Openair MAC
Openair MAC
The MAC layer is responsible for scheduling control plane and user traffic on the physical OFDMA resources. On transmission, the inputs to the MAC layer are connected to data queues originating in the RLC layer which form the set of logical channels. The control plane traffic is represented by the following logical channels:
The user plane traffic is represented by the following logical channels:
It is important to note that although dedicated resources are configured at the input of the MAC-layer, the physical resources allocated in the scheduling entities (with exception of the CHBCH) are dynamically allocated with the granularity of the mini-frame (nominally 2ms), and thus all physical resources are shared. Furthermore, in the case of TDD deployments, the portion of bandwidth allocated to uplink and downlink traffic is also dynamically adjusted at the granularity of the mini-frame.
As a general rule, MCCH, DCCH and MTCH do not use HARQ, or equivalently at most a single transmission round is used. DTCH generally use HARQ with a maximum number of retransmission rounds determined by higher layer configuration (i.e. the delay class in mac_lchan_desc), which is at most 8, corresponding to the number of parallel HARQ processes.
SACH scheduling makes use of up to 8 parallel HARQ processes per logical channel in order to maximize throughput and benefit from superior channel conditions.
At the start of each mini-frame, the Node-B scheduler determines the physical allocations (OFDM symbol, OFDM sub-bands, transmit antennas) for downlink and uplink logical channels and correspondingly parametrizes the DL_SACCH_PDU and UL_ALLOC_PDU data structures. For logical channels using HARQ (DCCH/DTCH), it manages the HARQ retransmission rounds in conjunction with the PHY channel decoder and packet integretity verification algorithm. At each HARQ round a new coding format and power level can be chosen for the redundancy bits to be transmitted, which are applied uniformly to all HARQ processes. UL power control and HARQ acknowledgements are also computed for corresponding UL flow.
The DL_SACCH_PDU contains the HARQ sequencing information which indicates the active HARQ processes and their progress indices.
The UE SACH scheduler parses the UL_ALLOC_PDU to find its allocations and processes the next retransmission round of the HARQ process for the allocated logical channels as well as performing DL power control on the correspoding DL flows, acknowledging receipt of a HARQ PDU, and relaying the quantized logical channel PDU backlog. The latter are reflected by the UL_SACCH_FB data structure. The UE SACH scheduler can select transmit power and coding format with the granularity of the mini-frame. These allocations are reflected in the UL_SACCH_PDU which precedes the corresponding SACH resources. The UL_SACCH_PDU must use the lowest spectral-efficiency coding format and is not subject to HARQ since it must be correctly decoded so that the HARQ process of the corresponding SACH can make use of the coded symbols in current mini-frame.
Processing of the UL_ALLOC_PDU at the UE must be sufficiently efficient for the UL_SACH to be configured in the same mini-frame.
Adjacent cell interference should be managed by the Node-Bs in a given region in a decentralized fashion using DL power control coupled with resource randomization across HARQ retransmission rounds combined with dual-antenna reception at the UE.
Modules | |
| MAC Layer Primitives for Communications with RRC | |
| This subclause describes the primitives for communications between the RRC and MAC sub-layers. | |
| MAC Layer Primitives for Communications with RLC | |
| This subclause describes the primitives for communications between the RLC and MAC sub-layers. | |
| MAC Layer Primitives for Communications with PHY | |
| This subclause describes the primitives for communications between the MAC and PHY sub-layers. | |
1.4.7