Patent Publication Number: US-11394487-B2

Title: Non-orthogonal multiple access (NOMA) using rate based receivers

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is a 35 U.S.C. § 371 National Stage of International Patent Application No. PCT/EP2018/058833, filed Apr. 6, 2018, designating the United States, the disclosure of which is incorporated by reference. 
     TECHNICAL FIELD 
     Disclosed are embodiments related to non-orthogonal multiple access (NOMA) networks. 
     BACKGROUND 
     The design of multiple access schemes is of interest in the design of cellular telecommunication systems. The goal of multiple access schemes is to provide multiple user equipments (UEs) (i.e., wireless communication devices, such as, for example, smartphones, tablets, phablets, smart sensors, wireless Internet-of-Things (IoT) devices, etc., that are capable of wirelessly communicating with an access point) with radio resources in a spectrum, cost, and complexity-efficient manner. In 1G-3G wireless communication systems, frequency division multiple access (FDMA), time division multiple access (TDMA) and frequency division multiple access (CDMA) schemes have been introduced. Long-Term Evolution (LTE) and LTE-Advanced employ orthogonal frequency division multiple access (OFDMA) and single-carrier (SC)-FDMA as orthogonal multiple access (OMA) schemes. Such orthogonal designs have the benefit that there is no mutual interference among UEs, leading to high system performance with simple receivers. 
     Recently, non-orthogonal multiple access (NOMA) has received considerable attention as a promising multiple access technique for LTE and 5G systems. With NOMA, two or more UEs may share the same radio resources (e.g., time resources, frequency resources, and/or code resources). Particularly, 3GPP has considered NOMA in different applications. For instance, NOMA has been introduced as an extension of the network-assisted interference cancellation and suppression (NAICS) for intercell interference (ICI) mitigation in LTE Release 12 as well as a study item of LTE Release 13, under the name of “Downlink multiuser superposition transmission.” Also, in recent 3GPP meetings, it is decided that new radio (NR) should target to support (at least) uplink NOMA, in addition to the OMA approach. 
     SUMMARY 
     NOMA outperforms OMA in terms of sum rate. This performance gain, however, comes at the cost of higher decoding delay and receiver complexity. In downlink NOMA, a “cell-center” UE (i.e., a UE having a relatively good channel quality) may be pairs with a “cell-edge” UE (i.e., a UE with a comparatively lower channel quality) and the cell-center UE may use successive interference cancellation (SIC) to first decode and remove the signal of the cell-edge UE and then decode its own signal free of interference. This two-step decoding process by the cell-center UE results in a larger end-to-end transmission delay for the cell-center UE. It also may lead to larger end-to-end delay for the cell-edge UE in cases where their signals need to be synchronized. NOMA-based data transmission also leads to higher receiver complexity compared to conventional OMA-based data transmission. 
     Certain embodiments disclosed herein provide an adaptive receiver for cell-center UEs using NOMA. The objective is to reduce the complexity and the decoding delay of the receivers. Importantly, the decoding scheme in a UE may be determined based on a demanded rate of downlink transmission. In some embodiments, different decoding schemes may be considered by the cell-center UE depending on the rate demands of the UEs. In such embodiments, the network node may adapt its transmission power and synchronize the signals of the UEs according to the selected decoding scheme. 
     For instance, in one aspect there is provided a method performed by a network node, wherein the network node serves a first UE and a second UE. The method includes the network node obtaining, for a first decoding scheme, a first set of data points, each data point included in the first set of data points identifying a maximum achievable rate for the first UE and a maximum achievable rate for the second UE. The network node obtains a first rate demand for the first UE and a second rate demand for the second UE. Using the first set of data points, the first rate demand, and the second rate demand, the network node determines a decoding scheme for decoding a message transmitted by one of the first UE and a transmission point of the network node. In some embodiments, the decoding scheme includes using a successive interference cancellation (SIC) receiver to decode a message. 
     In some embodiments, the step of using the set of data points, the first rate demand, and the second rate demand to determine the decoding scheme includes selecting a data point from the set of data points, wherein selected data points identifies a maximum achievable rate for the first UE that is equal to the rate demand for the first UE; and determining whether the maximum achievable rate for the second UE identified by the selected data point is greater than or equal to the rate demand for the second UE. 
     In some embodiments, the step of using the set of data points, the first rate demand, and the second rate demand to determine the decoding scheme includes selecting a data point from the set of data points, wherein selected data points identifies a maximum achievable rate for the second UE that is equal to the rate demand for the second UE; and determining whether the maximum achievable rate for the first UE identified by the selected data point is greater than or equal to the rate demand for the first UE. 
     In another aspect there is provided a method performed by a network node, wherein the network node serves a first UE and a second UE. The method includes the network node obtaining a first rate demand for the first UE and a second rate demand for the second UE. The network node determines a first channel gain for the first UE and a second channel gain for the second UE. Using the first rate demand, the second rate demand, the first channel gain, and the second channel gain, the network node determines a decoding scheme for decoding a message transmitted by one of the first UE and a transmission point of the network node. In some embodiments, the decoding scheme includes using a successive interference cancellation (SIC) receiver to decode a message. 
     Compared to the conventional NOMA techniques, the embodiments disclosed herein considerably reduce the receiver complexity of the cell-center UE. The proposed method further reduces the end-to-end transmission delay of the network, thereby increasing the end-to-end throughput. For example, the proposed method leads to lower end-to-end transmission delay for both the cell-edge and the cell-center UEs. While the embodiments described in the current disclosure relate to downlink transmission, the same approach is applicable for uplink transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments. 
         FIG. 1  illustrates a TRP communicating with a UE using a high-gain beam. 
         FIGS. 2A-2B  illustrate NOMA setups according to some embodiments. 
         FIG. 3  illustrates the achievable rates for UEs using different data transmission schemes and decoding methods according to one embodiment. 
         FIG. 4  is a flow chart illustrating a process according to one embodiment. 
         FIG. 5  is a flow chart illustrating a process according to one embodiment. 
         FIG. 6  is a block diagram of a network node according to one embodiment. 
         FIG. 7  is a diagram showing functional units of a network node according to one embodiment. 
         FIG. 8  is a diagram showing functional units of a network node according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a network  100  having a network node  105  (e.g., access point (AP) such as, for example, a 4G or 5G base station or other access point) serving a large number of UEs—e.g., UE  101 , UE  102 , etc. While only two UEs are shown, network node  150  may serve N number of UEs, where N&gt;&gt;2. The UEs connect to the network node  105  using a limited number of spectrum resource blocks, i.e., time-frequency chunks. 
     In some embodiments, the network  100  may be a conventional downlink NOMA-based network where UE  101  and UE  102  are served by the network node  105  in common spectrum resource blocks. In some instances, the UE  101  may experience a better channel quality compared to the UE  102 . In such instances, |h 1 |≥|h 2 |, where h 1  and h 2  represents a channel coefficient of the link between the network node  105  and the UE  101  and the UE  102 , respectively. The channel gain for each UE is defined as g i =|h i | 2 , where i=1, 2. 
     Using NOMA, the network node  105  may generate and transmit a superimposed signal to both UEs in the same resources in a time slot t. The superimposed signal is given as S(t)=√{square root over (P 1 )}X 1 (t)+√{square root over (P 2 )}X 2 (t). Here, X 1 (t) and X 2 (t) are the unit-variance message signals and P 1  and P 2  are the allocated transmit power for UE  101  and UE  102 , respectively. Here, P represents a total power of the network node  105  and P=P 1 +P 2 . Accordingly, the signal received by the UEs is given by
 
 Y   i ( t )= h   i (√{square root over ( P   1 )} X   1 ( t )+√{square root over ( P   2 )} X   2 ( t ))+ Z   i ( t ), i= 1,2,  (1)
 
where Z i (t) denotes the Gaussian white noise added in UE i , i=1, 2 (UE  101  and UE  102 , respectively).
 
       FIG. 2A  depicts a first NOMA setup (also referred to as SIC receiver based NOMA) according to some embodiments. As shown in  FIG. 2A , UE 1 , i.e., the UE  101  experiencing a better channel quality, uses a SIC receiver to first decode and remove the message of UE 2 , i.e., the UE  102  experiencing a worse channel quality, and then decode its own message with no interference. UE  102  uses a non-SIC receiver to decode its own message in the presence of interference due to the UE  101  signal. As shown by T SIC  in  FIG. 2A , using the SIC receiver results in higher decoding delay compared to using non-SIC receivers. For synchronization of the UEs signals, some delay may be considered by UE  102  or the network node  105  may perform the synchronization. For example, UE  102  may enter a sleep mode until UE  101  completes decoding its message as shown in  FIG. 2A . 
     The goal of each UE is to decode its own message. In some embodiments, a UE may first decode the message of the other UE to reduce the interference. In accordance with the first NOMA setup shown in  FIG. 2A , UE  101  uses the SIC receiver to first decode and remove the message of UE  102  and decodes its own message free of interference. UE  102  uses OMA-based receivers to decode its own message and considers the UE  101  message as interference. UE  102  uses OMA-based receivers because it can be theoretically shown that there is no chance that UE  102  can first decode and remove the message of UE  101  and subsequently decode its own message free of interference in the first NOMA setup shown in  FIG. 2A . Accordingly, in the first NOMA setup, the maximum achievable rates for UE  101  and UE  102  is given by: 
     
       
         
           
             
               
                 
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     The SIC receiver is a high-complexity receiver compared to conventional OMA-based receivers. Using the SIC receiver results in larger decoding delay due to the two step decoding process. Such decoding delay of UE  101  affects the end-to-end transmission delay of both UEs in cases where the signals of UE  101  and UE  102  need to be synchronized. Different methods can be applied to synchronize the signals. For example, UE 2  may enter a sleep mode, as shown in  FIG. 2A , or the network node  105  may perform the synchronization. 
       FIG. 2B  depicts a second NOMA setup (also referred to as OMA receiver based NOMA) according to some embodiments. In the second NOMA setup shown in  FIG. 2B , both UEs utilize a conventional OMA-based receiver. That is, UE  101  does not use a SIC receiver as opposed to the first NOMA setup shown in  FIG. 2A . In this NOMA setup, the maximum achievable rates for UE  101  and UE  102  is given by: 
     
       
         
           
             
               
                 
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     As shown in  FIG. 2B , each of the UEs use OMA-based receivers to decode its own message in the presence of interference caused by the other UE message. The use of conventional receivers and decoders allows lower implementation complexity and decoding delay compared to using SIC based receivers, as shown by T Conv  in  FIG. 2B . 
     In the second NOMA setup, each of the UEs decodes its message of interest in one step and considers the other UE message as interference. This allows decoding delay due to the two-step decoding process shown in  FIG. 2A  to be removed which considerably reduces the decoder complexity and network end-to-end transmission delay. 
     In some embodiments, the network node  105  may allocate all power and frequency resources to one of the UEs in a time slot. In such embodiments, the other UE is not served in that time slot. When the network node  105  allocates all the power and time-frequency resources to UE  101 , the maximum achievable rates for UE  101  and UE  102  is given by: 
     
       
         
           
             
               
                 
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     When the network node  105  allocates all the power and time-frequency resources to UE  102 , the maximum achievable rates for UE  101  and UE  102  is given by: 
     
       
         
           
             
               
                 
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       FIG. 3  illustrates the achievable rates given by (2)-(5) and described above for different data transmission schemes and decoding methods for the UEs. As shown in  FIG. 3 , the achievable rates given by (2) for the SIC receiver based NOMA correspond to region AOBD. The achievable rates given by (3) for the OMA receiver based NOMA correspond to region AOBC. Point A corresponds to the achievable rates given by (4) where the network node  105  allocates all power and time-frequency resources to UE  101 . Point B corresponds to the achievable rates given by (5) where the network node  105  allocates all power and time-frequency resources to UE  102 . In some embodiments, the achievable rates shown in  FIG. 3  may be based on a NOMA setup where the total power of the network node  105  (P) is 40 dBm, a channel gain for UE  101  (g 1 ) is 0.2, and a channel gain for UE  102  (g 2 ) is 0.1. 
     An embodiment of adaptive decoding schemes where the SIC receiver is only used when required is explained as follows. In this embodiment, the appropriate receiver for UE  101  is determined based on the rate demands of the UEs and the quality of the links between the network node  105  and the UEs. Here, the rate demand of UE  101  and UE  102  is given as r i , i=1, 2. Further, (r 1 , r 2 )∈ (X) indicates that rate demands r 1  and r 2  are in region X. 
     Step 1: The UEs (e.g., UE  101  and UE  102 ) send pilot signals to the network node  105 . 
     Step 2: Using the pilot signals, the network node  105  determines the channel gains (g 1  and g 2 ) for the channels between the network node  105  and UE  101  and UE  102 . 
     Step 3: Using the rate demands of the UEs (r 1  and r 2 ), i.e., the data rate of interest or the buffered data size, the network node  105  uses achievable rates given by (2)-(5) to determine the appropriate decoding scheme of UE  101 . Referring now to  FIG. 3 , an embodiment of the procedure for determining the appropriate decoding scheme of UE  101  is as follows: 
     Step 3(a): If (r 1 , r 2 )∈ (AOBC), the network node  105  selects an OMA-based receiver for UE  101 . Accordingly, UE  101  does not decode the UE  102  message and considers it as interference. The network node  105  further optimizes the power allocation for the UEs based on the achievable rates given by (3) to find the optimal power allocation which guarantees the rate demands. 
     Step 3(b): If (r 1 , r 2 )∈ (ACBD), the network node  105  selects a SIC-based receiver for UE  101 . Accordingly, UE  101  first decodes and removes the UE  102  message and subsequently decodes the UE  101  message free of interference. The network node  105  further optimizes the power allocation for the UEs based on the achievable rates given by (2) to find the optimal power allocation which guarantees the rate demands. 
     Step 3(c): If (r 1 , r 2 )∈ (ADBE), it is not possible for the network node  105  to support the rate demands of both UEs simultaneously. In order to provide the UEs according to their rate demands, different schemes may be considered. For example, the total transmit power (P) of the network node  105  can be increased in one scheme. As another exemplary scheme, the UEs may be scheduled in different time slots where their rate demands can each be supported based on the achievable rates given by (4) and (5). The network node  105  may adapt power allocation for the UEs depending on the selected scheme. 
     For simplicity of explanation, the rate demands of the UEs are limited to their maximum achievable rates, i.e., r 1 ≤R 1,max  and r 2 ≤R 2,max , in the embodiments described herein. In some embodiments, however, the rate demands of the UEs may be higher than the maximum achievable rates. In order to support the rate demands of the UEs in such embodiments, region ADBE is expanded by increasing the transmit power (P) of the network node  105  or step 3(c) is performed. 
     Step 4: The network node  105  informs both UEs about the selected decoding scheme of UE  101 . In some embodiments, the network node  105  transmits an indication to the UEs indicating the selecting decoding scheme of UE  101 . 
     Step 5: Depending on the selected decoding scheme of UE  101 , both UEs synchronize transmit and/or receive timings. For example, UE  102  may enter a sleep mode until UE  101  completes decoding the UE  101  message as shown in  FIG. 2A . 
     With the proposed scheme, both the end-to-end transmission delay of the NOMA-based setup and the implementation complexity of the UE  101  receiver are considerably decreased. 
     In the embodiments described herein, the proposed scheme describes extreme cases where the cell-center UE, i.e., UE  101 , either uses SIC or conventional OMA-based receivers. This is not required, however, and different suboptimal decoding schemes with different complexities and decoding delays may be used in alternative embodiments of NOMA transmission. The proposed scheme described herein is applicable to every combination of different decoders at UE  101  where each decoding scheme corresponds to a specific region for the achievable rates of the UEs as shown in  FIG. 3 . 
     While the embodiments described herein are directed to downlink transmission, the proposed scheme is applicable to uplink transmission. In some embodiments, the network node  105  may consider different decoding schemes based on the rate demands of the UEs and the quality of the channels. In such embodiments, the UEs may adapt their transmit power according to the decoding scheme selected by the network node  105 . 
       FIG. 4  is a flow chart illustrating a process  400 , according to some embodiments, that is performed by a network node  105 , wherein the network node serves a first UE (UE  101 ) and a second UE (UE  102 ). Process  400  may begin with step s 402  in which network node  105  obtains, for a first decoding scheme, a first set of data points, each data point included in the first set of data points identifying a maximum achievable rate for the first UE and a maximum achievable rate for the second UE. In step s 404 , the network node obtains a first rate demand for the first UE. In step s 406 , the network node obtains a second rate demand for the second UE. In step s 408 , the network node determines, using the first set of data points, the first rate demand, and the second rate demand, a decoding scheme for decoding a message transmitted by one of the first UE and a transmission point of the network node. In some embodiments, the decoding scheme includes using a successive interference cancellation (SIC) receiver to decode a message. 
     In some embodiments, the step of using the set of data points, the first rate demand, and the second rate demand to determine the decoding scheme includes selecting a data point from the set of data points, wherein selected data points identifies a maximum achievable rate for the first UE that is equal to the rate demand for the first UE; and determining whether the maximum achievable rate for the second UE identified by the selected data point is greater than or equal to the rate demand for the second UE. 
     In some embodiments, the step of using the set of data points, the first rate demand, and the second rate demand to determine the decoding scheme includes selecting a data point from the set of data points, wherein selected data points identifies a maximum achievable rate for the second UE that is equal to the rate demand for the second UE; and determining whether the maximum achievable rate for the first UE identified by the selected data point is greater than or equal to the rate demand for the first UE. 
       FIG. 5  is a flow chart illustrating a process  500 , according to some embodiments, that is performed by a network node  105 , wherein the network node serves a first UE (UE  101 ) and a second UE (UE  102 ). Process  500  may begin with step s 502  in which network node obtains a first rate demand for the first UE. In step s 504 , the network node obtains a second rate demand for the second UE. In step s 506 , the network node determines a first channel gain for the first UE. In step s 508 , the network node determines a second channel gain for the second UE. In step s 510 , the network node determines, using the first rate demand, the second rate demand, the first channel gain, and the second channel gain, a decoding scheme for decoding a message transmitted by one of the first UE and a transmission point of the network node. In some embodiments, the decoding scheme includes using a successive interference cancellation (SIC) receiver to decode a message. 
       FIG. 6  is a block diagram of network node  105  according to some embodiments. As shown in  FIG. 6 , network node  105  may comprise: a processing circuit (PC)  602 , which may include one or more processors (P)  655  (e.g., a general purpose microprocessor and/or one or more other processors, such as an application specific integrated circuit (ASIC), field-programmable gate arrays (FPGAs), and the like); a network interface  648  comprising a transmitter (Tx)  645  and a receiver (Rx)  647  for enabling network node  105  to transmit data to and receive data from other nodes connected to a network  110  (e.g., an Internet Protocol (IP) network) to which network interface  648  is connected; circuitry  603  (e.g., radio transceiver circuitry comprising an Rx  605  and a Tx  606 ) coupled to an antenna system  604  for wireless communication with UEs); and local storage unit (a.k.a., “data storage system”)  608 , which may include one or more non-volatile storage devices and/or one or more volatile storage devices (e.g., random access memory (RAM)). In embodiments where PC  602  includes a programmable processor, a computer program product (CPP)  641  may be provided. CPP  641  includes a computer readable medium (CRM)  642  storing a computer program (CP)  643  comprising computer readable instructions (CRI)  644 . CRM  642  may be a non-transitory computer readable medium, such as, but not limited, to magnetic media (e.g., a hard disk), optical media, memory devices (e.g., random access memory, flash memory), and the like. In some embodiments, the CRI  644  of computer program  643  is configured such that when executed by data processing apparatus  602 , the CRI causes network node  105  to perform steps described herein (e.g., steps described herein with reference to the flow charts and/or message flow diagrams). In other embodiments, network node  105  may be configured to perform steps described herein without the need for code. That is, for example, PC  602  may consist merely of one or more ASCs. Hence, the features of the embodiments described herein may be implemented in hardware and/or software. 
       FIG. 7  is a diagram showing functional units of network node  105  according to some embodiments. As shown in  FIG. 7 , network node  105  includes a first obtaining unit  702  for obtaining, for a first decoding scheme, a first set of data points, each data point included in the first set of data points identifying a maximum achievable rate for the first UE and a maximum achievable rate for the second UE; a second obtaining unit  704  for obtaining a first rate demand for the first UE; a third obtaining unit  706  for obtaining a second rate demand for the second UE; and a determining unit  708  for determining, using the first set of data points, the first rate demand, and the second rate demand, a decoding scheme for decoding a message transmitted by one of the first UE and a transmission point of the network node. 
       FIG. 8  is a diagram showing functional units of network node  105  according to some embodiments. As shown in  FIG. 8 , network node  105  includes a first obtaining unit  802  for obtaining a first rate demand for the first UE; a second obtaining unit  804  for obtaining a second rate demand for the second UE; a first determining unit  806  for determining a first channel gain for the first UE; a second determining unit  808  for determining a second channel gain for the second UE; and a third determining unit  810  for determining, using the first rate demand, the second rate demand, the first channel gain, and the second channel gain, a decoding scheme for decoding a message transmitted by one of the first UE and a transmission point of the network node. 
     Also, while various embodiments of the present disclosure are described herein, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Additionally, while the processes described above and illustrated in the drawings are shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed in parallel.