LTE Scheduling Feature
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LTE Scheduling Feature
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LTE Scheduling Feature
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LTE Scheduling Feature
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LTE Scheduling Feature
LTE air interface scheduling is the responsibility of the eNB, however additional scheduling and QoS (Quality of Service) handling could take place in the EPC (Evolved Packet Core).
Typically, the main goal of scheduling is to meet the different users’ expectations. Historically the radio interface is the “weak link” or “bottle neck” in the overall end-to-end end-to-end service. This is typically due to limited physical resources, i.e. limited bandwidth or channels. The scheduling in previous systems, such as GSM and UMTS, was easier. This was due to the fact that voice was the main service and required a dedicated channel. As such, the number of channels (or elements) on the base station limited the number of simultaneous calls.
The eNodeB implements scheduling at the Medium Access Control (MAC) layer and provides time-and-frequency resources for uplink and downlink through scheduling. On the premise of guaranteed Quality of Service (QoS), scheduling aims to transmit data on the channel with better quality and maximize system throughput by using different channel qualities among UEs.
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LTE Scheduling Feature
LTE Default and Dedicated EPS bearers are capable of transporting a large variety of traffic types between the UE and the PDN. This could range from regular Internet browsing based on HTTP, through to real time voice services based on RTP. Above table outlines the traffic types which can potentially be encountered, including detail on the characteristics of the traffic and its associated QCI (QoS Class Identifier) value.
The QCI is a parameter associated with each EPS bearer which will determine the bearer level packet forwarding treatment e.g. scheduling weights, admission thresholds, queue management etc. The QCI value of an EPS bearer will be established during the Default or Dedicated EPS bearer setup procedure.
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LTE Scheduling Feature
Packet switched technologies are designed to provide enhance network utilization and converge multiple data types (multimedia). Unfortunately, services such as voice and multimedia have various issues associated with delay and jitter. To combat this, the LTE packet switches / bearer managers are QoS aware, in that they are able to classify packets, as well as enforce forwarding characteristics. The eNB (Evolved Node B), S-GW (Serving Gateway) and PDN-GW (Packet Data Network - Gateway) all get involved in the managing of QoS.
In eNodeB, DSCP header is used to identify the different QoS level, and determine the schedule weight.
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LTE Scheduling Feature
Scheduling is a very complicated algorithm that involve a lot of input parameters, as shown in the above figure.
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LTE Scheduling Feature
In dynamic scheduling, scheduling is performed every Transmission Time Interval (TTI) of 1 ms and all the UEs to be scheduled are notified with the scheduling information through control signaling within this TTI. Dynamic scheduling has no requirements on the size and arrival time of data packets. Therefore, dynamic scheduling is applicable for all services.
Semi-persistent scheduling is introduced to reduce the overhead of control signaling. Semipersistent scheduling is a process where one user uses the same time-and-frequency resources in a specified semi-persistent scheduling period (20 ms in Huawei eNodeB) until they are released. Semi-persistent scheduling is mainly used for processing services with a constant rate, regular packet arrival, and low delay requirements, such as the Voice over IP (VoIP). By adopting semi-persistent scheduling, VoIP services can save the overhead of control signaling and increase the VoIP capacity.
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LTE Scheduling Feature
Huawei eNodeB supports four scheduling strategies:
Max C/I
Round Robin (RR)
Proportional Fair (PF)
Enhanced Proportional Fair (EPF)
The downlink scheduling strategy is decided by the Dlschesw parameter, and the uplink scheduling strategy is decided by the UlschStrategy parameter. Max C/I, RR, and PF are basic features. EPF is an optional feature. With Max C/I, RR, and PF scheduling strategies, dynamic scheduling is used for all services. With the EPF scheduling strategy, only the VoIP services use semi-persistent scheduling.
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LTE Scheduling Feature
The signaling required for scheduling downlink resources is firstly dependent on the type of resources being scheduled. The LTE system defines various DCI (Downlink Control Information). These enable both downlink and uplink scheduling, as well as linking to different MIMO and diversity options.
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LTE Scheduling Feature
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LTE Scheduling Feature
The type 1 resource block assignment information consists of three fields:
The first field is used to indicate the selected RBG subset among P RBG subsets
The second field with one bit is used to indicate a shift of the resource allocation span within a subset. A bit value of 1 indicates a shift is triggered. Otherwise a shift is not triggered.
The third field includes a bitmap, where each bit of the bitmap addresses a single PRB in the selected RBG subset in such a way that MSB to LSB of the bitmap are mapped to the PRBs in the increasing frequency order
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LTE Scheduling Feature
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LTE Scheduling Feature
Downlink scheduling allocates time-and-frequency resources at the Physical Downlink Shared Channel (PDSCH) for transmission of system messages and downlink data. Downlink scheduling described in this chapter is based on the EPF scheduling strategy.
Downlink scheduling calculates available scheduling resources based on the current remaining power. In addition, the scheduling priority and Modulation and Coding Scheme (MCS) are determined based on the amount of data at the Radio Link Control (RLC) layer, QoS requirements of bearers, and UE channel quality. In downlink scheduling, the UE channel quality information is obtained through the CQIs reported by the UE. The prioritization and MCS selection of scheduling depend on the CQI information. Therefore, if reported CQIs cannot properly reflect the actual channel conditions, the downlink resource efficiency is low.
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LTE Scheduling Feature
VoIP service
Control-plane data and IMS signaling
Control-plane data consists of common control messages and UE-level control messages. Common control messages consist of broadcast messages, paging messages, and random access response messages. UE-level control messages consist of Signaling Radio Bearer 0 (SRB0), SRB1, and SRB2. The scheduling of IMS signaling is the same as that of UE-level control messages.
HARQ retransmission data
The VoIP service experiencing semi-persistent scheduling has the highest priority. Semi-persistent scheduling is used in the talk spurts of the VoIP services.
HARQ retransmiss retransmissions ions of Huawei Huawei eNodeB are classified classified into urgent urgent HARQ retransmissions and non-urgent HARQ retransmissions. HARQ retransmissions that are not scheduled in the X TTIs since the last transmission are defined as urgent. The other HARQ retransmissions are non urgent (X=X=N+M+ DrxReTxTimer -2)
Other initial transmission services
Other initial transmission services refer to the initial transmission services of other QCIs excluding VoIP services and IMS signaling.
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LTE Scheduling Feature
Generally, VoIP services adopt dynamic scheduling for the transient state and silent period and semi-persistent scheduling for the talk spurts.
In downlink scheduling, if the VoIP service is in the talk spurts, semi-persistent scheduling is activated. If the VoIP service is in the silent period, the resources allocated to semipersistent scheduling are released. If the VoIP service transits from the talk spurts to the silent period, semi-persistent scheduling should be activated again. If PDCCH resources are insufficient in this case and the semi-persistent scheduling indication fails to be delivered, the UE uses dynamic scheduling. In a TTI of semi-persistent scheduling, dynamic scheduling is used instead of semi-persistent scheduling if large-size data packets appear
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LTE Scheduling Feature
The resource of semi-persistent scheduling is a semi-persistent scheduling period in the time domain and is the system wideband in the frequency domain. When semi-persistent scheduling is activated, associated RBs are allocated to semi-persistent scheduling. When the VoIP service enters the silent period, related RBs are released. If a new VoIP service is admitted at this time, it can use these RBs.
The eNodeB sets the upper and lower thresholds for time-and-frequency resources of semi-persistent scheduling in each TTI. Thus, failures of scheduling other services caused by excessive resource usage of semi-persistent scheduling can be prevented. The threshold algorithm is fixed setting in Huawei eNodeB. If resources of semi-persistent scheduling exceed the upper threshold, admission requests of new VoIP services are rejected. If resources of semi-persistent scheduling are fewer than the lower threshold, new VoIP services are admitted.
VoIP services are prioritized by the waiting time. The VoIP service with longer waiting time has a higher priority. The process for selecting the MCS and determining the number of RBs is described as shown above.
If initial transmission in semi-persistent scheduling of the VoIP service fails, retransmission of data is required. Data retransmissions of downlink VoIP services use dynamic scheduling and the downlink asynchronous adaptive HARQ retransmission.
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LTE Scheduling Feature
The scheduling priority of control-plane data is only lower than that of VoIP services. Control-plane data is subject to dynamic scheduling. Control-plane data consists of common control messages and UE-level control messages. The scheduling of IMS signaling is the same as that of UE-level control messages.
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LTE Scheduling Feature
HARQ retransmissions of Huawei eNodeB are classified into urgent HARQ retransmissions and non-urgent HARQ retransmissions.
The scheduling priority of the urgent HARQ retransmission is lower than that of SRB0 and higher than that of SRB1 and SRB2. The scheduling schedu ling priority of the non-urgent non -urgent HARQ retransmission is lower than that of control-plane messages and higher than that of other initial transmission services. The HARQ retransmission (both urgent and non urgent) with longer waiting time has a higher scheduling priority. If all the retransmissions have the same waiting time, a retransmission is randomly selected.
If the UE has VoIP services for semi-persistent scheduling, HARQ retransmissions of other services cannot be performed on the UE during the TTI of the semi-persistent scheduling. If SRB1, SRB2, and IMS signaling are scheduled in the current TTI, non-urgent HARQ retransmissions cannot be scheduled in this TTI.
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LTE Scheduling Feature
Other initial transmission services refer to the initial transmission services of other QCIs excluding VoIP services and IMS signaling. The following sections detail the scheduling process of initial transmission services.
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LTE Scheduling Feature
Eliminate the following UEs that do not need nee d prioritization
UEs that experience semi-persistent scheduling in the current TTI
UEs that experience HARQ retransmission scheduling in the current TTI
UEs that run out of HARQ process numbers
UEs that enter the measurement gap
UEs that enter the DRX dormant period
UEs that stay out of synchronization and have failed radio links
Eliminate services (both non-GBR and GBR services) whose rates have met the guaranteed rates. These services do not need prioritization. The decision of whether rates meet the guaranteed rate is not made on the GBR services with QCI of 1. Such GBR services are prioritized directly.
Prioritize the remaining services
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LTE Scheduling Feature
The following factors are weighted in the priority calculation of non-GBR services in Huawei eNodeB:
CQI: The service with higher spectral efficiency of the corresponding wideband CQI has a higher priority.
Average rate of non-GBR services: The non-GBR service with a greater average rate has a lower priority
UE differentiation factor: The UE differentiation factor reflects the priority of UEs of different levels. The UE with a higher level set by operators has a higher priority in scheduling.
Weight factor: In downlink scheduling, the weight factor is determined by the DlschGammaQci parameter. A greater value of the weight factor leads to a higher
priority of scheduling. The standard QCI and extension QCI can be configured with the weight factor respectively.
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LTE Scheduling Feature
The following factors are weighted in the priority calculation of GBR services in Huawei eNodeB:
Channel quality: The instantaneous channel quality of the UE is taken into account. The UE with better instantaneous channel quality has a higher priority. In the case of the same channel quality, the GBR service with QCI of 1 has a higher priority than other GBR services.
Delay: The closer the waiting time of the first packet in the buffer is to the Packet Delay Budget (PDB), the higher the priority is.
Relative priority: The prioritization of GBR services is different from that of non-GBR services. This factor is added to compare the priority of GBR services with that of non-GBR services.
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LTE Scheduling Feature
After services are prioritized, the eNodeB selects candidate UEs for scheduling based on the number of remaining RBs, services waiting for scheduling, and whether the number of candidate UEs reaches the maximum number. The maximum number of candidate UEs in a TTI depends on the cell bandwidth.
During the selection of candidate UEs for scheduling, services are selected by the priority in descending order and the number of RBs of this service required in the current TTI is calculated. If there are sufficient RBs for this service and the number of candidate UEs does not reach the maximum number, the service is placed in the candidate UE set for scheduling. This procedure is repeated until the number of available RBs in the current TTI is 0. If a UE has multiple services in the candidate set, the UE is considered to be one candidate UE instead of multiple candidate UEs.
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LTE Scheduling Feature
The procedure of selecting the MCS is described as follows:
The eNodeB obtains the ITBS according to the adjusted CQI.
The eNodeB obtains the number of RBs by consulting the TBS sheet, based on the ITBS and the amount of data to be scheduled.
The eNodeB selects IMCS according to the mapping from ITBS to IMCS.
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LTE Scheduling Feature
The CQI adjustment algorithm is enabled or disabled through the CqiAdjAlgoSwitch parameter.
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LTE Scheduling Feature
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LTE Scheduling Feature
Uplink scheduling selects an appropriate UE at a proper time and allocates appropriate resources on the PUSCH to the UE.
After the scheduling request from the UE is received, uplink scheduling is performed on the UE, and MCS selection and RB allocation are performed on the basis of the current c urrent channel quality of the UE, amount of data to be scheduled, and power headroom. In uplink scheduling, the channel quality of the UE is indicated by the SINR measured at the physical layer of the eNodeB. The amount of data to be scheduled depends on the Buffer Status Report (BSR) reported by the UE. The power headroom depends on the Power Headroom Report (PHR) reported by the UE.
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LTE Scheduling Feature
Uplink scheduling allocates resources on the PUSCH. The following data is involved in the priority handling in uplink scheduling:
VoIP Service: The following data is involved in the priority handling in uplink scheduling:
Retransmission data: Retransmission data includes data in retransmiss ions of TTI bundling, retransmissions in semi-persistent scheduling, suspended retransmissions, and retransmissions in dynamic scheduling. The suspended retransmissions are retransmissions that are not performed in the current TTI. Retransmissions are suspended in the following cases:
The resource allocation in the last retransmission fails because of resource conflicts. Therefore, the eNodeB sends ACK to suspend the retransmission.
The time of transmitting the NACK collides with the measurement gap. Therefore, the UE assumes that the eNodeB sends the ACK to suspend the retransmission.
Control-plane data: The UEs with control-plane data to be transmitted include the UE with the first uplink transmission, UE with SRB or IMS signaling to be transmitted, and UE with the Scheduling Request (SR) to be transmitted.
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LTE Scheduling Feature
Other initial transmission data: The UEs with other initial transmission data include the UEs with the initial transmission data in dynamic scheduling of services excluding IMS signaling and VoIP services, and the UEs with pre-allocation data.
Virtual MIMO pairing: Virtual MIMO pairing is a feature where the eNodeB schedules two single-antenna UEs at the same time to enable the two UEs to transmit data on the same time-and-frequency resources. UEs are scheduled flexibly through the optimal virtual MIMO pairing and appropriate UEs are selected for pairing transmission.
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LTE Scheduling Feature
In uplink scheduling, the PDCP checks the first arrived data packet after the VoIP service is set up.
If the VoIP service is in the talk spurts, semi-persistent scheduling is activated. If there is no data transmission on the resources of semi-persistent scheduling for consecutive times after the activation, the resources of semi-persistent scheduling are implicitly released. Implicit release of resources means that the eNodeB directly releases resources without notifying the UE.
If the VoIP service is in the silent period, dynamic scheduling is performed. When the VoIP service transits from the silent period to the talk spurts, the PDCP checks the data packet size and determines that the VoIP service is in the talk spurts. In this case, semi-persistent scheduling is activated.
In uplink scheduling, whether semi-persistent scheduling is used for the VoIP service in the talk spurts is set through the SpsScheSw parameter. If the VoIP service does not use semipersistent scheduling, it can use dynamic scheduling with the same priority as other GBR services.
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LTE Scheduling Feature
Uplink retransmissions use the synchronous non-adaptive HARQ re transmission. In the FDD system, the interval between retransmissions is fixed to eight TTIs. If TTI bundling is used, the interval is changed to 16 TTIs
. If resource conflicts occur in the uplink retransmission, the eNodeB re-allocates resources to the UE. If resource allocation fails, the eNodeB sends ACK to the UE to suspend the retransmission. UEs with suspended retransmissions are sorted by the number of retransmissions. The UE with a larger number of retransmissions has a higher priority in scheduling. If resources for the retransmission in semi-persistent scheduling are in conflict with those of the PUCCH, the resources for the initial transmission in semi-persistent scheduling are activated again.
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LTE Scheduling Feature
The UE with control-plane data to be transmitted includes the UE with the first uplink transmission, UE with SRB or IMS signaling to be transmitted, and UE with the SR to be transmitted.
The UE with the first transmission refers to the UE that needs to transmit msg3. These UEs are scheduled in the order of msg3 transmission time. The UE with the first transmission uses the resource allocation scheme of frequency non-selective scheduling. Four RBs are allocated to a UE with the first firs t transmission. In case of non-contention non- contention based random access, IMCS = 1. In case of contention based random access, IMCS = 1 if group A is used or IMCS = 5 if group B is used.
UEs with SRB or IMS signaling to be transmitted are sorted by the average Signal-to-Noise Ratio (SNR) in descending order. The UE with a larger average SNR has a higher priority in scheduling. For UEs with Radio Resource Control (RRC) messages or IMS signaling to be transmitted, resource allocation adopts frequency selective scheduling and the MCS is the same as that in dynamic scheduling.
UEs with the SR to be transmitted are sorted by the number of received SRs since the last scheduling. The UE with a greater number of SRs has a higher priority in scheduling. For UEs with the SR to be transmitted, the resource allocation adopts the frequency selective scheduling and the MCS is the same as that in dynamic scheduling.
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LTE Scheduling Feature
UEs with other initial transmission services include the UEs with the initial transmission data in dynamic scheduling of services excluding VoIP services and IMS signaling and the UEs with pre-allocation data.
The UE reporting CQIs in event-triggered mode refers to the UE that reports the CQI through the PUSCH in case of no valid CQIs. The happy user refers to the UE with non-GBR services that meets the Min_GBR but fails to meet the Aggregate Maximum Bit Rate (AMBR). The Min_GBR in uplink scheduling is controlled through the UlMinGbr parameter. Pre-allocation refers to a process where the eNodeB reserves resources for UEs with high requirements for delay based on the uplink load status.
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LTE Scheduling Feature
In uplink scheduling, the token bucket algorithm is used to determine whether the rate of services meets the guaranteed rate. The principle of the token bucket is as follows: Water injected into the bucket in a specified period is the size of the bucket. During this period, the remaining amount of water in the bucket is equal to the accumulated water in the bucket minus the amount of the scheduled data. The water injection rate is in proportion to the service rate. When the remaining amount of water is greater than 0, the rate of the service does not meet the required rate.
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LTE Scheduling Feature
the resource allocation of the UE with unsatisfied GBR, UE with unsatisfied Min_GBR, and happy users adopts frequency selective scheduling. The following factors are weighted in the scheduling priority of these UEs:
Effective rate: The UE with a low effective rate has a higher priority in scheduling. The effective rate refers to the average rate of the UE data received by the eNodeB in a specified period.
Total guaranteed rate of services with unsatisfied rates: The UE with a higher total guaranteed rate has a higher priority in scheduling. The UEs with the unsatisfied rate include the UE with unsatisfied GBR, UE with unsatisfied Min_GBR, and happy user. The guaranteed rate of the happy user is the AMBR.
Average channel quality: The UE with a greater average SINR has a higher priority in scheduling.
Weight factor: The UE with a greater weight factor value has a higher priority in scheduling. When services with more than one QCI are running on a UE, the weight factor of the QCI with the highest priority is used. The weight factor has no impact on the priority of the GBR services. In the priority calculation of non-GBR services, the weight factor is determined by the UlschGammaQci parameter, which supports the extension QCI.
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LTE Scheduling Feature
Uplink virtual MIMO is also called uplink multiple-UE MIMO. In virtual MIMO pairing, the eNodeB schedules two single-antenna UEs to transmit data at the same time-andfrequency resources. UEs are scheduled flexibly through the pairing strategy, and appropriate UEs are selected for pairing transmission. The virtual MIMO pairing can be enabled or disabled through the UlVmimoSw parameter.
The virtual MIMO pairing has requirements on the UE bandwidth, channel quality, and amount of data to be transmitted. The virtual MIMO pairing is performed when these requirements are met. When the shortest time of the virtual MIMO pairing is met, the eNodeB determines whether to disable the virtual MIMO pairing in each TTI. If the virtual MIMO pairing is not disabled, the pairing continues. The virtual MIMO pairing is disabled in the following cases:
The channel spectral efficiency does not meet the requirements of pairing .
There are conflicts with the TTI bundling retransmissions.
If there is a conflict with semi-persistent scheduling, the pairing is not performed in the current TTI. The pairing can be continued in the next TTI.
If there is a conflict with the retransmission, the pairing is not performed in the current TTI. The pairing can be continued in the next TTI.
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LTE Scheduling Feature
In uplink scheduling, TTI bundling can be used to improve the transmission quality when the UE channel quality is poor or the Transmit (TX) power is limited (such as at the cell edge). TTI bundling refers to a scenario where a single transport block is coded and transmitted in a set of consecutive subframes. The bundled subframes are handled as one unit. Thus, signaling overhead can be reduced. The TtiBundlingSwitch parameter is used to enable or disable the TTI bundling.
In Huawei eNodeB, the TTI bundling size is fixed to four subframes. Users can transmit the same data in the four subframes. If the retransmission is required for the data transmitted through TTI bundling, the retransmission is also a type of TTI bundling. Accordingly, the retransmission interval changes and the number of Hybrid Automatic Repeat Request (HARQ) progresses decreases. In the FDD system, the retransmission interval is changed from 8 TTIs to 16 TTIs
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LTE Scheduling Feature
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