1. TTI (Transmission Time Interval)
- What it is: The “tick” or heartbeat of the LTE network.
- The Detail: In LTE, time is divided into 10-millisecond (ms) Radio Frames. Each frame is divided into 10 Subframes of 1ms each. One Subframe equals one TTI.
- Why you care: Your MAC scheduling C function must execute exactly once every TTI (every 1 millisecond). It looks at the network state, makes a decision, and waits for the next tick.
2. PRB (Physical Resource Block)
- What it is: The currency of the LTE network.
- The Detail: LTE bandwidth is a 2D grid of Time and Frequency. A PRB is a block of 12 subcarriers (frequency) over 0.5ms (time). A 10 MHz LTE cell has exactly 50 PRBs available per TTI. A 20 MHz cell has 100 PRBs.
- Why you care: The sole output of your scheduler is an array deciding which UE gets which PRBs (e.g., UE_1 gets PRBs 0–24, UE_2 gets PRBs 25–49).
3. CQI & MCS (The Signal & Speed Translation)
- CQI (Channel Quality Indicator): The UE constantly measures the radio signal’s Signal-to-Noise Ratio (SNR) and sends a CQI score (from 1 to 15) back to the tower. 1 is terrible; 15 is perfect.
- MCS (Modulation and Coding Scheme): An index from 0 to 28 chosen by the eNodeB based on the CQI.
- Why you care: If a UE has a CQI of 15, the MAC layer selects a high MCS (like 64-QAM). This means the scheduler can pack hundreds of bytes into a single PRB. If the CQI is 1, the scheduler must use a low MCS (QPSK), fitting only a few bytes into a PRB. Your algorithm must prefer giving PRBs to users with high CQI to maximize total network throughput.
4. BSR (Buffer Status Report)
- What it is: The UE telling the tower, “I have data to upload.”
- The Detail: The eNodeB knows exactly how much data is waiting to be downloaded (because the data sits in the eNodeB’s own memory). However, it cannot see the UE’s memory. The UE sends a BSR to request Uplink PRBs.
- Why you care: For Downlink scheduling, you look at the eNB’s internal buffers. For Uplink scheduling, you look at the BSRs received from the UEs.
5. HARQ (Hybrid Automatic Repeat Request)
- What it is: The MAC layer’s ultra-fast error correction.
- The Detail: When the MAC layer sends a packet, it keeps a copy in a “HARQ Process buffer”. If the UE fails to decode it due to interference, it sends a NACK (Negative Ack). The MAC layer then resends the packet.
- Why you care: HARQ retransmissions always have the highest priority. Your scheduler must allocate PRBs to HARQ retransmissions before it gives PRBs to brand new data.