Patent Publication Number: US-11646768-B2

Title: Method and apparatus for downlink transmission in a cloud radio access network

Description:
FIELD 
     The subject matter herein generally relates to radio communications. 
     BACKGROUND 
     The technology of cloud radio access network (C-RAN) is part of the main technology in the fifth generation of mobile communication (5G). The C-RAN centralizes baseband units (BBUs) of distributed base stations in an area to form a BBU resource pool. Baseband signals of the remote radio heads (RRHs) in this area are processed in the same BBU pool. 
     Faced with the huge data-transmission amounts between RRHs and a number of user equipment (UEs), the overall performance can be enhanced through cooperative transmission between RRHs. The cooperative transmission between RRHs can improve the overall performance but requires higher fronthaul capacity compared to non-cooperative transmissions. 
     Thus, there is room for improvement within the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present technology will now be described, by way of embodiment, with reference to the attached figures, wherein: 
         FIG.  1    is a block diagram of one embodiment of a cloud radio access network (C-RAN). 
         FIG.  2    is an example of one embodiment of a coordinated multi-point downlink transmission scenario in the C-RAN. 
         FIG.  3    is flowchart of one embodiment of a method for downlink transmission in C-RAN. 
         FIG.  4    is a flowchart of another embodiment of a method for downlink transmission. 
         FIG.  5    is a block diagram of one embodiment of a central unit for downlink transmission in the C-RAN. 
         FIG.  6    is a block diagram for one embodiment of a radio remote head for downlink transmission in the C-RAN. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. 
     References to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one”. 
     In general, the word “module” as used hereinafter, refers to logic embodied in computing or firmware, or to a collection of software instructions, written in a programming language, such as Java, C, or assembly. One or more software instructions in the modules may be embedded in firmware, such as in an erasable programmable read only memory (EPROM). The modules described herein may be implemented as either software and/or computing modules and may be stored in any type of non-transitory computer-readable medium or another storage device. Some non-limiting examples of non-transitory computer-readable media include CDs, DVDs, BLU-RAY, flash memory, and hard disk drives. The term “comprising”, when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series, and the like. 
       FIG.  1    illustrates a high-level architecture of cloud radio access network (C-RAN)  100  according to one embodiment. The C-RAN  100  comprises a core network  110 , a central unit  120 , a plurality of remote radio heads (RRHs), for example, RRHs  130   a ,  130   b , and  130   c , and a plurality of user equipment (UEs), for example, UEs  140   a ,  140   b ,  140   c , and  140   d . The set of RRHs  130   a ,  130   b , and  130   c  are connected to a baseband unit (BBU) pool, which comprises the central unit  120 , through high bandwidth transport links known as fronthaul (FH). The set of RRHs  130   a ,  130   b , and  130   c  can send and receive radio signals from the plurality of UEs  140   a ,  140   b , and  140   c  through wireless interfaces. The central unit  120  can be in communication with the core network  110 . In one example, data from the core network  110  to the UE  140   a , which is in a coverage area of the RRH  130   a , can first be sent to the central unit  120 . The central unit  120  then sends the data to the RRH  130   a  through a fronthaul link. The data can finally be sent through a radio signal from the RRH  130   a  to the UE  140   a . This is referred to as a downlink transmission. 
     In one embodiment, the C-RAN  100  performs coordinated multi-point (CoMP) transmission on the downlink to enhance system performance and user-experienced service quality. The CoMP transmission means that the data is transmitted to a UE jointly from the set of RRHs, thereby not only reducing the interference but also increasing the received power. The transmission from the set of RRHs can also take channel conditions at the different UEs into account to enhance the received signal strength, while the same time reducing the interference between different transmissions.  FIG.  2    illustrates an example of a CoMP downlink transmission scenario in the C-RAN  100 . In the example, the N=3 RRHs are geographically distributed while the K=4 UEs are in a group requiring wireless data services. For simplicity, single antenna is used at the RRHs and the UEs. In order to set up CoMP, there are several steps needing to be executed by the RRHs and the UEs: Step (1), the RRHs send pilots in the downlink so that the UEs can acquire the channel state information (CSI) for this link. Step (2), the UEs feed the CSI back to their serving RRH, typically its strongest BS. Step (3), the CSI acquired at the RRHs is forwarded to the central unit  120  of the C-RAN  100  to compute the precoding weights to mitigate interference. Step (4), the UE data is routed to the RRHs based on the precoding weights. Finally, the UEs are served. That is, each one of the UEs participating in CoMP will feed the CSI back over the air to its serving RRH, then forwarding to the central unit  120  for precoding, imposing a heavy burden on the fronthaul traffic. As the precoding weights computed at the central unit  120  needs to be transmitted to all the RRHs along with the UE data, this can further overwhelm the backhaul, especially for a scenario which includes a large number of UEs. In fact, it is not necessary for all the RRHs to serve one UE at the same time. In one example, the first UE (that is, UE 1  in the  FIG.  2   ) can select the second RRH (that is, RRH 2  in  FIG.  2   ) to perform no service. In the example, the central unit  120  can design the precoding weights with f 2,1 =0 and format the aggregated channel matrix with h 2,1 =0, where f 2,1  is the precoding weight for the first UE at the second RRH, and h 2,1  is the channel matrix from the second RRH to the first UE. Then the fronthaul traffic between the second RRH and the central unit  120  can be compressed. If each UE selects a subset of RRHs to provide service, both the precoding weights and the aggregated channel matrix are sparse, thus reducing the fronthaul load. In one embodiment, the data compression ratio can be calculated by the number of zero elements in the precoding weights, the data compression ratio becoming greater as the number of the zero elements becomes greater. In the example, the first UE selecting the second base station to provide no service will achieve a 33% data compression ratio effect. On the other hand, since the second base station does not service the first UE, the control unit  120  will not receive full channel state information. In one embodiment, the control unit  120  can calculate a null space matrix according to an interference channel matrix of the first UE, and then design the precoding weights for the first UE for each of the RRH serving the first UE based on the null space matrix. 
       FIG.  3    illustrates a method for downlink transmission performed by the central unit  120  according to one embodiment. In the embodiment, the central unit  120  schedules CoMP with a predetermined data compression ratio. 
     At step S 302 , the central unit  120  determines a specific number of RRHs as non-serving RRHs based on the predetermined data compression ratio. 
     At step S 304 , for each one of the plurality of UEs, the central unit  120  determines a combination of RRHs which are non-serving in CoMP from the plurality of RRHs, based on the determined specific number. 
     In one embodiment, the determination as to the combination is performed by exhaustion in order to achieve maximum sum-rate in the C-RAN  100 . 
     Let there be total N RRHs and total K UEs, and the central unit  120  needs to determine a combination of Z RRHs which are non-serving in CoMP for each UE. For each one of the plurality of UEs, there are C Z   N  possible combinations of Z RRHs non-serving in CoMP. For each one of the possible combinations, the central unit  120  first collects CSI of the UE. The central unit  120  then forms an interference channel matrix of the UE based on the CSI, calculates a null space matrix of the interference channel matrix, and designs precoding weights for the UE based on the null space matrix. Finally, the central unit  120  calculates sum rate of the UE for the combination based on the precoding weights and the collected CSI. The central unit  120  collects sum rates of all the possible combinations, and determines the combination of RRHs which are non-serving in CoMP from the possible combinations based on the collected sum rates. For example, the central unit  120  compares sum rates of all the possible combinations for the UE, and determines one combination which has the maximal sum rate of the UE. 
     In another embodiment, the central unit  120  obtains C Z   N  possible combinations of Z RRHs which are non-serving in CoMP for each one of the K UEs, and evaluates channel orthogonality for each possible combination. After the evaluation, the central unit  120  determines a combination of RRHs which are non-serving in CoMP from the plurality of RRHs for each one of the K UEs, based on the evaluation. For example, the central unit  120  can determine a combination of RRHs which are non-serving in CoMP from the plurality of RRHs for each one of the K UEs with strong channel orthogonality. 
     In this embodiment, the central unit  120  needs to design precoding weights only once for each one of the plurality of UEs for the plurality of RRHs. 
     In another embodiment, the central unit  120  obtains C Z   N  possible combinations of Z RRHs which are non-serving in CoMP for each one of the K UEs, evaluates channel orthogonality and estimates channel gain for each possible combination. After making the evaluations and estimating channel gain for each possible combination for the K UEs, the central unit  120  determines a combination of RRHs which are non-serving in CoMP from the plurality of RRHs for each one of the K UEs based on the evaluations and the estimated channel gain. For example, the central unit  120  can determine a combination of RRHs which are non-serving in CoMP from the plurality of RRHs for each one of the K UEs, with strong channel orthogonality and maximal channel gain. 
     In this embodiment, the central unit  120  needs to design only once precoding weights for each one of the plurality of UEs for the plurality of RRHs. 
     At step S 306 , for each one of the plurality of UEs, the central unit  120  transmits the combination of RRHs which are non-serving in CoMP to the plurality of RRHs. 
     At step S 308 , for each one of the plurality of UEs, the central unit  120  performs CoMP downlink transmission based on the combination of RRHs which are non-serving in CoMP. 
     In one embodiment, before step S 308 , for each one of the plurality of RRHs, the central units  120  further determines a number of bits allocated for each one of the plurality of UEs, based on a bit budget. For example, the bit allocation problem can be formulated as a sum of symbol error rate (SER) at all the UEs in the C-RAN. The central units  120  can determine a number of bits allocated for each one of the UEs for each one of the plurality of RRHs based on the bit budget, the collected CSI, and the designed precoding weights. 
       FIG.  4    illustrates a method for downlink transmission performed by each one of the plurality of RRHs according to one embodiment. In the embodiment, the plurality of UEs feedback CSI to all the RRHs. Therefore, each RRH collects all the available CSI including those related to other RRHs. In the embodiment, each RRH schedules CoMP with a predetermined data compression ratio. 
     At step S 402 , each RRH determines a specific number of UEs as to-be-served UEs based on the predetermined data compression ratio. 
     At step S 404 , each RRH determines a combination of UEs to be served in CoMP from the plurality of UEs, based on the predetermined specific number. 
     In one embodiment, the determination as to the combination is performed by exhaustion in order to achieve maximum sum-rate in the C-RAN  100 . 
     Let there be total K UEs, and each RRH needs to determine a combination of K′ UEs to-be-served in CoMP. There are C K′   K  possible combinations of K′ UEs to be served in CoMP. For each one of the UEs of the possible combinations, RRH first collects CSI of the UE of the possible combination. The RRH then forms an interference channel matrix of the UE based on the CSI, calculates a null space matrix of the interference channel matrix, and designs precoding weights for the UE based on the null space matrix. Finally, the RRH calculates sum rate of the UE for the possible combination based on the precoding weights and the collected CSI. The RRH collects sum rates of all the possible combinations, and determines the combination of UEs which are to be served in CoMP from all possible combinations, based on the collected sum rates. For example, the RRH compares sum rates of all the possible combinations, and determines one combination which has the maximal sum rate. 
     In another embodiment, the RRH obtains C K′   K  possible combinations of K′ UEs to be served in CoMP, and evaluates channel orthogonality for each possible combination. After evaluating channel orthogonality of each possible combination for the K′ UEs, the RRH determines a combination of UEs to be served in CoMP from the plurality of UEs based on the evaluation. For example, the RRH can determine a combination of K′ UEs to be served in CoMP from the plurality of UEs, with strong channel orthogonality. 
     In this embodiment, the RRH needs to design precoding weights for each one of the to-be-served UEs only once. 
     In another embodiment, the RRH obtains C K′   K  possible combinations of K′ UEs to be served in CoMP, evaluates channel orthogonality and estimates channel gain for each possible combination. After evaluating and estimating channel gain for each possible combination for the K′ UEs, the RRH determines a combination of UEs to be served in CoMP from the plurality of UEs based on the evaluation and the estimation. For example, the RRH can determine a combination of UEs to be served in CoMP from the plurality of UEs with strong channel orthogonality and maximal channel gain. 
     In this embodiment, the RRH designs precoding weights for each one of the to-be-served UEs only once. 
     At step S 406 , each RRH performs CoMP downlink transmission based on the combination of UEs to be served in CoMP. 
     In one embodiment, before step S 406 , each RRH further determines a number of bits allocated for each one of the combinations of UEs to be served in CoMP based on a bit budget. For example, the bit allocation problem can be formulated as a sum of symbol error rate (SER) at all the plurality of UEs in the C-RAN must been minimize. Each RRH can determine a number of bits allocated for each one of the combinations of UEs to be served in CoMP based on the bit budget, the collected CSI, and the designed precoding weights. 
       FIG.  5    illustrates a block diagram of the central unit  120  according to an embodiment. The central unit  120  comprises a processing unit  122 , and a computer readable storage medium  124 . The processing unit  122  is electrically connected to the computer readable storage medium  124 . The processing unit  122  comprises a microcontroller, a microprocessor, or other circuit with processing capabilities, and executes or processes instructions, data, and computer programs stored in the computer readable storage medium  124 . The computer readable storage medium  124  comprises a read-only memory (ROM), a random access memory (RAM), a magnetic disk storage medium device, an optical storage medium device, a flash memory device, electrical, optical, or other physical/tangible (e.g., non-transitory) memory device, etc. A computer-readable storage medium is used to store one or more computer programs that control the operation of the central unit  120  and executed by the processing unit  122 . In the embodiment, the computer readable storage medium  124  stores or encodes one or more computer programs, and stores models, configurations, and computing parameters data, for the processing unit  120 , to execute the method shown in  FIG.  3   . 
       FIG.  6    illustrates a block diagram of an RRH  600  according to an embodiment. The RRH  600  comprises a processing unit  610 , and a computer readable storage medium  620 . The processing unit  610  is electrically connected to the computer readable storage medium  620 . The processing unit  610  comprises a microcontroller, a microprocessor, or another circuit with processing capabilities, and executes or processes instructions, data, and computer programs stored in the computer readable storage medium  620 . The computer readable storage medium  620  comprises a read-only memory (ROM), a random access memory (RAM), a magnetic disk storage medium device, an optical storage medium device, a flash memory device, electrical, optical, or other physical/tangible (e.g., non-transitory) memory device, etc. A computer-readable storage medium is used to store one or more computer programs that control the operation of the RRH  600  and executed by the processing unit  610 . In the embodiment, the computer readable storage medium  620  stores or encodes one or more computer programs, and stores models, configurations, and computing parameters data for the processing unit  610  to execute the method shown in  FIG.  4   . 
     The method and apparatus for downlink transmission in the C-RAN achieves predetermined data compression ratio while maintaining optimal overall performance. 
     The embodiments shown and described above are only examples. Many details are often found in the art; therefore, many such details are neither shown nor described. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, especially in matters of shape, size, and arrangement of the parts within the principles of the present disclosure, up to and including the full extent established by the broad general meaning of the terms used in the claims. It will, therefore, be appreciated that the embodiments described above may be modified within the scope of the claims.