Architecture of the E-UTRA Absolute Radio Frequency Channel Number (EARFCN)
In the 3GPP Long Term Evolution (LTE) and LTE-Advanced specifications (3GPP TS 36.101), radio frequency carriers are identified globally using a standardized integer known as the E-UTRA Absolute Radio Frequency Channel Number (EARFCN). Unlike legacy 2G GSM systems which used irregular 200 kHz ARFCN rasters or 3G UMTS UARFCNs with 200 kHz channel increments, LTE standardized its frequency mapping on a uniform 100 kHz (0.1 MHz) channel raster. This allows seamless integer indexing across all spectrum allocations worldwide.
1. The 3GPP TS 36.101 Carrier Frequency Equation
Per 3GPP TS 36.101 Section 5.7.3, the relationship between the carrier center frequency in Megahertz ($F_{\text{carrier}}$) and the corresponding EARFCN ($N_{\text{REF}}$) is governed by two fundamental linear equations:
F_{\text{UL}} = F_{\text{UL\_low}} + 0.1 \cdot (N_{\text{UL}} - N_{\text{Offs-UL}})
Where:
- $F_{\text{DL\_low}}$ and $F_{\text{UL\_low}}$: The lower edge frequency (in MHz) of the downlink and uplink band allocations as specified in 3GPP Table 5.7.3-1.
- $N_{\text{Offs-DL}}$ and $N_{\text{Offs-UL}}$: The minimum offset channel number defined for that specific band.
- $0.1$: The channel raster multiplier, reflecting the 100 kHz step size ($0.1\text{ MHz} = 100\text{ kHz}$).
2. FDD Paired Spectrum vs. TDD Single-Carrier Allocation
The architecture of LTE bands is divided into two primary duplex operational schemes:
-
Frequency Division Duplex (FDD): Operates using separate, paired frequency bands for transmission and reception. Every FDD channel defines two linked EARFCNs: a downlink channel ($N_{\text{DL}}$) and an uplink channel ($N_{\text{UL}}$).
- Standard Duplex: The downlink frequency is higher than the uplink frequency ($F_{\text{DL}} > F_{\text{UL}}$), providing superior isolation between the high-power base station transmitter and terminal receivers (e.g., Band 1 with $+190\text{ MHz}$ spacing; Band 3 with $+95\text{ MHz}$ spacing).
- Reversed Duplex: In certain spectrum clearing scenarios—such as Europe's 800 MHz Digital Dividend (Band 20) or US 700 MHz (Band 13)—the uplink band sits physically higher in frequency than the downlink band ($F_{\text{DL}} < F_{\text{UL}}$), resulting in a negative duplex spacing (e.g., $-41.0\text{ MHz}$ for Band 20). This reversed geometry was required to prevent high-power mobile uploads from desensitizing adjacent digital terrestrial television (DTT) broadcast receivers. - Time Division Duplex (TDD): Downlink and uplink transmissions time-share the exact same RF carrier frequency (Bands 33 through 54). In TDD bands, $F_{\text{DL}} = F_{\text{UL}}$ and $N_{\text{DL}} = N_{\text{UL}}$, with a duplex spacing of exactly $0.0\text{ MHz}$.
3. Extended EARFCN Range Architecture (Release 9 to Release 13)
In early 3GPP LTE releases (Rel 8 and Rel 9), EARFCN values were encoded as 16-bit unsigned integers, limiting the channel space to values between 0 and 65,535. However, as international spectrum auctions opened new bands—such as Band 65 (extended IMT), Band 66 (AWS-3 extending Band 4 up to 2.2 GHz), and Band 70—the 16-bit range became fully exhausted.
Starting in 3GPP Release 10 and expanded in Release 13, 3GPP introduced the Extended EARFCN Range, expanding the field to a 24-bit integer capable of addressing channels up to 262,143. To maintain backwards compatibility with legacy Category 3/4 user devices that cannot decode 24-bit EARFCN fields in RRC signaling, 3GPP employs Multi-Frequency Band Indicators (MFBI): the base station broadcasts both legacy overlapping band IDs (such as Band 4) and extended band IDs (such as Band 66) within System Information Block Type 1 (SIB1).
4. Channel Raster Alignment and RRC Measurement Reporting
In LTE, mobile user equipments (UEs) perform initial cell search by scanning the radio spectrum in discrete 100 kHz increments. The UE searches for the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS), which are always centered precisely on the 6 central Physical Resource Blocks (72 subcarriers) of the channel:
- If an RF center frequency deviates from an integer multiple of 100 kHz (e.g., $1842.55\text{ MHz}$ instead of $1842.50\text{ MHz}$), the mobile receiver's internal local oscillator (LO) frequency synthesizer will fail to lock to the DC subcarrier, preventing cell camping.
- During connected mode mobility, the eNodeB configures
MeasObjectEUTRAin RRC signaling containing the exact target EARFCN. The UE continuously monitors RSRP and RSRQ on that specific carrier to trigger intra-frequency (A3 event) or inter-frequency (A4/A5 event) handovers.