Robert Schmidt
a8de9733e6
Refactor DLSCH scheduler Refactor the DLSCH post processor similarly to what has been done in !3521 with the goal of saving an additional iteration through all UEs, hopefully making the DLSCH scheduler faster. It further includes changes to make working with and debugging multi-UE operation easier: - add some measurements in L1 (for total TX/RX time) - a new option MACRLCs.[0].stats_max_ue to limit the number of UEs in the periodic output (default: 8, 0=disable) - avoid stack overflow in the MAC - fix the writing of stats to nrMAC_stats.log and nrL1_stats.log when the written output reduces by truncating the files - avoid to schedule retransmission with 0 RBs.
423 lines
22 KiB
Markdown
423 lines
22 KiB
Markdown
This document describes the basic functioning of the 5G MAC scheduler,
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describes the periodic output, and explains
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out the various configuration options that influence its behavior.
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[[_TOC_]]
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# General
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The 5G MAC scheduler is a proportional fair (PF) scheduler, "approximating
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wide-band CQI" (for lack of a better term, but CQI is typically used for PF)
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through the selection of an MCS to use.
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Concretely, the scheduler loops through all UEs and calculates the PF
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coefficient using the currently selected MCS, and the historical achieved rate.
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The UE with highest coefficient wins and is scheduled RBs until all resources
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are used, or until it has no more data to fill RBs, in which the scheduler
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continues with the UE with the second-best coefficient.
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UEs with retransmissions are allocated first; similarly, UEs that have not been
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scheduled for some time in UL are scheduled automatically in UL and have
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therefore priority over data with "normal" traffic.
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The MCS selection is done in `get_mcs_from_bler()` in file
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[`gNB_scheduler_primitives.c`](../../openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_primitives.c).
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It consider two thresholds for a "BLER" that is computed from the number of
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first-round retransmissions over total transmissions in the last window (50ms).
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If that ratio is higher than an "upper" threshold (see
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`dl/ul_bler_target_upper` in the configuration section below), it is
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interpreted as "bad channel" and MCS is decremented by 1. If the ratio is
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lower than a "lower" threshold (see `dl/ul_bler_target_lower`), it is
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interpreted as "good channel" and MCS is incremented by 1. This happens each
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window.
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The actual scheduler implementation can be found in functions `pf_dl()` and
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`pf_ul()` in files
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[`gNB_scheduler_dlsch.c`](../../openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_dlsch.c)
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(for DL) and
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[`gNB_scheduler_ulsch.c`](../../openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_ulsch.c)
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(for UL), respectively.
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## PDCCH aggregation level
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PDCCH aggregation level is selected using closed loop controller, where DL HARQ
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feedback is the controller feedback signal. It is used to increment `pdcch_cl_adjust`
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variable if no feedback is detected and decrement the variable when feedback is detected.
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`pdcch_cl_adjust` is later mapped to the PDCCH aggregation level range.
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The value of `pdcch_cl_adjust` is clamped to range <0,1>, the increment value is 0.05 while
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the decrement value is 0.01. These values are selected to ensure PDCCH success rate is high.
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See Examples below for futher explaination.
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The possible values of aggregation level on UE SS can be configured via
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`uess_agg_levels` configuration option. By default the gNB uses two candidates
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in aggregation level 2 which translates to `uess_agg_levels` set to `[0, 2, 0,
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0, 0]`. For example, to enable one candidate on aggregation levels 2 and 4 set
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`uess_agg_levels` to `[0, 1, 1, 0, 0]`.
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### Examples:
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#### Example 1:
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Say we have 90% PDCCH success rate at aggregation level 1, `pdcch_cl_adjust` will stay at 0
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for most of the time. 2 consecutive PDCCH failures will not result in increasing the aggregation
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level (because (0.05 + 0.05) * 4 = 0.4 which is closer to 0 than to 1). If PDCCH fails 3 times
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in a row the aggregation level will change to 2 and hopefully back to 1 once more PDCCH successes
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happen.
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### Example 2
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Say we have 0% PDCCH success rate (radio link failure scenario) but `pdcch_cl_adjust` is 0 indicating
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perfect PDCCH channel. it would take ~18 PDCCH failures to reach maximum aggregation level.
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# Periodic output and interpretation
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The scheduler periodically outputs statistics that can help you judge the radio
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channel quality, i.e., why a UE does not perform as you would expect. The
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scheduler outputs this info in log level "INFO". The same information is also
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available in the the file `nrMAC-stats.log` in the same directory in which
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`nr-softmodem` is run.
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Example:
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```
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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)
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UE 2460: CQI 15, RI 2, PMI (14,1)
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UE 2460: UL-RI 2 TPMI 0
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UE 2460: dlsch_rounds 32917/5113/1504/560, dlsch_errors 211, pucch0_DTX 1385, BLER 0.19557 MCS (1) 23 CCE fail 3
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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
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UE 2460: MAC: TX 1530943191 RX 194148 bytes
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UE 2460: LCID 1: TX 651 RX 3031 bytes
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UE 2460: LCID 2: TX 0 RX 0 bytes
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UE 2460: LCID 4: TX 1526169592 RX 16152 bytes
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```
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In the first line,
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* `UE RNTI` (here `2460`): this is also used as the DU UE ID over F1; each line
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is prepended with the RNTI
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* `CU UE ID` (`2`): separate identifier from the RNTI to handle multiple
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UEs across DUs (in which case RNTI conflicts are possible)
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* whether a UE is `in-sync` (actively being scheduled) or `out-of-sync` (the UE
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is not being scheduled, because it did not respond when being scheduled in UL
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or since it has radio-link failure
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* `PH` (`28`): Power Headroom, the amount of power the UE has left. If it is > 40 you
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can achieve full UL throughput. `PCMAX` (`24 dBm`) is what the UE reported as
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maximum UL transmit power it can output in the channel.
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* `RSRP` (`-74`): measured power of the DL reference signals at the UE. >-80dBm
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you should have full DL throughput. <-95 dBm, you are very limited in terms
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of connectivity.
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* `SINR` (`40.0`): measured signal to interference and noise ratio of the SSB
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received at the UE. Maximum value that can be reported by the UE is 40.0 dB.
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The second and third line reflect channel state information (CSI) as
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reported by the UE, and only appear if CSI-RS/SRS are enabled and _received_
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(for some bands, they cannot be enabled):
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* `CQI` (`15`): the channel quality indicator is a number between 0 and 15. It
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indicates the achievable spectral efficiency of the UE. 15 means highest, 0
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is lowest. This corresponds to a 4-bit table in 38.214 with actual spectral
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efficiencies in bits/s/Hz
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* `RI`: Rank indicator 1-4. If 1 it means limited to 1 layer transmission on DL. 2 two layers, etc.
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* `PMI`: precoding matrix indicator. It's a measure of the spatial-direction of
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the gNBs transmit array as seen by the UE. It indicates the precoding that
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the gNB applies. It should be more or less stationary unless the UE is moving
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around quickly. It can jump when objects move around the UE
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* `UL-RI`, `TPMI`: same as DL.
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The fourth and fifth line show HARQ-related information:
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* `dlsch_rounds A/B/C/D` (`32917/5113/1504/560`). This is the number of
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transmissions by the gNB for each round of the HARQ protocol. `A` is the first
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round, B is the second, etc. If `A` is high and `B` is low, `C` is lower and `D` is
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around 0, then things work well. If `B` is around the same as `A`, then the first
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round is almost always in error. This is not usually good unless the UE is
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far from the gNB.
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* `dlsch_errors` (`211`) is the number of errors, meaning that after 4
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transmissions the MAC transport block was not received by the UE. A high
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number is not good/it should be a very low number, again unless there are
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severe radio conditions.
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* `pucch0_DTX`: this is the number of PUCCH format 0 (or 1 later) missed
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detections (unreceived ack/nak). This means that either the UL is very bad
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and ACK/NAK cannot be conveyed properly or DL DCIs are missed by the UE. This
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is also something that should be very small compared to `A` in
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`dlsch_rounds`.
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* `DLSCH BLER` is the current measured block-error rate of the DLSCH. Basically
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a moving average of `B`/`A` in `dlsch_rounds`. This is something that should always
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be close to the target bler that the MAC scheduler uses. typically 10-30% if
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we want a high throughput scheduling policy.
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* `MCS (Q) M`: M (0-28) is the current MCS used by the MAC scheduler and Q is
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the mcs table: 0=64QAM, 1=256QAM, 2=low SE table for URLLC
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* `ulsch_rounds`/`ulsch_errors`: same as DLSCH but for UL.
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* `ulsch_DTX` (`285`): number of PUSCH missed detection (i.e. signal energy below
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threshold in configuration file, `L1s.pusch_dtx_threshold`, which is 10 times the
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actual unnormalized digital signal level). This is an indication of either
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very poor radio conditions (UE cannot reach the gNB), power control problems,
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or missed DCI 0\_x DCIs. It should be low compared to `A` in `ulsch_rounds`
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when things are working properly.
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* ULSCH `BLER/MCS`: same as DLSCH but for UL.
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* ULSCH `Qm X deltaMCS Y dB`: modulation order: 2=QPSK, 4=16QAM, 6=64QAM,
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8=256QAM and the dB offset for deltaMCS component in PUSCH power control law.
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If deltaMCS is disabled (this is the default) then it indicates 0. When
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deltaMCS is enabled it indicates the current power offset applied by the UE
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on the UL corresponding to the scheduled MCS
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* ULSCH `NPRB`: current number of PRBs scheduled by the gNB. This will
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fluctuate a lot but when doing high throughput with iperf, it indicates the
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number of PRBs that the UE is actually able to use with its power budget and
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should be high.
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* ULSCH `SNR`: the current SNR that the gNB receives the UE signal with. It
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should be close to the target in the gNB configuration file,
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`pusch_TargetSNRx10`, which should be around 10 times the value shown in the
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log
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* Both ULSCH/DLSCH `CCE fail`: lists the number of failed CCE attempts. If this
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number gets high, it signifies that the scheduler tried to scheduled this UE,
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but could not allocate the DCI.
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In the last lines:
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* `MAC` shows the amount of MAC PDU bytes scheduled in transmit (`TX`,
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`1530943191`) and receive (`RX`, `194148`) directions
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* `LCID X` shows the amount of MAC SDU/RLC PDU data for Logical Channel ID with
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ID `X` in transmit and receive directions. LCIDs 1 and 2 are mapped to SRBs 1
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and 2. LCIDs 4 and onward are mapped to DRBs 1 onward. If you have an LCID 4,
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it means you have a PDU session.
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# Configuration of the MAC
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## Split-related options (running in a DU)
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See [nFAPI documentation](../nfapi.md) or [Aerial
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tutorial](../Aerial_FAPI_Split_Tutorial.md) for information about the (n)FAPI
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split.
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See [F1 documentation](../F1AP/F1-design.md) for information about the F1 split.
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## MAC scheduler-related configuration options
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The following options have an influence on the MAC scheduler operation (for all
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UEs, if applicable), either on the MAC scheduler operation directly or how a UE
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is configured.
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In the `MACRLCs` section of the gNB/DU configuration file:
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* `ulsch_max_frame_inactivity` (default 10): number of frames before a UE is
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scheduled automatically in UL. Lowering can improve latency at the expense of
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more resources in idle (which limits the number of UEs, and increases UE
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power consumption)
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* `pusch_TargetSNRx10` (default 200): target SNR in PUSCH times 10 (200
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corresponds to 20dB target SNR)
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* `pucch_TargetSNRx10` (default 150): target SNR in PUCCH times 10 (as above)
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* `ul_prbblack_SNR_threshold` (default 10): target SNR for disabling RB
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scheduling in uplink on affected RBs.
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* `pucch_FailureThres` (default 10): number of DTX on PUCCH after which
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scheduler declares UE in radio link failure and moves it to "out-of-sync
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state". **Currently not used**.
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* `pusch_FailureThres` (default 10): number of DTX on PUSCH after which
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scheduler declares UE in radio link failure and moves it to "out-of-sync
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state"
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* `dl_bler_target_upper` (default 0.15): upper threshold of BLER (first round
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retransmission over initial transmission) to decrease MCS by 1
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* `dl_bler_target_lower` (default 0.05): lower threshold of BLER (first round
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retransmission over initial transmission) to increase MCS by 1
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* `dl_min_mcs` (default 0): minimum MCS to use for any UE
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* `dl_max_mcs` (default 28): maximum MCS to use for any UE
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* `ul_bler_target_upper` (default 0.15): as `dl_bler_target_upper`
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* `ul_bler_target_lower` (default 0.05): as `dl_bler_target_lower`
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* `ul_min_mcs` (default 0): as `dl_min_mcs`
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* `ul_max_mcs` (default 28): as `dl_max_mcs`
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* `dl_harq_round_max` (default 4): maximum number of HARQ rounds, i.e.,
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retransmissions to perform, in DL
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* `ul_harq_round_max` (default 4): as `dl_harq_round_max`
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* `min_grant_prb` (default 5): number of PRBs to schedule for UE after activity
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(see `ulsch_max_frame_inactivity`) or after scheduling request (SR)
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* `identity_precoding_matrix` (default 0=false): flag to enable to use only
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the identity precoding matrix in DL precoding
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* `set_analog_beamforming` (default 0=false): flag to enable analog
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beamforming (for more information [`analog_beamforming.md`](../analog_beamforming.md))
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* `beam_duration` (default 1): duration/number of consecutive slots for a given set of
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beams, depending on hardware switching performance
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* `beams_per_period` (default 1): set of beams that can be simultaneously allocated in a
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period (`beam_duration`)
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* `pusch_RSSI_Threshold`: Value between -1280 and 0 which maps to range
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from -128.0 dBm/dBFS to 0.0 dBm/dBFS. This limits PUSCH TPC commands in
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case RSSI reaches the threshold and prevents ADC railing. Unit depends on
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RSSI reporting config.
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* `pucch_RSSI_Threshold`: Same as above but for PUCCH
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* `stats_max_ue` (default 8): maximum number of UEs to show in periodical
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stats; beyond this number, periodical statistics will be disabled (it can
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still be seen in `nrMAC_stats.log`. Use `0` to disable periodical stats.
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In the `gNBs` section of the gNB/DU configuration file: some of the parameters
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affect RRC configuration (CellGroupConfig) of a UE, and are therefore listed
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here, *also they pertain to the DU*, i.e., the scheduler. Note also that some
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SIBs are configured at the DU and some at the CU; please consult the [RRC
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configuration](../RRC/rrc-usage.md) as well for SIB configuration.
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* `pdsch_AntennaPorts_XP` (default 1): number of XP logical antenna
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ports in PDSCH (see also [`RUNMODEM.md`](../RUNMODEM.md))
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* `pdsch_AntennaPorts_N1` (default 1): number of horizontal logical antenna
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ports in PDSCH (see also [`RUNMODEM.md`](../RUNMODEM.md))
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* `pdsch_AntennaPorts_N2` (default 1): number of vertical logical antenna
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ports in PDSCH (see also [`RUNMODEM.md`](../RUNMODEM.md))
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* `pusch_AntennaPorts` (default 1): number of antenna ports in PUSCH
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* `maxMIMO_layers` (default -1=unlimited): maximum number of MIMO layers to use
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in downlink
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* `sib1_tda` (default 1): time domain allocation (TDA) indices to use for SIB1
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(38.214 section 5.1.2.1.1)
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* `do_CSIRS` (default 0): flag whether to use channel-state information
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reference signal (CSI-RS)
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* `do_SRS` (default 0): flag whether to use sounding reference signal (SRS)
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* `CSI_report_type` (default `ssb_rsrp`): parameter to enable different CSI reporting (options: `ssb_rsrp`, `ssb_sinr` and `cri_rsrp`)
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Default setting of CSI reporting quantity is SSB-RSRP.
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* `min_rxtxtime` (default 2): minimum feedback time for UE to respond to
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transmissions (k1 and k2 in 3GPP spec)
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* `ul_prbblacklist`: PRBs that should not be used for UL scheduling. Cf with
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parameter `ul_prbblack_SNR_threshold` that does a similar thing based on
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measurements
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* `force_256qam_off` (default 0=false): flag whether to disable 256QAM (limit to
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64QAM) in DL
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* `force_UL256qam_off` (default 0=false): flag whether to disable 256QAM (limit to
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64QAM) in DL
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* `disable_harq` (default 0=false): flag whether to disable HARQ completely
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(useful for NTN operation, see <../RUNMODEM.md>). **this is a Rel-17 feature
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and you need to have a capable UE for this**
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* `use_deltaMCS` (default 0=false): flag whether to enable deltaMCS (**this is not fully tested
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and might not work** and you might need to adjust other parameters such as
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target SNRs)
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* `num_dlharq` (default 16): number of HARQ processes to use in DL (other valid
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options are 2, 4, 6, 8, 10, 12, 32; **32 is a Rel-17 features**)
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* `num_ulharq` (default 16): as `num_dlharq` for UL (other valid option is 32;
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**32 is i Rel-17 feature**)
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- `du_sibs` (default `[]`): list of SIBs to transmit in the cell. Currently,
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SIB19 (for NTN) is supported.
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| DL MIMO |`do_CSIRS`|`CSI_report_type`| CSI report Quantity |
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| ------------------------------ | -------- | --------------- | --------------------------------------------------|
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| any | any | `ssb_rsrp` | SSB-RSRP |
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| any | 0 | `cri_rsrp` | SSB-RSRP (no CSI-RS configured) |
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| any | 1 | `cri_rsrp` | CRI-RSRP |
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| any | any | `ssb_sinr` | SSB-SINR |
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| ON (`pdsch_AntennaPorts` > 1) | 1 | any | cri-RI-PMI-CQI |
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Note that activating `cri-RI-PMI-CQI` will result in that report to be produced
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in addition to either `SSB-SINR`, `SSB-RSRP` or `CRI-RSRP`.
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DL-MIMO is configured using following parameters:
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`pdsch_AntennaPorts_XP` , `pdsch_AntennaPorts_N1` , `pdsch_AntennaPorts_N2`, `maxMIMO_layers`
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(see also [`RUNMODEM.md`](../RUNMODEM.md))
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## ServingCellConfigCommon parameters
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The `gNBs` configuration section has a big structure `servingCellConfigCommon`
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that has an influence on the overall behavior of MAC and L1. As the name says,
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this structure is a flat representation of the ServingCellConfigCommon
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structure, specified by 3GPP. Describing all of these parameters would be too
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exhaustive; more information about the individual fields can be found in 3GPP
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TS 38.331, section 6.3.2 "Radio resource control information elements".
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Below is a description of some of these parameters.
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### Frequency configuration
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There are many parameters, such as `absoluteFrequencySSB`, etc., that have an
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impact on the frequency used by the gNB. For more information, please check the
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[corresponding document](../gNB_frequency_setup.md).
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### TDD pattern configuration
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The TDD configuration parameters allow to use one or two TDD patterns.
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#### Single TDD pattern
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Configure the TDD pattern through these options:
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- `dl_UL_TransmissionPeriodicity`: Refers to the UL/DL slots periodicity for
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the TDD pattern. See below for valid numbers.
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- `nrofDownlinkSlots`: Refers to the number of consecutive DL slots in the TDD
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pattern. The number of DL slots depends on the TDD period.
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- `nrofDownlinkSymbols`: Indicates the number of consecutive DL symbols within
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the special slot that follows the downlink slots in the TDD pattern. The
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special slot is needed only to switch from DL to UL. The maximum number of
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symbols in this slot is 14.
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- `nrofUplinkSlots`: Refers to the number of consecutive UL slots in the TDD
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pattern, depending on the desired TDD period.
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- `nrofUplinkSymbols`: Indicates the number of consecutive UL symbols within
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the special slot that follows the downlink slots in the TDD pattern. The sum
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of downlink and uplink symbols should not exceed 14.
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- `prach_ConfigurationIndex`: index for PRACH according to tables 6.3.3.2-2 to
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6.3.3.2-4 in TS 38.211.
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As an example, the below figure shows a single TDD pattern, consisting of 3 DL
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slots, 1 mixed slots (with 10 DL, 2 guard, 2 UL symbols), and 1 UL slot.
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To configure this pattern in the configuration file, use
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```plaintext
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#dl_UL_TransmissionPeriodicity 0=ms0p5, 1=ms0p625, 2=ms1, 3=ms1p25, 4=ms2, 5=ms2p5, 6=ms5, 7=ms10, 8=ms3, 9=ms4
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dl_UL_TransmissionPeriodicity = 5;
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nrofDownlinkSlots = 3;
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nrofDownlinkSymbols = 10;
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nrofUplinkSlots = 1;
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nrofUplinkSymbols = 2;
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```
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The `dl_UL_TransmissionPeriodicity` is set to `5` (2.5ms). The above figure
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shows two TDD periods over 5ms. The 10 ms frame period must be strictly
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divisible by the sum of the TDD pattern periods.
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#### Two TDD patterns
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An optional `pattern2` structure is used to signal a TDD pattern 2.
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In this case, the TDD pattern may have two extended values of 3 ms and 4 ms.
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These values are standardized as `dl-UL-TransmissionPeriodicity-v1530`. For the
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sake of simplicity of the gNB configuration file, we have extended the set of
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`dl-UL-TransmissionPeriodicity` values to support the new TDD periods, as
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explained in the table below. However, these values will later be encoded as
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`dl-UL-TransmissionPeriodicity-v1530` to match the specifications.
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| `dl_UL_TransmissionPeriodicity` | TDD period |
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|---------------------------------|------------|
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| 0 | 0.5 ms |
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| 1 | 0.625 ms |
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| 2 | 1 ms |
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| 3 | 1.25 ms |
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| 4 | 2 ms |
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| 5 | 2.5 ms |
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| 6 | 5 ms |
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| 7 | 10 ms |
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| 8 | 3 ms |
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| 9 | 4 ms |
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The 10 ms frame period must be strictly divisible by the sum of the TDD pattern
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periods. For example, the 3 ms TDD pattern should be used with a second TDD
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pattern of 2 ms. Additionally, the 4 ms TDD pattern should be used with a
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second TDD pattern of 1 ms.
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As an example, this configuration might be used, where the first TDD pattern is
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followed by a second, consisting of 4 DL slots.
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```
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# pattern1
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# dl_UL_TransmissionPeriodicity
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# 0=ms0p5, 1=ms0p625, 2=ms1, 3=ms1p25, 4=ms2, 5=ms2p5, 6=ms5, 7=ms10
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# ext: 8=ms3, 9=ms4
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dl_UL_TransmissionPeriodicity = 8;
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nrofDownlinkSlots = 3;
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nrofDownlinkSymbols = 6;
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nrofUplinkSlots = 2;
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nrofUplinkSymbols = 4;
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# pattern2
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pattern2: {
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dl_UL_TransmissionPeriodicity2 = 4;
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nrofDownlinkSlots2 = 4;
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nrofDownlinkSymbols2 = 0;
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nrofUplinkSlots2 = 0;
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nrofUplinkSymbols2 = 0;
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};
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```
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#### UL-heavy TDD patterns
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"UL-heavy TDD patterns", i.e., TDD patterns that have many UL slots are
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supported. Examples for such patterns would be DSUUU or DDDSUUUUUU.
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Note that you should increase the aggregation level candidates as described in
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[the corresponding section above](#pdcch-aggregation-level). This is because the
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scheduler has to schedule multiple DCIs in a single DL slots for multiple UL
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slots. As a suggestion, you could try `uess_agg_levels = [4, 2, 2, 0, 0]`.
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