There are 55 LTE bands in existence, partly due to spectrum fragmentation and market strategies. The proliferation of LTE bands is also a manifestation of government allocation and the auctioning of frequency space. LTE is also split into two categories that are not compatible:
- Time Division Duplex (TDD): TDD uses a single frequency space for uplink and downlink data. The transmission direction is controlled via time slots.
- Frequency Division Duplex (FDD): In an FDD configuration, the base station (eNodeB) and the user equipment (UE) will open a pair of frequency spaces for uplink and downlink data. An example may be LTE band-13, which has an uplink range of 777 to 787 MHz and a downlink range of 746 to 756 MHz. Data can be sent simultaneously to both the uplink and downlink.
Combination TDD/FDD modules exist that combine both technologies into a single modem allowing for multiple carrier usages.
A few other terms specific to LTE are needed to understand the spectrum usage:
- Resource Element: This is the smallest transmission unit in LTE. The RE consists of one subcarrier for exactly one unit of symbol time (OFDM or SC-FDM).
- Subcarrier spacing: This is the space between subcarriers. LTE uses 15 KHz spacing with no guard bands.
- Cyclic prefix: Since there are no guard bands, a cyclic prefix time is used to prevent multipath inter-symbol interference between subcarriers.
- Time slot: LTE uses a 0.5 ms time period for LTE frames. That equals six or seven OFDM symbols depending on the cyclic prefix timing.
- Resource block: One unit of transmission. Contains 12 subcarriers and seven symbols, which is equal to 84 resource elements.
An LTE frame which is 10 ms long would consist of 10 subframes. If 10% of the total bandwidth of a 20 MHz channel is used for a cyclic prefix, then the effective bandwidth is reduced to 18 MHz. The number of subcarriers in 18 Mhz is 18 MHz/15 kHz = 1200. The number of resource blocks is 18 MHz/180 kHz = 100:
The bands allocated for 4G-LTE are specific to regional regulations (North America, Asia-Pacific, and so on). Each band has a set of standards developed and ratified by the 3GPP and ITU. Bands are split between FDD and TDD areas and have common name acronyms routinely used in the industry, such as Advanced Wireless Service (AWS). The following is a table separating the FDD and TDD bands for the North American region only:
LTE also has been developed to work in the unlicensed spectrum. Originally a Qualcomm proposal, it would operate in the 5 GHz band with IEEE 802.11a. The intent was for it to serve as an alternative to Wi-Fi hotspots. This frequency band of between 5150 MHz and 5350 MHz typically requires radios to operate at 200 mW maximum and indoors only. To this date, only T-Mobile supports the unlicensed use of LTE in areas of the USA. AT&T and Verizon are conducting public tests with LAA mode. There are three categories of unlicensed spectrum use for cellular:
- LTE Unlicensed (LTE-U): As mentioned, this would coexist in the 5 GHz band with Wi-Fi devices. The control channel of LTE would remain the same while voice and data would migrate to the 5 GHz band. Within LTE-U is the concept that user equipment may only support unidirectional downlinks in the unlicensed band or full duplex.
- Licensed-Assisted Access (LAA): This is similar to LTE-U but governed and designed by the 3GPP organization. It uses a contention protocol called listen-before-talk (LBT) to assist in coexisting with Wi-Fi.
Multifire uses the same LBT coexistence technology as LAA. In reality, Multifire would need to rely on a blanket of small cells to provide coverage (similar to Wi-Fi access points). Another benefit of Multifire is that the band is globally used by Wi-Fi and allows for consistency between regions.