Method and Apparatus for Combining Frames in WLAN

Abstract

The present invention relates to a method performed by a station of a wireless local area network for combining information from received frames. The method comprises receiving (420) a first frame from another station of the wireless local area network, and determining (430) a radio characteristic of the first frame. The method further comprises comparing (440) the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame, and determining (450) whether to combine information from the first frame with information from the previously received second frame based on the comparison. When it is determined to combine the information, the method comprises combining (460) information from the first frame with information from the previously received second frame.

Claims

1-20. (canceled)

21. A method performed by a station of a wireless local area network for combining information from received frames, the method comprising: receiving a first frame from another station of the wireless local area network, determining a radio characteristic of the first frame, comparing the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame, determining whether to combine information from the first frame with information from the previously received second frame based on the comparison, and when it is determined to combine the information: combining information from the first frame with information from the previously received second frame.

22. The method according to claim 21, further comprising when failing to decode the previously received second frame: determining the radio characteristic of the previously received second frame, storing information from the previously received second frame and the radio characteristic of the previously received second frame, and wherein the radio characteristic of the first frame is compared with the stored radio characteristic of the previously received second frame, and information from the first frame is combined with the stored information from the previously received second frame when it is determined to combine the information.

23. The method according to claim 21, wherein determining the radio characteristic of the first frame comprises obtaining a physical layer parameter from a header of the first frame.

24. The method according to claim 21, wherein determining the radio characteristic of the first frame comprises measuring the radio characteristic of the first frame.

25. The method according to claim 21, wherein the radio characteristic of the first frame is determined when failing to decode the first frame.

26. The method according to claim 21, wherein comparing comprises: estimating a difference between the radio characteristic of the first frame and the radio characteristic determined for the previously received second frame, and wherein it is determined to combine the information when the estimated difference is below a threshold value.

27. The method according to claim 26, wherein the radio characteristic comprises at least one of: an average received power; an angle of arrival; an impulse response; and a frequency response.

28. The method according to claim 21, wherein the radio characteristic comprises a modulation and coding scheme, and wherein it is determined to combine the information when the modulation and coding scheme of the first frame is equal to or more robust than the modulation and coding scheme of the previously received second frame.

29. The method according to claim 21, wherein the radio characteristic comprises a frame length, and wherein it is determined to combine the information when the frame length of the first frame is equal to the frame length of the previously received second frame.

30. A station for a wireless local area network adapted to combine information from received frames, the station being configured to: receive a first frame from another station of the wireless local area network, determine a radio characteristic of the first frame, compare the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame, determine whether to combine information from the first frame with information from the previously received second frame based on the comparison, combine information from the first frame with information from the previously received second frame when it is determined to combine the information.

31. The station according to claim 30, further configured to, when failing to decode the previously received second frame: determine the radio characteristic of the previously received second frame, store information from the previously received second frame and the determined radio characteristic of the previously received second frame, compare the determined radio characteristic of the first frame with the stored radio characteristic of the previously received second frame, and combine information from the first frame with the stored information from the previously received second frame when it is determined to combine the information.

32. The station according to claim 30, further configured to determine the radio characteristic of the first frame by obtaining a physical layer parameter from a header of the first frame.

33. The station according to claim 30, further configured to determine the radio characteristic of the first frame by measuring the radio characteristic of the first frame.

34. The station according to claim 30, further configured to determine the radio characteristic of the first frame when failing to decode the first frame.

35. The station according to claim 30, further configured to compare by estimating a difference between the radio characteristic of the first frame and the radio characteristic determined for the previously received second frame, and to determine to combine the information when the estimated difference is below a threshold value.

36. The station according to claim 35, wherein the radio characteristic comprises at least one of: an average received power; an angle of arrival; an impulse response; and a frequency response.

37. The station according to claim 30, wherein the radio characteristic comprises a modulation and coding scheme, and wherein the station is further configured to determine to combine the information when the modulation and coding scheme of the first frame is equal to or more robust than the modulation and coding scheme of the previously received second frame.

38. The station according to claim 30, wherein the radio characteristic comprises a frame length, and wherein the station is further configured to determine to combine the information when the frame length of the first frame is equal to the frame length of the previously received second frame.

39. A station for a wireless local area network, the station comprising a processor and a memory, said memory containing instructions executable by said processor whereby said station is operative to: receive a first frame from another station of the wireless local area network, determine a radio characteristic of the first frame, compare the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame, determine whether to combine information from the first frame with information from the previously received second frame based on the comparison, combine information from the first frame with information from the previously received second frame when it is determined to combine the information.

40. The station according to claim 39, further operative to, when failing to decode the previously received second frame: determine the radio characteristic of the previously received second frame, store information from the previously received second frame and the determined radio characteristic of the previously received second frame, compare the determined radio characteristic of the first frame with the stored radio characteristic of the previously received second frame, and combine information from the first frame with the stored information from the previously received second frame when it is determined to combine the information.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1a is a schematic illustration of the sensing, deferral, and back-off features used in a WLAN.

[0026] FIG. 1b is a schematic illustration of the IEEE 802.11n MAC frame format.

[0027] FIG. 1c is a schematic illustration of the SIG field of the IEEE 802.11n PLCP header.

[0028] FIG. 2a is a block diagram schematically illustrating an example WLAN scenario with three transmitting stations and one receiving station.

[0029] FIGS. 2b-c are diagrams schematically illustrating results from average received power measurements and angle of arrival measurements at the receiving station of the example scenario illustrated in FIG. 2a.

[0030] FIG. 3a is a block diagram schematically illustrating an example WLAN scenario with two transmitting stations, a reflecting object and one receiving station.

[0031] FIGS. 3b-c are diagrams schematically illustrating results from impulse response measurements and frequency response measurements at the receiving station of the example scenario illustrated in FIG. 3a.

[0032] FIGS. 4a-b are flowcharts illustrating the method in the station of a WLAN according to embodiments.

[0033] FIGS. 5a-b are block diagrams schematically illustrating the station of a WLAN according to embodiments.

DETAILED DESCRIPTION

[0034] In the following, different aspects will be described in more detail with references to certain embodiments and to accompanying drawings. For purposes of explanation and not limitation, specific details are set forth, such as particular scenarios and techniques, in order to provide a thorough understanding of the different embodiments. However, other embodiments that depart from these specific details may also exist.

[0035] Embodiments are described in a non-limiting general context in relation to an example scenario in a WLAN, where the station is an access point receiving frames from transmitting mobile terminals, also referred to as stations, in the area served by the access point. However, it should be noted that the embodiments may be applied to any network technology compliant with IEEE 802.11. Furthermore, the station receiving frames may be any type of station such as a mobile terminal receiving frames from transmitting access points.

[0036] In embodiments of the invention, the problem of not being able to use combining techniques for frames in a WLAN due to that the transmitter addresses are not available, is addressed by a solution where the receiving station identifies if two frames are transmitted from a same transmitting station by comparing radio characteristics of the received frames. Frames from different transmitting stations will in many cases be received with different radio characteristics such as power or frequency response. They may also be received with different radio characteristics in terms of physical layer parameters, such as Modulation and Coding Schemes (MCS). Frames received with similar or equal radio characteristics may therefore be assumed to originate from the same transmitting station, and may thus be combined for improved performance.

[0037] In one exemplary embodiment of the invention, the method in the receiving station may comprise the following:

[0038] 1) Receiving a first frame Tx.sub.1.

[0039] 2) If decoding of Tx.sub.1 on the MAC layer fails, storing information I.sub.1 for Tx.sub.1 from the decoding.

[0040] 3) Determining a radio characteristic C.sub.1 for Tx.sub.1.

[0041] 4) Receiving a second frame Tx.sub.2.

[0042] 5) If decoding of Tx.sub.2 on the MAC layer fails, storing information I.sub.2 for Tx.sub.2 from the decoding.

[0043] 6) Determining a radio characteristic C.sub.2 for Tx.sub.2, where C.sub.2 is the same type of radio characteristic as C.sub.1.

[0044] 7) Comparing C.sub.2 with C.sub.1.

[0045] 8) if C.sub.2 and C.sub.1 are similar, combing I.sub.1 and .sup.I.sub.2.

[0046] 9) Attempting to decode Tx.sub.2 using the combination of I.sub.1 and I.sub.2.

[0047] The radio characteristics C.sub.2 and C.sub.1 of a frame used in the comparison in step 7) above may comprise one of the following: [0048] Average received power: The output power of a transmitting station is stable. Furthermore, the distance between transmitting and receiving stations does not vary much between retransmissions, nor does the power loss. This means that similar received power for received frames may indicate that the frames come from a same transmitting station. [0049] Angle of Arrival: Same assumptions as for average received power are possible. Transmission of two subsequent frames from a same transmitting station should not result in substantially different angle of arrival measurements at the receiving side. [0050] Frequency shifts: May comprises either shifts due to imperfections in the transmitter, or Doppler shifts due to that the transmitting or receiving station is moving. A frequency shift typically differs between different transmitters. As the receiver needs to estimate the frequency error between the transmitter and the receiver in order to perform a demodulation, this frequency error could be used as the radio characteristic type that is compared for two transmissions. The same estimated frequency error would thus indicate that the transmissions come from the same transmitting station. [0051] Received power as a function of frequency or frequency response of the channel. [0052] Power delay profile or the impulse response of the channel. [0053] Difference in the above listed parameters between different receive antennas of a station.

[0054] The types of radio characteristics listed above are determined by the receiving station based on measurements. However, the radio characteristics of the frame may also comprise physical layer parameters such as MCS, bandwidth, and length or duration of a frame, which may be obtained from the SIG field in the PLCP header of a WLAN frame and does thus not require a measurement. For example, in the 802.11n standard the PLCP header comprises a parameter indicating a number of octets of data in the frame in the so called High Throughput (HT) Length field.

[0055] FIG. 2a schematically illustrates an example scenario where three stations 110a, 110b, and 110c all transmit to a receiving station 110d. With similar transmit power at all the stations 110a, 110b, and 110c, the power received at station 110d from station 110a is smaller than that from stations 110b and 110c, due to the longer distance to the receiving station 110d. This is illustrated in the diagram of FIG. 2b. The received powers from stations 110b and 110c are similar as their distances to the receiving station 110d are similar. However, the signals carrying frames from stations 110b and 110c have different angles of arrival at receiving station 110d, as illustrated in the diagram of FIG. 2c. These radio characteristic types, i.e. average received power and angle of arrival may thus be used together to assess if two frames are received from a same station or not.

[0056] FIG. 3a schematically illustrates an example scenario where two stations 110b and 110c both transmit to the receiving station 110d. The signal is reflected by an object 120 on its path to the receiving station 110d. The extra path length for the reflected path 130b, 130c, is longer for station 110c than for station 110b. This results in a longer delay between the direct path 140c and the reflected path 130c for station 110c than between the direct path 140b and the reflected path 130b for station 110b. This can be seen in the channel impulse response, or the power delay profile illustrated in the diagram of FIG. 3b. The longer delay between direct path and reflected path may also be seen as faster variations in a frequency response for the channel between station 110b and station 110d, compared to the channel between station 110c and station 110d, as illustrated in the diagram of FIG. 3c.

[0057] When using measured radio characteristics for the comparison, such as average received power or angle of arrival, the similarity check of the radio characteristics may comprise estimating a difference in the measured radio characteristic for the first and the second frame. If the estimated difference is below a selected threshold, the first and the second frames are assumed to be transmitted by the same station. As an example, if a first radio characteristic for Tx.sub.1 and Tx.sub.2 are denoted Ca.sub.1 and Ca.sub.2 respectively, the relative difference Da of the parameters may be calculated as:

Da=abs(Ca.sub.1Ca.sub.2)/((Ca.sub.1+Ca.sub.2)/2)[1]

[0058] Similarity in the radio characteristic may be assessed as the difference being below a certain threshold Da.sub.max, i.e. Da<Da.sub.max.

[0059] By making more than one comparison of radio characteristics, the combining procedure may be further assured at the cost of an increased complexity. One example scenario where this may be needed is described above with reference to FIG. 2a where both received power and angle of arrival is measured in order to be able to differentiate between station 110b and 110c. Eq. [1] may thus be used for other types of measured radio characteristics respectively denoted Cb.sub.1, Cb.sub.2; Cc.sub.1, Cc.sub.2; etc. In one embodiment, overall similarity based on several radio characteristic comparisons may be assumed when at least a fraction, e.g. 75%, of the radio characteristic comparisons are similar.

[0060] As mentioned above, the measured radio characteristic may comprise a frequency response for a received frame. If the frequency responses for two received frames are denoted h1(f) and h2(f), a mean square difference may be calculated as an integral over f:

.sub.f[h.sub.1(f)h.sub.2(f)].sup.2df [2]

[0061] If the frequency response is discrete, i.e. if only a few samples are measured in frequency, a sum of the discrete values may be calculated instead of the integral. It may then be determined to combine the two frames if the calculated mean square difference is lower than a threshold value. If the measured radio characteristic comprises the impulse response for the received frame, the impulse response may be transformed to the frequency domain before using the same mean square difference calculation as above.

[0062] When comparing radio characteristics obtainable from the PLCP header, the comparison may be straight forward, as with the example of a comparison of the HT length field parameter. Two frames may be determined to be combined if the HT length field parameters of the two frames are equal. When comparing MCS parameters, it may be taken into account that the MCS may often be changed to a more robust MCS for a retransmission. Therefore, a frame with a less robust MCS in the header compared to an MCS parameter of a previously received frame may indicate that the two frames are received from different transmitting stations. It may thus be determined to combine two frames only when the MCS parameter of one frame is equal to or more robust than that of a previously received frame.

Methods and Nodes

[0063] FIG. 4a is a flowchart illustrating one embodiment of a method performed by a station 110d of a wireless local area network 100 for combining information from received frames. The method comprises: [0064] 420: Receiving a first frame from another station 110a of the wireless local area network. [0065] 430: Determining a radio characteristic of the first frame. This determining may comprise obtaining a physical layer parameter from a header of the first frame. Examples are obtaining the HT length field parameter or the MCS from the SIG field of the PLCP header. The determining may also comprise measuring the radio characteristic of the first frame. Examples are measuring the average received power or the angle of arrival. In embodiments, the radio characteristic is determined when failing to decode the first frame. [0066] 440: Comparing the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame. [0067] 450: Determining whether to combine information from the first frame with information from the previously received second frame based on the comparison.

[0068] When it is determined to combine the information, the method further comprises: [0069] 460: Combining information from the first frame with information from the previously received second frame.

[0070] FIG. 4b is a flowchart illustrating another embodiment of the method in the station 110d. The method optionally comprises when failing to decode the previously received second frame: [0071] 410: Determining the radio characteristic of the previously received second frame. [0072] 415: Storing information from the previously received second frame and the radio characteristic of the previously received second frame. The information and the radio characteristic may be stored such that it may be retrieved when a subsequently received frame's radio characteristic is to be compared with the previously received frame's radio characteristic.

[0073] The method further comprises, as in the previously described embodiment, receiving 420 the first frame from another station 110a of the wireless local area network and determining 430 the radio characteristic of the first frame. The radio characteristic may be determined 430 when failing to decode the first frame. Furthermore, the method comprises the following: [0074] 440: Comparing the radio characteristic of the first frame with the stored radio characteristic of the previously received second frame. [0075] 450: Determining whether to combine the information.

[0076] When it is determined to combine the information, the method further comprises: [0077] 460: Combining information from the first frame with the stored information from the previously received second frame.

[0078] In one embodiment, the comparing 440 comprises estimating a difference between the radio characteristic of the first frame and the radio characteristic determined for the previously received second frame. The estimation may for example be done based on eq. [1] above. In such an embodiment, it may be determined 450 to combine the information when the estimated difference is below a threshold value, such as Da.sub.max in the previously described example. The radio characteristics for which the estimation of a difference may be relevant is the measured radio characteristics. The radio characteristics may in embodiments comprise at least one of an average received power; an angle of arrival; an impulse response; and a frequency response.

[0079] In embodiments, the radio characteristic may comprise an MCS. The parameter indicating the MCS used by the transmitting station may be obtained from the SIG field in the PLCP header of the frame. It may be determined 450 to combine the information of the frames when the MCS of the first frame is equal to or more robust than the MCS of the previously received second frame.

[0080] In embodiments, the radio characteristic may comprise a frame length. The parameter indicating the length of the transmitted frame may be obtained from the SIG field in the PLCP header of the frame. It may be determined 450 to combine the information when the frame length of the first frame is equal to the frame length of the previously received second frame.

[0081] An embodiment of a station 110d for a wireless local area network (100) adapted to combine information from received frames, is schematically illustrated in the block diagram in FIG. 5a. The station 110d is configured to receive a first frame from another station 110a of the WLAN, to determine a radio characteristic of the first frame, to compare the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame, to determine whether to combine information from the first frame with information from the previously received second frame based on the comparison, and to combine information from the first frame with information from the previously received second frame when it is determined to combine the information.

[0082] In embodiments, the station 110d may be further configured towhen failing to decode the previously received second framedetermine the radio characteristic of the previously received second frame, and store information from the previously received second frame and the determined radio characteristic of the previously received second frame. The station may be configured to compare the determined radio characteristic of the first frame with the stored radio characteristic of the previously received second frame, and to combine information from the first frame with the stored information from the previously received second frame when it is determined to combine the information.

[0083] The station 110d may be further configured to determine the radio characteristic of the first frame by obtaining a physical layer parameter from a header of the first frame, such as the PLCP header. The station 110d may also be configured to determine the radio characteristic of the first frame by measuring the radio characteristic of the first frame. Furthermore, the station 110d may be configured to determine the radio characteristic of the first frame when failing to decode the first frame.

[0084] In embodiments, the station 110d may be further configured to compare by estimating a difference between the radio characteristic of the first frame and the radio characteristic determined for the previously received second frame, and to determine to combine the information when the estimated difference is below a threshold value. The radio characteristic may comprise at least one of: an average received power; an angle of arrival; an impulse response; and a frequency response.

[0085] In one embodiment, the radio characteristic comprises an MCS. The station 110d may be further configured to determine to combine the information when the MCS of the first frame is equal to or more robust than the MCS of the previously received second frame.

[0086] In another embodiment, the radio characteristic may comprise a frame length, and the station 110d may be further configured to determine to combine the information when the frame length of the first frame is equal to the frame length of the previously received second frame.

[0087] In embodiments of the invention, the station 110d comprises a processor 111 and a memory 112. The station 110d may also comprise a radio interface circuit 113 configured to communicate with the other stations 110a, 110b or 110c of the WLAN 100, and connected to the processor 111. The memory 112 contains instructions executable by said processor 111 whereby said station 110d is operative to receive a first frame from another station 110a, 110b, or 110c of the wireless local area network 100. The first frame may be received via the radio interface circuit 113. The station 110d is further operative to determine a radio characteristic of the first frame, to compare the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame, to determine whether to combine information from the first frame with information from the previously received second frame based on the comparison, and to combine information from the first frame with information from the previously received second frame when it is determined to combine the information.

[0088] The station 110d may be further operative to, when failing to decode the previously received second frame, determine the radio characteristic of the previously received second frame, and to store information from the previously received second frame and the determined radio characteristic of the previously received second frame. The station 110d may be still further operative to compare the determined radio characteristic of the first frame with the stored radio characteristic of the previously received second frame, and combine information from the first frame with the stored information from the previously received second frame when it is determined to combine the information.

[0089] In an alternative way to describe the embodiment in FIG. 5a, illustrated in FIG. 5b, the station 110d comprises a receiving module 115 adapted to receive a first frame from another station of the wireless local area network, a first determining module 116 adapted to determine a radio characteristic of the first frame, a comparing module 117 adapted to compare the radio characteristic of the first frame with a radio characteristic determined for a previously received second frame, a second determining module 118 adapted to determine whether to combine information from the first frame with information from the previously received second frame based on the comparison, and a combining module 119 adapted to combine information from the first frame with information from the previously received second frame when it is determined to combine the information. The modules described above are functional units which may be implemented in hardware, software, firmware or any combination thereof. In one embodiment, the modules are implemented as a computer program running on a processor.

[0090] In an alternative way to describe the embodiment in FIG. 5a, the station 110d comprises a Central Processing Unit (CPU) which may be a single unit or a plurality of units. Furthermore, the station 110d comprises at least one computer program product (CPP) in the form of a non-volatile memory, e.g. an EEPROM (Electrically Erasable Programmable Read-Only Memory), a flash memory or a disk drive. The CPP comprises a computer program, which comprises code means which when run on the station 110d causes the CPU to perform steps of the procedure described earlier in conjunction with FIGS. 4a-b. In other words, when said code means are run on the CPU, they correspond to the processor 111 of FIG. 5a.

[0091] The above mentioned and described embodiments are only given as examples and should not be limiting. Other solutions, uses, objectives, and functions within the scope of the accompanying patent claims may be possible.