1. Field of the Invention
The present invention relates generally to a User Equipment (UE), and in particular, to adaptation of data unit size in a UE.
2. Description of the Related Art
The present invention relates to the Third Generation Partnership Project (3GPP) Specifications for Layer 2 Protocols. The latest 3GPP Release-8 versions 8.5.0, of 25.321 Media Access Control (MAC) and 8.4.0 of 25.322 Radio Link Control (RLC) are referred to in context of the explanation of various protocols and state of the art herein.
A communication device such as a UE includes multiple protocol layers or stacks. The data link layer or layer 2 (L2), is layer responsible for such tasks as handling errors in the physical layer (or L1), flow control, and frame synchronization. The functions of L2 are shared by two sub layers of L2, known as the MAC sub layer and the RLC sub layer. The MAC sub layer controls the access to the network by granting permission to the communicating entities to transmit the data and the RLC sub layer controls the data frame synchronization, flow control, and error checking.
The service provided by the RLC is called Radio Bearer (RB). MAC is connected to L1 via a transport channel and to RLC via a logical channel. Release 6 of 3GPP introduced the Enhanced Dedicated CHannel (E-DCH) in the UpLink (UL). In conventional systems, the E-DCH is configured with specific E-DCH Transport Format Combinations (E-TFCs). E-TFCs are the data rate combinations that are controlled by a UE in High Speed Uplink Packet Access (HSUPA) and are used by the UE. The data rate for an E-DCH is selected using a procedure called the E-TFC selection to transmit data from the logical channel. Every logical channel is associated with some absolute priority and transmission of higher priority data is maximized.
The size of an RLC PDU, that is to be transmitted by a UE in either a current Transmission Time Interval (TTI) or in a future TTI, is adapted according to the current radio conditions of the UE. Most of the commonly known schemes for RLC PDU size adaptation are either E-TFC based or Grant based. In the E-TFC based schemes, the E-TFC selected in the current TTI is considered as the guiding metric for determining the PDU size in the next or future TTI. Accordingly, if an RB has little or no data to transmit in the current TTI, then little or no data would be taken from it for transmission in a future TTI. Thus, the data size from this RB would be minimal or zero. However, the amount of data for an RB in the current TTI may not remain the same for a future TTI. In a future TTI, when this RB has data available for transmission, the existing E-TFC based schemes would incorrectly result into a zero (or a smaller size) data transmission due to its reliance on the selected E-TFC. This is an incorrect estimation resulting in an incorrect adaptation of the data size.
The Grant based schemes consider the grant received in the current TTI as the guiding metric for the data size estimation for a future TTI. However, the data size for an RB cannot be directly derived from the grant and is affected by presence of data on higher priority RBs, buffer occupancy of the higher priority RB when the higher priority RB is using a scheduled grant, and applicability for or restriction to a TTI of the relevant non-scheduled grant, when the higher priority RB is using a non-scheduled grant.
All of these may lead to an incorrect estimation of the data size for a future TTI and subsequently, result in an incorrect adaptation of the data size for a future TTI. Further, the existing schemes also fail to regulate the number of RLC PDUs that could be created prior to the actual transmission. One of the commonly known approaches is to specify an arbitrary limit on the number of RLC PDUs that can be created, which either causes too many or too few RLC PDUs to be created. When the grant increases and an insufficient number of PDUs are created, then it is possible that the grants would not be properly used, resulting in a waste of scarce radio resources. However, if the grant decreases and too many PDUs are created using this approach, then the PDUs would have to be segmented into many parts across TTIs, resulting in an increased probability of loss and header overheads. Thus, there is a need to provide a solution to estimate the size of the RLC data with improved correctness that may be adapted by a UE for transmitting the data in the next or a future TTI.
Further, as per the conventional methods, a constant number of RLC PDUs are created in advance by the UE using the grant allowed in the current TTI. However, the scheduled grant for the UE may decrease over a period of time, which would eventually allow less data to be transmitted in the future TTIs. Therefore, all the RLC PDUs generated in the intermediate TTIs may not be transmitted by the UE and could remain in the buffer as the cumulative size of the generated RLC PDUs may be larger than the size allowed by a prevailing scheduled grant. Thus, there is a need to efficiently generate the RLC PDUs by a UE for the future TTI.