCU to the transmitting unit CCU for controlling retransmission of data frames, wherein the transmitting unit CCU is controlled by the control unit PCU to retransmit data frames stored in the transmitting unit CCU to the receiver MS. Moreover, the above-mentioned object is achieved by a transmitting unit CCU in a distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver MS, the distributed transmitting apparatus comprising the transmitting unit CCU and a control unit PCU, wherein the transmitting unit CCU receives data frames from the control unit PCU for transmission to a receiver MS, transmits data frames to the receiver MS and stores the transmitted data frames; and wherein the transmitting unit CCU receives control data from the control unit PCU for controlling retransmission of data frames, and retransmits data frames stored to the receiver MS upon retransmission commands from the control unit PCU. Furthermore, the object is achieved by a control unit PCU in a distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver MS, the distributed transmitting apparatus comprising the control unit PCU and a transmitting unit CCU, wherein the control unit PCU transmits data frames to the transmitting unit CCU for transmission to a receiver MS, and transmits control data to the transmitting unit CCU for controlling retransmission of data frames; and wherein the control unit PCU controls the transmitting unit CCU to retransmit data frames stored in the transmitting unit CCU to the receiver MS. In case the receiver MS has not correctly received a data frame, the control unit PCU receives a retransmission request from the transmitting unit CCU, which retransmission request is sent to the transmission unit CCU by the receiver MS. According to an embodiment of the present invention, the control unit PCU adds an identification number to each data frame which is transmitted from the control unit PCU to the transmitting unit CCU and commands a retransmission of a data frame using the identification number. That is, the control unit PCU controls retransmission by transmitting the identification number of the previously transmitted requested data frame to the transmitting unit CCU so as to cause the transmitting unit CCU to retransmit the requested data frame which has been stored in the transmitting unit CCU at the time of first transmission. Alternatively, the control unit PCU defines a numbering sequence for the data frames transmitted from the control unit PCU to the transmitting unit CCU and commands a retransmission of a data frame on the basis of the numbering sequence. To be precise, numbers of data frames are not transmitted, but they are based for example on counters provided in both the control unit PCU and the transmitting unit CCU. Each counter is incremented when a data frame is transmitted from the control unit PCU to the transmitting unit CCU. Retransmission is controlled by transmitting the number of the requested data frame or the difference between the current value of the counter in the control unit PCU and the number of the requested data frame from the control unit PCU to the transmitting unit CCU, so as to cause the transmitting unit CCU to retransmit the requested data frame to the receiver MS. Moreover, according to the present invention, the control unit is placed remotely to the transmitting unit. The present invention is applicable to different ARQ protocols used in communications where retransmission (s) are requested by a receiver when a data frame is received errorneously . For example, the present invention can be applied to a stop-and-wait ARQ protocol, go-back-N ARQ protocol, selective-reject ARQ protocol and hybrid ARQ protocol . In the present invention, the transmitted packets are stored in a memory of the transmitting unit CCU instead of a memory of the control unit PCU. Therefore, the PCU does not need to transmit the packets to the CCU each time a retransmission is needed, but rather commands the CCU to transmit the packet from the memory of the CCU to the receiver. Hence, the PCU still remains in full control of the ARQ protocol. An advantage of the present invention is that the need for communication between the PCU and the CCU is reduced, i.e. the capacity of the secondary link is not wasted on retransmissions. In the case of the cellular network shown in Fig. 1, the required Abis capacity is reduced. Moreover, the present invention is advantageous in EGPRS (Enhanced General Packet Radio Service) where the use of the IR (Incremental Redundancy) mode causes a high number of retransmissions and where the amount of data in each frame is large. In the following the present invention will be described by way of preferred embodiments thereof with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic block diagram of components of a cellular communications network, which are relevant for the present invention. Fig. 2 shows a flowchart of a retransmission method according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the GPRS network as shown in Fig. 1, the Packet Control Unit PCU is located in the Base Station Controller BSC and the Channel Codec Unit CCU is located in the Base Transceiver Station BTS, forming a distributed transmitter. The PCU communicates with the CCU via the Abis interface. The transmission of data frames from the GPRS network to the mobile station MS according to the present invention will be described in the following with reference to Fig. 2. It is to be noted that in the description of the present invention, the unit of transmission is called a frame. However, the term frame should be understood to include also the meaning of the words packet and block. When a data frame is to be transmitted for the first time from the network to the MS, the PCU sends it to the CCU for transmission (step SI) . Then, the CCU transmits the data frame to the MS via the radio interface and stores the data frame in a memory provided in the CCU (step S2) . In case of a retransmission requested by the MS from the PCU via the BTS according to the ARQ protocol used, the PCU identifies the data frame whose retransmission has been requested (step S4). After having identified the requested data frame, the PCU does not resend the entire frame via the Abis interface, but only a signaling element that can be used by the CCU to identify the requested data frame (step S5) . The CCU then locates the requested data frame in its memory and transmits it via the primary link to the MS (step S6) . The PCU remains in full control of the ARQ protocol. Next, it will be described how the PCU signals the CCU which frame is to be retransmitted. According to an embodiment of the present invention, the PCU adds an identification number to each frame transmitted from the PCU to the CCU. When a retransmission is requested by the MS, the PCU sends the identification number of the previously transmitted requested data frame to the CCU. The CCU identifies the requested data frame in its memory using the identification number and retransmits the data frame to the MS. According to another embodiment of the present invention, the PCU defines a numbering sequence for the data frames sent to the CCU for transmission. The numbers are not transmitted to the CCU, but are based for example on counters in both the PCU and the CCU. Each counter is incremented when a data frame is sent from the PCU to the CCU via the Abis interface. In case a retransmission is requested by the MS, the PCU sends the number of the requested data frame or the difference between the current value of the counters and the number of the requested data frame to the CCU. The CCU identifies the requested data frame in its memory on the basis of the number sent from the PCU and retransmits the requested data frame to the MS. The CCU stores the transmitted data frames until the PCU instructs the CCU to erase them. The time of erasure depends on the acknowledgment frames received by the PCU from the MS and the ARQ protocol used. Moreover, according to an alternative way of erasing the data frames in the CCU, the data frames are kept in the CCU for a certain period of time or, alternatively, the number of frames in the CCU memory is fixed and old frames are replaced by new frames. In either way, no frame erasure signaling is needed between the PCU and the CCU because the PCU is able to calculate whether a certain frame is still in the CCU memory. If the PCU determines that a frame is not in the memory, then it will fall back into the conventional method, i.e. the PCU will retransmit the entire frame. The present invention also is useful in EGPRS, especially in case the PCU is placed remote to the BTS. In EGPRS, the use of the IR (Incremental Redundancy) mode causes a high number of retransmissions and the amount of data in each frame is large. In applying the present invention, the need for communicating entire data frames from the PCU to the CCU or BTS is reduced and the Abis interface capacity can be saved, for example for other users. While the invention has been described with reference to preferred embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims .

Claims: CLAIMS : 1. A distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver (MS) , said apparatus comprising: a transmitting unit (CCU) for transmitting data frames to a receiver (MS) and for storing the transmitted data frames; and a control unit (PCU) for transmitting data frames to said transmitting unit (CCU) for transmission to the receiver (MS) , and for transmitting control data to said transmitting unit (CCU) for controlling retransmission of data frames, wherein said control unit (PCU) controls said transmitting unit (CCU) to retransmit data frames stored in said transmitting unit (CCU) to the receiver (MS) . 2. The apparatus according to claim 1, wherein, when the receiver (MS) has not correctly received a data frame, said control unit (PCU) receives a retransmission request from said transmitting unit (CCU) , which retransmission request is sent to said transmission unit (CCU) by the receiver (MS) . 3. The apparatus according to claim 2, wherein said control unit (PCU) adds an identification number to each data frame transmitted from said control unit to said transmitting unit and commands a retransmission of a data frame using the identification number. 4. The apparatus according to claim 2, wherein said control unit (PCU) defines a numbering sequence for the data frames transmitted from said control unit to said transmitting unit and commands a retransmission of a data frame on the basis of the numbering sequence. 5. The apparatus according to claim 1, wherein said control unit is placed remotely to said transmitting unit. 6. A transmitting method in a distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver (MS) , said method comprising the steps of: transmitting (SI) data frames from a control unit (PCU) to a transmitting unit (CCU) for transmission to a receiver (MS) ; transmitting (S2) data frames from said transmitting unit (CCU) to the receiver (MS); storing (S2) the transmitted data frames in said transmitting unit (CCU) ; transmitting (S5) control data from said control unit (PCU) to said transmitting unit (CCU) for controlling retransmission of data frames, wherein said transmitting unit (CCU) is controlled by said control unit (PCU) to retransmit data frames stored in said transmitting unit (CCU) to the receiver (MS) . 7. The method according to claim 6, wherein, when the receiver (MS) has not correctly received a data frame, said transmitting unit (CCU) sends a retransmission request to said control unit (PCU) , which retransmission request is sent to said transmission unit (CCU) by the receiver (MS) . 8. The method according to claim 7, further comprising the steps of: adding an identification number to each data frame transmitted from said control unit to said transmitting unit; and commanding a retransmission of a data frame using the identification number. 9. The method according to claim 7, further comprising the steps of: defining a numbering sequence for the data frames transmitted from said control unit to said transmitting unit; and commanding a retransmission of a data frame on the basis of the numbering sequence. 10. The method according to claim 6, wherein said control unit is placed remotely to said transmitting unit. 11. A transmitting unit (CCU) in a distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver (MS) , the distributed transmitting apparatus comprising said transmitting unit (CCU) and a control unit (PCU) , wherein said transmitting unit (CCU) receives data frames from the control unit (PCU) for transmission to a receiver (MS), transmits data frames to the receiver (MS) and stores the transmitted data frames; and wherein said transmitting unit (CCU) receives control data from the control unit (PCU) for controlling retransmission of data frames, and retransmits data frames stored to the receiver (MS) upon retransmission commands from the control unit (PCU) . 12. A control unit (PCU) in a distributed transmitting apparatus in a communications system in which retransmission of data frames can be requested by a receiver (MS) , the distributed transmitting apparatus comprising said control unit (PCU) and a transmitting unit (CCU) , wherein said control unit (PCU) transmits data frames to the transmitting unit (CCU) for transmission to a receiver (MS) , and transmits control data to the transmitting unit (CCU) for controlling retransmission of data frames; and wherein said control unit (PCU) controls the transmitting unit (CCU) to retransmit data frames stored in the transmitting unit (CCU) to the receiver (MS) .

Inventors: SIPOLA JUSSI, SIPOLA, JUSSI
Applicants: NOKIA NETWORKS OY, SIPOLA JUSSI, SIPOLA, JUSSI
Application year: 1999
Main classification: H04L1
IPC codes: H04B7, H04L1, H04L12, H04L29, H04W88