WO2020225473A1

WO2020225473A1 - Shared transmission opportunity

Info

Publication number: WO2020225473A1
Application number: PCT/FI2019/050358
Authority: WO
Inventors: Olli Alanen; Mika Kasslin; David LOPEZ-PEREZ; Lorenzo GALATI GIORDANO; Adrian GARCIA RODRIGUEZ
Original assignee: Nokia Technologies Oy
Priority date: 2019-05-07
Filing date: 2019-05-07
Publication date: 2020-11-12

Abstract

This document discloses a solution for a shared transmission opportunity in a wireless network. According to an aspect, a method comprises as performed by an apparatus of a first wireless network: transmitting a transmission opportunity sharing request to an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network and duration of at least one transmission period of a shared transmission opportunity; acquiring a transmission opportunity sharing response from the access node, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network; transmitting an announcement frame indicating the at least one station of the second wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

Description

Shared Transmission Opportunity

Field

Various embodiments described herein relate to the field of wireless communications and, particularly, to sharing a transmission opportunity between wireless devices.

Background

Constant demand of higher throughput and capacity and density of various wireless networks sets a demand for improving spectral efficiency. Beamforming is a technique where a transmitter focuses radio energy towards an intended recipient and/or reduces radio energy towards an unintended recipient. The benefit may be improved link quality and/or reduced interference towards other radio receivers, as well as an improved spatial reuse.

Brief description

Some aspects of the invention are defined by the independent claims.

Some embodiments of the invention are defined in the dependent claims.

According to an aspect, there is provided an apparatus for a first wireless network, comprising means for performing: transmitting a transmission opportunity sharing request to an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network and duration of at least one transmission period of a shared transmission opportunity; acquiring a transmission opportunity sharing response from the access node, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network; transmitting an announcement frame indicating the at least one station of the second wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

In an embodiment, the transmission opportunity sharing request further indicates a transmission sequence comprising a plurality of transmission periods comprising: at least a first downlink period of a shared transmission opportunity; and a first uplink period of the shared transmission opportunity, and wherein the duration of at least one transmission period comprises a duration of at least one of the first downlink period and the first uplink period.

In an embodiment, the transmission opportunity sharing request indicates an order of the transmission periods in the transmission sequence.

In an embodiment, the transmission opportunity sharing request further indicates a maximum number of nulls for the null steering beamforming transmission.

In an embodiment, the transmission opportunity sharing response indicates at most a number stations of the second wireless network equal to the maximum number of nulls.

In an embodiment, the means are configured to perform at least one of said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using null steering beamforming such that a null is directed to the at least one station of the second wireless network.

In an embodiment, the means are configured to perform said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using the null steering beamforming such that the null is directed to the at least one station of the second wireless network.

In an embodiment, the shared transmission opportunity comprises a first time interval during which the apparatus and the access node are allowed to transmit subsequently but not simultaneously, and further comprises a second time interval during which the apparatus and the access node are allowed to transmit at least partially simultaneously.

In an embodiment, the means are configured to perform the null steering beamforming transmission in the first wireless network simultaneously with a null steering beamforming transmission by the access node during the shared transmission opportunity.

According to another aspect, there is provided an apparatus for a first wireless network, comprising means for performing: receiving a transmission opportunity sharing request from an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the second wireless network and duration of at least one transmission period of a shared transmission opportunity; transmitting a transmission opportunity sharing response to the access node, the transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the first wireless network; transmitting an announcement frame indicating the at least one station of the first wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the first wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

In an embodiment, the transmission opportunity sharing request further indicates a maximum number of nulls, and wherein the means are configured to indicate in the transmission opportunity sharing response at most a number stations of the first wireless network equal to the maximum number of nulls.

According to another aspect, there is provided an apparatus for a first wireless network, comprising means for performing: detecting a transmission opportunity sharing message for a shared transmission opportunity, the transmission opportunity sharing message indicating the apparatus and comprising an identifier of a first access node of the first wireless network and an identifier of a second access node of a second wireless network different from the first wireless network; adding the identifier of the second access node to a beamforming address set stored in the apparatus; receiving an announcement frame indicating the apparatus from the second access node; receiving a channel sounding signal from the second access node; measuring the channel sounding signal on the basis of the second access node comprised in the beamforming address set of the apparatus, and determining channel state information on the basis of the measured channel sounding signal; transmitting a beamforming report comprising the channel state information; and receiving a null steering beamforming transmission from the first access node during the shared transmission opportunity shared between the first access node and the second access node.

In an embodiment, the means are further configured to, upon receiving the null steering beamforming transmission from the first access node, transmit an acknowledgement frame to the first access node during the shared transmission opportunity.

In an embodiment, the above-described means comprises: at least one processor; and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the respective apparatus.

According to another aspect, there is provided a method for an apparatus of a first wireless network, comprising: transmitting, by the apparatus, a transmission opportunity sharing request to an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network and duration of at least one transmission period of a shared transmission opportunity; acquiring, by the apparatus, a transmission opportunity sharing response from the access node, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network; transmitting, by the apparatus, an announcement frame indicating the at least one station of the second wireless network; transmitting, by the apparatus, a channel sounding signal; receiving, by the apparatus, a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing, by the apparatus on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

In an embodiment, the apparatus performs at least one of said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using null steering beamforming such that a null is directed to the at least one station of the second wireless network.

In an embodiment, the apparatus performs said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using the null steering beamforming such that the null is directed to the at least one station of the second wireless network.

In an embodiment, the apparatus performs the null steering beamforming transmission in the first wireless network simultaneously with a null steering beamforming transmission by the access node during the shared transmission opportunity.

According to another aspect, there is provided a method for an apparatus of a first wireless network, comprising: receiving, by the apparatus, a transmission opportunity sharing request from an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the second wireless network and duration of at least one transmission period of a shared transmission opportunity; transmitting, by the apparatus, a transmission opportunity sharing response to the access node, the transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the first wireless network; transmitting, by the apparatus, an announcement frame indicating the at least one station of the first wireless network; transmitting, by the apparatus, a channel sounding signal; receiving, by the apparatus, a beamforming report comprising channel state information from the at least one station of the first wireless network; and performing, by the apparatus on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

According to another aspect, there is provided a method for an apparatus of a first wireless network, comprising means for performing: detecting, by the apparatus, a transmission opportunity sharing message for a shared transmission opportunity, the transmission opportunity sharing message indicating the apparatus and comprising an identifier of a first access node of the first wireless network and an identifier of a second access node of a second wireless network different from the first wireless network; adding, by the apparatus, the identifier of the second access node to a beamforming address set stored in the apparatus; receiving, by the apparatus, an announcement frame indicating the apparatus from the second access node; receiving, by the apparatus, a channel sounding signal from the second access node; measuring, by the apparatus, the channel sounding signal on the basis of the second access node comprised in the beamforming address set of the apparatus, and determining channel state information on the basis of the measured channel sounding signal; transmitting, by the apparatus, a beamforming report comprising the channel state information; and receiving, by the apparatus, a null steering beamforming transmission from the first access node during the shared transmission opportunity shared between the first access node and the second access node.

In an embodiment, the apparatus transmits, upon receiving the null steering beamforming transmission from the first access node, an acknowledgement frame to the first access node during the shared transmission opportunity.

According to another aspect, there is provided a computer program product embodied on a computer-readable medium and comprising a computer program code readable by a computer for an apparatus of a first wireless network, wherein the computer program code configures the computer to carry out a computer process comprising: transmitting a transmission opportunity sharing request to an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network and duration of at least one transmission period of a shared transmission opportunity; acquiring a transmission opportunity sharing response from the access node, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network; transmitting an announcement frame indicating the at least one station of the second wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

A computer program product embodied on a computer-readable medium and comprising a computer program code readable by a computer for an apparatus of a first wireless network, wherein the computer program code configures the computer to carry out a computer process comprising: receiving a transmission opportunity sharing request from an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the second wireless network and duration of at least one transmission period of a shared transmission opportunity; transmitting a transmission opportunity sharing response to the access node, the transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the first wireless network; transmitting an announcement frame indicating the at least one station of the first wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the first wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

According to another aspect, there is provided a computer program product embodied on a computer-readable medium and comprising a computer program code readable by a computer for an apparatus of a first wireless network, wherein the computer program code configures the computer to carry out a computer process comprising: detecting a transmission opportunity sharing message for a shared transmission opportunity, the transmission opportunity sharing message indicating the apparatus and comprising an identifier of a first access node of the first wireless network and an identifier of a second access node of a second wireless network different from the first wireless network; adding the identifier of the second access node to a beamforming address set stored in the apparatus; receiving an announcement frame indicating the apparatus from the second access node; receiving a channel sounding signal from the second access node; measuring the channel sounding signal on the basis of the second access node comprised in the beamforming address set of the apparatus, and determining channel state information on the basis of the measured channel sounding signal; transmitting a beamforming report comprising the channel state information; and receiving a null steering beamforming transmission from the first access node during the shared transmission opportunity shared between the first access node and the second access node.

List of drawings

Embodiments are described below, by way of example only, with reference to the accompanying drawings, in which

Figure 1 illustrates a wireless communication scenario to which some embodiments of the invention may be applied;

Figures 2 to 4 illustrate some embodiment of processes for setting up and operating a transmission opportunity shared between two wireless networks;

Figure 5 illustrates a signalling diagram according to an embodiment employing the shared transmission opportunity;

Figure 6 illustrates frame transmissions within the shared transmission opportunity according to an embodiment;

Figure 7 illustrates another embodiment employing the shared TXOP; and

Figures 8 and 9 illustrate block diagrams of structures of apparatuses according to some embodiments of the invention.

Description of embodiments

The following embodiments are examples. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Furthermore, words "comprising" and "including" should be understood as not limiting the described embodiments to consist of only those features that have been mentioned and such embodiments may contain also features/structures that have not been specifically mentioned.

A general wireless communication scenario to which embodiments of the invention may be applied is illustrated in Figure 1. Figure 1 illustrates wireless communication devices comprising a plurality of access points (AP) 110, 112 and a plurality of wireless terminal devices or stations (STA) 100 to 106. Each AP may be associated with a basic service set (BSS) which is a basic building block of an IEEE 802.11 wireless local area network (WLAN). The most common BSS type is an infrastructure BSS that includes a single AP together with all STAs associated with the AP. The AP may be a fixed AP or it may be a mobile AP, and a general term for an apparatus managing a wireless network such as the BSS and providing the stations with wireless services is an access node. The APs 102, 104 may also provide access to other networks, e.g. the Internet. In another embodiment, the BSS may comprise a plurality of APs to form an extended service set (ESS), e.g. the AP 110 or 112 may belong to the same ESS with another AP and have the same service set identifier (SS1D) . While embodiments of the invention are described in the context of the above- described topologies of IEEE 802.11 based networks, it should be appreciated that these or other embodiments of the invention may be applicable to networks based on other specifications, e.g. different versions of the IEEE 802.11, WiMAX (Worldwide Interoperability for Microwave Access), UMTS LTE (Long-term Evolution for Universal Mobile Telecommunication System), and other networks having cognitive radio features, e.g. transmission medium sensing features and adaptiveness to coexist with radio access networks based on different specifications and/or standards.

IEEE 802.11 specifications specify a data transmission mode that includes a primary channel and secondary channels. The primary channel is used in all data transmissions and, in addition to the primary channel, one or more secondary channels may be employed for additional bandwidth. The transmission band of a BSS may contain the primary channel and zero or more secondary channels. The secondary channels may be used to increase data transfer capacity of a transmission opportunity (TXOP). The secondary channels may be called a secondary channel, a tertiary channel, a quaternary channel, etc. However, let us for the sake of simplicity use the secondary channel as the common term to refer also to the tertiary or quaternary channel, etc. The primary channel may be used for channel contention, and a TXOP may be gained after successful channel contention on the primary channel.

Some IEEE 802.11 networks employ channel contention based on carrier sense multiple access with collision avoidance (CSMA/CA) for channel access. Every device attempting to gain a TXOP is reducing a backoff value while the primary channel is sensed to be idle for a certain time interval. The backoff value may be selected randomly within a range defined by a contention window parameter. The contention window may have different ranges for different types of traffic, thus affecting priority of the different types of traffic. The channel sensing may be based on sensing a level of radio energy in the radio channel. The sensed level may be compared with a threshold: if the sensed level is below the threshold level, the channel may be determined to be idle (otherwise busy). Such a procedure is called clear channel assessment (CCA) in 802.11 specifications. When the backoff value reaches zero, the STA gains the TXOP and starts frame transmission. If another STA gains the TXOP before that, the backoff value computation may be suspended, and the STA continues the backoff computation after the TXOP of the other STA has ended and the primary channel is sensed to be idle. The time duration (the backoff value) may not be decremented during the TXOP of the other STA, but the time duration that already lapsed before the suspension may be maintained, which means that the device now has a higher probability of gaining the TXOP. A secondary channel may be used in the transmission if it has been free for a determined time period (may be the same or different time period than that used for gaining the TXOP) just before TXOP start time in order for the contending device to take the secondary channel in use.

The STA 100 to 106 may be considered to be a terminal device or a station capable of connecting or associating to any one of the APs 110, 112. The STA may establish a connection with any one of APs it has detected to provide a wireless connection within the neighbourhood of the STA. The connection establishment may include authentication in which an identity of the STA is established in the AP. The authentication may comprise setting up an encryption key used in the BSS. After the authentication, the AP and the STA may carry out association in which the STA is fully registered in the BSS, e.g. by providing the STA with an association identifier (AID). A separate user authentication may follow association, which may also comprise building an encryption key used in the BSS. It should be noted that in other systems terms authentication and association are not necessarily used and, therefore, the association of the STA to an AP should be understood broadly as establishing a connection between the STA and the AP such that the STA is in a connected state with respect to the AP and waiting for downlink frame transmissions from the AP and monitoring its own buffers for uplink frame transmissions. A STA not associated to the AP is in an unassociated state. An unassociated STA may still exchange some frames with the AP, e.g. discovery frames.

For the sake of the following description, let us assume a situation where the stations 100 and 102 are associated to the access node 110 while the stations 104 and 106 are associated to the access node 112. Further, the access nodes 110, 112 manage different wireless networks having different network identifiers, e.g. different SSIDs. Figure 1 illustrates coverage areas of the respected networks. Stations 100 to 104 are located within the coverage area of the access node 110 while the stations 102 to 106 are located within the coverage area of the access node 112. Stations 104 and 102 are thus within a communication range of both access nodes 110, 112. It means that the access node 112 is a potential source of interference to the station 102 and the access node 110 is a potential source of interference to the station 104. The interference may be present in terms of increasing frame losses, reducing channel access, etc. A device not associated to the access node 112 and attempting channel access and sensing a channel in a clear channel assessment process may detect a signal from the access node 112 and, thus, consider the channel to be busy. Overall performance of the wireless network of the access node 110 could be improved if the access node 112 reduced interference towards the station 102. The same applies to the station 104 and the wireless network of the access node 112: reduction of interference from the access node 110 towards the station 104 would improve the performance of a link between the access node 112 and the station 104.

The access nodes 110, 112 may comprise an antenna array that enables beamforming. As known in the art, beamforming or spatial filtering is a signal processing technique used for directional signal transmission or reception. The spatial filtering is achieved by combining signals in the antenna array in such a way that signals transmitted to particular angles experience constructive interference while signals transmitted to other angles experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. The improvement is directivity of radio signal to desired directions and reduction in the emitted radio energy to undesired directions.

Null steering is a beamforming technique where a transmission null is directed to the undesired directions. For example, the access node 112 may try to steer a transmission null towards the station 102. Effective use of null steering would require channel state information on a radio channel between the access node 112 and the station 102. Since the station 102 is in an unassociated state with respect to the access node 112, the access node 112 may not acquire the channel state information directly from the station 102. Figures 2 to 4 illustrate processes according to some embodiments for performing inter-network beamforming cooperation and sharing a TXOP. Figure 2 illustrates a process executed in one of the access nodes, e.g. the access node 112, Figure 3 illustrates a process executed in another access node, e.g. the access node 110, and Figure 4 illustrates a process executed in a station, e.g. the station 102.

Referring to Figure 2, a method performed by the access node 112 of a first wireless network comprises: transmitting (block 202) a transmission opportunity sharing request to the access node 110 of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network (e.g. the STA 104) and duration of at least one transmission period of a shared transmission opportunity; acquiring (block 204) a transmission opportunity sharing response from the access node 110, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network (e.g. the STA 102); transmitting (block 206) an announcement frame indicating the at least one station of the second wireless network; transmitting (block 206) a channel sounding signal; receiving (block 208) a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing (block 210), on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

Upon receiving the beamforming report in block 208, the access node 112 may determine a beamforming configuration for the shared TXOP, wherein the beamforming configuration may define at least one null directed towards the at least one station of the wireless network of the access node 110.

Let us next describe the process of Figure 2 from the viewpoint of the access node 110 with reference to Figure 3 that comprises as performed by the access node 110: receiving (block 302) the transmission opportunity sharing request from the access node 112, wherein the transmission opportunity sharing request indicates at least one station (e.g. the STA 104) of the wireless network of the access node 110 and the duration of the at least one transmission period of the shared transmission opportunity; transmitting (block 304), in response to the transmission opportunity sharing request, a transmission opportunity sharing response to the access node 112, indicating acceptance of the shared transmission opportunity and the at least one station of the wireless network of the access node 110 (e.g. the STA 102); transmitting (block 306) an announcement frame indicating the at least one station of the wireless network of the access node 112; transmitting a channel sounding signal (block 306); receiving (block 308) a beamforming report comprising channel state information from the at least one station of the wireless network of the access node 112; and performing (block 310), on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

Upon receiving the beamforming report in block 308, the access node 110 may determine a beamforming configuration for the shared TXOP, wherein the beamforming configuration may define at least one null directed towards the at least one station of the wireless network of the access node 112. Blocks 202, 204, 302, 304 may be comprised in initiation of the TXOP sharing (blocks 200 and 300). The initiation may prepare both access nodes 110, 112 for the shared TXOP and comprise further steps or information elements exchanged, as described below.

Let us then describe the process from the viewpoint of the station with reference to Figure 4. The station may be the station 102 associated to the access node 110 although the station 104 may support the same procedure in a straightforward manner. Referring to Figure 4, the station 102 performs at least the following: detecting (block 400) a transmission opportunity sharing message for a shared transmission opportunity, the transmission opportunity sharing message indicating the station and comprising an identifier of the access node 110 and an identifier of access node 112; adding (block 402) the identifier of the access node 112 to a beamforming address set (405) stored in the station; receiving (block 404) an announcement frame indicating the apparatus from the access node 112; receiving (block 408) a channel sounding signal from the access node 112; measuring (block 408) the channel sounding signal on the basis of the access node 112 comprised in the beamforming address set of the station, and determining channel state information on the basis of the measured channel sounding signal; transmitting (block 410) a beamforming report comprising the channel state information; and receiving a null steering beamforming transmission from the access node 110 during the shared transmission opportunity shared between the access nodes 110, 112.

The transmission opportunity sharing message for the shared transmission opportunity may be any one of the TXOP sharing request and the TXOP sharing response described above in connection with Figures 2 and 3.

The beamforming address set 405 may comprise an identifier of at least one access node to which the station is not associated. Upon receiving a frame from the at least one access node indicated in the beamforming address set, the station may extract the frame and respond to the frame. For example, upon receiving the announcement frame from the unassociated access node 112 in block 404, the station may, upon detecting that the access node is comprised in the beamforming address set, proceed with the process and measure and report the channel state information in blocks 408 and 410. If the announcement frame was received from an access node to which the station is not associated and which is not indicated in the beamforming address set, the station would discard the frame.

In an embodiment, the access nodes 110, 112 first setup or establish the inter-network beamforming cooperation before the execution of the processes of Figures 2 and 3. During the establishment, the station(s) of the wireless networks may be assigned to the inter-network beamforming configuration as well, e.g. on the basis of measurements carried out by the stations. Stations determined to potentially suffer from interference from the access node of the neighbouring network may be added to the inter-network beamforming configuration, e.g. interference between the STA 102 and the access node 112 and between the STA 104 and the access node 110. The station may also setup the inter-network beamforming configuration before the process of Figure 4. In the station, the setup may comprise the block 402, for example. Therefore, block 402 may in such embodiments precede block 400. Execution of block 402 may be subjected to the station receiving, from the associated access node, a beamforming address set update request message comprising an identifier of the unassociated access node with which the associated access node established the inter-network beamforming configuration. Upon entering the unassociated access node to the beamforming address set 405, the associated access node may inform the unassociated access node that the station has added the identifier of the unassociated access node to the beamforming address set. As a consequence, the unassociated access node is aware that the station is capable of responding to the frames transmitted by the unassociated access node, e.g. the announcement frame and associated channel sounding signal.

In an embodiment the at least one station is indicated in the TXOP sharing request and associated response by a medium access control address (es) of the at least one station.

In an embodiment, the transmission of the beamforming report is subjected to the access node that transmitted the channel sounding signal transmitting a beamforming report request message that triggers the transmission of the beamforming report in block 410. In the embodiment of Figures 2 to 4 where the beamforming report request is omitted, the transmission of the channel sounding signal may serve as the trigger for the transmission of the beamforming report, and all necessary information for transmitting the beamforming report may be provided in the announcement frame and in the channel sounding signal. For example, the announcement frame may indicate the station and, as a consequence, the station acquires the instruction to monitor for the channel sounding signal transmitted by the unassociated access node. Upon detecting the channel sounding signal, the station may measure and generate the channel state information and transmit the beamforming report comprising the channel state information without a separate trigger after the channel sounding signal. An advantage of using the beamforming report request is that the access node may coordinate and schedule the stations to transmit the beamforming reports. However, such a feature is not essential to the general scope of the invention.

In an embodiment, the announcement frame is a null data packet announcement (NDPA) frame of 802.11 specifications. The NDPA frame may indicate stations from which the access node requests channel state information (CSI) and contains information on the requested CSI. In an embodiment, the announcement frame indicates at least one station from the network of the access node 112 transmitting the announcement frame and at least one station from the network of the other access node 110. Below, an example of the NDPA frame is illustrated:

The number below each item represents the length of the respective item in octets. Frame Control field specifies the type of the frame, and Duration field specifies the duration of the frame. RA is a receiver address, and TA is a transmitter address. The RA may be a medium access control (MAC) address of a target STA in case of unicast transmission, or it may be a broadcast address. When the frame is targeted only to the stations associated to the access node 112, the TA may be a MAC address of the access node 112. When the frame is targeted to one or more unassociated stations, the TA field may comprise a SSID of the access node 112, i.e. an identifier of the wireless network of the access node 112. Alternatively, the TA field may comprise the MAC address of the access node 112 when the frame is targeted to one or more unassociated stations. The Sounding Dialog Token may announce that the frame is a high-efficiency (HE) NDPA frame. The STA1 to STA N fields may identify the N stations that are requested to perform the channel sounding measurements, i.e. to execute block 310. The stations may comprises stations associated to the access node 112 and/or stations not associated to the access node 112. The table below illustrates an embodiment of contents of the field STA N. Frame Check Sequence (FCS) may be used for error detection/correction.

The number below each item represents the length of the respective item in bits. AID11 contains the least significant bits of an association identifier of the station identified by the field. In this case, 11 least significant bits is used but the number of bits may be different. In another embodiment, the determined number of bits of the association identifier of a station may be combined with an SSID of the access node 110 to which the station is associated so that the identification is globally unique and unambiguous. Partial bandwidth (BW) information field may be used to specify a measurement band in terms of resource units. Disambiguation bit may be set to value Ύ for a HE frame. The Feedback Type & Ng and Codebook size sub-fields define the type of channel state information to be determined in block 310, such as quantization resolution, single- user/multi-user feedback type, and precoding codebook size.

In an embodiment, the channel sounding signal is a null data packet (NDP) of 802.11 specifications. In an embodiment where the announcement frame is the NDPA frame and the channel sounding signal the NDP, the NDP may be transmitted a short inter-frame space (SIFS) after the NDPA frame. The NDP may carry no payload, i.e. no data field. It may yet comprise training sequence fields to enable the measurements in block 310 and, additionally, one or more signalling fields.

In an embodiment, the beamforming report request is a beamforming report poll (BFRP) trigger frame of 802.11 specifications. In an embodiment where the announcement frame is the NDPA frame, the channel sounding signal is the NDP, and the beamforming report request is the BFRP trigger frame, the access node 112 may transmit the BFRP trigger frame the SIFS after the NDP. The station 102 may then execute block 314 after the SIFS has expired from the reception of the BFRP trigger frame. The channel contention described above may be used for the transmission of the beamforming report.

As described above, the at least one station of the wireless network of the access node 110 may form a subset of stations of a wireless network, e.g. a subset of terminal devices served by the access node 110. For example, the station 100 not within the coverage area of the access node 112 may be omitted from the inter network beamforming cooperation. One or more stations within the coverage area of the access node 112 may equally be omitted for various reasons, e.g. low batter status.

Figure 5 illustrates a signalling diagram combining the embodiments of Figures 2 to 4, i.e. illustrating the inter-network beamforming cooperation during the shared TXOP. Referring to Figure 5, the station 102 may associate to the access node 110 in step 500 and the station 104 may associate to the access node 112 in step 501 in the above-described manner. In step 502, the access nodes 110, 112 may establish the inter-network beamforming cooperation, e.g. in the above-described manner. The establishment may include the access node 110 instructing the station 102 to add the identifier of the access node 112 to the beamforming address set of the station 102 (step 504). In a similar manner, the access node 112 may instruct the station 104 to add the identifier of the access node 110 to the beamforming address set of the station 104 (step 505).

As described above, the procedure for establishing the shared TXOP may be initiated by the access node 112 transmitting and the access node 110 receiving the TXOP sharing request (steps 202, 302). The TXOP sharing request indicates the station 104 so that the access node 110 becomes aware that the station 104 shall be protected by the access node 110 during the shared TXOP. The access node 110 responding to the TXOP sharing request may similarly indicate the station 102 in the TXOP sharing response (steps 204, 304) so that the access node 112 becomes aware that the station 104 shall be protected by the access node 112 during the shared TXOP.

As described above, the TXOP sharing may indicate the duration of the shared TXOP. The shared TXOP may be considered to start from the transmission of the TXOP sharing response. A determined time interval at the start of the shared TXOP may be reserved for acquiring the CSI(s) from the unassociated station(s). During this determined time interval, subsequent frame transmissions may be allowed but not simultaneous transmissions. Accordingly, the access node 112 may transmit the announcement frame and the channel sounding signal addressed to the station 102 in steps 206. The other devices 110, 102, 104 detecting the announcement frame may withdraw from channel access. In the embodiment of Figure 5, also the beamforming report request is illustrated to trigger the transmission of the beamforming report in block 208. Upon receiving the CSI from the station 102 in step 208, the access node 112 may determine the beamforming configuration for the shared TXOP by steering a transmission (TX) and/or reception (RX) null towards the station 102 (block 508). In a similar manner, the access node 110 may transmit the announcement frame and the channel sounding signal addressed to the station 104 in steps 206. Upon receiving the CSI from the station 104 in step 208, the access node 112 may determine the beamforming configuration for the shared TXOP by steering a transmission (TX) and/or reception (RX) null towards the station 104 (block 510). Upon performing the CSI measurements necessary to protect the station(s) of the neighbouring network, the access nodes may carry out data frame transmissions simultaneously during the shared transmission opportunity in blocks 512 and 514.

After the determined time interval has expired, simultaneous transmissions may be allowed during the remaining time interval of the shared TXOP. The simultaneous transmissions may comprise or consist of transmissions of data frames and, optionally, associated acknowledgments. Block 512 may comprise the access node 110 transmitting one or more frames to the station 102 such that a transmission null is directed towards the station 104 and/or the access node 110 receiving one or more frames from the station 102 such that a reception null is directed towards the station 104. Accordingly, the downlink transmissions of the access node 110 will not interfere the station 104 and the uplink transmissions by the station 104 will not interfere the access node 110.

Block 514 may comprise the access node 112 transmitting one or more frames to the station 104 such that a transmission null is directed towards the station 102 and/or the access node 112 receiving one or more frames from the station 104 such that a reception null is directed towards the station 102. Accordingly, the downlink transmissions of the access node 112 will not interfere the station 102 and the uplink transmissions by the station 102 will not interfere the access node 112.

From another perspective, the shared TXOP may be configured to start after the CS1 acquisition in step 208 or 308, and include only the time interval where the simultaneous transmissions are allowed.

The access nodes 110, 112 may be capable of or willing to form a limited number of nulls, thus being able to protect a limited number of stations of the neighbouring network during the shared TXOP. In an embodiment, the TXOP sharing request indicates a maximum number of nulls for the null steering beamforming transmission, thus informing the access node receiving the TXOP sharing request about the number of stations that can be protected during the shared TXOP. The access node may then determine whether or not to participate in the shared TXOP. For example, if the maximum number of nulls is lower than the number of stations the access node would allocate to the shared TXOP, e.g. multicasting TXOP or any other multi-user TXOP employing frequency-division or spatial division through multi-user multiple- input-multiple-output (MU-MIMO) communication, the access node may indicate in the TXOP sharing response rejection of the shared TXOP. If the maximum number of nulls indicates sufficient protection for the station (s) associated to the access node, the access node may indicate in the TXOP sharing response approval of the shared TXOP.

In an embodiment, the access node may respond to such a TXOP sharing request with a TXOP sharing response that indicates at most a number stations of the wireless network of the access node equal to the maximum number of nulls indicated in the TXOP sharing request. In other words, the access node may request protection to only the number of stations that the other access node can protect.

Figure 5 illustrates an embodiment of actions during the shared TXOP, e.g. blocks 512 and 514 of Figure 5. The shared TXOP may be started by carrying out the CSI measurement with respect to the station(s) with which the access node 110 or 112 intends to transmit one or more frames during the shared TXOP. With reference to Figure 6, Let us now assume for the sake of simplicity that the access node 110 intends to transmit a downlink frame to the station 102 and that the access node 112 intends to transmit a downlink frame to the station 104. The access node 110 may start the shared TXOP by transmitting an announcement frame indicating the station 102 in step 600. The announcement frame may be transmitted by using a current beamforming configuration determined in block 510, e.g. a transmission null being steered towards the station 104. That would enable the access node 112 to transmit a frame simultaneously to the station 104 (step 620), e.g. a similar announcement frame that indicates the station 104. The access node 112 may also use the current beamforming configuration determined in block 508 such that a transmission null is directed towards the station 102. The announcement frame(s) transmitted in steps 600, 620 may be the above-described NDPA frame.

In connection with transmitting the announcement frame, the access node 110 may transmit a channel sounding signal to enable the station 102 to measure the CSI. The channel sounding signal may also be transmitted by using the current beamforming configuration directing the transmission null towards the station 104. In block 604, the access node transmits a beamforming report request (optional) indicating the station 102, and the beamforming report request may also be transmitted by directing the transmission null towards the station 104. When receiving the corresponding beamforming report from the station 102 in step 606, the access node 110 may use the beamforming configuration to direct a reception null towards the station 104 so that a transmission by the station 104 will not interfere the reception of the beamforming report. Upon acquiring the CSI from the station 102 in step 606, the access node may direct the data transmission better towards the station 102 in block 608. A data frame transmission may also be transmitted by directing the transmission null towards the station 104. When receiving an acknowledgment to the data frame transmission from the station 102 in step 610, the access node 110 may again use the beamforming configuration to direct the reception null towards the station 104.

A similar procedure may be carried out by the access node 112 with the station 104 simultaneously during the shared TXOP. The access node 112 may direct the transmission null towards the station 102 when transmitting frames to the station 104 and, further, direct a reception null towards the station 102 when receiving frames from the station during the shared TXOP. In an embodiment, the TXOP sharing request further indicates a transmission sequence comprising a plurality of transmission periods for the shared TXOP. The transmission periods may comprise at least a first downlink period and a first uplink period of the shared TXOP. The above-described duration indicated in the TXOP sharing request may comprise a duration of at least one of the first downlink period and the first uplink period or the total duration of the shared TXOP. In an embodiment, the transmission opportunity sharing request indicates an order of the transmission periods in the transmission sequence. Figure 7 illustrates such an embodiment. In Figure 7, the same reference numbers as used in the Figures above represent the same or substantially similar functions.

Referring to Figure 7, steps 500 to 502 may be carried out in the above- described manner, and above-described steps 504, 505 and 404 may also be carried out. In step 700, the access node 112 transmits the TXOP sharing request. The access node 112 may schedule the TXOP to comprise a determined number of uplink periods and/or a determined number of downlink periods, and the TXOP sharing request may indicate the scheduling, e.g. in the form of order and duration of the transmission periods. In the embodiment of Figure 7, the access node 112 has scheduled the shared TXOP to start with a downlink period for 100 microseconds (us), followed by an uplink period for a duration of 400 us. The TXOP sharing response may have the format described above. A channel sounding procedure in step 702 may be directed to the unassociated stations and comprise the steps 206, 208, 306, 308 and optionally 506. A channel sounding procedure in steps 704 and 706 may be directed to the associated stations with which the access nodes 110, 112 intends to exchange frames during the shared TXOP, and it may include the steps 600 to 606 and 620 to 626.

With respect to the data transmissions during the shared TXOP, the access node 110 may transmit one or more downlink data frames to the station 102 during the scheduled lOOus downlink period 720 in block 708 such that a transmission null is directed towards the station 104 and, further, receive one or more acknowledgments from the station 102 in step 712 such that a reception null is directed towards the station 104. Similarly, the access node 112 may transmit one or more downlink data frames to the station 104 during the scheduled lOOus downlink period 720 in block 710 such that a transmission null is directed towards the station 102 and, further, receive one or more acknowledgments from the station 104 in step 714 such that a reception null is directed towards the station 102.

After the downlink period, the scheduled 400us uplink period 722 follows. During the uplink period 722, the access node 110 receives one or more data frames from the station 102 in block 716 such that a reception null is directed towards the station 104 and, further, transmits one or more acknowledgments to the station 102 in step 720 such that a transmission null is directed towards the station 104. Simultaneously, the access node 112 receives one or more data frames from the station 104 in block 718 such that a reception null is directed towards the station 102 and, further, transmits one or more acknowledgments to the station 102 in step 722 such that a transmission null is directed towards the station 102. In this manner, the access nodes may share the TXOP even in the case where unassociated station (s) reside in the coverage areas of the access nodes.

The reader is advised that a simplified version of the above-described embodiments can be realized in a straightforward manner. For example, if the station 102 was replaced by the station 100 for the shared TXOP, there would be no need for the access node 112 to direct a null towards the station 100 because the station is not in the coverage area of the access node. The negligible probability for interference between the station 100 and the access node 112 may be determined through measurements. Accordingly, steps 206, 208, and 506 performed by the access node 112 in the process of Figure 6 may be omitted, there would be no need for the access node 112 to direct a null in the processes of Figures 6 and 7.

Figure 8 illustrates an embodiment of a structure of the above-mentioned functionalities of an apparatus executing the functions of the station 102 or 104 in the process of Figure 4 or any one of the embodiments described above for the station 102 or 104. The apparatus may be a terminal device or a client device of a wireless network, e.g. the 802.11 network. In other embodiments, the apparatus may be a circuitry or an electronic device realizing some embodiments of the invention in the wireless device. The apparatus may comply with 802.11 specifications. The apparatus may be or may be comprised in a computer (PC), a laptop, a tablet computer, a cellular phone, a palm computer, a sensor device, or any other apparatus provided with radio communication capability. In another embodiment, the apparatus carrying out the above-described functionalities is comprised in such a device, e.g. the apparatus may comprise a circuitry such as a chip, a chipset, a processor, a micro controller, or a combination of such circuitries in any one of the above-described devices. The apparatus may be an electronic device comprising electronic circuitries for realizing some embodiments of the present invention.

Referring to Figure 8, the apparatus may comprise a station entity 50 providing the apparatus with capability of communicating in the wireless network of the access node 110 and/or 112. The station entity may comprise a radio interface 52 providing the apparatus with radio communication capability. The radio interface 52 may comprise radio frequency converters and other radio frequency components such as an amplifier, filter, and frequency-converter circuitries and one or more antennas. The station entity 50 may further comprise a radio modem 58 configured to carry out transmission and reception of messages in the wireless network. The radio modem 58 may comprise encoder and decoder circuitries, modulator and demodulator circuitries, etc.

The station entity 50 may further comprise a channel estimation circuitry 54 configured to measure a signal strength from a received radio signal and to determine other channel state information such as precoding parameters for the beamforming. The channel estimation circuitry 54 may be configured to execute block 408 and to generate the channel state information to be transmitted in block 410.

The station entity 50 may further comprise a controller 56 configured to control transmissions and functions of the station entity 50. The controller 56 may, for example, control the establishment of the association in step 500 or 501, to control the channel estimation circuitry 54 to perform the channel estimation, and to control the radio modem 58 to respond to the beamforming report request received from an access node to which the apparatus is not currently associated. The station entity 50 may comprise at least one processor comprising the controller 56 and the channel estimation circuitry 54 and, optionally, at least some of the circuitries of the radio modem 58.

The apparatus may further comprise an application processor 56 executing one or more computer program applications that generate a need to transmit and/or receive data through the station entity 50. The application processor may form an application layer of the apparatus. The application processor may execute computer programs forming the primary function of the apparatus. For example, if the apparatus is a sensor device, the application processor may execute one or more signal processing applications processing measurement data acquired from one or more sensor heads. If the apparatus is a computer system of a vehicle, the application processor may execute a media application and/or an autonomous driving and navigation application. The application processor may generate data to be transmitted in the wireless network.

The apparatus may further comprise a memory 60 storing one or more computer program products 62 configuring the operation of said processor(s) of the apparatus. The memory 60 may further store a configuration database 64 storing operational configurations of the apparatus. The configuration database 64 may, for example, store the beamforming address set 405. Figure 9 illustrates an embodiment of a structure of the above-mentioned functionalities of an apparatus executing the functions of the access node in the process of Figure 2 and/or Figure 3 any one of the embodiment described above for the access nodes 110 and 112. The access nodes 110 and 112 may be both support all the functions described above for either access node 110, 112. In another embodiment, the apparatus carrying out the above-described functionalities of the access node is comprised in such a device, e.g. the apparatus may comprise a circuitry, e.g. a chip, a chipset, a processor, a micro controller, or a combination of such circuitries in the access node. The apparatus may be an electronic device comprising electronic circuitries for realizing some embodiments of the access node.

Referring to Figure 9, the apparatus may comprise a first communication interface 22 or a communication circuitry configured to provide the apparatus with capability for bidirectional communication with stations over a radio interface. The communication interface may comprise radio frequency circuitries for processing received control frames and data frames and control frames and data frames to be transmitted. The communication interface 22 may comprise standard well-known components such as an antenna array, amplifier, a filter, a frequency converter, and encoder/decoder circuitries.

The apparatus may further comprise a second communication interface 30 or a communication circuitry configured to provide the apparatus with capability for bidirectional communication with other networks, e.g. the Internet or another computer network. In some embodiments, the communication interface 30 is used for the communication between the access nodes. In other embodiments, the access nodes communicate over a radio interface through the communication interface 22. The communication interface 30 or 22 may comprise circuitries for processing messages described above in connection with steps 202, 204 and 302, 304. The communication interface 22 may comprise standard well-known components such as an amplifier, a filter, and encoder/decoder circuitries.

The apparatus may further comprise a memory 20 storing one or more computer program products 24 configuring the operation of at least one processor 10 of the apparatus. The memory 20 may further store a configuration database 26 storing operational configurations of the apparatus, e.g. channel state information received from the stations and the beamforming configuration 212, 312.

The apparatus may further comprise the at least one processor 10 configured to carry out the process of Figure 2 or any one of its embodiments, or the process of Figure 3 or any one of its embodiments. The processor may comprise a communication controller controlling the operation of the access node. Referring to Figure 9, the processor(s) 10 comprise(s) a beamforming controller 12 and a transmission circuitry 15. The transmission circuitry may carry out frame transmissions in a wireless network managed by the apparatus. The frame transmissions may include transmissions of frames to stations associated to the apparatus in which case the transmission circuitry may employ the beamforming configuration 212, 312 currently stored in the configuration database 26. The frame transmissions may include the setup of the beamforming cooperation, as described above. In such a case, the transmission circuitry 15 may employ a beamforming configuration that does or does not use the null steering, e.g. omnidirectional transmission.

The beamforming controller 12 may comprise a beamforming setup circuitry 14 configured to perform the setup of the beamforming cooperation in block 200 or 300, depending on whether the apparatus is requesting the setup of the inter network beamforming cooperation or responding to such a request. The beamforming setup circuitry may support both blocks 200 and 300. For the generation of the beamforming configuration, the beamforming controller 12 may comprise a channel state information (CSI) acquisition circuitry 16 configured to select stations to be involved in the generation, control the transmission of the announcement frame and, optionally, the beamforming report request in step 206, 306. Upon receiving the beamforming reports from the stations, a null steering circuitry 15 may compute the beamforming configuration such that transmission null(s) is/are directed towards unassociated stations and transmission energy is directed towards associated stations. The null steering circuitry 18 may then store the beamforming configuration in the configuration database 26.

As used in this application, the term 'circuitry' refers to one or more of the following: (a) hardware-only circuit implementations such as implementations in only analog and/or digital circuitry; (b) combinations of circuits and software and/or firmware, such as (as applicable): (i) a combination of processor(s) or processor cores; or (ii) portions of processor(s)/software including digital signal processor(s), software, and at least one memory that work together to cause an apparatus to perform specific functions; and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present.

This definition of 'circuitry' applies to uses of this term in this application. As a further example, as used in this application, the term "circuitry" would also cover an implementation of merely a processor (or multiple processors) or portion of a processor, e.g. one core of a multi-core processor, and its (or their) accompanying software and/or firmware. The term "circuitry" would also cover, for example and if applicable to the particular element, a baseband integrated circuit, an application- specific integrated circuit (ASIC), and/or a field-programmable grid array (FPGA) circuit for the apparatus according to an embodiment of the invention.

The processes or methods described in Figures 2 to 7 may also be carried out in the form of one or more computer processes defined by one or more computer programs. A separate computer program may be provided in one or more apparatuses that execute functions of the processes described in connection with the Figures. The computer program(s) may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. Such carriers include transitory and/or non- transitory computer media, e.g. a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package. Depending on the processing power needed, the computer program may be executed in a single electronic digital processing unit or it may be distributed amongst a number of processing units.

Embodiments described herein are applicable to wireless networks defined above but also to other wireless networks. The protocols used, the specifications of the wireless networks and their network elements develop rapidly. Such development may require extra changes to the described embodiments. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. An apparatus for a first wireless network, comprising means for performing: transmitting a transmission opportunity sharing request to an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network and duration of at least one transmission period of a shared transmission opportunity; acquiring a transmission opportunity sharing response from the access node, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network; transmitting an announcement frame indicating the at least one station of the second wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

2. The apparatus of claim 1, wherein the transmission opportunity sharing request further indicates a transmission sequence comprising a plurality of transmission periods comprising: at least a first downlink period of a shared transmission opportunity; and a first uplink period of the shared transmission opportunity, and wherein the duration of at least one transmission period comprises a duration of at least one of the first downlink period and the first uplink period.

3. The apparatus of claim 2, wherein the transmission opportunity sharing request indicates an order of the transmission periods in the transmission sequence.

4. The apparatus of any preceding claim, wherein the transmission opportunity sharing request further indicates a maximum number of nulls for the null steering beamforming transmission.

5. The apparatus of claim 4, wherein the transmission opportunity sharing response indicates at most a number stations of the second wireless network equal to the maximum number of nulls.

6. The apparatus of any preceding claim, wherein the means are configured to perform at least one of said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using null steering beamforming such that a null is directed to the at least one station of the second wireless network.

7. The apparatus of claim 6, wherein the means are configured to perform said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using the null steering beamforming such that the null is directed to the at least one station of the second wireless network.

8. The apparatus of any preceding claim, wherein the shared transmission opportunity comprises a first time interval during which the apparatus and the access node are allowed to transmit subsequently but not simultaneously, and further comprises a second time interval during which the apparatus and the access node are allowed to transmit at least partially simultaneously.

9. The apparatus of any preceding claim, wherein the means are configured to perform the null steering beamforming transmission in the first wireless network simultaneously with a null steering beamforming transmission by the access node during the shared transmission opportunity.

10. An apparatus for a first wireless network, comprising means for performing: receiving a transmission opportunity sharing request from an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the second wireless network and duration of at least one transmission period of a shared transmission opportunity; transmitting a transmission opportunity sharing response to the access node, the transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the first wireless network; transmitting an announcement frame indicating the at least one station of the first wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the first wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

11. The apparatus of claim 10, wherein the transmission opportunity sharing request further indicates a maximum number of nulls, and wherein the means are configured to indicate in the transmission opportunity sharing response at most a number stations of the first wireless network equal to the maximum number of nulls.

12. The apparatus of claim 10 or 11, wherein the means are configured to perform at least one of said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using null steering beamforming such that a null is directed to the at least one station of the second wireless network.

13. The apparatus of any preceding claim 10 to 12, wherein the shared transmission opportunity comprises a first time interval during which the apparatus and the access node are allowed to transmit subsequently but not simultaneously, and further comprises a second time interval during which the apparatus and the access node are allowed to transmit at least partially simultaneously.

14. An apparatus for a first wireless network, comprising means for performing: detecting a transmission opportunity sharing message for a shared transmission opportunity, the transmission opportunity sharing message indicating the apparatus and comprising an identifier of a first access node of the first wireless network and an identifier of a second access node of a second wireless network different from the first wireless network; adding the identifier of the second access node to a beamforming address set stored in the apparatus; receiving an announcement frame indicating the apparatus from the second access node; receiving a channel sounding signal from the second access node; measuring the channel sounding signal on the basis of the second access node comprised in the beamforming address set of the apparatus, and determining channel state information on the basis of the measured channel sounding signal; transmitting a beamforming report comprising the channel state information; and receiving a null steering beamforming transmission from the first access node during the shared transmission opportunity shared between the first access node and the second access node.

15. The apparatus of claim 14, wherein the means are further configured to, upon receiving the null steering beamforming transmission from the first access node, transmit an acknowledgement frame to the first access node during the shared transmission opportunity.

16. The apparatus of any preceding claim 1 to 15, wherein the means comprises: at least one processor; and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus.

17. A method for an apparatus of a first wireless network, comprising: transmitting, by the apparatus, a transmission opportunity sharing request to an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network and duration of at least one transmission period of a shared transmission opportunity; acquiring, by the apparatus, a transmission opportunity sharing response from the access node, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network; transmitting, by the apparatus, an announcement frame indicating the at least one station of the second wireless network; transmitting, by the apparatus, a channel sounding signal; receiving, by the apparatus, a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing, by the apparatus on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

18. The method of claim 17, wherein the transmission opportunity sharing request further indicates a transmission sequence comprising a plurality of transmission periods comprising: at least a first downlink period of a shared transmission opportunity; and a first uplink period of the shared transmission opportunity, and wherein the duration of at least one transmission period comprises a duration of at least one of the first downlink period and the first uplink period.

19. The method of claim 18, wherein the transmission opportunity sharing request indicates an order of the transmission periods in the transmission sequence.

20. The method of any preceding claim 17 to 19, wherein the transmission opportunity sharing request further indicates a maximum number of nulls for the null steering beamforming transmission.

21. The method of claim 20, wherein the transmission opportunity sharing response indicates at most a number stations of the second wireless network equal to the maximum number of nulls.

22. The method of any preceding claim 17 to 21, wherein the apparatus performs at least one of said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using null steering beamforming such that a null is directed to the at least one station of the second wireless network.

23. The method of claim 22, wherein the apparatus performs said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using the null steering beamforming such that the null is directed to the at least one station of the second wireless network.

24. The method of any preceding claim 17 to 23, wherein the shared transmission opportunity comprises a first time interval during which the apparatus and the access node are allowed to transmit subsequently but not simultaneously, and further comprises a second time interval during which the apparatus and the access node are allowed to transmit at least partially simultaneously.

25. The method of any preceding claim 17 to 24, wherein the apparatus performs the null steering beamforming transmission in the first wireless network simultaneously with a null steering beamforming transmission by the access node during the shared transmission opportunity.

26. A method for an apparatus of a first wireless network, comprising: receiving, by the apparatus, a transmission opportunity sharing request from an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the second wireless network and duration of at least one transmission period of a shared transmission opportunity; transmitting, by the apparatus, a transmission opportunity sharing response to the access node, the transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the first wireless network; transmitting, by the apparatus, an announcement frame indicating the at least one station of the first wireless network; transmitting, by the apparatus, a channel sounding signal; receiving, by the apparatus, a beamforming report comprising channel state information from the at least one station of the first wireless network; and performing, by the apparatus on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

27. The method of claim 26, wherein the transmission opportunity sharing request further indicates a maximum number of nulls, and wherein the means are configured to indicate in the transmission opportunity sharing response at most a number stations of the first wireless network equal to the maximum number of nulls.

28. The method of claim 26 or 27, wherein the apparatus performs at least one of said transmitting the announcement frame, transmitting the channel sounding signal, and receiving the beamforming report by using null steering beamforming such that a null is directed to the at least one station of the second wireless network.

29. The method of any preceding claim 26 to 28, wherein the shared transmission opportunity comprises a first time interval during which the apparatus and the access node are allowed to transmit subsequently but not simultaneously, and further comprises a second time interval during which the apparatus and the access node are allowed to transmit at least partially simultaneously.

30. A method for an apparatus of a first wireless network, comprising means for performing: detecting, by the apparatus, a transmission opportunity sharing message for a shared transmission opportunity, the transmission opportunity sharing message indicating the apparatus and comprising an identifier of a first access node of the first wireless network and an identifier of a second access node of a second wireless network different from the first wireless network; adding, by the apparatus, the identifier of the second access node to a beamforming address set stored in the apparatus; receiving, by the apparatus, an announcement frame indicating the apparatus from the second access node; receiving, by the apparatus, a channel sounding signal from the second access node; measuring, by the apparatus, the channel sounding signal on the basis of the second access node comprised in the beamforming address set of the apparatus, and determining channel state information on the basis of the measured channel sounding signal; transmitting, by the apparatus, a beamforming report comprising the channel state information; and receiving, by the apparatus, a null steering beamforming transmission from the first access node during the shared transmission opportunity shared between the first access node and the second access node.

31. The method of claim 30, wherein the apparatus transmits, upon receiving the null steering beamforming transmission from the first access node, an acknowledgement frame to the first access node during the shared transmission opportunity.

32. A computer program product embodied on a computer-readable medium and comprising a computer program code readable by a computer for an apparatus of a first wireless network, wherein the computer program code configures the computer to carry out a computer process comprising: transmitting a transmission opportunity sharing request to an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the first wireless network and duration of at least one transmission period of a shared transmission opportunity; acquiring a transmission opportunity sharing response from the access node, a transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the second wireless network; transmitting an announcement frame indicating the at least one station of the second wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the second wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

33. A computer program product embodied on a computer-readable medium and comprising a computer program code readable by a computer for an apparatus of a first wireless network, wherein the computer program code configures the computer to carry out a computer process comprising: receiving a transmission opportunity sharing request from an access node of a second wireless network, wherein the transmission opportunity sharing request indicates at least one station of the second wireless network and duration of at least one transmission period of a shared transmission opportunity; transmitting a transmission opportunity sharing response to the access node, the transmission opportunity sharing response indicating acceptance of the shared transmission opportunity and at least one station of the first wireless network; transmitting an announcement frame indicating the at least one station of the first wireless network; transmitting a channel sounding signal; receiving a beamforming report comprising channel state information from the at least one station of the first wireless network; and performing, on the basis of the received channel state information, null steering beamforming transmission in the first wireless network during the shared transmission opportunity.

34. A computer program product embodied on a computer-readable medium and comprising a computer program code readable by a computer for an apparatus of a first wireless network, wherein the computer program code configures the computer to carry out a computer process comprising: detecting a transmission opportunity sharing message for a shared transmission opportunity, the transmission opportunity sharing message indicating the apparatus and comprising an identifier of a first access node of the first wireless network and an identifier of a second access node of a second wireless network different from the first wireless network; adding the identifier of the second access node to a beamforming address set stored in the apparatus; receiving an announcement frame indicating the apparatus from the second access node; receiving a channel sounding signal from the second access node; measuring the channel sounding signal on the basis of the second access node comprised in the beamforming address set of the apparatus, and determining channel state information on the basis of the measured channel sounding signal; transmitting a beamforming report comprising the channel state information; and receiving a null steering beamforming transmission from the first access node during the shared transmission opportunity shared between the first access node and the second access node.