Method of coordinated uplink communication for co-existing aerial and terrestrial users

Abstract

A method of providing a way to non-orthogonally multiplex aerial UEs with terrestrial UEs in a network with terrestrial gNBs is presented. Multiple gNBs from a group of interconnected gNBs are composed into coordinated sets. Using aerial UEs CSI information, one gNB of a coordinated set is selected for decoding private RSMA data streams of the aerial UE, while all gNBs of the set decode the public data stream. Likewise, all gNBs decode the UL transmissions from the attached terrestrial UEs. Coordinated resource allocation for optimised RSMA-based transmissions from aerial UEs mitigates the interference at the terrestrial gNBs transmitting in the same RB as the aerial UEs.

Description

[0001] METHOD OF COORDINATED UPLINK COMMUNICATION FOR CO-EXISTING AERIAL AND TERRESTRIAL USERS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to optimising communication of both urban air mobility (UAM) user equipment (UE) and terrestrial UE with terrestrial base stations, e.g., in accordance with any of the standards stipulated by the 3rdGeneration Partnership Project’ (3GPP), such as 4G / LTE and 5G / NR.

[0004] NOTATIONS AND DEFINITIONS

[0005] 3GPP, the abbreviation of ‘3rdGeneration Partnership Project’, is an umbrella term for a number of standards organizations which develop protocols for mobile telecommunications. In the context of 3GPP the term ‘random’ emphasizes that user equipment (UE) may request access to a network resource at any time, as the UE is not bound by a predetermined or fixed access scheme. The expression ‘terrestrial wireless communication networks’ is meant to refer to wireless communication networks whose base stations are located on ground-based structures, and whose UEs are typically likewise located in or on ground-based structures. The abbreviation ‘gNB’, specifically used for 5G NR base station, is used interchangeably herein with the abbreviation BS for base stations irrespective of the naming conventions in various standards, which may use ‘eNB’ or other abbreviations for base stations. UAM UEs may also be referred to herein as aerial UEs.

[0006] BACKGROUND

[0007] When integrating aerial UEs into a terrestrial cellular network, interference to existing terrestrial UEs’ signals needs to be handled. A large part of the interference may be caused by aerial UEs’ signals. Since UAM UEs fly at considerable height from the ground, they enjoy line-of-sight (LOS) paths to several base stations. Thus, when aerial users like aircraft for UAM communicate with terrestrial gNBs, their signals cause interference at many gNBs due to the generally favourable LOS propagation conditions, which may not be easily mitigated with conventional interference management schemes. The interference from the UL transmission of aerial UEs affecting multiple terrestrial gNBs instigates a strong need for effective methods to manage the interference.

[0008] A traditional approach to interference management is to let the users transmit on orthogonal, i.e. , non-overlapping, time-frequency resource blocks (RB), e.g., collections of subcarriers (SCs) in an orthogonal frequency division multiple access (OFDMA) system. However, allocating different time-frequency RBs of an OFDMA system to different UEs is inefficient as it fails to use all the available degrees of freedom (DoF) in the wireless channel, i.e., does not fully utilize all the available time- frequency resources.

[0009] It is possible to achieve higher DoF if the receiver and / or the transmitter have multiple antennas, which permits using receive and / or transmit beamforming techniques to separate different users’ signals transmitted on the same time- frequency RB. In this technique, known as space division multiple access (SDMA), the UEs use different precoding vectors or matrices to spatially separate their signals arriving at the gNBs. Although interference from aerial UEs may be managed by scheduling aerial and terrestrial UEs on orthogonal RBs or orthogonally multiplexing them on the same RB by separating their signals using the multiple receive antennas at the gNB, there are certain limitations to these approaches, amongst them the requirement of a sufficient number of antennas at the gNB to discriminate between the transmissions from different UEs, which translates into an higher hardware cost.

[0010] Even if the transceivers have single antennas, the achievable DoF can be further improved by using nonorthogonal multiple access (NOMA) techniques, where signals from multiple users can be superimposed in the same time-frequency RB, and more sophisticated interference cancellation techniques can be used at the receiver to extract these signals. In NOMA, in which some UEs share the same RB, some UEs’ signals are completely decoded and cancelled before decoding other UEs signals. In the specific use case of terrestrial and aerial UEs sharing time-frequency RBs discussed herein, NOMA requires that the gNBs that are subject to interference from the aerial UEs decode and cancel their signals completely before decoding terrestrial UEs’ signals or vice-versa. Various aspects of NOMA are discussed by L. Lei, D. Yuan, C. K. Ho and S. Sun, in "Power and Channel Allocation for Non-Orthogonal Multiple Access in 5G Systems: Tractability and Computation," IEEE Transactions on Wireless Communications, vol. 15, no. 12, pp. 8580-8594, Dec. 2016, by S. Sruthy and N. B. Mehta, in "Power and Discrete Rate Adaptation in Wideband NOMA in Frequency-Selective Channels," IEEE Transactions on Wireless Communications, doi: 10.1109 / TWC.2023.3315709, by W. Mei and R. Zhang, in "Uplink Cooperative NOMA for Cellular-Connected UAV," in IEEE Journal of Selected Topics in Signal Processing, vol. 13, no. 3, pp. 644-656, June 2019, by Y. Liu, Z. Qin, Y. Cai, Y. Gao, G. Y. Li and A. Nallanathan, in "UAV Communications Based on Non-Orthogonal Multiple Access," in IEEE Wireless Communications, vol. 26, no. 1 , pp. 52-57, February 2019, by L. Liu, S. Zhang and R. Zhang, in "Exploiting NOMA for Multi-Beam UAV Communication in Cellular Uplink," ICC 2019 - 2019 IEEE International Conference on Communications (ICC), Shanghai, China, 2019, pp. 1-6, and by Mustafa Eroz, Lin-Nan Lee, in “System And Method For Asynchronous Multi-stream Transmission For NOMA”, Hughes Network Systems LLC, 2018, US 2019140770 A1 .

[0011] Yet another known technique is rate splitting multiple access (RSMA). As mentioned before, traditional NOMA techniques require the receiver to fully decode and cancel an interfering user’s signal in order to achieve higher performance compared to orthogonal transmission schemes or multiple antenna schemes, which may not always be feasible, depending on the wireless channel conditions. Note that throughout this specification the expressions ‘cancelling signals’ and ‘cancelling streams’ may be used interchangeably, unless explicitly stated otherwise or readily apparent to the skilled person from the respective context. The technique of RSMA, in which the transmitting user splits its data stream into a common, or public, part which can be decoded and cancelled by receivers other than its intended one, and a private part, which is meant to be decoded only by its intended receiver, improves on traditional NOMA by enabling an optimal part of the interfering user’s signal to be cancelled by the receiver, and can achieve a smooth trade-off between the two extremes of treating interference as noise - which is optimal when the interference is weak - and traditional NOMA - which is optimal when the interference is strong - by adjusting the signal power allocated to the common and private data streams. By allocating different power levels to the “common” and “private” messages, the level of interference to non-intended receivers can be controlled dynamically.

[0012] Various aspects of RSMA are discussed by B. Clerckx et al., in "A Primer on Rate- Splitting Multiple Access: Tutorial, Myths, and Frequently Asked Questions," IEEE Journal on Selected Areas in Communications, vol. 41 , no. 5, pp. 1265-1308, May 2023, by Y. Mao, B. Clerckx and V. 0. K. Li, in "Rate-Splitting Multiple Access for Coordinated Multi-Point Joint Transmission," 2019 IEEE International Conference on Communications Workshops (ICC Workshops), Shanghai, China, 2019, pp. 1-6, by M. M. Șahin, 0. Dizdar, B. Clerckx and H. Arslan, in "Multicarrier Rate-Splitting Multiple Access: Superiority of OFDM-RSMA Over OFDMA and OFDM-NOMA," IEEE Communications Letters, vol. 27, no. 11 , pp. 3088-3092, Nov. 2023, by L. Yin and B. Clerckx, in "Rate-Splitting Multiple Access for Satellite-Terrestrial Integrated Networks: Benefits of Coordination and Cooperation," IEEE Transactions on Wireless Communications, vol. 22, no. 1 , pp. 317-332, Jan. 2023, by W. Jaafar, S. Naser, S. Muhaidat, P. C. Sofotasios and H. Yanikomeroglu, in "On the Downlink Performance of RSMA-Based UAV Communications," IEEE Transactions on Vehicular Technology, vol. 69, no. 12, pp. 16258-16263, Dec. 2020, by X. Liu, J. Feng, F. Li and V. C. M. Leung, in "Downlink Energy Efficiency Maximization for RSMA- UAV Assisted Communications," IEEE Wireless Communications Letters, vol. 13, no. 1 , pp. 98-102, Jan. 2024, by Mao Xiaomao, Qiu Peiliang, in “Rate splitting based method for allocating user rates in wireless multiple access channels”, Zhejiang University, 2010, CN 101827452 A, by Aasheesh Shukla, Abhay Chaturvedi Nira, in “Rate Splitting Multiple Access (RSMA) System”, GLA University Mathura, IN 202111053727 A, by Chao Dong, Zhiqiang He, Xinrui Huang, Kai Niu, Zhongwei Si, in “Method And Device For Transmitting Data Based On Rate Division Non- orthogonal Multiple Access Technology”, Beijing University of Posts and Telecommunications, 2016, WO2018023732 A1 , by Jiang Hao, Li Hao, Wu Jing, Zhou Changjia, Zhou Jianguo, in “Rate optimization method for uplink rate division multiple access system”, Wuhan University, 2022, CN 115866638 A, by Daesung Yu, Junbeom Kim, Seok Hwan Park, in “Common Message Generation Method for Rate- Splitting Multiple Access Based on Hierarchical User Clustering”, Industrial Cooperation Foundation Jeonbuk National University, 2020, KR 102270993 B1 , by Kwak Kyung Sup, Liu Hongwu, in “An Adaptive Rate Splitting Method and System for Uplink Non-Orthogonal Multiple Access Systems”, KR 102074217 B1 , and by Liu Hongwu, in “Rate splitting method suitable for non-orthogonal multiple access uplink”, Shandong Jiaotong University, 2019, CN 110366234 A.

[0013] While a plethora of academic papers exist on general aspects of NOMA and of RSMA, none of those discusses the integration of RSMA for co-existence with OFDMA, the latter found in traditional terrestrial communication systems.

[0014] Similarly, there are a few academic papers that discuss applying RSMA for aerial base stations, but not for the co-existence of aerial UEs in traditional terrestrial communication networks.

[0015] So far, interference management between aerial and terrestrial UEs in the same traditional communication networks has considered NOMA, but not RSMA.

[0016] As already mentioned above, when serving both aerial and terrestrial UEs the cellular network needs to intelligently allocate resources to accommodate both types of UEs. Therefore, there is a need for techniques to manage this unique challenge faced when integrating aerial UEs into terrestrial cellular networks.

[0017] SUMMARY OF THE INVENTION

[0018] The present invention aims to address this need by the method of claim 1 , the base station of claim 7, the central unit of claim 8, the communication system of claim9, and the computer program product of claim 10. A corresponding computer-readable storage medium is presented in claim 11 . Advantageous embodiments and developments are described in respective dependent claims.

[0019] The method proposes implementing coordinated reception for terrestrial UEs using OFDMA and aerial UEs with OFDMA-based RSMA. As the initially mentioned problem mainly occurs in uplink (UL) transmission, i.e. , transmission from UE to gNB, the present method only considers UL transmission.

[0020] The proposed method enables gNBs to operate in a coordinated set, for effectively implementing RSMA-based UL communication for aerial UEs in co-existence with terrestrial UEs, permitting those aerial UEs to transmit on the same RBs as terrestrial UEs. This is achieved by gNBs in the coordinated set of gNBs cooperating with each other to share the channel state information (CSI) of the aerial UEs, which enables all the gNBs in the coordinated set to decode and cancel the “common” data stream of aerial UEs scheduled on the same RB as other terrestrial UEs.

[0021] The method of the present invention assumes the following conditions as given:

[0022] 1 ) A set of gNBs is configured for communicating among themselves to acquire UL CSI information of the aerial UEs in their coverage range on the same RBs. This can be achieved using the concept of a coordinated set of gNBs, as available in current 3GPP specifications. The gNBs can be organised as multiple distributed units (DU) connected to the same central unit (CU) or multiple sets of CUs and DUs.

[0023] In the first case the CU computes and coordinates resource allocation for all the DUs, while in the second case one of the CUs can compute and coordinate the resource allocation to other CUs belonging to the coordination set.

[0024] 2) Use of OFDMA-based communication system with many subcarriers (SC), where users are allocated time-frequency RBs as stipulated in current 3GPP standards. The transmission and reception of aerial UEs’ signals is synchronized at all the gNBs in the coordination set via joint resource allocation.

[0025] 3) For terrestrial UEs, the gNBs may or may not form a coordination set.

[0026] 4) Both, UEs and gNBs, may have one or more antenna elements, and can transmit more than one data stream on their allocated RB. The number of data streams to be transmitted is determined by the network based on CSI feedback as stipulated in current 3GPP standards.

[0027] In accordance with a first aspect of the invention, a method 100 of coordinated uplink, UL, communication for co-existing aerial and terrestrial user equipment, UE, at least temporarily communicatively attached to a terrestrial communication network is proposed. The method will be described in the following with reference to figure 1 . In step 110 channel status information, CSI, measurements related to one or more terrestrial UEs and one or more aerial UEs are performed amongst a set of interconnected base stations, gNB, of the terrestrial communication network. In step 120 resource allocation for the terrestrial and the aerial UEs is performed amongst the set of interconnected gNBs of the terrestrial communication network. Resource allocation may comprise transmitting information about the resource allocation to the aerial and terrestrial UEs, e.g., in a corresponding broadcast. Resource allocation may be based on information about the effective distance between gNBs and UEs. In step 130 one or more coordinated sets of gNBs are formed or composed from gNBs of the set of interconnected gNBs. Individual gNBs may be part of multiple coordinated sets. Each of the gNBs from a respective coordinated set of gNBs processes signals from one or more aerial UEs along with those from one or more terrestrial UEs, in particular is configured for decoding common, or public, RSMA data streams for at least one of one or more aerial UEs that may at least temporarily be communicatively attached to the coordinated set. The signals from the aerial UEs are multiplexed on the same RB as the signals from terrestrial UEs using RSMA. While a coordinated set is typically formed with respect to each aerial UE, two or more aerial UEs may have associated coordinated sets that share the same gNBs. In step 140, out of the gNBs of a coordinated set, a selected gNB is determined that is configured for also decoding private RSMA data streams, in addition to the common or public data streams, for at least one of the one or more aerial UEs at least temporarily attached to the respective coordinated set of gNBs. Selection may comprise sharing CSI information related to the associated aerial UE amongst the gNBs of the respective coordinated set, for determining the selected gNB . The resource allocation amongst the gNBs comprised in a respective coordinated set of gNBs is performed in a joint manner. Resource allocation includes UE-RB or UE- subcarrier (SC) allocation, transmit precoding and / or power allocation for UEs allocated on each RB or SC, determining the members of a coordinated set of gNBs for each aerial UE, and determining a data stream decoding order for UEs allocated on a particular RB or SC. The coordinated sets of gNBs can be different for each aerial UE and can be optimally chosen based on the measured CSI and other parameters of the UE like location, signal-to-noise ratio, signal level, and the like, from the larger set of interconnected gNBs. Combinations of the parameters may be used for composing the coordinated sets. Typically, gNBs selected for forming a coordinated set for an aerial UE do not comprise all gNBs that are capable of receiving transmissions from the respective aerial UE, i.e. , are within radio range.

[0028] The aerial UE transmits by splitting its message into one or more "common", or “public”, data streams and one or more "private" data streams, as in RSMA. The split data streams are transmitted simultaneously as per the RSMA technique, while terrestrial UEs can retain the same transmission format as in the current standard and need not use RSMA. The method 100, thus, further comprises receiving, in step 150, at all gNBs of a coordinated set of gNBs, data from the one or more aerial UEs associated with the coordinated set of gNBs and, at each respective gNB, data from terrestrial UEs attached or associated with the respective gNB.

[0029] The received common data streams of the one or more aerial UEs are first decoded, in step 160, and subsequently cancelled, in step 170, from the received signal on a particular RB at least by all the gNBs in the coordinated set associated with the one or more aerial UEs. Ideally, the common data streams are cancelled by all interconnected gNBs. Since the common data streams can be decoded and cancelled at the gNBs in the coordinated set of gNBs, the aerial UEs can be multiplexed on the same RBs as the terrestrial UEs, with reduced interference to terrestrial UE’s signals. Further, after cancelling the common data streams of the aerial UE in step 170, each gNB of the coordinated set of gNBs proceeds to decode, in step180, the data streams of their own terrestrial UEs, i.e., the terrestrial UEs attached to or associated with the respective gNB. Finally, in step 190, the private data streams of the aerial UEs are decoded, in addition to the common data streams, exclusively by the selected gNB in the coordinated set which had previously been collectively determined, e.g., based on parameters like the measured CSI, location of the UEs, etc., and had been associated with the aerial UE. The selection of the gNB configured for decoding the private data streams of the aerial UE can be chosen dynamically within the coordinated set of gNBs, compensating for the movement of the aerial UE over time. The other gNBs of a respective coordinated set only decode the common data streams of the aerial UE. The common data streams may include data about the aerial UE altitude, or if the aerial node is an IAB node, the number of other UEs attached to it, etc. In one or more embodiments the method further comprises receiving, in step 125, information about respective spatial locations from the one or more aerial UEs and providing this information to the composing step 130 and / or to the selecting step 140. The information relating to the spatial locations may be indicated absolute, e.g., as geo-locations comprising latitude and longitude, or relative to the one or more gNBs.

[0030] In embodiments of the method the composing step 130 comprises selecting, for each aerial UE, those gNBs that are located closest to the aerial UE, those gNBs that have the highest signal-to-noise ratio and / or signal level, or a combination thereof, while not selecting all gNBs that are capable of receiving transmissions from the respective aerial UE.

[0031] In one or more embodiments of the method composing one or more coordinated sets of gNBs from the set of interconnected gNBs comprises selecting, for each aerial UE, those gNBs that are located closest to the aerial UE, those gNBs that have the highest signal-to-noise ratio and / or signal level, or a combination thereof, while not selecting all gNBs that are capable of receiving transmissions from the respective aerial UE.

[0032] In one or more embodiments the method further comprises repeating the CSI measuring and the selecting step while a gNB has been selected in a coordinated set of gNBs, for dynamically adapting the selection. The measurements may be carried out within a coordinated set of gNBs or within the group of interconnected gNBs. The latter may result in a coordinated set associated with an aerial UE being re- composed, e.g., if a UAM has moved out of the radio range of some of the gNBs of the prior coordinated set of gNBs and is now within radio range of a gNB that did not belong to the prior coordinated set of gNBs.

[0033] In one or more embodiments the method further comprises adjusting the power allocation to the common and private data streams and / or the modulation and coding schemes for the common data streams, private data streams and / or data streams not using rate splitting multiple access, targeting maximising a weighted sum rate over all data streams while maintaining a block error rate and / or a total transmit power, of each transmitter that transmits a data stream within the same subchannel and / or resource block, below respective predetermined maximum values.

[0034] Figure 2 shows a schematic representation of an exemplary scenario implementing the method in accordance with the first aspect of the invention. The outermost circle represents an overall set of interconnected gNBs. The gNBs are represented by the radio tower icons. The smaller circle numbered “1” represents a set of coordinated gNBs for the aerial UE1 , represented by a helicopter icon, and the smaller circle numbered “2” represents a set of coordinated gNBs for the aerial UE2, likewise represented by a helicopter icon. In each of the coordinated sets of gNBs, one gNB is selected for decoding all data streams of the aerial UE associated with the respective coordinated set of gNBs, indicated by the dashed circle around the gNB.

[0035] Figure 3 shows a further schematic representation of an exemplary scenario implementing the method in accordance with the invention. A number of aerial UEs, represented by the helicopter icons, are present in the airspace above the various gNBs, represented by the radio tower icons, which form a coordinated set of gNBs. Each gNB serves a number of terrestrial UEs, indicated by the smartphone symbols, as indicated by the dashed encirclement. Aerial UE1 transmits common and private data stream parts to all gNBs in the coordinated set, indicated by the dash-dotted arrows, but only the gNB selected for that aerial UE will decode the public and the private data stream parts from aerial UE 1.

[0036] Figure 4 shows a more descriptive schematic flow diagram of an embodiment of the method in accordance with the first aspect of the present invention. First, the interconnected gNBs perform CSI measurements to all aerial UEs and their own terrestrial UEs. Next, the interconnected gNBs perform RB allocation for terrestrial and aerial UEs, before selecting subsets of gNBs to form respective coordinated sets of gNBs for common data streams decoding for each aerial UE. Then, the gNBs in the coordinated sets of gNBs may perform additional CSI measurements for the aerial UE, if required. In the next step, the gNBs in each coordinated set of gNBs decide which gNB should decode the aerial UE’s private data streams. The aerial UEs and the terrestrial UEs scheduled on the same RB transmit their UL data. The aerial UEs use RS, while the terrestrial UEs do not use RS. All gNBs in a coordinated set of gNBs decode and cancel the common data streams from aerial UEs, and each gNB further decodes data streams of its own associated or attached terrestrial UEs scheduled in an RB. Finally, only the selected gNB in the coordinated set of gNBs decodes the private data streams of the associated aerial UE.

[0037] In a first alternative embodiment of the method the terrestrial UEs may split their UL data using RS, while the aerial UEs don’t. In this case, the coordination set is only used to select the best gNB to decode each data stream of an aerial UE. Each gNB first decodes and cancels the common data streams of its own terrestrial UEs scheduled on the RB or subchannel (SC) and then proceeds to decode the data streams of the aerial UEs followed by decoding the private data streams of the terrestrial UEs.

[0038] In a second alternative embodiment of the method both the aerial UEs and the terrestrial UEs employ RS for their UL data transmissions. In this case, each gNB may choose the decoding and cancellation order between the aerial UEs data streams and its own terrestrial UEs data streams.

[0039] The underlying mathematic concept of coordinated reception with OFDMA transmission by terrestrial UEs and RSMA transmission by aerial UEs will now be presented in greater detail.

[0040] A. 1 Resource Allocation for OFDMA-based Single Input Single Output Transmission

[0041] This Section provides the detailed system model and problem formulation for the OFDMA-based system with single-input-single-output (SISO) and potential non- orthogonal transmission by aerial UEs and terrestrial UEs on any SC with rate- splitting (RS) employed by the aerial UEs. Consider a system where a set of

[0042] U = {1, ... , U} aerial UEs and K. = {1, ... , K} terrestrial UEs transmit data in the UL to a set of M = {1, terrestrial BSs. Each terrestrial UE is associated to only one BS, while for each aerial UEu on each SCn there would be a coordination set denoted by The BSs in the coordination set perform channel estimation to the UEu jointly, while one BS denoted by m will decode the private data stream of UEu. Assume that a set of terrestrial UEs are associated with a BSm where and All the users and BSs are assumed to have single antennas. The extension to multiple antennas will be considered later. We assume that the BSs have CSI of their own terrestrial UEs and the aerial UEs for which they form a coordinated set. Thus, if denotes the set of aerial UEs for which BSm is part of a coordination set on all the SCs, then BSm has CSI for all the UEs in Umon the respective SCs. This may be acquired in practice by coordinated allocation and transmission of CSI-RSs on the same RBs by the BSs to the aerial UEs.

[0043] Assume OF DMA-based transmission by the UEs, where the bandwidth B of the system is divided among N OFDM SCs, each SC occupying a bandwidth B / N. However, different from a traditional OFDMA system where only one user transmits on each SC, consider that the network scheduler may multiplex one terrestrial UE and one aerial UE on each SC, using RSMA, where the RS is performed by the respective aerial UE. Let

[0044] Similarly, let

[0045] Then and

[0046] The transmitted signal by the aerial UEu on SC n is given by where is the complex-valued common data stream transmitted by the aerial UEu with transmit power is the complex valued private data stream transmitted by the aerial UEu with transmit power

[0047] The transmitted signal by the terrestrial user is given by where mis the single data stream transmitted by the user with power

[0048] For now, assume that one terrestrial UE and one aerial UE are scheduled on all SCs at every BS, i.e., Assume that there is negligible inter-cell interference due to two different terrestrial users transmitting to two different cells on the same SC. The received signal at BS m on SCn is then given by where is the additive white Gaussian noise (AWGN) at the BS receiver with variance assumed to be same on all the SCsn On each SC, each BS is assumed to first decode and cancel the common data stream of the aerial UE and then decode the terrestrial UE's signal, treating the private data stream of the aerial UE as noise. On the other hand, one BS on each SC decodes the aerial UE's signal (i.e., on different SCs, potentially different BSs can decode the aerial UEs' data streams). The signal to interference and noise ratio (SINR) for decoding the common data stream of aerial UE u on SC n at BS m as a function of the transmit powers of both the terrestrial UE and aerial UE is then given by

[0049] The rate of the common data stream of UAM UE U on SC n at BS m is then given by

[0050] After decoding and cancelling the common data stream of the aerial UE on SC n, the BS decodes the data stream transmitted by the terrestrial UE treating the private data stream of the aerial UE as noise. The SINR for decoding the terrestrial UE's signal, as a function of the transmit powers of both the terrestrial UE and aerial UE is then given by:

[0051] Hence the achievable rate for terrestrial UE on SC n is:

[0052] Let if BSm decodes the private data stream of aerial UE u on SCn otherwise. (A 13)

[0053] Since only one BS in needs to decode aerial UE it 's signal on SC n, we have

[0054] If BSm decodes the private data stream of aerial UEit on SCn, i.e. , then the SNR at BS m as a function of the transmit power of the private data stream of aerial UE is given by:

[0055] The rate of the private data stream of aerial UE on SC n e when decoded by BSm is given by

[0056] Let the rate of the common data stream of the aerial UE u on SC n b be denoted by Then, it must be such that it is decodable at all BSs in m

[0057] Then, the achievable rate for aerial UEu is given by

[0058] A.1.1 Problem Formulation

[0059] It is now desirable maximize the weighted sum rate of the terrestrial and aerial UEs subject to transmit power constraints at the UEs and the RSMA common data stream decoding constraints at the BSs. Let denote the weight assigned to terrestrial user and denote the weight assigned to the aerial UE Let denote the vector of common data stream rates of aerial UE u on all SCs. Similarly, let and denote the vector of common and private data stream power allocations for aerial UE u on all SCs, and denote the vector of power allocation for the terrestrial UE data streams on all SCs. Further, let denote the vector of indicator variables which indicate if aerial UE u transmits on SC n and denote the corresponding vector for terrestrial UE on SC n. Similarly, let denote the vector of indicator variables which indicate if BSm decodes the private data stream of UE u on SC n. The overall problem A.19 can then be formulated as given below: subject to Problem A.19 is non-convex due to the binary constraints on the variables and and due to the non-concavity of the function for the achievable rate of terrestrial user

[0060] A. 2 Modulation and Coding Scheme and Power Allocation for a Single Physical Resource Block or Set of PRBs In NR, = 12SCs form a physical RB (PRB). In current systems, the same power allocation is used for the whole set of PRBs allocated to a user. We focus on a single PRB which consists of 12 SCs and assume that an aerial UE and a terrestrial UE are multiplexed on it, with the aerial UE transmitting using RSMA.

[0061] A.2.1 Single BS

[0062] First consider the situation with only a single BS. Then the channel gains on each SC in the PRB are denoted by respectively for the aerial UE and the terrestrial UE. The power allocation is restricted to be the same for all the SCs within that RB, i.e. , power allocation is performed on a per RB basis rather than a per SC basis. Note that the same decoding order as used in section A discussed further above is assumed. Let denote the power allocations for the common and private data streams of the aerial UE on the RB and denote the power allocation for the terrestrial UE on the RB. Then, the SINR on SC n in the PRB for decoding the common data stream of the aerial UE is given by

[0063] Similarly, the SINR for the terrestrial UE is and the SNR for the private data stream of the aerial UE is

[0064] Let denote the vector of SINRs for the common data streams of the aerial UE on the PRB, and similarly for the terrestrial UE and for the private data streams of the aerial UE. Let denote the set of discrete modulation and coding schemes (MCSs) that can be used by any UE on each SC. Then, we wish to choose the tuple of MCSs for the three data streams corresponding to the common and private data streams of the aerial UE and the data stream of the terrestrial UE. Note that the same MCS is used for each type of data stream on all the SCs belonging to the same PRB. e.g., is the MCS used for all the common data streams of the aerial UE on the Nsc= 12SCs in the PRB, and similarly for and For each choice of MCS tuple c = there is a corresponding rate tuple achieved on the PRB. Our objective is to choose the MCS tuple and the power allocation such that the weighted sum rate of the two users scheduled on this PRB is maximized. The transport block error rates (BLERs) for each data stream transmission is a function of both the receive SINRs at the BS corresponding to each data stream, and the MCS used for each data stream. Let and denote the BLERs of the respective data streams. Then, we want the BLERs of each data stream to be below a certain threshold e. Thus, the problem can be written as s.t.

[0065] A.2.2 Multiple BSs

[0066] Now, consider the case when there are multiple BSs coordinating to estimate the channel to the aerial UE and therefore able to decode and cancel the common data stream of the aerial UE. In this case, there are different terrestrial UEs sharing the same RB at different BSs. Let denote the terrestrial UE at BS m Then denote the channel gains on SC n for the aerial UE and the terrestrial UEs respectively. Then, the SINR for decoding the common stream of the aerial UE at BS m is given by

[0067] Similarly, the SINR for decoding the terrestrial UE signal at BS m is given by

[0068] Finally, let indicate if BS m decodes the aerial UEs private data stream also or not. The SNR at BS m e M for decoding the private data stream of the aerial UE is given by

[0069] Similar to the single BS case, let denote the vector of SINRs at BS m for the common data streams of the aerial UE on the PRB. Similarly, let denote the vector of

[0070] SNRs at BS m for the terrestrial UE and denote the vector SINRs at BS m for the private data streams of the aerial UE.

[0071] Similar to the single BS case, we wish to choose the tuple of for two streams corresponding to the common and private data streams of the aerial UE, and the data stream of each terrestrial UE. Note that the same MCS is used for each type of data stream on all the SCs belonging to the same PRB, e.g. is the MCS used for all the common data streams of the aerial UE on the Nsc= 12 SCs in the PRB, and similarly for and ckmfor the UE km. For each choice of MCS tuple c = there is a corresponding rate tuple achieved on the PRB. Hence, the problem is to choose the discrete MCS and power allocations for the aerial UE's common and private data streams, the decoding BS for the aerial UE's private data stream, and the discrete MCS and power allocations for each of the terrestrial UEs. Let and denote the BLERs of the respective data streams if they are decoded at BS m. Then, the problem can be written as

[0072] In accordance with a second aspect of the invention a gNB of a set of communicatively interconnected gNBs of a terrestrial communication network comprises one or more antennas coupled with a first, wireless communication interface, a second communication interface adapted for communicating with gNBs of the set of communicatively interconnected gNBs of the terrestrial communication network and / or with a central unit (CU) communicatively connected to said set, and further comprising one or more microprocessors and associated volatile and non- volatile memory. The non-volatile memory stores computer program instructions which, when executed by the one or more microprocessors, configure the gNB to execute embodiments of the method in accordance with the first aspect of the invention as presented above.

[0073] In accordance with a third aspect of the invention a central unit of a terrestrial communication network is presented. The central unit comprises one or more second communication interfaces configured for communicating with one or more base stations in accordance with the second aspect of the invention, and further comprises one or more microprocessors and associated volatile and non-volatile memory. The aforementioned elements are communicatively coupled via one or more signal and / or data lines or buses. The non-volatile memory stores computer program instructions which, when executed by the one or more microprocessors, configure the central unit to execute embodiments of the method in accordance with the first aspect of the invention as presented above.

[0074] In accordance with a fourth aspect of the invention a communication system comprises a plurality of gNBs in accordance with the second aspect of the invention as presented above that are communicatively connected with each other. Each gNB has one or more antennas, and is configured for communicating with one or more terrestrial UE via OFDMA within a coverage range. The communicatively connected gNBs, or a central unit (CU) connected thereto, are configured for composing one or more coordinated sets of gNBs. Each of the one or more coordinated sets of gNBs is associated with one or more terrestrial UEs and one or more aerial UEs. Further, each of the one or more coordinated sets of gNBs is configured for communicating with the one or more terrestrial UEs and the one or more associated aerial UEs in using the respective steps of the method in accordance with the first aspect of the invention.

[0075] The method described hereinbefore may be represented by computer program instructions. Thus, in accordance with a third aspect of the present invention a computer program product comprises computer program instructions which, when executed by a microprocessor of or functionally coupled with a gNB and / or a CU connected thereto, of a communication system in accordance with the second aspect of the invention, cause the processor and / or the gNB and / or the CU to carry out a method in accordance with the first aspect of the invention.

[0076] The computer program instructions may be retrievably stored or transmitted on a computer-readable medium or data carrier. The medium or the data carrier may by physically embodied, e.g., in the form of a hard disk, solid state disk, flash memory device or the like. However, the medium or the data carrier may also comprise a modulated electro-magnetic, electrical, or optical signal that is received by the computer by means of a corresponding receiver, and that is transferred to and stored in a memory of the computer. The present invention provides a way to non-orthogonally multiplex aerial UEs with terrestrial UEs in a network with terrestrial gNBs, while requiring minimal cooperation among the terrestrial gNBs in terms of sharing CSI and resource allocation among the gNBs in the coordinated set, which can be different for each aerial UE. To this end, a method of coordinated reception of UL transmissions from aerial and terrestrial UEs that are scheduled in the same RB is proposed. One embodiment of the method employs rate splitting for the aerial UEs to achieve non-orthogonal multiplexing with terrestrial UEs using the same RBs. Here, coordinated reception refers to the formation of a "coordination set" among interconnected gNBs from the perspective of an aerial UE, which then proceed to jointly obtain CSI for the aerial UEs and proceed to perform resource allocation for both aerial and terrestrial UEs. Coordinated resource allocation for optimised RSMA-based transmissions from aerial UEs mitigates the interference at the terrestrial gNBs.

[0077] The interference level when decoding the terrestrial UEs signals on each RB can be adjusted by adjusting the precoding and / or power allocation for the common and private data streams of the aerial UEs. Since the common data streams of the aerial UEs reach multiple gNBs in the coordination set, they can be cancelled at all of them. From the perspective of a terrestrial UE, the gNBs need not form a coordination set.

[0078] The present invention provides a number of benefits over traditional multiplexing using SDMA while treating interfering signals as noise, inter alia, the capability of multiplexing a larger number of UEs for the same number of antennas and data stream transmissions at the gNBs and UEs. Further, even though only one gNB needs to decode the signal of an aerial UE, when decoding terrestrial UEs’ signals using traditional SDMA treating signals from aerial UEs as noise, multiple terrestrial gNBs need to optimize their receive processing to align the aerial UE’s signals so that its interfering effect is reduced. In the proposed method using RSMA, since the common data streams are decoded and cancelled, only the private data streams would cause residual interference to terrestrial UEs.

[0079] The present invention further provides a benefit over traditional NOMA by completely decoding and cancelling interfering signals from aerial UEs. When using traditional NOMA, due to the favourable channel conditions of the aerial UE to multiple terrestrial gNBs, all gNBs need to be able to completely decode and cancel the aerial UE’s signal, even though essentially only one gNB requires the aerial UE’s signal. This reduces the achievable rate for the aerial UE, since the rate of transmission cannot be larger than that achievable on the worst aerial UE-gNB link. In the proposed method with RSMA only the common data stream rate is constrained by this predicament, while the private data stream rate can generally be higher.

[0080] The present invention may be used in current 4G and 5G networks and future 6G networks and beyond.

[0081] BRIEF DESCRIPTION OF THE DRAWING

[0082] In the following the invention will be described with reference to the attached drawing, in which

[0083] Fig. 1 shows a schematic flow diagram of an exemplary method in accordance with the invention,

[0084] Fig. 2 shows a schematic representation of an exemplary scenario implementing the method in accordance with the invention,

[0085] Fig. 3 shows a further schematic representation of an exemplary scenario implementing the method in accordance with the invention,

[0086] Fig. 4 shows a more descriptive schematic flow diagram of an embodiment of the method in accordance with the first aspect of the present invention, and

[0087] Fig. 5 shows a schematic block diagram of a gNB in accordance with the second aspect of the present invention.

[0088] DETAILED DESCRIPTION OF EMBODIMENTS

[0089] Figures 1 to 4 have been described further above and will not be discussed again.

[0090] Figure 5 shows a schematic block diagram of a gNB in accordance with the second aspect of the present invention. The gNB comprises one or more antennas coupled with a first, wireless communication interface 501 , and a second communication interface 502 adapted for communicating with gNBs of the set of communicatively interconnected gNBs of the terrestrial communication network (not shown in the figure) and / or with a central unit communicatively connected to said set (not shown in the figure). The gNB further comprises one or more microprocessors 503 and associated volatile 504 and non-volatile memory 505. The aforementioned elements are communicatively coupled via one or more signal and / or data lines or buses 506. The non-volatile memory 505 stores computer program instructions which, when executed by the one or more microprocessors 503, configure the gNB to execute embodiments of the method in accordance with the first aspect of the present invention.

Claims

CLAIMS1 . A method (100) of coordinated uplink, UL, communication for co-existing aerial and terrestrial user equipment, UE, at least temporarily communicatively attached to a terrestrial communication network, comprising:- performing (110), amongst a set of interconnected base stations, gNB, of the terrestrial communication network, channel status information, CSI, measurements related to one or more terrestrial UEs and one or more aerial UEs,- performing (120), amongst the set of interconnected gNBs of the terrestrial communication network, resource allocation for the one or more terrestrial and the one or more aerial UEs,- composing (130) one or more coordinated sets of gNBs from the set of interconnected gNBs, all gNBs from a respective coordinated set of gNBs being configured for decoding common RSMA data streams received from one or more terrestrial UEs and for decoding data streams from at least one aerial UE,- selecting (140), in each coordinated set of gNBs, one gNB that is tasked with decoding data streams from at least one of the one or more aerial UEs associated with the respective coordinated set of gNBs,- receiving (150), at all gNBs of a coordinated set of gNBs, data from the one or more aerial UEs associated with the coordinated set of gNBs and, at each respective gNB, data from terrestrial UEs attached or associated with the respective gNB, wherein the method (100) further comprises, if terrestrial UEs transmit data to the gNB using rate splitting, while aerial UEs don’t:- decoding (160a) and cancelling (170a), in the selected gNB, the common data streams of terrestrial UEs received at the selected gNB, prior to decoding (165a) data streams of the one or more aerial UEs assigned thereto, followed by decoding (180a) the private data streams of the terrestrial UEs, or wherein the method (100) further comprises, if the aerial UEs transmit data to the gNB using rate splitting, while the terrestrial UEs don’t:- decoding (160), at all gNBs of a coordinated set of gNBs, the RSMA commondata streams received from the one or more aerial gNBs, and cancelling (170) the decoded RSMA common data streams,- decoding (180), at each gNB of a coordinated set of gNBs, data received from the respective attached or associated terrestrial UEs,- decoding (190), at the selected gNB of each coordinated set of gNBs, the RSMA private data streams received from the one or more aerial UEs, or wherein the method (100) further comprises, if both the terrestrial and the aerial UEs transmit data to the gNB using rate splitting:- each gNB individually choosing the decoding (160) and cancelling (170) order for common and private data streams, respectively, received from aerial UEs and terrestrial UEs.

2. The method (100) of claim 1 , wherein selecting (140) one gNB that is tasked with decoding data streams from at least one of the one or more aerial UEs associated with the respective coordinated set of gNBs is based on CSI measurements performed in step (110), and / or a further additional CSI measurement performed by gNBs of the coordinated set of gBNs in respect of the one or more aerial UEs..

3. The method (100) of claim 1 or 2, further comprising receiving (125), from the one or more aerial UEs, information about their respective spatial locations (absolute or relative to the one or more gNBs), and providing this information to the composing step (130) and / or to the selecting step (140).

4. The method (100) of one of claims 1 to 3, wherein composing (130) one or more coordinated sets of gNBs from the set of interconnected gNBs comprises selecting, for each aerial UE, those gNBs that are located closest to the aerial UE, those gNBs that have the highest signal-to-noise ratio and / or signal level, or a combination thereof, while not selecting all gNBs that are capable of receiving transmissions from the respective aerial UE.

5. The method (100) of one of claims 1 to 4, further comprising repeating the CSI measuring (110) and the selecting step (140) while a gNB has been selected in a coordinated set of gNBs, for dynamically adapting the selection.

6. The method (100) of one or more of claims 1 to 5, further comprising adjusting the power allocation to the common and private data streams and / or the modulation and coding schemes for the common data streams, private data streams and / or data streams not using rate splitting multiple access, targeting maximising a weighted sum rate over all data streams while maintaining a block error rate and / or a total transmit power, of each transmitter that transmits a data stream within the same subchannel and / or resource block, below respective predetermined maximum values.

7. A base station, gNB, of a set of communicatively interconnected gNBs of a terrestrial communication network, comprising one or more antennas coupled with a first, wireless communication interface (501 ), a second communication interface (502) adapted for communicating with gNBs of the set of communicatively interconnected gNBs of the terrestrial communication network and / or with a central unit communicatively connected to said set, further comprising one or more microprocessors (503) and associated volatile (504) and non-volatile memory (505), the aforementioned elements being communicatively coupled via one or more signal and / or data lines or buses (506), wherein the non-volatile memory (505) stores computer program instructions which, when executed by the one or more microprocessors (503), configure the gNB to execute the method of one or more of claims 1 to 6.

8. A central unit of a terrestrial communication network, comprising one or more second communication interfaces (502) configured for communicating with one or more base stations in accordance with claim 7, further comprising one or more microprocessors (503) and associated volatile (504) and non-volatile memory (505), the aforementioned elements being communicatively coupled via one or more signal and / or data lines or buses (506), wherein the non- volatile memory (505) stores computer program instructions which, whenexecuted by the one or more microprocessors (503), configure the gNB to execute the method of one or more of claims 1 to 6.

9. A communication system comprising a plurality of base stations, gNB, that are communicatively connected with each other, each gNB having one or more antennas, each gNB being configured for communicating with one or more terrestrial user equipment, UE, within a coverage range, wherein the communicatively connected gNBs, or a central unit connected thereto, are configured for composing one or more coordinated sets of gNBs, each of the one or more coordinated sets of gNBs being associated with one or more terrestrial UEs and one or more aerial UEs, and each of the one or more coordinated sets of gNBs being configured for communicating with the one or more terrestrial UEs and the one or more associated aerial UEs in accordance with the method of one or more of claims 1 to 6.

10. A computer program product comprising computer program instructions, which, when executed by a microprocessor (503) of or functionally coupled with a gNB or a central unit of a system in accordance with claim 6, cause the processor (503), the gNB and / or the central unit to carry out the method of one or more of claims 1 to 6.

11. Computer readable medium or data carrier retrievably transmitting or storing the computer program product of claim 10.

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