In the 3rd Generation Partnership Project (3GPP) standardization body, technologies like GSM, HSPA, and LTE have been and are currently developed for cellular communication networks that are providing higher data rates together with improved capacity and coverage. In LTE, the access technology is based on Orthogonal Frequency Division Multiplexing (OFDM) for the downlink (DL) and Single Carrier FDMA (SC-FDMA) for the uplink (UL). Radio resource allocation to user equipments (UEs) on both the downlink (DL) and the uplink (UL) is performed adaptively using “fast scheduling” taking into account the current traffic pattern and radio propagation characteristics associated with each UE. Assigning radio resources in both the DL and UL is performed in the scheduler situated in the base station which is referred to in LTE as an eNodeB.
The technology in this application is related to UL scheduling in LTE and in systems that employ uplink scheduling similar to LTE. One of the challenges in assigning resources for UL transmissions is that the UE must make the eNodeB aware that there is data pending or waiting in the UE buffer for UL transmission. One way to do this in LTE for example is for the UE to transmit a scheduling request (SR) to the eNodeB. The SR can be sent on a dedicated SR channel (D-SR) or on a contention based Random Access Channel (RACH). A D-SR requires that the UE be UL-synchronized and that the UE has been assigned a SR channel on the Physical Uplink Control Channel (PUCCH). Both of these procedures result in delay. Then, the eNodeB responds with a grant including information on what time/frequency resources the UE will use for the UL transmission. The grant is sent on the Physical Downlink Control Channel (PDCCH). With support from the link adaptation function in the eNodeB, the transport block size, modulation, coding, and antenna scheme are selected, and the selected transport format is signaled together with user ID information to the UE.
The resource granted by the eNodeB can be of variable size so that the UL transmission that follows from the UE can contain various numbers of bits. At a minimum, the UL transmission should include a buffer status report (BSR). Other information may be included along with the BSR.
Sending a scheduling request (SR) informs the eNodeB uplink scheduler of the UE's need for UL transmission resources. In LTE, triggering a scheduling request (SR) is related to the different logical channels in LTE. Those logical channels are normally grouped together into logical channel groups (LGC) that share similar characteristics. More specifically, a transmission of a buffer status report (BSR) is triggered when UL data arrives in the UE transmission buffer and that data belongs to a logical channel group (LCG) with a higher priority than the priority for data already existing in the UE transmission buffer. In turn, a scheduling request (SR) is triggered if the UE does not have an UL resource allocated for the current transmission time period. A dedicated scheduling request (SR) (D-SR) is transmitted on the PUCCH if this resource is allocated to the UE, or alternatively, a random access scheduling request (SR) (RA-SR) is transmitted on the RACH.
LTE also offers the opportunity to use semi-persistent scheduling in which a UE is allocated an UL resource with some periodicity. A benefit of semi-persistent scheduling is that it saves scarce radio resources on the Physical Downlink Control
Channel (PDCCH) by avoiding the transmission of UL grants for every resource allocation. One service likely to benefit from a semi-persistent scheduling configuration is voice over IP (VoIP). When a UE has an UL semi-persistent radio resource configured for a VoIP flow or the like, each packet arriving to an empty buffer triggers a RA-SR or a D-SR unless the timing of the resource is perfectly aligned with the arrival of the VoIP data. In other words, there will likely be many instances when each packet arriving to an empty buffer triggers a RA-SR or a D-SR even though a SR is unnecessary given the semi-persistent scheduling of UL resource for the VoIP flow.
In this situation, the UL scheduler cannot distinguish between an RA-SR or a D-SR triggered (1) by a VoIP frame (that typically does not need the scheduler to respond to the SR because the semi-persistent scheduling already has a resource ready for the VoIP frame in the near future) or (2) by some higher priority data (e.g. related to a signaling radio bearer (SRB) which does need the scheduler to respond to the SR). The scheduler either ignores all SRs from the UE or schedules the UE dynamically for all SRs. In the first case, the transmission of higher priority data, like signaling radio bearer (SRB) data, might be delayed until the next semi-persistent resource comes up. If the delayed higher priority data takes the next semi-persistent resource, then the buffered, lower priority VoIP data is delayed until the next semi-persistent grant comes along. If VoIP frames are bundled, extra delay time could be 40 ms or more, which may be unacceptable. In the second case, there is not much benefit from using semi-persistent resource scheduling because both PDCCH grants and SRs will be sent extensively regardless.