WO2005022818A1

WO2005022818A1 - A hybrid arq method and apparatus for use in wlan

Info

Publication number: WO2005022818A1
Application number: PCT/IB2004/051647
Authority: WO
Inventors: Wei Li
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-09-02
Filing date: 2004-09-01
Publication date: 2005-03-10
Also published as: CN1592244A

Abstract

A hybrid ARQ (automatic repeat request) method in WLAN performed by a wireless terminal is provided, comprising steps of: (a) receiving a data frame and checking whether the received data frame can meet the QoS requirement; storing the data frame if the received data frame can't meet the QoS requirement; receiving a retransmitted data frame, and selecting the data frame having the same content with said retransmitted data frame from the stored data frames; combining said selected data frame and said retransmitted data frame, so as to get a data frame which meets the QoS requirement.

Description

A Hybrid ARQ Method and Apparatus for Use in WLAN

Field of the Invention

The present invention relates to a hybrid ARQ method and apparatus for use in WLAN.

Background of the Invention

WLAN is a flexible data communication system, by using radio waves to t ransmit and receive data. Thus it minimizes requirement for wired connection and combines data connectivity with user mobility. Furthermore, WLAN is easy to be deployed, so it is widely used in buildings or on campus as an expansion to, or as an alternativ e for wired LAN. WLAN is a packet -based (data packet) data transmission communication system, and one of its key QoS requirements is to provide reliable transmission service. Unfortunately, radio links are known to be prone to location -dependent time -varying errors and burst errors. To reduce radio link errors and obtain reliable transmission service, ARQ (automatic repeat request) scheme is widely used in WLAN. ARQ works in the way that when a packet arrives, the receiver checks its reliability by means of an error detecting code, such as CRC (cyclic redundancy check code) , i.e.: whether the packet can satisfy the QoS requirement. If the packet is not reliable, the receiver will reject the packet and request a retransmission through a feedback channel , or just keep silent without sending an ACK message so that the sender retransmits the packet after a certain time period . If the packet is reliable, the receiver accepts the packet and sends an ACK message to the sender. Fig.l is a schematic diagram illustrat ing how packets are processed in the ARQ scheme. First, the receiver receives packet 10 checks its error detecting code such as CRC and the checking fails, hence the receiver discards packet 10 and requests for retransmission. Then, the receiver receives p acket 20 that is retransmitted for the first time, checks its error detecting code such as CRC and the checking still fails, hence the receiver discards packet 20 and requests for retransmission again. Last, the receiver receives packet 30 that is retransm itted for the second time, checks its error detecting code such as CRC and the checking succeeds, thus the receiver accepts packet 30. When the channel condition is good or moderately good, there will be very few packets requesting for retransmission due t o unreliability, so the effective data rate in the WLAN is very high. In this case, ARQ scheme can guarantee reliable transmission service of the WLAN and maintain a high effective data throughput. When the channel condition is poor, there will be a lot of packets requesting for retransmission due to unreliability, so the effective data rate in the WLAN will be very low. In this case, ARQ scheme can 't provide the WLAN with a high effective data throughput although it can guarantee reliable transmission serv ice. To provide WLAN with reliable transmission service and high effective data throughput regardless of various channel conditions, it 's necessary to offer a more advanced ARQ method.

Summary of the Invention An object of the present invention is to provi de a hybrid ARQ method and apparatus for use in WLAN, wherein unreliable packets are kept and combined through time diversity with the subsequently retransmitted packets that have the same content, to get the reliable packet, thus the number of the retrans mitted packets can be reduced. The WLAN can obtain reliable transmission service, as well as improve effective data throughput considerably. A hybrid ARQ method in WLAN to be executed by a wireless terminal, comprises: (a) receiving a data frame and detec ting whether the received data frame packet can meet the QoS requirement; (b) storing the data frame packet if it can 't meet the QoS requirement; (c) receiving a retransmitted data frame packet, and selecting the data frame packet having the same content w ith the retransmitted data frame packet from the stored data frame packets; (d) combining said selected data frame packet and said retransmitted data frame packet so as to get a data frame packet meeting the QoS requirement.

Brief Description of the Drawin gs Fig.l is a schematic diagram illustrating how packets are processed in existing ARQ scheme; Fig.2 illustrates the hybrid ARQ method in the present invention; Fig.3 illustrates the basic state machine of the hybrid ARQ method in the present invention; Fig.4 is the flow char showing the hybrid ARQ method in the present invention; Fig.5 is a schematic diagram illustrating the system structure of the hybrid ARQ method in the present invention.

Detailed Description of the Invention The main idea of the hybrid ARQ method in the present invention can be summarized as: the unreliable packets can be stored and later combined with additional copies of the packet, creating a single packet that is more reliable than any of its constituent packets. MRC (Maximal ratio combining) is used, meaning that each copy pertaining to the same packet is weighted by a weight coefficient corresponding to its reliability. Fig.2 displays the principle of the hybrid ARQ method in the present invention. As shown in figure, to data frame 40 that is CRC failure from transmitter and data frame 50 that is retransmitted for the first time and, the receiver won 't discard them, but stores them in buffer T10 along with their corresponding combination coefficients (i.e. weight coefficients) computed according to their frame headers. On receipt of data frame 60 that is retransmitted for the second time after CRC checking fails, the receiver computes its corresponding combination coefficient (i.e. weight coefficients) according to the frame header of data frame 60, and then picks data frames 40 and 50 and their corresponding combination coefficients (i.e. weight coefficients) from buffer T10. Last, in MRC unit T20, data frames 40, 50 and 60 are respectively multiplied with their combination coefficients (i.e. weight coefficients) and combined with MRC criterion to output a data frame whose CRC checking succeeds. Fig.3 illustrates the basic state machine of the hybrid ARQ method in the present invention. State A or namely the success state is the defau It state that the hybrid ARQ method works in. On receipt of a frame, the hybrid ARQ method checks its CRC. If the checking succeeds, it will continue to work in state A; otherwise, it will go to the duplicate state or namely state B. In state B, the hybrid ARQ method stores the frames whose CRC checking fails, and waits for receiving the retransmitted frame. After receiving the retransmitted frame, the stored frames whose CRC checking fails, together with the retransmitted frame, will be weighted and combined to output a new combined frame. If the CRC checking of the combined frame succeeds, the hybrid ARQ method returns to work in state A; otherwise, the hybrid ARQ method keeps in state B, stores said retransmitted frame and waits for the next retransmitted frame. A detailed description will be given below to the hybrid ARQ method in the present invention, in conjunction with Fig.4, exemplifying WLAN based on IEEE 802.11 protocols. As shown in figure, when receiving a frame, the wireless terminal checks its CRC in FCS field in the frame (step S10). If CRC checking fails, the frame is stored in a buffer serially (step S20), wherein the stored frame doesn 't include the preamble (that is a midamble for data synchronization and channel estimation) and the preambl e sector is replaced by the channel estimation result so as to be used as weight coefficient later when the frame is weighted and combined. In addition to the channel estimation result, the stored frame further includes PLCP header, MAC header, frame body and FCS field, wherein PLCP header and MAC header are for frame identification later when the frame is weighted and combined. Structure of the above frame stored in the buffer is shown in Fig.5. When receiving a new retransmitted frame (step S30), the wire less terminal first checks its CRC (step S32). If CRC checking fails, the wireless terminal selects the frames with the same content as the retransmitted frame from the buffer (step S35), and respectively multiplies the selected frames and the currently re ceived frame with their corresponding weight coefficients (step S40). Then the weighted frames are combined with MRC criterion to get a weighted and combined frame (step S50). The wireless terminal checks the CRC code of the combined frame again(step S60). If CRC checking still fails, the wireless terminal stores the newly received retransmitted frame in the buffer and waits for the next new retransmitted frame (step S70). If CRC checking succeeds, then outputting the combined frame while the wireless termi nal instructs a buffer management unit to delete the selected frames from the buffer (step S80). In the above hybrid ARQ method, channel estimation result is used as the weight coefficient to weight the received retransmitted data frame and stored the data frames. The channel estimation module is an indispensable component of all WLAN terminals, so the hybrid ARQ method in the present invention can obtain the needed weight coefficient without adding additional hardware resources. Moreover, implementation of this method doesn 't need any modification to current WLANs or adding complicated computation units. Only a buffer unit needs to be added, for storing some frames. As to the above hybrid ARQ method described in Fig.4, it can be implemented in software module or in hardware module. When implemented in hardware module, the hybrid ARQ apparatus comprises: a checking unit, for checking the CRC in FCS field in the data frame when receiving the data frame; a buffer unit, for storing the data frame when CRC checking fails; a frame identifying unit, for selecting the data frames with the same content as the retransmitted data frame from those stored in the buffer when receiving a new retransmitted data frame; a weighting unit, for multiplying the selected data frames and the currently received data frame respectively with their corresponding weight coefficients; a combing unit, for combing the weighted data frames with MRC method to get a weighted and combined data frame; a buffer management unit, for deleting the selected data frames from the buffer when CRC checking of the combined data frame succeeds. Fig. 5 illustrates a system structure of implementing the hybrid ARQ method by using hardware module in a wireless terminal. As shown in the figure, when the wireless terminal receives a data frame, if CRC checking in FCS field of the data frame fails after the checking unit checks, preamble estimating unit 200 performs channel estimation to the preamble in the data frame to get the channel estimation result, then take s it as the weight coefficient to replace the preamble in the data frame, and last stores the altered data frame in buffer 210. When the wireless terminal receives a retransmitted frame: (1) first, preamble estimating unit 200 performs channel estimation to the preamble in the retransmitted data frame to get the channel estimation result, and takes it as weight coefficient to replace the preamble in the data frame; (2) next, frame identifying unit 220 selects the data frame who have the same PLCP header and MAC header as the retransmitted data frame (as the dashed line arrows LI and L2 show); (3) then, MRC unit 230 multiplies the body of the retransmitted data frame and the bodies (as dashed line arrow L3 shows) of the selected data frames from buffer 210 with their respective weight coefficients (as dashed line arrow L4 shows), combines them and outputs a data frame; (4) last, checking unit 240 checks the CRC of the combined data frame. If the checking fails, the wireless terminal stores the retransmitted data frame in buffer 210 and waits for new retransmitted data frames; if the checking succeeds, the wireless terminal outputs the combined data frame, and instructs buffer management unit 250 to delete the selected data frames from buffer 210.

Beneficial Results of the Invention As described above, with regard to the hybrid ARQ method and apparatus for use in

WLAN as provided in the present invention, unreliable packets are kept and combined according to a certain rule with the subsequently received retransmi tted packets that have the same content to get the reliable packet, thus the number of retransmitted packets can be reduced. The WLAN can obtain reliable transmission service, as well as improve effective data throughput considerably. Furthermore, no modification is needed to current WLANs to implement this apparatus, and this apparatus doesn 't need complicated computation units to be added. Only a buffer unit needs to be added, for storing some frames. It is to be understood by those skilled in the art that the hybrid ARQ method and apparatus for use in WLAN as disclosed in this invention can be modified considerably without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

What is claimed is:

1. A hybrid ARQ (automatic repeat request) method in WLAN performed by a wireless terminal, comprising: (a) receiving a data frame and checking whether the received data frame can meet the QoS requirement; (b) storing the data frame if the rece ived frame can 't meet the QoS requirement; (c) receiving a retransmitted data frame, and selecting the data frame having the same content with said retransmitted data frame from the stored data frames; (d) combining said selected data frame and said retransmitted data frame so as to get a data frame meeting the QoS requirement.

2. The method according to claim 1, further comprising: (e) checking whether the combined data frame meets the QoS requirement; (f) storing said retransmitted data frame if the combined data frame can't meet the QoS requirement.

3. The method according to claim 2, further comprising: (g) deleting said selected data frame if the combined data frame meets the QoS requirement.

4. The method according to any one of claim 1 to 3, wherein step (d) includes: (i) multiplying said selected data frame and said retransmitted data frame respectively with the estimated weight coefficient; (ii) combining said selected data frame and said retransmitted data frame after weighting so as to get a data frame meeting the QoS requirem ent.

5. The method according to claim 4, wherein said weight coefficient is obtained according to the information included in the data frame related with the transmission quality of the data frame.

6. The method according to claim 5, wherein said weight co efficient is the channel estimation result calculated according to the preamble in the data frame.

7. The method according to claim 4, wherein said combining method adopts MRC (Maximal Ratio Combining ).

8. The method according to any one of claim 1 to 3, wherein said stored data frame at least includes data frame identification information and the information related with the transmission quality of the data frame.

9. A wireless terminal for performing the hybrid ARQ method, comprising: a receiving unit, fo r receiving data frames; a checking unit, for checking whether the received data frame meets the QoS requirement; a storing unit, for storing the data frame when the received data frame can't meet the QoS requirement; a frame identifying unit, for selectin g the data frame having the same content with the received retransmitted data frame from the stored data frames; a combining unit, for combing said selected data frame and said retransmitted data frame so as to get a data frame meeting the QoS requirement.

10. The wireless terminal according to claim 9, further comprising: a control unit, for deleting said selected data frames when the combined data frame meets the QoS requirement.

11. The wireless terminal according to claim 9 or 10, wherein said combining unit further includes: a weighting unit, for multiplying said selected data frame and said retransmitted data frame respectively with the estimated weight coefficients.

12. The wireless terminal according to claim 11, wherein said weight coefficient is th e channel estimation result calculated according to the preamble in the data frame.

13. The wireless terminal according to claim 9 or 10, wherein said stored data frame at least includes data frame identification information and the information related wit h the transmission quality of the data frame.