3GPP TSG-RAN WG1 Ad-Hoc
`Cannes, France
`June 21-24, 2004
`
`Agenda item:
`
`6.3
`
`Source:
`
`Lucent
`
`Title:
`
`Scheduling Schemes for EDCH
`
`Document for:
`
` Decision
`
`
`
`1
`
`Introduction
`
`
`
`R1-040705
`
`
`
`In this contribution, we compared the two scheduling modes available: Rate/Time and Rate scheduling and
`discuss the different options of operations when consider together with the different modes of
`retransmissions. As many issues have been raised in previous meetings on the schemes to be adopted for
`further studies during this work item phase, this document presents our view on the preferred scheduling
`mode for enhanced uplink.
`
`2
`
`Rate/Time and Rate Scheduling
`
`As discussed in detail in TR25.896, Rate and Time scheduling mode involves selecting a subset of UEs,
`based on some scheduling algorithm, that are allowed to send in the uplink for a certain period of time. The
`UEs would transmit for a time interval using a signaled TFCS that it receives in the downlink scheduling
`assignment sent by the NodeB. In WG1#37, some simulation performance of Rate/Time over Rate
`scheduling have been presented and while the results still needs to be further verified, potential gains of
`Rate/Time scheduling over Rate scheduling were reported. The following lists the benefits of Rate and Time
`scheduling:
`
`1. Performance Gain over Rate Scheduling: By selecting a subset of UEs scheduled for transmission in
`the uplink, a better control of the RoT is achieved in addition of the reduced latency as well. The
`RoT management is improved due to the tighter restriction on which UEs are allowed to send and
`the duration of transmissions as well. Certain UEs with larger buffer occupancy, higher priority
`and/or data with less delay tolerance is able to take advantage of this mode since a higher TFC can
`be assigned, up to the maximum TFC, to these UEs for a limited period of time;
`
`2. Downlink signaling: This depends on the number of UEs that need to be scheduled for simultaneous
`transmissions. Since the UEs are selected based some criteria to fulfill the system RoT operating
`point, depending on the scheduling algorithm used, the number of UEs that need to be scheduled to
`meet the RoT utilization needs to be studied further. In comparison, in Rate scheduling, even though
`the exact TFCS signaling is not needed, the incremental scheduling signaling needs to be sent to all
`UEs. Based on the gain versus complexity tradeoffs, the optimum maximum number of UEs that can
`be scheduled for simultaneous transmission in EDCH needs to be studied further with simulations
`and a recommendation can then me made.
`
`3. The option of using a Cell wide signaling for scheduling grant is available in Rate scheduling. Such
`an option would reduce the signaling overhead in the downlink. However, with Rate and Time
`scheduling, UE specific DL signaling is mandatory as only a subset of UEs are chosen and each
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`UE’s TFC is more likely to be different due to the greater granularity in the TFC adjustments for
`Rate and Time mode.
`
`3
`
`Autonomous or Scheduled Retransmission
`
`As already currently described in Section 7.1 of the TR [1], there are currently two options for
`retransmissions:
`
`Autonomous Retransmissions: The UE is allowed to send its retransmissions at any subsequent
`retransmission timing without the need to be scheduled by the NodeB. This lets the UE to send its
`retransmission with less latency that in the case of scheduled retransmissions but with the potential
`disadvantage of undesirable RoT fluctuation at the NodeB. If its TFC subset at the time of retransmission
`does not include the TFC of the first transmission, the UE has the option of using the TFC of the first
`transmission with or without transmission power adjustment. This mode is similar to the operation of Rate
`scheduling since both rate and transmission time is UE controlled;
`
`Scheduled Retransmissions: The UE retransmissions are scheduled. If the assigned TFC subset for the
`retransmission does not include the TFC of the first transmission, the UE has the option of using the TFC of
`the first transmission with or without transmission power adjustment. This mode is similar to Rate and Time
`scheduling but with the possibility of UE needing to adjust its TFC so that the initial transmission TFC is
`included as part of the TFC subset for retransmissions.
`
`The above two options when combined with the two scheduling modes available would yield the following
`options:
`
`1. Rate/Time Scheduling for 1st and all retransmissions (Scheduled retransmissions): The newly
`assigned TFC subset and duration of transmission are both signalled to the UE. As the channel
`variations may very likely changes during the retransmissions, the NodeB would have to rely on
`transmission power adjustment at the UE if the original TFC for first transmission could not be
`assigned to the UE. Hence, the objective of more RoT control may be less effective during HARQ
`retransmissions;
`
`2. Rate/Time Scheduling for 1st transmission and Autonomous retransmissions: Only the first
`transmission is Rate and Time scheduled. The retransmissions are allowed to be autonomously
`controlled by the UE. The UE could choose to perform power adjustment on the original TFC or
`retransmit with the same power level as the first transmission. The retransmission behaviour in the
`scenario where the UE would need to adjust its transmission power would be very similar to above
`scenario of scheduled retransmissions. However, with autonomous retransmissions, the signalling for
`retransmissions is reduced;
`
`3. Rate Scheduling for 1st and all retransmission (Autonomous retransmissions): All transmissions from
`the UE are autonomous so that the UE transmits at its chosen time within the assigned TFC subset;
`
`4. Rate Scheduling for 1st and Scheduled retransmissions: Only the retransmissions are scheduled. This
`combination implies that the retransmissions contribute to a significant RoT variation compared to
`the first transmission and need to be controlled tighter compared to the first transmission. Depending
`on the usefulness of such a scenario, this combination may be excluded.
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`4
`
`Summary
`
`In this contribution, the various scheduling combinations for first and retransmissions are discussed. Based
`on the above discussions, it is proposed that only
`
`Option 2: Rate/Time for 1st transmission and Autonomous retransmission, and
`
`Option 3: Rate for 1st transmission and Autonomous retransmissions
`
`should be for further performance comparisons during the work item phase.
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