449 lines
23 KiB
Markdown
449 lines
23 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;
|
|
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](#pdcch-aggregation-level). 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.
|