Low power millimeter wave (mmwave) receiver for beam tracking

Abstract

A wireless device (WD) and method for low power millimeter wave (mmWave) receiving for beam tracking are disclosed. According to one aspect, a secondary WD is configured to receive an amplitude shift keying (ASK) modulated signal from a primary WD using at least three antenna branches of a low power receiver. In each antenna branch, the secondary WD down-converts the ASK modulated signal using a local oscillator signal from a free-running multi-phase oscillator applied in a multi-phase mixer to provide a multi-phase intermediate frequency signal. The secondary WD determines a beam direction of the ASK modulated signal by correlating phase adjusted combinations of the intermediate frequency signals from the antenna branches with a reference sequence and provides the beam direction to the a main receiver of the secondary WD. The beam direction may be used for an initial communication between the second receiver and the primary WD.

Description

TECHNICAL FIELD The present disclosure relates to wireless communications, and in particular, to low power millimeter wave (mmWave) receivers for beam tracking. BACKGROUND The Third Generation Partnership Project (3GPP) has developed and is developing standards for Fourth Generation (4G) (also referred to as Long Term Evolution (LTE)) and Fifth Generation (5G) (also referred to as New Radio (NR)) wireless communication systems. Such systems provide, among other features, broadband communication between network nodes, such as base stations, and mobile wireless devices (WD), as well as communication between network nodes and between WDs. The 3GPP is also developing standards for Sixth Generation (6G) wireless communication networks. Sidelink communication is a direct data communication between User Equipments (UEs), hereafter referred to as wireless devices (WDs). In sidelink communications, the data traffic does not go through a base station or network node. Sidelink communication is beneficial for low-latency and high reliability data communication. 5G / NR sidelink has been introduced since 3GPP Technical Release 16 (3GPP Rei 16). Both Frequency Range 1 (FR1) and Frequency Range 2 (FR2) are supported in NR sidelink. In mmWave SL, beam tracking between WDs is very challenging. The existing mmWave beamforming management in NR standard is more suitable for a centralized cellular network where the base station controls the beam tracking procedure for all its associated WDs. The regular NR beam management procedure is generally based on synchronization signal blocks (SSBs) and channel state information reference signals (CSLRS). SSBs are periodically broadcast from a transmitter via beamforming. Each SSB is mapped to a given angular direction which may be identified by a unique SSB index. A WD receiver may perform beam detection by SSB reception and SSB index decoding. Then CSLRS resource sets may be configured and received by the WD to refine the transmit and receive beam selection, to further tune the beam alignment. At present, mmWave SL beam management is still lacking detailed specification in 3GPP. Millimeter wave beam management in today’s NR standard is more suitable for a centralized cellular network. Simply applying the existing 3GPP mmWave beam management mechanism for SL may result in high WD energy consumption and long latency to setup SL between WDs. SUMMARY Some embodiments advantageously provide methods, systems, and apparatuses for low power millimeter wave (mmWave) receivers for beam tracking. In one example, a method is provided for mmWave SL beam management to achieve lower WD power consumption and shorter time latency for beam tracking / alignment as compared with known arrangements. A beam tracking method for mmWave SL communication is disclosed. In some embodiments, a primary WD broadcasts a single-tone signal modulated by amplitude shift keying (ASK), such as an On-Off Keying (OOK) sequence, by beamforming. A secondary WD includes a low power receiver and a main transceiver. The low power receiver (LPR) detects and tracks the beam direction by detecting the sequence transmitted by the primary WD, and shares the information of the detected beam direction with the main transceiver. The LPR uses a local free-running multi-phase oscillator shared by the down-conversion mixers in all branches of the beamforming receiver. The LPR may be implemented with either an analog option or a digital option. • (Analog option) The LPR equipped with multiple Rx branches starts to detect the sequence by applying various phase rotation values on the IF output signals of each Rx branch. After combination of the different branch signals, followed by conversion to a digital signal, via a comparator, for example, the digital baseband circuit performs sequence correlation for each phase rotation value. By finding the maximum correlation result, the Tx beam direction may be decided. The low correlation results may also be used in determining the beam direction with improved resolution, similar to a null-scan. • (Digital option) The LPR equipped with multiple Rx branches performs digital beamforming by applying complex weights to a digital intermediate frequency (IF) for each separate Rx signal and summing the result of all Rx branches. The beam directions are searched by correlating the received sequences with a reference sequence. This may be done for multiple concurrent beam directions, which reduces time required to synchronize and makes beam tracking more agile. However, multiple analog to digital converters (ADCs) as well as automatic gain control (AGC) may result in higher power consumption compared to the analog option. Some embodiments provide, an energy efficient beam tracking method for mmWave SL wireless communication between a primary device and one or several secondary devices. Some embodiments may include one or more of the following features: 1. A primary WD broadcasts a single-tone signal modulated by an ASK sequence, such as an On-Off Keying (OOK) sequence, by beamforming and / or by broadcast; 2. A low power receiver (LPR) in a secondary WD detects and tracks the beam direction by detecting the sequence transmitted by the primary WD, and shares the information of the detected beam direction with a main transceiver in the secondary WD; 3. The LPR may be active continuously or may operate with a duty cycle, while the main transceiver is kept in low power or sleep mode; 4. The implementation of the LPR may be implemented with either an analog option or a digital option; 5. In the LPR, the local RF carrier frequency (LO) may be generated from a free-running multi-phase oscillator (FRO), e.g., a voltage control oscillator (VCO) implemented with a ring oscillator; and 6. In the LPR, the baseband circuit may be clocked from a stable low-frequency oscillator, e.g., a 20MHz crystal oscillator (XO). Some embodiments may be extended for a WD tracking beam direction from a base station or access point which broadcasts the single-tone signal modulated by an ASK sequence by beamforming and / or by broadcast. In the LPR capable of beam direction finding and tracking, a low power free-running multi-phase oscillator is used as an LO to down-convert the RF signal to intermediate (IF) signals. Accurate RF carrier frequency generation or synchronization is not required, which reduces overhead, energy consumption and time latency for beam finding. Some embodiments are applicable to beam direction finding using a receiver with a free-running multi-phase oscillator combined with the techniques and signals described in some known methods, where a receiver with a free running multi-phase oscillator provides a frequency that may be corrected by frequency synchronization. Thus, a receiver with a free-running multi-phase oscillator that may be implemented with low-power multi-phase oscillators. Some embodiments provide for time / frequency synchronization and beam tracking for mmWave SL without beam detecting realized by digital null-forming angle of arrival (AoA) estimation from 4 receive (Rx) branches. According to one aspect, a secondary wireless device, WD, configured to communicate with a primary WD is provided. The secondary WD includes a first receiver and a second receiver, the first receiver comprising at least three antenna branches, each antenna branch including a multi-phase mixer, the first receiver including a free-running multi-phase oscillator. The secondary WD is configured to receive an amplitude shift keying, ASK, modulated signal from the primary WD using at least three different antenna branches of the first receiver of the secondary WD. In each of the at least three antenna branches, the ASK modulated signal is down-converted using a multi-phase local oscillator signal from the free-running multi-phase oscillator applied in the multi-phase mixer to provide an intermediate frequency signal comprising individual intermediate frequency signals for each of a plurality of phases.. The secondary WD is configured to determine a beam direction of the ASK modulated signal by correlating phase adjusted combinations of the intermediate frequency signals from the antenna branches with a reference sequence. The secondary WD is also configured to provide the beam direction to the second receiver, and use the determined beam direction for an initial communication between the second receiver and the primary WD. According to this aspect, in some embodiments, the ASK modulated signal is an OOK modulated signal. In some embodiments, the correlation is performed in a digital domain and phase adjusting includes multiplying the intermediate frequency signals by complex weights. In some embodiments, the correlation is performed in a digital domain and phase adjusting includes performing phase rotations of the intermediate frequency signals. In some embodiments, the first receiver is configured to consume less power than the second receiver. In some embodiments, the second receiver operates in a low power mode while the first receiver operates to determine the beam direction. In some embodiments, the free running multi-phase oscillator includes a voltage controlled oscillator, VCO, with a ring oscillator. In some embodiments, the correlation is performed according to a clock of a crystal oscillator. In some embodiments, determining the beam direction includes comparing a correlation result to a threshold. In some embodiments, the secondary WD is configured to scan a beam until the correlation result exceeds the threshold. According to another aspect, a method in a secondary wireless device, WD, configured to communicate with a primary WD is provided. The secondary WD includes a first receiver and a second receiver, the first receiver comprising at least three antenna branches, each antenna branch including a multi-phase mixer, the first receiver including a free-running multi-phase oscillator. The method includes receiving an amplitude shift keying, ASK, modulated signal from the primary WD using at least three different antenna branches of the first receiver of the secondary WD. The method includes, in each of the at least three antenna branches, down-converting the ASK modulated signal using a multiphase local oscillator signal from the free-running multi-phase oscillator applied in the multi-phase mixer to provide an intermediate frequency signal comprising individual intermediate frequency signals for each of a plurality of phases. The method includes determining a beam direction of the ASK modulated signal by correlating phase adjusted combinations of the intermediate frequency signals from the antenna branches with a reference sequence. The method also includes providing the beam direction to the second receiver, and using the determined beam direction for an initial communication between the second receiver and the primary WD. According to this aspect, in some embodiments, the ASK modulated signal is an OOK modulated signal. In some embodiments, the correlation is performed in a digital domain and phase adjusting includes multiplying the intermediate frequency signals by complex weights. In some embodiments, the correlation is performed in a digital domain and phase adjusting includes performing phase rotations of the intermediate frequency signals. In some embodiments, the first receiver is configured to consume less power than the second receiver. In some embodiments, the second receiver operates in a low power mode while the first receiver operates to determine the beam direction. In some embodiments, the free running multi-phase oscillator includes a voltage controlled oscillator, VCO, with a ring oscillator. In some embodiments, the correlation is performed according to a clock of a crystal oscillator. In some embodiments, determining the beam direction includes comparing a correlation result to a threshold. In some embodiments, the method includes scanning a beam until the correlation result exceeds the threshold. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: FIG. lisa schematic diagram of an example network architecture illustrating a communication system according to principles disclosed herein; FIG. 2 is a diagram of a network node in communication with a primary wireless device over a wireless connection, the primary WD being in wireless communication with a secondary WD, according to some embodiments of the present disclosure; FIG. 3 is a block diagram of a network node a primary WD and a secondary WD; FIG. 4 is a flowchart of an example process in a secondary wireless device (WD) for low power millimeter wave (mmWave) receivers for beam tracking; FIG. 5 is a transmitter with multiple antennas for transmitting multiple beams; FIG. 6 is a block diagram of a radio interface constructed in accordance with principles set forth herein; FIG. 7 is a block diagram of a low power receiver (LPR) with analog processing configured according to principles disclosed herein; FIG. 8 is a flowchart of an example process in a low power receiver according to principles disclosed herein; FIG. 9 is a block diagram of a low power receiver (LPR) with digital processing configured according to principles disclosed herein; and FIG. 10 is a schematic of a free running multi-phase ring oscillator. DETAILED DESCRIPTION Before describing in detail example embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to low power millimeter wave (mmWave) receivers for beam tracking. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Like numbers refer to like elements throughout the description. As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the concepts described herein. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and / or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. In embodiments described herein, the joining term, “in communication with” and the like, may be used to indicate electrical or data communication, which may be accomplished by physical contact, induction, electromagnetic radiation, radio signaling, infrared signaling or optical signaling, for example. One having ordinary skill in the art will appreciate that multiple components may interoperate and modifications and variations are possible of achieving the electrical and data communication. In some embodiments described herein, the term “coupled,” “connected,” and the like, may be used herein to indicate a connection, although not necessarily directly, and may include wired and / or wireless connections. In some embodiments, the non-limiting terms wireless device (WD) or a user equipment (UE) are used interchangeably. The WD herein may be any type of wireless device capable of communicating with a network node or another WD over radio signals, such as wireless device (WD). The WD may also be a radio communication device, target device, device to device (D2D) WD, machine type WD or WD capable of machine to machine communication (M2M), low-cost and / or low-complexity WD, a sensor equipped with WD, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer Premises Equipment (CPE), an Internet of Things (loT) device, or a Narrowband loT (NB-IOT) device, etc. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Some embodiments provide low power millimeter wave (mmWave) receivers for beam tracking. Referring now to the drawing figures, FIG. 1 is a schematic overview depicting a wireless communications network 10 wherein embodiments herein may be implemented. The wireless communications network 10 comprises one or more RANs 12 and one or more CNs 14. The wireless communications network 10 may use a number of different technologies, such as Wi-Fi, Long Term Evolution (LTE), LTE-Advanced, 5G, New Radio (NR), Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications / enhanced Data rate for GSM Evolution (GSMZEDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. Embodiments herein relate to recent technology trends that are of particular interest in a 5G context, however, embodiments are also applicable in further development of other existing wireless communication systems such as e.g. WCDMA and LTE and in future wireless communication systems, such as 6G systems. Access nodes operate in the wireless communications network 10 such as a radio access node such as a radio base station, shown as network node 16. The network node 16 provides radio coverage over a geographical area, a service area referred to as a cell 18, which may also be referred to as a beam or a beam group of a first radio access technology (RAT), such as 5G, LTE, Wi-Fi or similar. There may be more than one cell. For example, there may be a first cell 18a and a second cell 18b. There may be more than two cells. The radio coverage may further be provided by one or more narrow beams, specifically when mmWave frequencies are used for communication. The radio access nodes may each be a NR-RAN node, transmission and reception point e.g. a base station, a radio access node such as a Wireless Local Area Network (WLAN) access point or an Access Point Station (AP STA), an access controller, a base station, e.g. a radio base station such as a NodeB, an evolved Node B (eNB, eNode B), a gNB, a base transceiver station, a radio remote unit, an Access Point Base Station, a base station router, a transmission arrangement of a radio base station, a stand-alone access point or any other network unit capable of communicating with a wireless communications device within the service area depending e.g. on the radio access technology and terminology used. The respective radio access node may be referred to as a serving radio access node and communicates with a WD with Downlink (DL) transmissions to the WD and Uplink (UL) transmissions from the WD. A number of WDs 22, 24 operate in the wireless communication network 10, such as a primary WD 22 and a secondary WD 24. Both the primary WD 22 and the secondary WD 24 are configured for D2D communication. The primary WD 22 may further be connected to the wireless communications network 10 through the network node 16. The primary WD 22 may communicate with the network node 16 by transmitting data or control signals on an UL communications link UL-26 illustrated in Figure 2. The primary WD 22 may also receive data or control signals from the network node 16 on a DL communications link DL-28 illustrated in FIG. 1. Further, the WDs 22, 24 may each be a mobile station, a non-access point (non-AP) STA, a STA, a user equipment and / or a wireless terminal, that communicate via one or more Access Networks (AN), e.g. RAN, e.g. via the network node 16 to one or more core networks (CN) e.g. comprising a core node 30, for example comprising an Access Management Function (AMF). It should be understood by the skilled in the art that “WD” is a non-limiting term which means any terminal, wireless communication terminal, user equipment, Machine Type Communication (MTC) device, D2D terminal, or node e.g. smart phone, laptop, mobile phone, sensor, relay, mobile tablets or even a small base station communicating within a cell. Embodiments herein disclose a method for a wireless communications device to support improved synchronization of D2D communication with another wireless communications device. FIG. 2 illustrates embodiments of a D2D communication system. In FIG. 2, the primary WD 22 is connected to multiple WDs such as the secondary WD 24. The multiple secondary WDs 24 may include, for example, mobile phones, augmented reality (AR) or virtual reality (VR) glasses, smart watches, etc. A connection between the primary WD 22 and the secondary WD 24 is a D2D connection such as an SL connection. Thus, the primary WD 22 may communicate with the secondary WDs 24 over a D2D wireless communications channel 32 illustrated in FIG. 1. The primary WD 22 may communicate with the secondary WD 24 over the D2D wireless communications channel 32 using one or more transmit or receive beams, of which a transmit beam 34 is illustrated in FIG. 1. The primary device 22 and / or the secondary device 24 may include a low power receiver 36 which utilizes a free running multi-phase oscillator (FRO) 38. The free running multiphase oscillator 38 may be a voltage controlled oscillator, VCO, for example, that may not be synchronized to the network and may not be part of a phase or frequency locked loop. The primary WD 22 may further be connected to the wireless communications network 10 through the network node 16. The primary WD 22 may communicate with the network node 16 by transmitting data or control signals on an UL communications link UL-26 illustrated in FIG. 1. The primary WD 22 may also receive data or control signals from the network node 16 on a DL communications link DL-28 illustrated in FIG. 2. However, it is not necessary for the primary WD 22 to be connected to the wireless communications network 10. For example, in a scenario in which embodiments herein may be implemented the primary WD 22 may be out of coverage of the wireless communications network 10. A carrier for the D2D connection, such as a SL carrier, may be a mmWave-frequency radio signal, e.g. in NR FR2. Embodiments herein are particularly advantageous at mmWave frequencies since receivers at mmWave frequencies are very power hungry and embodiments herein lower the power consumption of receivers. A link between the primary WD 22 and the wireless communications network 10 may be either on a low-frequency band, e.g. NR FR1, or on the mmWave-frequency band, e.g. NR FR2. In some embodiments herein the primary WD 22 is able to perform frequency or time synchronization or both with the wireless communications network 10 by monitoring reference signals broadcast from the network node 16, via standardized methods, e.g. 3GPP NR standardized synchronization approach. The primary WD 22 may also select Global Navigation Satellite System (GNSS) as a synchronization source in case of being out of network coverage. A primary WD 22 is configured to include an ASK modulator which may be configured to modulate a carrier frequency using ASK modulation according to a different modulation sequence for each of a plurality of beams and / or a single modulation sequence that is broadcast in multiple directions. In some embodiments, a secondary wireless device 24 is configured to include an LPR 26 which may be configured to adjust via LPR 26, a plurality of intermediate frequency signals according to a correlation of the intermediate frequency signals with a reference sequence to determine a beam direction associated with a highest correlation result. Example implementations, in accordance with an embodiment, of the WD 22, 24 and network node 16 discussed in the preceding paragraphs will now be described with reference to FIG. 3. FIG. 3 shows a wireless communications system 10 that includes a network node 16. The network node 16 includes hardware 40 enabling it to communicate with the WD 22, 24. The hardware 40 may include a radio interface 42 for setting up and maintaining at least a wireless connection 26, 28 with a WD 22, 24 located in a coverage area 18 served by the network node 16. The radio interface 42 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and / or one or more RF transceivers. The radio interface 42 includes an array of antennas 46 to radiate and receive signal(s) carrying electromagnetic waves. In the embodiment shown, the hardware 40 of the network node 16 further includes processing circuitry 48. The processing circuitry 48 may include a processor 50 and a memory 52. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 48 may comprise integrated circuitry for processing and / or control, e.g., one or more processors and / or processor cores and / or FPGAs (Field Programmable Gate Array) and / or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 50 may be configured to access (e.g., write to and / or read from) the memory 52, which may comprise any kind of volatile and / or nonvolatile memory, e.g., cache and / or buffer memory and / or RAM (Random Access Memory) and / or ROM (Read-Only Memory) and / or optical memory and / or EPROM (Erasable Programmable Read-Only Memory). The wireless communications system 10 further includes the primary WD 22 and the secondary WD, 24 already referred to. The WD 22, 24 may have hardware 54 that may include a radio interface 56 configured to set up and maintain the wireless connection 26, 28 with a network node 16 serving a coverage area 18 in which the WD 22, 24 is currently located. The radio interface 56 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and / or one or more RF transceivers such as a main transceiver 58. The radio interface 56 includes an array of antennas 55 to radiate and receive signal(s) carrying electromagnetic waves. For example, the radio interface 56 of the wireless device 22, 24 may include an ASK modulator 60 which may be configured to modulate a carrier frequency using an on-off keying (OOK) modulation according to a different modulation sequence for each of a plurality of beams and / or a single modulation sequence broadcast in multiple directions. The LPR 36 may be referred to herein as the first receiver 36 and the main transceiver 58 may be referred to herein as the second receiver 58, The hardware 54 of the WD 22 further includes processing circuitry 62. The processing circuitry 62 may include a processor 64 and memory 66. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 62 may comprise integrated circuitry for processing and / or control, e.g., one or more processors and / or processor cores and / or FPGAs (Field Programmable Gate Array) and / or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 64 may be configured to access (e.g., write to and / or read from) memory 66, which may comprise any kind of volatile and / or nonvolatile memory, e.g., cache and / or buffer memory and / or RAM (Random Access Memory) and / or ROM (Read-Only Memory) and / or optical memory and / or EPROM (Erasable Programmable Read-Only Memory). Thus, the WD 22 may further comprise software 68, which is stored in, for example, memory 66 at the WD 22, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the WD 22. The software 56 may be executable by the processing circuitry 62. The software 68 may include a client application 70. The client application 70 may be operable to provide a service to a human or non-human user via the WD 22. The processing circuitry 62 may be configured to control any of the methods and / or processes described herein and / or to cause such methods, and / or processes to be performed, e.g., by WD 22. The processor 64 corresponds to one or more processors 64 for performing WD 22 functions described herein. The WD 22 includes memory 54 that is configured to store data, programmatic software code and / or other information described herein. In some embodiments, the software 68 and / or the client application 58 may include instructions that, when executed by the processor 64 and / or processing circuitry 62, causes the processor 64 and / or processing circuitry 62 to perform the processes described herein with respect to WD 22. In some embodiments, the radio interface 56 is configured to include an LPR 36, including free running multi-phase oscillator 38, which may be configured to adjust, a plurality of intermediate frequency signals according to a correlation of the intermediate frequency signals with a reference sequence to determine a beam direction associated with a highest correlation result. In some embodiments, the inner workings of the network node 16 and WD 22, 24 may be as shown in FIG. 3 and independently, the surrounding network topology may be that of FIG. 1. The wireless connection 32 between the WD 22, 24 and the network node 16 is in accordance with the teachings of the embodiments described throughout this disclosure. More precisely, the teachings of some of these embodiments may improve the data rate, latency, and / or power consumption and thereby provide benefits such as reduced user waiting time, relaxed restriction on file size, better responsiveness, extended battery lifetime, etc. In some embodiments, a measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. Although FIGS. 1-3 show various “units” such as ASK modulator 24 and LPR 26 as being within a respective processor and / or radio interface, it is contemplated that these units may be implemented such that a portion of the unit is stored in a corresponding memory within the processing circuitry. In other words, the units may be implemented in hardware or in a combination of hardware and software within the processing circuitry and / or radio interface. FIG. 4 is a flowchart of an example process in a secondary wireless device 24 according to some embodiments of the present disclosure. One or more blocks described herein may be performed by one or more elements of secondary wireless device 24 such as by one or more of processing circuitry 62 (including the LPR 36), processor 64, and / or radio interface 56. Secondary wireless device 24 such as via processing circuitry 62 and / or processor 64 and / or radio interface 56 is configured to receive an amplitude shift keying, ASK, modulated signal from the primary WD 22 using at least three different antenna branches of a first receiver 36 of the secondary WD 24. The method includes, in each of the at least three antenna branches, down-converting the ASK modulated signal using a multi-phase local oscillator signal from the free running multi-phase oscillator 38 applied in the multi-phase mixer 80 to provide an intermediate frequency signal having a phase, the phase in each branch being different. The method includes determining a beam direction of the ASK modulated signal by correlating phase adjusted combinations of the intermediate frequency signals from the antenna branches with a reference sequence. The method also includes providing the beam direction to the second receiver, and using the determined beam direction for an initial communication between the second receiver 58 and the primary WD 22. In some embodiments, the ASK modulated signal is an OOK modulated signal. In some embodiments, the correlation is performed in a digital domain and phase adjusting includes multiplying the intermediate frequency signals by complex weights. In some embodiments, the correlation is performed in a digital domain and phase adjusting includes performing phase rotations of the intermediate frequency signals. In some embodiments, the first receiver 38 is configured to consume less power than the second receiver 58. In some embodiments, the second receiver 58 operates in a low power mode while the first receiver 38 operates to determine the beam direction. In some embodiments, the free multi-phase running oscillator 38 includes a voltage controlled oscillator, VCO, with a ring oscillator 115. In some embodiments, the correlation is performed according to a clock of a crystal oscillator 90. In some embodiments, determining the beam direction includes comparing a correlation result to a threshold. In some embodiments, the method includes scanning a beam until the correlation result exceeds the threshold. Having described the general process flow of arrangements of the disclosure and having provided examples of hardware and software arrangements for implementing the processes and functions of the disclosure, the sections below provide details and examples of arrangements for low power millimeter wave (mmWave) receivers for beam tracking. In some embodiments, the functionality of a wakeup receiver is combined with a beam tracking functionality for mmWave sidelink (SL) communications, so that the SL beam tracking may be more energy efficient. FIG. 5 is a radio interface 56 of a primary WD 22 which includes an ASK modulator 60 which may be configured to apply ASK modulation to signals transmitted on a plurality of beams to secondary WDs 24. FIG. 6 is a radio interface 56 in a secondary WD 22 that includes a low power receiver (LPR) 36 and a main transceiver 58. It is noted that main transceiver 58 may include a receiver and transmitter. However, implementations are not limited solely to transceivers. Implementations may be accomplished using separate receivers and transmitters. The LPR 36 tracks the beam direction by detecting the sequence transmitted by the primary WD 22 and shares the information of the detected beam direction with the main transceiver 58 in the secondary WD 24. The LPR 36 may be active continuously while the main transceiver is kept in low power or sleep mode or it may operate with a duty-cycle while the main transceiver is kept in low power or sleep mode. An ASK modulation pattern of the synchronization signal may not be detected in the null direction. However, the ASK modulation pattern may be detected in beam directions adjacent to a null direction. Once the null direction is found, the angle of arrival (AoA) of the transmitted beam may be estimated as the null direction. Then the secondary WD 24 may form a beam in the estimated beam direction when the sidelink (SL) communication is initialized. In some embodiments, the LPR 36 may be implemented on a same integrated circuit chip on which the main transceiver 58 is located. LPR Architecture Generally speaking, there are two architecture options that may be used to implement the LPR 36: an analog architecture and a digital architecture. In both analog and digital architectures, the local oscillator frequency (LO) may be generated from the free-running multi-phase oscillator (FRO) 38, e.g., a voltage control oscillator (VCO) implemented with a ring oscillator, while the baseband circuit may be clocked from a stable low-frequency oscillator, e.g., a 20MHz crystal oscillator (XO). The LPR 26 may perform baseband clock correction by using a low power scheme. Of note, although solutions and embodiments herein show four branches in the drawing figures, such is provided only to aid understanding of the disclosure. It is understood that embodiments may be implemented using at least three channels. Analog Solution FIG. 7 shows an example of LPR architecture implemented using an analog solution. The LPR 36 includes an RF front end 72 with multiple Rx branches (e.g., 4 Rx branches) and a baseband unit 74. The RF front end 72 includes RF filters 76, low noise amplifiers 78, multi-phase mixers 80 and intermediate frequency (IF) filters 82. The RF front end 72 also includes an intermediate frequency (IF) combiner 84, envelope detector 86, comparator 88 and the free running multi-phase oscillator 38. The baseband unit 74 receives a clock from a crystal oscillator 90. In some embodiments, the same free running multi-phase oscillator 38 provides signals of different phases to the multi-phase mixers 80. In each Rx branch, the low power free-running multi-phase oscillator 38 is used to down convert the carrier frequency signal to a multi-phase IF signal. The baseband unit 74 includes a correlator 92 to correlate the signal received from the comparator 88 with a reference sequence to produce a correlation result that is received by a beam tracking unit 94. The filtered IF signal is phase rotated via a phase rotator 96. The phase rotation of the phase rotator 96 is controlled by the beam tracking unit 94. The IF combiner 84 is used to combine the phase rotated IF signals of the 4 Rx branches. Note that in some embodiments there may be more or less than four Rx branches. In some embodiments, there are at least three RX branches. From the output of the IF combiner 84, the envelope detector 86 and comparator 88 sample the baseband signal. Then the baseband unit 74 performs sequence correlation via the correlator 92 on the received baseband signal. There may also be a filter between the envelope detector 86 and the comparator 88 to suppress extraneous frequencies or other frequencies not needed for detection of the ASK signal. The beam tracking unit 94 may apply various phase rotation values on the multiphase IF signals. For each phase rotation value, the correlator 92 performs sequence correlation. By finding the maximum correlation result among the phase rotation values, the transmitted beam direction may be determined and reported to the main transceiver 58. The low correlation results may also be used in determining the beam direction with improved resolution, similar to a null-scan. An example embodiment of beam tracking is described with respect to FIG. 8. A set of IF phase rotation values are determined (Block S20). A correlation is performed on a received sequence (Block S22). The beam tracking unit 110 analyzes the correlation results (Block S24). If a peak correlation value is not above a threshold (Block S26), the beam tracking unit 948 increments the IF phase rotations (Block S28). If the peak correlation value does exceed the threshold (Block S26), a null scan in azimuth and in elevation may be performed to improve resolution (Block S30). The LPR 36 may report the IF phase rotation value to the main transceiver 58 (Block S32). Digital Solution FIG. 9 shows an example of an LPR 36 with a digital architecture. The LPR 36 includes an RF front end 98 having multiple Rx branches (e.g., 4 Rx branches) and a digital baseband unit 100. Each Rx branch includes RF filter 76, low noise amplifier 78, multi-phase mixer 80, IF filter 82 and an ADC 102. A baseband unit 100 provides digital filtering of each digital signal received from the RF front end 98 via digital filters 104. A digital beamforming unit 106 applying a complex weight value to each digitized IF signal and combines the digital output of all the Rx branches, which is followed by an envelope detector 108, optional comparator 110, correlator 112 and beam tracking unit 114. In this case, multiple beam settings may be investigated in parallel, by forming multiple beams and correlating the results. The optional comparator 110 may be a single level or multilevel comparator. With multi-bit ADCs used to implement the ADCs 104 of the RF front end 98, information regarding the signal strength of beams may also be obtained. All this information may be used to make beam finding faster and the beam tracking more agile. However, the increased performance may come at a cost, as the multiple multi-bit ADCs will increase power consumption, and AGC may be needed before each ADC 1046. FIG. 10 is one example of a multi-phase inductor-coupled ring oscillator 115 that may be utilized to implement the free running multi-phase oscillator 38. The multi-phase inductor-coupled ring oscillator 115 includes two single-ended ring oscillators 116 and 118, each featuring three active stages acting as delay cells 120. As shown in the example of FIG. 11, some embodiments are of low complexity and low power consumption by using as few active elements as possible. The delay cells 120 introduce a 120-degree phase shift between their respective input and output and thus, three phases are available at the three outputs of the delay cells 120 for each single-ended ring oscillator 116 and 118. The output node of each delay cell 120 of the multi-phase inductor-coupled ring oscillator 115 is coupled to an output of a corresponding delay cell 120 of the ring oscillator using a differential inductor 122, which is a passive element. The differential inductor 122 creates a 180-degree phase shift between the output nodes of the corresponding delay cells 120 of each single-ended ring oscillator 116, 118. The use of passive elements to induce this phase shift has no impact on the power consumption and does not add significant complexity to the circuit. This is in contrast to known methods where active elements are used, which adds complexity and increases overall oscillator power consumption. By using passive inductors to tune the oscillator nodes, as disclosed herein, the multi-phase inductor-coupled ring oscillator 38 may operate at higher frequency with low power. The phase noise is also reduced compared to known ring oscillators without inductive tuning. A benefits, is that the differential inductors improve performance over a frequency band, reduce power consumption and reduce phase noise. This is achieved without significant additional circuit complexity or power consumption. In some embodiments, the multi-phase inductor-coupled ring oscillator 115 has six different phases available, one phase for each delay cell output of the six delay cells 120. The differential inductors 122 may have the same size and a same coupling factor k between their primary and secondary windings. Each differential inductor 122 may have their center taps connected to the AC ground through a fixed capacitor Cmid 124. A variable capacitor Cm 126 is placed between ground and the output node of each active delay cell 120. The value of variable capacitor Cm 126, together with the supply voltage, may be used to set the oscillation frequency of the delay cell 120. The resonant frequency of each LC tank is: fo =            + Cm) o v where Cinv is the sum of the input capacitance of a loading inverter 128 and the output capacitance of a driving inverter 128, where each inverter 128 may be a loading inverter for a previous delay cell 120, and each inverter 128 may be the driving inverter for a subsequent delay cell 120. As will be appreciated by one of skill in the art, the concepts described herein may be embodied as a method, data processing system, computer program product and / or computer storage media storing an executable computer program. Accordingly, the concepts described herein may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a “circuit” or “module.” Any process, step, action and / or functionality described herein may be performed by, and / or associated to, a corresponding module, which may be implemented in software and / or firmware and / or hardware. Furthermore, the disclosure may take the form of a computer program product on a tangible computer usable storage medium having computer program code embodied in the medium that may be executed by a computer. Any suitable tangible computer readable medium may be utilized including hard disks, CD-ROMs, electronic storage devices, optical storage devices, or magnetic storage devices. Some embodiments are described herein with reference to flowchart illustrations and / or block diagrams of methods, systems and computer program products. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer (to thereby create a special purpose computer), special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks. These computer program instructions may also be stored in a computer readable memory or storage medium that may direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instruction means which implement the function / act specified in the flowchart and / or block diagram block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks. It is to be understood that the functions / acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality / acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows. Computer program code for carrying out operations of the concepts described herein may be written in an object oriented programming language such as Python, Java® or C++. However, the computer program code for carrying out operations of the disclosure may also be written in conventional procedural programming languages, such as the “C” programming language. The program code may execute entirely on the user’s computer, partly on the user’s computer, as a stand-alone software package, partly on the user’s computer and partly on a remote computer or entirely on the remote computer. In the latter scenario, the remote computer may be connected to the user's computer through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, all embodiments may be combined in any way and / or combination, and the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination. Abbreviations that may be used in the preceding description include: Abbreviation Explanation ADC Analog Digital Converter AGC Automatic Gain Control ASK Amplitude Shift Keying FRO Free-Running Oscillator IF Intermediate Frequency LNA Low Noise Amplifier LO Local Oscillator LPR Low Power Receiver NR New Radio OOK On OFF Keying SL Sidelink UE User Equipment VCO Voltage Controlled Oscillator WD Wireless Device XO Crystal Oscillator It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the 5 accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.

Claims

1. A secondary wireless device, WD (24), configured to communicate with a primary WD (22), the secondary WD (24) comprising a first receiver (36) and a second receiver (58), the first receiver (36) comprising at least three antenna branches, each antenna branch including a multi-phase mixer (80), the first receiver (36) including a free-running multi-phase oscillator (38), the secondary WD (24) configured to:receive an amplitude shift keying, ASK, modulated signal from the primary WD (22) using at least three different antenna branches of the first receiver (36) of the secondary WD (24);in each of the at least three antenna branches, down-convert the ASK modulated signal using a multi-phase local oscillator signal from the free-running multi-phase oscillator (38) applied in the multi-phase mixer (80) to provide a multi-phase intermediate frequency signal comprising individual intermediate frequency signals for each of a plurality of phases;determine a beam direction of the ASK modulated signal by correlating phase adjusted combinations of the intermediate frequency signals from the antenna branches with a reference sequence;provide the beam direction to the second receiver (58); anduse the determined beam direction for an initial communication between the second receiver (58) and the primary WD (22).

2. The secondary WD (24) of Claim 1, wherein the ASK modulated signal is an OOK modulated signal.

3. The secondary WD (24) of any of Claims 1 and 2, wherein the correlation is performed in a digital domain and phase adjusting includes multiplying the intermediate frequency signals by complex weights.

4. The secondary WD (24) of any of Claims 1 and 2, wherein the correlation is performed in a digital domain and phase adjusting includes performing phase rotations of the intermediate frequency signals.

5. The secondary WD (24) of any of Claims 1-4, wherein the first receiver(36) is configured to consume less power than the second receiver (58).

6. The secondary WD (24) of any of Claims 1-5, wherein the second receiver (58) operates in a low power mode while the first receiver (36) operates to determine the beam direction.

7. The secondary WD (24) of any of Claims 1-6, wherein the free runningmulti-phase oscillator (38) includes a voltage controlled oscillator, VCO, with a ring oscillator (115).

8. The secondary WD (24) of any of Claims 1-7, wherein the correlation is performed according to a clock of a crystal oscillator (90).

9. The secondary WD (24) of any of Claims 1-8, wherein determining the beam direction includes comparing a correlation result to a threshold.

10. The secondary WD (24) of any of Claims 1-9, wherein the secondary WD (24) is configured to scan a beam until the correlation result exceeds the threshold.

11. A method in a secondary wireless device, WD (24), configured to communicate with a primary WD (22), the secondary WD (24) comprising a first receiver (36) and a second receiver (58), the first receiver (36) comprising at least three antenna branches, each antenna branch including a multi-phase mixer (80), the first receiver (36) including a free-running multi-phase oscillator (38), the method comprising:receiving (S10) an amplitude shift keying, ASK, modulated signal from the primary WD (22) using at least three different antenna branches of the first receiver (36) of the secondary WD (24);in each of the at least three antenna branches, down-converting (S12) the ASK modulated signal using a multi-phase local oscillator signal from the free-running multiphase oscillator (38) applied in the multi-phase mixer (80) to provide an intermediate multi-phase frequency signal comprising individual intermediate frequency signals for each of a plurality of phases;determining (SI 4) a beam direction of the ASK modulated signal by correlating phase adjusted combinations of the intermediate frequency signals from the antenna branches with a reference sequence;providing (SI6) the beam direction to the second receiver (58); andusing (SI8) the determined beam direction for an initial communication between the second receiver (58) and the primary WD (22).

12. The method of Claim 11, wherein the ASK modulated signal is an OOK modulated signal.

13. The method of any of Claims 11 and 12, wherein the correlation is performed in a digital domain and phase adjusting includes multiplying the intermediate frequency signals by complex weights.

14. The method of any of Claims 11 and 12, wherein the correlation is performed in a digital domain and phase adjusting includes performing phase rotations of the intermediate frequency signals.

15. The method of any of Claims 11-14, wherein the first receiver (36) is configured to consume less power than the second receiver (58).

16. The method of any of Claims 11-15, wherein the second receiver (58) operates in a low power mode while the first receiver (36) operates to determine the beam direction.

17. The method of any of Claims 11-16, wherein the free running multi-phase oscillator (38) includes a voltage controlled oscillator, VCO, with a ring oscillator (115).

18. The method of any of Claims 11-17, wherein the correlation is performed according to a clock of a crystal oscillator (90).

19. The method of any of Claims 11-18, wherein determining the beam direction includes comparing a correlation result to a threshold.

20. The method of any of Claims 11-19, further comprising scanning a beam until the correlation result exceeds the threshold.

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