MIMO antenna

Abstract

Antenna arrays for use in MIMO devices are disclosed. Such antenna arrays can include a plurality of array elements, where every two elements in a first direction are digitally predistortionless linear elements, and every two elements in the first direction are nonlinear elements. In the antenna array, the spacing between adjacent elements in the first direction is less than one half of a free space wavelength. Nonlinear precoding is applied to transmissions from the antenna array, the nonlinear precoding converting out-of-band emissions of the transmissions to reactive power in the near field around the antenna array while ensuring that in-band signals generated by the elements are not affected.

Description

TECHNICAL FIELD

[0001] Various example embodiments relate to wireless communication. BACKGROUND

[0002] Wireless communication systems are continuously evolving. Advanced antenna technology, e.g. multiple-input multiple-output, MIMO, antennas, are used to increase the capacity of wireless connections without requiring more spectrum. SUMMARY

[0003] According to an apparatus comprising at least one antenna array, the antenna array comprises a plurality of array elements, wherein every two elements in a first direction are digital pre-distortion free linear elements, every two elements in the first direction are non-linear elements, and a spacing between adjacent elements in the first direction is less than half a free space wavelength; at least one processor; and at least one memory including computer program code and reference information, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: applying a non-linear precoding to a transmission from the antenna array, the non-linear precoding converting out-of-band emissions of the transmission into reactive power in a near field around the antenna array while ensuring that in-band signals generated by the elements are not affected; and transmitting the in-band signals from the antenna array.

[0004] In an embodiment, the spacing between adjacent elements in the first direction is equal to or less than a quarter of the free space wavelength.

[0005] In an embodiment, the antenna array is a two-dimensional array, the two-dimensional array comprising elements of the same type in a second direction, and a spacing between adjacent elements in the second direction is equal to or greater than half the free space wavelength.

[0006] In an embodiment, the apparatus further comprises a linear power amplifier for amplifying the out-of-band signals after precoding.

[0007] In an embodiment, the plurality of digital pre-distortion free linear elements are hybrid elements, wherein outputs of a first number of power amplifiers in a transmission chain are connected through phase shifters and combiners to a second number of radiating antennas, the second number being greater than the first number.

[0008] In an embodiment, the at least one memory and the computer program code are configured to, with the at least one processor, further cause the apparatus to perform the non-linear precoding using results of a process, the process comprising initializing a matrix representation of time domain signals generated by the non-linear elements to constant envelope signals closest to a zero-forcing solution, and performing a predefined number of iteration rounds.

[0009] In an embodiment, the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to further perform, during the iteration rounds: randomly selecting coordinates for elements in a matrix; calculating values for the elements such that the values have a magnitude of one while other elements have a fixed value by minimizing a sum of mean squared errors between a received signal and an expected signal at the serving apparatus under constraints that a Lagrange multiplier weighted for out-of-band emissions is added to the sum of mean squared errors, one constraint ensures that linear elements do not transmit in-band, another constraint limits the power spectral density of the linear elements, and yet another constraint ensures that a signal transmitted by the nonlinear elements has a constant envelope in time domain; repeating the selecting and calculating until all elements have calculated values; and updating a matrix representation of a frequency domain signal transmitted by the linear elements accordingly.

[0010] According to an aspect, there is provided a method comprising: applying a nonlinear precoding to a transmission from at least one antenna array, the antenna array comprising a plurality of array elements, wherein every two elements in a first direction are digital pre-distortionless linear elements, every two elements in the first direction are nonlinear elements, and a spacing between adjacent elements in the first direction is less than half a free-space wavelength, the nonlinear precoding converting out-of-band emissions of the transmission to reactive power in a near-field around the antenna array while ensuring that in-band signals generated by the elements are not affected; and transmitting the in-band signals from the antenna array.

[0011] In an embodiment, the method further comprises performing the nonlinear precoding using results of a process, the process comprising initializing a matrix representation of time domain signals generated by the nonlinear elements to constant envelope signals closest to a zero-forcing solution, and performing a predefined number of iteration rounds.

[0012] In an embodiment, the method further comprises performing, during the iteration rounds: randomly selecting coordinates for elements in a matrix; calculating values for the elements such that the values have a magnitude of one while other elements have a fixed value by minimizing a sum of mean squared errors between a received signal and an expected signal at the serving apparatus under constraints that a Lagrange multiplier weighted for out-of-band emissions is added to the sum of mean squared errors, one constraint ensures that linear elements do not transmit in-band, another constraint limits the power spectral density of the linear elements, and yet another constraint ensures that a signal transmitted by the nonlinear elements has a constant envelope in time domain; repeating the selecting and calculating until all elements have calculated values; and updating a matrix representation of a frequency domain signal transmitted by the linear elements accordingly.

[0013] According to an aspect, there is provided a computer readable medium comprising program instructions stored thereon for performing at least the following: applying a non-linear precoding to a transmission from at least one antenna array, the antenna array comprising a plurality of array elements, wherein every two elements in a first direction are digital pre-distortion free linear elements, every two elements in the first direction are non-linear elements, and a spacing between adjacent elements in the first direction is less than half a free space wavelength, the non-linear precoding converting out-of-band emissions of the transmission to reactive power in a near field around the antenna array while ensuring that in-band signals generated by the elements are not affected; and transmitting the in-band signals from the antenna array.

[0014] In an embodiment, the computer readable medium further comprises program instructions stored thereon for performing the non-linear precoding using a process comprising initializing a matrix representation of time domain signals generated by the non-linear elements to a constant envelope signal closest to a zero-forcing solution, and performing a predefined number of iteration rounds.

[0015] In an embodiment, the computer readable medium is a non-transitory computer readable medium.

[0016] According to an aspect, there is provided a computer program comprising instructions for performing at least the following: applying a non-linear precoding to a transmission from at least one antenna array, the antenna array comprising a plurality of array elements, wherein every two elements in a first direction are digital pre-distortion free linear elements, every two elements in the first direction are non-linear elements, and a spacing between adjacent elements in the first direction is less than half a free space wavelength, the non-linear precoding converting out-of-band emissions of the transmission to reactive power in a near field around the antenna array while ensuring that in-band signals generated by the elements are not affected; and transmitting the in-band signals from the antenna array.

[0017] In an embodiment, the computer program further comprises instructions for performing the non-linear precoding using a process comprising initializing a matrix representation of time domain signals generated by the non-linear elements to a constant envelope signal closest to a zero-forcing solution, and performing a predefined number of iteration rounds. BRIEF DESCRIPTION OF DRAWINGS

[0018] Embodiments are described below by way of example only and with reference to the accompanying drawings, in which:

[0019] Figure 1 Fig. illustrates an exemplary wireless communication system;

[0020] Figure 2 Fig. illustrates an example of an array arrangement and a transmission chain;

[0021] Figure 3 Fig. illustrates an array arrangement from another perspective;

[0022] Figure 4 Figure illustrates an example of a mixed linear transmission chain;

[0023] Figure 5 Figure illustrates a power spectral density;

[0024] Figure 6 、 Figure 7 and Figure 8 is a flowchart illustrating functionality;

[0025] Figure 9 Figure illustrates another power spectral density; and

[0026] Figure 10 is a schematic block diagram. DETAILED DESCRIPTION

[0027] The following embodiments are examples. Although the specification can

[0028] In the following, different exemplary embodiments will be described as examples of access architectures to which the embodiments can be applied, however, without restricting the embodiments to such architectures. The embodiments can also be applied to other kinds of communication networks with suitable components by adjusting parameters and procedures accordingly. Some examples of other options for applicable systems are the Universal Mobile Telecommunication System (UMTS) radio access network (UTRAN or E-UTRAN), Long Term Evolution (LTE, same as E-UTRA), Wireless Local Area Network (WLAN or WiFi), Worldwide Interoperability Microwave Access (WiMAX), Bluetooth Wideband Code Division Multiple Access (WCDMA), systems using Ultra- Wideband (UWB) technology, sensor networks, Mobile Ad-Hoc Networks (MANET) and Internet Protocol Multimedia Subsystem (IMS) or any combination thereof.

[0029] Figure 1 An example of a simplified system architecture is depicted, only some elements and functional entities are shown, all being logical units whose implementation can differ from what is shown. Figure 1 The connections to the logical elements are shown as lines, which represent the interaction of logical elements. Figure 1 The shown functionalities and structures.

[0030] However, embodiments are not limited to the system 100 given as an example, but a person skilled in the art can apply the solution to other communication systems provided with the necessary properties.

[0031] Figure 1 An example shows a part of an exemplary radio access network.

[0032] Figure 1 User equipment 101, 101' configured to wirelessly connect with a node 102 over one or more communication channels is shown. The node 102 is further connected to a core network 105. In one example, the node 102 can be an access node such as an eNodeB / gNodeB (e / g) which provides or serves devices in a cell. In one example, the node 102 can be a non-3GPP access node. The physical link from a device to an (e / g)NodeB is called uplink or reverse link and the physical link from the (e / g)NodeB to the device is called downlink or forward link. It should be understood that the (e / g)NodeB or its functionalities can be implemented by using any entity, host, server or access point etc. suitable for such usage.

[0033] A communication system typically comprises more than one (e / g)node B, which in this case can also be configured to communicate with each other over links, wired or wireless, designed for the purpose. These links can be used for transmission of signalling purposes. The (e / g)node B is a computing device configured to control the radio resources of the communication system it has been coupled to. The node B can also be referred to as a base station, access point or any other type of interfacing device including a relay station capable of operating in a wireless environment. The (e / g)node B includes or is coupled to a transceiver. From the transceiver of the (e / g)node B, a connection is provided to an antenna unit that establishes the double directional radio link to the device. The antenna unit can comprise a plurality of antennas or antenna elements. The (e / g)node B is further connected to a core network 105 (CN or Next Generation Core NGC). Depending on the different systems, the counterpart on the CN side can be a serving gateway (S-GW, routing and forwarding user data packets), a packet data network gateway (P-GW) for providing connectivity of the user equipment (UE) to external packet data networks, or a mobile management entity (MME), or an access and mobility management function (AMF), etc.

[0034] A user equipment (also referred to as UE, user equipment, user terminal, terminal device, etc.) illustrates one type of apparatus to which resources on the air interface are allocated and assigned, so any features described herein with respect to a user equipment can be implemented with a corresponding apparatus, such as a relay node. An example of such a relay node is a layer 3 relay towards a base station (self-backhauled relay).

[0035] User equipment generally refers to a device (e.g., a portable or non-portable computing device) that includes a wireless mobile communication device operating or not operating in a subscription identification module (SIM) of the device, including, but not limited to, the following types of devices: a mobile station (mobile phone), a smartphone, a personal digital assistant (PDA), a handset, a device using a wireless modem (alarm or measurement device, etc.), a laptop and / or touch screen computer, a tablet, a game console, a notebook, and a multimedia device. It should be appreciated that a device can also be a nearly exclusive uplink only device, an example of which is a camera or video camera that loads images or video clips to a network. A device can also be a device that has the capability to operate in an Internet of Things (IoT) network, which is a scenario in which objects are provided with the ability to transfer data over a network, without requiring human-to-human or human-to-computer interaction, such as for smart grids and interconnected vehicles. A user equipment can also utilize cloud. In some applications, a user equipment can include a user portable device with a radio part, such as a watch, earphone, glasses, other wearable accessory or wearable device, and the computing is implemented in the cloud. The device (or in some embodiments a layer 3 relay node) is configured to perform one or more user equipment functions. A user equipment can also be referred to as a subscriber unit, mobile station, remote terminal, access terminal, user terminal, or user equipment (UE), to name a few names or apparatuses.

[0036] The various techniques described herein can also be applied to cyber-physical systems (CPS) (systems of collaborating computational elements controlling physical entities). CPS can enable the implementation and exploitation of massive amounts of interconnected ICT devices (sensors, actuators, processors microcontrollers, etc.) embedded in physical objects at different locations. Mobile cyber-physical systems, where the physical systems move, are a subclass of cyber-physical systems. Examples of mobile cyber-physical systems include moving robots and electronics transported by humans or animals.

[0037] Furthermore, although the apparatus is depicted as a single entity, different units, processors and / or memory units can be implemented (not all shown in the figure). Figure 1

[0038] ​5G enables the use of multiple input - multiple output (MIMO) antennas, many more base stations or nodes than the LTE (the so-called small cell concept), including macro sites operating in co-operation with smaller stations and adopting a variety of radio technologies depending on service needs, use cases and / or available frequency spectrum. 5G mobile communications supports a wide range of use cases and related applications including video streaming, augmented reality, different ways of data sharing and various forms of machine type communications such as (massive) machine type communications (mMTC), including vehicular safety, different sensors and real-time control. 5G is expected to have multiple radio interfaces, i.e. below 6GHz, cmWave and mmWave, and it can also be integrated with the existing legacy radio access technologies, such as the LTE. The integration with the LTE can be implemented as a system at least in the early phase, where macro coverage is provided by the LTE and 5G radio interfaces access from small cells by aggregating to the LTE. In other words, 5G plans to support inter-RAT operability (such as LTE-5G) and inter-RI operability (radio interface operability, such as below 6GHz-cmWave, below 6GHz-cmWave-mmWave). One of the concepts considered for use in 5G networks is network slicing, where multiple independent and dedicated virtual subnets (network instances) can be created within the same infrastructure to run services that have different requirements on latency, reliability, throughput and mobility.

[0039] The current architecture in the LTE network is fully distributed in radio and fully centralized in the core network. The low latency applications and services requirements in 5G bring the content close to the radio, resulting in local breakouts and multi-access edge computing (MEC). 5G enables analytics and knowledge generation to occur where data is generated, close to the sources themselves. Such an approach requires leveraging resources that can not have persistent connectivity to the network, such as laptops, smartphones, tablets and sensors. MEC provides a distributed computing environment for application and service hosting. It is also capable of storing and processing content in close proximity to the cell's subscribers for faster response time. Edge computing encompasses a wide range of technologies such as wireless sensor networks, mobile data acquisition, mobile signature analysis, cooperative distributed peer-to-peer networks and processing, also classifiable as local cloud / fog computing and grid / mesh computing, dew computing, mobile edge computing, subcloud, distributed data storage and retrieval, self-healing networks, remote cloud services, augmented and virtual reality, data caching, Internet of Things (massive connectivity and / or delay critical), critical communications (autonomous vehicles, traffic safety, real-time analytics, time-critical control, medical applications).

[0040] The communication system is also able to communicate with other networks, such as a public switched telephone network or the Internet 106, or with services provided by them. The communication system can also support the use of cloud services, for example, at least part of the core network operations can be executed as a cloud service (this is illustrated in Fig. 1 by the cloud 107). Figure 1The communication system also comprises a core network 106 operated by the same or another operator than the RAN 100. The core network 106 can comprise a Home Subscriber Server (HSS) 107 that keeps track of the subscribers registered to the communication system. The communication system also comprises a central control entity, etc. that provides facilities for networks of different operators to cooperate, e.g. in terms of spectrum sharing.

[0041] By utilizing Network Function Virtualization (NFV) and Software-Defined Networking (SDN), edge cloud technology can be introduced in the Radio Access Network (RAN). Using edge cloud technology can mean that node operations are performed at least partly in servers, hosts, or nodes that are operably coupled to remote radio heads or base stations that include radio parts. It can also be possible that node operations are distributed among a number of servers, nodes, or hosts. The application of cloud RAN architecture enables RAN real-time functions to be implemented at the RAN side (in the Distributed Unit, DU 102) and non-real-time functions to be implemented in a centralized manner (in the Centralized Unit, CU 104).

[0042] It should also be understood that the distribution of labor between core network operations and base station operations can be different from the LTE, or even non-existent. Some other technological advancements that can be used are Big Data and All-IP, which can change the way networks are built and managed. 5G (or New Radio, NR) networks are designed to support multiple hierarchies, where MEC servers can be placed between the core and the base station or the core and the NodeB (gNB). It should be understood that MEC can also be applied to 4G networks.

[0043] 5G can also utilize satellite communication to enhance or complement the coverage of 5G services, for example by providing backhauling. Possible use cases include providing service continuity for machine-to-machine (M2M) or Internet of Things (IoT) devices or for passengers on board of vehicles, or ensuring service availability for critical communications, and future railway / ship / aeronautical communications. Satellite communication can utilize Geostationary Earth Orbit (GEO) satellite systems, but also Low Earth Orbit (LEO) satellite systems, particularly mega-constellations (systems that deploy hundreds of (nano)satellites). Each satellite 103 in a mega-constellation can cover several network entities of the support satellite that create ground cells. Ground cells can be created by ground relay nodes 102 or by gNBs located in the ground or in the satellite.

[0044] It is obvious for a person skilled in the art that the depicted system is only an example of a part of a radio access system and in practice the system can comprise a plurality of (e / g)node Bs, the user equipment can access a plurality of radio cells and the system can also comprise other apparatuses such as physical layer relay nodes or other network elements. At least one of the (e / g)node Bs can be a home (e / g)node B. In addition, in the geographical area of the radio communication system a plurality of different kinds of radio cells can be provided as well as a plurality of radio cells. The radio cells can be macro cells (or umbrella cells), which are large cells, typically with a diameter of up to tens of kilometers, or smaller cells such as micro cells, femto cells or pico cells. Figure 1 The (e / g)node Bs can provide any of these cells. The cellular radio system can be implemented as a multi-tier network comprising a plurality of cells. Typically, in a multi-tier network one access node provides one or more cells, thus requiring a plurality of (e / g)node Bs to be required to provide such a network structure.

[0045] To meet the need for improving the deployment and performance of communication systems, the concept of a "plug and play" (e / g)node B has been introduced. Typically, a network capable of using "plug and play" (e / g)node Bs comprises a home node B gateway or HNB-GW (not shown in the figure) in addition to home (e / g)node Bs, H(e / g)node Bs. Figure 1 The HNB gateway (HNB-GW), which is typically installed within the operator's network, can aggregate traffic from a large number of HNBs back to the core network.

[0046] Figure 2 is a schematic block diagram illustrating an example of an antenna array arrangement for a two-dimensional antenna array 200, describing the basic structure and concepts on a generalized level for illustrative purposes only. It should be appreciated that the antenna array can also comprise other elements and comprise any number of the illustrated elements. Furthermore, an apparatus comprising one or more antenna arrays, such as a radio unit or a base station or an access node, can also comprise other elements, such as elements for controlling the operation. The principles disclosed for the two-dimensional antenna array are applicable to one-dimensional antenna arrays and their implementation is simple and obvious for a person skilled in the art.

[0047] The antenna array 200 comprises a plurality of array elements of two different types, which types are non-linear elements 211 and digital non-pre-distorted linear elements 212, or simply linear elements. The non-linear elements are conventional antenna elements connected to a non-linear radio frequency chain. The linear elements are conventional antenna elements connected to a linear radio frequency chain. In the antenna array 200, the first direction (x-direction) is the direction of the array elements and the second direction (y-direction) is the direction of the rows of the array elements. Figure 2Each two elements 211a, 211b in the horizontal direction (in the plane of the drawing) is a nonlinear element, and each two elements 212a, 212b in the first direction is a digital predistortion-free linear element. The spacing 201 between adjacent elements in the first direction (inter-element spacing), i.e. the spacing between a nonlinear element and a digital predistortion-free linear element, is less than half the wavelength (wavelength of the radiation signal). Such a spacing results in adjacent antenna elements coupling to each other. The spacing 202a between elements of the same type in the first direction (intra-element spacing) is at least the sum of two consecutive inter-element spacings 201. It should be understood that even though the elements are uniformly spaced in the first direction in the illustrated example, the elements can be non-uniformly spaced, provided that the inter-element spacing is less than half the wavelength.

[0048] In the illustrated example, the antenna array 200 comprises antenna elements of the same type in the second direction (vertical direction in the plane of the drawing). In other words, in the second direction, adjacent antenna elements are either nonlinear elements 211a, 211c or digital predistortion-free linear elements 212a, 212c. The spacing 202b between antenna elements in the second direction is independent of the spacing in the first direction. Typically, the spacing is greater than or equal to half the wavelength. Figure 2

[0049] In an implementation, the inter-element spacing 201 is equal to or less than a quarter of the wavelength, and the elements are uniformly spaced in the first direction, resulting in an intra-element spacing 202a equal to or less than half the wavelength, and the spacing 202b in the second direction can be the same as the intra-element spacing 202a, i.e. equal to or greater than half the wavelength. In other words, in said implementation, the antenna array 200 is up-sampled by a factor of 2 in the first direction.

[0050] Figure 2 Different types of components in a transmission chain, i.e. in a radio frequency chain, are also illustrated according to an implementation.

[0051] The transmission chain of a nonlinear element 211 comprises a local oscillator (LO) 211-1, a phase modulator 211-2, e.g. a voltage controlled oscillator, a power amplifier 211-3, a band pass filter (BPF) 211-4, and an antenna 211-5. The local oscillator 211-1 is connected to the phase modulator 211-2, which in turn is connected to the power amplifier 211-3. The power amplifier 211-3 is connected to the band pass filter 211-4, to which the antenna 211-5 is connected. The power amplifier can be a nonlinear power amplifier, such as a class E / F power amplifier, or a linear power amplifier, e.g. a class AB power amplifier. The transmission chain can also comprise a power amplifier driver before the power amplifier, and / or a circulator (radio frequency circulator) after the band pass filter for isolating the transmitter from the receiver, i.e. for preventing the transmitter from interfering with the receiver. Figure 2 ​These elements are not shown in the figure.

[0052] In the illustrated example, the transmission chain of the digital non-pre-distortion linear element 212 comprises digital-to-analog converters (DAC) 212-6, 212-6' for the in-phase (I) and quadrature (Q) signals, two mixers 212-7, 212-7', a phase shifter 212-8, a local oscillator (LO) 212-1, a combiner (adder) 212-9, a power amplifier 212-3, for example a non-linear power amplifier such as a class E / F power amplifier or a linear power amplifier such as a class AB power amplifier, a band-pass filter 212-4 and an antenna 212-5. More precisely, each converter has one mixer, one 212-7' of the mixers directly receives the output of the local oscillator 212-1, the other mixer 212-7 receives the output of the local oscillator 212-1 via the phase shifter 212-8, the outputs of the mixers are combined by the combiner 212-9, the combined output signal is input to the power amplifier 212-3 which is then filtered by the filter 212-4 before being radiated from the antenna 212-5. Also in this document, the transmission chain can also comprise a power amplifier driver before the power amplifier and / or a circulator after the band-pass filter, even if these elements are not shown in the figure. Figure 2

[0053] In other words, the linear elements 212a, 212b, 212c do not contain digital pre-distortion elements and feedback receiver elements that linearize the power amplifier. For example, the power amplifier can be biased in the linear region.

[0054] It should be understood that the two-dimensional antenna array 200 can comprise any number of elements in the first direction, for example 8 or 12, and any number of elements in the second direction, for example 3 or 4, without being limited to the examples of solutions.

[0055] The components after the power amplifier, for example the band-pass filters 211-4, 212-4 and the antennas 211-5, 212-5 are passive, and thus linear and time-invariant. Therefore, it can be assumed that all components after the power amplifier in the transmission chain, whether the transmission chain is non-linear or linear, are linear and time-invariant. Given the relationship assumptions of an antenna array comprising M NL non-linear elements and M L linear elements, the relationship between the current and the voltage at the output of the power amplifier can be expressed as:

[0056] v(f) = Z(f)i(f) (1)

[0057] where

[0058] ​is the voltage on the M output antenna elements at frequency f,

[0059] is the corresponding current, and

[0060] is the impedance matrix

[0061] i.e. the total number of antenna elements (nonlinear elements and linear antenna elements)

[0062] the real part of the impedance matrix is the resistance component, the imaginary part is the reactance component. The power radiated by the antenna array is the resistive power, given by

[0063]

[0064] where

[0065] P rad (f) is the radiated power, and

[0066] is the resistive element.

[0067] Due to the mutual coupling, the real part of the M L +M NL has the following interesting mathematical property:

[0068]

[0069] In other words, the matrix is rank-deficient and has a M L dimensional null space. This means that the current in the out-of-band transmission frequencies can be chosen such that the radiated power in the out-of-band transmission frequencies can be made zero, i.e. i(f) can be chosen such that:

[0070] out-of-band frequencies

[0071] In essence, all the power in the out-of-band frequencies is converted to reactive power in the near field around the antenna array without dissipation, thereby reducing the out-of-band emission. However, since the current from the linear transmission chain in the in-band frequencies is zero, the mutual coupling does not affect the radiated power in the in-band frequencies.

[0072] Figure 3 illustrates the antenna array arrangement of Figure 2 or simply the antenna array.

[0073] Reference is made to Figure 3The antenna array includes a shared nonlinear pre-encoder element 301, which converts the out-of-band emissions generated by nonlinear elements 211a and 211c into reactive power, as will be discussed below. Figure 6 and Figure 7 To describe in more detail.

[0074] Figure 4 The diagram illustrates the process of... Figure 2 Chain 212 illustrates alternative implementations of the transmission chain (RF chain) for different linear elements. The alternative solution is based on a hybrid arrangement where multiple linear elements are driven by a single element via a phase shifter (RF phase shifter) transmission chain.

[0075] exist Figure 4 In the example shown, the components preceding the power amplifier are not shown, but it is assumed that power amplifier 212-3 receives output 401 from the combiner. Figure 4 Not shown in the image, in Figure 2 (Shown in 212-9). In the example shown, four radiating antennas are fed via three power amplifiers 212-3. An output 402 from one power amplifier 212-3 is fed to four phase shifters 212-10 to be phase-shifted. The phase shifter outputs are input to a combiner 212-11, such that a phase-shifted output from each power amplifier is fed to the combiner 212-11, which combines the three phase-shifted outputs. The combined output is then input to antenna 212-5 via a bandpass filter 212-4 to be radiated. More precisely, in the example shown, for a linear element, each amplified output 402, the output is phase-shifted, and three phase-shifted outputs from different amplified outputs are combined. At a more general level, there is a first number of power amplifiers, a second number of antennas, a second number of combiners, and a third number of phase shifters, the second number being greater than the first number, and the third number being the product of the first and second numbers. In other words, the outputs of the power amplifiers are connected in parallel to a second number of phase shifters, and the outputs of the phase shifters are connected to a combiner, such that the combiner combines a first number of phase-shifted outputs, one phase-shifted output for each power amplifier.

[0076] Figure 5 The diagram illustrates the power spectral density of an antenna array comprising linear and nonlinear antenna elements according to the examples above.

[0077] refer to Figure 5 The power spectral density 501 of the nonlinear element is depicted by a dashed line, and the power spectral density 502 of the digital predistortion-free linear element is also depicted by a dashed line. Figure 5In the middle, the system bandwidth, i.e. the in-band, is indicated by the band arrowed line 503 and for clarity of the figure, the spectrum mask 504 is depicted by its start part. The spectrum mask will continue on the start level until it rises by an amount 505 at the start of the system bandwidth and it will fall by an amount 505 at the end of the system bandwidth and continue from there on the start level. In case the start level is zero, the amount 505 depicts the in-band transmission power level.

[0078] The power spectral density 501 of the nonlinear element has a shoulder in the out-of-band region, i.e. the region not within the system band 503, which is the generated out-of-band emission. From the power spectral density 502 of the digital pre-distortionless linear element it can be seen that the linear element only transmits in the out-of-band region and at the same location the power level from the nonlinear element is transmitted as out-of-band emission. For example, if the in-band transmission level is 45 dB, then the out-of-band transmission level is approximately 15 dB, i.e. approximately 30 dB lower than the in-band transmission power level, as indicated by the amount 506. Due to the small transmission power of the linear element, the power efficiency of the power amplifier in the linear element transmission chain can be low. Therefore, a class AB power amplifier can be used.

[0079] Figure 6 is a flowchart of the function performed by an antenna array comprising linear and nonlinear elements as described above (antenna array arrangement) or by an apparatus comprising one or more such antenna arrays to ensure that the in-band signal is unaffected and radiated from the antenna array with maximum efficiency.

[0080] Reference Figure 6 In block 601 nonlinear precoding is applied to the transmission from the antenna array, which converts the out-of-band emission of the transmission into reactive power in the near field around the antenna array and is not dissipated while ensuring that the in-band signal is generated by the elements, or from Figure 5 It can be seen in that the in-band signal generated by the nonlinear elements remains unaffected. The in-band signal is then transmitted (radiated) from the antenna array in block 602.

[0081] Figure 7 Fig. illustrates an example of how the nonlinear precoder obtains a nonlinear precoding algorithm, i.e. the algorithm that can be used in block 601 in Figure 6 The example of the obtained nonlinear precoding algorithm is based on the assumption that all components in the transmission chain after the power amplifier, whether the transmission chain is nonlinear or linear, are linear and time-invariant, as discussed in Figure 2 Figure 7 The nonlinear precoding algorithm shown in solves the optimization problem using a greedy coordinate descent to determine the currents i(f) so that the radiated power at the out-of-band frequencies is zero. ​

[0082] Suppose that K devices are served using an antenna array comprising M NL nonlinear elements and M L linear elements, and that it transmits a signal, e.g., an orthogonal frequency-division multiplexing (OFDM) signal, using N subcarriers, where N U occupied subcarriers and N Z empty subcarriers result in an oversampling rate of N / N U , the optimization problem can be expressed as:

[0083]

[0084] subject to the constraints:

[0085] X = AF

[0086]

[0087]

[0088]

[0089]

[0090] where

[0091] is a matrix representation of the time-domain signal generated by the nonlinear elements,

[0092] is a matrix representation of the frequency-domain signal transmitted by the linear elements,

[0093] is a time-domain representation of the transmitted signal,

[0094] is a channel between the M NL nonlinear elements and the K devices,

[0095] is a matrix representation of the frequency-domain signal generated by the nonlinear elements,

[0096] e[n] is the nth column of the matrix E,

[0097] x[n] is the nth column of the matrix X,

[0098] is a discrete Fourier transform (DFT) matrix,

[0099] a[n] is the nth column of the matrix A,

[0100] a mn is the (m, n)th element of the matrix A

[0101] N = total number of subcarriers (including zero and used subcarriers),

[0102] L is the number of samples of the channel impulse response, and

[0103] a is a factor.

[0104] The first constraint of the optimization problem mentioned first, i.e. X = AF, relates to the time and frequency domain signals transmitted by the linear element. The second constraint, i.e. ensures that the linear element does not transmit in-band. The third constraint will limit the out-of-band (OOB) emission on the subcarriers in to be less than e. The fourth constraint, i.e. limits the power spectral density of the linear element to be less than h. The last constraint, i.e. ensures that the signal transmitted by the nonlinear element has a constant envelope in the time domain.

[0105] The Lagrangian function of the optimization problem described above is as follows:

[0106]

[0107] Constraints:

[0108]

[0109]

[0110]

[0111] where

[0112] λ OOB is the Lagrange multiplier that weights the out-of-band (OOB) emission, whose value can be set freely.

[0113] Then, as described above, the above Lagrangian of the optimization problem is used to determine the nonlinear precoding algorithm by repeating Figure 7 the predefined iteration rounds described. In other words, the sum of the mean square errors between the received signal at the serving device and the desired signal (i.e. the best signal) is minimized under the following constraints, to which the Lagrange multiplier that weights the out-of-band emission is added, one constraint ensures that the linear element does not transmit in-band, another constraint limits the power spectral density of the linear element and other constraints that ensure that the signal transmitted by the nonlinear element has a constant envelope in the time domain.

[0114] Reference is made to Figure 7The matrix representation of the time domain signal generated by the nonlinear element, i.e. A, is initialized in block 701 to the constant envelope signal closest to the zero-forcing solution. Furthermore, in the illustrated example, the iteration round counter round number is also initialized to zero in block 701. However, it should be appreciated that any other way can be used to keep track of the number of iteration rounds. The iteration rounds are then performed. The iteration rounds are started by randomly selecting a coordinate (m, n) in the matrix coordinates of the matrix A that have not been selected during the current iteration round in block 702. In another implementation, the coordinates can be selected in the matrix coordinates regardless of whether they have been selected earlier during the current iteration round.

[0115] Then in block 703, the optimal value of the (m, n)th element of the matrix A, a mn is calculated using the optimization problem described above, such that all other elements in the matrix A have fixed values (initial values, or values calculated during the iterations). Then in block 704 the element value of the element a mn in the matrix A is replaced with the value calculated in block 703. Then in block 705 it is checked whether all coordinates of the matrix A have undergone blocks 703 and 704 during the current iteration round. If not (block 705: No), the process returns to block 702 to randomly select a coordinate.

[0116] When all coordinates of the matrix A have undergone blocks 703 and 704 during the current iteration round (block 705: Yes), the matrix representation E and a of the frequency domain signal transmitted by the linear element is updated in block 706. The matrix representation E is initialized to zero and, at the update, the above optimization problem for e[n] is solved by keeping x[n] and the matrix A fixed. Furthermore, in the illustrated example, the iteration round counter round number is also updated by incrementing its value by one. Then in step 707 it is checked whether the iteration round number indicated by the iteration round counter is the same as a preset target iteration round number. The preset target number can be freely set, the larger the value, the longer the time to determine the nonlinear precoding algorithm, but the more accurate. For example, a sufficiently accurate precoding algorithm that allows real-time use can be obtained by performing 10 rounds or 20 rounds of iterations.

[0117] If the iteration round number has not reached the target iteration round number (block 707: No), the process starts another iteration round and returns to block 702 to randomly select a coordinate. If the target number of iteration rounds is performed (block 707: Yes), the solution thus obtained can be used as (block 708) the nonlinear precoder (nonlinear precoding algorithm).

[0118] Figure 7 how to obtain the nonlinear precoding algorithm for the nonlinear precoder illustrated in the above-described example, i.e. can be performed in the following way: Figure 6The algorithm used in block 601 in the above description describes a very specific way. Figure 8 A more general approach of how to obtain a nonlinear precoding algorithm is illustrated.

[0119] With reference to Figure 8 In block 801 the matrix representation of the time domain signal generated by the nonlinear element (i.e. A) is initialized to a constant envelope signal closest to the zero-forcing solution. Then in block 802 a predefined number of iteration rounds are performed resulting in an algorithm that can be used in block 601.

[0120] By changing the used set of subcarriers N u and the set of empty subcarriers N z The nonlinear precoding algorithm can be adjusted to have a wider operational bandwidth or a smaller operational bandwidth also including out-of-band emission by setting the sets such that the signal generated by the nonlinear element is such that the out-of-band emission is unilaterally with respect to the system bandwidth as shown in Figure 9 Figure 9 The same reference signs are used. Figure 5 The difference between Figure 9 and Figure 5 is that the power spectral density 501 of the nonlinear and linear transmission chains is unilateral with out-of-band emission to the right of the system bandwidth 503. In such a solution the operational bandwidth of the linear element can then be unilateral of the out-of-band emission to allow the linear transmission chain to be confined to a small bandwidth, in the shown example by the out-of-band emission. Non-limiting examples related to the used sets of subcarriers and sets of empty subcarriers include, if N = 64, one set with wider operational bandwidth can be N u = {17,..., 48} and N z = {1,.., 16, 49,.., 64} and the other set changing the smaller operational band can be N u = {1,..., 32} and N z = {33,..., 64}.

[0121] The blocks and related functions described above by Figures 2 to 9 are not in absolute time order and some of them can be executed simultaneously or in a different order than given. Other functions can also be executed between them or within them and other information is sent between the screenshots and / or other rules are applied or selected. Some of the blocks or parts of the blocks or one or more pieces of information can also be omitted or replaced by corresponding blocks or parts of the blocks or one or more pieces of information.

[0122] Figure 10 ​Fig illustrates an apparatus 1000 comprising means for implementing any of the above described embodiments / examples / realizations. The apparatus can comprise at least one processor or processing circuitry, and at least one memory 1020 including computer program code (software, algorithms) ALG 1021, wherein the at least one memory and the computer program code (software, algorithms) are configured to, with the at least one processor, cause the apparatus to implement any of the above described embodiments, examples and realizations. The apparatus 1000 can be an electronic device, such as a network apparatus, a user equipment, a radio unit, a base station, an access node or a terminal device in a vehicle, other examples are listed above together with the description of the electronic device. The at least one processor or processing circuitry can implement a communication controller 1010 controlling communication in the radio interface. Figure 1

[0123] The apparatus 1000 can further comprise an application processor (not shown) executing one or more computer program applications generating requirements for transmitting and / or receiving data via the communication controller 1010. The application processor can form an application layer of the apparatus. The application processor can execute computer programs forming main functions of the apparatus. For example, if the apparatus is a computer system of a vehicle, the application processor can execute media applications and / or autonomous driving and navigation applications.

[0124] Referring to Figure 10 , the memory 1020 can be implemented using any suitable data storage technology, such as semiconductor-based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The memory can comprise a configuration storage CONF. 1022, such as a configuration database, for storing configuration parameters, e.g. for the non-linear precoding algorithm. The memory 1020 can also store other data, such as a data buffer for data waiting for processing (including transmission).

[0125] Referring to Figure 10 , the apparatus comprises a communication interface 1030 comprising hardware and / or software for implementing communication connections according to one or more wireless and / or wired communication protocols. The communication interface 1030 provides the apparatus with radio communication capabilities. The communication interface 1030 comprises one or more antenna arrangements with linear and non-linear elements (L and n-LAA) 1031 according to any of the above described embodiments / examples / realizations, and it can comprise other standard well-known components. Digital signal processing regarding transmission and reception of signals can be performed in the communication controller 1010. The communication controller 1010 comprises a non-linear precoding circuit 1011 (n-L prec) configured to apply non-linear precoding for transmission according to any of the above described embodiments / examples / realizations. The communication controller 1010 can control the non-linear precoding circuit 1011. ​

[0126] In an embodiment, Figure 10 At least some functions of the apparatus can be shared between two physically separate devices, e.g. between a radio unit and / or radio head and a corresponding counter-party (e.g. a distributed unit), forming one operational entity. Thus, the apparatus can be seen to depict an operational entity comprising one or more physically separate devices for comprising the described antenna arrangement and / or for performing at least some of the described procedures.

[0127] As used in this application, the term "circuitry" refers to all of the following: (a) hardware-only circuitry such as only analog and / or digital circuitry, including only analog and / or digital circuitry; (b) a combination of circuits and software (and / or firmware), such as (as appropriate): (i) a combination of processor(s) or (ii) portions of storage (digital and / or otherwise) that are used to implement one or more components of the device, including one or more digital signal processors, software, and memory that execute one or more portions of software to perform various functions, and (c) to circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of "circuitry" applies to all uses of this term in this application. As a further example, as used in this application, the term "circuitry" would also cover an implementation that includes one or more processors and / or processors' portion(s) and accompanying software and / or firmware. The term "circuitry" would also cover, for example and if applicable, a baseband integrated circuit or applications processor integrated circuit in a mobile phone, or a similar integrated circuit in a server, a cellular network device, or other network device.

[0128] In an embodiment, in connection with Figures 2 to 9 At least some of the procedures described can be implemented by an apparatus comprising respective means for performing at least some of the described procedures. The apparatus can comprise separate means for separate stages of the procedures, or can perform means for several stages or the whole procedure. Some example means for implementing these procedures can comprise at least one of the following: a detector, a processor (including dual-core and multi-core processors), a digital signal processor, a controller, a receiver, a transmitter, an encoder, a decoder, a memory, a RAM, a ROM, software, firmware, a display, a user interface, display circuitry, user interface circuitry, user interface software, display software, circuitry, an antenna, antenna circuitry, and circuit means. In an embodiment, at least one processor, memory, and computer program code form a processing means or comprise one or more computer program code portions for performing one or more operations according to any of the embodiments / examples / implementations described herein.

[0129] According to yet another embodiment, an apparatus implementing the embodiments / examples comprises circuitry comprising at least one processor and at least one memory including computer program code. When activated, the circuitry causes the apparatus to perform at least some of the functions or operations in accordance with any of the embodiments / examples / realizations described herein. Figures 2 to 9

[0130] The techniques and methods described herein can be implemented by various means. For example, these techniques can be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or combinations thereof. For a hardware implementation, the apparatus implementing the embodiments can be implemented within one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. For firmware or software, the implementation can be carried out through modules of at least one chipset) that are performed by at least one

[0131] The described embodiments / examples / realizations can also be carried out in the form of a computer process defined by a computer program or portions thereof. The computer program can be stored in a computer program distribution medium readable by a computer or a processor. The computer program medium can be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. The computer program can be distributed ahead of time to the computer or the processor. It is to be understood that the computer program can be implemented in a plurality of forms including a type of downloadable content adapted to be stored on the computer or a type of software that is executed in the computer or the processor. The computer program medium can be realized in various forms and the embodiments are not limited to a specific type of the computer program medium. Figures 2 to 9 Embodiments of the described methods can be implemented by executing at least part of a computer program comprising corresponding instructions. The computer program can be in source code form, object code form, or in some intermediate form. The computer program can be stored in some sort of carrier, which can be any entity or device capable of carrying the program. For example, the computer program can be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium can be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package. The computer program medium can be realized in various forms and the embodiments are not limited to a specific type of the computer program medium. The code for implementing the embodiments shown and described can be completely within the scope of a person of ordinary skill in the art. In an embodiment, the computer readable medium comprises the computer program.

[0132] ​It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The example embodiments are not limited to the examples described above, but can vary within the scope of the claims. The word 'comprising' does not exclude not excluding other elements or steps, and the word 'a' or 'an' does not exclude a plurality. The word 'comprising' includes the case of words 'consisting of'. Furthermore, the words 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', 'comprising', 'comprise', '

Claims

1. An apparatus for communication, comprising at least one antenna array, the antenna array comprising a plurality of array elements, wherein every two elements in a first direction are digitally pre-distortionless linear elements, every two elements in the first direction are non-linear elements, and a spacing between adjacent elements in the first direction is less than half a free-space wavelength; at least one processor; and at least one memory including computer program code and reference information, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: applying a non-linear precoding to a transmission from the antenna array, the non-linear precoding converting out-of-band emissions of the transmission into reactive power in a near-field around the antenna array while ensuring that an in-band signal generated by the elements remains unaffected; and sending the in-band signal from the antenna array.

2. The apparatus according to claim 1, wherein the spacing between adjacent elements in the first direction is equal to or less than a quarter of the free-space wavelength.

3. The apparatus according to claim 1 or 2, wherein the antenna array is a two-dimensional array, the two-dimensional array comprising elements of the same type in a second direction, and a spacing between adjacent elements in the second direction is equal to or greater than the half of the free-space wavelength.

4. The apparatus according to claim 1 or 2, wherein the apparatus further comprises a linear power amplifier for amplifying out-of-band signals after precoding.

5. The apparatus according to claim 1 or 2, wherein a plurality of digitally pre-distortionless linear elements are hybrid elements, wherein outputs of a first number of power amplifiers in a transmission chain are connected through phase shifters and combiners to a second number of radiating antennas, the second number being greater than the first number.

6. The apparatus according to claim 1 or 2, wherein the at least one memory and the computer program code are configured to, with the at least one processor, further cause the apparatus to perform the non-linear precoding using results of a procedure, the procedure comprising initializing a matrix representation of a time-domain signal generated by the non-linear elements to a constant envelope signal closest to a zero-forcing solution, and performing a predefined number of iteration rounds.

7. The apparatus according to claim 6, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus further to perform, during an iteration round: randomly selecting coordinates for an element in the matrix; calculating a value for the element by minimizing a sum of mean square errors between a received signal and a desired signal at a serving apparatus, a Lagrange multiplier weighted on out-of-band emissions being added to the sum of mean square errors, one constraint ensuring that the linear elements do not transmit in-band, another constraint limiting a power spectral density of the linear elements, and a further constraint ensuring that the signal transmitted by the non-linear elements has a constant envelope in the time domain, such that the value is of a size one while other elements have a fixed value. repeating the selection and the computation until all elements have computed values; and updating a matrix representation of the frequency domain signal transmitted by linear elements accordingly.

8. A method for communication, comprising: applying nonlinear precoding to a transmission from at least one antenna array, the antenna array comprising a plurality of array elements, wherein every two elements in a first direction are digital pre-distortion free linear elements, every two elements in the first direction are nonlinear elements, and a spacing between adjacent elements in the first direction is less than half a free space wavelength, the nonlinear precoding converting out-of-band emissions of the transmission into reactive power in a near field around the antenna array while ensuring that in-band signals generated by the elements remain unaffected; and transmitting the in-band signals from the antenna array.

9. The method of claim 8, further comprising: performing the nonlinear precoding using results of a procedure, the procedure comprising initializing a matrix representation of time domain signals generated by the nonlinear elements to constant envelope signals closest to a zero-forcing solution and performing a predefined number of iteration rounds.

10. The method of claim 9, further comprising performing the following during an iteration round: randomly selecting a coordinate for an element in the matrix; computing a value for the element such that a magnitude of the value is one while other elements have fixed values by minimizing a sum of mean square errors between a received signal and a desired signal at a serving device under constraints that a Lagrange multiplier weighted by out-of-band emissions is added to the sum of mean square errors, one constraint ensures that the linear elements do not transmit in-band, another constraint limits a power spectral density of the linear elements, and yet another constraint ensures that the signals transmitted by the nonlinear elements have constant envelopes in the time domain; repeating the selection and the computation until all elements have computed values; and updating a matrix representation of the frequency domain signal transmitted by linear elements accordingly.

11. A computer readable medium comprising program instructions stored thereon for performing at least the following: applying nonlinear precoding to a transmission from at least one antenna array, the antenna array comprising a plurality of array elements, wherein every two elements in a first direction are digital pre-distortion free linear elements, every two elements in the first direction are nonlinear elements, and a spacing between adjacent elements in the first direction is less than half a free space wavelength, the nonlinear precoding converting out-of-band emissions of the transmission into reactive power in a near field around the antenna array while ensuring that in-band signals generated by the elements remain unaffected; and transmitting the in-band signals from the antenna array.

12. The computer readable medium of claim 11, further comprising program instructions stored thereon for performing the nonlinear precoding using results of a procedure, the procedure comprising initializing a matrix representation of time domain signals generated by the nonlinear elements to constant envelope signals closest to a zero-forcing solution and performing a predefined number of iteration rounds.

13. The computer readable medium of claim 11 or 12, wherein the computer readable medium is a non-transitory computer readable medium.

14. A computer program product comprising instructions, wherein the instructions, when executed by a processor, perform a method comprising: applying a nonlinear precoding to a transmission from at least one antenna array, the antenna array comprising a plurality of array elements, wherein every two elements in a first direction are digital pre-distortionless linear elements, every two elements in the first direction are nonlinear elements, and a spacing between adjacent elements in the first direction is less than half a free space wavelength, the nonlinear precoding converting out-of-band emissions of the transmission to reactive power in a near field surrounding the antenna array while ensuring that in-band signals generated by the elements remain unaffected; and transmitting the in-band signals from the antenna array.

15. The computer program product of claim 14, wherein the method further comprises performing the nonlinear precoding using results of a process, the process comprising initializing a matrix representation of time domain signals generated by the nonlinear elements to constant envelope signals closest to a zero-forcing solution, and performing a predefined number of iteration rounds.

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