Contention based access optimization

The mobile station described herein comprises a transmission buffer, a receiver, a transmitter, and a controller. The controller triggers a scheduling request and buffer status report when data enters the transmission buffer. When the receiver receives a contention-based scheduling grant allocating contention-based uplink resources, the transmitter transmits one or more data packets retrieved from the transmission buffer along with the buffer status report on the contention-based resources to the network station. If one or more data packets remain in the transmission buffer after transmission of the buffer status report on the contention-based resources, the controller maintains the pending status of the buffer status report and the corresponding scheduling request.

TECHNICAL FIELD

The present invention relates generally to uplink transmissions, and more particularly to resource scheduling for uplink transmissions in LTE systems.

BACKGROUND

In LTE systems, the base station dynamically allocates dedicated uplink resources to user terminals operating within the wireless network. The basic assumption is that uplink resources are allocated only when the user terminal has data to send. When the user terminal has data to send, it sends a Scheduling Request (SR) to the base station at the first opportunity to request resources for the uplink transmission and waits for a Scheduling Grant (SG). The base station dynamically allocates dedicated uplink resources to the requesting user terminal and sends an uplink Scheduling Grant (SG) to the user terminal identifying the allocated uplink resources. Buffer Status Reports (BSRs) are transmitted from the user terminals to the base station to provide information about buffer levels of the user terminal and to assist in the allocation of uplink resources.

The dynamic allocation of uplink resources can significantly increase the transmission efficiency so that uplink resources are not left unused if some of the user terminals have no data or only a small amount of data to send. In some scenarios, however, dedicated resource scheduling takes too long. For example, the user plane latency for scheduling uplink transmission on dedicated uplink resources is in the order of 11.5 ms due to uplink control channel cycle times, transmission times, and processing delays associated with the generation, transmission, and decoding of the SR and SG. The 3GPP LTE-Advanced target for user plane latency is 10 ms or less. Thus, there remains a need for improved resource scheduling processes to reduce the time required to access uplink resources.

SUMMARY

The present invention improves resource scheduling procedures by incorporating contention-based resource scheduling operations with existing dedicated resource scheduling operations. To reduce the impact of at least some of the reliability issues associated with contention-based transmissions, the present invention controls the status of a pending scheduling request after the transmission of data packets on contention-based uplink resources based on the status of a transmission buffer.

A mobile station according to one exemplary embodiment comprises a transmission buffer, a receiver, a transmitter, and a controller. The controller triggers a scheduling request and buffer status report when data enters the transmission buffer. When the receiver receives a contention-based scheduling grant that schedules contention-based uplink resources, the transmitter transmits one or more data packets retrieved from the transmission buffer along with the buffer status report on the contention-based resources to the network station. If one or more data packets remain in the transmission buffer after transmission of the buffer status report on the contention-based resources, the controller maintains the pending status of the buffer status report and the corresponding scheduling request. In so doing, the controller ensures that the network station will eventually be made aware of a need for uplink transmission resources for the remaining data (e.g., by the subsequent transmission of the pending scheduling request), even if the buffer status report transmitted on the contention-based resources is lost or otherwise not correctly received by the network station.

DETAILED DESCRIPTION

Referring now to the drawings,FIG. 1illustrates a mobile communication network10including a base station20and a plurality of user terminals30. A scheduler22at the base station20coordinates the transmissions from the user terminals30to the base station20on the uplink. The scheduler22determines when the user terminals30are allowed to transmit on the uplink and allocates uplink resources to the user terminals30. The scheduler22may allocate either dedicated resources or contention-based resources to the user terminal30. Scheduling uplink resources is a function of the medium access control (MAC) layer in LTE systems. For illustrative purposes, the exemplary embodiment is described in the context an LTE system; however, those skilled in the art will appreciate that the present invention is applicable to communication systems using other standards now known or later developed where transmission on uplink resources are scheduled by the network.

LTE systems use a request-grant procedure for scheduling user terminals30on dedicated uplink resources. To briefly summarize, a user terminal30sends a scheduling request (SR) to the base station20at the first opportunity when the user terminal30has data to send. The scheduler22at the base station20allocates uplink resources to the user terminal30and sends a scheduling grant (SG) to the user terminal30that gives permission to the user terminal30to transmit data on the resources identified in the SG. Because the transmission buffers for the uplink are located at the user terminals30, the user terminals30provide information about transmission buffer levels to the base station20by sending a buffer status report (BSR). A BSR is triggered when data arrives in the transmission buffer containing a higher priority than data already in the transmission buffer. A BSR can also be triggered when a predetermined period of time has elapsed since the last BSR, or when the serving cell for the user terminal30changes. The user plane latency for scheduling uplink transmission on dedicated uplink resources is in the order of 11.5 ms in the best case due to uplink control channel cycle times, transmission times, and processing delays associated with the generation, transmission, and decoding of the SR and SG.

In order to reduce the amount of time it takes to access uplink resources, a mechanism is also provided for contention-based access on uplink resources that have not been allocated to a specific user terminal30by the scheduler22. There will be times when uplink resources are still available after allocating dedicated resources to the user terminals30. When unallocated uplink resources are available, the base station20may allow all eligible user terminals30to send data on a contention basis without first requesting resources by sending a contention-based scheduling grant on a control channel to all user terminals30. For example, time-aligned user terminals30that have data packets to transmit may use the resources allocated by the CBG to transmit the data packets.

FIG. 2illustrates an exemplary contention-based access procedure100for the uplink in a wireless communication system according to one exemplary embodiment of the present invention. In the scenario depicted inFIG. 2, data arriving at the transmit buffer of a user terminal30triggers a BSR and SR (110). While the BSR and SR are pending, the base station20sends a Contention-Based scheduling Grant (CBG) indicating that uplink resources are available (120). The base station20may, for example, transmit the CBG on the PDCCH. While the CBG is not addressed to a specific user terminal30and is not generated responsive to an SR, the CBG may use a Contention-Based Radio Network Temporary Identifier (CB-RNTI) to identify the CBG to the user terminals30. The user terminals30may be made aware of the CB-RNTI as part of system broadcast information, or the CB-RNTI may be signaled to each user terminal30during connection setup. The CBG identifies the uplink resources available for contention-based access and may have the same format as a dedicated SG, e.g., may specify a modulating and coding scheme, transport format, etc., to be used for the contention-based uplink transmission. Alternatively, a new format could be defined to efficiently signal several CBGs. In any event, each user terminal30compatible with contention-based operations listens for both a CBG addressed to a CB-RNTI and an SG addressed to their dedicated C-RNTI. It will be appreciated that contention-based operations are backwards compatible as any user terminals30incompatible with contention-based operations will not decode the CBG addressed to a CB-RNTI.

Responsive to the CBG, the user terminal30transmits data retrieved from the transmission buffer on the uplink resources identified in the CBG and appends the BSR to one or more of the transmitted data packets (130). In contrast to transmissions on dedicated resources, the hybrid automatic repeat request (HARQ) retransmission protocol is typically not used for transmissions on contention-based resources. If the transmission buffer is emptied by the contention-based transmission, the BSR and SR may be cancelled. However, when data remains in the transmission buffer, it may be beneficial to keep the BSR and SR pending because transmissions on the contention-based resources are not as reliable as a scheduled transmission on dedicated resources. More particularly, data transmitted from different user terminals30on the same contention-based uplink resources may collide and at least some of the transmitted data is likely to be lost. Such transmission errors are particularly problematic when they result in the loss of the Buffer Status Report (BSR), which is generally used by the base station20to determine whether or not to schedule additional dedicated resources to one or more of the user terminals30.

Because there is a possibility of collisions on the contention-based resources, the user terminal30cannot be certain that the BSR will be received. The present invention reduces the impact of such contention-based transmission errors by maintaining the pending status of the BSR, and therefore the pending status of the corresponding SR, when one or more data packets remain in a transmission buffer after the transmission of data packets on the contention-based resources (140). At the next opportunity, the user terminal30transmits the pending SR to the base station20(150). Maintaining the pending status of the BSR and SR when data packets remain in the transmission buffer, and subsequently sending the SR to the base station20, speeds up the process of receiving a dedicated SG at the user terminal30. In so doing, the present invention potentially enables a faster recovery of any failed transmissions associated with contention-based operations by speeding up the process necessary to enable retransmissions, e.g., RLC or MAC retransmissions, of any missing data packets

In some circumstances, a user terminal30may transmit data packets on contention-based resources after sending a SR to the base station20. If the transmission buffers are emptied by a CB transmission, the user terminal30may not have data to send when it receives the dedicated SG from the base station20. The user terminal30is still required to transmit something on the dedicated resources. The user terminal30may, for example, transmit padding to the base station20on the dedicated resources. However, the uplink resource is essentially wasted in this case.

According to one embodiment, the user terminal30may implement a pro-active retransmission of an unacknowledged radio link control (RLC) packet when it has no data left in its transmission or retransmission buffer. The RLC layer is above the MAC layer where HARQ operations are performed. The RLC layer is responsible for segmentation of Packet Data Convergence Protocol (PDCP) packets into RLC packet data units (PDUs), which are passed to the MAC layer. In an acknowledged mode (AM), the RLC layer also implements a retransmission protocol to ensure error free delivery of data packets to higher layers. AM is typically used for TCP-based services where error-free data delivery is important.

When the user terminal30receives a SG and has no data in its transmission buffer, the user terminal30may, instead, retrieve data waiting to be acknowledged from the RLC transmission buffer and pro-actively retransmit the retrieved RLC PDUs. It will be appreciated that these pro-active retransmissions of RLC PDUs are not requested by the base station20. To ensure that such unprompted pro-active retransmission do not interfere with normal RLC operations, the RLC retransmission counter should not be incremented when a pro-active retransmission is performed at the MAC level.

FIG. 3shows one exemplary pro-active retransmission process200according to the present invention. In the depicted scenario the user terminal30transmits an SR (210) to the base station20to request uplink resources and waits for a SG. While the user terminal30is waiting for the SG, the base station20sends a CBG to the user terminals30on the PDCCH as previously described (220). Because the user terminal30has data to send, it transmits data contained in its transmission buffer on the uplink resources (230) identified in the CBG. It is presumed in this example that the transmission buffer is emptied by the contention-based transmission. Although the user terminal30no longer has any MAC PDUs to send, the base station20allocates uplink resources to the user terminal30(240) and sends an SG to the user terminal30(250). Because the user terminal30no longer has any data in its transmission buffers, the user terminal30retrieves data packets from the RLC transmission buffer (260) that were already sent and are awaiting acknowledgement, and pro-actively retransmits the retrieved RLC data packets to the base station20on the dedicated resources allocated to the user terminal30by the SG (270). To ensure that the unprompted retransmission does not interfere with normal RLC operations, the user terminal30does not increment the RLC retransmission counter. The RLC entities at the base station20will delete any duplicate RLC PDUs. According to normal HARQ operations, the base station20will acknowledge the packets sent (280). However, the acknowledgement is only of the MAC layer packets containing the RLC PDU, and not of the RLC PDU.

FIG. 4shows one exemplary user terminal30according to the present invention. User terminal30comprises antenna32, transceiver34, a controller36, and buffer system40. Transceiver34configured to transmit data packets to the base station and receive data packets from the base station via antenna32. Controller36processes the signals transmitted and received by the user terminal30and controls the operation of user terminal30, and particularly implements MAC and RLC layer protocols. In particular, the controller36performs physical layer processing such as coding/decoding, modulation/demodulation, etc. and implements higher layer protocols including MAC protocols and RLC protocols. The controller36may comprise one or more microprocessors, firmware, hardware, or a combination thereof. The buffer system40comprises memory for storing data to be transmitted or retransmitted. In the exemplary embodiment, the buffer system includes a MAC transmission and retransmission buffers42,44for HARQ operations, as well as a RLC transmission and retransmission buffers46,48for AM operations. The buffer system40may also contain the PDCP-level buffers, but for simplicity reasons, the PDCP-level buffers are left out of this example.

The controller36implements MAC layer and RLC layer protocols as herein described. In particular, the controller36implements MAC layer operations, such as generating and sending BSRs and SRs, maintaining MAC transmission and retransmission buffers42,44, processing received SGs or CBGs, and retrieving data from the transmission and retransmission buffers40for transmission on the scheduled resources. Controller36may also include a RLC retransmission counter38that tracks the number of retransmissions of RLC PDUs. The MAC transmission buffer42stores new data (e.g. transport blocks) waiting for transmission. After transmission on dedicated uplink resources, the transmitted data packets await acknowledgement by the base station20. When the user terminal30receives a NACK for transmitted data packets, the data packets are put in the retransmission buffer44and the controller36resends the data packets. Because HARQ retransmission is not typically used for contention-based transmissions, data transmitted on CB resources does not typically await acknowledgement and is typically removed from the transmission buffer42.

Similarly, the RLC transmission buffer46stores new RLC PDUs waiting for transmission. After transmission on uplink resources, the RLC PDUs are kept in the transmission buffer46while waiting for acknowledgement by the base station20. When the user terminal30receives a NACK for a transmitted RLC PDU, the PDU is moved to the RLC retransmission buffer48. The controller36increments the RLC counter38and resends the PDU in the retransmission buffer48unless the RLC counter38indicates the maximum number of retransmissions has been reached. Further, RLC PDUs stored in the RLC transmission buffer46awaiting acknowledgement can be retransmitted proactively in response to an SG from the base station20when there is no other data in the transmission buffers. In this case, the RLC counter is not incremented.

The contention-based access procedure described herein reduces the time it takes to access uplink resources and signaling overhead by allowing a synchronized user terminal30to send data on the uplink without first requesting resources for the transmission. The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.