Patent Publication Number: US-2020296735-A1

Title: Retransmission of messages using a non-orthogonal multiple access (noma) communication system

Description:
TECHNICAL FIELD 
     Disclosed are embodiments related to non-orthogonal multiple access (NOMA) communication systems. 
     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 time resource and frequency resource as well as, if applicable, the same code resource and beam resource. 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 exploits channel difference between or among UEs to improve spectrum efficiency. Generally, the highest gain of NOMA is observed in the cases where a “strong” UE (i.e., a UE experiencing a good channel condition with a base station, such as, for example, a UE located in the center of a cell) and a “weak” UE (i.e., a UE having a poor channel condition with the base station, such as, for example, a UE located at or near a cell edge) are grouped (i.e., use the same radio resources). However, the implementation of NOMA implies: 1) use of more advanced and complex receivers to enable multiuser signal separation, 2) more difficult synchronization, and 3) a higher signal decoding delay 
     For example, considering downlink NOMA, the strong UE typically uses successive interference cancellation (SIC) to first decode and remove the message for the weak UE and then decode its own message interference-free. As a result, compared to conventional OMA scheme, NOMA-based data transmission leads to higher receiver complexity. Also, compared to OMA-based systems, the two-step decoding process of the strong UE may lead to larger end-to-end transmission delay for the strong UE, as well as for the weak UE (e.g. in scenarios in which their signals should be synchronized). Also, there is a probability that the strong UE cannot correctly decode the message of the weak UE affecting the successful decoding probability of its own message. 
     Also, while using NOMA outperforms OMA in terms of sum rate, the sum rate gain of NOMA is at the cost possible rate loss for the weak UE (e.g., the cell-edge UE). This is because, with downlink NOMA, the weak UE considers the signal of the strong UE as interference and uses the typical OMA-based decoder to decode its own message. Thus, there is a significant probability that neither the strong UE nor the weak UE can decode the message intended for it, thus requiring the network to re-transmit the messages. 
     This disclosure describes, among other things, a method that improves downlink (DL) and uplink (UL) message throughput in a NOMA system. The method may be referred to as a “smart” hybrid automatic repeat request (HARM) based method. In a downlink embodiment of the smart HARQ based method, a network node, using a set of one or more radio resources, transmits a first superimposed signal containing a message for a first UE and a second message for a second UE. If the network node receives a NACK from the first UE indicating that it could not decode either the first message or the second message and also receives a NACK from the second UE indicating that it could not decode the second message, the network node initially only retransmits one of the messages. More specifically, for example, the network node transmits a second superimposed signal containing the second message for the second UE and a new (third) message for the first UE but not containing the first message for the first UE. In this scenario, it is possible that, as a result of receiving the second superimposed signal, the first UE is able to decode the second message and then use this decoded message to decode both the first message and the new message. 
     Hence, throughput is greatly improved because the network does not need to retransmit the first message even though the first UE was initially unable to obtain the first message from the first superimposed signal. While embodiments are exemplified using the simplest case of two UEs and downlink transmission, the embodiments can be adapted to the cases with arbitrary number of UEs and uplink transmission as well. 
     Accordingly, in one embodiment there is provided a method for transmitting messages to a first UE and a second UE. The method is performed by a network node (NN) and includes the NN transmitting first superimposed signal comprising a first message for the first UE and a second message for the second UE. The method also includes the NN determining that the first UE was not able to successfully decode either the first message or the second message. The method also includes the NN determining that the second UE was not able to successfully decode the second message. The method also includes the NN, in response to determining that the first UE was not able to successfully decode either the first message or second message and that the second UE was not able to successfully decode the second message, deciding to retransmit the second message but not the first message. The method also includes the NN retransmitting the second message by transmitting a second superimposed signal comprising the second message for the second UE and a third message for the first UE but not including the first message, wherein the third message is different than the first message. 
     In another embodiment there is provided a method for receiving messages transmitted by a network node. The method is performed by a first UE and includes the first UE receiving a first superimposed signal transmitted by the network node, the first superimposed signal comprising a first message for the first UE and a second message for a second UE. The method also includes the first UE attempting to decode the second message for the second UE prior to attempting to decode the first message for the first UE. The method also includes, after attempting to decode the second message, the first UE providing an indication to the network node indicating that the second message has not been successfully decoded. The method also includes the first UE buffering the first superimposed signal. The method also includes, after providing the indication to the network node, the first UE receiving a second superimposed signal transmitted by the network node, the second superimposed signal comprising the second message for the second UE and a third message for the first UE but not including the first message for the first UE, wherein the third message is different than the first message. The method also includes, after receiving the second superimposed signal, the first UE successfully decoding the second message for the second UE. The method also includes, after successfully decoding the second message for the second UE, the first UE using the decoded second message and the buffered first superimposed signal to decode the first message from the first superimposed signal. 
     In another embodiment there is provided a method for obtaining a first message transmitted by a first UE and a second message transmitted by a second UE. The method is performed by a network node (NN) and includes the NN scheduling the first UE to transmit the first message using a first time and frequency resource and scheduling the second UE to transmit the second message using the first time and frequency resource. The method also includes the NN receiving a first signal comprising the first message and the second message, and, as a result of not being able to obtain either the first message or the second message from the signal, the NN performs performing steps comprising: buffering the first signal; scheduling the first UE to retransmit the first message using a second time and frequency resource; and scheduling the second UE to transmit a third message using the second time and frequency resource, wherein the third message is different than the second message. 
     In another embodiment there is provided a method for transmitting messages to a network node. The method is performed by a UE and includes the UE receiving a first scheduling message transmitted by the network node. The method also includes, as a result of receiving the first scheduling message, the UE transmitting a first signal comprising a first message. The method also includes the UE, after transmitting the first signal, buffering the first message in case the network node requires the UE to retransmit the first message. The method also includes the UE, after buffing the first message, receiving a second scheduling message transmitted by the network node, the second scheduling message instructing the UE to transmit a second message. The method also includes the UE, as a result of receiving the second scheduling message, transmitting a second signal comprising the second message but not comprising the first message. The method also includes the UE, after transmitting the second signal, receiving: i) acknowledgment information transmitted by the network node, wherein the acknowledgment information indicates that the network node has been able to obtain the first message from the first signal and the second message from the second signal, or ii) a request to retransmit the first message. 
    
    
     
       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 network node communicating simultaneously with a first UE and a second UE. 
         FIG. 2  illustrates processing that occurs during a time slot. 
         FIG. 3  illustrates processing, according to one embodiment, that occurs during first and second time slots. 
         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 flow chart illustrating a process according to one embodiment. 
         FIG. 7  is a flow chart illustrating a process according to one embodiment. 
         FIG. 8  is a block diagram of a network node according to one embodiment. 
         FIG. 9A  is a diagram showing functional units of a network node according to an embodiment. 
         FIG. 9B  is a diagram showing functional units of a network node according to an embodiment. 
         FIG. 10  is a block diagram of a UE according to one embodiment. 
         FIG. 11A  is a diagram showing functional units of a UE according to one embodiment. 
         FIG. 11B  is a diagram showing functional units of a UE according to one embodiment. 
         FIG. 12  schematically illustrates a telecommunication network connected via an intermediate network to a host computer. 
         FIG. 13  is a generalized block diagram of a host computer communicating via a base station with a user equipment over a partially wireless connection. 
         FIG. 14  is a flowchart illustrating a method implemented in a communication system including a host computer, a base station and a user equipment. 
         FIG. 15  is a flowchart illustrating a method implemented in a communication system including a host computer, a base station and a user equipment. 
         FIG. 16  is a flowchart illustrating a method implemented in a communication system including a host computer, a base station and a user equipment. 
         FIG. 17  is a flowchart illustrating a method implemented in a communication system including a host computer, a base station and a user equipment 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a network  100  having a network node (NN)  105  (e.g., a system comprising a 4G or 5G base station or other access point) serving two UEs: UE  101  and UE  102 . The two UEs have different channel (or “link”) qualities. In this scenario, UE  102  is a “weak” UE (e.g., a cell-edge UE) and UE  101  is a “strong” UE (e.g. a cell-center UE). 
     With respect to uplink OMA transmissions, the UE  101 &#39;s and UE  102 &#39;s signals are transmitted in orthogonal resources, for instance at the same time but in different frequency bands, and NN  105  decodes the two transmitted signals separately. With respect to downlink OMA transmissions, NN  105  transmits for UE  101  a first signal using for example a first frequency band and transmits for UE  102  a second signal using for example a second frequency band that does not overlap with the first frequency band. 
     With respect to uplink NOMA, on the other hand, the UEs share the same frequency (or “spectrum”), time resources, and code or spreading resources, if any, to send their messages simultaneously. That is, NN  105  receives a superimposed signal containing the message transmitted by UE  101  and the message transmitted by UE  102 . In such a NOMA scenario, NN  105 , using for example a SIC receiver, first decodes the message of UE  101  (the “strong” UE), considering the message of UE  102  as noise. Then, after successfully decoding UE  101 &#39;s message, NN  105  subtracts UE  101 &#39;s message from the received signal and decodes UE  102 &#39;s signal with no interference from UE  101 . 
     Likewise, with respect to downlink NOMA, UE  101  and UE  102  are served by NN  105  in common radio resources, i.e., time-frequency chunks, as well as common code and/or beam resources, if applicable. We shall consider a frequency slot so that the time-frequency chunks refer to different time slots. Then, with no loss of generality, suppose that UE  101  experiences a better channel quality compared to UE  102  (i.e., UE  101  is the strong UE and UE  102  is the weak UE). That is, we have |h 2 |≤|h 1 |, where h 1  represents the channel coefficient of the NN  105 -UE  101  link and h 2  represents the channel coefficient of the NN-UE link. We define the channel gains as g i =|h i | 2 , i=1, 2. 
     Using NOMA, in time slot t NN  105  generates and transmits a superimposed signal S(t)=√{square root over (P 1 )}M 1 (t)+√{square root over (P 2 )}M 2 (t) to both UEs in the same resources. Here, M 1 (t) and M 2  (t) are the unit-variance messages for UE  101  and UE  102 , respectively, and P i , i=1, 2, are their corresponding transmit powers with P 1 +P 2 =P where P is the NN total power. In this way, the signal received by UE  101  (i.e., Y i (t)) and the signal received by UE  102  (i.e., Y 2 (t)) is given by: 
         Y   i ( t )= h   i (√{square root over ( P   1 )} M   1 ( t )+√{square root over ( P   2 )} M   2 ( t ))+ Z   i ( t ),  i= 1,2,   (1)
 
     where 
     Z i (t) denotes a noise signal (e.g., Gaussian white noise). 
     In the above scenario, which is illustrated in  FIG. 2 , UE  101  uses a SIC receiver to first decode-and-remove the message for UE  102  (i.e., M 2 ) and then obtain its own message (M 1 ) with no interference. The UE with the worse channel quality, i.e., UE  102  uses typical decoders to decode its own message in the presence of interference of the signal for UE  101 . 
     The goal of each UE is to decode its own message, although they may decode the message of the other UE to reduce the interference. With conventional NOMA, UE  102  considers the signal for UE  101  as interference and uses OMA-based receivers to decode its own message. This is 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 then, decode its own message interference-free). UE  101 , on the other hand, uses a SIC receiver to first decode-and-remove the message of UE  102  and then decode its own message interference-free. 
     Compared to conventional OMA-based receivers, SIC is a high-complexity scheme. Also, because the desired signal is decoded in two steps, SIC implies larger decoding delay which affects, e.g., the HARQ feedback process and, thereby, may increase the end-to-end transmission delay for both UEs in the situations where UE  102 &#39;s signal should be synchronized with the signal of the UE  101  (different methods can be applied to synchronize the signals—for instance, some sleeping period may be considered by UE  102  (as illustrated in  FIG. 2 ) or NN  105  may synchronize the signals of the UEs). Finally, with SIC, there is a probability of error propagation. This is because, if the message of UE  102  is not correctly decoded in the first step, the interference is not removed which reduces the probability that the cell-center can successfully decode its own message. 
     With this setup, the achievable rate for UE  101  (i.e., R 1 ) and the achievable rate for UE  102  (i.e., R 2 ) is given by: 
     
       
         
           
             
               
                 
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     Due to the interference signal of UE  101 , UE  102  experiences a low channel quality and may need retransmissions to decode its messages. 
     Depending on the channels quality, there is a probability that UE  101  can not decode-and-remove the message for UE  102 . In this case, the error propagates, and the additional interference increases the probability that UE  101  can not decode its own message correctly. 
     In this way, compared to OMA-based systems, there may be a higher probability that the UEs need retransmissions for successful message decoding. However, HARQ-based retransmissions reduce the network throughput, which is the main winning point of NOMA compared to OMA. Thus, to implement an efficient NOMA-based setup, it would be useful to reduce retransmissions. 
     Assume that UE  101  (denoted “UE 1 ”) has been able to decode none of the messages that are intended for U 1  and UE  102  (denoted “UE 2 ”), and UE 2  cannot decode its own message. However, similar approach is applicable if UE 1  can decode the message of UE 2  but none of the UEs can decode their own messages. 
     In a conventional system, with no successful message decoding at the UEs, both of their signals should be retransmitted by NN  105 . Proposed herein is that NN  105  delays the message retransmission of UE 1  while UE 1  buffers the undecoded signal. That is, if none of the UEs have been able to decode the messages correctly, NN  105  retransmits the message for UE 2  while sending a new messages for UE 1 . Then, buffering the undecoded message, in the next time slot UE 1  utilizes the signal retransmitted for UE 2  to decode and remove the interference. If it is successful to remove the interference, it retries to decode its own message. This is because the removed interference improves the quality of the received useful signal and, as a result, UE 1  has a better chance to decode its own signal with no retransmissions. In other words, typical non-NOMA systems utilize the retransmissions to improve the power of the useful signal. With NOMA, however, one can use the retransmitted message of the other UE to reduce the interference power, which improves the received signal-to-interference-and noise ratio (SINR). 
       FIG. 3  illustrates an embodiment. In this embodiment, in time slot tl NN  105  transmits a first superimposed signal containing a message for UE 1  (i.e., M 1  and a message for UE 2  (i.e., M 2 ). Accordingly, in slot tl, UE 1  receives Y 1  (t 1 ), which contains M 1  and M 2 , and UE 2  receives Y 2  (t 1 ), which also contains M 1  and M 2 . Assume that in time slot tl, using a SIC-based decoder, UE 1  cannot decode M 1  or M 2  and buffers the signal that it received (i.e., Y 1  (t 1 )), and using a non-SIC-based decoder UE 2  cannot decode M 2  and buffers the signal that it received (i.e., Y 2  (t 1 )). Hence, as shown in  FIG. 3 , UE 1  informs NN  105  that it could not obtain either M 1  or M 2  (e.g., UE 1  sends two NACKs to NN  105 ), and UE 2  informs NN  105  that it could not obtain M 2  (e.g. UE 2  sends a NACK to NN  105 ). 
     Subsequently, in time slot t 2 , NN  105  retransmits M 2  but sends a new message M 3  for UE 1 . That is, in slot t 2 , NN  105  transmits a second superimposed signal containing M 2  and M 3 , but not containing M 1 . Accordingly, in slot t 2 , UE 1  receives Y 1  (t 2 ), which contains M 3  and M 2 , and UE 2  receives Y 2  (t 2 ), which also contains M 3  and M 2 . 
     Then, using the SIC-based decoding approach, UE 1  tries to decode and remove M 2  using its two received copies of this signal. If UE 1  decodes M 2  correctly, it has the chance to decode M 1  and M 3  with no retransmission of M 1 . Assuming that UE 1  is able to obtain M 1  and M 3  in time slot t 2 , UE 1  informs NN  105  (e.g., as shown in  FIG. 3 , UE 1  transmits two ACKs to NN  105 , one for each message). Assuming that UE 1  is still unable to obtain M 1 , NN  105  may retransmit M 1  when a) retransmission of M 2  stops (either because U 2  has decoded M 2  correctly or the maximum number of retransmission rounds is reached) or b) UE 1  is able to decode M 2  but is not able to decode M 1 . Accordingly, for this embodiment, in each time slot: 1) UE 1  attempts to decode all different, buffered and recently received, signals, 2) UE 1  sends acknowledgement/negative acknowledgement (ACK/NACK) feedbacks for all messages it tries to decode and 3) NN  105  informs the UEs if it is retransmitting a specific signal (or the UEs are informed by other means). 
     In summary, the following signaling procedure may be applied by NN  105  and UE 1 . In each time slot, UE 1  tries to decode all recently received and undecoded-and-buffered signals. Then, it sends separate ACK/NACK signals to inform NN  105  about the message decoding status of each signal. Depending on the messages decoding status, NN  105  may delay the retransmission of the signals for UE 1 . Also, if it retransmits a signal, it informs the UEs about the index of the message which is retransmitted. 
     In the above example, UE 1  cannot decode either M 1  or M 2  and UE 2  cannot decode M 2 . A similar approach is applicable if UE 1  can decode M 2  but not M 1  and UE 2  cannot decode M 2 . In this scenario, NN  105  delays the retransmission of M 2  while it retransmits Ml. Then, utilizing the two copies of the interference signal, UE 2  has the chance to decode and remove the interference signal of UE 1 , which gives UE 2  the chance to decode its own message (M 2 ) interference-free and with no need for retransmissions. 
     As the above demonstrates, an advantage provided by the above embodiments is that they reduce the decoding complexity at the UEs and increase throughput because there is a chance that the UEs decode the undecoded messages with no need for retransmissions. 
     Also, the above described embodiments illustrate the DL NOMA transmission scenario, but the same approach is applicable for UL NOMA transmissions. With respect to UL NOMA transmission (i.e., wherein UE 1  transmits a message (M 1 ) using radio resources and UE 2  transmits a message (M 2 ) using the same radio resources and NN  105  uses a SIC-based decoding approach to obtain M 1  and M 2 ), if NN  105  fails to decode both messages, it first asks one of the UEs for a retransmission, while the other UE sends a new message (M 3 ). For example, NN  105  instructs UE 1  to retransmit M 1  and instructs UE 2  to transmit M 3  using the same radio resources. Then, again using the SIC-based decoding approach, NN  105  tries to decode the retransmitted message M 1 , and if NN  105  is successful in obtaining M 1 , NN  105  will have a chance to of decoding M 2  from the first received signal with no need for retransmission of M 2 . 
       FIG. 4  is a flow chart illustrating a process  400 , according to an embodiment, that is performed by NN  105 . Process  400  may begin in step s 402 . 
     In step s 402 , NN  105  transmits, during a first time slot (t 1 ), a first superimposed signal (S(t 1 )) comprising a first message (M 1 ) for a first UE (e.g., UE  101  or UE  102 ) and a second message (M 2 ) for a second UE (e.g., UE  101  or UE  102 ). 
     In step s 404 , NN  105  determines that the first UE was not able to successfully decode either the first message or the second message. 
     In step s 406 , NN  105  determines that the second UE was not able to successfully decode the second message; 
     In step s 408 , in response to determining that the first UE was not able to successfully decode either the first message or the second message and that the second UE was not able to successfully decode the second message, NN  105  decides to retransmit the second message but not the first message. 
     In step s 410 , NN  105  retransmits the second message by transmitting a second superimposed signal comprising the second message for the second UE and a third message for the first UE but not including the first message, wherein the third message is different than the first message (e.g., the third message does not comprise any portion of the first message). 
     In some embodiments, prior to transmitting the first superimposed signal comprising M 1  and M 2 , NN  105  obtains (receives, generates or otherwise obtains) M 1  and M 2  and then generates the first superimposed signal (S 1 ) (i.e., S 1 =M 1 +M 2 ). For instance, NN  105  may receive M 1  from a first host computer  111  (see  FIG. 1 ) and may receive M 2  from the first host computer or a second host computer (not shown). 
     In some embodiments, process  400  further includes, after transmitting the second superimposed signal and without any retransmission of the first message, NN  105  receives a positive acknowledgement (ACK) transmitted by the first UE, the ACK indicating that the first UE has successfully decoded the first message. 
     In some embodiments, process  400  further includes, after retransmitting the second message, determining that the first UE is still unable to decode the first message, but the second UE has successfully decoded the second message; and as a result of determining that the first UE is still unable to decode the first message, but the second UE has successfully decoded the second message, retransmitting the first message. 
     In some embodiments, process  400  further includes, after deciding to retransmit the second message but not the first message, informing the first UE that the second messing is being retransmitted. 
     In some embodiments, NN  105  determines that the first UE was not able to successfully decode either the first message or the second message by receiving a NACK corresponding to the first message and a second NACK corresponding to the second message, the first NACK indicating that the first UE was not able to successfully decode the first message, and the second NACK indicating that the first UE was not able to successfully decode the second message. 
       FIG. 5  is a flow chart illustrating a process  500 , according to an embodiment, that is performed by UE 1 . Process  500  may begin in step s 502 . 
     In step s 502 , UE 1  receives a first superimposed signal transmitted by the network node, the first superimposed signal comprising a first message (M 1 ) for UE 1  and a second message (m 2 ) for UE 2 . 
     In step s 504 , UE 1  attempts to decode the second message prior to attempting to decode the first message. 
     In step s 506 , UE 1 , after attempting to decode the second message, UE 1  provides an indication to the network node indicating that the second message has not been successfully decoded. 
     In step s 508 , UE 1  buffer the first superimposed signal. 
     In step s 510 , after providing the indication to the network node, UE 1  receives a second superimposed signal transmitted by the network node, the second superimposed signal comprising the second message and a third message for UE 1  but not including the first message for UE 1 , wherein the third message is different than the first message. 
     In step s 512 , after receiving the second superimposed signal, UE 1  successfully decodes the second message for UE 2 . 
     In step s 514 , after successfully decoding the second message for UE 2 , UE 1  uses the decoded second message and the buffered first superimposed signal to decode the first message from the first superimposed signal. 
     In some embodiments, process  500  further includes, after receiving the second superimposed signal and without receiving any retransmission of the first message, UE 1  transmits a positive acknowledgement, ACK, the ACK indicating that UE 1  has successfully decoded the first message. 
     In some embodiments, process  500  further includes, UE 1  receiving information transmitted by the network node, the information indicating that the second messing is being retransmitted together with the third message. 
     In some embodiments, providing the indication to the network node comprises UE 1  transmitting a negative acknowledgement, NACK, indicating that it was not able to successfully decode the second message. 
       FIG. 6  is a flow chart illustrating a process  600 , according to an embodiment, that is performed by NN  105  for obtaining a first message (M 1 ) transmitted by a first UE (e.g., UE 1 ) and a second message (M 2 ) transmitted by a second UE (e.g, UE 2 ). Process  600  may begin in step s 602 . 
     In step s 602 , NN  105  schedules the first UE to transmit the first message using a first time and frequency resource. 
     In step s 604 , NN  105  schedules the second UE to transmit the second message using the first time and frequency resource. 
     In step s 606 , NN  105  receives a first signal comprising the first message and the second message. 
     As a result of not being able to obtain either the first message or the second message from the first signal, NN  105  performs steps comprising: buffering the first signal (step s 608 ); scheduling the first UE to retransmit the first message using a second time and frequency resource (step s 610 ); and scheduling the second UE to transmit a third message using the second time and frequency resource (step s 612 ), wherein the third message is different than the second mes sage. 
     In some embodiments, process  600  also includes NN  105  performing steps comprising: receiving a second signal comprising the first message and the third message; obtaining the first message from the second signal; obtaining the third message from the second signal; and using the first message obtained from the second signal and the buffered first signal, obtaining the second message from the first signal. In some embodiments, obtaining the second message from the first signal comprises: removing the first message from the first signal, thereby producing a residual signal comprising the second message; and obtaining the second message from the residual signal. 
     In some embodiments, process  600  also includes NN  105  receiving a second signal comprising the first message and the third message; and, as a result of not being able to obtain either the first message or the third message from the second signal, performing steps comprising: buffering the second signal; scheduling the first UE to retransmit the first message using a third time and frequency resource; and scheduling the second UE to transmit a fourth message using the second time and frequency resource, wherein the fourth message is different than the second and third message. 
     In some embodiments, scheduling the second UE to transmit a third message comprises transmitting to the second UE a scheduling message (e.g., a Downlink Control Information (DCI) message) comprising information for causing the second UE to buffer the second message in case the second UE needs to retransmit the second message. 
     In some embodiments, after successfully obtaining the first message (M 1 ) and the second message (M 2 ), NN  105  may forward M 1  towards a first host computer  111  and may forward M 2  towards a second host computer (or the first host computer  111 ). 
       FIG. 7  is a flow chart illustrating a process  700 , according to an embodiment, that is performed by UE 1  for transmitting messages to a network node. Process  700  may begin in step s 702 . 
     In step s 702 , UE 1  receives a first scheduling message (e.g., DCI) transmitted by the network node. 
     In step s 704 , as a result of receiving the first scheduling message, UE 1  transmits a first signal comprising a first message. 
     In step s 706 , after transmitting the first signal, UE 1  buffers the first message in case the network node requires the UE to retransmit the first message (e.g., stores the first message in a retransmit queue). 
     In step s 708 , after buffing the first message, UE 1  receives a second scheduling message transmitted by the network node, the second scheduling message instructing the UE to transmit a second message. 
     In step s 710 , as a result of receiving the second scheduling message, UE 1  transmits a second signal comprising the second message but not comprising the first message. 
     In step s 712 , after transmitting the second signal, UE 1  receives: i) acknowledgment information transmitted by the network node, wherein the acknowledgment information indicates that the network node has been able to obtain the first message from the first signal and the second message from the second signal, or ii) a request to retransmit the first message. 
     In response to receiving the acknowledgment information indicating that the network node has been able to obtain the first message from the first signal and the second message from the second signal, UE 1  de-buffers the first message (e.g., removes the first message from the retransmit queue). 
       FIG. 8  is a block diagram of network node  150 , according to some embodiments for performing methods disclosed herein. As shown in  FIG. 8 , network node  150  may comprise: processing circuitry (PC)  802 , which may include one or more processors (P)  855  (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), which processors may be co-located or distributed in different locations; a network interface  848  comprising a transmitter (Tx)  845  and a receiver (Rx)  847  for enabling network node  150  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  848  is connected; circuitry  803  (e.g., radio transceiver circuitry comprising an Rx  805  and a Tx  806 ) coupled to an antenna system  804  for wireless communication with UEs); and a local storage unit (a.k.a., “data storage system”)  808 , which may include one or more non-volatile storage devices and/or one or more volatile storage devices. In embodiments where PC  802  includes a programmable processor, a computer program product (CPP)  841  may be provided. CPP  841  includes a computer readable medium (CRM)  842  storing a computer program (CP)  843  comprising computer readable instructions (CRI)  844 . CRM  842  may be a non-transitory computer readable medium, such as, 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  844  of computer program  843  is configured such that when executed by PC  802 , the CRI causes network node  150  to perform steps described herein (e.g., steps described herein with reference to the flow charts). In other embodiments, network node  150  may be configured to perform steps described herein without the need for code. That is, for example, PC  802  may consist merely of one or more ASICs. Hence, the features of the embodiments described herein may be implemented in hardware and/or software. 
       FIG. 9A  is a diagram showing functional units of network node  105  according to an embodiment. In the embodiment shown, network node  105  includes: a transmission unit  902  for employing a transmitter to transmit a first superimposed signal comprising a first message for UE 1  and a second message for UE 2 ; and a receiver unit  904  for employing a receiver to i) obtain a message transmitted by UE 1  indicating that UE 1  was unable to decode the first message for UE 1  and the second message for UE 2  and ii) obtain a message transmitted by UE 2  indicating that UE 2  was unable to decode the second message; and a retransmitting unit  906  for delaying the retransmission of the first message, but not delaying the retransmission of the second message by transmitting a second superimposed signal comprising a third message for UE 1  and the second message for UE 2 . 
       FIG. 9B  is a diagram showing functional units of network node  105  according to an embodiment. In the embodiment shown, network node  105  includes: a scheduling unit  922  for scheduling a first UE to transmit a first message using a first time and frequency resource and scheduling a second UE to transmit a second message using the first time and frequency resource; a receiver unit  924  configured to receive via a receiver a first signal comprising the first message and the second message; a buffering unit  926 ; and a determining unit  930 . The determining unit  930  is operable to determine whether the NN  105  is able to obtain either the first message or the second message from the first signal. As a result of the determining unit  930  determining that NN  105  is not able to obtain either the first message or the second message from the first signal, the buffering unit  926  buffers the first signal and the scheduling unit  922  schedules the first UE to retransmit the first message using a second time and frequency resource and schedules the second UE to transmit a third message using the second time and frequency resource, wherein the third message is different than the second message. 
       FIG. 10  is a block diagram of a UE (e.g. UE  101  or UE  102 ), according to some embodiments. As shown in  FIG. 10 , the UE may comprise: processing circuitry (PC)  1002 , which may include one or more processors (P)  1055  (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); circuitry  1003  (e.g., radio transceiver circuitry comprising an Rx  1005  and a Tx  1006 ) coupled to an antenna system  1004  for wireless communication); and a local storage unit (a.k.a., “data storage system”)  1008 , which may include one or more non-volatile storage devices and/or one or more volatile storage devices. In embodiments where PC  1002  includes a programmable processor, a computer program product (CPP)  1041  may be provided. CPP  1041  includes a computer readable medium (CRM)  1042  storing a computer program (CP)  1043  comprising computer readable instructions (CRI)  1044 . CRM  1042  may be a non-transitory computer readable medium, such as, 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  1044  of computer program  1043  is configured such that when executed by PC  1002 , the CRI causes the UE to perform steps described herein (e.g., steps described herein with reference to the flow charts). In other embodiments, the UE may be configured to perform steps described herein without the need for code. That is, for example, PC  1002  may consist merely of one or more ASICs. Hence, the features of the embodiments described herein may be implemented in hardware and/or software. 
       FIG. 11A  is a diagram showing functional units of a UE (e.g., UE  101  or UE  102 ) according to an embodiment. In the embodiment shown, the UE includes: a receiver unit  1102  for employing a receiver to receive a first superimposed signal transmitted by the network node, the first superimposed signal comprising a first message for the first UE and a second message for the second UE; a decoding unit  1104  for attempting to decode the second message from the first superimposed signal prior to attempting to decode the first message for the first UE; an indication providing unit  1106  for providing an indication to the network node indicating that the second message has not been successfully decoded; and a buffering unit  1108  for buffering the first superimposed signal. The receiver unit  1102  is further operable to employ the receiver to receive a second superimposed signal transmitted by the network node, the second superimposed signal comprising the second message for the second UE and a third message for the first UE but not including the first message for the first UE, wherein the third message is different than the first message. The decoding unit  1104  is further operable to decode the second message for the second UE and, after successfully decoding the second message for the second UE, use the decoded second message and the buffered first superimposed signal to decode the first message from the first superimposed signal. 
       FIG. 11B  is a diagram showing functional units of a UE (e.g., UE  101  or UE  102 ) according to an embodiment. In the embodiment shown, the UE includes: a receiver unit  1122  for receiving a first scheduling message transmitted by a network node; a transmission unit  1124  for employing a transmitter to transmit a first signal comprising a first message as a result of the UE receiving the first scheduling message; and a buffering unit  1126  for buffering the first message, after transmitting the first signal, in case the network node requires the UE to retransmit the first message. The receiver unit  1122  is further operable to receive a second scheduling message transmitted by the network node, the second scheduling message instructing the UE to transmit a second message, and the transmission unit  1124  is further operable to, as a result of the UE receiving the second scheduling message, employ the transmitter to transmit a second signal comprising the second message but not comprising the first message. The receiver unit  1122  is further operable to receive i) acknowledgment information transmitted by the network node, wherein the acknowledgment information indicates that the network node has been able to obtain the first message from the first signal and the second message from the second signal or ii) a request to retransmit the first message. The buffering unit  1126  is further configured such that, as a result of the UE receiving the acknowledgment information indicating that the network node has been able to obtain the first message from the first signal, the buffering unit  1126  de-buffers the first message. 
       FIG. 12  illustrates a telecommunication network connected via an intermediate network to a host computer  111  in accordance with some embodiments. With reference to  FIG. 12 , in accordance with an embodiment, a communication system includes telecommunication network  1210 , such as a  3 GPP-type cellular network, which comprises access network  1211 , such as a radio access network, and core network  1214 . Access network  1211  comprises a plurality of APs (hereafter base stations)  1212   a,    1212   b,    1212   c,  such as NB s, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area  1213   a,    1213   b,    1213   c.  Each base station  1212   a,    1212   b,    1212   c  is connectable to core network  1214  over a wired or wireless connection  1215 . A first UE  1291  located in coverage area  1213   c  is configured to wirelessly connect to, or be paged by, the corresponding base station  1212   c.  A second UE  1292  in coverage area  1213   a  is wirelessly connectable to the corresponding base station  1212   a.  While a plurality of UEs  1291 ,  1292  are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole UE is in the coverage area or where a sole UE is connecting to the corresponding base station  1212 . 
     Telecommunication network  1210  is itself connected to host computer  111 , which may be embodied in the hardware and/or software of a standalone server, a cloud-implemented server, a distributed server or as processing resources in a server farm. Host computer  111  may be under the ownership or control of a service provider, or may be operated by the service provider or on behalf of the service provider. Connections  1221  and  1222  between telecommunication network  1210  and host computer  111  may extend directly from core network  1214  to host computer  111  or may go via an optional intermediate network  1220 . Intermediate network  1220  may be one of, or a combination of more than one of, a public, private or hosted network; intermediate network  1220 , if any, may be a backbone network or the Internet; in particular, intermediate network  1220  may comprise two or more sub-networks (not shown). 
     The communication system of  FIG. 12  as a whole enables connectivity between the connected UEs  1291 ,  1292  and host computer  111 . The connectivity may be described as an over-the-top (OTT) connection  1250 . Host computer  111  and the connected UEs  1291 ,  1292  are configured to communicate data and/or signaling via OTT connection  1250 , using access network  1211 , core network  1214 , any intermediate network  1220  and possible further infrastructure (not shown) as intermediaries. OTT connection  1250  may be transparent in the sense that the participating communication devices through which OTT connection  1250  passes are unaware of routing of uplink and downlink communications. For example, base station  1212  may not or need not be informed about the past routing of an incoming downlink communication with data originating from host computer  111  to be forwarded (e.g., handed over) to a connected UE  1291 . Similarly, base station  1212  need not be aware of the future routing of an outgoing uplink communication originating from the UE  1291  towards the host computer  111 . 
     Example implementations, in accordance with an embodiment, of the UE, base station and host computer discussed in the preceding paragraphs will now be described with reference to  FIG. 13 , which illustrates a host computer communicating via a base station with a user equipment over a partially wireless connection in accordance with some embodiments. In communication system  1300 , host computer  1310  comprises hardware  1315  including communication interface  1316  configured to set up and maintain a wired or wireless connection with an interface of a different communication device of communication system  1300 . Host computer  1310  further comprises processing circuitry  1318 , which may have storage and/or processing capabilities. In particular, processing circuitry  1318  may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. Host computer  1310  further comprises software  1311 , which is stored in or accessible by host computer  1310  and executable by processing circuitry  1318 . Software  1311  includes host application  1312 . Host application  1312  may be operable to provide a service to a remote user, such as UE  1330  connecting via OTT connection  1350  terminating at UE  1330  and host computer  1310 . In providing the service to the remote user, host application  1312  may provide user data which is transmitted using OTT connection  1350 . 
     Communication system  1300  further includes base station  1320  provided in a telecommunication system and comprising hardware  1325  enabling it to communicate with host computer  1310  and with UE  1330 . Hardware  1325  may include communication interface  1326  for setting up and maintaining a wired or wireless connection with an interface of a different communication device of communication system  1300 , as well as radio interface  1327  for setting up and maintaining at least wireless connection  1370  with UE  1330  located in a coverage area (not shown in  FIG. 13 ) served by base station  1320 . Communication interface  1326  may be configured to facilitate connection  1360  to host computer  1310 . Connection  1360  may be direct or it may pass through a core network (not shown in  FIG. 13 ) of the telecommunication system and/or through one or more intermediate networks outside the telecommunication system. In the embodiment shown, hardware  1325  of base station  1320  further includes processing circuitry  1328 , which may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. Base station  1320  further has software  1321  stored internally or accessible via an external connection. 
     Communication system  1300  further includes UE  1330  already referred to. Its hardware  1335  may include radio interface  1337  configured to set up and maintain wireless connection  1370  with a base station serving a coverage area in which UE  1330  is currently located. Hardware  1335  of UE  1330  further includes processing circuitry  1338 , which may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. UE  1330  further comprises software  1331 , which is stored in or accessible by UE  1330  and executable by processing circuitry  1338 . Software  1331  includes client application  1332 . Client application  1332  may be operable to provide a service to a human or non-human user via UE  1330 , with the support of host computer  1310 . In host computer  1310 , an executing host application  1312  may communicate with the executing client application  1332  via OTT connection  1350  terminating at UE  1330  and host computer  1310 . In providing the service to the user, client application  1332  may receive request data from host application  1312  and provide user data in response to the request data. OTT connection  1350  may transfer both the request data and the user data. Client application  1332  may interact with the user to generate the user data that it provides. 
     It is noted that host computer  1310 , base station  1320  and UE  1330  illustrated in  FIG. 13  may be similar or identical to host computer  111 , one of base stations  1212   a,    1212   b,    1212   c  and one of UEs  1291 ,  1292  of  FIG. 12 , respectively. This is to say, the inner workings of these entities may be as shown in  FIG. 13  and independently, the surrounding network topology may be that of  FIG. 12 . 
     In  FIG. 13 , OTT connection  1350  has been drawn abstractly to illustrate the communication between host computer  1310  and UE  1330  via base station  1320 , without explicit reference to any intermediary devices and the precise routing of messages via these devices. Network infrastructure may determine the routing, which it may be configured to hide from UE  1330  or from the service provider operating host computer  1310 , or both. While OTT connection  1350  is active, the network infrastructure may further take decisions by which it dynamically changes the routing (e.g., on the basis of load balancing consideration or reconfiguration of the network). 
     Wireless connection  1370  between UE  1330  and base station  1320  is in accordance with the teachings of the embodiments described throughout this disclosure. One or more of the various embodiments improve the performance of OTT services provided to UE  1330  using OTT connection  1350 , in which wireless connection  1370  forms the last segment. More precisely, the teachings of these embodiments may improve one or more of message throughput, SINR, latency, overhead, and power consumption and thereby provide benefits such as reduced user waiting time, better responsiveness, extended battery lifetime, etc. 
     A measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. There may further be an optional network functionality for reconfiguring OTT connection  1350  between host computer  1310  and UE  1330 , in response to variations in the measurement results. The measurement procedure and/or the network functionality for reconfiguring OTT connection  1350  may be implemented in software  1311  and hardware  1315  of host computer  1310  or in software  1331  and hardware  1335  of UE  1330 , or both. In embodiments, sensors (not shown) may be deployed in or in association with communication devices through which OTT connection  1350  passes; the sensors may participate in the measurement procedure by supplying values of the monitored quantities exemplified above, or supplying values of other physical quantities from which software  1311 ,  1331  may compute or estimate the monitored quantities. The reconfiguring of OTT connection  1350  may include message format, retransmission settings, preferred routing etc.; the reconfiguring need not affect base station  1320 , and it may be unknown or imperceptible to base station  1320 . Such procedures and functionalities may be known and practiced in the art. In certain embodiments, measurements may involve proprietary UE signaling facilitating host computer  1310 &#39;s measurements of throughput, propagation times, latency and the like. The measurements may be implemented in that software  1311  and  1331  causes messages to be transmitted, in particular empty or ‘dummy’ messages, using OTT connection  1350  while it monitors propagation times, errors etc. 
       FIG. 14  is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIG. 12  and  FIG. 13 . In step S 1410 , the host computer provides user data. In substep S 1411  (which may be optional) of step S 1410 , the host computer provides the user data by executing a host application. In step S 1420 , the host computer initiates a transmission carrying the user data to the UE. In step S 1430  (which may be optional), the base station transmits to the UE the user data which was carried in the transmission that the host computer initiated, in accordance with the teachings of the embodiments described throughout this disclosure. In step S 1440  (which may also be optional), the UE executes a client application associated with the host application executed by the host computer. 
       FIG. 15  is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIG. 12  and  FIG. 13 . For simplicity of the present disclosure, only drawing references to  FIG. 15  will be included in this section. In step S 1510  of the method, the host computer provides user data. In an optional substep (not shown) the host computer provides the user data by executing a host application. In step S 1520 , the host computer initiates a transmission carrying the user data to the UE. The transmission may pass via the base station, in accordance with the teachings of the embodiments described throughout this disclosure. In step S 1530  (which may be optional), the UE receives the user data carried in the transmission. 
       FIG. 16  is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIG. 12  and  FIG. 13 . For simplicity of the present disclosure, only drawing references to  FIG. 16  will be included in this section. In step S 1610  (which may be optional), the UE receives input data provided by the host computer. Additionally or alternatively, in step S 1620 , the UE provides user data. In substep S 1621  (which may be optional) of step S 1620 , the UE provides the user data by executing a client application. In substep S 1611  (which may be optional) of step S 1610 , the UE executes a client application which provides the user data in reaction to the received input data provided by the host computer. In providing the user data, the executed client application may further consider user input received from the user. Regardless of the specific manner in which the user data was provided, the UE initiates, in substep S 1630  (which may be optional), transmission of the user data to the host computer. In step S 1640  of the method, the host computer receives the user data transmitted from the UE, in accordance with the teachings of the embodiments described throughout this disclosure. 
       FIG. 17  is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIG. 12  and  FIG. 13 . For simplicity of the present disclosure, only drawing references to  FIG. 17  will be included in this section. In step S 1710  (which may be optional), in accordance with the teachings of the embodiments described throughout this disclosure, the base station receives user data from the UE. In step S 1720  (which may be optional), the base station initiates transmission of the received user data to the host computer. In step S 1730  (which may be optional), the host computer receives the user data carried in the transmission initiated by the base station. 
     Any appropriate steps, methods, features, functions, or benefits disclosed herein may be performed through one or more functional units or modules of one or more virtual apparatuses. Each virtual apparatus may comprise a number of these functional units. These functional units may be implemented via processing circuitry, which may include one or more microprocessor or microcontrollers, as well as other digital hardware, which may include digital signal processors (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as read-only memory (ROM), random-access memory (RAM), cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory includes program instructions for executing one or more telecommunications and/or data communications protocols as well as instructions for carrying out one or more of the techniques described herein. In some implementations, the processing circuitry may be used to cause the respective functional unit to perform corresponding functions according one or more embodiments of the present disclosure. 
     While various embodiments 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 this 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.