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RAN 7.3 functional Split

Several options for splitting RAN functions have been proposed by 3GPP, going from option 1 to option 8. Every split function has its own pros and cons. It turns out that the 7.2x split offers the best balance between RU complexity and inter-cell cooperation (Read more here: Why most of the telecom companies are interested in RAN 7.2x functional split?).

This post discusses RAN split 7.3 in detail. In the 7.3 split, modulation/demodulation functions are more close to the antennas.

7.3 Downlink Split

7.3 Uplink Split

In this split, hard bits are sent in the downlink while soft bits are sent in the uplink. These soft bits are LLRs (Log-Likelihood Ratio).

The required fronthaul capacity for 7.3 split in DL is:

( Nsc * Nlayers * Qm )/ Ts


Nsc: Number of subcarriers
Nlayers: Number of layers
Qm: Modulation order
Ts: Symbol time

For a single carrier, 100 MHz bandwidth & 30 kHz subcarrier spacing (Nsc = 3276, Ts = 33.33 us), 256 QAM modulation with 16 layers multiplexed the required fronthaul capacity in downlink is = (3276 * 16 * 8)/33.33 ~ 12.58 Gbps. This is a huge gain in capacity compared to split 7.2x and 8.

In uplink instead of hard bits, LLRs are sent from RU to DU, therefore the required fronthaul capacity for uplink is different from downlink. The same can be given by:

( Nsc * Nlayers * Qm * Sbw)/ Ts

Where Sbw is the soft bit size. Common soft bit sizes are 6 bits, 8 bits, 12 bits, and 16 bits.

For the same case above with a soft bit size of 8 bits, the required fronthaul capacity in the uplink is = (3276 * 16 * 8 * 8)/33.33 ~ 100.64 Gbps which is 8 times higher than the required capacity in the downlink.

In 7.3 split bits are transmitted compared to the I/Q samples in 7.2x or split 8. This yields a significant reduction in the required fronthaul capacity.

The fronthaul capacity requirement in the 7.3 split depends on the modulation order, by decreasing the modulation order capacity can be reduced, wherein 7.2x split fronthaul capacity doesn’t depend on the modulation order.


  • 3GPP, 3rd Generation Partnership Project, Study on new radio access technology Radio access architecture and interfaces