Communication devices and methods

Abstract

A first communication device configured to operate as initiator and communicate with a second communication device configured to operate as responder. The first communication device comprising circuitry configured to connect to the second communication device via at least two links including a non-mmWave link and a mmWave link; transmit a beam-forming (BF) request frame to the second communication device via the non-mmWave link; perform BF training (BFT) with the second communication device by subsequently transmitting a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link; and receive, at the end of the BFT, a BFT feedback frame from the second communication device via the non-mmWave link if it is not busy at the end of the BFT.

Description

Communication devices and methodsBACKGROUNDFIELD OF THE DISCLOSURE

[0001] The present disclosure relates to first and second communication devices for use as initiator and responder. The present disclosure relates further to corresponding first and second communication methods.DESCRIPTION OF RELATED ART

[0002] In IEEE 802.11 standards, beamforming (BF) is required prior to transmitting data on a mmWave link to establish a link with sufficient quality. Although many variants of BF areavailable, these known variants of BF do generally consume a huge amount of time, which is undesirable for low-latency and high-throughput applications.

[0003] The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.SUMMARY

[0004] It is an object to reduce the amount of time required for establishing a link between two communication devices for data exchange required in low-latency and high-throughput applications. It is a further object to provide corresponding communication devices and methods as well as a corresponding computer program and a non-transitory computer- readable recording medium for implementing the communication methods.

[0005] According to an aspect there is provided a first communication device configured to operate as initiator and communicate with a second communication device configured to operate as responder, the first communication device comprising circuitry configured to: connect to the second communication device via at least two links including a non- mmWave link and a mmWave link; transmit a beamforming (BF) request frame to the second communication device via the non-mmWave link; perform BF training (BFT) with the second communication device by subsequently transmitting a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link; and receive, at the end of the BFT, a BFT feedback frame from the second communication device via the non-mmWave link if it is not busy at the end of the BFT.

[0006] According to a further aspect there is provided a second communication device configured to operate as responder and communicate with a first communication device configured to operate as initiator, the second communication device comprising circuitry configured to: connect to the first communication device via at least two links including a non- mmWave link and a mmWave link; receive a beamforming (BF) request from the first communication device via the non-mmWave link; perform BF training (BFT) with the first communication device by subsequently receiving a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link, wherein during the reception of a training field different receive sectors are used; and transmit, at the end of the BFT, a BFT feedback frame to the first communication device via the non-mmWave link if it is not busy at the end of the BFT.

[0007] According to still further aspects a computer program comprising program means for causing a computer to carry out the steps of the method disclosed herein, when said computer program is carried out on a computer, as well as a non-transitory computer- readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method disclosed herein to be performed are provided.

[0008] Embodiments are defined in the dependent claims. It shall be understood that the disclosed communication methods, the disclosed computer program and the disclosed computer-readable recording medium have similar and / or identical further embodiments as the claimed communication devices and as defined in the dependent claims and / or disclosed herein.

[0009] One of the aspects of the disclosure is the recognition that BF for asymmetric links takes a long time and is not useful in use cases requiring low latency and high throughput, because conventionally an initiator (herein also called “first communication device”) is required to wait a certain amount of time with different receive sectors in the feedback phase. The presented disclosure applies a much faster and efficient BF approach in a multi-link operation (MLO) using a non-mmWave link (e.g. a Sub-7GHz link) and ammWave link (e.g. a link using 45 GHz, 60 GHz or 70 GHz). The proposed BF is also called multi-link-assisted BF hereinafter. One of the elements of the present disclosure is that feedback information for BF is preferably transmitted on the non-mmWave link at the end of the BF training (BFT) to make the BF duration shorter.

[0010] Hereinafter, the term "sector" refers to one of predetermined beam patterns of communication devices. The term "TX sector" refers to a sector, which is used in transmission by communication devices, and the term "RX Sector" refers to a sector, which is used in reception by communication devices. Transmitters can use TX sectors and receivers can use RX sectors to obtain sufficient link quality. The term "beamforming (BF)" refers to the process of determining the optimal TX / RX sector(s) for the link prior to data transmission. The term "BF training (BFT)" refers to the training process within beamforming.

[0011] The foregoing paragraphs have been provided by way of general introduction and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWING

[0012] A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:Fig. 1 shows a diagram of an example of conventional beamforming for asymmetric links.Fig. 2 shows a diagram of an embodiment of a proposed beamforming according to a first case for asymmetric link leveraging MLO.Fig. 3 shows a first embodiment of a BFT feedback frame.Fig. 4 shows a second embodiment of a BFT feedback frame.Fig. 5 shows an embodiment of a BFT acknowledgement frame.Fig. 6 shows a diagram of a first embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO.Fig. 7 shows a diagram of a second embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO.Fig. 8 shows a diagram of a third embodiment of the proposed beamforming according to the first case f and option A or asymmetric link leveraging MLO.Fig. 9 shows a diagram of a fourth embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO.Fig. 10 shows a diagram of a fifth embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO.Fig. 11 shows a diagram of a first embodiment of the proposed beamforming according to the first case and option B for asymmetric link leveraging MLO.Fig. 12 shows a diagram of a second embodiment of the proposed beamforming according to the first case and option B for asymmetric link leveraging MLO.Fig. 13 shows a diagram of an embodiment of the proposed beamforming according to case 2 for asymmetric links leveraging MLO.Fig. 14 shows a diagram of an embodiment of the proposed beamforming according to case 2 and option A for asymmetric links leveraging MLO.Fig. 15 shows a diagram of an embodiment of the proposed beamforming according to case 2 and option B for asymmetric links leveraging MLO.Fig. 16 shows a flow chart of an embodiment of a first communication method according to the present disclosure.Fig. 17 shows a flow chart of an embodiment of a second communication method according to the present disclosure.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0013] Both low latency and high throughput may be required at the same time for future applications such as virtual reality (VR), where a console device and head mount display have untethered connectivity. According to IEEE 802.11ad and 802.1 lay standards, Sector Sweep (SSW) is performed prior to data transmission, and in SSW phase two variants of BF can be performed: 1) Transmit Sector Sweep (TXSS) and 2) Receive Sector Sweep (RXSS). Each of TXSS and RXSS can be performed for beamforming initiator’s sector as well as beamforming responder’s sector. Typically, in TXSS for the initiator, the initiator transmits training signals using different TX sectors while responder receives the signals with quasi-omnidirectional pattern. After the transmission of the training signals, the responder feeds back the best (or several of the best) transmit sector(s) to the initiator, and vice versa in RXSS for the initiator.

[0014] According to IEEE 802.11 ay, BFs for communication devices with antenna reciprocity are standardized as well. The background of antenna reciprocity is that TX sector and RX sector typically are not identical due to imperfect TX / RX circuitry within each communication device, but that calibration allows the communication device to compensate this mismatch so that the TX sector and the RX sector are the same beam pattern.

[0015] One of the BFs leveraging antenna reciprocity in IEEE 802.11 ay is called "beamforming for asymmetric links", where the link of the initiator to the responder has better quality than the link of the responder to the initiator during the SSW phase. The asymmetric links essentially happen from the differences of sector gains between the initiator and the responder. For example, if the initiator is a station (STA) with high gain of TX sectors and the responder is a STA with less gain of TX sectors, training signals from the initiator to the responder can be decoded correctly, but training signals from the responder to the initiator cannot be decoded correctly. This is because the device should set a quasi- omnidirectional pattern during SSW. Feedback for the TXSS for the initiator can neither be decoded at the initiator because the initiator sets quasi-omnidirectional patterns for receiving the feedback. In view of the above, both initiator and responder set their sectors during the SSW phase as beamforming for asymmetric links.

[0016] Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, Fig. 1 shows a diagram of an example of conventional beamforming for asymmetric links. In this example, TXSS for the initiator is performed as part of beacon signaling.

[0017] In the beacon transmission from the initiator, several beacon frames, each of which can be appended with training field (TRN-R), are sent using different TX sectors. The responder can set and switch RX sectors while receiving each TRN-R, and thus can train which tuple of initiator’s TX sector ID, responder’s RX sector ID> is the best (or better). In Fig. 1 , the responder sets RX sectors from RX sector #1 to RX sector #m during receiving each TRN-R. Each communication device has predetermined sectors with sector IDs.

[0018] Since the initiator does not know which initiator’s RX sector should be set to receive feedback signals from the responder, feedback of the best (or better) tuple of <i nitiator’s TX sector, responder’s RX sector> is sent in a feedback phase. In the feedback phase, feedback is sent by the responder in scheduled time slot(s), in which the initiator’s RX sector, which is estimated as the best TX sector by the responder, is used. The schedule information of the feedback phase, for example which RX sector of initiator is set at which time slot, is included in a beacon frame. The decision whether the link is asymmetric ornot is made by responders by estimating link quality with 1) parameters contained in the beacon frames from initiators and 2) RSSI (Received Signal Strength Indicator) of the received beacon frame at the responders.

[0019] As depicted in Fig. 1, it may be assumed that initiator’s TX sector #N, responder’s RX sector #1 > is estimated as the best tuple of initiator’s TX sector ID, responder’s RX sector ID> by the responder at the end of the beacon transmission. In the feedback phase, as indicated in the beacon frames, the initiator allocates different durations in each of which the initiator sets different RX sectors. The responder sends feedback frame(s) (in this case SSW frame(s)) in the duration where the initiator sets RX sector #N.

[0020] BF for asymmetric links takes a long time and is hardly applicable because the conventional communication systems require the initiator to wait an amount of time with different RX sectors in the feedback phase. The approach disclosed herein proposes efficient BF leveraging MLO, where mmWave and non-mmWave links are used. Each of the proposed BFs is called multi-link-assisted BF hereinafter. One of the elements of the present disclosure is that feedback information for BF is preferably sent on non-mmWave links to make the BF duration shorter.

[0021] In the following two main cases will be described, both assuming that BFT signals are transmitted on mmWave links: According to case 1, transmissions of training signals are not suspended in the middle of the BFT. According to case 2, transmissions of training signals can be suspended in the middle of the BFT.

[0022] Fig. 2 shows a diagram of a first embodiment of a proposed beamforming according to case 1 for asymmetric links leveraging MLO. Two STAs are associated with each other, wherein STA1 operates as initiator and STA2 operates as responder. As illustrated in Fig. 2, both STA1 and STA2 have two links, one of which is a mmWave link 10 and the other is a non-mmWave link 11, e.g. a Sub-7GHz link (e.g. having a center frequency below 7.125 GHz). In the following, the non-mmWave link is referred to as Sub-7GHz link, but essentially it is a link where beamforming is not required for association ad / or controlmessage exchange, and other non-mmWave links may be used instead of the Sub-7GHz link.

[0023] In an MLO setup phase 20, STA1 and STA2 exchange their capabilities and establish association with each other. The capabilities exchange is typically performed on the Sub- 7GHz link, because in general mmWave links, especially asymmetric links, require performing BF prior to exchange of information with each other. The MLO setup essentially allows STA1 and STA2 to know which functions each of them supports (e.g. BF for asymmetric links, number of TX / RX Sectors, etc.), and to make the initial configuration, e.g. which links to be used afterwards. One of the examples for the capabilities exchange leveraging MLO is exchanging an STA Profile subelement. It shall be assumed that STA1 and STA2 are configured to use least one Sub-7GHz link and one mmWave link for data transmission through MLO setup below. In the MLO setup, it can also be included that a specific channel is allocated to the Sub-7GHz link to ensure that any other communication devices do not make interference with the link. The link for the MLO setup may be different from the Sub-7GHz link where the BF request and the response are exchanged.

[0024] After the MLO setup, a BFT request and response phase 21 is carried out. STA1 transmits a request for BF (BF Request frame 210 in Fig. 2) to STA2 on the Sub-7GHz link 11. After the BF request frame reception, STA2 transmits to STA1 an optional response (BFT response frame 211 in Fig. 2) to the request. RTS (Request-to-send) frame 212 and CTS (Clear-to-send) frame 213 exchange (not shown in Fig. 2) on the Sub-7GHz link 11 may additionally be performed as TXOP (Transmission Opportunity) obtaining prior to transmitting BF request frame. The RTS frame 212 on the Sub-7GHz link 11 may be identical to an RTS frame according to the IEEE 802.11 standard, and the CTS frame 213 on the Sub-7GHz link 11 may be identical to a CTS frame according to the IEEE 802.11 standard.

[0025] The BF request frame 210 may include the following indications: The number of STATs TX sectors which will be trained in the following BFT phase 22 on the mmWave link 10 and the number of STATs TX antennas which will be trained in the following BFT phase 22 on the mmWave link 10. The BF response frame to the BF request is transmitted as an acknowledgement and may include the following requests for the following BFT phase 22:The number ( / VRx Sector) of STA2’s RX sectors which will be trained in BFT phase 22 and the number ( / VRx ant) of STA2’s RX antennas which will be trained in BFT phase 22.

[0026] As the nature of MLO, it is worth to mention how TXOP can be obtained on both links. Three exemplary options of signaling on the mmWave link are mentioned below. Each signaling takes place at the same time with the exchange of BF request 210 and BFT response 211 . The RTS frame 212 on the mmWave link can be identical to the RTS frame or DMG-RTS frame according to the IEEE 802.11 standard, and CTS frame 213 on mmWave link can be identical to CTS frame or DMG-CTS frame according to the IEEE 802.11 standard.

[0027] According to a first option, RTS / CTS frame exchange on the mmWave link 10 is performed as shown in Fig. 2. STA1 transmits a RTS frame 212 to STA2 on the mmWave link 10 to obtain TXOP on the mmWave link. In the transmission of the RTS frame, at least one of the following conditions can be met: a) The BF request frame also includes an indication that an RTS frame is sent on the mmWave link; b) The RTS frame on the mmWave link is sent at the same time with the transmission of the BF request; and d) Each PPDll (PHY Data Protocol Data Unit) of the RTS frame on the mmWave link and the BF request has substantially the same length by, for example, padding. After receiving the BF request frame 210 from STA1 , STA2 sends a CTS frame 213 on the mmWave link. In the transmission of the CTS frame 213 on the mmWave link, at least one of the following conditions can be met: a) The CTS frame on mmWave link is sent at the same time with the response for BF request; and b) Each PPDU of the both the response and the CTS frame has substantially the same length by, for example, padding. The RTS / CTS frame exchange shall not be performed if the inter frame space (IFS) between the RTS frame and the CTS frame on the mmWave link is different from IFS between the BF request frame and the response frame on the Sub-7GHz link.

[0028] According to a second option, a CTS frame is transmitted from the STA1 on the mmWave link 10 (not shown in Fig. 2) at the same time with the BF request frame to obtain TXOP on the mmWave link. In this case, STA1 can include an indication that STA1 sends a CTS frame on the mmWave link into the BF request frame.

[0029] According to a third option, no frame exchange is performed on the mmWave link (not shown in Fig. 2), i.e., STA1 does not transmit any frames on the mmWave link. In this case, STA1 can include an indication that STA1 does not send any frame on the mmWave link into the BF request frame.

[0030] After exchanging BF request and BFT response on the Sub-7GHz link, a BFT phase 22 is performed, in which STA1 and STA2 perform BFT for their TX / RX sectors on the mmWave link in the TXOP that was established the BFT request and response phase 21. As illustrated in Fig. 2, STA1 transmits SSW frames 220a-220N each of which is appended with a TRN field 221a-221N.

[0031] A common TX sector is used for transmission of the SSW frame and appended TRN field, but different TX sectors can be used for different SSW frames. Each SSW frame may contain one or more of the following indications:Control frame: indication that the frame is which frame.Length: number of bits or octets of the frame.RA (Receiving STA Address): indication of the MAC address of the communication device that is the intended destination of the frame.TA (Transmitting STA Address): indication of the MAC address of the communication device transmitting the frame.CDOWN: down-counter indicating the number of remaining SSW frame transmission to the end of training phase.Sector ID: indication of the TX sector ID used in transmission of the SSW frame. Antenna ID: indication of mmWave antenna which the transmitter is currently using for the transmission.FCS (Frame Check Sequence): extra bits for error detection.

[0032] Each TRN field may contain the same TRN subfields, but all TRN subfields within a TRN field should be transmitted through a common TX sector and mmWave antenna, which are indicated and also used for the prepended SSW frame. The number of TRN subfields may be a multiple of NRx_antx NRx-Sector. where NRx_antand NRx Sector are defined as explained above.

[0033] Generally, known fields are prepended to any frames for AFC (Auto Frequency Correction), AGC (Auto Gain Control), channel estimation etc. so that the receiver can decode the frames, and the TRN subfield may be identical to the known field for channel estimation, and the TRN subfield should be known to STA2.

[0034] In receiving each SSW frame 220, STA2 sets the sector to a quasi-omnidirectional pattern, but when receiving each appended TRN field, STA2 set different RX sectors for each TRN subfield in the TRN field so that STA2 can estimate the optimal Rx sector(s). As illustrated in Fig. 2, STA 2 changes Rx sector from #1 to #m while receiving TRN.

[0035] After the BFT phase 22, a feedback phase 23 is performed. STA2 transmits a BFT feedback frame 230 to STA1, and STA1 transmits BFT Ack frame 231 as an acknowledgement in response to the BFT feedback frame 230, both on the Sub-7GHz link 11 , as illustrated in Fig. 2. Typically, a separate TXOP is established on the Sub-7GHz link 11.

[0036] An embodiment of a BFT feedback frame 230a is illustrated in Fig. 3. Generally, the BFT feedback frame 230 may contain one or more of the following indications:Control frame 2301 : indication that the frame is which frame.Length 2302: number of bits or octets of the frame.RA (Receiving STA Address) 2303: indication of the MAC address of the communication device that is the intended destination of the frame.TA (Transmitting STA Address) 2304: indication of the MAC address of the communication device transmitting the frame.Decode Flag 2305: indication that SSW frame, which was transmitted using the TX Sector indicated in TX Sector Select field within the BFT Feedback, was decoded correctly at STA 2 or not.Tx Sector Select 2306: indication that which SSW frame was received with the best quality at STA 2 in the BFT phase.Antenna Select 2307: indication that the value of initiator’s Antenna ID of the SSW frame that was received with best quality in the BFT phase.RSSI / SNR Report 2308: indication of SNR (Signal-to-Noise Ratio) of the selected TX sector. It is estimated from the received TRN field for that TX sector, which was received with best quality during BFT phase, if the Decode Flag field indicates that SSWframe was decoded correctly. Otherwise, it is the indication of RSSI (Received Signal Strength Indicator) of the selected TX sector. It is estimated from the received TRN field that was received with best quality during BFT phase.FCS (Frame Check Sequence) 2309: extra bits for error detection.

[0037] Another embodiment of a BFT feedback frame 230b is illustrated in Fig. 4. The major difference from the BFT feedback frame 230a is that multiple tuples 2310 of <Decode Flag, Tx Sector Select, Antenna Select, RSSI / SNR Report> are indicated to let STA1 know alternative initiator’s sectors and antenna sets in this case. In this embodiment, the BFT feedback frame 230b contains Decode Flag fields 2303, Tx Sector fields 2306, Antenna Select fields 2307 and RSSI / SNR Report fields 2308 (preferably in each tuple). Further, the following field is included:Number of Tx Sector Select 2311 : indication of the number of tuples 2310 of <Decode Flag, Tx Sector Select, Antenna Select, RSSI / SNR Report> within the frame.

[0038] After receiving the BFT feedback frame 230, STA1 transmits a BFT Ack frame 231 to announce which Tx sector and antenna ID will be used in the following transmission. Fig. 5 shows an embodiment of a BFT Ack frame 231 which may contain one or more of the following indications:Control frame 2310: indication that the frame is which frame.Length 2311 : number of bits or octets of the frame.RA (Receiving STA Address) 2312: indication of the MAC address of the communication device that is the intended destination of the frame.TA (Transmitting STA Address) 2313: indication of the MAC address of the communication device transmitting the frame.Tx Sector Select 2314: indication that which initiator’s Sector to be used in the following transmissionAntenna Select 2315: indication that the value of initiator’s Antenna ID to be used in the following transmission.FCS (Frame Check Sequence) 2316: extra bits for error detection.

[0039] As illustrated in Fig. 2, the BFT feedback frame 230 and the BFT Ack frame 231 are transmitted on the Sub-7GHz link 11, but several options can be assumed. According to afirst option A (illustrated in Figs. 6 to 10), STA1 sends data to STA2 on the Sub-7GHz link 11 during the BFT phase. According to a second option B (illustrated in Figs. 11 and 12), the Sub-7GHz link 11 is busy due to other transmissions during at least the feedback phase. In other word, according to the present disclosure the BFT feedback is transmitted, at the end of the BFT, via the non-mmWave link 11 if it is not busy at the end of the BFT.

[0040] Fig. 6 shows a diagram of a first embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO. The MLO setup phase 20 is not shown in this figure. According to this embodiment data frames 223, 224 (PPDlls in this example) are transmitted to STA2 from STA1 on the Sub-7GHz link 11. The last PPDll 224 ends at the same time as the BFT phase 22. If the data size of the PPDlls 223, 224 is so large that the transmission time is estimated to exceed the end of the BFT phase 22, STA1 transmits a part of the data so that the BFT feedback frame and the BFT Ack frame can exchange on the Sub-7GHz link 11 right after the BFT phase. Receipt of the data frames 223, 224 may be acknowledged by STA2 by transmitting acknowledgement (Ack) frames 225. The acknowledgement for at least the last PPDll 224 may be transmitted separately (not shown) or in conjunction with the BFT feedback frame 230a, e.g. as part of the BFT feedback frame (as shown in Fig. 6) or attached to the BFT feedback frame.

[0041] Fig. 7 shows a diagram of a second embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO. The major difference the second embodiment shown in Fig. 6 is that STA1 obtains a TXOP 24 up to the end of the feedback phase 23 on the Sub-7GHz link 11 by exchange of RTS and CTS frames 214, 215 prior to the transmission of the BF request frame 210. This case can be considered based on the following assumptions: The data has been already queued before the RTS frame 214 is transmitted on the Sub-7GHz link 11. Further, the above data size is sufficient such that the transmission time is expected to be substantially identical to or longer than the BFT phase 22 using the worst code rate of candidate MCS (Modulation and Coding Scheme). Still further, the TXOP 24 for the feedback phase 23 should be ensured due to low latency demands.

[0042] According to this embodiment, the RTS frame 214 and / or the CTS frame 215 indicate(s) that the TXOP 24 has a duration such that the TXOP 24 covers at least the estimated feedback phase 23. To ensure that the time of the feedback phase 23 is fixed before transmitting the BF request, the BF request frame 210 may indicate at least one of the following: a) the number of STA2’s RX sectors and antennas to be trained in the BFT phase 22; b) the indication that the response frame 211 to the BF request frame 210 shall not include the request of the number of STA2’s RX sectors and antennas to be trained in the BFT phase 22; and c) the indication that STA1 does not consider STA2’s request of the number of STA2’s RX sectors and antennas to be trained in the BFT phase 22.

[0043] Fig. 8 shows a diagram of a third embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO. According to this embodiment separate TXOPs are obtained. For example, there are two TXOPs 25, 26, the first TXOP 24 having a duration from the RTS / CTS exchange to the end of the BFT phase 22, and the second TXOP 25 covering the feedback phase 23. In this case, another RTS / CTS exchange exchanging RTS frame 232 and CTS frame 233 may occur just before the feedback phase 23 as shown in Fig. 8.

[0044] According to another embodiment, the first TXOP 25 may cover the transmission of the Ack+BFT feedback frame 230a, and the second TXOP 26 may only cover the BFT Ack transmission 231. This is illustrated in Fig. 9 showing a fourth embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO.

[0045] The above embodiments assume that PPDlls are transmitted during the BFT phase 22, but it may be considered that STA1 does not have sufficient traffic so that the transmitted PPDll(s) on the Sub-7GHz link 11 may not occupy this link up to the end of the BFT phase 22. Fig. 10 shows a diagram of a fifth embodiment of the proposed beamforming according to the first case and option A for asymmetric link leveraging MLO. According to this embodiment PPDU(s) 223 are transmitted during the BFT phase 22, but the PPDUs do not occupy the whole duration of the BFT phase 22. In this case, another RTS / CTS exchange exchanging RTS frame 226 and CTS frame 227 may be performed to ensurethat the BFT feedback 230 can be transmitted after the BFT phase. 22 The end time of the CTS frame 227 may be aligned with the end time of the last TRN field 221 N.

[0046] The CTS frame 227 may be transmitted earlier than the reception of the last TRN field 221 N by (TIFS+ TCTS- Ti), where TIFSis inter frame space (IFS) between the CTS frame 227 and BFT feedback frame 230, TCTSis duration of the CTS frame 227, and T is the duration between reception of the last TRN field 221 N and transmission of BFT feedback frame 230, as illustrated in Fig. 10. TIFScan be identical to SIFS (Short IFS) defined in the IEEE 802.11 standard.

[0047] Fig. 11 shows a diagram of a first embodiment of the proposed beamforming according to the first case and option B for asymmetric link leveraging MLO. According to this embodiment, STA2 assesses that the channel on the Sub-7GHz link 11 is busy due to other transmissions during at least the feedback phase 23. Hence, STA2 cannot transmit the BFT feedback frame to STA1 during the busy status but transmits the BFT feedback frame 230a after STA2 assesses that channel is clear.

[0048] Fig. 12 shows a diagram of a second embodiment of the proposed beamforming according to the first case and option B for asymmetric link leveraging MLO. According to this embodiment, the BFT feedback 230 is transmitted on the mmWave link. In an asymmetric link scenario, STA1 cannot decode any signal from STA2 unless the optimal sectors at both STA1 and STA2 are set, and the feedback phase 23 is performed on the mmWave link like "BF for asymmetric link" defined in IEEE 802.1 lay, according to which STA1 sets specific RX sectors in predetermined time slots.

[0049] STA1 and STA2 make decisions whether this process is performed. If STA 1 does not receive any signal from STA2 within a certain duration, Tthr, after transmitting the lastTRN field to STA2, STA1 recognizes that STA2 assesses that the channel is busy. Also, since STA2 assesses that the busy condition continues for the certain duration, Tthr, STA2 tries to send the BF feedback frame 230 to STA1 on the mmWave link 10.

[0050] In the embodiment shown in Fig. 12, it is assumed that STA2 estimates <Tx Sector #2, Rx Sector #1> as the best tuple of <Tx Sector ID, Rx Sector I D>, and the BFT feedback is transmitted on STA2’s Tx Sector #1 in the time slot where STA1 sets Rx Sector #2. After receiving the BFT feedback, STA 1 transmits BFT Ack to STA2 and the feedback phase 23 is terminated. Although it is illustrated that the time slot for each STATs Tx sector is one, the number of time slots for each STATs Rx sector can be more than one.

[0051] The BF Request frame 210 may further include a) an indication of potential scheduling information of the time slots and / or b) the value of Tthr. The scheduling information within the BF request 210 may contain a) the duration and start time of each time slot and / or b) the Initiator’s Rx sector ID and antenna ID to be used in each time slot.

[0052] With reference to Figs. 2 to 12 different embodiment for a case 1 , according to which transmissions of training signals are not suspended in the middle of the BFT, have been illustrated. In the following, with reference to Figs. 13 to 15, different embodiments for a case 2, according to which transmissions of training signals can be suspended in the middle of the BFT, will be illustrated.

[0053] Fig. 13 shows a diagram of an embodiment of the proposed beamforming according to case 2 for asymmetric links leveraging MLO. According to this embodiment, transmissions of BFT in the BFT phase 22 are suspended. The major difference point from the embodiments according to case 1 is that BFT is suspended after STA1 receives the BFT feedback indication 230 from STA2 on the Sub-7GHz link 11.

[0054] In the MLO setup phase 120, STA1 and STA2 exchange their capabilities and establish association with each other. This phase is similar or identical with the MLO setup phase 20 in case 1, but STA1 and STA2 may exchange capabilities as follows: Initially, STA1 may suspend BFT if STA1 receives the indication that STA2 estimates at least one of the tuples of <STA1’s TX Sector ID, STA2’s RX Sector ID> with a certain link quality. Subsequently, STA2 may send the feedback information to STA 1 during BFT transmission on the Sub-7GHz link 11. The feedback information may also be preliminary feedback and indication that STA1 can stop BFT.

[0055] After the MLO setup, in the BF request and response phase 21 , STA1 sends a request 210 for BFT to STA2 on the Sub-7GHz link 11 if STA1 obtains TXOP on the Sub-7GHz link. STA2 the sends the response 211 in response to the BF request frame 210. The BF request and response phase 21 may thus be similar or identical to the BF request and response phase 21 in case 1 but may contain further information as explained below.

[0056] To allow STA2 to estimate the link quality, the BF request frame 210 may include one or more of the following indications: the indication that the corresponding BFT can be suspended when STA2 transmits the indication that STA2 estimates at least one of tuples of <STA1’s TX Sector ID, STA2’s RX Sector ID> with certain link quality, Sank quauty', the parameter of the above certain link quality, which can indicate either minimum required RSSI, minimum required estimated SNR for link from STA1 to STA2, or required estimated SNR for link from STA2 to STA1 ; the indication that the certain link quality, Sunk quauty, is intended to be one of the following: required RSSI estimated by STA2, required SNR for link from STA1 to STA2 or required SNR for link from STA2 to STAI .

[0057] If the BF request frame 210 contains indication a) or b) mentioned above, STA2 estimates RSSI of the received SSW frame 220 or SNR of the received SSW frame 220 within the BFT phase 22. If the BF request frame 210 contains indication c) mentioned above, the way of calculating the estimated SNR for the link from STA2 to STA1 by STA2 may be as follows:SNRest= PSTA2 + STA2-TX + RSSI — C — GSTA2-Rx) , (1) where SNRestis an estimation value by STA2 of the expected SNR that STA2’s transmission is received by the STA 1, PSTA2and GSTA2-Txare transmit power and antenna gain for the expected STA2’s transmission, RSSI is the power that was measured by STA2 during reception of the TRN field within the BFT phase 22, and C is the value contained in the received SSW frame 220 within the BFT phase. The indicated value of C can be set to as follows:G = PSTAI + GSTAI-TX + PNOISB > (2)where PSTAI and GSTAI-TXarethe transmit power and antenna gain used in the SSW frame transmission and PNolseis the estimated noise power level by STA1. Each unit of the above parameters is dB or dBm.

[0058] After exchanging BF request and response on the Sub-7GHz link 11, STA1 and STA2 perform BF training in the BFT phase 122 for their TX / RX sectors. BFT may be similar or identical as the BFT phase 22 in case 1, i.e., STA1 transmits SSW frames 220a-220K+1 each of which is appended with a TRN field 221a-221 K+1. SSW frames and TRN fields may be similar or identical as in case 1 , but the SSW frames may additionally contain the parameter C mentioned above.

[0059] During the BFT phase 122, STA2 estimates the link quality of what is indicated in the BF request frame 210, i.e. RSSI, SNR for the link from STA1 to STA2, or SNR for the link from STA2 to STA1. If the estimated link quality is equal to or greater than a threshold sunk quality that may be contained within the BF request frame 210, STA2 transmits the BFT feedback frame 230 to STA1. As an exemplary case, Fig. 13 illustrates that K+1 SSW frames are transmitted by STA1 , but STA2 estimates that at least one of sector ID #1 to #K, which are used for the first K SSW frames, is a candidate to satisfy that the link quality is equal to or greater than Stink quauty If STA2 receives SSW frames after STA2 transmits the BFT feedback frame 230, STA2 does not have to decode those SSW frames.

[0060] It is not illustrated in Fig. 13, but STA1 can stop transmitting the (K+1)-th SSW frame 220K+1 appended with the TRN field 221 K+1 during the transmission of the frame, during which STA1 receives the BFT feedback frame 230.

[0061] In the feedback phase 123 a BFT end frame 234 may be transmitted by STA1 on the Sub- 7GHz link 11 that contains the indication which Tx sector and antenna ID will be used in the following transmission and that the BFT phase 122 on the mmWave link 10 is ended. A CF (Contention Free) end frame (not shown in Fig. 13) may be transmitted on the Sub- 7GHZ link 11 after the BFT end frame 234 or contained in BFT end frame 234.

[0062] STA2 may also send a CF end frame 235 on the mmWave link 10 at the same time with the BFT Ack frame (not shown in Fig. 13) or after the current SSW frame transmission to announce that the TXOP on the mmWave link 10 is ended. The CF end frame may be similar or identical as described in the IEEE 802.11 standard.

[0063] As illustrated in Fig. 13, BFT feedback and BFT end frames are transmitted on the Sub- 7GHz link 11, but several options, where the Sub-7GHz link is occupied by another transmission, can be assumed as follows: According option A STA 1 sends data to STA 2 on the Sub-7GHz link during the BFT phase 122 and according to option B the Sub-7GHz link 11 is busy due to other transmissions during at least the feedback phase 123.

[0064] Fig. 14 shows a diagram of an embodiment of the proposed beamforming according to case 2 and option A for asymmetric links leveraging MLO. According to this embodiment data frames 223, 224 are transmitted to STA 2 from STA 1 on the Sub-7GHz link 11. In Fig. 14 it is also illustrated that the last PPDll 225 ends at the time with or after the N-th SSW frame 220N on the mmWave link 10 has been completely transmitted. In this case, STA2 transmits the BFT feedback frame 230a to STA1 after the PPDll transmission. The Ack frame to the PPDll(s) may be transmitted separately (as the Ack frame 225) or can be transmitted with the BFT feedback frame 230a.

[0065] To allow STA2 to send the BFT feedback frame 230a on the Sub-7GHz link 11 while STA1 obtains TXOP on the Sub-7GHz link 11, STA1 may include RDG (Reverse Direction Grant) into a PPDll 223, 224 on the Sub-7GHz link 11 during the BFT phase 122. RDG may be similar or identical as described in the IEEE 802.11 standard.

[0066] If the data size is so large that the transmission time is estimated to exceed the end of the M-th SSW frame transmission, where M is maximum number of the SSW frames to be transmitted in the BFT phase, STA1 may transmit only a part of the data so that the BFT feedback frame 230a can be transmitted on the Sub-7GHz link 11 at latest after the M-th SSW transmission.

[0067] STA2 can decode SSW frames and estimate link qualities up to the BFT feedback transmission even if STA2 has already estimated at least one of the tuples of <STA1’s Tx Sector ID, STA2’s Rx Sector ID> with required link qualities.

[0068] Fig. 15 shows a diagram of an embodiment of the proposed beamforming according to case 2 and option B for asymmetric links leveraging MLO. According to this embodiment the Sub-7GHz link at STA 2 is busy due to other transmissions during at least the feedback phase 123. STA 2 cannot transmit the BFT feedback frame 230 to STA 1 during the busy status but transmits the BFT feedback frame 230 after STA 2 assesses that the channel on Sub-7GHz link 11 is clear. After receiving the BFT feedback frame 230 from STA2, STA1 transmits a BFT end frame 234 on the Sub-7GHz link 11 and, optionally, a CF end frame 235 on the mmWave link 10. Another exemplary case is that the BFT feedback frame 230 is transmitted on the mmWave link 10. This case is similar or identical to what has been described above with reference to case 1.

[0069] Fig. 16 shows a flow chart of an embodiment of a first communication method 300 that may be executed by the first communication device (STA1) that operates as initiator and communicates with the second communication device (STA2) that operates as responder. In a first step 301 the first communication device connects to the second communication device via at least two links including a non-mmWave link and a mmWave link. In a second step 302 it transmits a BF request frame to the second communication device via the non-mmWave link. In a third step 303 it performs BF training with the second communication device by subsequently transmitting a plurality of SSW frames and training fields in different transmit sectors via the mmWave link. In a fourth step 304 it receives, at the end of the BFT, a BFT feedback frame from the second communication device via the non-mmWave link if it (the non-mmWave link) is not busy at the end of the BFT.

[0070] Fig. 17 shows a flow chart of an embodiment of a second communication method 400 that may be executed by the second communication device (STA2) operating a responder. In a first step 401 the second communication device connects to the first communication device via the at least two links including a non-mmWave link and a mmWave link. In a second step 402 it receives a BF request from the first communication device via the non- mmWave link. In a third step 403 it performs BF training with the first communicationdevice by subsequently receiving a plurality of SSW frames and training fields in different transmit sectors via the mmWave link, wherein during the reception of a training field different receive sectors are used. In a fourth step it transmits, at the end of the BFT, a BFT feedback frame to the first communication device via the non-mmWave link if it (the non-mmWave link) is not busy at the end of the BFT.

[0071] In summary, the present disclosure aims at quick beamforming for multi-link operation using non-mmWave (e.g. Sub-7GHz) and mmWave links. To establish a link for data communication with sufficient quality without consuming a huge amount of time, mechanisms of shorter BF time leveraging MLO, especially an asymmetric link case in mmWave, are present, particularly for low-latency and high-throughput applications.

[0072] Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. As will be understood by those skilled in the art, the present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present disclosure is intended to be illustrative, but not limiting of the scope of the disclosure, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.

[0073] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0074] In so far as embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Further, such a software may also be distribut-ed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0075] The elements of the disclosed devices, apparatus and systems may be implemented by corresponding hardware and / or software elements, for instance appropriated circuits. A circuit is a structural assemblage of electronic components including conventional circuit elements, integrated circuits including application specific integrated circuits, standard integrated circuits, application specific standard products, and field programmable gate arrays. Further a circuit includes central processing units, graphics processing units, and microprocessors which are programmed or configured according to software code. A circuit does not include pure software, although a circuit includes the above-described hardware executing software.

[0076] It follows a list of further embodiments of the disclosed subject matter:1 . First communication device (STA 1) configured to operate as initiator and communicate with a second communication device (STA 2) configured to operate as responder, the first communication device comprising circuitry configured to: connect (20) to the second communication device via at least two links including a non-mmWave link (11) and a mmWave link (10); transmit (21) a beamforming (BF) request frame (210) to the second communication device via the non-mmWave link; perform (22) BF training (BFT) with the second communication device by subsequently transmitting a plurality of sector sweep (SSW) frames (220A ... 220N) and training fields (221 A ... 221 N) in different transmit sectors via the mmWave link; and receive (23), at the end of the BFT, a BFT feedback frame (230) from the second communication device via the non-mmWave link if it is not busy at the end of the BFT.2. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to use the same beam pattern for transmission and reception in the mmWave link., wherein a transmit / receive sector is defined by a specific beam pattern3. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to include multiple training subfields in a training field.4. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to transmit a BFT acknowledgement frame to the second communication device via the non-mmWave link in response to the BFT feedback frame received from the second communication device via the non-mmWave link.5. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to include into the BFT acknowledgement frame one or more of: a control frame indication indicating the frame is a BFT acknowledgement frame; a length indication indicating the number of bits or octets of the BFT acknowledgement frame; a destination indication indicating the communication device to which the BFT acknowledgement frame is addressed; a source indication indicating the first communication device; a sector select indication indicating which transmit sector to be used in the following transmission via the mmWave link; an antenna select indication indicating which antenna to be used in the following transmission via the mmWave link; and a frame check sequence (FCS) for use for error detection.6. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to include into an SSW frame one or more of: a control frame indication indicating the frame is an SSW frame; a length indication indicating the number of bits or octets of the SSW frame; a destination indication indicating the communication device to which the SSW frame is addressed; a source indication indicating the first communication device; a counter indication indicating the number of remaining SSW frames to be transmitted up to the end of the BFT ;a sector select indication indicating which transmit sector has been used for the SSW frame transmission; an antenna select indication indicating which antenna has been used for the SSW frame transmission; and a frame check sequence (FCS) for use for error detection.7. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to use a Sub-7 GHz link as non-mmWave link, the Sub- 7 GHz link having a center frequency below 7.125 GHz.8. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to transmit one or more data frames to the second communication device during the BFT via the non-mmWave link, wherein the transmission of data frames ends at the latest at the end of the BFT.9. First communication device according to embodiment 8, wherein the circuitry is configured to transmit, via the non-mmWave link, i) no ready-to-send (RTS) frame or ii) an RTS frame before the transmission of the BF request, and / or before, during and / or after the reception of the BFT feedback frame, and / or during the BFT.10. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to transmit to the second communication device before the BFT, in particular as part of the BF request frame, a clearance period information indicating a clearance period for use by the second communication device to decide if the BFT feedback frame shall be transmitted via the mmWave link if the if the non-mmWave link does not become idle within the clearance period, and / or a link quality information indicating a link quality threshold for use by the second communication device to decide if the BFT feedback frame shall be transmitted during the BFT.11. First communication device according to any one of the preceding embodiments, wherein the circuitry is configured to suspend the BFT if the BFT feedback frame is received during the BFT.12. Second communication device (STA 2) configured to operate as responder and communicate with a first communication device (STA 1) configured to operate as initiator, the second communication device comprising circuitry configured to: connect (20) to the first communication device via at least two links including a non-mmWave link (11) and a mmWave link (10); receive (21) a beamforming (BF) request (210) from the first communication device via the non-mmWave link; perform (22) BF training (BFT) with the first communication device by subsequently receiving a plurality of sector sweep (SSW) frames (220A ... 220N) and training fields (221 A ... 221 N) in different transmit sectors via the mmWave link, wherein during the reception of a training field different receive sectors are used; and transmit (23), at the end of the BFT, a BFT feedback frame (230) to the first communication device via the non-mmWave link if it is not busy at the end of the BFT.13. Second communication device according to embodiment 12, wherein the circuitry is configured to use the same beam pattern for transmission and reception in the mmWave link.14. Second communication device according to any one of embodiments 12 to 13, wherein the circuitry is configured to transmit a BF response to the first communication device via the non-mmWave link in response to the BF request received from the first communication device via the non-mmWave link.15. Second communication device according to any one of embodiments 12 to 14, wherein the circuitry is configured to receive multiple training subfields included in a training field, wherein each training subfield is received with a different receive sector.16. Second communication device according to any one of embodiments 12 to 15,wherein the circuitry is configured to include into the BFT feedback frame a decode flag indicating if a transmitted SSW frame has been decoded correctly by the second communication device; and / or two or more decode flag fields indicating two or more alternative transmit sectors for use by the first communication device.17. Second communication device according to embodiment 16, wherein the circuitry is configured to further include into the BFT feedback frame one or more of: a control frame indication indicating the frame is a BFT feedback frame; a length indication indicating the number of bits or octets of the BFT feedback frame; a destination indication indicating the communication device to which the BFT feedback frame is addressed; a source indication indicating the second communication device;- a decode flag indicating if a transmitted SSW frame, which is indicated in a corresponding sector select indication, has been decoded correctly by the second communication device; a sector select indication indicating which SSW frame was received with the best quality at the second communication device in the BFT phase; an antenna select indication indicating an antenna from which the SSW frame was received with best quality in the BFT phase; an RSSI / SNR report indication indicating the signal-to-noise ratio (SNR) or received signal strength indicator (RSSI) of the best transmit sector in the BFT phase; and a frame check sequence (FCS) for use for error detection.18. Second communication device according to any one of embodiments 12 to 17, wherein the circuitry is configured to transmit, via the non-mmWave link, in response to a ready-to-send (RTS) frame from the first communication device, a CTS frame before the transmission of the BF request, and / or before, during and / or after the reception of the BFT feedback frame, and / or during the BFT.19. Second communication device according to any one of embodiments 12 to 18, wherein the circuitry is configured to transmit the BFT feedback frame, if the non- mmWave link is busy at the end of the BFT, via the non-mmWave link as soon as the non- mmWave link becomes idle or, if the non-mmWave link does not become idle within a clearance period, after the clearance period via the mmWave link.20. Second communication device according to embodiment 19, wherein the circuitry is configured, if the non-mmWave link is busy at the end of the BFT and does not become idle within a clearance period, to estimate a best combination of a best transmit sector of the first communication device and a best receive sector of the second communication device; and transmit the BFT feedback frame after the clearance period via the mmWave link using a transmit sector estimated as best receive sector of the second communication device at a time slot at which the first communication device receives using a receive sector estimated as best transmit sector of the first communication device.21. Second communication device according to any one of embodiments 12 to 20, wherein the circuitry is configured to transmit the BFT feedback frame during the BFT if the estimated link quality become equal to or greater than a link quality threshold.22. Second communication device according to any one of embodiments 12 to 21 , wherein the circuitry is configured to transmit the BFT feedback frame, if the non- mmWave link is busy due to transmission of data frames by the first communication device to the second communication device or due to other traffic on the non-mmWave link, as soon as the last data frame has been transmitted by the first communication device or the non-mmWave link becomes idle.23. First communication method of a first communication device configured to operate as initiator and communicate with a second communication device configured to operate as responder, the first communication method comprising: connecting to the second communication device via at least two links including a non-mmWave link and a mmWave link;transmitting a beamforming (BF) request frame to the second communication device via the non-mmWave link; performing BF training (BFT) with the second communication device by subsequently transmitting a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link; and receiving, at the end of the BFT, a BFT feedback frame from the second communication device via the non-mmWave link if it is not busy at the end of the BFT.24. Second communication method of a second communication device configured to operate as responder and communicate with a first communication device configured to operate as initiator, the second communication method comprising: connecting to the first communication device via at least two links including a non- mmWave link and a mmWave link; receiving a beamforming (BF) request from the first communication device via the non-mmWave link; performing BF training (BFT) with the first communication device by subsequently receiving a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link, wherein during the reception of a training field different receive sectors are used; and transmitting, at the end of the BFT, a BFT feedback frame to the first communication device via the non-mmWave link if it is not busy at the end of the BFT.25. A non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method according to embodiment 23 or 24 to be performed.26. A computer program comprising program code means for causing a computer to perform the steps of said method according to embodiment 23 or 24 when said computer program is carried out on a computer.

Claims

CLAIMS1. First communication device configured to operate as initiator and communicate with a second communication device configured to operate as responder, the first communication device comprising circuitry configured to: connect to the second communication device via at least two links including a non- mmWave link and a mmWave link; transmit a beamforming (BF) request frame to the second communication device via the non-mmWave link; perform BF training (BFT) with the second communication device by subsequently transmitting a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link; and receive, at the end of the BFT, a BFT feedback frame from the second communication device via the non-mmWave link if it is not busy at the end of the BFT.

2. First communication device according to claim 1, wherein the circuitry is configured to use the same beam pattern for transmission and reception in the mmWave link., wherein a transmit / receive sector is defined by a specific beam pattern and / or to suspend the BFT if the BFT feedback frame is received during the BFT.

3. First communication device according to claim 1, wherein the circuitry is configured to include multiple training subfields in a training field.

4. First communication device according to claim 1, wherein the circuitry is configured to transmit a BFT acknowledgement frame to the second communication device via the non-mmWave link in response to the BFT feedback frame received from the second communication device via the non-mmWave link, and / or to include into the BFT acknowledgement frame one or more of: a control frame indication indicating the frame is a BFT acknowledgement frame; a length indication indicating the number of bits or octets of the BFT acknowledgement frame;a destination indication indicating the communication device to which the BFT acknowledgement frame is addressed; a source indication indicating the first communication device; a sector select indication indicating which transmit sector to be used in the following transmission via the mmWave link; an antenna select indication indicating which antenna to be used in the following transmission via the mmWave link; and a frame check sequence (FCS) for use for error detection.

5. First communication device according to claim 1, wherein the circuitry is configured to include into an SSW frame one or more of: a control frame indication indicating the frame is an SSW frame; a length indication indicating the number of bits or octets of the SSW frame; a destination indication indicating the communication device to which the SSW frame is addressed; a source indication indicating the first communication device; a counter indication indicating the number of remaining SSW frames to be transmitted up to the end of the BFT ; a sector select indication indicating which transmit sector has been used for the SSW frame transmission; an antenna select indication indicating which antenna has been used for the SSW frame transmission; and a frame check sequence (FCS) for use for error detection.

6. First communication device according to claim 1, wherein the circuitry is configured to use a Sub-7 GHz link as non-mmWave link, the Sub- 7 GHz link having a center frequency below 7.125 GHz.

7. First communication device according to claim 1, wherein the circuitry is configured to transmit one or more data frames to the second communication device during the BFT via the non-mmWave link, wherein the transmission of data frames ends at the latest at the end of the BFT.

8. First communication device according to claim 7, wherein the circuitry is configured to transmit, via the non-mmWave link, i) no ready-to-send (RTS) frame or ii) an RTS frame before the transmission of the BF request, and / or before, during and / or after the reception of the BFT feedback frame, and / or during the BFT.

9. First communication device according to claim 1, wherein the circuitry is configured to transmit to the second communication device before the BFT, in particular as part of the BF request frame, a clearance period information indicating a clearance period for use by the second communication device to decide if the BFT feedback frame shall be transmitted via the mmWave link if the if the non-mmWave link does not become idle within the clearance period, and / or a link quality information indicating a link quality threshold for use by the second communication device to decide if the BFT feedback frame shall be transmitted during the BFT.

10. Second communication device configured to operate as responder and communicate with a first communication device configured to operate as initiator, the second communication device comprising circuitry configured to: connect to the first communication device via at least two links including a non- mmWave link and a mmWave link; receive a beamforming (BF) request from the first communication device via the non-mmWave link; perform BF training (BFT) with the first communication device by subsequently receiving a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link, wherein during the reception of a training field different receive sectors are used; and transmit, at the end of the BFT, a BFT feedback frame to the first communication device via the non-mmWave link if it is not busy at the end of the BFT.

11. Second communication device according to claim 10, wherein the circuitry is configured to use the same beam pattern for transmission and reception in the mmWave link, and / or to transmit a BF response to the first communication device via the non-mmWave link in response to the BF request received from the first communication device via the non-mmWave link.

12. Second communication device according to claim 10, wherein the circuitry is configured to receive multiple training subfields included in a training field, wherein each training subfield is received with a different receive sector.

13. Second communication device according to claim 10, wherein the circuitry is configured to include into the BFT feedback frame a decode flag indicating if a transmitted SSW frame has been decoded correctly by the second communication device; and / or two or more decode flag fields indicating two or more alternative transmit sectors for use by the first communication device.

14. Second communication device according to claim 13, wherein the circuitry is configured to further include into the BFT feedback frame one or more of: a control frame indication indicating the frame is a BFT feedback frame; a length indication indicating the number of bits or octets of the BFT feedback frame; a destination indication indicating the communication device to which the BFT feedback frame is addressed; a source indication indicating the second communication device; a decode flag indicating if a transmitted SSW frame, which is indicated in a corresponding sector select indication, has been decoded correctly by the second communication device; a sector select indication indicating which SSW frame was received with the best quality at the second communication device in the BFT phase; an antenna select indication indicating an antenna from which the SSW frame was received with best quality in the BFT phase;an RSSI / SNR report indication indicating the signal-to-noise ratio (SNR) or received signal strength indicator (RSSI) of the best transmit sector in the BFT phase; and a frame check sequence (FCS) for use for error detection.

15. Second communication device according to claim 10, wherein the circuitry is configured to transmit, via the non-mmWave link, in response to a ready-to-send (RTS) frame from the first communication device, a CTS frame before the transmission of the BF request, and / or before, during and / or after the reception of the BFT feedback frame, and / or during the BFT.

16. Second communication device according to claim 10, wherein the circuitry is configured to transmit the BFT feedback frame, if the non- mmWave link is busy at the end of the BFT, via the non-mmWave link as soon as the non- mmWave link becomes idle or, if the non-mmWave link does not become idle within a clearance period, after the clearance period via the mmWave link, in particular, , if the non-mmWave link is busy at the end of the BFT and does not become idle within a clearance period, to estimate a best combination of a best transmit sector of the first communication device and a best receive sector of the second communication device; and transmit the BFT feedback frame after the clearance period via the mmWave link using a transmit sector estimated as best receive sector of the second communication device at a time slot at which the first communication device receives using a receive sector estimated as best transmit sector of the first communication device.

17. Second communication device according to claim 10, wherein the circuitry is configured to transmit the BFT feedback frame a) during the BFT if the estimated link quality become equal to or greater than a link quality threshold, and / or b) if the non-mmWave link is busy due to transmission of data frames by the first communication device to the second communication device or due to other traffic on the non- mmWave link, as soon as the last data frame has been transmitted by the first communication device or the non-mmWave link becomes idle.

18. First communication method of a first communication device configured to operate as initiator and communicate with a second communication device configured to operate as responder, the first communication method comprising: connecting to the second communication device via at least two links including a non-mmWave link and a mmWave link; transmitting a beamforming (BF) request frame to the second communication device via the non-mmWave link; performing BF training (BFT) with the second communication device by subsequently transmitting a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link; and receiving, at the end of the BFT, a BFT feedback frame from the second communication device via the non-mmWave link if it is not busy at the end of the BFT.

19. Second communication method of a second communication device configured to operate as responder and communicate with a first communication device configured to operate as initiator, the second communication method comprising: connecting to the first communication device via at least two links including a non- mmWave link and a mmWave link; receiving a beamforming (BF) request from the first communication device via the non-mmWave link; performing BF training (BFT) with the first communication device by subsequently receiving a plurality of sector sweep (SSW) frames and training fields in different transmit sectors via the mmWave link, wherein during the reception of a training field different receive sectors are used; and transmitting, at the end of the BFT, a BFT feedback frame to the first communication device via the non-mmWave link if it is not busy at the end of the BFT.

20. A non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method according to claim 18 or 19 to be performed.

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