In LTE, the bandwidth is divided into smaller units for efficient scheduling and data transmission. This division starts with the smallest unit called the Resource Element (RE), which is essentially one subcarrier with a bandwidth of 15 kHz. An RE represents the fundamental logical unit of the LTE spectrum used for modulating data.
Resource Block Concept in LTE.
Resource Elements (REs).
- Definition: The smallest division of the LTE spectrum.
- Bandwidth: Each RE has a bandwidth of 15 kHz in both normal and extended cyclic prefix (CP) modes. There is also a special mode with 7.5 kHz subcarrier spacing used with extended CP.
- Usage: All physical LTE channels utilize REs for data modulation.
- Modulation: Data on REs can be modulated using schemes like Quadrature Phase Shift Keying (QPSK) to 64QAM, determined by the scheduling algorithm of the eNodeB (eNB).
- Adaptive Modulation and Coding (AMC): This process dynamically adjusts the modulation and coding scheme to optimize data transmission based on current channel conditions, ensuring robustness against transmission errors by adding redundancy.
Note: To understand more clearly Resource Block please read LTE Frame Structure.
Physical Resource Blocks (RBs).
- Definition: The smallest unit allocated by the scheduling algorithm for user transmissions on shared channels.
- Frequency Domain: An Resource Block consists of 12 adjacent REs, giving it a total bandwidth of 180 kHz.
- Time Domain: An RB spans one subframe, which is 1 ms long and divided into two 0.5 ms slots.
Resource Block Grid Structure.
Subframes and OFDM Symbols
- Subframe Duration: 1 ms, containing two slots of 0.5 ms each.
- OFDM Symbols per Subframe:
- Normal CP: 14 OFDM symbols per subframe.
- Extended CP: 12 OFDM symbols per subframe.
- REs in an RB:
- Normal CP: An RB covers 12 × 14 REs (168 REs).
- Extended CP: An RB covers 12 × 12 REs (144 REs).
LTE Resource Blocks and MIMO.
In LTE, resource blocks (RBs) are essential units used for scheduling and transmitting data. They are defined in both time and frequency dimensions. In the time dimension, an RB spans one subframe, which lasts 1 millisecond (ms) and is further divided into two slots of 0.5 ms each. In the frequency dimension, an RB consists of 12 subcarriers, each with a bandwidth of 15 kHz, totaling 180 kHz. This two-dimensional structure of RBs forms the basic grid on which LTE data transmission occurs.
With the introduction of MIMO (Multiple Input Multiple Output) technology, a third dimension is added to the RB structure. MIMO uses multiple antenna ports to enhance data transmission, effectively adding layers to the existing time-frequency grid. Each antenna port layer correlates with the number of transmit antenna ports used, significantly improving the data throughput and reliability of the LTE network.
RBs play a crucial role in the LTE scheduling process, as they are the smallest unit used by the eNodeB (evolved Node B) scheduler to allocate resources to users. The total bandwidth of an LTE cell is described in terms of the number of RBs, which is broadcasted in the Physical Broadcast Channel (PBCH). This mapping helps in defining the overall capacity and configuration of the LTE cell, ensuring efficient utilization of the available spectrum.
To provide more flexibility in user allocations, LTE employs virtual resource blocks (VRBs), which are of equal size to physical RBs. There are two types of VRBs: localized VRBs and distributed VRBs. Localized VRBs map directly to physical RBs, while distributed VRBs split a physical RB at the slot boundary and hop between different slots. This hopping mechanism enhances frequency diversity, improving the robustness of data transmission against frequency-selective fading.
Resource Element Groups (REGs) are another important aspect of LTE’s physical layer. REGs are used to map physical channels to OFDM symbols, particularly for the Physical Downlink Control Channel (PDCCH). By spreading information over a larger frequency range, REGs ensure that control signaling remains robust, even in challenging radio conditions. This frequency diversity is crucial for maintaining reliable communication in LTE networks.
In summary, LTE resource blocks are fundamental building blocks for data scheduling and transmission. The combination of time, frequency, and MIMO dimensions allows for efficient and reliable use of the available spectrum. Virtual resource blocks and resource element groups add further flexibility and robustness, ensuring that LTE networks can meet the high demands of modern wireless communication.