Solutions and signaling enabling self-free multiple-input multiple-output transmission

Abstract

A system, method, apparatus, and computer program product are disclosed for enabling efficient cell-free MIMO transmission. One method includes receiving a downlink reference signal, determining at least one cell-free multiple-input multiple-output transmission strategy associated with at least one access point based at least on the downlink reference signal, and transmitting at least one of the at least one cell-free multiple-input multiple-output transmission strategy and enablement information to a network entity.

Description

Technical Field

[0001] Some exemplary embodiments generally relate to mobile or wireless communication systems such as Long-Term Evolution (LTE), Fifth Generation (5G) Radio Access Technology (RAT), New Radio (NR) access technology, Sixth Generation (6G), and / or other communication systems. For example, certain exemplary embodiments relate to systems and / or methods for enabling efficient cell-free Multiple-Input Multiple-Output (MIMO) transmission.

Background Art

[0002] Examples of mobile communication systems or wireless communication systems include Radio Frequency (RF) 5G RAT, Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN), LTE Evolved UTRAN (E-UTRAN), LTE-Advanced (LTE-A), LTE-A Pro, NR access technology, and / or the MultiFire Alliance. 5G wireless systems refer to the next generation (NG) of wireless systems and network architectures. 5G systems are typically built on 5G NR, although 5G (or, NG) networks may also be built on E-UTRA radio. NR is expected to support service categories such as enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communications (URLLC), and Massive Machine-Type Communications (mMTC). NR is expected to provide extreme broadband, ultra-robust and low-latency connectivity, and large-scale networks to support the Internet of Things (IoT). The Next Generation Radio Access Network (NG-RAN) represents the 5G RAN and can provide radio access for NR, LTE, and LTE-A. Note that 5G nodes that provide radio access capabilities to user equipment (e.g., similar to Node B of UTRAN or evolved Node B (eNB) of LTE) may be called Next Generation Node B (gNB) when built with NR radio and may be called Next Generation eNB (NG-eNB) when built with E-UTRA radio.

Summary of the Invention

[0003] In some exemplary embodiments, the method may include a user equipment receiving a downlink reference signal. The method may further include the user equipment determining at least one cell-free multiple-input multiple-output (MIMO) transmission strategy associated with at least one access point based at least on the downlink reference signal. The method may further include the user equipment transmitting to a network entity at least one of at least one cell-free MIMO transmission strategy and enablement information.

[0004] In certain exemplary embodiments, the apparatus may comprise means for receiving a downlink reference signal. The apparatus may further comprise means for determining at least one cell-free MIMO transmission strategy associated with at least one access point based at least on the downlink reference signal. The apparatus may further comprise means for transmitting to a network entity at least one of at least one cell-free MIMO transmission strategy and enablement information.

[0005] In various exemplary embodiments, the non-transitory computer-readable medium may be encoded with instructions that, when executed in hardware, may perform a method. The method may include receiving a downlink reference signal. The method may further include determining at least one cell-free MIMO transmission strategy associated with at least one access point based at least on the downlink reference signal. The method may further include transmitting to a network entity at least one of at least one cell-free MIMO transmission strategy and enablement information.

[0006] In some exemplary embodiments, a computer program product can execute a method. The method may include receiving a downlink reference signal. The method may further include determining, based at least on the downlink reference signal, at least one cell-free multiple-input multiple-output (MIMO) transmission strategy associated with at least one access point. The method may further include transmitting to a network entity at least one of the at least one cell-free MIMO transmission strategy and enablement information.

[0007] In certain exemplary embodiments, an apparatus may comprise at least one processor and at least one memory including computer program code. The at least one memory and the computer program code may be configured by the at least one processor to cause the apparatus to at least receive a downlink reference signal. The at least one memory and the computer program code may be further configured by the at least one processor to at least determine, based at least on the downlink reference signal, at least one cell-free MIMO transmission strategy associated with at least one access point. The at least one memory and the computer program code may be further configured by the at least one processor to cause the apparatus to transmit to a network entity at least one of the at least one cell-free MIMO transmission strategy and enablement information.

[0008] In various exemplary embodiments, an apparatus may comprise circuitry configured to receive a downlink reference signal. The circuitry may be further configured to determine, based at least on the downlink reference signal, at least one cell-free MIMO transmission strategy associated with at least one access point. The circuitry may be further configured to transmit to a network entity at least one of the at least one cell-free MIMO transmission strategy and enablement information.

[0009] In some exemplary embodiments, the method may include transmitting, by a network entity, at least one downlink reference signal to a user equipment. The method may further include receiving, by the network entity, at least one self-free multiple-input multiple-output transmission strategy based on at least one downlink reference signal from the user equipment. The method may further include determining, by the network entity, a role of each of a plurality of access points based on at least one of the strategy and the enable information from the user equipment. The method may further include transmitting, by the network entity, a display of the association of each of the plurality of access points with the strategy to the user equipment.

[0010] In an exemplary embodiment, the apparatus may comprise means for transmitting at least one downlink reference signal to a user equipment. The apparatus may further comprise means for receiving at least one self-free multiple-input multiple-output transmission strategy based on at least one downlink reference signal from the user equipment. The apparatus may further comprise means for determining a role of each of a plurality of access points based on at least one of the strategy and the enable information from the user equipment. The apparatus may further comprise means for transmitting a notification of the association of each of the plurality of access points with the strategy to the user equipment.

[0011] In various exemplary embodiments, the non-transitory computer-readable medium may be encoded with instructions that, when executed in hardware, may perform a method. The method may include transmitting at least one downlink reference signal to a user equipment. The method may further include receiving at least one self-free multiple-input multiple-output transmission strategy based on at least one downlink reference signal from the user equipment. The method may further include determining a role of each of a plurality of access points based on at least one of the strategy and the enable information from the user equipment. The method may further include transmitting a notification of the association of each of the plurality of access points with the strategy to the user equipment.

[0012] In some exemplary embodiments, a computer program product can execute a method. The method may include transmitting at least one downlink reference signal to a user equipment. The method may further include receiving at least one free-space multiple-input multiple-output (FSMIMO) transmission strategy from the user equipment. The method may further include determining, based on at least one of the strategy and the enable information from the user equipment, a role of each of a plurality of access points. The method may further include transmitting, to the user equipment, a notification of the relevance between each of the plurality of access points and the strategy.

[0013] In certain exemplary embodiments, an apparatus may comprise at least one processor and at least one memory including computer program code. The at least one memory and the computer program code may be configured, by the at least one processor, to cause the apparatus to transmit at least one downlink reference signal to a user equipment. The at least one memory and the computer program code may be further configured, by the at least one processor, to cause the apparatus to receive at least one FSMIMO transmission strategy from the user equipment. The at least one memory and the computer program code may be further configured, by the at least one processor, to cause the apparatus to at least determine a role of each of a plurality of access points based on at least one of the strategy and the enable information from the user equipment. The at least one memory and the computer program code may be further configured, by the at least one processor, to cause the apparatus to at least transmit, to the user equipment, a notification of the relevance between each of the plurality of access points and the strategy.

[0014] In various exemplary embodiments, the apparatus may comprise circuitry configured to transmit at least one downlink reference signal to a user equipment. The circuitry may further be configured to receive at least one self-free multiple-input multiple-output (MIMO) transmission strategy from the user equipment. The circuitry may further be configured to determine the role of each access point of a plurality of access points based on at least one of the strategy and the enablement information from the user equipment. The circuitry may further be configured to transmit to the user equipment a notification of the relevance of each of the plurality of access points to the strategy.

Brief Description of the Drawings

[0015] To properly understand the exemplary embodiments, please refer to the accompanying drawings.

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[0016] It will be readily understood that the components of the particular exemplary embodiments generally depicted and illustrated in the figures in this example can be arranged and designed in a wide variety of different configurations. Accordingly, the following detailed description of some exemplary embodiments of a system, method, apparatus, and computer program product for enabling efficient cell-free MIMO transmission is not intended to limit the scope of the particular exemplary embodiments, but rather to be representative of the selected exemplary embodiments.

[0017] Figure 1 shows a general network architecture for cell-free MIMO transmission. Cell-free MIMO is one of many 6G capabilities that can provide high system capacity in both the sub-6G and millimeter wave (mmW) frequency bands. The network side shown in Figure 1 can include at least two types of nodes: an access point (AP) and a central processing unit (CPU). The functions of the AP node and the CPU node are being continuously developed, especially with regard to the split of layer 1 (L1) procedures.

[0018] One of the important features of self-free MIMO is that one or more APs can provide services to user equipment (UE) in both uplink (UL) communication and downlink (DL) communication, and the selection of APs to improve UE communication is being continuously developed, especially in static cluster schemes and / or UE-centered dynamic cluster schemes. For the static cluster scheme, a given AP connected to the same CPU may be configured to serve some or all of the UEs within a given area regardless of the channel quality between the AP and the target UE. In the case of a UE-centered dynamic cluster scheme, an AP that promotes data transmission can be selected based on the radio quality between the UE and the AP, and a list of candidate serving APs can be updated according to the movement of the UE and / or changes in the radio channel quality.

[0019] The detailed transmission method may include coherent joint transmission (CJT) and / or non-coherent joint transmission (NCJT). Similarly, self-free MIMO transmission may also include spatial multiplexing (SM) to improve system capacity, especially in situations where the radio channel quality is high. For example, in CJT, the APs can accurately synchronize in the DL with a small phase error between the APs so that the signal-to-interference-plus-noise ratio (SINR) increases at the UE side. Furthermore, in NCJT, although the requirements regarding the phase error may be relaxed, synchronization may be required (similar to CJT) to ensure that the DL signals from the relevant APs are received by the UE within the cyclic prefix (CP) length.

[0020] As described above, different APs can support different transmission strategies for different situations for the purpose of providing optimal system performance. Therefore, the network and the UE can coordinate which AP follows which transmission strategy to enable the UE to receive data optimally and accurately. Accordingly, certain embodiments can provide a way to achieve such coordination.

[0021] Certain exemplary embodiments in this example may have various advantages and / or benefits for overcoming the above-mentioned drawbacks. For example, certain exemplary embodiments may provide the UE with the flexibility to accurately recommend an appropriate transmission strategy for each AP. When the phase error information is ignored, the UE may need to report only the role of each AP without reporting detailed information regarding the phase error and DL synchronization (SYN) status among all APs, thereby reducing the UL signaling overhead. Furthermore, the availability of the phase error information may provide the network with important information for making appropriate CJT decisions regarding the appropriate frequency band for optimal system performance. Thereby, the UE and the network can also obtain sufficient flexibility to achieve a trade-off between performance and signaling overhead.

[0022] Furthermore, the proposed architecture of certain embodiments may enable flexible hybrid cell-free MIMO transmission suitable for UEs in the situation of multiple antenna ports and / or multiple subpanels. If the UE has multiple antenna ports, the UE can simultaneously recommend different transmission strategies for different antenna ports. For example, assuming that a 6G UE has three antenna ports, the UE can recommend CJT_AP for antenna port 1 and SM_AP for antenna ports 2 and 3. This hybrid transmission strategy may enable the network to optimally schedule transmissions among APs within each serving cluster. Other exemplary embodiments may include UEs having only one antenna / RF chain configuration where only one data stream can be transmitted on the air interface for each transmission time interval (TTI), in which case the UE can recommend only one AP per transmission or propose multiple APs for CJT transmission. Thus, the UE operation and UL signaling framework can support different UE-side configurations to improve system performance. Thus, the specific exemplary embodiments described below are directed towards improvements in computer-related technologies.

[0023] Some of the exemplary embodiments in this specification relate to recommending and configuring the AP to use appropriate transmission strategies in the time domain simultaneously or individually. Further, certain exemplary embodiments define new UL / DL signaling solutions for adjusting the network and the UE on the adopted transmission strategies of the corresponding AP for the proper operation of the UE, and enable a hybrid transmission scheme between APs to achieve optimal system performance.

[0024] Various exemplary embodiments may schedule / determine an appropriate transmission strategy for each AP and provide solutions and related UL / DL signals for realizing the adjustment between the UE and the network. Some exemplary embodiments may also provide a hybrid self-free MIMO transmission strategy in which the same or different types of self-free MIMO transmission strategies between serving APs can be scheduled simultaneously.

[0025] The UE can start the procedure by requesting the CPU to update the DL self-free MIMO information. Since the CPU can respond by simultaneously transmitting different reference signals to the UE via the corresponding AP within the current serving cluster of the CPU, the UE can measure the phase error information, DL SYN status information, and radio channel quality for each AP between these APs.

[0026] Thereafter, the UE can recommend a specific role for each AP and the corresponding appropriate transmission strategy. Based on the new UL signaling (example shown in FIG. 2), the UE can report the determined proposal / recommendation to the CPU, specifically, which AP applies which transmission strategy. Further, the UE can report the phase error for each sub-band or wide-band between the APs to the CPU to support the final decision by the CPU regarding the role of each AP.

[0027] Based on the proposals / recommendations received from the UE and the actual situation, the CPU can make a final decision and notify the UE, for example, through DL signaling, which AP should perform which type of DL transmission. With this information, the UE can take appropriate actions when receiving DL data.

[0028] Figure 3 shows an example of a signaling diagram illustrating a system that enables efficient cell-free MIMO transmission. UE310, AP320, and CPU330 may be similar to NE1010 and UE1020 as shown in FIG. 10 according to certain exemplary embodiments. As described above, UE310, AP320, and CPU330 may be configured to use multiple cell-free MIMO transmission strategies such as CJT, NCJT, and SM. Only one AP is shown, but AP320 may include any number of APs.

[0029] In various exemplary embodiments, different types of AP sub-clusters may be specialized to implement different types of cell-free MIMO transmission strategies. For example, the AP_CJT sub-cluster may indicate that the APs within this sub-cluster may perform CJT transmission, in which case all APs may transmit the same DL data stream to improve the DL reception SINR by UE310. Furthermore, there may be cases where UE310 does not need to distinguish each wireless link between UE310 and each AP320 within this sub-cluster.

[0030] Furthermore, the AP_NCJT sub-cluster may indicate that the AP320 within this sub-cluster may perform NCTJ transmission and may transmit the same DL data stream. In this sub-cluster, UE310 can first distinguish each wireless link between UE310 and the AP320 of this sub-cluster, and then UE310 can demodulate these wireless links to the soft bit level and combine them to combine the gains.

[0031] Furthermore, the AP_SP subcluster can indicate that APs within this subcluster implement a SM that enables multiple stream transmissions for higher throughput. In this subcluster, UE310 can process each radio link independently without performing a combining operation.

[0032] Similarly, the AP_NO subcluster can indicate that even when an AP is within the current serving cluster, the AP within this subcluster may not be scheduled to serve UE310 within a predetermined period. For example, the DL transmission arrival times of these APs may be outside the CP range, or else may lead to significant interference. If all APs are DL SYN, this subcluster may not exist, and all APs within this subcluster may belong to one of the first three subclusters.

[0033] UE310 and CPU330 can adjust regarding the determination of the serving AP cluster. For example, an AP320 connected to the same CPU (CPU330) having a received RSRP above a predefined threshold can be selected to create the serving cluster for UE310. Within this serving cluster, the type(s) of appropriate transmission strategy may be determined for each AP based on optimal system performance.

[0034] At 301, UE310 can send a request for DL self-free MIMO transmission update to CPU330. This request may be triggered, for example, due to an update of the serving cluster, a change in radio channel quality, and / or a degradation of the decoding performance on the UE side.

[0035] In 302, the CPU 330 can configure and trigger different DL wideband reference signal transmissions simultaneously on the APs 320 within the current serving cluster of the UE 310 (i.e., AP(1-6) and AP(15) of UE(1), AP(7) and AP(9-11) of UE(2), AP(13-17) of UE(3), AP(16) and AP(18-20) of UE(4)). In some exemplary embodiments, prior to the self-free MIMO transmission, the UE 310 can select a serving cluster, for example, based on large-scale information.

[0036] In 303, the CPU 330 can notify the UE 310 via the AP 320 of a configured DL reference signal resource that includes resources in the frequency domain, time domain, and code domain so that the CPU 330 can recognize from which AP 320 the UE 310 receives which reference signal and at what time. This operation can be performed in advance. For example, the DL reference signal frequency resource and code domain information may be transmitted to the UE 310 in advance by RRC signaling. For the notification of the time domain information, the UE 310 and the CPU 330 may be synchronized on the reference signal transmission time slot based on the X+n information, where X is the time slot for UE request transmission / reception in the UE 310 and the AP 320 respectively, and n may be a previously set parameter related to the link transmission delay between the CPU 330 and the AP 320.

[0037] In various exemplary embodiments, the CPU 330 can notify the UE 310 as to which of the APs among the APs 320 is the master AP, and in response, the non-master APs within the serving cluster can then become slave APs. This information can support the UE 310 in measuring and estimating the phase error between the master AP and all the slave APs. The CPU 330 can configure each AP 320 with a different reference signal in order to distinguish each radio link between the UE 310 and the AP 320. Further, the APs 320 may be triggered to simultaneously transmit different reference signals, enabling the UE 310 to measure the phase error between the APs 320 in a simultaneous transmission situation where the measured phase error may be accurate enough to support CJT at a later stage.

[0038] At 304, the UE 310 can estimate related information (e.g., enablement information, channel quality, network load, and / or resource utilization information) used in the proposal creation process (at 305), such as the phase error and radio link quality of each AP 320. Specifically, after the UE 310 receives a wideband reference signal from the AP 320, the UE 310 may estimate the phase error between the relevant APs. For example, the UE 310 may estimate the phase error between the master AP and all slave APs. For example, the UE 310 can use one signal, such as the signal of the master AP, as a reference. Phase rotation can be applied to the reference signal of each slave AP with an offset value based on a predetermined phase offset book. Phase rotation may be performed for each sub-band so that the UE 310 can derive the phase error information for each sub-band. The UE 310 may then, at 305, combine the baseline signal with the processed signal after phase rotation to determine their joint signal power. This procedure may be repeated until all offset values of the predetermined phase offset book are tested. In some exemplary embodiments, the "offset value" may refer to the maximum joint signal power considered as the phase error between the master AP and the corresponding slave AP on the target sub-band. Alternatively, the UE 310 can stop the test when the joint signal power exceeds a predetermined threshold, and in that case, the corresponding offset value may be considered as the phase error.

[0039] By performing phase error estimation, UE 310 can accurately determine the phase error for each sub-band between APs 320 and can improve CJT transmission. For example, if the maximum phase error among all sub-bands is less than a predetermined threshold, UE 310 can propose CJT transmission for those APs. Alternatively, if the smallest phase error among all sub-bands is greater than or equal to a predetermined threshold, UE 310 may not propose a CJT strategy because the CJT strategy may provide minimal benefit. CJT proposals for each sub-band may be possible, and UE 310 and / or CPU 330 may specify CJT for one sub-band if the associated phase error is below a predetermined threshold. Further, a wideband phase error may be estimated based on all sub-band phase error information and may be used as a matrix for CJT determination, and CJT and / or NCJT may be proposed if the calculated wideband phase error is below or above a predetermined threshold.

[0040] In 306, UE 310 can transmit a proposal indicating the determined role of each AP 320. Specifically, after measuring and calculating the phase error information, DL SYN status, and radio link quality information, UE 310 can propose a transmission strategy for AP 320 to optimize system performance.

[0041] For an AP320 with a phase error within a predetermined threshold, the UE310 can further determine the corresponding radio link quality to confirm which strategy, CJT or SM, should be proposed for these APs. For an AP320 with good radio link quality, the UE310 can roughly estimate the corresponding throughput (assuming that CJT or SM is implemented respectively) and recommend these APs to the strategy with higher throughput. For example, if CJT results in higher throughput than SM, the UE310 can recommend AP_CJT for these APs, and if not, it can recommend AP_SP. In the case of an AP with non-optimal radio link quality, since there is a possibility that the UE310 does not decode each radio link independently, sufficient performance gain may not be obtained with SM. Therefore, the UE310 can recommend CJT transmission for these APs to improve the SINR combining gain.

[0042] In the case of an AP with a small phase error referring to the master AP, the proposal by the UE310 may cover three potential situations, namely AP_CJT, AP_SP, or both AP_CJT and AP_SP. For both AP_CJT and AP_SP, some of the AP320s can perform CJT transmission together, and the other APs can perform SM independently. This hybrid self-free MIMO transmission concept may be effective for multiple antenna ports and / or sub-arrays in the UE310. For example, one antenna port can be used to receive CJT transmission from the associated AP, and the other ports and / or sub-arrays can be used to receive SM transmission from other AP320s.

[0043] In various exemplary embodiments, for AP320s where the phase error exceeds a predetermined threshold, since CJT may not provide optimal performance gain, UE310 may consider only these AP320s for NCJT or SM that may depend on the radio link quality. For example, for AP320s in this embodiment, UE310 can check the radio link quality. For AP320s with sufficient radio link quality, UE310 can check whether SM can improve the performance gain when multiple data streams are transmitted. If so, UE310 can propose AP_SP for these AP320s, and if not, UE310 can propose AP_NCJT for these AP320s. Similarly, for AP320s with insufficient radio link quality, since UE310 may not be able to normally decode each radio link independently, UE310 may propose only NCJT based on the combined gain to improve successful data reception. Therefore, in the case of AP320s with large phase errors, the proposal by UE310 may include three potential situations, namely AP_NCJT, AP_SP, or both AP_NCJT and AP_SP. In AP_NCJT and AP_SP, for AP320s within this category, UE310 can propose AP_NCJT for some of these AP320s and AP_SP for other AP320s.

[0044] Various exemplary embodiments may include AP320s where the DL reception time difference of UE310 exceeds a pre-set CP length. For these AP320s, UE310 may not propose that these AP320s provide DL transmission due to potential interference. Instead, UE310 may propose AP_NO for these AP320s, but AP_NO may not exist if all AP320s within the serving cluster are sufficiently synchronized.

[0045] In some exemplary embodiments, UE 310 can classify all APs 320 within the current serving cluster into four categories: AP_CJT, AP_NCJT, AP_SP, and AP_NO. However, similar to the categories for AP classification, more transmission mechanisms may be possible. UE 310 can recommend only one of the four sub-clusters for each of the APs 320 within the current serving cluster.

[0046] Furthermore, at 306, as shown in FIG. 2, UE 310 can explicitly propose four categories to CPU 330 for data transmission based on UL signaling. When UE 310 proposes AP_CJT, UE 310 can also report the corresponding phase error information of the corresponding AP 320 to CPU 330. Alternatively, UE 310 can report only the phase error information of the slave AP to CPU 330, enabling CPU 330 to design appropriate precoding and scheduling operations to mitigate the effects of such phase errors. When the phase error is related to the sub-band situation, the proposal from UE 310 can also include such sub-band information. As a result, with the proposal, CPU 330 can schedule CJT based on the reported sub-band information, thereby optimizing the performance of UE 310.

[0047] In some exemplary embodiments, appropriate UL signaling can be used so that the UE 310 can explicitly indicate four types of AP sub-clusters to the CPU 330. For example, the UL signaling can be sent to the CPU 330 by UL L1 signaling or media access control (MAC) layer signaling according to the feedback speed required by the CPU 330. In the case of L1 UL signaling, the CPU 330 can receive the proposal from the UE 310 as soon as possible and can immediately handle it to optimize the performance. In the case of UL L2 signaling, the CPU 330 may receive the feedback after a time delay, but since the change in the phase error may not be dynamic, the overhead of L1 signaling may have a greater impact on the UL system coverage, so layer 2 (L2) signaling may be used.

[0048] In various exemplary embodiments, the UE 310 can report using a structure similar to that illustrated in FIG. 2, which may include two parts of the role of the AP 320 and the related enabling information. The first part can include a proposal from the UE 310 on which AP should be used for data transmission with which strategy. The second part can include the phase error information for the proposed AP for CJT or for all APs within the current serving cluster of the UE 310. This phase error can be at the sub-band level or at the wide-band level. When the phase error is reported at the sub-band level, the UE 310 can also include the sub-band information corresponding to the UL report, indicating the frequency band for scheduling the CJT transmission to the CPU 330.

[0049] Figure 4 shows an example of UL signaling for reporting the role of each AP. For the first part shown in Figure 4, the variable-size signaling may include X fields, where X is the identifier of the AP 320 within the current serving cluster. The detailed value of each field can explicitly indicate the corresponding role of the AP 320 in the next transmission. The bits for each field can be determined by the supported cell-free MIMO transmission strategy. For example, at present, three strategies may be supported, so it may be sufficient to use 2 bits per field to identify each strategy. The detailed signal value of each field may correspond to a specific strategy according to the corresponding role of the AP recommended by the UE 310. For example, 00 may indicate that the corresponding AP may perform CJT with other APs 320 marked as "00", 01 may indicate that the corresponding AP 320 may perform NCJT with other APs 320 having the same value marked as "01", 10 may indicate that the corresponding AP 320 may transmit a dedicated data stream to the UE 310, and 11 may indicate that the corresponding AP 320 does not transmit a data stream to the UE 310 (even if it is within the current serving cluster).

[0050] In various exemplary embodiments, if additional transmission strategies are added to cell-free MIMO, the number of bits per field may also change without affecting the structure shown in Figure 2.

[0051] The second part shown in FIG. 2 may include the phase error of the AP recommended for CJT transmission. Alternatively, the phase error information may be reported for all slave APs, improving flexibility and providing the CPU 330 with an opportunity to schedule CJT transmission. Specifically, this phase error may be at the sub-band level or at the wide-band level. In the case of the sub-band level, the UE 310 can include the corresponding sub-band information in this report. Thereafter, the CPU 330 can perform CJT scheduling in the corresponding sub-band to improve system performance. The detailed design and the required number of bits may vary after determining the sub-band or wide-band phase error and the granularity of the phase error.

[0052] The UL signaling architecture may follow the examples depicted in FIGS. 2 and 4. As shown in FIG. 4, AP1, AP3, and the last AP may be candidates to coherently transmit the same data stream to the UE 310 to improve the SINR, and the UE 310 may not need to change the corresponding wireless links between these APs, but may combine the signals for demodulation and decoding. Further, AP4 may be recommended not to provide service to the UE 310, perhaps due to a large phase error and / or lack of DL SYN with other APs within the current serving cluster.

[0053] In 307, after the CPU 370 receives a proposal from the UE 310, the CPU 330 can determine the role of each of the APs 320 within the recommended range of the UE 310. For example, as shown in FIG. 4, the CPU 330 can consider the load and resource status of each AP, and / or the side antenna port configuration of the UE 310, and determine which transmission strategy(ies) can be assigned to each AP. Thereafter, the CPU 370 can determine the demodulation reference signal (DMRS) corresponding to each selected AP.

[0054] At 308, after making the determination at 307, the CPU 330 can indicate to the UE 310 the final determination for improving the performance of the UE 310. For example, the CPU 330 can explicitly notify the UE 310 which AP will implement which transmission strategy. For each finally selected AP, the CPU 330 can explicitly or implicitly notify the UE 310 of the corresponding DMRS, thereby assisting the UE 310 to perform optimal operations. For example, for the AP 320 determined for CJT, the same DMRS signal may be transmitted from these APs, and the UE 310 may not need to distinguish each radio link between the UE 310 and these APs. For APs that perform NCJT or SM transmission, different DMRSs may be transmitted from these APs so that the UE 310 can distinguish each radio link from these APs. The CPU 330 may not include APs not recommended for DL transmission in the DL signaling, thereby reducing the overhead of the DL signaling.

[0055] In various exemplary embodiments, the DMRS settings for each AP may enable the UE 310 to correctly perform DL reception operations. Each AP can pre-set at least two types of DMRS, such as DMRS_CJT (one DMRS common to all APs within its serving cluster) and / or DMRS_other (different DMRSs for each AP to enable the UE to distinguish each radio link between the AP 320 and the UE 310).

[0056] Therefore, for each UE in a self-free MIMO transmission situation, the CPU can configure (or reconfigure) these two types of DMRS based on the serving AP(s) list. Furthermore, the definitions of DMRS_CJT and DMRS_other may mean that the UE 310 should recognize which type of DMRS can be used for the following DL data reception before self-free MIMO transmission.

[0057] The CPU 330 needs to notify the UE 310 via DL signaling which AP 320s may adopt which strategies, and two or more options (described in more detail below) can be used to perform the corresponding DL signaling. First, DL signaling similar to that illustrated in FIG. 2 may be used to explicitly notify the UE 310 which AP 320s may be involved in the next transmission and which transmission strategies they may be involved in. Alternatively, the DL signaling can explicitly indicate the AP 320s involved in the next transmission, but implicitly indicate which transmission strategies can be used for these APs by indicating DMRS information. FIG. 5 shows how the first and last APs in the CJT subcluster are scheduled for DL CJT, and FIG. 6 shows how two APs are scheduled for NCJT.

[0058] To explicitly notify each UE, the DL signaling can use an architecture similar to that depicted in FIG. 2. In this option, the CPU 330 can explicitly notify the UE 310 which AP 320s are selected to implement which transmission strategies in order to optimize system performance. One difference from FIG. 4 is that the detailed signaling size may be less than or equal to the size of FIG. 4. In the DL signaling, the selected APs may be indicated within the range of AP 320s recommended by the UE 310, but the AP 320s marked without transmission recommendation may be excluded. Thus, the DL signaling may include Y 2-bit fields, where Y is equal to the number of AP numbers recommended by the UE 310 excluding the APs marked with "11". Each 2-bit field can indicate the transmission strategy of the corresponding AP. Since it is possible that more than three self-free MIMO transmission strategies are not supported, 2 bits may be used for each AP. However, without affecting the DL signaling architecture, more bits designed to indicate the situation of each AP can be used to add more transmission strategies.

[0059] When explicitly notifying the UE310, the CPU330 may mark the AP320 that is within the recommended range of the UE310 but not selected for transmission as "11". In this way, some APs may be recommended by the UE310 for DL transmission but may not be finally selected by the CPU330 for various reasons. The CPU330 can notify this information to the UE310 so that the UE310 can recognize from which AP320 and by which mechanism it receives DL transmission.

[0060] Furthermore, by explicitly notifying each UE, since the UE310 recognizes the Y information based on previous recommendations, DL signaling decoding may be possible. Specifically, based on the previous UL signal of the UE310, the UE310 can recognize the value of Y in advance, and then can correctly decode the DL signaling to improve data demodulation and decoding. The DL signaling architecture is shown in FIGS. 5 and 6.

[0061] In FIG. 5, the CPU may make a final decision that the first, second, and last APs may jointly transmit the same data stream to the UE coherently. After the UE decodes this DL signaling, the UE can recognize that the same DMRS_CJT is transmitted from the first, second, and last APs and may not receive DMRS from other APs. The UE can perform a reception operation by combining DL transmissions from these APs.

[0062] In FIG. 6, the UE can recognize that only two APs are non-coherently co-transmitting the same data stream to the UE, and different DMRSs may be transmitted from these two APs. Since the other APs are marked as "11", the UE may not attempt to decode the information from the other APs and may not receive data transmissions from them. The UE 310 can distinguish the two wireless links from these two APs by different DMRSs. Then, the UE 310 can independently demodulate these two wireless links to the soft bit level and combine them to improve the combining gain and decoding performance. As a result, the UE 310 can clearly identify which AP 320 uses which transmission strategy and can execute the optimal receiving function.

[0063] The above two examples only show one transmission strategy per TTI, but such a signaling structure can be very flexible to indicate a hybrid transmission strategy in the time domain. For example, some APs can co-transmit the same data stream to the UE 310 coherently. Further, some APs can transmit the same data stream to the UE 310 non-coherently and even support SM. This may depend on the number of antenna ports set in the UE 310 to support the reception of one or more data streams.

[0064] Another advantage of explicit DL signaling is that the CPU 330 may have sufficient flexibility to ultimately determine which AP 320 should use which transmission strategy. For example, according to the proposal by the UE 310, some APs 320 may be recommended for NCJT due to their large phase errors. However, if such phase errors are recognized by the CPU 330 (e.g., based on the phase error information reported by the UE 310), the CPU 330 can assist in compensating for such phase errors through the precoding operation. Any such APs may be configured for CJT, which may be different from the recommendation by the UE 310. Instead, the CPU 330 can indicate the differences to the UE 310. Despite the greater burden of DL signaling overhead, each role of the AP 320 may be indicated. If such DL signaling is transmitted by L2 MAC signaling, such overhead may be minimal.

[0065] Explicit indication enables the indication of one transmission strategy per TTI, but such a signaling structure can also be flexible for indicating hybrid transmission strategies in the time domain. For example, some APs can jointly transmit the same data stream to the UE 310 coherently. Additionally, some APs can transmit the same data stream to the UE 310 non - coherently and support SM. This may depend on the number of antenna ports the UE 310 is configured to support for receiving one or more data streams.

[0066] Furthermore, explicit instructions may provide the CPU 320 with the flexibility to determine which AP 320 should use which transmission strategy. For example, following the recommendations by the UE 310, some APs 320 may be recommended for NCJT due to the phase error between them. If the phase error is known to the CPU 330, for example, based on the phase error information reported by the UE 310, the CPU 330 can assist in compensating such a phase error by a precoding operation. Such APs 320 may be configured for CJT, which may be different from the recommendations by the UE 310. For such a situation, the CPU 330 can clearly indicate such a difference to the UE 310. A greater burden of DL signaling overhead can clearly indicate the role of each AP. Such DL signaling may not be important if it is transmitted via L2 MAC signaling.

[0067] In implicit DMRS signaling, the 2-bit signal content of each field can indicate the corresponding AP320 to which the transmission strategy is assigned, and the UE310 can implicitly recognize which type of DMRS is transmitted from these AP320s for the next action. The burden of such signaling may be a signaling size of 2*Y bits in order to obtain sufficient flexibility to select an AP for any type of transmission. While using implicit DMRS signaling, the recommendations by the UE310 can follow the role of each AP320, and the CPU330 only needs to make a final decision on which AP320 is involved in the next transmission, and there may be no operation to change the role of the AP320. Since the UE310 understands the phase error information, DL SYN status, and DL channel status information of each AP320, this option may also be effective and feasible. The UE310 can recommend a role for the AP320 that is reliable and accurate enough for any of CJT, NCJT, SM, or no transmission. Furthermore, the CPU330 does not attempt to change the recommended role of each AP320 and can make a final decision on whether each AP320 is scheduled for transmission. In the case of this option, the DL signaling design (shown in Figure 7) only needs to indicate whether each AP320 is scheduled for transmission.

[0068] As shown in Figure 7, the DL signaling design may include multiple sections based on the recommendations from the UE310, and each section refers to the APs recommended by the UE310 for CJT, NCJT, and SM, respectively. Each section can include a number of fields equal to the AP number of the corresponding transmission strategy previously recommended by the UE310. The detailed value may be 1 bit for each field to indicate whether the corresponding AP320 is scheduled for transmission. For example, if the bit in this embodiment is "1", it means that the corresponding AP is scheduled to adopt the UE-recommended transmission strategy. On the other hand, the corresponding AP may not be incorporated into the next transmission. The total DL signal size may be Y bits, where Y is the number of APs recommended by the UE310 for transmission.

[0069] The technique depicted in FIG. 6 can improve the receiving operation of UE310 and support the execution of a hybrid transmission strategy in the time domain, but compared with the technique depicted in FIG. 5, the signaling size is half. There is one advantage of using implicit DMRS signaling with the burden that the CPU may not change the role of AP320 recommended by UE310. Since the recommendation by UE310 may be based on its accurate DL information, this burden may be minimal.

[0070] FIG. 8 shows an example of a flowchart of a method that can be executed by a UE such as UE1020 shown in FIG. 10 in various exemplary embodiments.

[0071] As described above, a UE, as well as an AP and a CPU (such as NE1010 shown in FIG. 10), can be configured to use a plurality of self-free MIMO transmission strategies such as CJT, NCJT, and SM. Although only one AP is shown, any number of APs can be included.

[0072] In various exemplary embodiments, different types of AP sub-clusters can be specialized to implement different types of self-free MIMO transmission strategies. For example, the AP_CJT sub-cluster may indicate that the APs within this sub-cluster may perform CJT transmission, and all APs may transmit the same DL data stream to improve the DL reception SINR by the UE. Further, there may be cases where the UE does not need to distinguish each wireless link between the UE within this sub-cluster and each AP.

[0073] Furthermore, the AP_NCJT subcluster may indicate that APs within this subcluster may perform NCTJ transmissions and transmit the same DL data stream. In this subcluster, the UE can first distinguish each radio link between the UE and the APs of this subcluster, and the UE can then demodulate these radio links at the soft bit level and combine them to combine the gains.

[0074] Furthermore, the AP_SP subcluster can indicate that APs within this subcluster implement a SM that enables multiple stream transmissions for higher throughput. In this subcluster, the UE can process each radio link independently without performing a combining operation.

[0075] Similarly, the AP_NO subcluster can indicate that even if an AP is within the current serving cluster, the APs within this subcluster may not be scheduled to serve the UE within a predetermined period. The DL transmission arrival times of these APs may be outside the range of the CP, or otherwise may lead to significant interference. If all APs are DL SYNs, this subcluster may not exist, and all APs within this subcluster may belong to any of the first three subclusters.

[0076] The UE and the CPU can adjust regarding the determination of the serving AP cluster. For example, when creating the serving cluster of the UE, APs connected to the same CPU having a received RSRP above a predefined threshold may be selected. Within this serving cluster, an appropriate type(s) of transmission strategy may be determined for each AP based on optimal system performance.

[0077] In 801, the method may include transmitting a request for DL self-free MIMO transmission update to the CPU. The request may be triggered, for example, due to an update of the serving cluster, a change in the radio channel quality, and / or a degradation of the decoding performance on the UE side.

[0078] At 802, the method may include receiving, from the CPU via the AP, a configured DL reference signal resource including resources in the frequency domain, time domain, and code domain so that the UE can recognize which reference signal to receive from which AP and when. This operation can be performed in advance. For example, the frequency resource and code domain information of the DL reference signal may be transmitted to the UE in advance by RRC signaling. For the notification of time domain information, the UE and the CPU may be synchronized on the reference signal transmission time slot based on X+n information (where X is the time slot for UE request transmission and reception in the UE and the AP respectively, and n may be a previously configured parameter related to the link transmission delay between the CPU and the AP).

[0079] In various exemplary embodiments, the CPU can notify the UE which AP is the master AP, and accordingly, the non-master APs within the serving cluster can then be slave APs. This information can support the UE to measure and estimate the phase error between the master AP and all slave APs. The CPU can configure each AP with a different reference signal to distinguish each radio link between the UE and the AP. Further, the APs may be triggered to transmit different reference signals simultaneously, enabling the UE to measure the phase error between the APs in a simultaneous transmission situation where the measured phase error may be accurate enough to support CJT at a later stage.

[0080] At 803, the method may include estimating related information (e.g., enabling information, channel quality, network load, and / or resource utilization information) used in the proposal creation process (at 804), such as the phase error of each AP and the wireless link quality. Specifically, after receiving a wideband reference signal from an AP, the UE can estimate the phase error between the master AP and all slave APs. For example, the UE can use one signal, such as the signal of the master AP, as a reference. The phase rotation may be applied to the reference signal of each slave AP with an offset value based on a predetermined phase offset book. The phase rotation may be performed for each sub-band so that the UE can derive the phase error information for each sub-band.

[0081] At 804, the method may include combining the baseline signals with the post-phase rotation processing to determine their combined signal power. This procedure may be repeated until all offset values of a predetermined phase offset book have been tested.

[0082] In some exemplary embodiments, the "offset value" may refer to the maximum combined signal power considered as the phase error between the master AP and the corresponding slave AP on the target sub-band. Alternatively, the UE can stop the test when the combined signal power exceeds a predetermined threshold and consider the corresponding offset value as the phase error.

[0083] By performing phase error estimation, the UE can accurately determine the phase error for each sub-band between APs and improve CJT transmission. For example, if the maximum phase error among all sub-bands is less than a predetermined threshold, the UE can propose CJT transmission for those APs. Alternatively, if the minimum phase error among all sub-bands exceeds a predetermined threshold, the UE may not propose a CJT strategy because the CJT strategy may bring only minimal benefits. CJT proposals for each sub-band are also possible, and the UE and / or CPU can specify CJT for one sub-band if the relevant phase error is below a predetermined threshold. Further, a wideband phase error may be estimated based on all sub-band phase error information and used as a matrix for CJT determination. CJT and / or NCJT may be proposed if the calculated wideband phase error is below or above a predetermined threshold.

[0084] At 805, the UE can send a proposal indicating the determined role of each AP. Specifically, after measuring and calculating the phase error information, DL SYN status, and radio link quality information, the UE can propose an AP's transmission strategy to optimize system performance.

[0085] For APs with phase error within a predetermined threshold, the UE can further determine the corresponding radio link quality and check which strategy, CJT or SM, should be proposed for these APs. For APs with good radio link quality, the UE can roughly estimate the corresponding throughput (assuming CJT or SM is implemented respectively) and recommend these APs to a strategy with higher throughput. For example, if CJT brings higher throughput than SM, the UE can recommend AP_CJT for these APs, otherwise it can recommend AP_SP. In the case of APs with suboptimal radio link quality, since the UE may not decode each radio link independently, SM may not achieve sufficient performance gain. Therefore, the UE can recommend CJT transmission for these APs to improve the SINR combining gain.

[0086] In the case of APs with small phase error referring to the master AP, the proposal by the UE may cover three potential situations: AP_CJT, AP_SP, or both AP_CJT and AP_SP. In both AP_CJT and AP_SP, some APs can perform CJT transmission together, and other APs can perform SM transmission independently. This hybrid cell-free MIMO transmission concept may be effective for multiple antenna ports and / or subarrays in the UE. For example, one antenna port may be used to receive CJT transmission from the associated AP, and other ports and / or subarrays may be used to receive SM transmission from other APs.

[0087] In various exemplary embodiments, for an AP whose phase error exceeds a predetermined threshold, since the CJT may not provide an optimal performance gain, the UE may consider only these APs for NCJT or SM that may depend on the radio link quality. For example, for the APs in this embodiment, the UE can check the radio link quality. For APs with sufficient radio link quality, when multiple data streams are transmitted, the UE can check whether the performance gain is improved by the SM. If so, the UE can propose AP_SP for these APs, and if not, the UE can propose AP_NCJT for these APs. Similarly, for APs with insufficient radio link quality, since the UE may not be able to normally decode each radio link independently, the UE can only propose NCJT based on the combined gain to improve the success rate of data reception. Therefore, for APs with a large phase error, the proposals by the UE may include three situations: AP_NCJT, AP_SP, or both AP_NCJT and AP_SP. In AP_NCJT and AP_SP, for the APs within this category, the UE can propose AP_NCJT for some of these APs and AP_SP for other APs.

[0088] Various exemplary embodiments may include APs for which the UE's DL reception time difference exceeds a predetermined CP length. For these APs, the UE may not propose that these APs provide DL transmission due to potential interference. Instead, the UE can propose AP_NO for these APs, but AP_NO may not exist if all the APs within the serving cluster are sufficiently synchronized.

[0089] In some exemplary embodiments, the UE can classify all APs in the current serving cluster into four categories: AP_CJT, AP_NCJT, AP_SP, and AP_NO. However, similar to the categories of AP classification, more transmission mechanisms may be possible. The UE can recommend only one of the above four sub-clusters for each AP in the current serving cluster.

[0090] Furthermore, at 805, as shown in FIG. 2, the UE can explicitly propose four categories to the CPU for data transmission based on UL signaling. When the UE proposes AP_CJT, the UE can also report the phase error information of the corresponding AP to the CPU. Alternatively, the UE can report only the phase error information of the slave AP to the CPU, enabling the CPU to design appropriate precoding and scheduling operations to mitigate the impact of such phase errors. When the phase error is related to the sub-band situation, the proposal from the UE may also include such sub-band information. As a result, the proposal enables the CPU to schedule CJT based on the reported sub-band information, potentially optimizing the UE's performance.

[0091] In some exemplary embodiments, appropriate UL signaling can be used so that the UE can explicitly indicate the four types of AP sub-clusters to the CPU. For example, the UL signaling may be transmitted to the CPU by UL L1 signaling or MAC layer signaling depending on the feedback rate required by the CPU. In the case of L1 UL signaling, the CPU can receive the proposal from the UE as soon as possible and immediately take action to optimize performance. In the case of UL L2 signaling, the CPU may receive the feedback with a time delay. However, since the change in the phase error may not be dynamic, layer 2 (L2) signaling, which may have a significant impact on the UL system coverage due to the overhead of L1 signaling, may be used.

[0092] In various exemplary embodiments, the UE can report using a structure similar to that illustrated in FIG. 2, including two parts of the role of the AP and the associated enabling information. The first part can include proposals from the UE regarding which AP should use which strategy for transmission. The second part may include phase error information regarding the proposed AP for CJT, or all APs within the current serving cluster of the UE. This phase error can be at the sub-band level or the wide-band level. When the phase error is reported at the sub-band level, the UE can also include sub-band information corresponding to the UL report, indicating the frequency band for scheduling CJT transmissions by the CPU.

[0093] FIG. 4 shows an example of UL signaling for reporting the role of each AP. For the first part of FIG. 2 shown in FIG. 4, the variable-size signaling may include X fields, where X is the identifier of the APs within the current serving cluster. The detailed value of each field can explicitly indicate the corresponding role of the AP in the next transmission. The bits for each field may be determined by the supported self-free MIMO transmission strategies. For example, since three strategies may be supported, two bits per field may be sufficient to identify each strategy. The detailed signal value of each field may correspond to a specific strategy according to the corresponding role of the AP recommended by the UE. For example, 00 may indicate that the corresponding AP may perform CJT with other APs marked as "00", 01 may indicate that the corresponding AP may perform NCJT with other APs having the same value marked as "01", 10 may indicate that the corresponding AP may transmit a dedicated data stream to the UE, and 11 may indicate that the corresponding AP does not transmit a data stream to the UE (even if it is within the current serving cluster).

[0094] In various exemplary embodiments, when a transmission strategy is further added to self-free MIMO, the number of bits per field may also change without affecting the structure shown in FIG. 2.

[0095] The second part shown in FIG. 2 can include the phase error of the AP recommended for CJT transmission. Alternatively, the phase error information may be reported for all slave APs, improving flexibility and providing the CPU with an opportunity to schedule CJT transmission. Specifically, this phase error may be at the sub-band level or the wide-band level. In the case of the sub-band level, the UE can include the corresponding sub-band information in this report. Subsequently, the CPU can perform CJT scheduling in the corresponding sub-band to improve system performance. The detailed design and the required number of bits may change after determining the sub-band or wide-band phase error and the granularity of the phase error.

[0096] The UL signaling architecture may follow the examples shown in FIGS. 2 and 4. As shown in FIG. 4, AP1, AP3, and the last AP may be candidates to coherently transmit the same data stream to the UE to improve the SINR, and the UE may not need to change the corresponding radio link between these APs, but may combine the signals for demodulation and decoding. Further, AP4 may be recommended not to provide service to the UE, perhaps due to a large phase error and / or the absence of a DL SYN with other APs in the current serving cluster.

[0097] At 806, the method may include receiving an instruction from the CPU for a final decision to improve the performance of the UE. For example, the UE may receive an explicit instruction regarding which AP implements which transmission strategy. For each finally selected AP, the UE may be explicitly or implicitly notified of the corresponding DMRS, which may assist the UE in optimal operation. For example, for the APs determined for CJT, the same DMRS signal may be transmitted from these APs, and the UE may not need to distinguish each radio link between the UE and these APs. For the APs performing NCJT or SM transmission, different DMRSs may be transmitted from these APs so that the UE can distinguish each radio link from these APs. The UE may not include in the DL signaling the APs that do not recommend DL transmission, thereby reducing the overhead of the DL signaling.

[0098] In various exemplary embodiments, the DMRS settings of each AP may enable the UE to correctly perform DL reception operations. Each AP can pre-set at least two types of DMRSs, such as DMRS_CJT (one DMRS common to all APs within its serving cluster) and / or DMRS_other (different DMRSs for each AP to enable the UE to distinguish each radio link between the AP and the UE).

[0099] Therefore, for each UE in the self-free MIMO transmission situation, the CPU can set (or re-set) these two types of DMRSs based on the serving AP(s) list. Furthermore, the definition of DMRS_CJT and DMRS_other may mean that the UE needs to know in advance which type of DMRS to use for the next DL data reception before the self-free MIMO transmission.

[0100] The CPU needs to notify the UE, via DL signaling, of which APs may adopt which strategies, and two or more options (described in more detail below) can be used to perform the corresponding DL signaling. First, DL signaling similar to that shown in Figure 2 may be used to explicitly notify the UE310 of which APs may be involved in the next transmission and which transmission strategies they may be involved in. Alternatively, the DL signaling can explicitly indicate the APs320 involved in the next transmission, but implicitly indicate which transmission strategies can be used for these APs by indicating the DMRS information. Figure 5 shows how the first and last APs in the CJT subcluster are scheduled for DL CJT, and Figure 6 shows how two APs are scheduled for NCJT.

[0101] To explicitly notify each UE, the DL signaling can use an architecture similar to that depicted in Figure 2. In this option, the CPU can explicitly notify the UE of which APs are selected to implement which transmission strategies in order to optimize system performance. One difference from Figure 4 is that the detailed signaling size may be less than that of Figure 4. In DL signaling, the selected APs may be shown within the range of APs recommended by the UE, but the APs marked without transmission recommendation may be excluded. Thus, the DL signaling may include Y 2-bit fields, where Y is equal to the number of AP numbers recommended by the UE, excluding the APs marked "11". Each 2-bit field can indicate the transmission strategy of the corresponding AP. Since more than three self-free MIMO transmission strategies are not supported, 2 bits can be used for each AP. However, without affecting the DL signaling architecture, more bits designed to indicate the situation of each AP can be used to add more transmission strategies.

[0102] When explicitly notifying the UE, the CPU may mark APs that are within the UE's recommended range but not selected for transmission as "11". In this way, some APs may be recommended by the UE for DL transmission but may ultimately not be selected by the CPU for various reasons. The CPU can notify the UE of this information so that the UE can recognize from which APs and by which mechanism it will receive DL transmission.

[0103] Furthermore, by explicitly notifying each UE, since the UE recognizes Y information based on previous recommendations, it may be possible to decode DL signaling. Specifically, based on the UE's previous UL signals, the UE can pre-recognize the value of Y and then correctly decode the DL signaling to improve data demodulation and decoding. The architectures of DL signaling are shown in FIGS. 5 and 6.

[0104] In FIG. 5, the CPU can make a final decision that the first, second, and last APs may jointly transmit the same data stream to the UE coherently. After the UE decodes this DL signaling, the UE can recognize that the same DMRS_CJT is transmitted from the first, second, and last APs and may not receive DMRS from other APs. The UE can perform a receiving operation by combining DL transmissions from these APs.

[0105] In FIG. 6, the UE can recognize that only two APs are jointly transmitting the same data stream to the UE non-coherently, and different DMRS may be transmitted from these two APs. The UE can distinguish the two wireless links from these two APs by different DMRS. The UE can then demodulate these two wireless links independently up to the soft bit level and combine them to improve the combining gain and decoding performance. Thereby, the UE can clearly identify which AP is using which transmission strategy and perform an optimal receiving function.

[0106] In the above two examples, only one transmission strategy is shown for each TTI. However, such a signaling structure can be made very flexible to indicate a hybrid transmission strategy in the time domain. For example, several APs can jointly transmit the same data stream to the UE coherently. Further, some APs can transmit the same data stream to the UE non - coherently and can also support SM. This may depend on the number of antenna ports configured at the UE to support the reception of one or more data streams.

[0107] Another advantage of explicit DL signaling is that the CPU may have sufficient flexibility to finally determine which AP should use which transmission strategy. For example, according to a proposal by the UE, some APs may be recommended to use NCJT because of a large phase error. However, if such a phase error is known to the CPU (e.g., based on the phase error information reported by the UE), the CPU may be useful in compensating such a phase error by the precoding operation. All such APs may be configured for CJT and may be different from the UE's recommendation. Instead, the CPU can indicate the difference to the UE. Despite the burden of more DL signaling overhead, each role of the AP may be indicated. If such DL signaling is transmitted by L2 MAC signaling, such overhead may be minimized.

[0108] Explicit indication enables the indication of one transmission strategy per TTI, but such a signaling structure can also be made flexible for indicating hybrid transmission strategies in the time domain. For example, several APs can jointly and coherently transmit the same data stream to the UE. Further, several APs can transmit the same data stream to the UE non-coherently and support SM. This may depend on the number of antenna ports configured at the UE to support the reception of one or more data streams.

[0109] Furthermore, explicit indication may provide the CPU with the flexibility to determine which AP should use which transmission strategy. For example, following the recommendation by the UE, some APs may be recommended for NCJT due to the phase error between APs. For example, if the phase error is known by the CPU based on the phase error information reported by the UE, the CPU can assist in compensating such a phase error by a precoding operation. Such APs may be configured for CJT and may be different from the recommendation by the UE. In such a situation, the CPU can clearly indicate such a difference to the UE. More DL signaling overhead may clarify the role of each AP. Such DL signaling may be minor if it is transmitted via L2 MAC signaling.

[0110] In implicit DMRS signaling, the 2-bit signal content of each field can indicate the corresponding AP to which the transmission strategy is assigned, and the UE can implicitly recognize which type of DMRS is transmitted from these APs for the next action. The burden of such signaling can be a 2*Y-bit signaling size to obtain sufficient flexibility to select an AP for any type of transmission. While using implicit DMRS signaling, the recommendation by the UE regarding the role of each AP is that the CPU only makes the final decision on which AP is involved in the next transmission, and there may be no operation to change the role of the AP. The UE can also have and realize this option to understand the phase error information, DL SYN status, and DL channel status information of each AP. The UE310 can recommend a sufficiently reliable and accurate role of the AP for any of CJT, NCJT, SM, or no transmission. Furthermore, the CPU does not attempt to change the recommended role of each AP and can make the final decision on whether each AP is scheduled for transmission. In the case of this option, the DL signaling design (shown in FIG. 7) only needs to indicate whether each AP is scheduled or not.

[0111] As shown in FIG. 7, the DL signaling design may include multiple sections based on the recommendations from the UE, and each section refers to the APs recommended by the UE for CJT, NCJT, and SM, respectively. Each section can include a number of fields equal to the AP number of the corresponding transmission strategy previously recommended by the UE. The detailed value can be 1 bit for each field to indicate whether the corresponding AP is scheduled for transmission. For example, if the bit in this embodiment is "1", it means that the corresponding AP is scheduled to take the transmission strategy recommended by the UE. On the other hand, the corresponding AP may not be incorporated into the next transmission. The total DL signal size can be Y bits, where Y is the number of APs recommended by the UE for transmission.

[0112] The technique shown in FIG. 6 can improve the receiving operation of the UE and support the execution of the hybrid transmission strategy in the time domain. However, compared with the technique shown in FIG. 5, the signaling size can be halved. One of the advantages of using implicit DMRS signaling is the burden that the CPU may not change the role of the AP recommended by the UE. Since the recommendation by the UE is based on accurate DL information, this burden may be minimized.

[0113] FIG. 9 shows an example of a flowchart of a method that can be executed by an NE such as NE1010 shown in FIG. 10 in various exemplary embodiments.

[0114] As described above, the NE, as well as the AP and the UE (such as NE1010 and UE1020 shown in FIG. 10), can be configured to use a plurality of self-free MIMO transmission strategies such as CJT, NCJT, and SM. Although only one AP is shown, any number of APs can be included.

[0115] In various exemplary embodiments, different types of AP sub-clusters can be specialized to implement different types of self-free MIMO transmission strategies. For example, the AP_CJT sub-cluster may indicate that the APs within this sub-cluster may perform CJT transmission, and all APs may transmit the same DL data stream to improve the DL reception SINR by the UE. Further, there may be cases where the UE does not need to distinguish each wireless link between the UE and each AP within this sub-cluster.

[0116] Furthermore, the AP_NCJT sub-cluster may indicate that the APs within this sub-cluster may perform NCTJ transmission and may transmit the same DL data stream. In this sub-cluster, the UE can first distinguish each wireless link between the UE and the APs of this sub-cluster, and then the UE can demodulate these wireless links to the soft bit level and combine them to combine the gains.

[0117] Furthermore, the AP_SP subcluster can indicate that the APs within this subcluster implement SMs that enable multiple stream transmissions for higher throughput. In this subcluster, the UE can process each radio link independently without performing a combining operation.

[0118] Similarly, the AP_NO subcluster may notify that even if an AP is within the current serving cluster, the APs within this subcluster may not be scheduled to provide service to the UE within a predetermined period. The arrival times of the DL transmissions of these APs may be outside the CP range, or otherwise may lead to significant interference. If all APs are DL SYNs, this subcluster may not exist, and all APs within this subcluster may belong to any of the first three subclusters.

[0119] The UE and the CPU can coordinate regarding the determination of the serving AP cluster. For example, when creating the UE's serving cluster, APs connected to the same CPU having a received RSRP above a predefined threshold may be selected. Within this serving cluster, the appropriate type(s) of transmission strategy may be determined for each AP based on optimal system performance.

[0120] At 901, the method may include receiving a request for DL self-free MIMO transmission update from the UE. The request may be triggered, for example, due to an update of the serving cluster, a change in radio channel quality, and / or a degradation of the decoding performance on the UE side.

[0121] At 902, the method may include transmitting, via an AP to a UE, a configured DL reference signal resource that includes resources in the frequency domain, time domain, and code domain so that the UE recognizes which reference signal from which AP to receive at what time. This operation can be performed in advance. For example, the frequency resource and code domain information of the DL reference signal may be transmitted to the UE in advance by RRC signaling. For notification of time domain information, the UE and the CPU may be synchronized on the reference signal transmission time slot based on X + n information (where X is the time slot for UE request transmission and reception at the UE and the AP respectively, and n may be a previously set parameter related to the link transmission delay between the CPU and the AP).

[0122] In various exemplary embodiments, the CPU can notify the UE which AP is the master AP, and accordingly, non-master APs within the serving cluster may then be slave APs. This information can support the UE to measure and estimate the phase error between the master AP and all slave APs. The CPU can configure each AP with a different reference signal to distinguish each radio link between the UE and the AP. Further, when the APs may be triggered to transmit different reference signals simultaneously, in a simultaneous transmission situation where the measured phase error may be accurate enough to support CJT at a later stage, the UE can measure the phase error between the APs.

[0123] At 903, the NE may receive a proposal indicating the role determined for each of the APs. Specifically, after measuring and calculating the phase error information, DL SYN status, and radio link quality information, the UE can propose a transmission strategy for the APs to optimize the system performance.

[0124] For APs with phase errors within a predetermined threshold, the UE can further determine the corresponding radio link quality and check which strategy, CJT or SM, should be proposed for these APs. For APs with good radio link quality, the UE can roughly estimate the corresponding throughput (assuming CJT or SM is implemented respectively) and recommend these APs to the strategy with higher throughput. For example, if CJT results in higher throughput than SM, the UE can recommend AP_CJT for these APs, and if not, it can recommend AP_SP. In the case of APs with suboptimal radio link quality, since the UE may not decode each radio link independently, SM may not achieve sufficient performance gain. Therefore, the UE can recommend CJT transmission for these APs to improve the SINR combining gain.

[0125] In the case of APs with small phase errors referring to the master AP, the proposal by the UE may cover three situations: AP_CJT, AP_SP, or both AP_CJT and AP_SP. In both AP_CJT and AP_SP, some APs can perform CJT transmission together, and other APs can perform SM transmission independently. This hybrid cell-free MIMO transmission concept may be effective for multiple antenna ports and / or subarrays in the UE. For example, one antenna port can be used to receive CJT transmission from the associated AP, and other ports and / or subarrays can be used to receive SM transmission from other APs.

[0126] In various exemplary embodiments, for APs where the phase error exceeds a predetermined threshold, since CJT may not provide optimal performance gain, the UE may consider only these APs for NCJT or SM that may depend on the radio link quality. For example, for the APs in this embodiment, the UE can check the radio link quality. For APs with sufficient radio link quality, when multiple data streams are transmitted, the UE can check whether the performance gain is improved by SM. If so, the UE can propose AP_SP for these APs, and if not, the UE can propose AP_NCJT for these APs. Similarly, for APs with insufficient radio link quality, since the UE may not be able to normally decode each radio link independently, the UE can only propose NCJT based on the combined gain to improve the success rate of data reception. Therefore, for APs with a large phase error, the proposals by the UE may include three situations: AP_NCJT, AP_SP, or both AP_NCJT and AP_SP. In AP_NCJT and AP_SP, for the APs within this category, the UE can propose AP_NCJT for some of these APs and AP_SP for other APs.

[0127] Various exemplary embodiments can include APs where the difference in the UE's DL reception time exceeds a predetermined CP length. For these APs, the UE may not propose that these APs provide DL transmission due to potential interference. Instead, the UE can propose AP_NO for these APs, but AP_NO may not be present if all the APs within the serving cluster are sufficiently synchronized.

[0128] In some exemplary embodiments, the UE can classify all APs within the current serving cluster into four categories: AP_CJT, AP_NCJT, AP_SP, and AP_NO. However, similar to the categories of AP classification, more transmission mechanisms may be possible. The UE can recommend only one of the above four sub-clusters for each AP within the current serving cluster.

[0129] Furthermore, in 903, the NE can receive, as shown in FIG. 2, four explicit category proposals for data transmission from the CPU based on UL signaling. When the UE proposes AP_CJT, the UE can also report the corresponding phase error information of the corresponding AP to the CPU. Alternatively, the UE can also report only the phase error information of the slave AP to the CPU. In order to mitigate the impact of such phase errors, the CPU can design appropriate precoding and scheduling operations. When the phase error is related to the sub-band situation, the proposal from the UE may also include such sub-band information. As a result, the proposal enables the CPU to schedule CJT based on the sub-band information reported, and the performance of the UE may be optimized.

[0130] In some exemplary embodiments, appropriate UL signaling can be used so that the UE can explicitly indicate four types of AP sub-clusters to the CPU. For example, the UL signaling may be transmitted to the CPU by UL L1 signaling or MAC layer signaling according to the feedback speed required by the CPU. In the case of L1 UL signaling, the CPU can receive the proposal from the UE as soon as possible and can immediately take action to optimize the performance. In the case of UL L2 signaling, the CPU may receive the feedback with a time delay. However, since the change in the phase error may not be dynamic, layer 2 (L2) signaling, which may have a significant impact on the UL system coverage due to the overhead of L1 signaling, may be used.

[0131] In various exemplary embodiments, the UE can report using a structure similar to that illustrated in FIG. 2, including two parts of the role of the AP and the associated enabling information. The first part can include a proposal from the UE regarding which AP should use which strategy for transmission. The second part may include phase error information regarding the proposed AP for CJT or all APs within the UE's current serving cluster. This phase error can be at the sub-band level or the wide-band level. When the phase error is reported at the sub-band level, the UE can also include sub-band information corresponding to the UL report, indicating the frequency band for scheduling CJT transmission by the CPU.

[0132] FIG. 4 shows an example of UL signaling for reporting the role of each AP. For the first part shown in FIG. 4, the variable-size signaling may include X fields, where X is the identifier of the APs within the current serving cluster. The detailed value of each field can explicitly indicate the corresponding role of the AP in the next transmission. The bits for each field may be determined by the supported self-free MIMO transmission strategies. For example, since three strategies may be supported, two bits per field may be sufficient to identify each strategy. The detailed signal value of each field may correspond to a specific strategy according to the role of the corresponding AP recommended by the UE. For example, 00 indicates that the corresponding AP may perform CJT with other APs marked "00", 01 indicates that the corresponding AP may perform NCJT with other APs having the same value marked "01", 10 indicates that the corresponding AP may transmit a dedicated data stream to the UE, and 11 may indicate that the corresponding AP does not transmit a data stream to the UE (even if it is within the current serving cluster).

[0133] In various exemplary embodiments, when a transmission strategy is further added to the self-free MIMO, the number of bits per field may also change without affecting the structure shown in FIG. 2.

[0134] The second part shown in FIG. 2 can include the phase error of the AP recommended for CJT transmission. Alternatively, the phase error information may be reported for all slave APs, improving flexibility and providing an opportunity for the CPU to schedule CJT transmission. Specifically, this phase error may be at the sub-band level or the wide-band level. In the case of the sub-band level, the UE can include the sub-band information corresponding to this report. Subsequently, the CPU can perform CJT scheduling in the corresponding sub-band and improve the system performance. The detailed design and the required number of bits may change after determining the sub-band or wide-band phase error and the granularity of the phase error.

[0135] The UL signaling architecture can follow the examples shown in FIGS. 2 and 4. As shown in FIG. 4, AP1, AP3, and the last AP may be candidates to coherently transmit the same data stream to the UE to improve the SINR, and the UE may not need to change the corresponding radio links between these APs, but may combine the signals for demodulation and decoding. Further, AP4 may be recommended not to provide service to the UE, perhaps because of a large phase error and / or because there is no DL SYN with other APs within the current serving cluster.

[0136] At 904, after the CPU receives a proposal from the UE, the method may further include determining the respective roles of each AP within the UE's recommended range. The respective roles of each AP may be determined based on at least one of the strategy and enablement information received from the user equipment at 805. Further, the channel quality information for each UE already available at the CPU may be considered, either alone or in combination with other information, when determining the respective roles of each AP. For example, as shown in FIG. 4, the CPU can consider the load and resource status of each AP, and / or the UE's side antenna port configuration, to determine which transmission strategy(ies) are assigned to each AP. Thereafter, the CPU can determine the demodulation reference signal (DMRS) corresponding to each selected AP.

[0137] At 905, the method may include transmitting, by the CPU, an instruction regarding a final decision to the UE to improve performance. For example, the NE can transmit an explicit instruction regarding which AP is to implement which transmission strategy. For each finally selected AP, the UE may be explicitly or implicitly notified of the corresponding DMRS, which may assist in the optimal operation of the UE. For example, for the APs determined for CJT, the same DMRS signal may be transmitted from these APs, and the UE may not need to distinguish between each radio link between the UE and these APs. For the APs performing NCJT or SM transmission, different DMRSs may be transmitted from these APs so that the UE can distinguish each radio link from these APs. The UE may not include APs that do not recommend DL transmission in DL signaling, thereby reducing the overhead of DL signaling.

[0138] In various exemplary embodiments, the DMRS configuration of each AP may enable the UE to correctly perform DL reception operations. Each AP can pre-configure at least two types of DMRS, such as DMRS_CJT (one DMRS common to all APs within its serving cluster) and / or DMRS_other (a different DMRS for each AP to enable the UE to distinguish each wireless link between the AP and the UE).

[0139] Therefore, for each UE in a self-free MIMO transmission scenario, the CPU can configure (or reconfigure) these two types of DMRS based on the serving AP(s) list. Furthermore, the definitions of DMRS_CJT and DMRS_other may mean that the UE needs to know which type of DMRS to use for the next DL data reception before self-free MIMO transmission.

[0140] The CPU needs to notify the UE by DL signaling which AP may adopt which strategy, and two or more options (described in more detail below) can be used to perform the corresponding DL signaling. First, DL signaling similar to that shown in FIG. 2 may be used to explicitly notify the UE310 which AP may be involved in the next transmission and which transmission strategy may be involved. Alternatively, the DL signaling can explicitly indicate the AP320 involved in the next transmission, but by indicating the DMRS information, it can implicitly indicate which transmission strategy can be used for these APs. FIG. 5 shows the first and last APs in the CJT subcluster being scheduled for DL CJT, and FIG. 6 shows two APs being scheduled for NCJT.

[0141] To explicitly notify each UE, DL signaling can use an architecture similar to that depicted in Figure 2. In this option, the CPU can explicitly notify the UE which AP is selected to implement which transmission strategy in order to optimize system performance. One difference from Figure 4 is that the detailed signaling size may be less than that of Figure 4. In DL signaling, the selected AP may be shown within the range of APs recommended by the UE, but the APs marked without transmission recommendation may be excluded. Therefore, DL signaling may include Y 2-bit fields, where Y is equal to the number of AP numbers recommended by the UE excluding those marked "11". Each 2-bit field can indicate the transmission strategy of the corresponding AP. Since more than three self-free MIMO transmission strategies are not supported, 2 bits can be used for each AP. However, without affecting the DL signaling architecture, more bits designed to indicate the situation of each AP can be used to add more transmission strategies.

[0142] When explicitly notifying the UE, the CPU may mark as "11" the APs that are within the UE's recommended range but not selected for transmission. Thus, some APs may be recommended by the UE for DL transmission but may not be finally selected by the CPU for various reasons. The CPU can notify this information to the UE so that the UE can recognize from which AP and by which mechanism it will receive DL transmission.

[0143] Furthermore, by explicitly notifying each UE, since the UE recognizes the Y information based on previous recommendations, DL signaling decoding may be enabled. Specifically, based on the UE's previous UL signal, the UE can pre-recognize the value of Y, and then correctly decode the DL signaling to improve data demodulation and decoding. The DL signaling architecture is shown in Figures 5 and 6.

[0144] In Figure 5, the CPU may make a final decision that the first, second, and last APs may jointly and coherently transmit the same data stream to the UE. After the UE decodes this DL signaling, the UE can recognize that the same DMRS_CJT is transmitted from the first, second, and last APs and that it may not receive DMRS from other APs. The UE can perform a receive operation by combining DL transmissions from these APs.

[0145] In Figure 6, the UE can recognize that only two APs are jointly and non - coherently transmitting the same data stream to the UE, and different DMRS may be transmitted from these two APs. The UE can distinguish the two radio links from these two APs by different DMRS. The UE can then demodulate these two radio links independently down to the soft - bit level and combine them to improve the combining gain and decoding performance. Thereby, the UE can clearly identify which AP uses which transmission strategy and perform an optimal receiving function.

[0146] The above two examples only show one transmission strategy per TTI, but such a signaling structure can be very flexible to show hybrid transmission strategies in the time domain. For example, several APs can jointly and coherently transmit the same data stream to the UE. Furthermore, some APs can transmit the same data stream to the UE non - coherently and may even support SM. This may depend on the number of antenna ports set in the UE to support the reception of one or more data streams.

[0147] Another advantage of explicit DL signaling is that the CPU may have sufficient flexibility to ultimately determine which AP should use which transmission strategy. For example, according to a proposal by the UE, some APs may be recommended to use NCJT because of a large phase error. However, if such a phase error is recognized by the CPU (e.g., based on the phase error information reported by the UE), the CPU can assist in compensating for such a phase error by a precoding operation. All such APs may be configured for CJT and may differ from the UE's recommendation. Instead, the CPU can indicate the difference to the UE. Despite the burden of more DL signaling overhead, each role of the AP may be indicated. If such DL signaling is transmitted by L2 MAC signaling, such overhead may be minimized.

[0148] Explicit indication enables the indication of one transmission strategy per TTI, but such a signaling structure can also be flexible for indicating a hybrid transmission strategy in the time domain. For example, some APs can jointly transmit the same data stream to the UE coherently. Further, some APs can transmit the same data stream to the UE non - coherently and support SM. This may depend on the number of antenna ports to which the UE is configured to support the reception of one or more data streams.

[0149] Furthermore, explicit instructions may provide the CPU with the flexibility to determine which AP should use which transmission strategy. For example, following the recommendation by the UE, due to the phase error between APs, some APs may be recommended for NCJT. For example, if the phase error is known by the CPU based on the phase error information reported by the UE, the CPU can assist in compensating such a phase error through pre-coding operations. Such APs may be set for CJT and may differ from the recommendation by the UE. In such a case, the CPU can clearly indicate such a difference to the UE. The burden of increasing the overhead of DL signaling may clarify the role of each AP. If such DL signaling is transmitted via L2 MAC signaling, such overhead may be minor.

[0150] In implicit DMRS signaling, the 2-bit signal content of each field can indicate the corresponding AP to which the transmission strategy is assigned, and the UE can implicitly recognize which type of DMRS is transmitted from these APs for the next action. The burden of such signaling can be a 2*Y-bit signaling size to obtain sufficient flexibility to select APs for any type of transmission. While using implicit DMRS signaling, it is possible to follow the recommendation by the UE regarding the role of each AP, and the CPU may only finally determine which AP is involved in the next transmission, without the operation of changing the role of the AP. Since the UE understands the phase error information, DL SYN status, and DL channel status information of each AP, this option is also valid and may be achievable. The UE310 can recommend a sufficiently reliable and accurate role of the AP for either CJT, NCJT, SM, or no transmission. Furthermore, the CPU can refrain from attempting to change the recommended role of each AP and can make the final decision on whether each AP is scheduled for transmission. In the case of this option, the DL signaling design (shown in Figure 7) only needs to indicate whether each AP is scheduled or not.

[0151] As shown in FIG. 7, the DL signaling design may include a plurality of sections based on recommendations from the UE, and each section refers to the APs recommended by the UE for CJT, NCJT, and SM, respectively. Each section can include a number of fields equal to the corresponding AP number of the transmission strategy previously recommended by the UE. The detailed value may be 1 bit for each field to indicate whether the corresponding AP is scheduled for transmission. For example, if the bit in this embodiment is "1", it means that the corresponding AP is scheduled to take the transmission strategy recommended by the UE. On the other hand, the corresponding AP may not be incorporated into the next transmission. The total DL signal size may be Y bits, where Y is the number of APs recommended for transmission by the UE.

[0152] The technique shown in FIG. 6 may improve the receiving operation of the UE and support the execution of the hybrid transmission strategy in the time domain. However, compared with the technique shown in FIG. 5, the signaling size can be halved. One of the advantages of using implicit DMRS signaling is the cost that the CPU may not change the role of the APs recommended by the UE. Since the recommendations by the UE are based on accurate DL information, this cost may be minimized.

[0153] FIG. 10 shows an example of a system according to a specific exemplary embodiment. In an exemplary embodiment, the system may include a plurality of devices such as, for example, NE1010 and / or UE1020.

[0154] NE1010 may be one or more of a base station such as an eNB or gNB, a serving gateway, a server, and / or any other access node or a combination thereof.

[0155] NE1010 may further include at least one gNB-CU that can be associated with at least one gNB-DU. The at least one gNB-CU and the at least one gNB-DU can communicate via at least one F1 interface, at least one X(n)-C interface, and / or at least one NG interface via 5GC.

[0156] UE1020 can include one or more of a mobile device such as a mobile phone, smartphone, personal digital assistant (PDA), tablet, or portable media player, a navigation device such as a digital camera, pocket video camera, video game console, global positioning system (GPS) device, a desktop or laptop computer, a single positioning device such as a sensor or smart meter, or any of their computing devices. Further, NE1010 and / or UE1020 may be one or more citizen broadband radio service devices (CBSDs).

[0157] NE1010 and / or UE1020 may each include at least one processor shown as 1011 and 1021 respectively. Processors 1011 and 1021 can be implemented by any computing or data processing device such as a central processing unit (CPU), an application specific integrated circuit (ASIC), or a device equivalent thereto. The processor may be implemented as a single controller or as multiple controllers or processors.

[0158] As shown by 1012 and 1022, at least one memory can be provided in one or more devices. The memory can be fixed or removable. The memory can contain computer program instructions or computer code contained therein. Memories 1012 and 1022 can each independently be any suitable storage device such as a non-transitory computer-readable medium. A hard disk drive (HDD), random access memory (RAM), flash (registered trademark) memory, or other suitable memory can be used. The memory may be combined on a single integrated circuit as a processor, or may be separated from one or more processors. Further, the computer program instructions stored in the memory and processable by the processor can be any suitable form of computer program code, for example, a compiled or interpreted computer program written in any suitable programming language.

[0159] Processor 1011 and 1021, memories 1012 and 1022, and any subset thereof can be configured to provide means corresponding to various blocks of FIGS. 3-9. Although not shown, the device can also include positioning hardware such as GPS or microelectromechanical system (MEMS) hardware and can be used to determine the position of the device. Other sensors can also be used and can be configured to determine position, altitude, speed, orientation, etc., such as a barometer, compass, etc.

[0160] As shown in FIG. 10, transceivers 1013 and 1023 may be provided, and one or more devices may each also include at least one antenna illustrated as 1014 and 1024, respectively. The device may have a number of antennas, such as an array of antennas configured for multiple-input multiple-output (MIMO) communication, or multiple antennas for multiple radio access technologies (RATs). For example, other configurations of these devices may be provided. Transceivers 1013 and 1023 may be units or devices that can be configured as a transmitter, a receiver, both a transmitter and a receiver, or both transmit and receive.

[0161] Memory and computer program instructions, together with a processor for a particular apparatus, can be configured to cause a hardware apparatus such as a UE to execute any of the above-described processes (i.e., FIGS. 3 to 9). Thus, in certain exemplary embodiments, a non-transitory computer-readable medium can be encoded with computer instructions that, when executed in hardware, execute a process such as one of the processes in this embodiment. Alternatively, certain exemplary embodiments may be executed entirely in hardware.

[0162] In one exemplary embodiment, the apparatus can include a circuit configured to execute any of the processes or functions shown in FIGS. 3 to 9. For example, the circuit may be a circuit implementation of only hardware such as an analog circuit and / or a digital circuit. In other examples, the circuit may be a combination of analog and / or digital hardware circuits and software or firmware, and / or any portion of a hardware processor and software (including a digital signal processor), software, and at least one memory that cooperates to cause the apparatus to execute various processes or functions, i.e., a combination of a hardware circuit and software. In yet other embodiments, the circuit may be a hardware circuit and / or a processor such as a microprocessor or a portion of a microprocessor that includes software such as firmware for operation. The software in the circuit may not be present if it is not necessary for the operation of the hardware.

[0163] Figure 11 shows an example of a 5G network and system architecture according to a particular exemplary embodiment. A plurality of network functions are shown that may be implemented as software operating as part of network equipment or dedicated hardware, as the network equipment itself or dedicated hardware, or as virtual functions operating as network equipment or dedicated hardware. The NEs and UEs illustrated in FIG. 11 may be similar to NE1010 and UE1020, respectively. The User Plane Function (UPF) can provide services such as in-RAT and inter-RAT mobility, routing and forwarding of data packets, packet inspection, user plane service quality (QoS) handling, buffering of downlink packets, and / or triggering of downlink data notifications. The Application Function (AF) is mainly connected to the core network, can facilitate the application utilization of traffic routing, and can interact with the policy framework.

[0164] According to some exemplary embodiments, processors 1011 and 1021, and memories 1012 and 1022 may be included in a processing circuit or a control circuit, or may form part of a processing circuit or a control circuit. Further, transceivers 1013 and 1023 in some exemplary embodiments may be included in a transceiver circuit or may form part of a transceiver circuit.

[0165] In some exemplary embodiments, an apparatus (e.g., NE1010 and / or UE1020) may include means for performing any of the methods, processes, or variations described in this example. Examples of means may include one or more processors, memories, controllers, transmitters, receivers, and / or computer program code for causing the operations to be performed.

[0166] In various exemplary embodiments, the apparatus 1020 is controlled by a memory 1022 and a processor 1021, receives a downlink reference signal, determines at least one cell-free multiple-input multiple-output (MIMO) transmission strategy associated with at least one access point based at least on the downlink reference signal, and can transmit at least one of the at least one cell-free MIMO transmission strategy and enablement information to a network entity.

[0167] Certain exemplary embodiments may be directed to an apparatus including means for performing any of the methods described herein, including, for example, means for receiving a downlink reference signal. The apparatus may further include means for determining at least one cell-free MIMO transmission strategy associated with at least one access point based at least on the downlink reference signal. The apparatus may further include means for transmitting at least one of the at least one cell-free MIMO transmission strategy and enablement information to a network entity.

[0168] In various exemplary embodiments, the apparatus 1010 is controlled by a memory 1012 and a processor 1011, transmits at least one downlink reference signal to a user equipment, receives at least one cell-free MIMO transmission strategy from the user equipment, determines the role of each of a plurality of access points based on at least one of the strategy and enablement information from the user equipment, and can transmit information indicating the association between each of the plurality of access points and the strategy to the user equipment.

[0169] Certain exemplary embodiments may be directed to an apparatus including means for performing any of the methods in this example, including means for transmitting at least one downlink reference signal to a user equipment. The apparatus may further include means for receiving at least one self-free multiple-input multiple-output transmission strategy from the user equipment. The apparatus may further include means for determining the role of each of a plurality of access points based on at least one of the strategy and enablement information from the user equipment. The apparatus may further include means for transmitting to the user equipment a display of the association of each of the plurality of access points with the strategy.

[0170] The features, structures, or characteristics of the exemplary embodiments described throughout this specification may be combined in any suitable way in one or more exemplary embodiments. For example, the use of phrases such as "various embodiments", "certain embodiments", "some embodiments", or other similar phrases throughout this example refers to the fact that the specific features, structures, or characteristics described in connection with the exemplary embodiments may be included in at least one exemplary embodiment. Thus, the phrases "in various embodiments", "in certain embodiments", "in some embodiments", or other similar expressions throughout this specification do not necessarily all refer to the same group of exemplary embodiments, and the described features, structures, or characteristics may be combined in any suitable way in one or more exemplary embodiments.

[0171] Furthermore, if necessary, the different functions or procedures described above can be performed in a different order and / or simultaneously with each other. Furthermore, if necessary, one or more of the described functions or procedures may be optional or combined. Thus, the above description should be considered as illustrative of the principles and teachings of specific exemplary embodiments and not as limiting.

[0172] A person skilled in the art will readily understand that the above-described exemplary embodiments can be implemented using procedures in a different order and / or hardware elements having a configuration different from those disclosed. Therefore, although some embodiments have been described based on these exemplary embodiments, it will be apparent to those skilled in the art that specific modifications, variations, and alternative configurations are apparent while remaining within the spirit and scope of the exemplary embodiments.

[0173] Part of the Glossary 3GPP (Registered Trademark) 3rd Generation Partnership Project 5G Fifth Generation 5GC Fifth Generation Core 5GS Fifth Generation System 6G Sixth Generation AMF Access and Mobility Management Function AP Access Point ASIC Application-Specific Integrated Circuit BS Base Station CBSD Citizen Broadband Radio Service Device CE Control Element CG Configured Grant CJT Coherent Joint Transmission CN Core Network CP Cyclic Prefix CPU Central Processing Unit DCI Downlink Control Information DL Downlink DMRS Demodulation Reference Signal DRB Data Radio Bearer DU Distributed Unit eMBB Enhanced Mobile Broadband eMTC Enhanced Machine-Type Communication eNB Evolved Node B EPS Evolved Packet System FDD Frequency Division Duplex FR Frequency Range gNB Next Generation Node B GPS Global Positioning System HDD Hard Disk Drive IoT Internet of Things L1 Layer 1 L2 Layer 2 LTE Long Term Evolution LTE-A Long Term Evolution Advanced MAC Media Access Control MBS Multicast and Broadcast System MC Multicast MCS Modulation and Coding Scheme MEMS Micro-Electro-Mechanical System MIMO Multiple-Input Multiple-Output MME Mobility Management Entity mMTC Massive Machine Type Communication mmW Millimeter NAS Non-Access Stratum NB-IoT Narrowband Internet of Things NCJT Non-Coherent Joint Transmission NE Network Entity NG Next Generation NG-eNB Next Generation evolved Node B NG-RAN Next Generation Radio Access Network NR New Radio NR-U New Radio Unlicensed PBR Priority Bit Rate PDA Personal Digital Assistant PHY Physical QoS Quality of Service RAM Random Access Memory RAN Radio Access Network RAT Radio Access Technology RE Resource Element RF Radio Frequency RRC Radio Resource Control RS Reference Signal RSRP Reference Signal Received Power SINR Signal-to-Interference-plus-Noise Ratio SM Spatial Multiplexing SR Scheduling Report SYNC Synchronization TDD Time Division Duplexing TTI Transmission Time Interval Tx Transmission UE User Equipment UL Uplink UMTS Universal Mobile Telecommunications System UPF User Plane Function URLLC Ultra-Reliable and Low-Latency Communication UTRAN Universal Mobile Telecommunications System Terrestrial Radio Access Network WLAN Wireless Local Area Network

Claims

1. The user equipment receives a downlink reference signal, The user equipment determines at least one cell-free multiple-input multiple-output (MIMO) transmission strategy related to at least one access point based on at least the downlink reference signal, The user equipment transmits at least one of the at least one cell-free MIMO transmission strategy and enablement information to a network entity, A method comprising:

2. The enablement information includes phase error information set for at least one of coherent joint transmission proposed for access points within the current serving cluster of the user equipment or coherent joint transmission proposed for all access points within the current serving cluster of the user equipment. The method according to claim 1.

3. The method according to claim 1 or 2, wherein the at least one cell-free MIMO transmission strategy is related to at least one of a coherent joint transmission strategy, a non-coherent joint transmission strategy, a spatial multiplexing strategy, or an undefined transmission strategy.

4. The user equipment further transmits a downlink cell-free MIMO transmission update request. The method according to any one of claims 1 to 3, further comprising:

5. The user equipment further receives different downlink wideband reference signal transmissions simultaneously. The method according to any one of claims 1 to 4, further comprising:

6. The user equipment further estimates phase error, radio link quality, or other related information for each of the plurality of access points. The method according to any one of claims 1 to 5, further comprising:

7. The user equipment further receives a notification of the relevance between each of the plurality of access points and the transmission strategy. The method according to any one of claims 1 to 6, further comprising:

8. A network entity transmits at least one downlink reference signal to a user equipment, The network entity receives at least one cell-free MIMO transmission strategy based on the at least one downlink reference signal from the user equipment, The network entity determines the role of each of a plurality of access points based on at least one of the strategy and the enable information from the user equipment; The network entity transmits to the user equipment a notification of the relevance between each of the plurality of access points and the strategy; A method comprising the steps of.

9. The method according to claim 8, wherein the at least one self-free multiple-input multiple-output transmission strategy is related to a coherent joint transmission strategy, a non-coherent joint transmission strategy, a spatial multiplexing strategy, or an undefined transmission strategy.

10. The method according to claim 8 or 9, further comprising the network entity receiving a downlink self-free multiple-input multiple-output transmission update request from the user equipment. The method according to claim 8 or 9, further comprising the network entity receiving a downlink self-free multiple-input multiple-output transmission update request from the user equipment.

11. The method according to any one of claims 8 to 10, further comprising the network entity simultaneously setting and triggering a plurality of downlink wideband reference signal transmissions for the plurality of access points within the current serving cluster. The method according to any one of claims 8 to 10, further comprising the network entity simultaneously setting and triggering a plurality of downlink wideband reference signal transmissions for the plurality of access points within the current serving cluster.

12. The method according to any one of claims 8 to 11, further comprising the network entity determining the role of each of the plurality of access points based on at least channel quality information. The method according to any one of claims 8 to 11, further comprising the network entity determining the role of each of the plurality of access points based on at least channel quality information.

13. An apparatus comprising: At least one processor; At least one memory including computer program code; The at least one memory and the computer program code cause the at least one processor to cause the apparatus to at least: Receive a downlink reference signal; Determine at least one self-free multiple-input multiple-output transmission strategy related to at least one access point based on at least the downlink reference signal; Transmit at least one of the at least one self-free multiple-input multiple-output transmission strategy and enable information to a network entity; An apparatus configured to perform the above operations.

14. The enable information includes phase error information set for at least one of coherent joint transmission proposed for access points within the current serving cluster of the device or coherent joint transmission proposed for all access points within the current serving cluster of the device. The device according to claim 13.

15. The at least one self-free multiple-input multiple-output transmission strategy is related to at least one of a coherent joint transmission strategy, a non-coherent joint transmission strategy, a spatial multiplexing strategy, or an undefined transmission strategy. The device according to claim 13 or 14.

16. The at least one memory and the computer program code further cause the device, by the at least one processor, to at least send a downlink self-free multiple-input multiple-output transmission update request The device according to any one of claims 13 to 15, configured to perform.

17. The at least one memory and the computer program code further cause the device, by the at least one processor, to at least simultaneously receive different downlink wideband reference signal transmissions The device according to any one of claims 13 to 16, configured to perform.

18. The at least one memory and the computer program code further cause the device, by the at least one processor, to at least estimate phase error, radio link quality, or other related information for each of the plurality of access points The device according to any one of claims 13 to 17, configured to perform.

19. The at least one memory and the computer program code further cause the device, by the at least one processor, to at least receive a notification of the relevance of each of the plurality of access points to the transmission strategy The device according to any one of claims 13 to 18, configured to perform.

20. A device comprising: at least one processor; at least one memory including computer program code; and comprising The at least one memory and the computer program code cause the apparatus, by the at least one processor, to at least transmit at least one downlink reference signal to a user equipment; receive at least one cell-free multiple-input multiple-output (MIMO) transmission strategy based on the at least one downlink reference signal from the user equipment; determine the respective roles of a plurality of access points based on at least one of the strategy and enablement information from the user equipment; send a notification of the relevance of each of the plurality of access points to the strategy to the user equipment; An apparatus configured to perform the above. **Claim 21** The apparatus according to claim 20, wherein the at least one cell-free MIMO transmission strategy is related to a coherent joint transmission strategy, a non-coherent joint transmission strategy, a spatial multiplexing strategy, or an undefined transmission strategy. **Claim 22** The at least one memory and the computer program code cause the apparatus, by the at least one processor, to further at least receive a downlink cell-free MIMO transmission update request from the user equipment; An apparatus according to claim 20 or 21, configured to perform the above. **Claim 23** The at least one memory and the computer program code cause the apparatus, by the at least one processor, to further at least simultaneously configure and trigger multiple downlink wideband reference signal transmissions at the plurality of access points within a current serving cluster; An apparatus according to any one of claims 20 to 22, configured to perform the above. **Claim 24** The at least one memory and the computer program code cause the apparatus, by the at least one processor, to further at least determine the respective roles of the plurality of access points based on at least channel quality information; An apparatus according to any one of claims 20 to 23, configured to perform the above. **Claim 25** means for receiving a downlink reference signal; means for determining at least one cell-free MIMO transmission strategy related to at least one access point based on at least the downlink reference signal; means for transmitting to a network entity at least one of the at least one self-free multi-input multi-output transmission strategy and the enable information; An apparatus comprising the same. **Claim 26** The apparatus according to claim 25, wherein the enable information includes phase error information set for at least one of coherent joint transmission proposed for access points within the current serving cluster of the apparatus or coherent joint transmission proposed for all access points within the current serving cluster of the apparatus. **Claim 27** The apparatus according to claim 25 or 26, wherein the at least one self-free multi-input multi-output transmission strategy is related to at least one of a coherent joint transmission strategy, a non-coherent joint transmission strategy, a spatial multiplexing strategy, or an undefined transmission strategy. **Claim 28** means for further transmitting a downlink self-free multi-input multi-output transmission update request; The apparatus according to any one of claims 25 to 27, further comprising the same. **Claim 29** means for further receiving different downlink wideband reference signal transmissions simultaneously; The apparatus according to any one of claims 25 to 28, further comprising the same. **Claim 30** means for further estimating, for each of the plurality of access points, a phase error, a radio link quality, or other related information; The apparatus according to any one of claims 25 to 29, further comprising the same. **Claim 31** means for further receiving a notification of the relevance of each of the plurality of access points to the transmission strategy; The apparatus according to any one of claims 25 to 30, further comprising the same. **Claim 32** means for transmitting at least one downlink reference signal to a user equipment; means for receiving at least one self-free multi-input multi-output transmission strategy based on the at least one downlink reference signal from the user equipment; means for determining the role of each of the plurality of access points based on at least one of the strategy and the enable information from the user equipment; means for transmitting to the user equipment a notification of the relevance of each of the plurality of access points to the strategy; An apparatus comprising the same. **Claim 33** The apparatus according to claim 32, wherein the at least one self-free multi-input multi-output transmission strategy is related to a coherent joint transmission strategy, a non-coherent joint transmission strategy, a spatial multiplexing strategy, or an undefined transmission strategy.

34. means for receiving a downlink cell-free multiple-input multiple-output transmission update request from the user equipment; The apparatus according to claim 32 or 33, further comprising the means.

35. means for simultaneously setting and triggering a plurality of downlink wideband reference signal transmissions at the plurality of access points within the current serving cluster; The apparatus according to any one of claims 32 to 34, further comprising the means.

36. means for determining each of the roles of the plurality of access points based at least on channel quality information; The apparatus according to any one of claims 32 to 35, further comprising the means.

37. A non-transitory computer-readable medium storing program instructions for executing the method according to any one of claims 1 to 12.

38. An apparatus including a circuit configured to execute the method according to any one of claims 1 to 12.

39. A computer program product encoded with instructions for executing the method according to any one of claims 1 to 12.

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