The present invention relates to a method for channel allocation in a radio communications system in which data is transmitted as packets in accordance with a protocol for the automatic re-transmission of erroneously transmitted data.  More specifically, the invention relates to a method for channel allocation in the transmission of erroneously transmitted data.  Packet transmission of data over radio channels is applied in a GSM system (GSM=Global System for Mobile communication) for instance.  The radio channels may be frequency division multiple access channels (FDMA), time division multiple access channels (TDMA) or code division multiple access channels (CDMA). 
The invention also relates to an arrangement and to a base station controller for carrying out the method. 
In a mobile radio communications system for data package transmission, a base station is able to communicate with one or more mobile stations through the medium of one or more time-divided channels between the base station and the mobile station.  A time-divided channel is divided into time slots.  A data burst comprising a plurality of information bits can be transmitted in each time slot.  Data that is transmitted in a packet switching radio communications system is divided into one or more packets, which in turn comprise one or more blocks.  Depending on the application and system concerned, the blocks may be the smallest data unit that are transmitted via the radio interface. 
In data transmission, where, in contradistinction to speech transmission, no errors can be tolerated in the transmitted messages there can be used a protocol for the automatic re-transmission of erroneously transmitted data.  The most common solution involves the use of an ARQ protocol (ARQ=Automatic Repeat reQuest).  This protocol implicates a return channel on which information concerning the status of a transmitted message can be transmitted.  The receiver discloses via the return channel whether or not a given message has been transmitted correctly.  When certain blocks in a packet have been transmitted erroneously, so-called selective ARQ enables solely the erroneous blocks to be retransmitted without needing to retransmit the remaining blocks in the packet. 
Increasing requirements on high bit rates and short delays have resulted in a greater need for communications systems of large bandwidth.  This need can either be satisfied with one single channel of very large bandwidth or by combining a plurality of narrowband channels such that the channels together provide the desired bit rate and delay.  One example of this latter solution is the general packet radio service (GPRS) which ETSI SMG (ETSI=European Technical Standards Institute; SMG=Special Mobile Group) is in the process of specifying as a part of GSM phase 2+.  Those channels that are used for data transmission within such a radio communications system, e.g. the cellular GSM system, will very probably have highly varying qualities. 
GB-A-2 279 205 teaches a packet data transmission radio system in which a mobile terminal monitors a parameter which discloses an anticipated communications quality for each channel.  The parameter is based on statistical measurements of the signal strength of the desired signal C in relation, C/I, to the signal strength of an interfering signal, I, preferably in those time slots in which data is transmitted.  The parameter is utilized when a mobile terminal initially informs a base station of those time slots in which the mobile terminal wishes to communicate data, when establishing a radio connection with said base station.  The base station then reserves these time slots for communication with the mobile terminal, provided that the desired time slots are available at that time. 
Patent Specification WO-A1-93/14579 discloses an algorithm according to which channels are allocated in a radio communications system.  The algorithm utilizes earlier registered events on the channels concerned for generating a list in which the channels are ranked in a descending order of quality.  When allocating a new channel for communication between a base station and a mobile station, the base station chooses the top available channel on the list.  Examples of events that are recorded during a given time period are the number of interrupted calls, the number of completed calls and the number of blocked requests for a call setup.  When allocating channels, the algorithm also takes into account whether or not a certain channel is heavily loaded locally. 
The present invention provides a solution to those problems caused by the aforesaid greatly varying channel qualities, and also constitutes an improvement in relation to the aforesaid known techniques.  In a radio communications system for transmitting data between two stations that communicate data over two or more channels in accordance with a protocol for the automatic re-transmission of erroneously transmitted data, the invention attacks the problem of allocating the most effective channels for the automatic re-transmission of erroneously transmitted data, in other words those channels on which there is the greatest probability of re-transmitting the data correctly. 
The channels utilized in the radio communications system may be frequency divided, such as in an FDMA system (FDMA=Frequency Division Multiple Access), for instance NMT (NMT=Nordic Mobile Telephone), or may be time divided, such as in pure TDMA (TDMA=Time Division Multiple Access).  One example of combined TDMA and FDMA is GSM, in which a given channel is characterized by a specific time slot on a separate carrier frequency.  The channels may also be separated with the aid of a spread code sprectrum which is unique for each channel, such as in a CDMA system (CDMA=Code Division Multiple Access), for instance IS-95. 
According to GB-A-2 279 205, the mobile terminal states in its access request to the base station those channels that are preferred by the mobile terminal.  The present-day TDMA system is unable to handle an access request of the length that this would require.  For instance, the access request in GSM is comprised solely of eight bits.  Thus, in order to apply the solution proposed by GB-A-2 279 205 in a GSM system, it would be necessary to lengthen the access request, which would, in turn, result in greater delays.  According to GB-A-2 279 205, the mobile terminal determines the quality of solely the downlink of the channel, i.e. when data is transmitted from the base station to the mobile terminal.  Consequently, the measurements do not provide sufficient basis on which the channel quality on the uplink can be estimated, i.e. when data is transmitted from the mobile terminal to the base station. 
The algorithm described in WO-A1-93/14579 is based on events recorded during a given time period and provides a mean value quality measurement.  In the transmission of data when relatively large volumes of information are transmitted in a relatively short time, it is essential to choose precisely that channel or that set of channels which will give the highest transmission quality at that moment in time.  Because the algorithm described in WO-A1-93/14579 gives a mean value of the historic quality of the radio channels, the algorithm does not provide a suitable solution for allocating channels for the re-transmission of data which were erroneously transmitted in a previous data transmission. 
Accordingly, one object of the present invention is to provide methods and arrangements for finding that channel or that set of channels which will, at that moment in time, provide the highest transmission quality in the re-transmission of erroneously transmitted data. 
This object is achieved in accordance with the proposed method, by selecting a transmission parameter with each transmission.  The transmission parameter is derived with the aid of information relating to the channel used for transmitting each given data.  At least one of the preceding channels for earlier transmitted data is used in the re-transmission of data. 
The proposed arrangement creates a transmission parameter in a control unit for each transmission, this parameter being derived with the aid of information relating to the channel used for transmitting each given data.  The channel allocating means in the control unit allocates at least one of the preceding channels used for earlier transmitted data for the re-transmission of erroneously transmitted data, in accordance with the transmission parameter. 
In the case of erroneously transmitted data, channels are allocated for re-transmission of data in accordance with the transmission parameter that has been created in a previous transmission.  According to one preferred embodiment of the first method according to the invention, the transmission parameter discloses those channels whose quality has exceeded a predetermined limit value in the transmission of data between a primary station and a specific secondary station. 
According to one advantageous embodiment, the aforementioned previous transmission may be the immediately preceding transmission of data between the primary station and the secondary station. 
According to another preferred embodiment, data is re-transmitted primarily on those channels whose transmission quality has exceeded a predetermined value.  This value can be given as the highest number of errors, n.sub. F, that may be accepted on a given channel in order for this channel to be allocated for the possible re-transmission of erroneously transmitted data.  When all of the previously used channels have transmitted data containing more errors than n.sub. F, at most one of these channels is allocated for re-transmission of the data.  The channel that has transmitted data with the lowest number of errors is preferably allocated.  In addition, at least one further channel that has not been used in an earlier transmission is allocated, provided that such a channel is available. 
According to an alternative embodiment, there can be calculated for each channel used in a previous transmission a quality measurement, Q, calculated in accordance with ##EQU1## where n.sub.tot represents the total number of blocks that have been transmitted on the channel, and where n.sub. Nack denotes the number of erroneously transmitted blocks on the channel.  When retransmitting erroneous data, there is allocated at most one of the previous channels whose quality measurements Q are below a predetermined quality limit Q.sub.1.  If the Q value of all the channels used in a previous transmission are below Q, there is allocated for re-transmission at least one further channel which has not been used in a previous transmission, provided that such a channel is available. 
The invention also relates to a base station switching centre in a radio communications system, which utilizes the method and the arrangement according to the invention.  Data is transmitted between a primary and a secondary station in the radio communications system via two or more channels and the data is transmitted in accordance with a protocol for the automatic re-transmission of erroneously transmitted data. 
One embodiment of the inventive arrangement presumes that each data message is divided into one or more packets, each of which includes one or more blocks.  In this case, the re-transmission of erroneously transmitted data is effected in each block, so as to avoid those channels on which an excessive number of blocks or an excessively high percentage of blocks have been transmitted erroneously. 
According to a further embodiment of the inventive arrangement, the arrangement includes a control unit in which the transmission parameter is created.  The control unit includes channel allocating means in the form of a processor and a memory unit.  The processor is used when creating the transmission parameter, this parameter then being stored in the memory unit at least until a receipt acknowledgement has been received to the effect that the whole packet in which the relevant blocks are included has been transmitted correctly. 
By re-transmitting erroneously transmitted data solely on those channels of good quality, it is possible to reduce the total data message transmission time and to therewith enhance the capacity of the data transmission system .