Sending data from a digital unit to a radio unit

Abstract

It is provided a method for sending data from a digital unit, DU, (20) to a radio unit, RU. The DU (20) and RU (22) form part of a network node (1). The method is performed by the DU (20). The method comprises: determining (40) a QAM, quadrature amplitude modulation, scheme to use for transmissions over a fronthaul interface (21) to the RU (22); obtaining (44) an input sample to be modulated and transmitted over a radio interface, the input sample not being a QAM constellation point of the QAM scheme; sending (48) a QAM symbol, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU over the fronthaul interface for transmission; and repeating the obtaining (44) a symbol, and sending (48) the QAM symbol for a plurality of input samples.

Description

SENDING DATA FROM A DIGITAL UNIT TO A RADIO UNITTECHNICAL FIELD

[0001] The present disclosure relates to the field of network nodes of a communication network and in particular to sending data from a digital unit to a radio unit, where the digital unit and the radio unit form part of a network node.BACKGROUND

[0002] Modern mobile communication systems rely on high data throughput and efficient resource utilization to meet ever-increasing user demands. The O-RAN (Open Radio Access Network) architecture is an approach to disaggregate components of the radio access network. In O-RAN’s functional split model, the baseband processing is partially centralized, while the radio front-end remains at remote sites. The communication between these two entities occurs over the fronthaul interface. This fronthaul interface typically uses packet -based protocols (e.g. eCPRI, enhanced common public radio interface) over Ethernet to transport in-phase / quadrature (IQ) samples.

[0003] Quadrature Amplitude Modulation (QAM) is widely used in these systems to achieve higher data rates by encoding multiple bits in each IQ sample. However, transmitting uncompressed IQ samples for high-order QAM constellations (e.g. 64-QAM or 256-QAM) still consumes a significant amount of fronthaul bandwidth. To address this, O-RAN specifications allow for modulation compression, which quantizes and compresses the IQ data. By reducing the bit-depth per sample and employing scaling schemes, the fronthaul bandwidth can be substantially lowered, enabling more cost-effective and scalable deployments without compromising overall link quality.

[0004] However, the modulation compression is only applicable for QAM constellation points. It would be of great value if the benefits of modulation compression could also be applied for IQ samples that are not QAM constellation points.SUMMARY

[0005] One object is to reduce bandwidth use over a fronthaul interface for samples that do not align with QAM constellation points.

[0006] According to a first aspect, it is provided a method for sending data from a digital unit, DU, to a radio unit, RU. The DU and RU form part of a network node. The method is performed by the DU. The method comprises: determining a QAM, quadrature amplitude modulation, scheme to use for transmissions over a fronthaul interface to the RU; obtaining an input sample to be modulated and transmitted over a radio interface, the input sample not being a QAM constellation point of the QAM scheme; sending a QAM symbol, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU over the fronthaul interface for transmission; and repeating the obtaining a symbol, and sending the QAM symbol for a plurality of input samples.

[0007] The method may further comprise: transmitting a message over a control plane to the RU in accordance with the determined QAM scheme.

[0008] The message may define the number of bits that are used to represent each QAM symbol over the fronthaul interface.

[0009] The determining a QAM scheme may comprise determining a QAM scheme of an order that is higher than what the RU is capable of supporting in transmission.

[0010] In the obtaining an input sample, each input sample may be an element that is used to support random access.

[0011] In the obtaining an input sample, each input sample may be used for integrated sensing and communication, ISAC.

[0012] In the obtaining an input sample, each input sample may be an element in a Zadoff-Chu sequence.

[0013] The method may comprise finding, in the QAM scheme, the QAM constellation point approximating the input sample.

[0014] According to a second aspect, it is provided a digital unit, DU, for sending data from a distributed unit, DU, to a radio unit, RU. The DU and RU form part of a network node. The DU comprises: processing circuitry; and memory circuitry storing instructions that, when executed by the processing circuitry, cause the DU to: determine a QAM, quadrature amplitude modulation, scheme to use for transmissions over a fronthaul interface to the RU; obtain an input sample to be modulated and transmitted over a radio interface, the input sample not being a QAM constellation point of the QAM scheme; send a QAM symbol, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU over the fronthaul interface for transmission; and repeat the instructions to obtain a symbol, and send the QAM symbol for a plurality of input samples.

[0015] The DU may further comprise instructions that, when executed by the processing circuitry, cause the DU to: transmit a message over a control plane to the RU in accordance with the determined QAM scheme.

[0016] The message may define the number of bits that are used to represent each QAM symbol over the fronthaul interface.

[0017] The instructions to determine a QAM scheme may comprise instructions that, when executed by the processing circuitry, cause the DU to determine a QAM scheme of an order that is higher than what the RU is capable of supporting in transmission.

[0018] In the instructions to obtain an input sample, each input sample may be an element that is used to support a random access.

[0019] In the instructions to obtain an input sample, each input sample may be used for integrated sensing and communication, ISAC.

[0020] In the instructions to obtain an input sample, each input sample may be an element in a Zadoff-Chu sequence.

[0021] The DU may further comprise instructions that, when executed by the processing circuitry, cause the DU to: find, in the QAM scheme, the QAM constellation point approximating the input sample.

[0022] According to a third aspect, it is provided a computer program for sending data from a digital unit, DU, to a radio unit, RU, the DU and RU forming part of a network node. The computer program comprises computer program code which, when executed on the DU causes the DU to: determine a QAM, quadrature amplitude modulation, scheme to use for transmissions over a fronthaul interface to the RU; obtain an input sample to be modulated and transmitted over a radio interface, the input sample not being a QAM constellation point of the QAM scheme; send a QAM symbol, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU over the fronthaul interface for transmission; and repeat the computer program code to obtain a symbol, and send the QAM symbol for a plurality of input samples.

[0023] According to a fourth aspect, it is provided a computer program product comprising a computer program according to the third aspect and a computer readable means comprising non-transitory memory in which the computer program is stored.

[0024] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

[0026] Fig 1 is a schematic diagram illustrating a communication network where embodiments presented herein may be applied;

[0027] Fig 2 illustrates the structure of the interface between the DU and the RU, together performing the function of the network node;

[0028] Figs 3A-B are schematic graphs illustrating how input samples are approximated by QAM constellation points;

[0029] Figs 4A-B are swimlane diagrams illustrating embodiments of methods for sending data from a DU to a RU;

[0030] Fig 5 is a schematic diagram illustrating components of the DU of Fig 2;

[0031] Fig 6 is a schematic diagram showing functional modules of the DU of Fig 2 according to one embodiment; and

[0032] Fig 7 shows one example of a computer program product comprising computer readable means.DETAILED DESCRIPTION

[0033] The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0034] According to embodiments presented herein, modulation compression of QAM for fronthaul communication is applied also for IQ samples that do not align with QAM constellation points. To make this word, the IQ sample is approximated with the nearest QAM constellation point, which can then be encoded using modulation compression. Compared to eight bits for I value and eight bits for Q value, this achieves great compression of data, with very low loss of accuracy.

[0035] Fig 1 is a schematic diagram illustrating a communication network 9 where embodiments presented herein may be applied. The communication network 9comprises a core network 3 and one or more network nodes 1 forming part of a radio access network (RAN). The communication network 9 can comply with any suitable current or future communication standard, e.g. sixth generation (6G) networks, next fifth generation (5G) networks, LTE (Long Term Evolution), as long as the principles described hereinafter are applicable.

[0036] The network node 1 can be the form of radio base stations being gNode Bs, gNBs, or evolved Node Bs, also known as eNode Bs or eNBs. The network node 1 provides radio connectivity over a wireless interface 4a-b to a plurality of UEs (User Equipments) 2a-b.

[0037] Over the wireless interface, downlink (DL) communication 4b occurs from the network node 1 to the UEs 2a-b and uplink (UL) communication 4a occurs from the UEs 2a-b to the network node 1. The quality of the wireless radio interface to each UE 2a-b can vary over time and depending on the position of the UE 2a-b, due to effects such as fading, multipath propagation, interference, etc.

[0038] The network node 1 can comprise several antennas, e.g. in a MIMO antenna array to thereby enable beamforming to the UEs 2a-b by applying beamforming coefficients, respectively for each one of the antennas. The beamforming can be applied separately for each UE 2a-b or simultaneously for multiple UEs. The beamforming can be applied in downlink 4b and / or uplink 4a. Abeam index defines the beamforming coefficients and may correspond to a direction from the radio to network node 1. UL beamforming from the UE is also possible.

[0039] The term UE is also known as mobile communication terminal, user device, mobile terminal, user terminal, user agent, wireless device, wireless terminal, machine-to-machine device etc., and can be, for example, what today are commonly known as a mobile phone, smart phone, a tablet / laptop with wireless connectivity, an internet-of-things (loT) device, etc.

[0040] The network node 1 is connected to the core network 3 for connectivity to network central functions and a wide area network, such as the Internet.

[0041] The network node 1 is here implemented in a distributed fashion, e.g. using an O-RAN (Open RAN) or equivalent distributed architecture. The network node 1 comprises a DU (digital unit) 20 and one or more RUs (radio units) 22a-b. When applied in the O-RAN architecture, the DU is termed an O-DU (open distributed unit) node, and the radio node is termed an O-RU (open radio unit) node. The DU 20 and the RUs 22a-b can be in different locations. The DU 20 is located uplink from the RUs 22a-b, i.e. towards the core network 3. Consequently, the RUs 22a-b are located downlink from the DU 20, towards the UEs 2a-b.

[0042] There are respective fronthaul interfaces 2ia-b between the DU 20 and each RU 22a-b. The fronthaul interfaces 2ia-b are bidirectional communication interfaces. The fronthaul interfaces 2ia-b can, for instance, be implemented using eCPRI (enhanced CPRI) over Ethernet. The DU 20 and the RUs 22a-b can split functionality of the network node 1 between them, such that each one of the DU 20 and the RUs 22a-b implements a subset of the functionality of the RAN for radio communication with the UEs 2a-b.

[0043] Fig 2 illustrates the structure of the interface between the DU 20 and the RU 22, together performing the function of the network node 1. Communication between the DU 20 and the RU 22 occurs over the fronthaul interface 21, using any one or more of a user plane (UP) 24, a control plane (CP) 25 and / or a management plane (MP) 26, as well over an synchronisation plane (not shown).

[0044] The user plane 24 carries the actual traffic-bearing IQ data (or compressed IQ samples) between the DU 20 and RU 22. The control plane 25 exchanges signalling messages e.g. for configuring and coordinating radio functions, such as scheduling, resource allocation, and other operational parameters. The management plane 26 handles the overall configuration, provisioning, and fault management of the network elements, supporting proper startup, monitoring, and maintenance.

[0045] The legacy CPRI-type fronthaul transports time-domain in-phase and quadrature (IQ) samples per antenna branch. As the number of antennas scales up in massive MIMO systems, the required fronthaul capacity also increases proportionally, which significantly drives up the fronthaul costs. To address this challenge, thefronthaul interface 21 has evolved from CPRI to eCPRI, a packet-based fronthaul interface. In eCPRI, more options for the functional split options between a DU 20 and a RU 22 are supported, referred to as different lower-layer split options. In the eCPRI standard specification, the terms eCPRI Radio Equipment Control (eREC) and eCPRI Radio Equipment (eRE) are sometimes used instead of DU and RU. By moving the frequency-domain beamforming function from DU 20 to RU 22, frequency samples or data of user-layers are transported over the fronthaul interface 21. In this way, the required fronthaul capacity, and thereby the fronthaul costs, are significantly reduced, as the number of user layers is typically much fewer than the number of antennas in massive MIMO. According to embodiments presented herein, additional ways to reduce bandwidth requirements over the fronthaul interface 21 are provided.

[0046] Figs 3A-B are schematic graphs illustrating how input samples are approximated by QAM constellation points. QAM is widely used for fronthaul communication to achieve higher data rates by encoding multiple bits in each IQ sample. However, transmitting uncompressed IQ samples for high-order QAM constellations (e.g. 64-QAM or 256-QAM) still consumes a significant amount of fronthaul bandwidth. To address this, O-RAN specifications allow for modulation compression, which quantizes and compresses the IQ data. By reducing the bit-depth per sample and employing scaling schemes, the fronthaul bandwidth can be substantially lowered, enabling more cost-effective and scalable deployments without compromising overall link quality.

[0047] In the graphs, the horizontal axis represents in-phase (I) amplitude, and the vertical axis represents quadrature-phase (Q) amplitude. The input samples are IQ values that are indicated with ‘+’ signs. The ‘x’ signs indicate QAM constellation points approximated with input samples, in accordance with the QAM scheme in question. The small dots are the available QAM constellation points in the QAM scheme. In the example illustrated by Figs 3A-B, the input samples are elements of Zadoff-Chu sequence.

[0048] In Fig 3A, the approximation is shown for approximating to QAM constellation points of a 256 QAM scheme. In Fig 3B, the approximation is shown for approximating to QAM constellation points of a 1024 QAM scheme. Since the number ofavailable constellation points is greater for the 1024 QAM scheme, those QAM constellation points are closer to their respective input samples.

[0049] The average error added using this methos is about 5 % for 256 QAM and less than 3 % for 1024 QAM. This type of error is often negligible compared to other sources of error when communicating over wireless networks.

[0050] Figs 4A-B are swimlane diagrams illustrating embodiments of methods for sending data from a DU 20 to a RU 22. As mentioned above, the DU 20 and RU 22 form part of a network node 1. The swimlane diagrams can be considered to comprise a flow chart for methods in the DU 20 on the left and a flow chart for methods in the RU 22 on the right. Communication between the DU 20 and the RU 22 is also shown. First, embodiments illustrated by Fig 4A will be described.

[0051] In a determine QAM scheme step 40, the DU 20 determines a QAM scheme to use for transmissions over a fronthaul interface to the RU. The determining a QAM scheme may comprise determining a QAM scheme of an order that is higher than what the RU is capable of supporting in transmission.

[0052] The QAM scheme can be determined by the DU 20 based on link quality with the UE. In general, a better link quality enables a higher order QAM, since the UE would then be able to better distinguish between the QAM constellation points.

[0053] In an obtain input sample step 44, the DU 20 obtains an input sample to be modulated and transmitted over a radio interface. The input sample is not a QAM constellation point of the QAM scheme. The input sample is represented as an IQ value to form part of a sub-carrier transmission.

[0054] In one embodiment, each input sample is an element that is used to support random access. Random access samples do not need to conform to QAM constellation points and are thus applicable for the input samples herein.

[0055] In one embodiment, each input sample is used for integrated sensing and communication (ISAC). Also samples for ISAC do not need to conform to QAM constellation points and are thus applicable for the input samples herein.

[0056] In one embodiment each input sample is an element in a Zadoff-Chu sequence. Zadoff-Chu sequences are usable both for random access supporting samples and for ISAC.

[0057] In a send QAM symbol step 48, the DU 20 sends a QAM symbol 32, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU 22 over the fronthaul interface 21 for transmission. The approximation of the input sample to the QAM constellation point can be performed by the DU 20 (as specified below) or can be performed elsewhere. The QAM symbol 32 can be determined based on modulation compression. Defined in O-RAN, modulation compression is a way to reduce the amount of data exchanged between the DU and RU over the fronthaul interface. Rather than sending full-resolution IQ samples for every subcarrier, the DU and RU use quantization and standardized encoding techniques to compress those samples. In other words, the QAM symbol 32 is the result of modulation compression of the QAM constellation point. It is to be noted that QAM symbol 32 is not the same as an OFDM (orthogonal frequency-domain multiplexing) symbol; each ODFM symbol can contain a large number of QAM symbols 32.

[0058] In a receive QAM symbol step 148, the RU 22 receives the QAM symbol 32 from the DU 20 over the fronthaul interface 21.

[0059] In a send transmission for QAM symbol step 150, the RU 22 processes the received (frequency domain) QAM symbol 32, and transforms to an over-the-air signal that is transmitted over the wireless interface. The method for the RU 22 then returns to the receive QAM symbol step 148 to be ready to receive another QAM symbol.

[0060] In a conditional repeat step 50, the DU 20 determines whether there are more input samples to process for transmission. If this is the case, the method for the DU 20 returns to the obtain input sample step 44. Otherwise, the method ends.

[0061] Looking now to Fig 4B, only new or modified steps compared to those of 4A will be described.

[0062] In an optional transmit QAM scheme message step 42, the DU 20 transmits a message 30 over a control plane 25 to the RU 22 in accordance with the determinedQAM scheme. The message 30 indicates the QAM scheme that will be used for subsequent transmissions over the fronthaul interface 21. For instance, the message can define the number of bits that are used to represent each QAM symbol over the fronthaul interface. In one embodiment, the parameter udlqWidth is used for the message, indicating i how many bits are used to represent each one of the I and Q component for chosen compression scheme.

[0063] In an optional receive QAM scheme message step 142, the RU 22 receives the QAM scheme message 30. The RU 22 then prepares to receive QAM symbols in accordance with the QAM scheme and compression specified in the QAM scheme message 30.

[0064] In an optional find approximating QAM symbol step 46, the DU 20 finds, in the QAM scheme, the QAM constellation point approximating the input sample. As illustrated in Figs 3A-B, the error introduced by this approximation is in many cases negligible.

[0065] If O-RAN compliant front haul were to be used, employing block floating point with 8 bits per I and Q sample, each sample would take up 16 bits. Compare this to approximating to constellation points for 256 QAM where modulation compression is applied, which can be reduced in size using modulation compression to 8 bits, rendering a bandwidth reduction of about 50 %. Even if increasing the modulation to 1024 QAM, the bandwidth reduction is still close to 40 %.

[0066] Using embodiments presented herein, it is sufficient that the fronthaul interface supports (compressed) QAM, as samples in any other format can be approximated with QAM constellation points. This greatly simplifies the situation, as there are many different potential formats on the fronthaul, that are hereby efficiently transmitted using the QAM constellation point approximation.

[0067] An embodiment applicable for a Zadoff-Chu sequence will now be described. Steps 44 and 48 (and optionally intermediate step 46) are repeated to transmit, over the fronthaul interface 21, a representation of a first sequence of QAM symbols. The first sequence of QAM symbols define an OFDM symbol to be transmitted over an airinterface. The first sequence has a first length. The first sequence approximates a second sequence which is a Zadoff-Chu sequence. The second sequence is the sequence out of the existing Zadoff-Chu sequences of the first length that best matches the first sequence. The first sequence is the sequence out of the possible QAM symbol sequences of the first length that best matches the second sequence. Once transmitted to the RU 22 over the fronthaul interface, the RU 22 transmits, over the air, an OFDM symbol according to the above QAM symbol sequence.

[0068] According to one embodiment, the corresponding procedure is applied in the uplink, from the RU to the DU. In this case, samples received over the air by the RU are approximated to QAM constellation points for transfer over the fronthaul interface to the DU. Based on the QAM scheme used, a significant reduction in bandwidth is achieved, from non-QAM RU internal bit-width, e.g. 16 bits per sample, to the number of bits used for each QAM constellation point.

[0069] Fig 5 is a schematic diagram illustrating components of the DU 20 of Fig 2. Processing circuitry 60 is provided using any combination of one or more of a suitable central processing unit (CPU), graphics processing unit (GPU), multiprocessor, neural processing unit (NPU), microcontroller, digital signal processor (DSP), etc., capable of executing software instructions 67 stored in memory circuitry 64, which can thus be a computer program product. The processing circuitry 60 could alternatively be implemented using an application specific integrated circuit (ASIC), field programmable gate array (FPGA), etc. The processing circuitry 60 can be configured to execute the methods illustrated with reference to Figs 4A-B above.

[0070] The memory circuitry 64 can be any combination of random-access memory (RAM) and / or read-only memory (ROM). The memory circuitry 64 also comprises non-transitory persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid-state memory or even remotely mounted memory.

[0071] A data memory 66 is also provided for reading and / or storing data during execution of software instructions in the processing circuitry 60. The data memory 66 can be any combination of RAM and / or ROM.

[0072] The further comprises an I / O interface 62 for communicating with external and / or internal entities. For instance, the I / O interface 62 implements one side of the fronthaul interface 21 to the RU 22. The I / O interface 62 can also implement one side of an interface to the core network 3.

[0073] Other components of the DU 20 are omitted in order not to obscure the concepts presented herein.

[0074] Fig 6 is a schematic diagram showing functional modules of the DU 20 of Fig 2 according to one embodiment. The modules are implemented using software instructions such as a computer program executing in the DU 20. Alternatively or additionally, the modules are implemented using hardware, such as any one or more of an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), or discrete logical circuits. The modules correspond to the steps in the methods illustrated in Figs 4A-B.

[0075] A QAM scheme determiner 70 corresponds to step 40. A message transmitter 72 corresponds to step 42. An input sample obtainer 74 corresponds to step 44. A QAM symbol finder 76 corresponds to step 46. A QAM symbol sender 78 corresponds to step 48. A repeater 80 corresponds to step 50

[0076] Fig 7 shows one example of a computer program product 90 comprising computer readable means. On this computer readable means, a computer program 91 can be stored in a non-transitory memory. The computer program can cause processing circuitry to execute a method according to embodiments described herein. In this example, the computer program product 90 is in the form of a removable solid-state memory, e.g. a Universal Serial Bus (USB) drive. As explained above, the computer program product could also be embodied in a memory of a device, such as the computer program product 64 of Fig 5. While the computer program 91 is here schematically shown as a section of the removable solid-state memory, the computer program can be stored in any way which is suitable for the computer program product, such as another type of removable solid-state memory, or an optical disc, such as a CD (compact disc), a DVD (digital versatile disc) or a Blu-Ray disc.

[0077] The aspects of the present disclosure have mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.

Claims

CLAIMS1. A method for sending data from a digital unit, DU, (20) to a radio unit, RU, (22) the DU (20) and RU (22) forming part of a network node (1), the method being performed by the DU (20), the method comprising:determining (40) a QAM, quadrature amplitude modulation, scheme to use for transmissions over a fronthaul interface (21) to the RU (22);obtaining (44) an input sample to be modulated and transmitted over a radio interface, the input sample not being a QAM constellation point of the QAM scheme; sending (48) a QAM symbol, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU over the fronthaul interface for transmission; andrepeating the obtaining (44) a symbol, and sending (48) the QAM symbol for a plurality of input samples.

2. The method according to claim 1, further comprising:transmitting (42) a message over a control plane to the RU in accordance with the determined QAM scheme.

3. The method according to claim 2, wherein the message defines the number of bits that are used to represent each QAM symbol over the fronthaul interface.

4. The method according to any one of the preceding claims, wherein the determining (40) a QAM scheme comprises determining a QAM scheme of an order that is higher than what the RU is capable of supporting in transmission.

5. The method according to any one of the preceding claims, wherein, in the obtaining (44) an input sample, each input sample is an element that is used to support random access.

6. The method according to any one of claims 1 to 4, wherein, in the obtaining (44) an input sample, each input sample is used for integrated sensing and communication, ISAC.

7. The method according to any one of the preceding claims, wherein, in the obtaining (44) an input sample, each input sample is an element in a Zadoff-Chu sequence.

8. The method according to any one of the preceding claims, wherein the method further comprises:finding (46), in the QAM scheme, the QAM constellation point approximating the input sample.

9. A digital unit, DU, (20) for sending data from a distributed unit, DU, (20) to a radio unit, RU, (22) the DU (20) and RU (22) forming part of a network node (1), the DU (20) comprising:processing circuitry (60); andmemory circuitry (64) storing instructions (67) that, when executed by the processing circuitry, cause the DU (20) to:determine a QAM, quadrature amplitude modulation, scheme to use for transmissions over a fronthaul interface (21) to the RU (22);obtain an input sample to be modulated and transmitted over a radio interface, the input sample not being a QAM constellation point of the QAM scheme;send a QAM symbol, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU over the fronthaul interface for transmission; and repeat the instructions to obtain a symbol, and send the QAM symbol for a plurality of input samples.

10. The DU (20) according to claim 9, further comprising instructions (67) that, when executed by the processing circuitry, cause the DU (20) to:transmit a message over a control plane to the RU in accordance with the determined QAM scheme.

11. The DU (20) according to claim 10, wherein the message defines the number of bits that are used to represent each QAM symbol over the fronthaul interface.

12. The DU (20) according to any one of claims 9 to 11, wherein the instructions to determine a QAM scheme comprise instructions (67) that, when executed by theprocessing circuitry, cause the DU (20) to determine a QAM scheme of an order that is higher than what the RU is capable of supporting in transmission.

13. The DU (20) according to any one of claims 9 to 12, wherein, in the instructions to obtain an input sample, each input sample is an element that is used to support a random access.

14. The DU (20) according to any one of claims 9 to 12, wherein, in the instructions to obtain an input sample, each input sample is used for integrated sensing and communication, ISAC.

15. The DU (20) according to any one of claims 9 to 14, wherein, in the instructions to obtain an input sample, each input sample is an element in a Zadoff-Chu sequence.

16. The DU (20) according to any one of claims 9 to 15, further comprising instructions (67) that, when executed by the processing circuitry, cause the DU (1) to: find, in the QAM scheme, the QAM constellation point approximating the input sample.

17. A computer program (67, 91) for sending data from a digital unit, DU, (20) to a radio unit, RU, (22) the DU (20) and RU (22) forming part of a network node (1), the computer program comprising computer program code which, when executed on the DU (20) causes the DU (20) to:determine a QAM, quadrature amplitude modulation, scheme to use for transmissions over a fronthaul interface (21) to the RU (22);obtain an input sample to be modulated and transmitted over a radio interface, the input sample not being a QAM constellation point of the QAM scheme;send a QAM symbol, approximating the input sample to a QAM constellation point of the QAM scheme, to the RU over the fronthaul interface for transmission; and repeat the computer program code to obtain a symbol, and send the QAM symbol for a plurality of input samples.

18. A computer program product (64, 90) comprising a computer program according to claim 16 and a computer readable means comprising non-transitory memory in which the computer program is stored.

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