1. Field of the Invention
The present invention relates to a wireless communication apparatus, and more particularly, to a wireless communication apparatus to implement the selective repeat scheme for ARQ.
2. Description of the Prior Art
In wireless communication system, the packets transmitting in the communication channels may have errors because of bad communication environments. The communication channel is influenced by noise, channel change, multiple path decay, or channel coupling so as to cause the packets transmitting errors. In most communication system, such as wireless communication, the technique that allows erroneous data to be retransmitted is used to increase the reliability of the data transmission. Automatic Repeat Request (ARQ) is generally used in wireless communication to ensure the protocol data units (PDUs) transmitting from a sender station to a receiver station. ARQ uses error detection codes during transmission. The receiver station determines if the PDUs are received correctly according to the error detection codes. In addition, ARQ uses a feedback mechanism so that the receiver station can notify the sender station if the PDUs are received correctly. The sender station determines if resend the PDUs according the feedback result provided by the receiver station. There are three ARQ schemes: stop-and-wait, go-back-N, and selective repeat.
In the stop-and-wait scheme, the sender station transmits a packet to the receiver station, starts a timer, and waits for an acknowledgment (ACK) from the receiver station. If the packet is received correctly, the receiver station sends a positive acknowledgment and the sender station can transmit the next packet. If the timer expires, the sender station retransmits the packet, restarts the timer, and waits for an acknowledgment. This scheme is quite simple but very inefficient because of the idle time spent in waiting for an acknowledgment after each retransmission.
In the go-back-N scheme, the sender station transmits packets one after another until it receives a negative acknowledgment. Upon reception of a negative acknowledgment, the sender station retransmits the negatively acknowledged packet and all the packets following it. This scheme is more efficient than the stop-and-wait protocol when the transmission error rate is not too high and the link propagation delay is small. The major cause of the inefficiency associated with this scheme is due to the retransmission of many correctly received packets.
In the selective repeat scheme, the sender station transmits packets one after another until it receives a negative acknowledgment. Upon reception of a negative acknowledgment, the sender station retransmits only the packets in error. The selective repeat scheme is the most efficient method. However, this scheme breaks the order of the packets, so the receiver station has to process the packets in a corresponding way.
In conclusion, in three ARQ schemes, the stop-and-wait scheme is the simplest but the most inefficient. The go-back-N scheme is more efficient than the stop-and-wait scheme, but the go-back-N scheme still transmits the packets in order. For an advanced efficiency, the selective repeat scheme is used. Thus, a wireless communication apparatus for the selective repeat scheme should be developed.