Resource Block Grid

A Resource Element (RE) is the smallest physical resource unit in the OFDM grid, occupying one subcarrier in the frequency domain and one OFDM symbol in the time domain. Each RE carries a single complex-valued modulation symbol (QPSK, 16QAM, 64QAM, or 256QAM).

• In LTE, a single RE with 15 kHz subcarrier spacing and normal CP occupies approximately 66.7 µs in time
• The total number of REs in a 20 MHz LTE carrier with normal CP is 84,000 per subframe
• REs are grouped into larger structures for scheduling: resource blocks, resource block groups, and control channel elements

A Resource Block (RB) is the minimum unit of resource allocation that the MAC scheduler can assign to a user. In LTE, an RB spans 12 consecutive subcarriers (180 kHz) in frequency and one slot (0.5 ms, 7 OFDM symbols with normal CP) in time, containing 84 REs.

• 5G NR introduces flexible RB definitions that scale with numerology, but the 12-subcarrier frequency span remains constant
• The total number of RBs depends on the channel bandwidth: 6 RBs for 1.4 MHz, 100 RBs for 20 MHz LTE
• RBs can be allocated as contiguous blocks or distributed across the bandwidth using resource allocation types 0, 1, or 2

The resource block grid forms a two-dimensional lattice where the horizontal axis represents OFDM symbols (time) and the vertical axis represents subcarriers (frequency). This orthogonal structure ensures that subcarriers do not interfere with each other despite overlapping in frequency.

• The grid dimensions for a 10 ms LTE radio frame with 20 MHz bandwidth: 100 RBs × 140 OFDM symbols (normal CP)
• Reference signals, synchronization signals, and control channels occupy specific, predefined positions within this lattice
• The lattice structure enables localized and distributed resource allocation schemes for frequency diversity

Specific REs within the resource block grid are reserved for reference signals (pilots) that enable channel estimation and coherent demodulation. These REs are transmitted at known power and phase, allowing the receiver to estimate the channel response.

• Cell-specific Reference Signals (CRS) in LTE occupy REs at fixed subcarrier and symbol offsets, with density depending on the number of antenna ports
• Demodulation Reference Signals (DMRS) in 5G NR are UE-specific and mapped to the scheduled RBs, supporting beamforming and MU-MIMO
• Reference signal overhead typically consumes 4.76% to 14.29% of REs per RB, depending on antenna configuration

REs are aggregated into Resource Element Groups (REGs) for control channel mapping. A REG consists of 4 consecutive REs within the same OFDM symbol that are not occupied by reference signals.

• Control Channel Elements (CCEs) are formed from 9 REGs (36 REs) and are the basic unit for PDCCH transmission
• PDCCH aggregation levels (1, 2, 4, or 8 CCEs) determine the coding rate and coverage of downlink control information
• The control region occupies the first 1–3 OFDM symbols of each subframe in LTE, while 5G NR uses the flexible CORESET structure

In 5G NR, the resource block grid scales with the numerology (µ), which defines the subcarrier spacing as 15 × 2^µ kHz. Larger subcarrier spacings compress the time-domain symbol duration while maintaining the 12-subcarrier RB width.

• µ=0 (15 kHz): 1 ms slot, 14 symbols — baseline for sub-6 GHz
• µ=1 (30 kHz): 0.5 ms slot — common for mid-band deployments
• µ=2 (60 kHz): 0.25 ms slot — enables low-latency URLLC
• µ=3 (120 kHz): 0.125 ms slot — used in mmWave bands above 24 GHz
• The frequency-domain RB count scales inversely with subcarrier spacing for a given bandwidth

The smallest physical unit in the OFDM grid, representing a single subcarrier for the duration of one OFDM symbol. Each RE carries one complex-valued modulation symbol (QPSK, 16QAM, 64QAM, or 256QAM).

• Frequency domain: 15 kHz (LTE) or configurable 15-240 kHz (5G NR)
• Time domain: ~66.7 µs symbol duration (excluding cyclic prefix)
• Capacity: 2, 4, 6, or 8 bits per RE depending on modulation order
• Mapping: REs are grouped into resource blocks for scheduling allocation

The minimum scheduling unit in LTE and 5G NR, consisting of 12 consecutive subcarriers in the frequency domain. An RB spans one slot in the time domain.

• LTE RB: 12 subcarriers × 7 OFDM symbols (normal CP) = 84 REs
• 5G NR RB: 12 subcarriers × 14 OFDM symbols (normal CP, 1 slot) = 168 REs
• Bandwidth: 180 kHz per RB in LTE (12 × 15 kHz)
• Scheduling: The MAC scheduler allocates RBs to UEs every Transmission Time Interval (TTI)

A contiguous set of resource blocks used as the granularity for downlink resource allocation Type 0 in LTE and 5G NR. RBG size depends on system bandwidth to balance scheduling flexibility with control signaling overhead.

• LTE RBG sizes: 1 RB (≤10 RBs), 2 RBs (11-26), 3 RBs (27-63), 4 RBs (64-110)
• Bitmap allocation: Each RBG is addressed by a single bit in the DCI
• Trade-off: Larger RBGs reduce DCI payload but decrease allocation precision
• Type 1 allocation: Uses individual RB addressing as an alternative to RBG-based scheduling

The scalable set of physical-layer parameters that define the subcarrier spacing and cyclic prefix length in 5G NR. Different numerologies enable the same frame structure to serve diverse use cases from enhanced mobile broadband to ultra-reliable low-latency communications.

• µ=0: 15 kHz SCS, normal CP (aligned with LTE)
• µ=1: 30 kHz SCS, normal CP (FR1 mid-band)
• µ=2: 60 kHz SCS, normal/extended CP (FR2 mmWave)
• µ=3: 120 kHz SCS, normal CP (FR2 high-band)
• µ=4: 240 kHz SCS, normal CP (FR2 for very low latency)
• Scaling: Higher µ reduces slot duration proportionally (1 ms / 2^µ)

A contiguous subset of carrier resource blocks configured per UE in 5G NR. BWPs enable bandwidth adaptation, allowing devices to operate on narrower bandwidths for power saving and switch to wider bandwidths for high throughput.

• Configuration: Up to 4 BWPs per UE per carrier (only 1 active at a time)
• Location and numerology: Each BWP can have independent SCS and CP configuration
• Power saving: Narrow BWP for monitoring PDCCH during inactivity
• UE capability: Accommodates devices that cannot support the full carrier bandwidth
• BWP switching: Triggered by DCI, MAC CE, or timers (bwp-InactivityTimer)

A time-frequency region configured for PDCCH transmission in 5G NR. Unlike LTE's fixed control region spanning the full bandwidth, CORESETs are flexibly placed within the resource grid.

• Frequency domain: Defined in multiples of 6 RBs (Resource Block Groups of 6)
• Time domain: 1-3 OFDM symbols within a slot
• CCE-to-REG mapping: Interleaved or non-interleaved mapping of Control Channel Elements
• TCI states: Each CORESET is associated with beamforming information via Transmission Configuration Indication
• Search space: UEs monitor specific CORESETs at configured monitoring occasions for DCI