In this option as shown in Fig below the lower physical layer functions and RF circuits are located in the DU(s). The upper protocol layers including the upper physical layer functions reside in the CU. There are multiple realizations of this option including asymmetrical implementation of the option in the downlink and uplink (e.g., option 7-1 in the uplink and option 7-2 in the downlink). A compression technique may be applied to reduce the required transport bandwidth between the CU and the DU.
This option can to some extent relax the fronthaul throughput requirements and allows centralized scheduling and joint processing in both transmit and receive sides.
Also this option will allow traffic aggregation from NR and LTE transmission points to be centralized and can facilitate the management of traffic load between NR and LTE transmission points.
The following represent different forms where this option can be implemented:
Option 7-1: In this variant, the fast Fourier transform (FFT), CP removal (OFDM processing), and possibly PRACH processing is implemented in the uplink and in the DUs, and the remaining physical layer functions reside in the CU. In the downlink, inverse FFT (IFFT) and CP insertion blocks (OFDM processing) reside in the DUs and the rest of physical layer functions will be performed in the CU. This variant would allow implementation of advanced receivers.
Option 7-2: In this variant, FFT, CP removal, resource de-mapping, and possibly MIMO decoding functions are implemented in the DU in the uplink and the remaining physical layer functional processing are performed in the CU. In the downlink, IFFT, CP addition, resource mapping, and MIMO precoding functions are performed in the DU, and the rest of physical layer processing is performed in the CU. This variant also allows the use of advanced receivers for enhanced performance.
Option 7-3: This downlink only option implements the channel encoder in the CU, and the rest of physical layer functions are performed in the DU(s). This option can reduce the fronthaul throughput requirements as the payloads consist of the encoded bits.
Split Option 7-2x: It is a specification for functional splitting between ORAN DU (O-DU) & ORAN RU (O-RU) adopted by ORAN fronthaul specification.
This split option has been introduced by ORAN Alliance to translate highly inefficient CPRI traffic to eCPRI using the low order physical layer 1 processing (Low PHY).
In the DL, OFDM phase compensation iFFT, CP addition, and digital beamforming functions reside in the O-RU as well as precoding for Category B O-RUs. The rest of the PHY functions including resource element mapping, precoding, layer mapping, modulation, scrambling, rate matching and coding reside in the O-DU. Precoding must be in the O-DU for Category A O-RU support but this only applies if the number of precoder output spatial streams is 8 or less, otherwise precoding must be in the (Category B) O-RU.
In the UL, OFDM phase compensation (for all channels except PRACH) , FFT, CP removal and digital beamforming functions reside in the O-RU. The rest of the PHY functions including resource element de-mapping, equalization, de-modulation, de-scrambling, rate de-matching and de-coding reside in the O-DU.
In general, the required fronthaul bandwidth becomes smaller as more functions become entrusted to the O-RU for example compared to CPRI in which O-RU handles only the RF function section only, placing IFFT/FFT processing in O-RU can prevent an increase in the fronthaul required bandwidth caused by over sampling applied to OFDM signal in the time domain. Similarly placing DL precoding in the O-RU can prevent the increase in the required fronthaul bandwidth that occurs when number of MIMO spatial stream is greater than the number of MIMO layers.
Next topic will be CPRI & eCPRI functionalities and advantages stay tuned..