1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method of transmitting or receiving a data packet in packet data communication system using a hybrid automatic repeat request.
2. Discussion of the Related Art
Generally, a hybrid automatic repeat request (HARQ) is used for transmission of packet data in a mobile communication system.
That is, the object of the HARQ is to improve the reliability and throughput during data transmission in the communication system by combining an automatic repeat request (ARQ) with a forward error correction (FEC).
The object of the ARQ is to improve the reliability in a manner that a receiving end requests repeat of the same information until the initially sent information is received in the receiving end without error. The object of the FEC is for the receiving end to compensate for the error occurring due to the channel environment using an error correction code.
If the channel environment is good and the error occurring frequency of the given information is low, only the use of the ARQ will suffice for the reliability.
However, if the channel environment gets worse, the error occurring frequency of the given information becomes greater, and thus the frequency of repeat request also becomes greater.
This deteriorates the throughput of the system. Thus, it has been proposed to use both the ARQ and the FEC, which is called the HARQ.
As a kind of the HARQ, there is a method that uses an incremental redundancy.
According to this method, a transmitting end first transmits information encoded with a high coding rate to a receiving end, encodes the information with the coding rate lowered whenever the repeat request is received from the receiving end, and then transmits only the additional redundancy bits to the receiving end. The receiving end combines the received redundancy bits with the previously sent information to decode the information.
These additional redundancy bits are bits added for error correction or detection of the previously transmitted data packet.
To do so, the receiving end can obtain the gain according to the combining. Also, since the combining coding rate of the packet data is gradually reduced whenever the repeat is performed, the transmitting end can adaptively give the redundancy to the data packet according to the change of the channel environment.
Currently, in a high data rate (HDR) mobile communication system, the incremental redundancy of the HARQ, particularly a synchronous incremental redundancy (SIR) is used.
According to the SIR method, as shown in FIG. 1A, information to be transmitted is encoded and repeated to construct one packet, and then the data packet is divided into sub packets of a fixed size for transmission.
Specifically, the transmitting end transmits a sub packet among sub packets that constitute a data packet. If the receiving end performs decoding of the transmitted sub packet without error and transmits an acknowledgment (ACK) to the transmitting end, the transmitting end transmits no further sub packet.
If the receiving end fails to perform the decoding of the transmitted sub packet and transmits a non-acknowledgment (NACK) to the transmitting end, the transmitting end transmits the next sub packet.
Meanwhile, in order to heighten the transmission rate in the changing channel environment, it is advantageous to change the length of the respective sub packets according to the changing channel environment.
Accordingly, in order to implement this, a method that uses a variable sub packet length has been proposed as shown in FIG. 2A.
During the process of dividing a data packet into several sub packets and transmitting the sub packets, this method enables the transmission of the respective sub packets to be performed with different sub packet lengths according to the channel environment.
At this time, there is provided a control channel for notifying information on the length of the sub packet, sub packet identifier (ID), etc.
If the transmitting end transmits the packet data to the receiving end through the above-described process, the receiving end extracts information on the sub packet through the control channel to receive the sub packet.
Then, the receiving end performs the decoding of the data packet by concatenating/combining the received sub packet with the data packet previously sent from the transmitting end using the extracted information.
In case of using the sub packets of the fixed length as shown in FIG. 1A, the length of all the sub packets is fixed, and thus the receiving end can grasp the transmission start point of the currently received sub packet if it knows the order of the sub packets even when it cannot receive any sub packet. Thus, it can perform the decoding by regenerating the packet with other sub packets (or even with only one sub packet) as shown in FIG. 1B.
Thus, if there is information on the order of the sub packet currently transmitted, the receiving end can recognize the transmission start point of the sub packet. In case of using the SIR method in the HDR system, the receiving end can always recognize information on when its own sub packet is received after the first sub packet is received, and thus does not require other control information in the event that the received sub packet is not the fifth sub packet. However, in case of using an asynchronous incremental redundancy, the transmission time of the next sub packet may randomly change even if the transmission time of the first sub packet is determined, and thus all the sub packets should be transmitted to the receiving end along with the information on the sub packet ID and the sub packet length that is the control information on the sub packet. In case that the length of the respective sub packet is fixed, the receiving end can recognize the transmission start point of the respective sub packet even if the transmitting end just transmits the sub packet IDs in order.
However, according to the method that uses the sub packets of the variable length as shown in FIG. 2A, the length of the sub packet is changed whenever the sub packet is transmitted, and thus if the control channel that includes the sub packet information is not received, the start point of the data packet cannot be recognized only through the respective sub packet ID.
That is, it cannot be recognized where the currently received sub packet starts. For this reason the variable length type method cannot decode the data packet even if only one sub packet is missing.
Specifically, if a sub packet 2 is not received and a sub packet 3 is received, there is no information on the previous sub packet 2 during the process of concatenating the sub packet 1 and the sub packet 3, and thus it cannot be recognized of which type between two types as shown in FIGS. 2B and 2C the sub packet 3 is received.
In the sequential transmitting method wherein the transmission of the next sub packet starts at a position where the transmission of the previous sub packet ends, if the receiving end misses even one sub packet, it cannot recognize the transmission position of the incoming sub packet after the missing sub packet, and thus the decoding cannot be performed.
To solve the above-described problem, a method is used as follows. That is, in transmitting variable sub packets, each sub packet is transmitted from the fixed start position in the encoded packet to the receiving end, and the information on the length and the starting position of the sub packet is transmitted to the receiving end through the control channel. The receiving end extracts the information on the sub packet through the control channel, and performs the decoding by concatenating/combining the received sub packet with the previously received sub packet using the extracted information.
However, in case of using this method, a certain portion in the encoded packet may be repeatedly transmitted, while another portion may not be transmitted, resulting in that the coding gain according to the HARQ cannot be obtained sufficiently.