Circuit switching is a method in which a connection is established between users by providing a predetermined amount of transmission capacity for the connection. Transmission capacity is exclusively in use of the connection in question for the total duration of the connection. Thus prior art mobile communications systems, such as the GSM-based GSM 900/DCS, 1800/PCS and 1900 systems and the U.S. radio system utilizing the CDMA technology are circuit switched systems. Packet switching is a method in which a connection is established between users by transmitting data in packets which in addition to the actual data contain address information and control information. Several connections may use the same transmission connection simultaneously. Use of packet switched radio systems especially for data transmission has been studied during the past years since the packet switching method suits well e.g. for the data transmission which is needed for interactive computer programs and in which the data to be transmitted is generated in bursts. Thus it is not necessary to reserve a data transmission connection for the total duration of transmission, but only for transmitting packets. This allows to save on costs and capacity considerably both when the network is built and used.
Researches on the packet radio networks began at the University of Hawaii in 1968 in connection with the ALOHA project in which remote equipment were connected to a central computer using a radio connection. In the further development of the GSM system, known as GPRS (General Packet Radio Service), special attention has focused on the packet radio networks. Solutions enabling packet transmission are planned in particular for mobile communications systems of the third generation, such as the UMTS (Universal Mobile Telephone System). The GPRS uses either the basic form or more advanced forms of the ARQ protocol which will be described next.
The ARQ protocol (Automatic Repeat Request) refers to a procedure in which retransmission of the information to be transmitted allows to improve the reliability of the data to be transmitted by improving its bit error rate. According to the protocol, the receiver sends a request for retransmitting the transmitted data to the transmitter if the receiver considers the received data unreliable. Unreliability of data can be found out e.g. by checking the checksum of the received packet. Until now the protocol has been used mainly in fixed networks. A major problem associated with radio networks is that the channels used for transmission on radio connections tend to fade. Fading (Rayleigh Fading) means that signal components that have propagated along multiple paths enter a receiver at reverse phases, and thus they partially cancel each other. In this case the power and the quality of the received signal decrease significantly. In addition to the normal background noise, the reception is hindered by interference caused to the radio connection by radio connections that are on the same channel and on the adjacent channels. The influence of interference and fading may be occasionally so harmful that the radio channel fades, i.e. its quality becomes so poor that the information transmitted on the channel cannot be recognized. On the other hand, the fading channel occasionally also has a very good quality.
A more advanced form of the ARQ basic protocol is the hybrid-ARQ which utilizes the combination of the ARQ and the FEC (Forward Error Correction). FEC means that the information to be transmitted is encoded using error correction coding. According to the improved type II hybrid-ARQ protocol, developed from the hybrid-ARQ, the data to be transmitted is coded so that the data is divided into several data blocks, and the data block to be transmitted first includes the data to be transmitted in uncoded form or in slightly encoded form. If the receiver considers the first data block erroneous, the receiver requests transmission of the next data block. In the subsequent data blocks the data to be transmitted is encoded differently than in the first data block. By combining the information of the data blocks the receiver can decode the coding and find the original data. The data to be transmitted can be encoded e.g. using 1/2-convolutional coding, and thus the amount of data doubles. Unfortunately, there are several problems related to the use of this protocol in mobile communications systems. Data blocks are not combined until after decoding, and therefore it is not possible to use the encoding and modulation methods suitable for a non-fading channel. Furthermore, when 1/2-convolutional coding is used, only the first retransmission can improve the possibility of decoding the received packet. If the degree of coding is increased, e.g. by using 1/4-convolutional coding, the probability of having to transmit several data blocks before decoding succeeds also increases.
Applications using packet switching require very low bit error rates, for example even a bit error rate of 10.sup.-9 is required of some data transmission services. Examples of such applications are wireless transmission of measurement data intended for medical purposes and wireless transmission of commands intended for controlling some device. Using the traditional ARQ protocol it is very difficult to achieve the above-mentioned bit error rate. The traditional ARQ protocol also wastes the system capacity since it does not take full advantage of erroneous data blocks. Furthermore, the procedure according to the protocol requires a lot of calculation capacity and thus more expensive equipment, since the error correction code has to be always decoded before it is possible to conclude whether to request transmission of the subsequent data block containing the same data.
When data is transmitted on a radio connection that fades occasionally, the signal quality can be improved by adding interleaving to convolutional coding. Interleaving scatters transmission errors, and thus they can be corrected by means of convolutional coding. When retransmission is used for correcting errors, interleaving periods should be short so that it would be possible to adjust to channel changes quickly and to avoid transmitting a lot of data that was received without errors because of a few erroneous sections. On the other hand, interleaving benefits from long interleaving benefits since the influence of channel conditions averages out.
It is problematic to combine retransmissions and interleaving effectively when the unit of data the retransmission of which is requested is shorter than the interleaving period. At the moment retransmission should be requested it is impossible to know whether it would be possible to correct the error later on by means of deinterleaving and decoding the convolutional coding. If errors are found in the received packet after deinterleaving and decoding of the convolutional coding, all transmission units that belong to the interleaving period have to be retransmitted, since after decoding it is no longer known which transmission units were erroneous.
For example, in a proposal relating to the above-mentioned GPRS, interleaving was performed over four subsequent GSM frames and the return channel for a retransmission request was located in each fifth GSM frame. Due to the problem described in the previous paragraph, the duration of interleaving has been reduced in the GPRS of the GSM from 19 transmission units used in circuit switched data transmission in the GSM to 4 transmission units, which weakens the error-averaging influence of interleaving. Even in this case all four transmission units have to be also retransmitted, even though only one transmission unit had contained errors.
In conclusion, different hybrid-ARQ protocols have been developed in order to solve the above-mentioned problems resulting from occasional fading of a radio connection. The solutions described do not, however, utilize the radio resource to be used effectively. Furthermore, they hinder the use of more efficient modulation and coding methods which would allow to improve the utilization of the available radio resources and the quality of the service provided.