The present disclosure relates to different communication devices, such as a mobile station and an access point, which are configured for RF-based communication with each other. The present disclosure further relates to corresponding communication methods.

DESCRIPTION OF RELATED ART

Communication systems in the 60 GHz frequency range suffer from strong free space path loss, which increases with frequency. For instance, a 60 GHz communication system has roughly 22dB higher attenuation compared to a communication system operating at 5 GHz. To overcome the increased path loss, 60 GHz or any mm-wave communication system employs beamforming, i.e. transmitter and/or receiver feature a steerable phased- array antenna (PAA), which can form directive beams towards the other communication device. Such beams have typically a high directivity and are spatially very narrow. The directivity in main direction increases with the number of antenna elements per PAA. In contrast, the half-power beam width (HPBW) defines the spatial width of a pattern decreases with increasing number of antennas. Thus, the more antennas per PAA, the higher the directivity and the smaller the HPBW. In order to make use of the PAA directivity for communications, beam alignment is crucial and of high importance for mm-wave communication systems and RF communication systems, methods and devices in general.

[0003] The goal of transmit beamforming techniques is to allow MIMO transmitters to

simultaneously send multiple streams to one or more receivers, based on the channel states, such that an adequate reception can be achieved. As compared to the sub 6GHz, mm-wave channels pose many additional challenges on the design of transmit beam- formers. Due to the very high frequency, mm-wave channels suffer from strong path loss, which can only be combatted by exploiting antenna arrays of multiple elements However, the use of many antenna elements can have prohibitive complexity both due to hardware constraint as well as channel estimation burden. Therefore, hybrid antenna architectures where only a limited number of RF chains are employed to steer antenna arrays of multiple elements are more useful.

[0004] Single carrier (SC) transmission has been chosen by the developing mm-wave standards

IEEE 802.1 1 ad and IEEE 802.1 l ay as the mandatory modulation mode, due to the reduced complexity. However, since large chunks of spectrum are available and envisioned to be used in mm-wave channels, the channels will experience a certain frequency selectivity and equalization techniques are required. Frequency domain equalization (FDE) is a common equalization technique to reduce inter-symbol interference.

[0005] Thus, there is a need for transmit beamforming schemes which are more appropriate for the wideband SC mm-wave, without needing overly complicated filters at the transmitter or excessive FFT/IFFT blocks and taking into account the equalization scheme at the receiver.

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 or impliedly admitted as prior art against the present disclosure.

SUMMARY

[0007] It is an object to provide communication devices and corresponding communication

methods enabling wideband SC mm-wave communication which do not require much additional or even complicated hard- or software, e.g. complex filters or FFT and IFFT blocks at the transmitter and which can be better adapted to the SC receiver structures

[0008] According to an aspect there is provided a (first) communication device (also called

initiator or transmitter), e.g. an access point, comprising:

digital beamforming circuitry configured to perform digital beamforming based on digital beamforming information to obtain RF data streams, and

analog beamforming circuitry configured to perform analog beamforming for the obtained RF data streams, and

wherein said analog beamforming circuitry is configured to perform analog beamforming training with the other communication device enabling the other communication device to compute the digital beamforming information for combinations of analog beams used in said analog beamforming training, and

wherein said digital beamforming circuitry is configured to receive the computed digital beamforming information and to use it for performing the digital beamforming.

[0009] According to a further aspect there is provided a (second) communication device (also called responder or receiver), e.g. a station, comprising:

analog beamforming circuitry configured to perform analog beamforming training with the other communication device for determining analog beams for use in receiving said F data streams, and

digital beamforming computation circuitry configured to compute digital beamfoim- ing information based on a predetermined metric computed for combinations of analog beams used in said analog beamforming training and to transmit the computed digital beamforming information to the other communication device enabling the other communication device to perform digital beamforming based on the digital beamforming information.

[0010] According to a further aspect there is provided a (third) communication device (which is an alternative to the first communication device) for RF-based communication with another communication device, comprising

digital beamforming circuitry configured to perform digital beamforming based on digital beamforming information to obtain RF data streams, and

analog beamforming circuitry configured to perform analog beamforming for the obtained RF data streams,

wherein said analog beamforming circuitry is configured to perform analog beamforming training with the other communication device enabling the communication device to compute the digital beamforming information corresponding to one or more combinations of analog beams used in said analog beamforming training, and

wherein said digital beamforming circuitry is configured to use the computed digital beamforming information for performing the digital beamforming.

[001 1 ] According to still further aspects a computer program comprising program means for causing a computer to carry out the steps of the method disclosed herein, when said computer program is carried out on a computer, as well as a non-transitory computer- readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method disclosed herein to be performed are provided.

[0012] Embodiments are defined in the dependent claims. It shall be understood that the

disclosed communication methods, the disclosed computer program and the disclosed

computer-readable recording medium have similar and/or identical further embodiments as the claimed communication devices and as defined in the dependent claims and/or disclosed herein.

One of the aspects of the disclosure is to provide low complexity digital beamforming, in particular precoding, for single user (SU) Ml MO mm-wave single carrier communications and feedback schemes to allow the application of the scheme. The beamforming information for use in the digital beamforming at the transmitter is computed at a receiver, particularly based on channel information in the frequency domain, and is preferably translated into a wideband precoder to be used by the transmitter. The obtained solution is then fed back to the transmitter, for which purpose required signaling is disclosed in embodiments. The disclosed solution leverages on the one hand the low complexity of the single carrier transmission and on the other hand the advantages of equalization techniques e.g., frequency domain equalization at the receiver to combat inter-symbol interference.

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

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:

shows a schematic diagram of a communication system including first and second communication devices according to the present disclosure;

Fig. 2 shows a more detailed schematic diagram of a communication system including a first communication device and a second communication device according to the present disclosure;

Fig. 3 shows a diagram illustrating a general embodiment of the operations of

communication methods according to the present disclosure;

Fig. 4 shows a diagram illustrating a first exemplary embodiment of the operations of communication methods according to the present disclosure;

Fig. 5 shows a diagram illustrating a second exemplary embodiment of the operations of communication methods according to the present disclosure;

Fig. 6 shows a diagram illustrating the operations of digital training after RTS/CTS exchange with acknowledgement;

Fig. 7 shows a diagram illustrating the operations of digital training after RTS/CTS exchange without acknowledgement; and

Fig. 8 shows a diagram illustrating the operations of digital beamforming without

RTS/CTS exchange directly after analog BF training and with acknowledgement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, Fig. 1 shows a schematic diagram of a communication system 100 including first communication devices 1 , 3 (stations STA1 , STA2) and a second communication device 2 (an access point AP). The communication devices are generally configured to carry out RF-based communication with each other. [0017] Each of said communication devices 1 , 3, serving as receiver, generally comprises digital beamforming circuitry 10, 30 (also called digital beamformer or digital beamforming unit herein) configured to perform digital beamforming based on digital beamforming information to obtain RF data streams, analog beamforming circuitry 1 1 , 31 (also called analog beamformer or analog beamforming unit herein) configured to perform analog beamforming for the obtained RF data streams, and antenna circuitry 12, 32 configured to transmit the obtained RF data streams using the analog beams formed by the analog beamforming circuitry. The analog beamforming circuitry 1 1 , 31 is hereby configured to perform analog beamforming training with the other communication device enabling the other communication device to compute the digital beamforming information based on a predetermined metric computed for combinations of analog beams used in said analog beamforming training. The digital beamforming circuitry 10, 30 is configured to receive the computed digital beamforming information and to use it for performing the digital beamforming

[0018] Said communication device 2, serving as transmitter, generally comprises antenna

circuitry 22 configured to receive RF data streams, analog beamforming circuitry 20 configured to perform analog beamforming training with the other communication device for determining analog beams for use by the antenna circuitry to receive said RF data streams, and digital beamforming computation circuitry 21 configured to compute digital beamforming information based on a predetermined metric computed for combinations of analog beams used in said analog beamforming training and to transmit the computed digital beamforming information to the other communication device enabling the other communication device to perform digital beamforming based on the digital beamforming information.

[0019] The disclosed communication devices enable transmitting beamforming schemes which are more appropriate for the wideband SC mm-wave. Precoding matrices may be computed and fed back based on the assumption that the precoding is done per subcarrier or per group of subcarriers. This does, however, not hold in the case of SC mm-wave systems, which are of interest here. Further, known mm-wave systems do not contain structures to allow for the feedback and application of digital beamformers.

An embodiment of this disclosure refers to a wideband hybrid beamforming solution for SU MI MO, which can be easily applied on conventional SC mm-wave communication links. Aspects of the present disclosure include a decoupled training, in which in a first stage analog beamforming training is performed to find the phase shifters configuration that best expose the channel and promises the largest capacity and a second stage in which the digital beamformers are found at the receivers based on frequency domain channel state information. A further aspect refers to a method to obtain an optimal transmit precoder, which is constant over frequency, based on the channel information in frequency domain available at the receiver. Further aspects refer to heuristic solutions which can be used for easier practical implementations and feedback schemes, e.g. mapped to the I EEE 802.1 1 ay signal flows and frame structure.

Fig. 2 depicts a detailed schematic diagram of an embodiment of a communication system 200 including a first communication device 4 (e.g. an access point, AP, serving as transmitter) and a second communication device 5 (e.g. a station STA serving as receiver) according to the present disclosure.

The first communication device 4 comprises a data mapping unit 40, a guard interval insertion unit 41 , a digital beamforming unit 42, several F chains 431 , 432 and, per RF chain, an analog beamforming unit 441 , 442, and antenna unit 45. RF signals are transmitted over the channel 6 to the second communication device 5, which comprises an antenna unit 50, several RF chains 521 , 522, an analog beamforming unit 51 1 , 512 per RF chain, a channel estimation unit 53, a digital beamforming computation unit 54 an equalization unit 55 and a detection unit 56. The operation of the communication devices 4 and 5 will be explained in more detail below. The second communication unit 5 may further comprise, before the channel estimation, a guard interval removing unit and a DFT unit and, after the equalization, an I FFT unit (not shown).

Assume that Ns streams are sent, which for ease of exposition are considered composed of just one block of M symbols and Nc— M guard interval symbols i.e.,

„ _ LCD „(2) (M+l) „GVC)1

ai L i ' i '—> 3i ' Λί >—> 3i J ■

For ease of exposition consider the transmitted symbol vector as =
. The transmitted signal is x = PAPDs, where PDand PA denote the transmit digital and analog beamforming matrices, respectively. More clearly, the analog matrix is represented as block diagonal, with all blocks equal, to model that the analog beamformer does not change during the transmission of a symbol block. On the other hand PD can model a filter with number of taps no larger than the guard interval length, thus it is chosen as a block circulant matrix. The received symbol after Gl removal is =
and WA is the receive analog beamforming matrix. WA is block diagonal matrix denoting the analog receive beamforming matrix. The effective channel which is seen after the analog transmit beamforming and receive beamforming can be written as

Heff = Μ^α(^ΑΗωΡΒΑ, ...,]ν^ΑΗ^^ΡΒΑ, 0ω, . . , 0Ν^]), where L_eff represents the number of effective taps remaining after applying the analog beamforming operations PBA and WBA denote repeating diagonal blocks of the precoding matrix and combining matrix respectively.

For a given precoding matrix, the receive strategy that minimizes the MMSE is

WD = {l + PD Hf,effHf,effPBD ) PD ^f,eff >

where H"effHf eff is a block diagonal matrix and each block k, corresponds to the channel matrix at subcarrier k. By imposing that the same digital transmit beamforming matrix is used for all subcarriers, is equivalent to assu = PBD (g) /

WD = (I + {PHBD ® I) HleffHfieff(PBD ®
® leff.

The digital transmit matrix which both maximizes the rate and minimizes the MMSE can be found by solving the following optimization problem (1 )

max V logdet(l + PBDHfkHfHkP^D) s. to Tr{PBDP»D}≤ P .

"BO *—i

k=l

Differently than conventional solutions, the mutual information maximization does not directly result from the SVD decomposition due to the coupling of PBD to all frequency domain channels. However, the mutual information maximization and the MMSE minimization are equivalent to a convex optimization problem, which can be solved within required precision by e.g. interior point solvers. To expose the convex structure, it can be noticed that problem (1 ) can be equivalently reformulated as an optimization problem in the positive semidefinite matrix QBD as

max