OEO100040 LTE Scheduling Feature and Parameters ISSUE1.00.pdf

LTE Scheduling Feature

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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

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.



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.



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.



Scheduling is a very complicated algorithm that involve a lot of input parameters, as shown

in the above figure.



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. Semi-

persistent 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.



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.



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.





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





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.



VoIP service

The VoIP service experiencing semi-persistent scheduling has the highest priority.

Semi-persistent scheduling is used in the talk spurts of the VoIP services.

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

HARQ retransmissions of Huawei eNodeB are classified into 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.



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 semi-

persistent 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



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.



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.



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 priority of the 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.



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.



Eliminate the following UEs that do not need 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



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.



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.



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.



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.



The CQI adjustment algorithm is enabled or disabled through the CqiAdjAlgoSwitch

parameter.





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 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.



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 retransmissions 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.



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.



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 semi-

persistent scheduling, it can use dynamic scheduling with the same priority as other GBR

services.



Uplink retransmissions use the synchronous non-adaptive HARQ retransmission. 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.



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 transmission. In case of 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.



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.



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.



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.



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-and-

frequency 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.



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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OEO100040 LTE Scheduling Feature and Parameters ISSUE1.00.pdf