OFDM-based backscatter radio with frequency and time shifting

Abstract

A method, backscattering device and network nodes for differential modulation and demodulation for backscatter radio are disclosed. According to one aspect, a method in a backscattering device includes receiving an orthogonal frequency division multiplexed, OFDM, signal in a first radio frequency channel from the network node. The method includes frequency shifting the received OFDM signal to a second radio frequency channel different from the first radio frequency channel. The method further includes reflecting the frequency shifted received OFDM signal. The method also includes synchronizing reflecting of the frequency shifted received OFDM signal to an OFDM time grid.

Description

[0001] OFDM-BASED BACKSCATTER RADIO WITH FREQUENCY AND TIME SHIFTING

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to wireless communications, and in particular, to differential modulation and demodulation for backscatter radio.

[0004] BACKGROUND

[0005] 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.

[0006] The Institute of Electrical and Electronic Engineers (IEEE) has developed and continues to develop standards for wireless communication networks, including Wireless Local Area Networks (WLANs), branded as “Wi-Fi” networks by the Wi-Fi Alliance. WLANs include wireless communication between access points (AP STAs) and non-access point stations (non-AP STAs). Such IEEE standards include IEEE 802.1 la / b / g / n / ac / ax / be and IEEE 802.15.

[0007] Backscatter radio is a technology well suited for ultra-low power and low-cost radios. For example, it is used in radio frequency identification (RFID) tags to track products, and it is being considered as a candidate technology for ultra-low power loT devices in both 3GPP and IEEE 802.11.

[0008] FIG. 1 illustrates an example of a baseband representation of a bistatic backscattering scenario. A transmitter TX sends an unmodulated carrier c0, which illuminates a backscatter tag. The carrier signal c0is reflected by the tag and has the form c0■ x, where x is a complex-valued reflection coefficient. The receiver RX receives the superposition of the carrier signal c0(often called the direct link or direct path) and the backscattered signal c0■ x. FIG. 1 illustrates bistatic backscattering where c0is the complex baseband representation of the carrier and x is a complex-valued reflection coefficient. In practice the backscattered signal is often much weaker than the carrier. For this reason, a receiver trying to demodulate the backscattered signal may often experience high levels of interference from the carrier. To alleviate this problem, it is possible to frequency shift the backscattered signal so that from the point of view of the receiver the backscattered signal modulates a carrier that is different from the transmitted carrier c0. Thus, the receiver may filter out the interference from c0by means of a channel selective filter. This is illustrated in FIG. 2.

[0009] FIG. 2 illustrates bistatic backscattering with frequency shift, where c0is the complex baseband representation of the carrier, x is a complex-valued reflection coefficient and is a baseband representation of a carrier at a different frequency from the carrier c0.

[0010] Backscattering may also be used in more complex scenarios where the transmitter sends communications signals (i.e., the carrier c0is modulated), and as in FIG. 2, the tag may introduce a frequency shift to the reflected signal, as illustrated in FIG. 3. In FIG. 3, s is the symbol modulating the carrier, while c0, and x are the same as shown in FIGS. 1 and 2.

[0011] FIG. 3 illustrates bistatic backscattering where the transmitter sends a modulated carrier. Here, c0is the complex baseband representation of the carrier, 5 is a complex symbol modulating the carrier, x is a complex-valued reflection coefficient and represents a carrier at a frequency different from c0.

[0012] In a monostatic scenario the TX and RX are co-located in the same node, so that the above description needs to be suitably modified. In this case the TX and RX are operating simultaneously in the same node, so that the TX self-jams the RX. The signals from the TX to the tag, and the reflection from the tag to the RX may be described as in FIGS. 1-3.

[0013] TX nodes in backscattering systems such as RFID send continuous waves. However, if backscattering technologies are to be supported by communication nodes in broadband systems such as NR or Wi-Fi, it is desirable to support TXs that transmit orthogonal frequency division multiplexed (OFDM) signals. One reason is that there may be local regulatory constraints on the TX power for narrowband signals so that peak output power may only be used with wideband signals. Another reason is that it may be desirable to share the band between backscattering devices and broadband devices. Another reason is that a pure carrier does not carry information so that a TX that sends only a continuous wave may not utilize the spectrum for communications while backscattering is ongoing.

[0014] There are known techniques for backscattering broadband signals (e.g., backscattered OFDM signals). Reception of backscattered OFDM signals is an extremely challenging problem and it is very difficult for the RX to handle the interference from the TX.

[0015] SUMMARY

[0016] Some embodiments advantageously provide methods, backscattering devices and network nodes for differential modulation and demodulation for backscatter radio.

[0017] In some embodiments, the TX illuminates the tag with an ordinary OFDM signal that may carry information addressed to broadband users. The tag modulates the reflections of the impinging OFDM signal in such a way that the reflected RF signal is shifted in frequency and does not overlap in the frequency domain with the signal sent by the TX. The RX may separate filter the direct link signal from the TX and reflect a backscattered signal at a frequency that is different than the frequency of the direct link signal.

[0018] In some embodiments, the RX first decodes the direct link OFDM signal. Afterwards it analyzes the backscattered signal in the frequency domain, and uses the decoded OFDM signal to remove the effect of the OFDM modulation symbols on the backscattered signal. The resulting signal is an OFDM signal including repetitions in time and frequency of the modulation symbols sent by the backscattering tag.

[0019] In some embodiments, the receiver is not a typical successive interference receiver because the interference to be removed is not additive but rather multiplicative. Also, if the direct link signal is decoded correctly, the impact of the interference may be perfectly suppressed because the removal process does not depend on channel estimates.

[0020] Some embodiments include simultaneous transmission of data from a TX node and from a backscattering tag. Moreover, the backscattered signals are well suited for reception by OFDM broadband receivers (as opposed to specialized receivers as in RFID). Since the backscattered signal is a slightly modified OFDM signal, the receiver may benefit from frequency diversity.

[0021] According to one aspect, a method is provided in a backscattering device configured to reflect signals received from a network node. The method includes receiving an orthogonal frequency division multiplexed, OFDM, signal in a first radio frequency channel from the network node. The method includes frequency shifting the received OFDM signal to a second radio frequency channel different from the first radio frequency channel. The method also includes reflecting the frequency shifted received OFDM signal. The method further includes synchronizing reflecting of the frequency shifted received OFDM signal to an OFDM time grid.

[0022] According to this aspect, in some embodiments, an amount of the frequency shifting is determined by a parameter received from the network node. In some embodiments, synchronizing reflecting of the frequency shifted OFDM signal to an OFDM time grid includes synchronizing to a timing signal received from the network node. In some embodiments, synchronizing to the OFDM time grid causes modulation symbols to start within a cyclic prefix of OFDM symbols in the received OFDM signal. In some embodiments, a duration of a reflected modulation symbol is not less than an OFDM symbol duration.

[0023] According to another aspect, a backscattering device configured to reflect signals received from a network node is provided. The backscattering device is configured to receive an orthogonal frequency division multiplexed, OFDM, signal in a first radio frequency channel from the network node. The backscattering device is configured to frequency shift the received OFDM signal to a second radio frequency channel different from the first radio frequency channel. The backscattered device is also configured to reflect the frequency shifted received OFDM signal, and synchronize reflecting of the frequency shifted received OFDM signal to an OFDM time grid.

[0024] According to this aspect, in some embodiments, an amount of the frequency shifting is determined by a parameter received from the network node. In some embodiments, synchronizing reflecting of the frequency shifted OFDM signal to an OFDM time grid includes synchronizing to a timing signal received from the network node. In some embodiments, synchronizing to the OFDM time grid causes modulation symbols to start within a cyclic prefix of OFDM symbols in the received OFDM signal. In some embodiments, a duration of a reflected modulation symbol is not less than an OFDM symbol duration.

[0025] According to yet another aspect, a method in a network node configured to illuminate a backscattering device is provided. The method includes configuring the backscattering device with a frequency shift parameter indicating an amount by which to frequency shift a received orthogonal frequency division multiplexed, OFDM, signal from a first carrier frequency to a second carrier frequency to be reflected by the backscattering device in synchronization with an OFDM time grid.

[0026] According to this aspect, in some embodiments, the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration. In some embodiments, the method includes configuring the backscattering device to reflect the received OFDM signal with a symbol duration that is an integer multiple of an OFDM symbol duration of the received OFDM signal. In some embodiments, the method includes transmitting an illumination signal at the first carrier frequency, the illumination signal comprising OFDM symbols. In some embodiments, the method includes configuring the backscattering device with an indication of a time to start a reflection of a received OFDM signal.

[0027] According to another aspect, a network node configured to illuminate a backscattering device is provided. The network node is configured to configure the backscattering device with a frequency shift parameter indicating an amount by which to frequency shift a received orthogonal frequency division multiplexed, OFDM, signal from a first carrier frequency to a second carrier frequency to be reflected by the backscattering device in synchronization with an OFDM time grid.

[0028] According to this aspect, in some embodiments, the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration. In some embodiments, the network node is further configured to configure the backscattering device to reflect the received OFDM signal with a symbol duration that is an integer multiple of an OFDM symbol duration of the received OFDM signal. In some embodiments, the network node is further configured to transmit an illumination signal at the first carrier frequency, the illumination signal comprising OFDM symbols. In some embodiments, the network node is further configured to configure the backscattering device with an indication of a time to start a reflection of a received OFDM signal.

[0029] According to yet another aspect, a method in a first network node includes simultaneously receiving a first orthogonal frequency division multiplexed, OFDM, signal, from a second network node at a first carrier frequency and a second OFDM signal reflected from a backscattering device at a second carrier frequency, the second OFDM signal being a frequency shifted reflection of the first OFDM signal. The method includes separating the first and second OFDM signals. The method also includes determining samples of OFDM symbols in the second OFDM signal based at least in part on samples of OFDM symbols in the first OFDM signal.

[0030] According to this aspect, in some embodiments, determining samples of the OFDM symbols in the second OFDM signal includes estimating samples of corresponding OFDM symbols in the first OFDM signal. In some embodiments, determining samples of the OFDM symbols in the second OFDM signal includes dividing OFDM samples in the second OFDM signal by corresponding estimates of the samples of OFDM symbols in the first OFDM signal. In some embodiments, a first symbol rate of the first OFDM signal is an integer multiple of a second symbol rate of the second OFDM signal. In some embodiments, the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration.

[0031] According to another aspect, a first network node is configured to: simultaneously receive a first orthogonal frequency division multiplexed, OFDM, signal, from a second network node at a first carrier frequency and a second OFDM signal reflected from a backscattering device at a second carrier frequency, the second OFDM signal being a frequency shifted reflection of the first OFDM signal. The first network node is configured to separate the first and second OFDM signals. The network node is also configured to determine samples of OFDM symbols in the second OFDM signal based at least in part on samples of OFDM symbols in the first OFDM signal.

[0032] According to this aspect, in some embodiments, determining samples of the OFDM symbols in the second OFDM signal includes estimating samples of corresponding OFDM symbols in the first OFDM signal. In some embodiments, determining samples of the OFDM symbols in the second OFDM signal includes dividing OFDM samples in the second OFDM signal by corresponding estimates of the samples of OFDM symbols in the first OFDM signal. In some embodiments, a first symbol rate of the first OFDM signal is an integer multiple of a second symbol rate of the second OFDM signal. In some embodiments, the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration.

[0033] 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:

[0034] FIG. 1 illustrates a baseband representation of a bistatic backscattering scenario;

[0035] FIG. 2 illustrates bistatic backscattering with frequency shift;

[0036] FIG. 3 illustrates bistatic backscattering where the transmitter sends a modulated carrier;

[0037] FIG. 4 is a schematic diagram of an example network architecture illustrating a communication system according to principles disclosed herein;

[0038] FIG. 5 is a block diagram of a network node in communication with a wireless device over a wireless connection according to some embodiments of the present disclosure;

[0039] FIG. 6 is a flowchart of an example process in a backscattering device for differential modulation and demodulation for backscatter radio;

[0040] FIG. 7 is a flowchart of an example process in a network node for differential modulation and demodulation for backscatter radio;

[0041] FIG. 8 is a flowchart of an example process in a network node for differential modulation and demodulation for backscatter radio; and

[0042] FIG. 9 illustrates frequency shifting according to principles disclosed herein.

[0043] DETAILED DESCRIPTION

[0044] Before describing in detail exemplary embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to differential modulation and demodulation for backscatter radio. 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The term “network node” used herein may be any kind of network node included in a radio network which may further include any of base station (BS), radio base station, base transceiver station (BTS), base station controller (BSC), radio network controller (RNC), g Node B (gNB), evolved Node B (eNB or eNodeB), Node B, multi-standard radio (MSR) radio node such as MSR BS, multi-cell / multicast coordination entity (MCE), relay node, donor node controlling relay, radio access point (AP), transmission points, transmission nodes, Remote Radio Unit (RRU) Remote Radio Head (RRH), a core network node (e.g., mobile management entity (MME), self- organizing network (SON) node, a coordinating node, positioning node, MDT node, etc.), an external node (e.g., 3rd party node, a node external to the current network), nodes in distributed antenna system (DAS), a spectrum access system (SAS) node, an element management system (EMS), etc. The network node may also include test equipment. The term “radio node” used herein may be used to also denote a wireless device (WD) such as a wireless device (WD) or a radio network node.

[0050] 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.

[0051] Also, in some embodiments the generic term “radio network node” is used. It may be any kind of a radio network node which may include any of base station, radio base station, base transceiver station, base station controller, network controller, RNC, evolved Node B (eNB), Node B, gNB, Multi-cell / multicast Coordination Entity (MCE), relay node, access point, radio access point, Remote Radio Unit (RRU) Remote Radio Head (RRH).

[0052] Note that although terminology from one particular wireless system, such as, for example, 3GPP LTE and / or New Radio (NR), may be used in this disclosure, this should not be seen as limiting the scope of the disclosure to only the aforementioned system. Other wireless systems, including without limitation Wide Band Code Division Multiple Access (WCDMA), Worldwide Interoperability for Microwave Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile Communications (GSM), may also benefit from exploiting the ideas covered within this disclosure.

[0053] Note further, that functions described herein as being performed by a wireless device or a network node may be distributed over a plurality of wireless devices and / or network nodes. In other words, it is contemplated that the functions of the network node and wireless device described herein are not limited to performance by a single physical device and, in fact, may be distributed among several physical devices.

[0054] 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.

[0055] Some embodiments are directed to differential modulation and demodulation for backscatter radio.

[0056] Returning to the drawing figures, in which like elements are referred to by like reference numerals, there is shown in FIG. 4 a schematic diagram of a communication system 10, according to an embodiment, such as a 3 GPP -type cellular network that may support standards such as LTE and / or NR (5G), which includes an access network 12, such as a radio access network, and a core network 14. The access network 12 includes a plurality of network nodes 16a, 16b, 16c (referred to collectively as network nodes 16), such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area 18a, 18b, 18c (referred to collectively as coverage areas 18). Each network node 16a, 16b, 16c is connectable to the core network 14 over a wired or wireless connection 20. A first wireless device (WD) 22a located in coverage area 18a is configured to wirelessly connect to, or be paged by, the corresponding network node 16a. A second WD 22b in coverage area 18b is wirelessly connectable to the corresponding network node 16b. While a plurality of WDs 22a, 22b (collectively referred to as wireless devices 22) are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole WD is in the coverage area or where a sole WD is connecting to the corresponding network node 16. Note that although only two WDs 22 and three network nodes 16 are shown for convenience, the communication system may include many more WDs 22 and network nodes 16.

[0057] Also, it is contemplated that a WD 22 may be in simultaneous communication and / or configured to separately communicate with more than one network node 16 and more than one type of network node 16. For example, a WD 22 may have dual connectivity with a network node 16 that supports LTE and the same or a different network node 16 that supports NR. As an example, WD 22 may be in communication with an eNB for LTE / E-UTRAN and a gNB for NR / NG-RAN. A network node 16 (eNB or gNB) may be configured to include an OFDM unit

[0058] 24 and / or a controller 25. The OFDM unit 24 may be configured to configure a backscattering device 26 with a frequency shift parameter indicating an amount by which to frequency shift a received orthogonal frequency division multiplexed, OFDM, signal from a first carrier frequency to a second carrier frequency to be reflected by the backscattering device 26 in synchronization with an OFDM time grid. The controller

[0059] 25 may be configured to separate the first and second OFDM signals, the first OFDM signal being received from another network node and the second OFDM signal being received from a backscattering device 26. The backscattering device 26 may be configured to include a frequency converter 60 and an OFDM synchronizer 62. The frequency converter 60 may be configured to frequency shift the received OFDM signal to a second radio frequency channel different from the first radio frequency channel. The OFDM synchronizer 62 may be configured to synchronize reflections of the backscattering device 36 with an OFDM time grid.

[0060] Example implementations, in accordance with an embodiment, of the WD 22 and network node 16 discussed in the preceding paragraphs will now be described with reference to FIG. 5.

[0061] The communication system 10 includes a network node 16 provided in a communication system 10 and including hardware 28 enabling it to communicate with the WD 22. The hardware 28 may include a radio interface 30 for setting up and maintaining at least a wireless connection 32 with a WD 22 located in a coverage area 18 served by the network node 16. The radio interface 30 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 30 includes an array of antennas 34 to radiate and receive signal(s) carrying electromagnetic waves.

[0062] In the embodiment shown, the hardware 28 of the network node 16 further includes processing circuitry 36. The processing circuitry 36 may include a processor 38 and a memory 40. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 36 may include 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 38 may be configured to access (e.g., write to and / or read from) the memory 40, which may include 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).

[0063] Thus, the network node 16 further has software 42 stored internally in, for example, memory 40, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the network node 16 via an external connection. The software 42 may be executable by the processing circuitry 36. The processing circuitry 36 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 network node 16. Processor 38 corresponds to one or more processors 38 for performing network node 16 functions described herein. The memory 40 is configured to store data, programmatic software code and / or other information described herein. In some embodiments, the software 42 may include instructions that, when executed by the processor 38 and / or processing circuitry 36, causes the processor 38 and / or processing circuitry 36 to perform the processes described herein with respect to network node 16. For example, processing circuitry 36 of the network node 16 may include an OFDM unit 24 and / or a controller 25. The OFDM unit may be configured to configure a backscattering device 26 with a frequency shift parameter indicating an amount by which to frequency shift a received orthogonal frequency division multiplexed, OFDM, signal from a first carrier frequency to a second carrier frequency to be reflected by the backscattering device 26 in synchronization with an OFDM time grid. The controller 25 may be configured to separate the first and second OFDM signals, the first OFDM signal being received from another network node and the second OFDM signal being received from a backscattering device 26.

[0064] The communication system 10 further includes the WD 22 already referred to. The WD 22 may have hardware 44 that may include a radio interface 46 configured to set up and maintain a wireless connection 32 with a network node 16 serving a coverage area 18 in which the WD 22 is currently located. The radio interface 46 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 46 includes an array of antennas 48 to radiate and receive signal(s) carrying electromagnetic waves.

[0065] The hardware 44 of the WD 22 further includes processing circuitry 50. The processing circuitry 50 may include a processor 52 and memory 54. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 50 may include 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 52 may be configured to access (e.g., write to and / or read from) memory 54, which may include 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).

[0066] Thus, the WD 22 may further include software 56, which is stored in, for example, memory 54 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 50. The software 56 may include a client application 58. The client application 58 may be operable to provide a service to a human or non-human user via the WD 22.

[0067] The processing circuitry 50 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 52 corresponds to one or more processors 52 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 56 and / or the client application 58 may include instructions that, when executed by the processor 52 and / or processing circuitry 50, causes the processor 52 and / or processing circuitry 50 to perform the processes described herein with respect to WD 22.

[0068] In some embodiments, the inner workings of the network node 16 and WD 22 may be as shown in FIG. 5 and independently, the surrounding network topology may be that of FIG. 4.

[0069] The wireless connection 32 between the WD 22 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. 4 and 5 show various “units” such as OFDM unit 24 and controller 25 as being within a respective processor, 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.

[0070] FIG. 6 is a flowchart of an example process in a backscattering device 26 for differential modulation and demodulation for backscatter radio. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the frequency converter 60 and / or OFDM synchronizer 62), processor 38, and / or radio interface 30. Network node 16 such as via processing circuitry 36 and / or processor 38 and / or radio interface 30 is configured to receive an orthogonal frequency division multiplexed, OFDM, signal in a first radio frequency channel from the network node (Block S10). The method includes frequency shifting the received OFDM signal to a second radio frequency channel different from the first radio frequency channel (Block S12). The method also includes reflecting the frequency shifted received OFDM signal (Block S14). The method further includes synchronizing reflecting of the frequency shifted received OFDM signal to an OFDM time grid (Block SI 6).

[0071] According to this aspect, in some embodiments, an amount of the frequency shifting is determined by a parameter received from the network node 16. In some embodiments, synchronizing reflecting of the frequency shifted OFDM signal to an OFDM time grid includes synchronizing to a timing signal received from the network node 16. In some embodiments, synchronizing to the OFDM time grid causes modulation symbols to start within a cyclic prefix of OFDM symbols in the received OFDM signal. In some embodiments, a duration of a reflected modulation symbol is not less than an OFDM symbol duration.

[0072] FIG. 7 is a flowchart of an example process in a network node 16 for differential modulation and demodulation for backscatter radio. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the OFDM unit 24 and / or controller 25), processor 38, and / or radio interface 30. Network node 16 such as via processing circuitry 36 and / or processor 38 and / or radio interface 30 is configured to configure the backscattering device 26 with a frequency shift parameter indicating an amount by which to frequency shift a received orthogonal frequency division multiplexed, OFDM, signal from a first carrier frequency to a second carrier frequency to be reflected by the backscattering device 26 in synchronization with an OFDM time grid (Block SI 8).

[0073] According to this aspect, in some embodiments, the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration. In some embodiments, the method includes configuring the backscattering device 26 to reflect the received OFDM signal with a symbol duration that is an integer multiple of an OFDM symbol duration of the received OFDM signal. In some embodiments, the method includes transmitting an illumination signal at the first carrier frequency, the illumination signal comprising OFDM symbols. In some embodiments, the method includes configuring the backscattering device 26 with an indication of a time to start a reflection of a received OFDM signal.

[0074] FIG. 8 is a flowchart of an example process in a first network node 16 according to some embodiments of the present disclosure. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the OFDM unit 24 and / or controller 25), processor 38, and / or radio interface 30. Network node 16 such as via processing circuitry 36 and / or processor 38 and / or radio interface 30 is configured to simultaneously receive a first orthogonal frequency division multiplexed, OFDM, signal, from a second network node at a first carrier frequency and a second OFDM signal reflected from a backscattering device 26 at a second carrier frequency, the second OFDM signal being a frequency shifted reflection of the first OFDM signal (Block S20). The method includes separating the first and second OFDM signals (Block S22). The method also includes determining samples of OFDM symbols in the second OFDM signal based at least in part on samples of OFDM symbols in the first OFDM signal (Block S24).

[0075] According to this aspect, in some embodiments, determining samples of the OFDM symbols in the second OFDM signal includes estimating samples of corresponding OFDM symbols in the first OFDM signal. In some embodiments, determining samples of the OFDM symbols in the second OFDM signal includes dividing OFDM samples in the second OFDM signal by corresponding estimates of the samples of OFDM symbols in the first OFDM signal. In some embodiments, a first symbol rate of the first OFDM signal is an integer multiple of a second symbol rate of the second OFDM signal. In some embodiments, the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration.

[0076] 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 differential modulation and demodulation for backscatter radio.

[0077] Consider the case illustrated in FIG. 3, where a first network node 16 that transmits a modulated carrier and the backscattering device 26 introduces a frequency shift to the reflected signal. The inventive principles disclosed herein may be applicable to both monostatic and bistatic deployments.

[0078] Specifically, consider a scenario where there is an underlying OFDM system (e.g., 4G, 5G or Wi-Fi) and re-use of the OFDM TX / RX infrastructure to support backscattering devices is desired. Embodiments disclosed herein involve modulation of the symbols transmitted by the backscattering device 26 and decoding these symbols at the receiver.

[0079] The carrier frequencies for c0and cxare chosen to coincide with subcarrier frequencies in the frequency raster of the OFDM system. Separation in frequency allows the receiver to effectively separate the two signals in the frequency domain, as shown in FIG. 9.

[0080] Consider an example where the signal sent by the network node 16a includes only one modulated subcarrier. Denote by T the OFDM symbol duration that includes a cyclic prefix (CP). The symbol duration for the backscattered signal is chosen to be an integer multiple N ■ T of the OFDM symbol duration, N > 1. Furthermore, assume that the separation in frequency between the subcarriers c0and is large enough so that the network node 16b may effectively filter out the interference from c0, and that the RX bandwidth is large enough to simultaneously receive both signals. Let xm,m = 1, ... , M, represent the complex data symbols transmitted by the backscattering device 26, let snrepresent the complex symbols sent by the network node 16a, let wnrepresent noise samples, and let be a complex number that models the channel, which includes the effect of the propagation environments between the network node 16a and the backscattering device 26 and between the backscattering device 26 and the network node 16b. Then, after applying filtering to filter out the subcarrier c0, the time domain received digital baseband signal rnsampled at rate 1 / T may be expressed as: rN= H1SNX1+ wNrN+l=^1SN+1X2 +WN + 1 rNM=^1SNMXM +WNM

[0081] Note that each desired data symbol from the backscattering device appears in N received samples since it is assumed that the symbol rate for the backscattering device 26 is 1 / N of the symbol rate for the transmitted signal.

[0082] Similarly, the receiver may filter out the subcarrier crDenoting by Gothe channel between the network node 16a and network node 16b and a>nthe noise samples, the time domain baseband received signal pncorresponding to the subcarrier c0may be written as: i = GQSI + <»i

[0083] P2=GQS2+ m2

[0084] PNM=GOSNM+ )NM

[0085] The signal pmmay be a conventional signal (e.g., it may be considered as a simple OFDM signal with only one active subcarrier) so that the receiver may estimate the channel and the transmitted symbols using any known method (e.g., by using zeroforcing or minimum mean square error (MMSE) equalizers). In summary, the receiver may employ the signal corresponding to the subcarrier c0to compute estimates smof the transmitted symbols.

[0086] Next, the receiver may remove the effect of the data symbols sent by the network node 16a as follows. First, it multiplies the received samples corresponding to subcarrier c0by the inverse of the estimated symbols sn where rnare referred to herein as modified samples. Note that if the data transmitted by the network node 16a in the subcarrier c0includes a cyclic redundancy code (CRC), the receiver may verify with high probability whether the transmitter symbol snis equal to the estimated symbol sn. In what follows, assume that sn= sn. Then:

[0087] This is a conventional signal from an OFDM subcarrier, and any known method (e.g., zero-forcing or MMSE equalization) may be used to estimate the desired data symbol xnfrom the backscattering device 26. Moreover, a group of N symbols may be coherently combined to obtain a processing gain of 10Zo^l0(A), since one backscattered symbol is spread over N modulated symbols.

[0088] The above discussion assumes that 1) the channel is known or has been estimated, and 2) that the backscattering device has synchronized in time to the TX signal. The channel Hi be obtained by using pilot signals. For example, the first M symbols xlt... , xmmay be known beforehand at the receiver, and they may be used, together with r15. . . , fMN, to estimate the channel. These pilots may also be used to solve the second issue. By choosing the complex sequence xlt... , xmwith good correlation properties it is possible perform matched filtering or autocorrelation tests to determine which modified sample rn0includes the first symbol xt. The index nO is called the synchronization position.

[0089] The description above may be easily generalized to the case where the network node 16a sends an OFDM signal including two or more active subcarriers. In this case the transmitter sends data symbols sn k, where the index k indicates the subcarrier and the index n indicates the OFDM symbol. As before, the network node 16b receives simultaneously two signals, one centered at subcarrier c0which comes directly from the network node 16a, and a second reflected signal centered at subcarrier from the backscattering device 26. As before, assume that the separation in frequency between the subcarriers c0and is large enough that the receiver may filter out the interference from each other. Since the signal corresponding to c0is an ordinary OFDM signal, the network node 16b may use it to produce estimates of the transmitted symbols. Also as before, assume that due to channel coding and the CRC, the receiver may determine with high probability the exact value of the transmitted symbols sn k. The signal from the backscattering device 26 may be modeled as the superposition of several narrowband signals, each of which may be analyzed as described above.

[0090] Suppose that the OFDM signal sent by the network node 16a includes K subcarriers. Further, suppose that the signal is sampled K times in every OFDM symbol. In order to simplify the notation, consider the transmission of the first data symbol x}by the backscattering device 26. This symbol is spread over the first N OFDM symbols in the OFDM signal sent by the network node 16a. The received samples arise from a superposition of subcarriers, and the reflection coefficient x affects equally the phase and amplitude of all the subcarriers in a group of N consecutive OFDM symbols. Applying the discrete Fourier transform (DFT) a frequency domain model of the following form is obtained:

[0091] Rp,k = XiHksp>k+ Wp k, k = 0, ... , M - l, p = 1, ... , N

[0092] Once again, the effect of the transmitted symbols sn kmay be removed as follows:

[0093] From this set of MN modified samples it is possible to obtain an estimate of the symbol X- transmitted by the backscattering device 26 and the receiver may benefit from frequency diversity. As before, the network node 16b may perform channel estimation and synchronization with the help of some pilot symbols xlt... , xm.

[0094] The backscattering device 26 may determine when to start its transmission by, for example receiving an explicit control message from the network node indicating the time to start the backscatter transmission. Alternatively, or in addition, the backscattering device 26 may use its receiver to determine the timings of the start of the CP. For example, the network may send a command instructing the backscattering device 26 to initiate backscattering at a given clock tick with respect to a time base counter(s). Then, at a few clock ticks before the intended initiation time, the backscattering device 26 may use its receiver to detect the arrival of the illuminating OFDM signal (e.g., using an energy detector), and use this detection to determine the time boundaries of the OFDM time grid. Then it may align its transmission to the determined grid.

[0095] If the clock accuracy of the backscattering device 26 is good enough to ensure initiation of backscattering within the cyclic prefix of an OFDM symbol, then the number of OFDM symbols N over which the backscatter is spread may be taken to be as small as N = 1. Otherwise, the first OFDM symbol(s) in each sequence of N backscattered OFDM symbols received may need to be discarded. The reason is that a phase shift in the middle of an OFDM symbol (i.e., after the CP) may corrupt the estimates. Some embodiments may include one or more of the following:

[0096] Embodiment 1. An OFDM wireless system supporting backscattering devices 26, including a network node 16a, a network node 16b, a backscattering tag and a controller node, characterized in that:

[0097] • The controller selects a center frequency and bandwidth for an OFDM transmission from the network node 16a, a receiver bandwidth for the network node 16b, and a frequency shift and modulation symbol duration parameters for the backscattering device 26;

[0098] • The backscattering device 26 backscatters the OFDM signal and imparts a frequency shift to it, as indicated by the frequency shift parameter, and the duration of the backscattered modulation symbol, as indicated by the duration parameter, is equal or exceeds the OFDM symbol duration; and / or

[0099] • The indicated RX bandwidth is equal to or exceeds twice the nominal bandwidth of the OFDM direct link signal sent by the network node 16a. The network node 16b receives the superposition of the direct link signal and the backscattered signal. It separates the signals in the frequency domain, estimates the modulation symbols transmitted by the network node 16a and uses these estimates to remove their effect on the backscattered signal; and / or

[0100] Embodiment 2. As in Embodiment 1, where the backscattering device synchronizes in time its transmissions to the OFDM time grid, such that the modulation symbols start within the cyclic prefix of the OFDM symbols.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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).

[0106] 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.

[0107] 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 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

Claims:

1. A method in a backscattering device (26) configured to reflect signals received from a network node (16), the method comprising: receiving (S10) an orthogonal frequency division multiplexed, OFDM, signal in a first radio frequency channel from the network node (16); frequency shifting (S12) the received OFDM signal to a second radio frequency channel different from the first radio frequency channel; reflecting (S14) the frequency shifted received OFDM signal; and synchronizing (SI 6) reflecting of the frequency shifted received OFDM signal to an OFDM time grid.

2. The method of Claim 1, wherein an amount of the frequency shifting is determined by a parameter received from the network node (16).

3. The method of any of Claims 1 and 2, wherein synchronizing reflecting of the frequency shifted OFDM signal to an OFDM time grid includes synchronizing to a timing signal received from the network node (16).

4. The method of any of Claims 1-3, wherein synchronizing to the OFDM time grid causes modulation symbols to start within a cyclic prefix of OFDM symbols in the received OFDM signal.

5. The method of any of Claims 1-4, wherein a duration of a reflected modulation symbol is not less than an OFDM symbol duration.

6. A backscattering device (26) configured to reflect signals received from a network node (16), the backscattering device (26) configured to: receive an orthogonal frequency division multiplexed, OFDM, signal in a first radio frequency channel from the network node (16); frequency shift the received OFDM signal to a second radio frequency channel different from the first radio frequency channel; reflect the frequency shifted received OFDM signal; andsynchronize reflecting of the frequency shifted received OFDM signal to an OFDM time grid.

7. The backscattering device (26) of Claim 6, wherein an amount of the frequency shifting is determined by a parameter received from the network node (16).

8. The backscattering device (26) of any of Claims 6 and 7, wherein synchronizing reflecting of the frequency shifted OFDM signal to an OFDM time grid includes synchronizing to a timing signal received from the network node (16).

9. The backscattering device (26) of any of Claims 6-8, wherein synchronizing to the OFDM time grid causes modulation symbols to start within a cyclic prefix of OFDM symbols in the received OFDM signal.

10. The backscattering device (26) of any of Claims 6-9, wherein a duration of a reflected modulation symbol is not less than an OFDM symbol duration.

11. A method in a network node (16) configured to illuminate a backscattering device (26), the method comprising: configuring (SI 8) the backscattering device (26) with a frequency shift parameter indicating an amount by which to frequency shift a received orthogonal frequency division multiplexed, OFDM, signal from a first carrier frequency to a second carrier frequency to be reflected by the backscattering device (26) in synchronization with an OFDM time grid.

12. The method of Claim 11, wherein the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration.

13. The method of any of Claims 11 and 12, further comprising configuring the backscattering device (26) to reflect the received OFDM signal with a symbol duration that is an integer multiple of an OFDM symbol duration of the received OFDM signal.

14. The method of any of Claims 11-13, further comprising transmitting an illumination signal at the first carrier frequency, the illumination signal comprising OFDM symbols.

15. The method of any of Claim 11-14, further comprising configuring the backscattering device (26) with an indication of a time to start a reflection of a received OFDM signal.

16. A network node (16) configured to illuminate a backscattering device (26), the network node (16) configured to: configure the backscattering device (26) with a frequency shift parameter indicating an amount by which to frequency shift a received orthogonal frequency division multiplexed, OFDM, signal from a first carrier frequency to a second carrier frequency to be reflected by the backscattering device (26) in synchronization with an OFDM time grid.

17. The network node (16) of Claim 16, wherein the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration.

18. The network node (16) of any of Claims 16 and 17, wherein the network node (16) is further configured to configure the backscattering device (26) to reflect the received OFDM signal with a symbol duration that is an integer multiple of an OFDM symbol duration of the received OFDM signal.

19. The network node (16) of any of Claims 16-18, wherein the network node (16) is further configured to transmit an illumination signal at the first carrier frequency, the illumination signal comprising OFDM symbols.

20. The network node (16) of any of Claim 16-19, wherein the network node (16) is further configured to configure the backscattering device (26) with an indication of a time to start a reflection of a received OFDM signal.

21. A method in a first network node (16), comprising: simultaneously receiving (S20) a first orthogonal frequency division multiplexed, OFDM, signal, from a second network node (16) at a first carrier frequency and a second OFDM signal reflected from a backscattering device (26) at a second carrier frequency, the second OFDM signal being a frequency shifted reflection of the first OFDM signal; separating (S22) the first and second OFDM signals; and determining (S24) samples of OFDM symbols in the second OFDM signal based at least in part on samples of OFDM symbols in the first OFDM signal.

22. The method of Claim 21, wherein determining samples of the OFDM symbols in the second OFDM signal includes estimating samples of corresponding OFDM symbols in the first OFDM signal.

23. The method of Claim 22, wherein determining samples of the OFDM symbols in the second OFDM signal includes dividing OFDM samples in the second OFDM signal by corresponding estimates of the samples of OFDM symbols in the first OFDM signal.

24. The method of any of Claims 21-23, wherein a first symbol rate of the first OFDM signal is an integer multiple of a second symbol rate of the second OFDM signal.

25. The method any of Claim 21-24, wherein the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration.

26. A first network node (16) configured to: simultaneously receive a first orthogonal frequency division multiplexed, OFDM, signal, from a second network node (16) at a first carrier frequency and a second OFDM signal reflected from a backscattering device (26) at a second carrier frequency, the second OFDM signal being a frequency shifted reflection of the first OFDM signal; separate the first and second OFDM signals; anddetermine samples of OFDM symbols in the second OFDM signal based at least in part on samples of OFDM symbols in the first OFDM signal.

27. The first network node (16) of Claim 26, wherein determining samples of the OFDM symbols in the second OFDM signal includes estimating samples of corresponding OFDM symbols in the first OFDM signal.

28. The first network node (16) of Claim 27, wherein determining samples of the OFDM symbols in the second OFDM signal includes dividing OFDM samples in the second OFDM signal by corresponding estimates of the samples of OFDM symbols in the first OFDM signal.

29. The first network node (16) of any of Claims 26-28, wherein a first symbol rate of the first OFDM signal is an integer multiple of a second symbol rate of the second OFDM signal.

30. The first network node (16) any of Claim 26-29, wherein the first and second carrier frequencies are selected to coincide with subcarrier frequencies of a frequency raster of an OFDM configuration.

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