-11 Tracking DDR operations in PDCP/RLC/MAC U-plane. 122 Open Radio Access Network (O-RAN) Systems Architecture and Design RLC enqueue The ciphered PDCP frame is enqueued into an RLC service data unit (SDU) queue for storage until the MAC scheduler algorithm allocates time/frequency resources in the OFDM frame for transmission. MAC slot-processing Each slot is triggered by a (slot boundary time aligned) interrupt. This triggers scheduler operation. We assume the scheduler algorithm to fetch each (active) user context for scheduler metric calculation, and to derive a slot context that includes elements like target scheduled users and their resource allocations. The MAC layer also builds a slot context that contains the information required for the (mostly stateless) Physical Layer to build the OFDM frame for transmission over-the- MAC/RLC PDU processing This processing includes (logical) dequeue of RLC SDU descriptors and (logical) RLC and MAC PDU generation steps to build a scatter/gather list that represents the Transport Block to be transmitted as a sequence of buffer descriptors. PHY processing Physical Layer processing reads the slot descriptor, Transport Block (TB) descriptor and (derived from the TB descriptor) individual PDU payload segments (representing PDCP SDUs) to build the TB for transmission over-the-air. Assessing DDR throughput is done by estimating (in granularity of cache lines) the memory transaction counts associated with all steps (like) described above. For example, assume 1 cache line (64B) associated with all buffer descriptors, 8 cache lines (512B) of storage for a 500B frame or PDU, 2 cache lines (128B) for scheduler UE contexts and 16 cache lines (1KB) for slot context. We count an aggregate: 6 Read or Write operations for frame payload (initial write for store, read/write for IPSec, read/write for PDCP and final read for Physical Layer transmit) 6 Read or Write operations for frame or TB descriptors that are operating per-frame < # active users 2 cache lines UE context Read 1 Write + 1