This document describes the basic functioning of the 5G MAC scheduler, describes the periodic output, and explains out the various configuration options that influence its behavior.

[[TOC]]

General

The 5G MAC scheduler is a proportional fair (PF) scheduler, “approximating wide-band CQI” (for lack of a better term, but CQI is typically used for PF) through the selection of an MCS to use.

Concretely, the scheduler loops through all UEs and calculates the PF coefficient using the currently selected MCS, and the historical achieved rate. The UE with highest coefficient wins and is scheduled RBs until all resources are used, or until it has no more data to fill RBs, in which the scheduler continues with the UE with the second-best coefficient.

UEs with retransmissions are allocated first; similarly, UEs that have not been scheduled for some time in UL are scheduled automatically in UL and have therefore priority over data with “normal” traffic.

The MCS selection is done in get_mcs_from_bler() in file gNB_scheduler_primitives.c. It consider two thresholds for a “BLER” that is computed from the number of first-round retransmissions over total transmissions in the last window (50ms). If that ratio is higher than an “upper” threshold (see dl/ul_bler_target_upper in the configuration section below), it is interpreted as “bad channel” and MCS is decremented by 1. If the ratio is lower than a “lower” threshold (see dl/ul_bler_target_lower), it is interpreted as “good channel” and MCS is incremented by 1. This happens each window.

The actual scheduler implementation can be found in functions pf_dl() and pf_ul() in files gNB_scheduler_dlsch.c (for DL) and gNB_scheduler_ulsch.c (for UL), respectively.

PDCCH aggregation level

PDCCH aggregation level is selected using closed loop controller, where DL HARQ feedback is the controller feedback signal. It is used to increment pdcch_cl_adjust variable if no feedback is detected and decrement the variable when feedback is detected. pdcch_cl_adjust is later mapped to the PDCCH aggregation level range.

The value of pdcch_cl_adjust is clamped to range <0,1>, the increment value is 0.05 while the decrement value is 0.01. These values are selected to ensure PDCCH success rate is high. See Examples below for futher explaination.

The possible values of aggregation level on UE SS can be configured via uess_agg_levels configuration option. By default the gNB uses two candidates in aggregation level 2 which translates to uess_agg_levels set to [0, 2, 0, 0, 0]. For example, to enable one candidate on aggregation levels 2 and 4 set uess_agg_levels to [0, 1, 1, 0, 0].

Examples:

Example 1:

Say we have 90% PDCCH success rate at aggregation level 1, pdcch_cl_adjust will stay at 0 for most of the time. 2 consecutive PDCCH failures will not result in increasing the aggregation level (because (0.05 + 0.05) * 4 = 0.4 which is closer to 0 than to 1). If PDCCH fails 3 times in a row the aggregation level will change to 2 and hopefully back to 1 once more PDCCH successes happen.

Example 2

Say we have 0% PDCCH success rate (radio link failure scenario) but pdcch_cl_adjust is 0 indicating perfect PDCCH channel. it would take ~18 PDCCH failures to reach maximum aggregation level.

Periodic output and interpretation

The scheduler periodically outputs statistics that can help you judge the radio channel quality, i.e., why a UE does not perform as you would expect. The scheduler outputs this info in log level “INFO”. The same information is also available in the the file nrMAC-stats.log in the same directory in which nr-softmodem is run.

Example:

UE RNTI 2460 CU-UE-ID 2 in-sync PH 28 dB PCMAX 24 dBm, average RSRP -74 (8 meas), average SINR 40.0 (32 meas)
UE 2460: CQI 15, RI 2, PMI (14,1)
UE 2460: UL-RI 2 TPMI 0
UE 2460: dlsch_rounds 32917/5113/1504/560, dlsch_errors 211, pucch0_DTX 1385, BLER 0.19557 MCS (1) 23 CCE fail 3
UE 2460: ulsch_rounds 3756/353/182/179, ulsch_errors 170, ulsch_DTX 285, BLER 0.33021 MCS (1) 27 (Qm 8  dB) NPRB 5  SNR 31.0 dB CCE fail 0
UE 2460: MAC:    TX     1530943191 RX         194148 bytes
UE 2460: LCID 1: TX            651 RX           3031 bytes
UE 2460: LCID 2: TX              0 RX              0 bytes
UE 2460: LCID 4: TX     1526169592 RX          16152 bytes

In the first line, * UE RNTI (here 2460): this is also used as the DU UE ID over F1; each line is prepended with the RNTI * CU UE ID (2): separate identifier from the RNTI to handle multiple UEs across DUs (in which case RNTI conflicts are possible) * whether a UE is in-sync (actively being scheduled) or out-of-sync (the UE is not being scheduled, because it did not respond when being scheduled in UL or since it has radio-link failure * PH (28): Power Headroom, the amount of power the UE has left. If it is > 40 you can achieve full UL throughput. PCMAX (24 dBm) is what the UE reported as maximum UL transmit power it can output in the channel. * RSRP (-74): measured power of the DL reference signals at the UE. >-80dBm you should have full DL throughput. <-95 dBm, you are very limited in terms of connectivity. * SINR (40.0): measured signal to interference and noise ratio of the SSB received at the UE. Maximum value that can be reported by the UE is 40.0 dB.

The second and third line reflect channel state information (CSI) as reported by the UE, and only appear if CSI-RS/SRS are enabled and received (for some bands, they cannot be enabled): * CQI (15): the channel quality indicator is a number between 0 and 15. It indicates the achievable spectral efficiency of the UE. 15 means highest, 0 is lowest. This corresponds to a 4-bit table in 38.214 with actual spectral efficiencies in bits/s/Hz * RI: Rank indicator 1-4. If 1 it means limited to 1 layer transmission on DL. 2 two layers, etc. * PMI: precoding matrix indicator. It’s a measure of the spatial-direction of the gNBs transmit array as seen by the UE. It indicates the precoding that the gNB applies. It should be more or less stationary unless the UE is moving around quickly. It can jump when objects move around the UE * UL-RI, TPMI: same as DL.

The fourth and fifth line show HARQ-related information: * dlsch_rounds A/B/C/D (32917/5113/1504/560). This is the number of transmissions by the gNB for each round of the HARQ protocol. A is the first round, B is the second, etc. If A is high and B is low, C is lower and D is around 0, then things work well. If B is around the same as A, then the first round is almost always in error. This is not usually good unless the UE is far from the gNB. * dlsch_errors (211) is the number of errors, meaning that after 4 transmissions the MAC transport block was not received by the UE. A high number is not good/it should be a very low number, again unless there are severe radio conditions. * pucch0_DTX: this is the number of PUCCH format 0 (or 1 later) missed detections (unreceived ack/nak). This means that either the UL is very bad and ACK/NAK cannot be conveyed properly or DL DCIs are missed by the UE. This is also something that should be very small compared to A in dlsch_rounds. * DLSCH BLER is the current measured block-error rate of the DLSCH. Basically a moving average of B/A in dlsch_rounds. This is something that should always be close to the target bler that the MAC scheduler uses. typically 10-30% if we want a high throughput scheduling policy. * MCS (Q) M: M (0-28) is the current MCS used by the MAC scheduler and Q is the mcs table: 0=64QAM, 1=256QAM, 2=low SE table for URLLC * ulsch_rounds/ulsch_errors: same as DLSCH but for UL. * ulsch_DTX (285): number of PUSCH missed detection (i.e. signal energy below threshold in configuration file, L1s.pusch_dtx_threshold, which is 10 times the actual unnormalized digital signal level). This is an indication of either very poor radio conditions (UE cannot reach the gNB), power control problems, or missed DCI 0_x DCIs. It should be low compared to A in ulsch_rounds when things are working properly. * ULSCH BLER/MCS: same as DLSCH but for UL. * ULSCH Qm X deltaMCS Y dB: modulation order: 2=QPSK, 4=16QAM, 6=64QAM, 8=256QAM and the dB offset for deltaMCS component in PUSCH power control law. If deltaMCS is disabled (this is the default) then it indicates 0. When deltaMCS is enabled it indicates the current power offset applied by the UE on the UL corresponding to the scheduled MCS * ULSCH NPRB: current number of PRBs scheduled by the gNB. This will fluctuate a lot but when doing high throughput with iperf, it indicates the number of PRBs that the UE is actually able to use with its power budget and should be high. * ULSCH SNR: the current SNR that the gNB receives the UE signal with. It should be close to the target in the gNB configuration file, pusch_TargetSNRx10, which should be around 10 times the value shown in the log * Both ULSCH/DLSCH CCE fail: lists the number of failed CCE attempts. If this number gets high, it signifies that the scheduler tried to scheduled this UE, but could not allocate the DCI.

In the last lines: * MAC shows the amount of MAC PDU bytes scheduled in transmit (TX, 1530943191) and receive (RX, 194148) directions * LCID X shows the amount of MAC SDU/RLC PDU data for Logical Channel ID with ID X in transmit and receive directions. LCIDs 1 and 2 are mapped to SRBs 1 and 2. LCIDs 4 and onward are mapped to DRBs 1 onward. If you have an LCID 4, it means you have a PDU session.

Configuration of the MAC

Split-related options (running in a DU)

See nFAPI documentation or Aerial tutorial for information about the (n)FAPI split.

See F1 documentation for information about the F1 split.

MAC scheduler-related configuration options

The following options have an influence on the MAC scheduler operation (for all UEs, if applicable), either on the MAC scheduler operation directly or how a UE is configured.

In the MACRLCs section of the gNB/DU configuration file:

• ulsch_max_frame_inactivity (default 10): number of frames before a UE is scheduled automatically in UL. Lowering can improve latency at the expense of more resources in idle (which limits the number of UEs, and increases UE power consumption)
• pusch_TargetSNRx10 (default 200): target SNR in PUSCH times 10 (200 corresponds to 20dB target SNR)
• pucch_TargetSNRx10 (default 150): target SNR in PUCCH times 10 (as above)
• ul_prbblack_SNR_threshold (default 10): target SNR for disabling RB scheduling in uplink on affected RBs.
• pucch_FailureThres (default 10): number of DTX on PUCCH after which scheduler declares UE in radio link failure and moves it to “out-of-sync state”. Currently not used.
• pusch_FailureThres (default 10): number of DTX on PUSCH after which scheduler declares UE in radio link failure and moves it to “out-of-sync state”
• dl_bler_target_upper (default 0.15): upper threshold of BLER (first round retransmission over initial transmission) to decrease MCS by 1
• dl_bler_target_lower (default 0.05): lower threshold of BLER (first round retransmission over initial transmission) to increase MCS by 1
• dl_min_mcs (default 0): minimum MCS to use for any UE
• dl_max_mcs (default 28): maximum MCS to use for any UE
• ul_bler_target_upper (default 0.15): as dl_bler_target_upper
• ul_bler_target_lower (default 0.05): as dl_bler_target_lower
• ul_min_mcs (default 0): as dl_min_mcs
• ul_max_mcs (default 28): as dl_max_mcs
• dl_harq_round_max (default 4): maximum number of HARQ rounds, i.e., retransmissions to perform, in DL
• ul_harq_round_max (default 4): as dl_harq_round_max
• min_grant_prb (default 5): number of PRBs to schedule for UE after activity (see ulsch_max_frame_inactivity) or after scheduling request (SR)
• identity_precoding_matrix (default 0=false): flag to enable to use only the identity precoding matrix in DL precoding
• set_analog_beamforming (default 0=false): flag to enable analog beamforming (for more information analog_beamforming.md)
• beam_duration (default 1): duration/number of consecutive slots for a given set of beams, depending on hardware switching performance
• beams_per_period (default 1): set of beams that can be simultaneously allocated in a period (beam_duration)
• pusch_RSSI_Threshold: Value between -1280 and 0 which maps to range from -128.0 dBm/dBFS to 0.0 dBm/dBFS. This limits PUSCH TPC commands in case RSSI reaches the threshold and prevents ADC railing. Unit depends on RSSI reporting config.
• pucch_RSSI_Threshold: Same as above but for PUCCH
• stats_max_ue (default 8): maximum number of UEs to show in periodical stats; beyond this number, periodical statistics will be disabled (it can still be seen in nrMAC_stats.log. Use 0 to disable periodical stats.

In the gNBs section of the gNB/DU configuration file: some of the parameters affect RRC configuration (CellGroupConfig) of a UE, and are therefore listed here, also they pertain to the DU, i.e., the scheduler. Note also that some SIBs are configured at the DU and some at the CU; please consult the RRC configuration as well for SIB configuration.

• pdsch_AntennaPorts_XP (default 1): number of XP logical antenna ports in PDSCH (see also RUNMODEM.md)
• pdsch_AntennaPorts_N1 (default 1): number of horizontal logical antenna ports in PDSCH (see also RUNMODEM.md)
• pdsch_AntennaPorts_N2 (default 1): number of vertical logical antenna ports in PDSCH (see also RUNMODEM.md)
• pusch_AntennaPorts (default 1): number of antenna ports in PUSCH
• maxMIMO_layers (default -1=unlimited): maximum number of MIMO layers to use in downlink
• do_CSIRS (default 0): flag whether to use channel-state information reference signal (CSI-RS)
• do_SRS (default 0): flag whether to use sounding reference signal (SRS)
• CSI_report_type (default ssb_rsrp): parameter to enable different CSI reporting (options: ssb_rsrp, ssb_sinr and cri_rsrp) Default setting of CSI reporting quantity is SSB-RSRP.
• min_rxtxtime (default 2): minimum feedback time for UE to respond to transmissions (k1 and k2 in 3GPP spec)
• ul_prbblacklist: PRBs that should not be used for UL scheduling. Cf with parameter ul_prbblack_SNR_threshold that does a similar thing based on measurements
• force_256qam_off (default 0=false): flag whether to disable 256QAM (limit to 64QAM) in DL
• force_UL256qam_off (default 0=false): flag whether to disable 256QAM (limit to 64QAM) in DL
• disable_harq (default 0=false): flag whether to disable HARQ completely (useful for NTN operation, see <../RUNMODEM.md>). this is a Rel-17 feature and you need to have a capable UE for this
• use_deltaMCS (default 0=false): flag whether to enable deltaMCS (this is not fully tested and might not work and you might need to adjust other parameters such as target SNRs)
• num_dlharq (default 16): number of HARQ processes to use in DL (other valid options are 2, 4, 6, 8, 10, 12, 32; 32 is a Rel-17 features)
• num_ulharq (default 16): as num_dlharq for UL (other valid option is 32; 32 is i Rel-17 feature)
• du_sibs (default []): list of SIBs to transmit in the cell. Currently, SIB19 (for NTN) is supported.

DL MIMO	do_CSIRS	CSI_report_type	CSI report Quantity
any	any	ssb_rsrp	SSB-RSRP
any	0	cri_rsrp	SSB-RSRP (no CSI-RS configured)
any	1	cri_rsrp	CRI-RSRP
any	any	ssb_sinr	SSB-SINR
ON (pdsch_AntennaPorts > 1)	1	any	cri-RI-PMI-CQI

Note that activating cri-RI-PMI-CQI will result in that report to be produced in addition to either SSB-SINR, SSB-RSRP or CRI-RSRP. DL-MIMO is configured using following parameters: pdsch_AntennaPorts_XP , pdsch_AntennaPorts_N1 , pdsch_AntennaPorts_N2, maxMIMO_layers (see also RUNMODEM.md)

ServingCellConfigCommon parameters

The gNBs configuration section has a big structure servingCellConfigCommon that has an influence on the overall behavior of MAC and L1. As the name says, this structure is a flat representation of the ServingCellConfigCommon structure, specified by 3GPP. Describing all of these parameters would be too exhaustive; more information about the individual fields can be found in 3GPP TS 38.331, section 6.3.2 “Radio resource control information elements”.

Below is a description of some of these parameters.

Frequency configuration

There are many parameters, such as absoluteFrequencySSB, etc., that have an impact on the frequency used by the gNB. For more information, please check the corresponding document.

TDD pattern configuration

The TDD configuration parameters allow to use one or two TDD patterns.

Single TDD pattern

Configure the TDD pattern through these options: - dl_UL_TransmissionPeriodicity: Refers to the UL/DL slots periodicity for the TDD pattern. See below for valid numbers. - nrofDownlinkSlots: Refers to the number of consecutive DL slots in the TDD pattern. The number of DL slots depends on the TDD period. - nrofDownlinkSymbols: Indicates the number of consecutive DL symbols within the special slot that follows the downlink slots in the TDD pattern. The special slot is needed only to switch from DL to UL. The maximum number of symbols in this slot is 14. - nrofUplinkSlots: Refers to the number of consecutive UL slots in the TDD pattern, depending on the desired TDD period. - nrofUplinkSymbols: Indicates the number of consecutive UL symbols within the special slot that follows the downlink slots in the TDD pattern. The sum of downlink and uplink symbols should not exceed 14. - prach_ConfigurationIndex: index for PRACH according to tables 6.3.3.2-2 to 6.3.3.2-4 in TS 38.211.

As an example, the below figure shows a single TDD pattern, consisting of 3 DL slots, 1 mixed slots (with 10 DL, 2 guard, 2 UL symbols), and 1 UL slot.

To configure this pattern in the configuration file, use

#dl_UL_TransmissionPeriodicity  0=ms0p5, 1=ms0p625, 2=ms1, 3=ms1p25, 4=ms2, 5=ms2p5, 6=ms5, 7=ms10, 8=ms3, 9=ms4

dl_UL_TransmissionPeriodicity = 5;
nrofDownlinkSlots             = 3;
nrofDownlinkSymbols           = 10;
nrofUplinkSlots               = 1;
nrofUplinkSymbols             = 2;

The dl_UL_TransmissionPeriodicity is set to 5 (2.5ms). The above figure shows two TDD periods over 5ms. The 10 ms frame period must be strictly divisible by the sum of the TDD pattern periods.

Two TDD patterns

An optional pattern2 structure is used to signal a TDD pattern 2. In this case, the TDD pattern may have two extended values of 3 ms and 4 ms. These values are standardized as dl-UL-TransmissionPeriodicity-v1530. For the sake of simplicity of the gNB configuration file, we have extended the set of dl-UL-TransmissionPeriodicity values to support the new TDD periods, as explained in the table below. However, these values will later be encoded as dl-UL-TransmissionPeriodicity-v1530 to match the specifications.

dl_UL_TransmissionPeriodicity	TDD period
0	0.5 ms
1	0.625 ms
2	1 ms
3	1.25 ms
4	2 ms
5	2.5 ms
6	5 ms
7	10 ms
8	3 ms
9	4 ms

The 10 ms frame period must be strictly divisible by the sum of the TDD pattern periods. For example, the 3 ms TDD pattern should be used with a second TDD pattern of 2 ms. Additionally, the 4 ms TDD pattern should be used with a second TDD pattern of 1 ms.

As an example, this configuration might be used, where the first TDD pattern is followed by a second, consisting of 4 DL slots.

# pattern1
# dl_UL_TransmissionPeriodicity
# 0=ms0p5, 1=ms0p625, 2=ms1, 3=ms1p25, 4=ms2, 5=ms2p5, 6=ms5, 7=ms10
# ext: 8=ms3, 9=ms4
dl_UL_TransmissionPeriodicity  = 8;
nrofDownlinkSlots              = 3;
nrofDownlinkSymbols            = 6;
nrofUplinkSlots                = 2;
nrofUplinkSymbols              = 4;

# pattern2
pattern2: {
    dl_UL_TransmissionPeriodicity2 = 4;
    nrofDownlinkSlots2             = 4;
    nrofDownlinkSymbols2           = 0;
    nrofUplinkSlots2               = 0;
    nrofUplinkSymbols2             = 0;
};

UL-heavy TDD patterns

“UL-heavy TDD patterns”, i.e., TDD patterns that have many UL slots are supported. Examples for such patterns would be DSUUU or DDDSUUUUUU.

Note that you should increase the aggregation level candidates as described in the corresponding section above. This is because the scheduler has to schedule multiple DCIs in a single DL slots for multiple UL slots. As a suggestion, you could try uess_agg_levels = [4, 2, 2, 0, 0].

Multiple Dedicated BWPs

A maximum of 4 dedicated BWPs can be configured for a UE per standard, but only 1 BWP can be active in UL and DL direction at a given time. In the code we only configure a single BWP for the UE at a given time and we would switch by reconfiguring this BWP. All this procedure is transparent for users and LOGs mark BWP switching according to the configuration file enumeration. It is possible to configure multiple dedicated BWPs and 1st active BWP via configuration file.

Setup of the Configuration files

In the configuration file you have the option to select the 1st active BWP, the BWP location and SCS of each BWP in the following way (example with 2 additional BWPs):

    first_active_bwp = 1;
    bwp_list = ({ scs = 1; bwpStart = 0; bwpSize = 106;},
                { scs = 1; bwpStart = 0; bwpSize = 24;});

This example configures 3 additional BWPs, with IDs from 1 to 3. A similar example can be found in configuration file ci-scripts/conf_files/gnb-du.sa.band78.106prb.usrpb200.conf tested in CI.