Band, Channel Bandwidth, EARFCN and Carrier Frequency

Radio frequency spectrum resources are the foundation of cellular networks. When dealing with products or systems related to cellular technologies such as IMSI catchers, cell signal repeaters and 5G/LTE CPEs, people can easily get confused with the terminologies related to radio frequency. In this article, we will explain the most common terminologies such as frequency band, channel bandwidth, EARFCN and carrier frequency, and also answer some frequently asked questions.  

 Page Contents

    1. What’s Frequency Band?

    2. What’s Channel Bandwidth?

    3. What’s Carrier Frequency?

    4. What’s EARFCN and how to Calculate it?

    5. Does EARFCN take into account the Channel Bandwidth?

    6. What’s the difference between the EARFCN of FDD and TDD bands?

Part 1. What’s Frequency Band?

The frequency band, or band, refers to a frequency range or the width of the spectrum, that is, the range between the lowest operating frequency and the highest operating frequency of the wireless codec, in Hz. For convenience, in LTE, the numbers 1-88 are used to represent different frequency bands (according to 3GPP protocol), thus referring to different frequency ranges.

The protocol specifies all current LTE frequency bands, the frequency range of the band and LTE network mode (FDD or TDD), as shown in the article 5G NR & LTE Frequency Bands and Terminology Explanation. It should be noted that the upstream and downstream frequency ranges of bands 1-32 and 65-88 are not overlapping, that is, the upstream and downstream data are transmitted in different frequencies. These bands are also called "Paired Frequency Bands", which are reserved for FDD-LTE networks. The upstream and downstream frequency ranges of bands 33-53 are the same. These bands are also called "Unpaired Frequency Band" and are reserved for TDD-LTE networks.

Part 2. What’s Channel Bandwidth?

Channel bandwidth limits the upper and lower frequencies that are allowed to pass through the channel, that is, limits a frequency passband. In a band, several different channel bandwidths can be flexibly allocated. LTE system supports flexible and variable channel bandwidth. There are 6 kinds of channel bandwidth that can be configured, which are 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz and 20MHz respectively. The lower bandwidths of 1.4MHz and 3MHz can facilitate the migration of spectrum used by 2G&3G to LTE, while helping facilitate the migration of 2G and 3G networks to LTE. The purpose of specifying multiple bandwidths is to adapt to different frequency usage scenarios. For example, when the available frequency band is less than 10MHz, the LTE system can be configured to use 5MHz.

3GPP also specifies the channel bandwidths that can be used in different frequency bands. Here below are some examples.

The full picture of channel bandwidths supported by each frequency band could also be find from the table of 4G LTE Frequency Bands in 5G NR & LTE Frequency Bands and Terminology Explanation.

Part 3. What’s Carrier Frequency?

In order to uniquely identify the frequency range of an LTE system, with only the two parameters, frequency band and channel bandwidth, you cannot define its frequency range. For example, Telekom in Germany occupies a 30MHz frequency range in band 3 (1800M), while the maximum channel bandwidth of LTE is 20MHz. Therefore, there is no way to define the specific location of 20MHz within the 30MHz range. At this time a new parameter is introduced: carrier center frequency Fc (carrier frequency for short).

Part 4. What’s EARFCN and how to calculate it?

As can be seen from the figure above, the specific frequency range of the LTE system can be uniquely determined by the three values of band, channel bandwidth and carrier frequency Fc. Because the carrier frequency Fc is a floating-point value, compared with the integer value, it is not suitable for air interface transmission. Therefore, an EARFCN number is used to represent the corresponding carrier frequency Fc when the 3GPP protocol is formulated.

The carrier frequency in the uplink and downlink is designated by the E-UTRA Absolute Radio Frequency Channel Number (EARFCN) in the range 0 - 65535. The relation between EARFCN and the carrier frequency in MHz for the downlink is given by the following equation, where FDL_low and NOffs-DL are given in the table below. FDL is the downlink carrier frequency, and NDL is the downlink EARFCN.

FDL = FDL_low + 0.1(NDL – NOffs-DL)

The relation between EARFCN and the carrier frequency in MHz for the uplink is given by the following equation where FUL_low and NOffs-UL are given in the table. FUL is the uplink carrier frequency and NUL is the uplink EARFCN.

FUL = FUL_low + 0.1(NUL – NOffs-UL)

For example, if the current LTE system uses frequency band 40, and the carrier frequency is 2320MHz. Then by looking up the table, you can get FDL_low=2300MHz, NOffs_Dl=38650, FUL_low=2300MHz, NOffs_Ul=38650. Then, by substituting the formula (note that MHz is not converted to Hz when substituting), the EARFCN number of the uplink (NUL) and downlink (NDL) can be obtained as 38850.

Part 5. Does EARFCN take into account the Channel Bandwidth?

The answer is NO. EARFCNs are basically LTE carrier channel numbers. They are used in LTE networks to define a particular carrier frequency and does not take into account the channel bandwidth. Therefore, given an EARFCN number of an LTE system, you can know its carrier frequency, but the channel bandwidth is not defined and could be 1.4MHz, 3MHz or up to 20MHz.

Part 6. What’s the difference between the EARFCN of FDD and TDD bands?

EARFCN of both FDD and TDD bands could be calculated from the same equations in Part 4. As FDD has paired frequency bands respectively for downlink and uplink, it’s FDL_low [MHz] and NOffs-DL for downlink are different from FUL_low [MHz] and NOffs-UL for uplink, therefore the EARFCN of downlink and uplink are different. Whereas for TDD, both uplink and downlink share the same frequency range, and thus the EARFCNs of the uplink and downlink are the same.