openairinterface5g/doc/oru-design.md

O-RU solution design

Overview

OAI O-RU shall implement a set of O-RU CAT-A and CAT-B on COTS hardware.

Procedure requirements for CAT-A

The procedures executed by the O-RU are defined in O-RAN.WG4.TS.CUS Section 4.2.1

DL procedures

For downlink, the O-RU processing chain is as follows:

1. Receive frequency domain IQ data from DU
2. Symbol rotation (if configured)
3. DFT
4. Transmission

PRACH procedures

TBD

PUSCH procedures

TBD

SRS procedures

TBD

Procedure requirements for CAT-B

TBD

Architecture

O-RU shall have two main threads are north_read and south_read. The threads shall read IQ data from the respective interfaces. The north interface is handled by xran and is a frequency domain interface The south interface is a split8 time domain device.

There are two processing triggers in the O-RU process:

• oran_tx_read - used to indicate to the application that the downlink UP data is available in xran buffers. Triggers DL FH processing and transmission.
• trx_read_func - indicates to the application that samples are available to on the south. Triggers PRACH, SRS and PUSCH FH processing.

North interface

The north interface shall be handles by using OAI split 7.2 library, which uses xran library.

Changes to xran adapter

The OAI xran adapter library has to be modified to adapt ot for usage with the O-RU. The existing function signatures which are used by the O-DU do not fulfill the processing time requirements of O-RU.

The proposed solution is to add a new set of sub-slot APIs to OAI xran adapter. This would ensure on-time transmission and delivery of frequency-domain IQ to xran library buffers.

Timing requirements

The timing requirements on O-RU processing come from the ORAN specification O-RAN.WG4.TS.CUS More directly the standard is implemented by the xran library used in OAI split7.2 adapter (radio/fhi_72) which shall be used by O-RU.

Timing requirements for DL

The main requirement for DL comes in the form of a timer: T1a_up min, max. The minimum value is the downlink UP reception window closing time. This value is given in uS before OTA. Therefore the downlink procedures must complete within this minimum timer value.

An additional requirement comes from the time domain split8 device. This requirement is due to the device requiring the samples to be provided before transmission.

Timing requirements for PRACH/PUSCH/SRS

TBD

Synchronization

xran library provides the main timing callback in the form of downlink UP reception window closing time. This infromation is received in north_read thread and should be communicated to south_read thread in order to determine slot and frame start sample. From there the south_read follows this synchronization information to produce samples for each slot and symbol.

Thread structure

The O-RU shall use threadPool and/or actors to parallelize the workload. The exact details are up to the implementation.

Actors are dedicated threads for a single job.

TheadPool is a pool of threads that can be shared for different tasks.

The OAI threadpool implementation disallows usage from another threadPool thread. Therefore if using threadpool, another form of parallelism is required.

DL

Each job shall process a subset of symbols in a given slot. The symbols processed by each job depend on the sense_of_time_t struct returned by xran call.

UL

UL should parallelize each task. This is due to the fact that south_read thread needs to poll the south interface for new IQ samples continuously.

PRACH

Each job should process at least 1 PRACH occasion. The job shall be triggered from south_read thread when the samples for the PRACH occasion are received

PUSCH

Each job should process a subset of RX antennas. The job shall be triggered from south_read thread once samples for at least one symbol are received.

Configuration

The O-RU shall use OAIs configuration subsystem and have its own configuration section. The changes to existing sections shall be kept to a minimum

Configuration elements required:

name	type	comment
phase_compensation	bool	Whether to perform symbol rotation.
prach_config_index	int	PRACH configuration index.