The LTE (Long-Term Evolution) MAC (Medium Access Control) layer is an essential component of the LTE protocol stack.3gpp spec for MAC is 36.321. It is responsible for managing the access to the shared radio resources and providing an efficient and fair allocation of these resources among multiple users in a wireless network.
MAC structure overview, UE side
Figure: 4.2.1-1 (3gpp 36.321)
MAC structure overview in LTE provides a detailed representation of the components and interactions within the MAC layer. Here’s a more comprehensive summary of the key elements depicted in the figure:
- Logical Channels:
- Control Channel (CCH): Carries control information such as scheduling requests, system information, and broadcast messages.
- Traffic Channel (TCH): Transports user data, including voice, video, or other application data.
- Transport Channels:
- Common Transport Channels: Used for broadcasting control information and system-wide notifications to all UEs within the cell.
- Dedicated Transport Channels: Reserved for specific UEs to carry their dedicated control and traffic information.
- MAC Control Elements:
- MAC Protocol Data Units (PDUs): Control information exchanged between the MAC layer and the physical layer.
- Examples include scheduling requests (SR), acknowledgments (ACK/NACK), and power control commands.
- MAC Procedures:
- Connection Establishment: MAC establishes connections between UEs and the network, including procedures for authentication and security setup.
- Connection Release: MAC handles the release of established connections when they are no longer needed.
- Scheduling: MAC determines the allocation of radio resources to UEs based on factors like channel conditions, QoS requirements, and fairness considerations.
- Hybrid Automatic Repeat Request (HARQ): MAC manages the error correction and retransmission process to ensure reliable data delivery.
- Quality of Service (QoS) Management: MAC enforces QoS requirements specified by higher layers, ensuring appropriate prioritization and resource allocation for different types of traffic.
- MAC Sublayer:
- The MAC sublayer encompasses all the logical and transport channels, MAC control elements, and procedures.
- It manages the interactions between the MAC layer and higher layers (such as RLC and PDCP) as well as the lower physical layer.
- MAC SDUs (Service Data Units):
- MAC SDUs are generated by the higher layers and encapsulated into MAC protocol data units (MAC PDUs) for transmission.
- They can include user data, control information, or signalling messages from higher layers.
- MAC Multiplexing:
- The MAC layer multiplexes multiple MAC SDUs from different logical channels or UEs onto transport channels.
- It organizes the MAC SDUs into MAC PDUs, which are then segmented and mapped onto the appropriate transport channel.
- MAC SDU Aggregation and Segmentation:
- MAC SDU aggregation refers to the process of combining multiple MAC SDUs into a single MAC PDU to improve efficiency.
- MAC SDU segmentation is the division of a large MAC SDU into smaller MAC PDUs for transmission over the air interface.
- MAC Subheader:
- The MAC subheader is included in MAC PDUs and provides information such as the UE identifier, channel identifier, and sequence numbers for proper processing and decoding.
- MAC Scheduler:
- The MAC scheduler is responsible for allocating radio resources to different UEs and their respective logical channels based on dynamic conditions.
- It takes into account factors like channel quality, QoS requirements, and fairness considerations to optimize resource utilization and improve overall system performance.
- Discontinuous Reception (DRX):
- DRX is a power-saving mechanism where UEs periodically turn off their receivers to conserve energy.
- The MAC layer handles DRX configurations and timing, allowing UEs to synchronize their sleep cycles and wake up when necessary for reception.
- MAC Control Elements (Control Channel):
- The Control Channel (CCH) carries various MAC control elements, including scheduling requests (SR), acknowledgments (ACK/NACK), and power control commands.
- These control elements facilitate efficient and reliable communication between the MAC layer and the physical layer.
- MAC Control Elements (Traffic Channel):
- The Traffic Channel (TCH) carries MAC control elements related to user data transmission, such as user data acknowledgments (ACK/NACK) and control signalling for retransmissions.
- Hybrid ARQ Buffer:
- The Hybrid ARQ (HARQ) buffer stores the copies of transmitted MAC PDUs awaiting acknowledgment or retransmission.
- The MAC layer manages the sequencing, retransmission, and discarding of MAC PDUs in the HARQ buffer based on feedback received from the receiving end.
Functions of MAC Layer
The MAC (Medium Access Control) layer in LTE (Long-Term Evolution) is responsible for various functionalities that are crucial for efficient and reliable communication in the wireless network. Here are the key functionalities of the MAC layer in LTE:
- Radio Resource Allocation: The MAC layer manages the allocation of radio resources, such as time and frequency slots, to different users in the network. It employs scheduling algorithms to optimize the utilization of available resources based on factors like channel conditions, QoS requirements, and fairness considerations.
- Hybrid Automatic Repeat Request (HARQ): HARQ is an error correction mechanism used in LTE. The MAC layer implements HARQ by combining automatic retransmissions with error detection and error correction techniques. It ensures reliable data transmission by detecting and correcting errors in received packets and retransmitting them when necessary.
- Logical Channel Mapping: The MAC layer maps logical channels, which carry control information or user data, to the appropriate transport channels. It organizes and multiplexes the data from different logical channels into MAC protocol data units (PDUs) for transmission over the air interface.
- Quality of Service (QoS) Management: The MAC layer enforces QoS requirements specified by higher layers, ensuring that different types of traffic (e.g., voice, video, data) receive the appropriate priority and resources. It allocates resources based on QoS parameters like packet delay, packet loss, and throughput, ensuring a satisfactory user experience for various applications.
- Connection Establishment and Release: The MAC layer handles the establishment, maintenance, and release of connections between user equipment (UE) and the base station (eNodeB) in LTE. It sets up the necessary parameters and procedures for data transmission and reception, ensuring seamless communication.
- Power Control: The MAC layer performs power control functions to optimize the transmission power of UEs. It adjusts the power levels based on channel conditions and distance from the base station, aiming to minimize interference and maximize network capacity while conserving battery life in UEs.
- Buffer Management: The MAC layer manages the buffer for storing data packets waiting to be transmitted. It ensures proper sequencing, prioritization, and timely transmission of the packets based on the scheduling decisions and available radio resources.
- Transport Format Selection: The MAC layer selects appropriate transport formats for data transmission based on channel conditions, modulation schemes, coding rates, and other factors. It adapts the transmission parameters to achieve the best possible data rates and reliability.
- Broadcast and Multicast Services: The MAC layer supports efficient delivery of broadcast and multicast services. It employs mechanisms like multicast group management and efficient distribution of control information to enable simultaneous transmission of data to multiple UEs
- Dynamic Channel Adaptation: The MAC layer continuously monitors channel conditions and adapts the transmission parameters accordingly. It adjusts modulation schemes, coding rates, and other transmission parameters to optimize performance in varying channel conditions, thereby maximizing data rates and minimizing errors.
- Random Access Procedure (RACH): The MAC layer handles the random-access procedure (RACH), which allows UEs to initiate communication with the network when establishing a connection or when there is a need to transmit unscheduled data. It manages contention-based access and collision resolution to ensure fair and efficient access to the network resources.
- Prioritization and QoS Differentiation: The MAC layer provides mechanisms for prioritizing traffic and differentiating quality of service (QoS) among different users and applications. It ensures that high-priority traffic, such as voice or emergency services, receives preferential treatment in terms of resource allocation and scheduling.
- Discontinuous Reception (DRX): The MAC layer incorporates DRX to optimize power consumption in UEs. It defines DRX cycles and parameters that allow UEs to periodically enter sleep mode, conserving energy by turning off their receivers while ensuring timely reception of incoming data.
- Multipoint Coordination Function (MPCF): The MAC layer supports coordination between multiple eNodeBs (base stations) through the MPCF. It facilitates efficient resource allocation and interference management in heterogeneous networks or multi-cell deployments, enhancing overall network performance.
- Dynamic Link Adaptation: The MAC layer performs dynamic link adaptation by adjusting transmission parameters based on the received signal quality and channel conditions. It determines the appropriate modulation and coding scheme (MCS) to optimize data rates and reliability while considering the channel quality and available resources.
- Resource Grant Control: The MAC layer manages the allocation and control of resource grants to UEs. It handles the signalling and procedures for granting and releasing resources, ensuring efficient utilization and fair distribution of radio resources based on the specific needs of UEs.
- HARQ Feedback Processing: The MAC layer processes HARQ feedback received from UEs regarding the success or failure of transmitted data packets. It uses this feedback to make decisions regarding retransmission, adaptive modulation and coding, and overall transmission strategies.
- Interference Management: The MAC layer incorporates interference management techniques to mitigate interference and enhance system performance. It employs strategies such as interference coordination, power control, and dynamic resource allocation to minimize interference effects and maximize overall network capacity.
MAC function location and link direction association.
The location of the different functions and their relevance for uplink and downlink respectively is illustrated in Table
MAC PDU Format for DL-SCH and UL-SCH
The MAC PDU (Protocol Data Unit) format in LTE varies depending on the specific transmission scenario, such as DL-SCH (Downlink Shared Channel), UL-SCH (Uplink Shared Channel), or control signaling. Here is a general overview of the MAC PDU format:
Mac Header consists of multiple subleaders.
Below are the MAC header components.
- RNTI (Radio Network Temporary Identifier): Identifies the UE or group of UEs associated with the MAC PDU.
- MCS (Modulation and Coding Scheme): Specifies the modulation and coding scheme to be used for the transmission.
- HARQ Process Information: Indicates the HARQ process number and related information for error detection and correction.
- Transport Block Size Information: Specifies the size of the transport blocks within the MAC PDU.
MAC header consists of multiple subheaders
- One subheader for each control element, MAC SDU, or padding
- Each subheader is 1, 2, or 3 bytes in length
Control element, fixed or variable length payload
- PDU Type: Indicates the type of MAC PDU, such as a DL-SCH PDU, UL-SCH PDU, or control PDU.
- Length: Specifies the length of the MAC PDU in bits or bytes.
- Control Information: Contains additional control information specific to the PDU type, such as channel information or special flags.
In Above figure we can see subheader of L filed 7 and 15 bits.
R=Reserved bit (1 bit) set to 0.
E=Extension (1 bit) defines if more fields are present in sub-header
LCID= Length is 5 bits
F= Format (1 bit)
L= Length 7/15 bits (size of SDU)
MAC CE (Control Element):
Here are some key details about LTE MAC CE:
- A MAC CE is a form of in-band control signaling in LTE.
- MAC CEs are identified by reserved values in the LCID (Logical Channel ID) field.
- The LCID value indicates the type of control information carried by the MAC CE.
- Both fixed-length and variable-length MAC CEs are supported, depending on the use case.
- For downlink transmissions, MAC CEs are located at the beginning of the MAC PDU.
- For uplink transmissions, MAC CEs are located at the end, immediately before the padding.
MAC CEs specified for different purposes:
- Consists of one or four octets, identified by MAC PDU subheaders with LCID indices 58 or 57, respectively.
- Contains C-fields (mapped to component carriers) and an R-field.
- DRX and long DRX command MAC CEs are identified by MAC PDU subheaders with LCID indices 60 and 59, respectively.
- Fixed size of zero bits.
Timing advance command:
- Identified by a MAC PDU subheader with LCID index 61.
UE contention resolution identity:
- Identified by a MAC PDU subheader with LCID index 62.
- Fixed size of 48 bits, containing UE contention resolution identities.
C-RNTI (Cell Radio Network Temporary Identifier):
- Identified by a MAC PDU subheader with LCID index 58.
- Fixed size with a single field containing the C-RNTI, 16 bits in length.
BSR (Buffer Status Report) MAC CEs:
- Short BSR format (fixed size), long BSR format (variable size), short truncated BSR format (fixed size), and long truncated BSR format (variable size).
- Identified by MAC PDU subheaders with LCID indices 59, 60, 61, and 62, respectively.
Service Data Units: Contains higher-layer data or control information, such as PDCP PDUs, RRC messages, or user data.
Length Indication: Indicates the size of each MAC SDU within the MAC PDU.
Segmentation: Allows larger MAC SDUs to be divided into smaller segments for transmission.
Padding Bits: If the MAC SDUs and MAC CE do not fully occupy the available space in the MAC PDU, padding bits may be added to fill the remaining bits.
Alignment: Padding is used to align the MAC PDU to the required size for efficient transmission and resource utilization.