WO2019068305A1

WO2019068305A1 - Receiving device and methods thereof

Info

Publication number: WO2019068305A1
Application number: PCT/EP2017/074991
Authority: WO
Inventors: Chaitanya TUMULA; Neng Wang; Fredrik RUSEK
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2017-10-02
Filing date: 2017-10-02
Publication date: 2019-04-11
Anticipated expiration: 2020-04-02

Abstract

L'invention concerne des procédés et des dispositifs destinés à être utilisés dans un système de communication sans fil. Un dispositif de réception est prévu, qui peut sélectionner le meilleur faisceau de réception en estimant la direction d'arrivée (DoA) du signal entrant avec un retard de traitement d'acquisition de faisceau court et pour améliorer le gain de formation de faisceau de réception à l'aide des DoA estimés.Methods and devices for use in a wireless communication system are disclosed. A receiving device is provided, which can select the best receiving beam by estimating the arrival direction (DoA) of the incoming signal with a short beam acquisition processing delay and to improve the reception beam forming gain. using the estimated DoAs.

Description

Receiving Device and Methods Thereof

TECHNICAL FIELD

The invention relates to methods and devices for use in a wireless communication system. In particular, the invention relates to a receiving device with Direction of Arrival (DoA) estimation and methods for use in such a receiving device.

BACKGROUND

Direction of arrival (DoA) or direction finding is a well-researched problem for applications such as radar, sonar, acoustic signal separation, electronic surveillance etc. With the use of large antenna arrays for multiple-input multiple-output (MIMO) communication and associated beamforming techniques, DoA estimation algorithms found their way into the wireless communications field.

In fifth generation (5G) systems, base station (BS) and/or user equipment (UE) or other client devices may be equipped with a large number of antenna elements. The transmission and reception of signals in the downlink (DL) and uplink (UL) are based on beam based transmissions, i.e., the data is transmitted using narrow beams with most of the signal energy transmitted in the direction of the receiver.

In case of millimeter wave (mmWave) frequency systems, the transmitter and receiver may use a hybrid antenna structure with analog-digital precoding at the transmitter and analog-digital combining at the receiver. In a hybrid receiver structure, the receiver may be equipped with multiple panels each comprising multiple antenna elements with different polarizations.

When the transmitter is using a certain transmit beam, the signal arrives at the receiver from a certain direction, the DoA. However, the receiver has no prior knowledge of the DoA of the incoming signal. Hence, the receiver has to use different receive beams and choose the best receive beam for a given transmit beam. A receive beam refers to a vector comprising of phase values associated with each of the phase shifters of antenna elements (connected to an ADC). This process of choosing the best receive beam is known as beam acquisition or beam selection.

If the receiving device can estimate the incoming signal DoA, it can choose the best receive beam based on such DoA information. The best receive beam is the beam that maximizes the receive beamforming gain in the direction of arrival of the signal.

In existing systems for determining DoA, the receiver selects the best receive beam by sweeping through different receive beams one after the other, and measuring the received signal power at the output of an ADC. The received signal power measurement can for example correspond to reference signal received power (RSRP) or reference signal received quality (RSRQ) or signal to noise ratio (SNR) or any other measurement to quantify the quality of the received signal. Typically, the receive beam that maximizes the RSRP can be selected as the best receive beam for a given transmit beam. Also, in the DoA estimation method described in US8754810, a phase information of the correlation coefficient is computed using the signal outputs from the ADCs connected to antenna elements from two separate panels for DoA estimation.

SUMMARY

In the existing systems for determining DoA, there is a drawback in terms of a delay caused by selecting the best receive beam by trying all possible receive beams in a sequential manner. Also, the maximum receive beamforming gain obtained by the receive antenna array is limited by the codebook size of receive beams, i.e. the number of beams that are used in the sweeping procedure. In order to achieve the largest receive beamforming gain in all directions, the receive beam codebook size should be very large, which can increase the delay in the beam acquisition process. Further, the correlation method is not applicable in practical systems as panels comprising multiple antennas may be placed with a larger separation in a receiving device and hence the channels observed by different panels are uncorrelated in a millimeter wave communication system. Hence, there is a need for an improvement of estimating the DoA of the incoming signal at a receiving device, in particular for a mmWave communication system with hybrid antenna array structure.

It is an object of the present invention to provide an improved estimation of the DoA in radio communication systems. This object and / or others are obtained by the receiving device as set out in the appended claims.

As has been realized by the inventors it would be beneficial to provide a receiving device that can select the best receive beam by estimating the DoA of the incoming signal with a short beam acquisition processing delay, and thereby obtain a better receive beamforming gain.

In accordance with the invention, a receiving device comprising an antenna array comprising at least two (receiving) antenna elements, the antenna elements being connected to a phase-shifter each, is provided. In the receiving device, each phase-shifter is connected to a combiner through a respective on-off switch associated with the respective phase shifter. An Analog to Digital Converter, ADC, for converting the combiner output signal from analog to digital domain is also provided. The receiving device is configured to receive Orthogonal Frequency Division Multiplexing, OFDM, symbols via the antenna array from a transmitter and to select at least a first receive beam direction for receiving a first OFDM symbol and a second receive beam direction for receiving a second OFDM symbol. Further, the receiving device is configured to measure at least an average received power value of the first OFDM symbol and an average received power value of the second OFDM symbol at the output of the ADC, and to estimate at least one parameter associated with Direction of Arrival, DoA, of the received OFDM symbols based on at least the measured averaged receive power values of the first OFDM symbol and the second OFDM symbol. The receiving device is configured to select a third receive beam direction for receiving a third OFDM symbol based on the estimated at least one parameter.

In a possible implementation the N antenna elements may be connected to only one single ADC. The receive beam direction corresponds to the center of the main lobe or main beam of the antenna radiation pattern. Moreover, the average received power values can be measured in the baseband, and can for example correspond to reference signal received power (RSRP) or reference signal received quality (RSRQ) or signal to noise ratio (SNR) or any other measurement of the associated OFDM symbol. Further, the on-off switches connected to each of the phase-shifters can be used to achieve the flexibility of using only a subset of antenna elements for making the measurements associated with the average received power values.

Hereby, an improved estimation of parameters associated with the DoA of the incoming signal can be achieved. The estimated parameters associated with the DoA information can be used for receiving further OFDM symbols at the receiving device. An improved reception of OFDM symbols can then be achieved since the receiving device is enabled to select the best receive beam based on the estimated parameters associated with the DoA, whereby the reception of the further OFDM symbols can be improved. In accordance with a first implementation of the invention, the receiving device is configured to select the at least first and second receive beam directions applied for different received OFDM symbols uniformly spaced over an angular range. Hereby an easy to implement and robust set-up of the receiving device can be achieved. The angular range can advantageously be set to [-180, +180] degrees. In accordance with a second implementation of the invention, the receiving device is configured to select the at least first and second receive beam directions applied for different received OFDM symbols adaptively based on a previously estimated value(s) of at least one parameter associated with the Direction of Arrival. Hereby, an alternative setup that can provide an efficient estimation of the at least one parameter associated with DoA is obtained.

In accordance with a third implementation of the invention, the antenna array at the receiving device comprises at least one of a uniform linear antenna array or a uniform planar antenna array. Based on the antenna array configuration, the receiving device can select receive beam directions in either two or three dimensions. In accordance with a fourth implementation of the invention, the receiving device comprises at least two antenna arrays, at least two ADCs, and at least two baseband processing units, wherein said at least two ADCs and at least two baseband processing units are associated with different antenna arrays, wherein the receiving device is configured to estimate the at least one parameter associated with the DoA of the received OFDM symbols independently for each antenna array. Hereby, a first alternative for multiple antenna arrays can be implemented. This alternative is advantageous if the two antenna arrays are placed at different places on the receiving device. For example, one array can be placed on one-side of the receiving device and another array on the other- side of the receiving device.

In accordance with a fifth implementation of the invention, the receiving device comprises at least two antenna arrays, at least two ADCs, and at least two baseband processing units, wherein said at least two ADCs and said at least two baseband processing units are associated with different antenna arrays, wherein the receiving device is configured to form a joint estimate of the at least one parameter associated with the DoA of the received OFDM symbols, for all antenna arrays. Hereby, a second alternative for multiple antenna arrays can be implemented. This alternative is advantageous if the two arrays are co-located on the receiving device. For example, the two antenna arrays are co- located with antenna elements in the two arrays having different polarizations. In accordance with a sixth implementation of the invention, the receiving device is configured to measure average received power values in time domain or in frequency domain of the first and the second received OFDM symbols. Hereby an advantageous implementation can be achieved. For example, if the received OFDM symbol comprises reference signals only, and the receiving device does not need to make any other measurements in the frequency domain, then the averaged received power values can be measured in the time domain. By using this implementation form, the receiving device can save the computational complexity associated with performing a fast Fourier transform (FFT) operation. In accordance with a seventh implementation of the invention, the receiving device is configured to perform estimation of the at least one parameter associated with the Direction of Arrival, DoA, comprising to determine a set of hypothetical values of the at least one parameter associated with the DoA of the received OFDM symbols, and to compute, in each OFDM symbol, a set of decision metric values for each of the hypothetical values of the at least one parameter. The receiving device is further configured to accumulate the decision metric values corresponding to each hypothetical value of the at least one parameter over multiple OFDM symbols, and to choose the estimate of the at least one parameter associated with the DoA of the received OFDM symbols using the accumulated decision metric values. Hereby, an estimation method is provided to estimate the parameters associated with the DoA of the received OFDM symbols. The estimation method of the seventh implementation can reduce or even minimize the error of the estimation of at least one parameter associated with the DoA. Thereby, the receiving device can select a better receive beam direction to receive further OFDM symbols.

In accordance with an eighth implementation of the invention, the receiving device is configured to compute the set of decision metric values in an OFDM symbol using at least one of: pre-computed array-factor values corresponding to each of the hypothetical values of the at least one parameter associated with the DoA (0) of the received OFDM symbols, receive beam direction used for receiving the OFDM symbol, determined channel

gain (a) value between the transmitter and the antenna array of the receiving device, determined noise power value at the receiving device, measured average received

power value of the OFDM symbol. Hereby, efficient steps to compute the

decision metric values used in the estimation of the at least one parameter associated with the DoA of the received OFDM symbols is provided.

In accordance with a ninth implementation of the invention, the receiving device is configured to determine the channel gain between the transmitter and the antenna array of the receiving device by connecting only one antenna element of the antenna array to the ADC using the on-off switches; measuring the average power of the received signal output in the baseband. Hereby, an efficient computation of the channel gain values used in the computation of decision metric values is provided.

In accordance with a tenth implementation of the invention, the receiving device comprises a uniform linear array with a number N antenna elements with an inter-element spacing of d and a wave-number of the received signal e, where the receiving device is configured to compute the decision metric value corresponding to a hypothetical value of the at least one parameter associated with the received signal DoA (0) and the receive beam direction

used for receiving the OFDM symbol using:

Hereby, an explicit equation for the computation of decision metric values is provided.

In accordance with an eleventh implementation of the invention, the receiving device is configured to set the at least first receive beam direction and second receive beam direction by adjusting phase values of the phase-shifters connected to each of the antennas in the antenna array. Hereby, an efficient implementation of configuring the receive device to set the first receive beam direction and the second receive beam direction is provided.

The invention in further aspects provide methods and computer program products that can be used in the devices set out above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example, and with reference to the accompanying drawings, in which: Fig. 1 a shows two example panels with different antenna array configurations that can be used in a receiving device.

Fig. 1 b shows an example receiving device comprising various elements,

Fig. 2 shows a flowchart illustrating steps performed according to an embodiment in a receiving device,

Fig. 3 illustrates uniformly spaced scanning beams used according to an embodiment in a receiving device,

Fig. 4 shows different scanning beams for receiving different OFDM symbols,

Fig. 5 illustrates communication between an estimation block and a receive beam selection block, and

Fig. 6 illustrates the performance of an exemplary DoA estimation algorithm.

DETAILED DESCRIPTION

The invention will now be described in detail hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

In the following a downlink scenario in which the Base Station (BS) is transmitting orthogonal frequency division multiplexing (OFDM) signal is considered. It is further assumed that the transmitter has selected a transmit beam and sends a data signal to a receiving device 100. The transmission can for example take place in a mmWave frequency system, where the transmitter, here the base station, and the receiving device can have a hybrid antenna structure with analog-digital precoding at the transmitter and analog-digital combining at the receiver. In a hybrid receiver structure, the receiver may be equipped with multiple panels each comprising multiple antenna elements with different polarizations as shown in Fig. 1 a. Each panel can comprise two-dimensional antenna array or one-dimensional antenna array. All antenna elements corresponding to a given polarization in an antenna panel are connected to an analog-to-digital converter (ADC) through a set of analog phase shifters as shown in Fig. 1 b. By adjusting the phase values of different phase shifters, the receive antenna beam can be steered in a given direction that can be referred to as the look angle.

The receiving device can be implemented in any device used for receiving OFDM signals over an air-interface such as a user equipment (UE) or any other client device. The signal arrives at the receiver at an elevation DoA Θ as shown in Fig. 1 b. The receiver is equipped with a number N antenna elements 102 connected to an ADC 104 through a set of phase shifters 106 as shown in Fig. 1 b. Each phase shifter 106 can be switched on and off by means of on-off switches 1 10 located in the phase shifters or connected to the phase shifters 106. In a possible implementation the N antenna elements may be connected to only one single ADC 104.

The receiving device 100 may be any of a User Equipment (UE) in Long Term Evolution (LTE) or New Radio (NR), mobile station (MS), wireless terminal or mobile terminal which is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The UE may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice or data via a radio access network with another entity, such as another receiver or a server. The UE can be a Station (STA) which is any device that contains an I EEE 802.1 1 - conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM).

The receiving device 100 may also be a transmission or reception point, TRP, or a 5G base station gNodeB, gNB. The receiving device 100 may be a base station, a (radio) network node or an access node or an access point or a base station, e.g., a Radio Base Station (RBS), which in some networks may be referred to as a transmitter, "eNB", "eNodeB", "NodeB" or "B node", depending on the technology and terminology used. The radio network nodes may be of different classes such as, e.g., macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. The radio network node can be a Station (STA) which is any device that contains an IEEE 802.1 1 -conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM).

For describing various embodiments of the invention, as shown in Fig. 1 b, a uniform linear array (ULA) is assumed to be placed along the Y-axis with inter-element spacing of d. For ease of explanation, it is further assumed that the signal has azimuth DoA φ = |, and that the receive beams are only functions of the elevation DoA Θ. The receive beam is represented by a vector of phase values to be used at phase shifters connected to different antenna elements. However, the underlying principles described herein can be extended to two-dimensional (2D) arrays with 2D DoA estimation, i.e., estimating both azimuth and elevation DoAs (φ, 0).

As shown in Fig. 1 b, the incoming signal from an elevation angle Θ is phase shifted by phase-shift values a\, .... , a_N ^* , at each of the N antenna elements, respectively. The phase- shifted signals are then combined in a combiner 108 and passed to the single ADC 104. The received signal during the /cth sample of tth OFDM symbol output from the ADC 104 can be expressed as:

Where M denotes the number of time domain samples in one OFDM symbol including the cyclic prefix _{js the} channel gain vector

observed at the respective antenna elements, a is the channel gain coefficient between the transmitter and the receiver. The channel gain coefficient a is assumed to be having complex circularly symmetric Gaussian distribution (CCSGD), with zero mean value and with unit power. The vector

is the receive beam used during the ah OFDM

symbol where is the wavenumber with λ denoting wavelength. The symbol θ_{

denotes the receive beam direction used for receiving the tth OFDM symbol. The vector

is the noise vector during the kth sample of tth OFDM symbol. It is assumed that the entries of

are independent and identically (i.i.d.) entries with complex circularly symmetric Gaussian distribution (CCSGD) with zero mean and power The quantity ) denotes the kth time-domain sample of the signal sent from the

transmitter during the tth OFDM symbol. If the knowledge of Θ is perfectly available at the receiving device, to maximize the Signal-to-Noise (SNR) or the receive beamforming gain, the optimal beam scanning vector can be chosen as

However, since the receiver does not know the

value of Θ, the receiving device must estimate the DoA value Θ, or a parameter associated with it. Even though various embodiments are described herein in terms of estimating the incoming signal direction Θ, it is also possible to use the described embodiments for estimating equivalent parameters associated with the incoming signal direction. For example, instead of estimating the parameter Θ, one may estimate the value of relative phase shift β = ed sin fl between any two adjacent antenna elements corresponding to the incoming signal direction.

To improve the DoA estimation, an antenna panel in which each antenna element is connected to an analog delay element is provided. The delayed signals of the antenna elements are then combined and connected to a single analog-to-digital converter (ADC). The estimated DoA information is then used in a beam acquisition process.

In Fig. 2, a flowchart illustrating some steps performed when performing a DoA estimation is shown. The method in Fig. 2 can be performed in a receiving device. The receiving device comprise an antenna array comprising at least two antenna elements, for instance receiving antenna elements, the antenna elements being connected to a phase-shifter each, wherein each phase-shifter is connected to a combiner through a respective on-off switch associated with the respective phase shifter. Further, each of the phase shifter's output signal is connected to a combiner via on-off switches, where each phase-shifter is associated with a respective separate on-off switch. Also, an Analog to Digital Converter, ADC, for converting the combiner signal output from analog to digital domain is provided. First, in such a receiving device, Orthogonal Frequency Division Multiplexing, OFDM, symbols are received via the antenna array from a transmitter in a step 21 . Next, in a step 23, at least a first receive beam direction for receiving a first OFDM symbol and a second receive beam direction for receiving a second OFDM symbol are selected. Then, in a step 25, at least an average received power value of the first OFDM symbol and an average received power value of the second OFDM symbol are measured at the output of the ADC. Next in a step 27, at least one parameter associated with a Direction of Arrival, DoA, of the received OFDM symbols based on at least the measured averaged receive power values of the first OFDM symbol and the second OFDM symbol is estimated. Finally, in a step 29 a third receive beam direction for receiving a third OFDM symbol based on the estimated at least one parameter is selected. The at least one parameter may be for instance the value of the DoA or a function of it. The method can be computer program implemented by computer instructions that when executed on a computer causes the computer to perform the method.

According to an embodiment of the invention, the received OFDM symbols may correspond to cyclic prefix (CP)-OFDM symbols or Discrete Fourier Transform-Spread (DFTS)-OFDM symbols. Detailed exemplary embodiments that implement the method in accordance with Fig. 2 will now be given. Fig. 3 illustrates by way of example an embodiment in which different receive scanning beams are used. By measuring the power of the signal in the baseband for each of the scanning beams, an estimation process can be initiated in which a matching of the measured power values to a function of the array factor (or receive beamforming gain) of the antenna array sampled at the scanning beam directions is performed. The matching can be performed by a Maximum Likelihood (ML) search.

In a typical implementation, as shown in Fig. 4, different scanning beams are used for receiving different OFDM symbols. Assume that the receiving device 100 uses a number N_Scan scanning beams, one per OFDM symbol as shown in Fig. 4. Let z_k denote the vector consisting of the

sample of all OFDM symbols. Using Equation 1 , the following

expression can be obtained:

where

matrix with and

being an N Note that entries of w' are

also i.i.d. with variance

Evaluating the covariance of z in Equation 2, gives

where is a diagonal matrix comprising of the diagonal elements of A

To arrive at Equation 3, it is assumed that:

The ith diagonal element of (or equivalent^ the tth diagonal element A

is

given by

It should be noted that

is equal to the channel gain of the incoming signal arriving at DoA Θ, measured through an antenna radiation pattern centered at the scanning angle e_t. The second term in Equation 5 is a function of the array factor of the ULA (antenna array). The tth diagonal element of is given as:

It should be noted that in Equations 2-6, the time-domain samples were used to construct the covariance matrix. However, the covariance matrix can be constructed using the received signals in frequency-domain as well.

The probability density function is the probability of observing z given

given by:

Given the observations the likelihood function is given by

Or the log-likelihood function is given by:

The maximum likelihood (ML) estimate of Θ is given by:

Equivalently, the ML estimate can be written as

where and it denotes the received power of the ah OFDM

symbol. The received power values can be measured either in time or frequency domain. Using Equation 5, the ML estimate can be expressed as:

Equation 7 The solution can be obtained by solving for Θ in the above equation. However, finding a closed-form solution for Θ can be difficult. Hence, a procedure for estimating the incoming signal DoA can be provided.

According an embodiment, the incoming signal DoA can be estimated by: i) determining a set of hypothetical values of the DoA of the received OFDM symbols; ii) computing in each OFDM symbol, a set of decision metric values for each of the hypothetical values of the DoA; iii) accumulating the decision metric values corresponding to each hypothetical DoA value over multiple OFDM symbols; and iv) choosing the estimate of the DoA of the received OFDM symbols using the accumulated decision metric values. Furthermore, according to another embodiment, the decision metric values can be computed using i) the pre-computed array-factor values corresponding to each of the hypothetical values of the DoA (0) of the received OFDM symbols; iii) the direction of the receive beam ( used for receiving the OFDM symbol; iii) the determined channel gain

(a) value between the transmitter and the antenna array of the receiving device; iv) the determined noise power

value at the receiving device; and v) the measured average received power value of the OFDM symbol.

The above described embodiments for performing the estimation of the DoA and the computation of decision metric values can be implemented in the receiving device by:

1 . Precompute store them. The

number of values is determined such that a required resolution of the estimate of the DoA is obtained.

2. Estimate the value This can be obtained, for example, by turning on only one

antenna element in the array and measure the power of the received signal in such a configuration.

3. For each OFDM symbol index

a. Compute using Equation 6 for different values of Θ by using the

calculated values from step 1 and step 2. This step typically requires Ν_θ real multiplications and Ν_θ real additions, where Ν_θ denotes the number of possible Θ values used for the ML search. For example, if the error is to be less than resolution degrees, then the value for Ν_θ can be set to Ν_θ = 360 /resolution. It is to be noted that this step can be performed independent of the OFDM symbol index, if the values of

are known a priori.

After using analog combiner

obtain the baseband samples z

is obtained, compute for different values of Θ. This

,

step requires Ν real multiplications.

Choose

The parameter N_Scan is a design parameter and can be selected based on the desired tolerance in Mean Square Error (MSE). To obtain a good result of the estimation, the scan beam directions such as the N_Scan beam directions can be chosen uniformly. In accordance with one embodiment the N_Scan beam directions are chosen uniformly between [-π, π] or equivalent^ in the range of [-180, 180] degrees. It is also possible to choose non-uniformly spaced scanning beams to obtain better estimation of the DoA. Moreover, the described embodiments can also be used in a receiving device consisting of multiple antenna panels and multiple ADCs with each ADC connected to a subset of antenna elements in each panel. According to an embodiment, a receiving device with multiple antenna arrays is provided, where each of the arrays is connected to an independent ADCs and to baseband processing units. The DoA of incoming signal can be estimated independently for each of the antenna arrays. This scenario can be useful in a receiving device with multiple panels placed at different places on the device. According to another embodiment, a receiving device with multiple antenna arrays is provided, where each of the arrays is connected to an independent ADCs and to baseband processing units. The DoA of incoming signal can be estimated jointly for the multiple antenna arrays. This scenario can be useful in a receiving device where multiple antenna arrays exist within a single panel.

Once the DoA information is obtained, the receiver can be configured to choose the next receive beam based the estimated DoA Θ. The next receive beam vector can in accordance with some embodiments be chosen as

The communication between the estimation block and the receive beam selection block of a receiving device 100 is illustrated in Fig. 5. Initially, the predefined receive beams stored in a receive beam selection block 120 are used for DoA estimation that is performed using an estimation block 1 18. The estimated DoA value from the estimation block will be sent back to receive beam selection block 120 to select the next receive beam.

In Fig. 6, the performance of an exemplary DoA estimation algorithm according to the above and of the conventional beam sweeping method for estimating the DoA of the incoming signal are illustrated. On the X-axis, the direction Θ of the incoming signal is shown and on the Y-axis, the root mean square error of the estimated DoA values is shown. Here, an 8-antenna uniform linear array with half-wavelength spacing between the antenna elements is considered. The SNR = l/σ² is set to 20 dB. In this illustration, 10 receive beams which are uniformly spaced between [Ο, π] rad are used. The value of Ν_θ is 1800 for the search procedure. In case of the beam sweeping approach, a resolution of 0.1 degree is used for the receive beams and estimated DoA is based on the measured received power. As can be seen from the illustration, the DoA estimation method as described herein is very effective in estimating the incoming signal DoA compared to the beam sweeping method. Using the methods and devices as set out herein can provide an efficient estimation of the DoA. The DoA can be used by a receiving device to select the best receive beam whereby the reception can be improved in the receiving device.

The functionality described herein can be performed, at least in part, by one or more computer program product components such as software components. According to an embodiment, the receiving device 100 can be configured by program code that when executed performs the operations and functionality described. Alternatively, or in addition, the functionality described herein can be performed, at least in part, by one or more hardware logic components. For example, and without limitation, illustrative types of hardware logic components that can be used include Field-programmable Gate Arrays (FPGAs), Program-specific Integrated Circuits (ASICs), Program-specific Standard Products (ASSPs), System-on-a-chip systems (SOCs), Complex Programmable Logic Devices (CPLDs), Graphics Processing Units (GPUs). Any range or device value given herein may be extended or altered without losing the effect sought. Also, any embodiment may be combined with another embodiment unless explicitly disallowed. It will further be understood that reference to 'an' item may refer to one or more of those items. The term 'and/or' may be used to indicate that one or more of the cases it connects may occur. Both, or more, connected cases may occur, or only either one of the connected cases may occur.

The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the embodiments described above may be combined with aspects of any of the other embodiments described to form further embodiments without losing the effect sought.

The term 'comprising' is used herein to mean including the method, blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements. It will be understood that the above description is given by way of example only and that various modifications can be made. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments.

Claims

1. A receiving device (100) comprising:

- an antenna array comprising at least two antenna elements (102), the antenna elements being connected to a phase-shifter (106) each,

-wherein each phase-shifter is connected to a combiner (104) through a respective on- off switch associated with the respective phase shifter,

- an Analog to Digital Converter, ADC, (104) for converting the combiner output signal from analog to digital domain, wherein the receiving device is configured to: receive Orthogonal Frequency Division Multiplexing, OFDM, symbols via the antenna array from a transmitter,

- select at least a first receive beam direction for receiving a first OFDM symbol and a second receive beam direction for receiving a second OFDM symbol; - measure at least an average received power value of the first OFDM symbol and an average received power value of the second OFDM symbol at the output of the ADC,

- estimate at least one parameter associated with Direction of Arrival, DoA, of the received OFDM symbols based on at least the measured averaged receive power values of the first OFDM symbol and the second OFDM symbol, and - select a third receive beam direction for receiving a third OFDM symbol based on the estimated at least one parameter.

2. The receiving device according to claim 1 , wherein the receiving device is configured to select the at least first and second receive beam directions applied for different received OFDM symbols uniformly spaced over an angular range.

3. The receiving device according to claim 2, wherein the angular range is [-180, +180] degrees.

4. The receiving device according to claim 1 , wherein the receiving device is configured to select the at least first and second receive beam directions applied for different received OFDM symbols adaptively based on a previously estimated value(s) of at least one parameter associated with the Direction of Arrival.

5. The receiving device according to any one of claims 1 - 4, wherein the antenna array comprises at least one of a uniform linear antenna array or a uniform planar antenna array.

6. The receiving device according to any one of claims 1 - 5 comprising at least two antenna arrays, at least two ADCs, and at least two baseband processing units, wherein said at least two ADCs and at least two baseband processing units are associated with different antenna arrays, wherein the receiving device is configured to:

- estimate the at least one parameter associated with the DoA of the received OFDM symbols independently for each antenna array.

7. The receiving device according to any one of claims 1 - 5 comprising at least two antenna arrays, at least two ADCs, and at least two baseband processing units, wherein said at least two ADCs and said at least two baseband processing units are associated with different antenna arrays, wherein the receiving device is configured to:

- form a joint estimate of the at least one parameter associated with the DoA of the received OFDM symbols, for all antenna arrays.

8. The receiving device according to any one of claims 1 - 7, wherein the receiving device is configured to measure average received power values in time domain or in frequency domain of the first and the second received OFDM symbols.

9. The receiving device according to claim any of claims 1 - 8, wherein the receiving device is configured to perform estimation of the at least one parameter associated with the Direction of Arrival, DoA, comprising to:

- determine a set of hypothetical values of the at least one parameter associated with the DoA of the received OFDM symbols, - compute, in each OFDM symbol, a set of decision metric values for each of the hypothetical values of the at least one parameter,

- accumulate the decision metric values corresponding to each hypothetical value of the at least one parameter over multiple OFDM symbols, and

- choose the estimate of the at least one parameter associated with the DoA of the received OFDM symbols using the accumulated decision metric values.

10. The receiving device according to claim 9, wherein the receiving device is configured to compute the set of decision metric values in an OFDM symbol using at least one of:

- pre-computed array-factor values corresponding to each of the hypothetical values of the at least one parameter associated with the DoA (0) of the received OFDM symbols and the receive beam direction

used for receiving the OFDM symbol,

- determined channel gain value between the transmitter and the

antenna array of the receiving device, - determined noise power

value at the receiving device, and

- measured average received power value of the OFDM symbol.

1 1 . The receiving device according to claim 9 or 10, wherein the receiving device is configured to determine the channel gain between the transmitter and the antenna array of the receiving device by

- connecting only one antenna element of the antenna array to the ADC using the on- off switches; measuring the average power of the received signal in the baseband.

12. The receiving device according to any one of claims 9 - 1 1 , wherein the receiving device comprises a uniform linear array with a number N antenna elements with an inter-element spacing of d and a wave-number of the received signal e, wherein the receiving device is configured to compute the decision metric value corresponding to a hypothetical value of the at least one parameter associated with the received signal DoA and the receive beam direction used for receiving the OFDM symbol as:

13. The receiving device according to any one of claims 1 - 12, wherein the receiving device is configured to set the at least first receive beam direction and second receive beam direction by adjusting phase values of the phase-shifters connected to each of the antennas in the antenna array.

14. A method in a receiving device, the receiving device comprising:

- an antenna array comprising at least two antenna elements, the antenna elements being connected to a phase-shifter each, wherein each phase-shifter is connected to a combiner through a respective on-off switch associated with the respective phase shifter, - wherein each of the phase shifters output signal is connected to a combiner via on-off switches, where each phase-shifter is associated with a respective separate on-off switch,

- an Analog to Digital Converter, ADC, for converting the combiner signal output from analog to digital domain, wherein the method comprises to: receive (21 ) Orthogonal Frequency Division Multiplexing, OFDM, symbols via the antenna array from a transmitter,

- select (23) at least a first receive beam direction for receiving a first OFDM symbol and a second receive beam direction for receiving a second OFDM symbol,

- measure (25) at least an average received power value of the first OFDM symbol and an average received power value of the second OFDM symbol at the output of the ADC,

- estimate at least one parameter associated with (27) a Direction of Arrival, DoA, of the received OFDM symbols based on at least the measured averaged receive power values of the first OFDM symbol and the second OFDM symbol, and

- select (29) a third receive beam direction for receiving a third OFDM symbol based on the estimated at least one parameter.

15. A computer program product comprising computer instructions that when executed on a computer causes the computer to perform a method according to claim 14.