The Third Generation Partnership Project (3GPP) Release 6 defines fast control of wireless transmit/receive unit (WTRU) transmissions through Node-B based scheduling in HSUPA. This faster control results in better control of the uplink (UL) noise rise, which allows operation at a higher average UL load without exceeding the threshold, thereby increasing system capacity. In HSUPA, control and feedback occurs through different physical control channels and information elements (IEs).
Node-B commands are conveyed by absolute or relative grant channels, while WTRU feedback is transmitted on an enhanced dedicated physical control channel (E-DPCCH), or “happy bit” within the E-DPCCH, where scheduling information (SI) is appended to the payload. The Node-B commands are expressed by a maximum power ratio over the power of the UL control channel (DPCCH). The happy bit is transmitted within the E-DPCCH along with 2 bits for retransmission sequence number (RSN) and 7 bits for the enhanced transport format combination indication (E-TFCI). All combinations of the 7 E-TFCI bits are defined to mean a specific size of the enhanced transport format combination (E-TFC). The value “0” (7 bits) is defined to mean the transmission of the SI alone. The E-DPCCH is always transmitted along with the enhanced dedicated physical data channel (E-DPDCH) except during compressed mode. Transmission of E-DPCCH alone does not occur.
The WTRU and Node-B are aware how much data can be transmitted for a given power ratio, and this correspondence is controlled by the radio network controller (RNC). Such a scheduled operation is particularly well suited to non-delay-sensitive types of applications, however it may also be used to support more delay-sensitive applications, given the fast resource allocation capabilities.
Under the current standard, data is optionally segmented, and buffered at the radio link control (RLC) layer. The set of possible RLC packet data units (PDU) sizes that are delivered to the medium access control (MAC) layer is configured by radio resource control (RRC) signaling. When segmentation takes place, generally the sizes of the PDUs are configured to be of the order of several hundreds of bits to avoid excessive overhead and obtain good coding performance. Currently, there is no further segmentation at the MAC layer. Accordingly, when a new transmission takes place, an integer number of PDUs, including zero, must be sent.
Since it is not possible to send out a fraction of an RLC PDU, a certain minimum instantaneous bit rate for the WTRU transmission is imposed. For instance, if the PDU size is 320 bits and the transmission time interval (TTI) is 2 milliseconds (ms), the instantaneous bit rate needs to be at least 160 kilo bits per second (kbps), without accounting for MAC overhead. Such an instantaneous bit rate translates into a certain minimum transmission power ratio, under which no RLC PDUs can be sent.
During scheduled operation, WTRU transmissions from a given MAC-d flow can be completely interrupted, or “blocked,” if the granted power ratio falls under the minimum required to transmit the RLC PDU at the head of buffer. Such a situation may occur out of the control of the serving radio link set, (i.e., Node-B) for a number of reasons. For example, the WTRU may have received a non-serving relative grant requesting a decrease of power from another Node-B, the WTRU may have erroneously decoded a relative or absolute grant command from the serving Node-B, or the WTRU may have several different configured RLC PDU sizes on a given MAC-d flow and a bigger than usual RLC PDU size is up for transmission.
When such a situation occurs, the WTRU cannot transmit until the time it is scheduled to transmit an SI. Until then, and unless the previous SI has been transmitted recently enough for the Node-B to infer that the WTRU buffer is not empty based on its subsequent transmissions, the Node-B has no ability to determine whether transmission stopped because the power ratio fell under the minimum, or simply because the WTRU has nothing to transmit. Accordingly, transmission from the WTRU is delayed until the SI can be transmitted.
This issue imposes a configuration of a small periodicity of SI transmission (T_SIG) for delay-sensitive applications, thereby increasing overhead. Furthermore, even if the Node-B was aware that transmission stopped because the power ratio is too low, when multiple RLC PDU sizes are configured, the Node-B does not know what power ratio to apply to correct the situation. Thus, the Node-B has to find out by trial and error what the correct power ratio is. This results in inefficient resource allocation and/or excessive scheduling delays.
In the current state of the art, transmission of scheduling information (SI) is only allowed under certain conditions such as those described in 3GPP TS 25.321, such as if the user has a grant (power ratio) of zero or has all its processes de-activated and has data to transmit, upon a change of E-DCH serving RLS (base station), or periodically, with a configurable period depending on whether the user has a grant or not. Accordingly, a solution to prevent blocking that would be compatible with the mechanisms defined in the current state of the art may include configuring periodic reporting of the SI with a very low period, such that the SI is transmitted along with almost every transmission of new data. However, overhead may be significantly increased since each SI takes up 18 bits. For instance, assuming a MAC service data unit (SDU) size of 280 bits and a MAC-e header size of 18 bits, this would represent an additional overhead of approximately 6%.
It would therefore be beneficial to provide a method and apparatus for transmission blocking in an HSUPA wireless communication system that is not subject to the limitations of the current state of the art.