ference from LTE where HARQ information was only transmitted on a TB basis. For determining code block size, refer to 3GPP 38.212, assuming Low Density Parallel Codes (LDPC) base graph 1 to find a nominal CB size of 8448 bits. The maximum transport block size is calculated from the frame configuration as follows: FR2/mmWave Assume (120KHz SCS, 125 microseconds slot time) 273 Resource Elements (RE) allocated, 12 SC/RE, 6 bit/subcarrier (64QAM), 14 symbol/slot, 2 layers/TB and 948/1024 code rate 273 X 12 X 6 X 14 X 2 X 948/1024 = 63690 Byte FR1/ <6 GHz CB (<8448B) CB (<8448B) CB (<8448B) FIGURE 3-12 Transport block segmentation to code blocks. 124 Open Radio Access Network (O-RAN) Systems Architecture and Design Assume (30KHz SCS, 500 microseconds slot time) 273 RE allocated, 12 SC/RE, 8 bit/subcarrier (256QAM), 14 symbol/slot, 2 layers/TB and 948/1024 code rate 273 X 12 X 8 X 14 X 2 X 948/1024 = 84920 Byte Note how in the receive side, each physical bit at the decoder output is represented by a so-called "soft bit" or LLR which is a 6- or 8-bit representation of each bit. In addition, given HARQ operation, the 3x datarate that is generated from a 3x LDPC decoder equates that the maximum uncoded data block going into the LDPC decoder can be approximately 3 times the size of the Transport Block. In aggregate, taking a 3x code rate and 8-bit LLRs, each TB has a peak memory footprint sized as: FR2/mmWave 273 12 X 6 X 14 1 Byte/LLR X 3 = 1.65 Mbyte FR1/ < GHz 273 12 X 8 X 14 X 2 X 1 Byte/LLR X 3 = 2.2 Mbyte For (worst-case) retransmissions, these buffers need to be stored and re-stored every decode opportunity (slot). This implies large buffers are typically too large to be stored in dedicated on-chip static random access memory and hence target external DDR memory. The bandwidth associated with this operation is similarly high: up to 24x (3x code rate, 8x LLR size) the MAC/PHY bit rate, driving high memory speed. Limited buffer rate matching Limited Buffer Rate Matching (LBRM) is a technique to reduce HARQ buffer size