Resource Grid, Resource Block & BWP

Resource Element — the smallest unit

A Resource Element (RE) is one subcarrier in one OFDM symbol — the smallest addressable unit of the resource grid. Each RE carries one complex modulation symbol. For 30 kHz SCS, one RE spans 30 kHz × 33.33 μs.

Resource Block — the scheduling unit

A Resource Block (RB) is 12 consecutive subcarriers in the frequency domain. It spans one slot (14 symbols) in time. The scheduler assigns frequency resources to UEs in units of RBs. For 30 kHz SCS: 1 RB = 12 × 30 kHz = 360 kHz.

One resource element (RE) is the basic unit of the resource grid, defined as one OFDM subcarrier during one OFDM symbol, indexed by (k, l) where k is the subcarrier index and l is the OFDM symbol index within a subframe. A resource block (RB) is defined as 12 consecutive subcarriers in the frequency domain.

3GPP TS 38.211, Section 4.4.1

1 subcarrier × 1 symbol

Carries 1 QAM symbol = 1–10 bits

12 subcarriers × 1 slot

168 REs. Scheduling granularity.

273

RBs in 100 MHz (30 kHz)

= 3276 active subcarriers

BWP

Bandwidth Part

Subset of RBs a UE monitors

Resource grid — time × frequency structure TS 38.211 §4.4

Channel bandwidth and RB counts

Guard bands at the channel edges prevent interference with adjacent channels. The number of active RBs is less than the total channel bandwidth divided by RB width.

RB counts — 30 kHz SCS, various bandwidths TS 38.101-1, Table 5.3.2-1

Channel BW	Active RBs	Active subcarriers	Active BW	Guard (each side)
5 MHz	11	132	3.96 MHz	0.52 MHz
10 MHz	24	288	8.64 MHz	0.68 MHz
20 MHz	51	612	18.36 MHz	0.82 MHz
40 MHz	106	1272	38.16 MHz	0.92 MHz
100 MHz	273	3276	98.28 MHz	0.86 MHz ← our cell

Point A — the absolute frequency reference

All RB positions are defined relative to Point A — the center of subcarrier 0 of RB 0. The MIB field ssb-SubcarrierOffset (k_SSB = 2) defines how far the SSB sits above Point A, allowing the UE to calculate Point A from the known SSB frequency.

Point A is the reference point for resource block grid and is located at the centre of subcarrier 0 of common resource block 0. The common resource block numbering starts at subcarrier 0 of common resource block 0 at Point A.

3GPP TS 38.211, Section 4.4.4.1

Bandwidth Parts (BWP)

A UE does not need to receive the full 100 MHz at all times. A Bandwidth Part is a configured subset of RBs. Up to 4 DL BWPs can be configured; only 1 is active at a time. BWP switching saves power during low-activity periods.

Resource grid numbers — n78, 100 MHz, 30 kHz SCSTS 38.211 §4.4, TS 38.101-1

// Frequency domain
Active RBs       = 273
Active subcarriers = 273 × 12 = 3276
Active BW        = 3276 × 30 kHz = 98.28 MHz
Guard bands      = (100 − 98.28) / 2 = 0.86 MHz each side

// Point A derivation (from k_SSB=2)
f_SSB_center = 3498.24 MHz
f_SSB_low    = 3498.24 − 60 × 0.030 = 3496.44 MHz
Point_A      = 3496.44 − 2 × 0.030 = 3496.38 MHz

// Initial DL BWP (from SIB1)
BWP start = CRB 0 = 3496.38 MHz
BWP width = 52 RBs = 52 × 360 kHz = 18.72 MHz

// REs per slot
Per RB per slot  = 12 × 14 = 168 REs
Total per slot   = 273 × 168 = 45,864 REs

Resource grid summary

RE → 1 SC × 1 symbol — smallest unit

RB → 12 SC = 360 kHz at 30 kHz SCS

Channel → 273 active RBs in 100 MHz

Point A → 3496.38 MHz — absolute reference

BWP → subset of RBs, 1 active per direction

Resource Grid —The time-frequency canvas

Resource Element — the smallest unit

Resource Block — the scheduling unit

Channel bandwidth and RB counts

Point A — the absolute frequency reference

Bandwidth Parts (BWP)

Resource Grid —
The time-frequency canvas