Communication devices and methods

Abstract

A first communication device that is configured to communicate with a second communication device comprises circuitry configured to generate data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences; map the generated data units onto a plurality of spatial streams; and transmit the generated data units. Generating the data units and transmitting the generated data units includes generating a first data unit including in its preamble a plurality of channel estimation sequences transmitted on the plurality of spatial streams, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; and generating one or more subsequent data units transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.

Description

COMMUNICATION DEVICES AND METHODSBACKGROUNDFIELD OF THE DISCLOSURE

[0001] The present disclosure relates to first and second communication devices that are configured to communicate with each other, particularly in a wireless local area network (WLAN). The present disclosure relates further to corresponding communication methods.DESCRIPTION OF RELATED ART

[0002] Current wireless LAN standards operate in time-division duplex (TDD) mode. For real-time applications like virtual or augmented reality, cloud gaming or industrial machine control, a low response delay is required.

[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 provide communication devices and methods that enable a shortening of the response delay, particularly in WLAN applications. It is a further object to provide a corresponding method as well as a corresponding computer program and a non-transitory computer-readable recording medium that stores therein a computer program product for implementing said method.

[0005] According to an aspect there is provided a first communication device configured to communicate with a second communication device, the first communication device comprising circuitry configured to: generate data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences; map the generated data units onto a plurality of spatial streams; and transmit the generated data units, wherein generating the data units and transmitting the generated data units includes generating a first data unit including in its preamble a plurality of channel estimation sequences transmitted on the plurality of spatial streams, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; and generating one or more subsequent data units transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.

[0006] According to a further aspect there is provided a second communication device configured to communicate with a first communication device, the second communication device comprising circuitry configured to: receive a first data unit from the first communication device, the first data unit comprising a preamble and a data portion and including in its preamble a plurality of channel estimation sequences transmitted on a plurality of spatial streams onto which the first data unit is mapped, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; estimate channels of the plurality of spatial streams using the channel estimation sequences included in the first data unit; receive one or more subsequent data units from the first communication device after receiving the first data unit, wherein each of the subsequent data units comprises a preamble and a data portion and includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams; estimate, using a received subsequent data unit, channels of a subset of spatial streams using the channel estimation sequences included in the said subsequent data unit; and update the channel estimate of a subset of the plurality of spatial streams obtained by use of channel estimation sequences included in the first or one or more subsequent data units by the channel estimate of the same subset of spatial streams obtained by use of a channel estimation sequence included in a subsequent data unit..

[0007] According to still further aspects corresponding methods, 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-transi- tory 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 methods, the disclosed computer program and the disclosed computer-readablerecording medium have similar and / or identical further embodiments as the claimed devices and as defined in the dependent claims and / or disclosed herein.

[0009] One of the aspects of the disclosure is to enable high-efficiency operation, in particular TDD (HE-TDD) operation, to reduce the response delay. Preamble shortening is proposed, which aims to save packet (e.g. PPDll) transmission time using a smaller (e.g. PHY) preamble. Spatial multiplexing is beneficial for short response delays since it increases bit rate and packets get shorter. However, in current wireless LAN standards the number of channel estimation sequences per packet increases with the number of spatial streams (SS) and packets have more preamble overhead. Thus, the present disclosure aims to shorten the preamble, in particular the transmit time required for channel estimation by sending less channel estimation sequences. In embodiments the problem arising with missing channel estimates for some spatial streams is solved.

[0010] In the context of the present disclosure the first communication device may be a transmitting communication device, which may in an embodiment be a station (STA) such as a virtual reality headset, glasses, remote controller or smartphone. The second communication device may be a receiving communication device, which may in an embodiment be an access point (AP) such as a network router, computer or game console.

[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 illustrating simplified packet transmission for four spatial streams with channel estimation sequences according to WLAN standards.Fig. 2 shows a schematic diagram of a conventional transmitter using channel estimation sequence transmission for four spatial streams according to WLAN standards.Fig. 3 shows a Nssx NtxDFT matrix.Fig. 4 shows a diagram of a scenario in which the present disclosure may be applied.Fig. 5 shows a flow chart of an embodiment of a first communication method according to the present disclosure.Fig. 6 shows a flow chart of an embodiment of a second communication method according to the present disclosure.Fig. 7 shows a diagram illustrating a first embodiment of a first data unit and four subsequent data units.Fig. 8 shows a diagram illustrating a first embodiment of the transmission of the data units shown in Fig. 7.Fig. 9 shows a diagram illustrating a second embodiment of the transmission of the data units shown in Fig. 7.Fig. 10 shows a diagram illustrating a third embodiment of the transmission of the data units shown in Fig. 7.Fig. 11 shows a diagram of the transmission of subsequent data units until all spatial streams are re-estimated.Fig. 12 shows a diagram of the transmission of subsequent data units until a transmission opportunity ends.Fig. 13 shows a diagram of the transmission of subsequent data units until the transmission of a new first data unit is desired.Fig. 14 shows a diagram of the transmission of data units interrupted by SIFSs and of the transmission of data units in a different communication direction.Fig. 15 shows the general layout of a data unit according to WLAN standards.Fig. 16 shows a schematic diagram of an embodiment of a transmitter at different processing steps.Fig. 17 shows a schematic diagram of another embodiment of a transmitter at different processing steps.Fig. 18 shows an exemplary Hadamard matrix.Fig. 19 shows a standard first data unit and several embodiments of a second data unit with and without change modulation and coding scheme applied on the data portion.Fig. 20 shows a diagram of the transmission of subsequent data units using configuration signaling for a subsequent PPDll.Fig. 21 shows a simplified flowchart of an embodiment of a first communication method of the first communication device according to the present disclosure.Fig. 22 shows a flowchart of an embodiment of a second communication method of the second communication device according to the present disclosure that corresponds to the first communication method illustrated in Fig. 21.Fig. 23 shows a flowchart of another embodiment of a second communication method of the second communication device according to the present disclosure.Fig. 24 shows a flowchart of another embodiment of a second communication method of the second communication device according to the present disclosure illustrating the behavior of the second communication device for a configuration request.Fig. 25 shows a flowchart of another embodiment of a first communication method of the first communication device according to the present disclosure illustrating the behavior of the first communication device in the case of configuration signaling.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0013] In current WLAN standards, the number N_LTF of transmitted channel estimation sequences, e.g., long training fields (LTFs). very high throughput long training fields (VHTLTFs), high-efficiency long training fields (HELTFs), extremely high throughput long training fields (EHTLTFs) (all generally having different properties, e.g. in terms of length), scales with the number N_ss of spatial streams (SS) that is rounded up to the next even number, except for N_ss=1 . Table 1 shows the relationship of N_ss to N_LTF in WLAN standard amendments since IEEE 802.11n.Table 1

[0014] In the WLAN standards, the N_LTF channel estimation sequences are concurrently transmitted on all SS. A simplified packet transmission with N_ss=4 spatial streams is shown in Fig. 1 , i.e. , each data unit D includes in its preamble 1 four channel estimation sequences 3, 4, 5, 6 (LTF on all SS) that are transmitted on all SS. In addition, each data unit D includes a (typically different) data portion 2. A transmitter block diagram for transmission of N_LTF=4 channel estimation sequences and N_ss =4 is shown in Fig. 2. The orthogonal mapping matrix in Fig. 2 is the so-called P matrix, as explained below.

[0015] In order to enable separation of the SS at the receiver, the channel estimation sequence S_k or equivalently an LTF is multiplied per subcarrier k with the orthogonal mapping matrix P, which is defined as follows for N_ss=8 and N_LTF=8:

[0016] If smaller values for N_ss or N_LTF are used, then a N_ss x N_LTF subset of this matrix is used except in the case of N_ss = 5 or 6 which utilize a different matrix P based on a 6x6 DFT matrix as shown in Fig. 3.

[0017] The transmitted signal X_k can then be expressed with X_k = P ■ S_k. In other words, the P matrix defines the phase of the transmitted estimation sequences per spatial stream. It shall be noted that cyclic shift delays are neglected here for simplicity.

[0018] Assuming the OFDM guard interval is sufficiently long, the wireless MIMO channel can be modeled as multiplication with an N_rx x N_ss matrix H, where N_rx is the number of receive antennas and Y_k is the received signal: Y_k = H ■ P ■ S_k. On the receiver side, the conjugate transpose of the P matrix is multiplied to the received estimation sequences.Since P is orthogonal and S_k is a scalar value, the P matrices are cancelled, and it is obtained:R_k = Y_k ■ PT= H ■ P ■ S_k ■ PT= H ■ S_k ■ N_LTF.

[0019] The channel estimate H_k per subcarrier k is determined as H_k = R_k I S_k I N_LTF. This N_rx x N_ss matrix contains a complex channel tap for each receive antenna and each transmit SS.

[0020] The number N_LTF of channel estimation sequences is equal to or larger than the number N_ss of spatial streams, since otherwise channel estimation would require solving for more unknowns than known measured values resulting in an underdetermined equation system. The drawback of this method is the increased packet duration and increased preamble overhead, which deteriorates response delay in TDD operation.

[0021] The present disclosure copes with these issues by using in a packet less channel estimation sequences than the number of SS to get a smaller preamble and packet duration. This has the consequence that the channel of only a subset of SS can be estimated with one packet. The channel estimates of the remaining subset of SS are taken from the past estimations. A possibly reduced channel estimation quality arising from using aged channel estimates may be compensated by adaption of the modulation and coding scheme.

[0022] It shall be noted that in the following, the term “LTF” is used as an example of a channel estimation sequence, but this term shall generally be understood as meaning “channel estimation sequence”, i.e., other implementations of channel estimation sequences than LTFs may be used as well. Similarly, the term “PPDll” is used as an example of a data unit, but this term shall generally be understood as meaning “data unit”, i.e., other implementations of data units than PPDlls may be used as well.

[0023] Fig. 4 shows a diagram of a scenario in which the present disclosure may be applied. It shows particularly a communication system including a first communication device 100 (which in an embodiment may be a station (STA), e.g. VR glasses or a controller of agame console) according to an aspect of the present disclosure for communicating with second communication device 200 (which in an embodiment may be an access point (AP) or STA, e.g. a computer or game console). The first communication device 100 is generally able to communicate with the second communication device 200, e.g. to exchange (receive and / or transmit) data with the second communication device 200. Each of the communication devices 100, 200 comprises circuitry 101, 201 that is configured to perform particular operations. The circuitries may be implemented by a respective processor or computer, i.e., as hardware and / or software, or by dedicated units or components. For instance, respectively programmed processors may represent the respective circuitries 101 , 201.

[0024] Fig. 5 shows a flow chart of an embodiment of a first communication method 300 of the first communication device 100 according to the present disclosure, which may be performed by the circuitry 101. In a first step 301 , the first communication device 100 generates data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences. In a second step 302, the first communication device 100 maps the generated data units onto a plurality of spatial streams. In a third step 303, the first communication device 100 transmits the generated data units. A first data unit is generated and transmitted that includes in its preamble a plurality of channel estimation sequences transmitted on the plurality of spatial streams, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream. One or more subsequent data units are generated and transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.

[0025] Fig. 6 shows a flow chart of an embodiment of a second communication method 400 of the second communication device 200 according to the present disclosure, which may be performed by the circuitry 201. In a first step 401 , the second communication device 200 receives a first data unit (DU) from the first communication device and, in a second step 402, estimates channels of the plurality of spatial streams using the channel estimationsequences included in the first data unit. In a third step 403, the second communication device 200 receives one or more subsequent data units from the first communication device after receiving the first data unit and, in a fourth step 404, estimates, using a received subsequent data unit, channels of a subset of spatial streams using the channel estimation sequences included in the said subsequent data unit. In a fifth step 405, the second communication device 200 updates the channel estimate of a subset of spatial streams obtained by use of channel estimation sequences included in the first data unit by the channel estimate of the same subset of spatial streams obtained by use of a channel estimation sequence included in a subsequent data unit.

[0026] Fig. 7 shows a diagram illustrating a first embodiment of a first data unit 10 and four subsequent data units 20, 30, 40, 50 (also called second data units). The data units may, e.g., be PPDlls. The first data unit 10 comprises a preamble 11 and a data portion 12. The preamble 11 includes a plurality (i.e., two or more) of channel estimation sequences, in this example four channel estimation sequences 13, 14, 15, 16 for four SS. The plurality of channel estimation sequences 13, 14, 15, 16 is transmitted on the plurality of SS and includes at least one channel estimation sequence per SS. These channel estimation sequences are transmitted to enable the second communication device to estimate all SS.

[0027] The data portion 12 is transmitted on all SS (If less SS for data transmission shall be used, then the plurality of SS would be chosen smaller), but this is independent of the channel estimation sequences. For example, a second PPDll can have channel estimation sequences for the first SS but uses first and second SS for the data portion. The relation between spatial stream and the transmit antenna is defined by spatial mapping. This mapping is generally unchanged between first and second PPDlls. A favorable implementation is that although just one spatial stream is occupied, all transmit antennas are active. This implies that the mapping matrix is not an identity matrix (e.g. a first SS is equal to a first transmit antenna), but a fully occupied matrix. Assuming there are two spatial streams, a mapping matrix to the transmit antennas could be as follows [1 , 1 ; 1 -1], Hence, the transmit antenna signals are given by t1 = s1+s2 and t2=s1-s2, wherein s1 and s2 denote the two spatial streams. "LTF on SS1” means s2=0, hence t1 = LTF and t2 = LTF. “LTF on SS2” means s1=0, hence t1 = LTF and t2 = -LTF. “LTF on SS1” thus means that the first SS is non-zero and the second SS is zero for example. There may bea difference among the LTF on SS1 in the second PPDll compared to the first PPDll. For instance, the tone spacing might change, e.g., using IxHELTF or 2xHELTF in first PPDll and using 4xHELTF in subsequent PPDUs. On top, a LTF may be multiplied by a P matrix entry with either +1 or -1 depending on the number of spatial streams that are estimated in parallel. For example, in a first PPDll when four SS are estimated, the fourth SS transmits -LTF in a first time slot (and +LTF in a second, third and fourth time slot), but in a fourth second PPDU, when just one SS is estimated, it would be +LTF.

[0028] The subsequent data units 20, 30, 40, 50 each comprise a preamble 21 , 31 , 41 , 51 and a data portion 22, 32, 42, 52, which are transmitted after the first data unit 10. The preamble includes one or more channel estimation sequences transmitted on a subset of the plurality of SS. Hereby, the number of channel estimation sequences transmitted on the subset of the plurality of SS is zero or more but less than the total number of SS. In the exemplary embodiment shown in Fig. 7, the preamble 21 of the first subsequent data unit 20 includes a single channel estimation sequence on a first SS (indicated as LTF on SS1 in Fig. 7), the preamble 31 of the second subsequent data unit 30 includes a single channel estimation sequence on a second SS (LTF on SS2), the preamble 41 of the third subsequent data unit 40 includes a single channel estimation sequence on a third SS (LTF on SS3), and the preamble 51 of the fourth subsequent data unit 50 includes a single channel estimation sequence on a fourth SS (LTF on SS4).

[0029] The number of channel estimation sequences transmitted on a subset of the plurality of SS may be zero, i.e. , there may be no channel estimation sequence included in one or more of the preambles 21 , 31 , 41 , 51 , which means that skipping a channel estimation sequence may be possible, which may be used in case the channel variation is slow compared to data unit (PPDU) duration and / or compared to the time spacing between PPDUs.

[0030] Fig. 8 shows a diagram illustrating a first embodiment of the transmission of the data units shown in Fig. 7. In Fig. 8 the OFDM resource grids (symbols slots over subcarriers) of PPDUs are shown. For the first PPDU 10, each channel estimation sequence of the preamble 11 occupies one symbol 13, 14, 15, 16, each transmitted on four subcarriers. The data portion 12 occupies two data symbols 17, 18, as an example. Each subsequent data unit 20, 30, 40, 50 comprises in its preamble 21 , 31 , 41 , 51 a single channel estimation se-quence 23, 33, 43, 53, i.e. , the subset of SS contains one SS. Per subsequent PPDll 20, 30, 40, 50, one SS of 4 SS is estimated and one symbol slot is occupied for each of the LTFs. As an example, each data portion 22, 32, 42, 52 of the subsequent PPDlls 20, 30, 40, 50 comprises two data symbols 27, 28, 37, 38, 47, 48, 57, 58. Other fields than LTF and data fields are neglected for simplicity. The data symbols 12, 22, 32, 42, 52 and the LTFs 12, 13, 14, 15 are generally sent on all SS. The LTF 23 in preamble 21 is sent on SS1, the LTF 33 in preamble 31 is sent on SS2, the LTF 43 in preamble 41 is sent on SS3, and the LTF 53 in preamble 51 is sent on SS4.

[0031] Fig. 9 shows a diagram illustrating a second embodiment of the transmission of the data units, wherein each subsequent data unit 20a, 30a, 40a, 50a comprises two channel estimation sequences, i.e., the subset of SS contains two SS. In Fig. 9, the first data unit 10 is identical as the first data unit 10 shown in Fig. 8. The subsequent data units 20a, 30a, 40a, 50a each comprise a data portion 22, 32, 42, 52 each comprising, as an example, two data symbols 27, 28 (like in Fig. 8), and a preamble 21 , 31, 41 , 51 each comprising two LTFs 23, 24, 33, 34, 43, 44, 53, 54, which may, e.g., have a different tone spacing. Thus, per subsequent PPDU 20a, 30a, 40a, 50a two SS of four SS are estimated and two symbol slots are occupied for the LTFs.

[0032] Fig. 10 shows a diagram illustrating a third embodiment of the transmission of the data units, wherein each subsequent data unit 20b, 30b, 40b, 50b comprises two channel estimation sequences, i.e., the subset of SS contains two SS. In Fig. 10, the first data unit 10 is identical with the first data unit 10 shown in Fig. 8. The subsequent data units 20b, 30b, 40b, 50b each comprise a data portion 22, 32, 42, 52 each comprising, as an example, two data symbols 27, 28 (like in Fig. 8), and a preamble 21 , 31, 41 , 51 each comprising two LTFs 23, 24, 33, 34, 43, 44, 53, 54. However, different from the second embodiment illustrated in Fig. 9, the LTFs of the two SS are transmitted on disjoint subsets of subcarriers. Thus, per subsequent PPDU 20b, 30b, 40b, 50b two SS of four SS are estimated and one symbol slot is occupied for the LTFs.

[0033] With the embodiments illustrated in Figs. 7 to 10, it is enabled that a first PPDU estimates at least one spatial stream, whereas a subsequent PPDU estimates zero or more spatial streams. The estimation of zero spatial streams may be applicable for very slow fadingscenario. Typically, at least one spatial stream is estimated in a subsequent PPDll and at least two spatial streams are estimated in a first PPDll. In general, the channel estimation sequences may be transmitted on all active subcarriers (as illustrated in Figs. 8 and 9) or on subsets of active subcarriers (as illustrated in Fig. 10), wherein the subsets of subcarriers may be disjoint for different SS.

[0034] Further second (= subsequent) PPDlls may be transmitted in a different configuration (i.e. , with a different subset of SS than the first second PPDll) and such that after two or more second PPDUs channel estimation sequences have been transmitted on all SS. The transmission of second PPDUs in different configuration may continue until a certain condition is fulfilled. Fig. 11 shows a diagram of the transmission of different second PPDUs 20, 30, 40, 50, each carrying a channel estimation sequence for a different SS, until all SS are re-estimated. Fig. 12 shows a diagram of the transmission of different second PPDUs 20, 30, 40, 50, 20’ (having a different data portion than PPDU 20), 30’ (having a different data portion than PPDU 30) until a transmission opportunity (TXOP) ends. In an embodiment, when a new TXOP is established, the new transmission should generally start with a new a first PPDU and any subsequent PPDUs should reside in the same TXOP. Fig. 13 shows a diagram of the transmission of second PPDU(s) (in this example only one second PPDU 20) until the transmission of a new first PPDU 10’ is desired, e.g. via configuration signaling 60 from the second communication device. Typically, the data portion of PPDU 10’ is different from the data portion of PPDU 10.

[0035] All second PPDUs should successively follow each other with a short time spacing, e.g., SIFS (short interframe spacing). The stream of PPDUs may be interrupted by a short PPDU in opposite communication direction such as Ack optionally followed by user data for opposite communication direction. Fig. 14 shows a diagram of the transmission of PPDUs interrupted by SIFSs and of the transmission of PPDUs 70, 71 in a different communication direction.

[0036] The general layout of a PPDU D according to WLAN standards is shown in Fig. 15 depicting more details than in Fig. 1. The PPDUs D generally comprise a physical layer (PHY) preamble 1 and a data field 2. The PHY preamble 1 comprises at least a legacy short training field L-STF, a legacy long training field L-LTF, a signaling field SIG, and channelestimation sequences LTF for different subsets of SS. The L-STF and L-LTF are transmitted on all SS irrespective if they belong to a first or second PPDll. The L-STF and L-LTF may have a different cyclic shift delay (CSD) per SS to avoid unwanted beamforming effects. For transmission of the L-STF and L-LTF the transmitter is configured as shown in Fig. 2 with an identity matrix being the orthogonal mapping matrix and an identity matrix being the spatial mapping. The CSDs in nanoseconds for transmission of L-STF and L- LTF for each SS are given in Table 2.Table 2

[0037] The SIG (either the L-SIG or the SIG fields of newer amendments) contains configuration information such asSubset of SS being occupied by the subsequent estimation sequences.Subset of subcarriers for each SS occupied by the subsequent estimation sequences.Type of subsequent channel estimation sequence. This includes the spacing of the occupied subcarriers of the estimation sequence.Power boost applied on the channel estimation sequenceIndication if channel estimates for all SS need to be reset. In this case the PPDU would contain a number of LTFs equal or larger than the number of SS.

[0038] In an embodiment, a power boost may be applied to the channel estimation sequences of a subsequent PPDll, i.e., the transmit power of the channel estimation sequences may be increased. The power boost may correspond to the total number of SS divided by number of occupied SS. Using this power boost, the total transmit power for the channel estimation sequences is unchanged between a first and a subsequent PPDU. Figs. 16A and 16B show schematic diagrams of a corresponding transmitter 500 at different processing steps, i.e., Fig. 16A shows the processing of a channel estimation sequence in a subsequent PPDU for estimation of one SS SS1 from four SS in total and Fig. 16B shows the processing of a channel estimation sequence in a subsequent PPDU for estimation of SS2.

[0039] In another embodiment, in a first PPDU, an orthogonal mapping matrix is applied to the channel estimation sequences as described above and as used in known systems. In a subsequent PPDU comprising only one channel estimation sequence no orthogonal mapping is required. In a subsequent PPDU comprising more than one channel estimation sequence an orthogonal mapping matrix shall be applied in order to enable separation of the SS. The orthogonal mapping matrix may be a subset of the matrix P defined above. Figs. 17A and 17B show schematic diagrams of a corresponding transmitter 600 at different processing steps, i.e., Fig. 17A shows the processing of a channel estimation sequence in a subsequent PPDU for estimation of two SS SS1 , SS2 from four SS in total and Fig. 17B shows the processing of a channel estimation sequence in a subsequent PPDU for estimation of two SS SS3, SS4.

[0040] In the embodiments of the transmitters 500, 600 shown in Figs. 16 and 17, after the power boost 501 and (in the transmitter 600) the orthogonal mapping matrix 601 , a different cyclic shift delay (CSD) 502 for each SS may be applied in order to prevent undesired beamforming effects. For the transmission of the channel estimation sequences of the subsequent PPDUs, a spatial mapping may be applied that keeps all transmit antennas at least partially active although a subset of SS is occupied. The first data unit and subsequent data units use the same spatial mapping: In a subsequent data unit the spatial streams, which do not belong to the subset of spatial streams occupied by channel estimation sequences, are non-occupied, i.e., zero (= no signal) for the duration of the channel estimation sequences.

[0041] The spatial mapping may be represented by a unitary matrix based on a Hadamard or Fourier matrix. An embodiment of a Hadamard matrix is shown in Fig. 18 for a number of spatial streams N_ss=8 and a number of transmit antennas N_tx=8. If smaller values for N_ss or N_tx are used, a N_ss x N_tx subset of the matrix shown in Fig. 18 may be used. It shall be noted that the matrix may be normalized by the square root of the number of transmit antennas. The Fourier matrix for variable values of N_ss and N_tx is shown in Fig. 3. The spatial mapping of the subsequent PPDlls is the same as the spatial mapping of the first PPDll. The spatial mapping used for transmission of the channel estimation sequences is used in the subsequent data field. Subsequently, inverse discrete Fourier transformation 504, insertion of a guard interval (Gl) 505, and analog and RF processing 506 are applied as generally known.

[0042] The modulation and coding scheme (MCS) of the data field of a subsequent PPDll may be adapted with respect to the first PPDll such that the data transmission is more robust. If a low response latency is desired, the MCS of the second PPDU may be chosen equal to the first PPDU as illustrated in Fig. 19A showing a standard first PPDU 10 and Fig. 19B showing an embodiment of a subsequent PPDU 20 without change of MCS. If a more robust data transmission is desired, the MCS may be decreased. Since this increases PPDU length and thus has a contrary effect to response latency, an MCS may be chosen such that the subsequent PPDU is not longer than the first PPDU. This is illustrated in Fig. 19C showing an embodiment of a second PPDU 20c, where a reduced MCS is applied on the data field and where the data field extension is smaller than the preamble reduction, and Fig. 19D showing an embodiment of a subsequent PPDU 20d, where a reduced MCS is applied on the data field and where the data field extension equals the preamble reduction.

[0043] A response message may be received from the second communication device, e.g., as part of an acknowledgement (ACK). The response message may configure the first or subsequent PPDUs transmitted subsequently by the first communication device. Fig. 13 shows a diagram of the transmission of PPDUs, where a configuration signaling 60 from the second communication device to the first communication device configuring a (new) first PPDU 10’ is used. Fig. 20 shows a diagram of the transmission of PPDUs, where a configuration signaling 61 from the second communication device to the first communica-tion device configuring a subsequent PPDll 20” is used. The subsequent PPDll 20” may then be adapted to the received configuration 61 , e.g., to include a channel estimation sequence on a subset of SS (SS1). Hereby, the data portion and / or the order may be different. This example is for four SS; hence, PPDll 20 occupies SS1 , PPDU 30 occupies SS2, However, PPDU 30 does not follow PPDU 20 but there is another PPDU 20” in between. Thus, the configuration signaling would indicate to repeat the LTF configuration of PPDU 20 again. Still, the data portion is typically different among all PPDUs.

[0044] The configuration signaling 60 of the first PPDU 10’ in Fig. 13 may include one or more of the following indications: the number of channel estimation sequences, which may be equal or larger than the number of total SS, the type of channel estimation sequences considering active subcarriers in the PPDU bandwidth, and the Gl length. The configuration signaling 61 of the subsequent PPDU 20” in Fig. 20 may include indications of one or more of the following indications: a desired subset of SS to be occupied by the channel estimation sequences in the subsequent PPDU 20” (the size of the subset defines the number of channel estimation sequences, which may be less than the number of total SS), the subset of subcarriers for each SS occupied by the estimation sequences, the type of channel estimation sequence (which may include the spacing of the occupied subcarriers of the estimation sequence), and the Gl length.

[0045] Fig. 21 shows a simplified flowchart of an embodiment of a first communication method 700 of the first communication device. In a first step 701 , the first PPDU is generated and transmitted, the first PPDU comprising channel estimation sequences for all SS. In a second step, one or more subsequent PPDUs are generated and transmitted, each subsequent PPDU comprising zero or more channel estimation sequences for a subset of zero or more SS, i.e. , less than all SS.

[0046] Fig. 22 shows a flowchart of an embodiment of a second communication method 800 of the second communication device according to the present disclosure that corresponds to the first communication method 700 illustrated in Fig. 21. Fig. 22 particularly illustrates the behavior of the second communication device upon reception of a first and subsequent PPDUs. In a first step 801 , the second communication device receives a first PPDU. The PPDU configuration may be obtained from the SIG field of the first PPDU. In a secondstep 802, the channel of all SS is estimated using the channel estimation sequences in the first PPDll. The channel estimates of all SS are stored in a memory 850 in step 803. The memory 850 to store the channel estimates may be erased before storing new values every time a first PPDll is received. Equalization of the received first PPDll is performed in step 804 using the stored channel estimate.

[0047] In step 805, the second communication device receives a subsequent PPDll. An auxiliary channel estimate is determined in step 806 from the L-LTF contained in the PHY preamble of the subsequent PPDU. As explained below, the auxiliary channel estimate may be used for a compensation process. The PPDU configuration may be obtained from the SIG field of the subsequent PPDU. This includes the subset of SS that is estimated with this PPDU. A compensation of any sample offset arising between a subsequent PPDU detection and a previous PPDU detection is performed on the stored channel estimates. The sample offset may arise from sample rate offset or time shift of the packet detection and causes a linear phase increment over subcarriers. The channel of a subset of SS is estimated using the channel estimation sequences in the received subsequent PPDU. The channel of the subset of SS may be estimated on only a subset of active subcarriers. An interpolation of channel estimates may be performed in a subset of active subcarriers to obtain channel estimates in all active subcarriers. Further, the power of the channel estimate may be normalized if a power boost was applied on the channel estimation sequence. The channel estimates of the subset of the SS stored in the memory are replaced or updated by the new channel estimates of the subset of SS in step 807. Finally, equalization of the received subsequent PPDU is performed in step 808 using the stored channel estimate, i.e. , the “partially renewed” channel estimate.

[0048] Fig. 23 shows a flowchart of another embodiment of a second communication method 900 of the second communication device according to the present disclosure. In this embodiment, steps 901, 903 to 906, 910, 912 and 913 correspond to steps 801 to 808 of the method 800 illustrated in Fig. 22. In steps 902 and 907, an auxiliary channel estimate is determined from the L-LTF, which can, e.g., be used to estimate a linear phase increment over subcarriers of the new channel estimate with respect to the old channel estimate in step 908. The linear phase increment over subcarriers may be determined via linear regression of the phase difference between the auxiliary channel estimate of a subsequentPPDll and the auxiliary channel estimate of the previous PPDll, which uses a memory 851 (or the memory 850) for storing the auxiliary channel estimate in steps 902 and 909. Alternatively, the linear phase increment over subcarriers may be determined via linear regression of the phase difference between the new channel estimate and the auxiliary channel estimate, which does not require a memory for the auxiliary channel estimate. The linear phase increment over subcarriers is compensated in step 911 by shifting the phase of the stored channel estimates with the inverse of the estimated linear phase increment over subcarriers. In the case of a total of two SS, but only one SS can be estimated with one subsequent PPDll, the difference between channel estimates in the subsequent PPDU carrying only one LTF and L-LTF may be used to update the channel estimation of the SS whose LTF was not transmitted in the subsequent PPDU.

[0049] Fig. 24 shows a flowchart of another embodiment of a second communication method 1000 of the second communication device according to the present disclosure illustrating the behavior of the second communication device for a configuration request. In this embodiment, steps 1001 to 1008 and 1012 correspond to steps 901 , 902, 904 to 907, 910, 911 and 913 of the method 900 illustrated in Fig. 23. In a default operation mode, the choice of subset of SS whose channel is estimated with a subsequent PPDU may be alternated in a round-robin fashion, i.e. , the oldest channel estimates are re-estimated. However, this may be suboptimal in some cases. The L-LTF contained in the PHY preamble can be used to determine the quality of the partially renewed channel estimate in step 1009. For example, the quality can be determined in terms of mean squared error of the auxiliary channel estimate obtained from the L-LTF to the partially renewed channel estimate. Since the auxiliary channel estimate is a single-stream channel estimate, all spatial streams of the partially renewed channel estimate are combined to a single-stream channel estimate considering the cyclic shifts applied at transmission of the L-LTF. Further, it may be necessary to reduce its frequency resolution in order to match it to the frequency resolution of the auxiliary channel estimate.

[0050] If this mean squared error exceeds a certain threshold, as checked in step 1010, the partially renewed channel estimate is considered to be obsolete or inaccurate. In this case, the second communication device may request from the first communication device in step 1011 the transmission of a first PPDU and / or a configuration of a first or subsequentPPDll that is transmitted subsequently by the first device. The configuration parameters have been described above. This request may be done by transmission of a PPDll, but may also be done, e.g., by an ACK, from the second communication device to the first communication device. Otherwise, equalization of the received subsequent PPDll is performed in step 1012.

[0051] Fig. 25 shows a flowchart of another embodiment of a first communication method 1100 of the first communication device according to the present disclosure illustrating the behavior of the first communication device in the case of configuration signaling from the second communication device. This corresponds to the behavior of the second communication device illustrated in Fig. 24. Steps 1101 and 1104 correspond to steps 701 and 702 of the method 700 illustrated in Fig. 21. In step 1102, if a PPDU is received from the second communication device, it is checked for configuration signaling (60 in Fig. 13) which is evaluated if present. In step 1103 it is checked if the configuration signaling requires a new first PPDU (10’ in Fig. 13). If yes, the method continues with step 1101. If no, in step 1104 a subsequent PPDU is transmitted. In step 1105, if a PPDU is received from the second communication device, it is checked for configuration signaling (60 in Fig. 13) which is evaluated if present. In step 1106 it is checked if the configuration signaling requires a new first PPDU (10’ in Fig. 13). If yes, the method continues with step 1101. If no, the method continues with step 1104.

[0052] Another embodiment is directed to a first communication device (and a corresponding communication method) configured to communicate with a second communication device, the first communication device comprising circuitry configured to: generate data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences; map the generated data units onto a plurality of spatial streams; and transmit the generated data units, wherein generating the data units and transmitting the generated data units includes generating a first data unit including in its preamble one or more channel estimation sequences transmitted on the plurality of spatial streams, wherein the one or morechannel estimation sequences includes at least one channel estimation sequence per spatial stream; and generating one or more subsequent data units transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.According to this embodiment, only a single SS is estimated in the first PPDll and zero SS are estimated in the subsequent PPDlls (i.e., zero SS is a subset of one SS). The corresponding second communication device and method may be configured accordingly.

[0053] The devices may be implemented by respective units or circuitry, e.g. a processor, processing circuitry, a computer, dedicated hardware, etc., that carries out the functions of the device. Alternatively, a common unit or circuitry, e.g. a common processor or computer, may implement the various functions of the device, or separate units or elements may be used that together represent the circuitry.

[0054] 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.

[0055] 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.

[0056] 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 distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0057] The elements of the disclosed devices, apparatus and systems may be implemented by corresponding hardware and / or software elements, for instance appropriate circuits or circuitry. 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. A circuit or circuitry may be implemented by a single device or unit or multiple devices or units, or chipset(s), or processor(s).

[0058] It follows a list of further embodiments of the disclosed subject matter:1. First communication device configured to communicate with a second communication device, the first communication device comprising circuitry configured to: generate data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences; map the generated data units onto a plurality of spatial streams; and transmit the generated data units, wherein generating the data units and transmitting the generated data units includes generating a first data unit including in its preamble a plurality of channel estimation sequences transmitted on the plurality of spatial streams, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; andgenerating one or more subsequent data units transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.2. First communication device according to embodiment 1 , wherein the circuitry is configured to generate a number of subsequent data units transmitted after the first data unit that corresponds to the number of spatial streams or to an integer fraction of the number of spatial streams.3. First communication device according to any preceding embodiment, wherein the circuitry is configured to include a single channel estimation sequence into the preamble of each subsequent data unit.4. First communication device according to any preceding embodiment, wherein the circuitry is configured to include channel estimation sequences transmitted on two or more spatial streams into the preamble of each subsequent data unit, wherein each channel estimation sequence included in a preamble of a subsequent data unit is mapped onto a different symbol and / or the channel estimation sequences included in a preamble of a subsequent data unit are mapped onto different subsets of subcarriers of a common symbol.5. First communication device according to any preceding embodiment, wherein the circuitry is configured to generate subsequent data units for transmission after the first data unit until either i) channel estimation sequences have been generated and transmitted on the plurality of spatial streams, or ii) a transmit opportunity ends, or iii) information is received from the second communication unit indicating a channel estimation error and / or requesting the transmission of a new first data unit and to generate a new first data unit afterwards.6. First communication device according to any preceding embodiment, wherein the circuitry is configured to transmit the first and / or subsequent data units with a time spacing in between and / or with reception of a response data unit from the second communication device in between.7. First communication device according to any preceding embodiment, wherein the circuitry is configured to control the transmit power of the channel estimation sequences included in the subsequent data units.8. First communication device according to embodiment 7, wherein the circuitry is configured to apply a power boost to the one or more channel estimation sequences included in a second data unit such that the transmit power used for transmitting said one or more channel estimation sequences is substantially equal to the transmit power used for transmitting the plurality of channel estimation sequences of a preceding first data unit.9. First communication device according to any preceding embodiment, wherein the circuitry is configured to include, into the preamble of the first data unit and / or one or more subsequent data units, information indicating if and / or which spatial streams are occupied by channel estimation sequences in the same data unit and / or information indicating which subcarriers are occupied by the channel estimation sequences for each of the said spatial streams.10. First communication device according to any preceding embodiment, wherein the circuitry is configured to include another channel estimation sequence into the preamble of each transmitted data unit and to transmit the other channel estimation sequence over all spatial streams, in particular with different cyclic shift per spatial stream.11. First communication device according to any preceding embodiment, wherein the circuitry is configured to map the generated data units onto a plurality of spatial streams with a spatial mapping matrix that is identical between a first and one or more subsequent data units.12. First communication device according to any preceding embodiment, wherein the circuitry is configured to apply orthogonal mapping to generate channel estimation sequences if two or more channel estimation sequences are included in a data unit.13. First communication device according to any preceding embodiment, wherein the circuitry is configured to apply a different cyclic shift to the channel estimation sequence for each spatial stream.14. First communication device according to any preceding embodiment, wherein the circuitry is configured to select which modulation and coding scheme to apply to the data portion of the one or more subsequent data units and the data portion of the first data unit.15. First communication device according to embodiment 14, wherein the circuitry is configured to apply a different modulation and coding scheme to the data portion of the one or more subsequent data units compared to the data portion of the first data unit.16. Second communication device configured to communicate with a first communication device, the second communication device comprising circuitry configured to: receive a first data unit from the first communication device, the first data unit comprising a preamble and a data portion and including in its preamble a plurality of channel estimation sequences transmitted on a plurality of spatial streams onto which the first data unit is mapped, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; estimate channels of the plurality of spatial streams using the channel estimation sequences included in the first data unit; receive one or more subsequent data units from the first communication device after receiving the first data unit, wherein each of the subsequent data units comprises a preamble and a data portion and includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channelestimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams; estimate, using a received subsequent data unit, channels of a subset of spatial streams using the channel estimation sequences included in the said subsequent data unit; and update the channel estimate of a subset of the plurality of spatial streams obtained by use of channel estimation sequences included in the first or one or more subsequent data units by the channel estimate of the same subset of spatial streams obtained by use of a channel estimation sequence included in a subsequent data unit.17. Second communication device according to embodiment 12, wherein the circuitry is configured to store channel estimates obtained by use of channel estimation sequences included in the first and subsequent data units.18. Second communication device according to embodiment 17, wherein the circuitry is configured to evaluate another channel estimation sequence transmitted by the first communication device, derive a phase compensation term and apply the phase compensation term to the stored channel estimates.19. Second communication device according to embodiment 17 or 18, wherein the circuitry is configured to equalize the received first data unit and / or one or more subsequent data units using the stored channel estimates.20. Second communication device according to any one of embodiments 16 to 19, wherein the circuitry is configured to compute an error of a channel estimate obtained by use of a channel estimate sequence included in a subsequent data unit and transmit information to the first communication unit indicating a channel estimation error and / or requesting the transmission of a new first data unit.21. Second communication device according to any one of embodiments 16 to 20, wherein the circuitry is configured to determine and transmit configuration signaling to the first communication device, the configuration signaling including one or more of:for the first data unit: number and / or type of channel estimation sequences and / or guard interval length; and / or for the one or more subsequent data units: number and / or type of channel estimation sequences and / or guard interval length and / or desired subset of spatial streams occupied by the channel estimation sequences included in a subsequent data unit and / or desired subset of subcarriers per spatial stream occupied by the channel estimation sequences included in a subsequent data unit.22. Second communication device according to any one of embodiments 16 to 21 , wherein the circuitry is configured to update the channel estimate of a subset of the plurality of spatial streams obtained by use of channel estimation sequences included in previously transmitted subsequent data units by the channel estimate of the same subset of spatial streams obtained by use of a channel estimation sequence included in a subsequent data unit.23. First communication method of a first communication device configured to communicate with a second communication device, the first communication method comprising: generating data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences; mapping the generated data units onto a plurality of spatial streams; and transmitting the generated data units, wherein generating the data units and transmitting the generated data units includes generating a first data unit including in its preamble a plurality of channel estimation sequences transmitted on the plurality of spatial streams, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; and generating one or more subsequent data units transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.24. Second communication method of a second communication device configured to communicate with a first communication device, the second communication method comprising: receiving a first data unit from the first communication device, the first data unit comprising a preamble and a data portion and including in its preamble a plurality of channel estimation sequences transmitted on a plurality of spatial streams onto which the first data unit is mapped, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; estimating channels of the plurality of spatial streams using the channel estimation sequences included in the first data unit; receiving one or more subsequent data units from the first communication device after receiving the first data unit, wherein each of the subsequent data units comprises a preamble and a data portion and includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams; estimating, using a received subsequent data unit, channels of a subset of spatial streams using the channel estimation sequences included in the said subsequent data unit; and updating the channel estimate of a subset of the plurality of spatial streams obtained by use of channel estimation sequences included in the first or one or more subsequent data units by the channel estimate of the same subset of spatial streams obtained by use of a channel estimation sequence included in a subsequent data unit.25. 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.26. 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.

Claims

CLAIMS1. First communication device configured to communicate with a second communication device, the first communication device comprising circuitry configured to: generate data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences; map the generated data units onto a plurality of spatial streams; and transmit the generated data units, wherein generating the data units and transmitting the generated data units includes generating a first data unit including in its preamble a plurality of channel estimation sequences transmitted on the plurality of spatial streams, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; and generating one or more subsequent data units transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.

2. First communication device according to claim 1 , wherein the circuitry is configured to generate a number of subsequent data units transmitted after the first data unit that corresponds to the number of spatial streams or to an integer fraction of the number of spatial streams.

3. First communication device according to claim 1 , wherein the circuitry is configured to include a single channel estimation sequence into the preamble of each subsequent data unit.

4. First communication device according to claim 1 , wherein the circuitry is configured to include channel estimation sequences transmitted on two or more spatial streams into the preamble of each subsequent data unit, wherein each channel estimation sequence included in a preamble of a subsequent data unit is mappedonto a different symbol and / or the channel estimation sequences included in a preamble of a subsequent data unit are mapped onto different subsets of subcarriers of a common symbol.

5. First communication device according to claim 1 , wherein the circuitry is configured to generate subsequent data units for transmission after the first data unit until either i) channel estimation sequences have been generated and transmitted on the plurality of spatial streams, or ii) a transmit opportunity ends, or iii) information is received from the second communication unit indicating a channel estimation error and / or requesting the transmission of a new first data unit and to generate a new first data unit afterwards.

6. First communication device according to claim 1 , wherein the circuitry is configured to transmit the first and / or subsequent data units with a time spacing in between and / or with reception of a response data unit from the second communication device in between.

7. First communication device according to claim 1 , wherein the circuitry is configured to control the transmit power of the channel estimation sequences included in the subsequent data units.

8. First communication device according to claim 7, wherein the circuitry is configured to apply a power boost to the one or more channel estimation sequences included in a second data unit such that the transmit power used for transmitting said one or more channel estimation sequences is substantially equal to the transmit power used for transmitting the plurality of channel estimation sequences of a preceding first data unit.

9. First communication device according to claim 1 , wherein the circuitry is configured to include, into the preamble of the first data unit and / or one or more subsequent data units, information indicating if and / or which spatial streamsare occupied by channel estimation sequences in the same data unit and / or information indicating which subcarriers are occupied by the channel estimation sequences for each of the said spatial streams.

10. First communication device according to claim 1 , wherein the circuitry is configured to include another channel estimation sequence into the preamble of each transmitted data unit and to transmit the other channel estimation sequence over all spatial streams, in particular with different cyclic shift per spatial stream.

11. First communication device according to claim 1 , wherein the circuitry is configured to map the generated data units onto a plurality of spatial streams with a spatial mapping matrix that is identical between a first and one or more subsequent data units.

12. Second communication device configured to communicate with a first communication device, the second communication device comprising circuitry configured to: receive a first data unit from the first communication device, the first data unit comprising a preamble and a data portion and including in its preamble a plurality of channel estimation sequences transmitted on a plurality of spatial streams onto which the first data unit is mapped, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; estimate channels of the plurality of spatial streams using the channel estimation sequences included in the first data unit; receive one or more subsequent data units from the first communication device after receiving the first data unit, wherein each of the subsequent data units comprises a preamble and a data portion and includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams; estimate, using a received subsequent data unit, channels of a subset of spatial streams using the channel estimation sequences included in the said subsequent data unit; andupdate the channel estimate of a subset of the plurality of spatial streams obtained by use of channel estimation sequences included in the first or one or more subsequent data units by the channel estimate of the same subset of spatial streams obtained by use of a channel estimation sequence included in a subsequent data unit.

13. Second communication device according to claim 12, wherein the circuitry is configured to store channel estimates obtained by use of channel estimation sequences included in the first and subsequent data units.

14. Second communication device according to claim 13, wherein the circuitry is configured to evaluate another channel estimation sequence transmitted by the first communication device, derive a phase compensation term and apply the phase compensation term to the stored channel estimates.

15. Second communication device according to claim 13, wherein the circuitry is configured to equalize the received first data unit and / or one or more subsequent data units using the stored channel estimates.

16. Second communication device according to claim 12, wherein the circuitry is configured to compute an error of a channel estimate obtained by use of a channel estimate sequence included in a subsequent data unit and transmit information to the first communication unit indicating a channel estimation error and / or requesting the transmission of a new first data unit.

17. Second communication device according to claim 12, wherein the circuitry is configured to determine and transmit configuration signaling to the first communication device, the configuration signaling including one or more of: for the first data unit: number and / or type of channel estimation sequences and / or guard interval length; and / or for the one or more subsequent data units: number and / or type of channel estimation sequences and / or guard interval length and / or desired subset of spatial streams occupied by the channel estimation sequences included in a subsequent data unit and / or de-sired subset of subcarriers per spatial stream occupied by the channel estimation sequences included in a subsequent data unit.

18. First communication method of a first communication device configured to communicate with a second communication device, the first communication method comprising: generating data units for transmission to the second communication device, a data unit comprising a preamble and a data portion, the preamble including one or more channel estimation sequences; mapping the generated data units onto a plurality of spatial streams; and transmitting the generated data units, wherein generating the data units and transmitting the generated data units includes generating a first data unit including in its preamble a plurality of channel estimation sequences transmitted on the plurality of spatial streams, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; and generating one or more subsequent data units transmitted after the first data unit, wherein each of the subsequent data units includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams.

19. Second communication method of a second communication device configured to communicate with a first communication device, the second communication method comprising: receiving a first data unit from the first communication device, the first data unit comprising a preamble and a data portion and including in its preamble a plurality of channel estimation sequences transmitted on a plurality of spatial streams onto which the first data unit is mapped, wherein the plurality of channel estimation sequences includes at least one channel estimation sequence per spatial stream; estimating channels of the plurality of spatial streams using the channel estimation sequences included in the first data unit; receiving one or more subsequent data units from the first communication device after receiving the first data unit, wherein each of the subsequent data units comprises apreamble and a data portion and includes in its preamble channel estimation sequences transmitted on a subset of the plurality of spatial streams, wherein the number of channel estimation sequences transmitted on the subset of the plurality of spatial streams is zero or more but less than the total number of spatial streams; estimating, using a received subsequent data unit, channels of a subset of spatial streams using the channel estimation sequences included in the said subsequent data unit; and updating the channel estimate of a subset of the plurality of spatial streams obtained by use of channel estimation sequences included in the first or one or more subsequent data units by the channel estimate of the same subset of spatial streams obtained by use of a channel estimation sequence included in a subsequent data unit.

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.

Images