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Thursday, June 30, 2011

Spectrum for LTE


LTE can be deployed both in existing IMT bands and in future bands that may be identified. The possibility of operating radio-access technology in different frequency bands is, in itself, nothing new. For example, quad-band GSM terminals are common, capable of operating in the 850, 900, 1800, and 1900 MHz bands. From a radio-access functionality perspective, this has no or limited impact and the LTE physical-layer specifications do not assume any specific frequency band.

What may differ, in terms of specification, between different bands are mainly the more specific RF requirements, such as the allowed maximum transmit power, requirements/limits on out-of-band (OOB) emission, and so on. One reason for this is that external constraints, imposed by regulatory bodies, may differ between different frequency bands. The frequency bands where LTE will operate are in both paired and unpaired spectrum, requiring flexibility in the duplex arrangement. For this reason, LTE supports both FDD and TDD.
Release 8 of the 3GPP specifications for LTE includes 19 frequency bands for FDD and nine for TDD. The paired bands for FDD operation are numbered from 1 to 21, as shown in the table above, while the unpaired bands for TDD operation are numbered from 33 to 41, as shown in in the table below;\
Note that the frequency bands for UTRA FDD use the same numbers as the paired LTE bands, but are labeled with Roman numerals. All bands for LTE are summarized in the illustrations, which also show the corresponding frequency allocation defined by the ITU.
Some of the frequency bands are partly or fully overlapping. In most cases this is explained by regional differences in how the bands defined by the ITU are implemented. At the same time, a high degree of commonality between the bands is desired to enable global roaming. The set of bands have first been specified as bands for UTRA, with each band originating in global, regional, and local spectrum developments. The complete set of UTRA bands was then transferred to the LTE specifications in release 8 and additional ones have been added in later releases. Bands 1, 33, and 34 are the same paired and unpaired bands that were defined first for UTRA in release 99 of the 3GPPP specifications, also called the 2 GHz “core band”. Band 2 was added later for operation in the US PCS1900 band and Band 3 for 3 G operation in the GSM1800 band. The unpaired Bands 35, 36, and 37 are also defined for the PCS1900 frequency ranges, but are not deployed anywhere today. Band 39 is an extension of the unpaired Band 33 from 20 to 40 MHz for use in China. Band 4 was introduced as a new band for the Americas following the addition of the 3 G bands at WRC-2000. Its downlink overlaps completely with the downlink of Band 1, which facilitates roaming and eases the design of dual Band 1  4 terminals. Band 10 is an extension of Band 4 from 2 x 45 to 2 x 60 MHz. Band 9 overlaps with Band 3, but is intended only for Japan. The specifications are drafted in such a way that implementation of roaming dual Band 3+ 9 terminals is possible. The 1500 MHz frequency band is also identified in 3GPP for Japan as Bands 11 and 21. It is allocated globally to mobile service on a co-primary basis and was previously used for 2 G in Japan.

With WRC-2000, the band 2500–2690 MHz was identified for IMT-2000 and it is identified as Band 7 in 3GPP for FDD and Band 38 for TDD operation in the “center gap” of the FDD allocation. The band has a slightly different arrangement in North America, where a US-specific Band 41 is defined. Band 40 is an unpaired band specified for the new frequency range 2300–2400 MHz identified for IMT and has a widespread allocation globally. WRC-2000 also identified the frequency range 806–960 MHz for IMT-2000, complemented by the frequency range 698–806 MHz in WRC’07. As shown in the illustration above, several bands are defined for FDD operation in this range. Band 8 uses the same band plan as GSM900. Bands 5, 18, and 19 overlap, but are intended for different regions. Band 5 is based on the US cellular band, while Bands 18 and 19 are restricted to Japan in the specifications. 2 G systems in Japan had a very specific band plan and Bands 18 and 19 are a way of partly aligning the Japanese spectrum plan in the 810–960 MHz range to that in other parts of the world. Note that Band 6 was originally defined in this frequency range for Japan, but it is not used for LTE. Bands 12, 13, 14, and 17 make up the first set of bands defined for what is called the digital dividend – that is, for spectrum previously used for broadcasting. This spectrum is partly migrated to be used by other wireless technologies, since TV broadcasting is migrating from analog to more spectrum-efficient digital technologies. Another regional band for the digital dividend is Band 20 that is defined in Europe.

Most of the frequency bands identified above for deployment of LTE are existing IMT-2000 bands and some bands also have legacy systems deployed, including WCDMA/HSPA and GSM. Bands are also in some regions defined in a “technology neutral” manner, which means that coexistence between different technologies is a necessity. The fundamental LTE requirement to operate in different frequency bands does not, in itself, impose any specific requirements on the radio-interface design. There are, however, implications for the RF requirements and how those are defined, in order to support the following:

• Coexistence between operators in the same geographical area in the band. These other operators may deploy LTE or other IMT-2000 technologies, such as UMTS/HSPA or GSM/EDGE. There may also be non-IMT-2000 technologies. Such coexistence requirements are to a large extent developed within 3GPP, but there may also be regional requirements defined by regulatory bodies in some frequency bands.
• Co-location of base station equipment between operators. There are in many cases limitations to where base-station equipment can be deployed. Often, sites must be shared between operators or an operator will deploy multiple technologies in one site. This puts additional requirements on both base-station receivers and transmitters.
• Coexistence with services in adjacent frequency bands and across country borders. The use of the RF spectrum is regulated through complex international agreements, involving many interests. There will therefore be requirements for coordination between operators in different countries and for coexistence with services in adjacent frequency bands. Most of these are defined in different regulatory bodies. Sometimes the regulators request that 3GPP includes such coexistence limits in the 3GPP specifications.
• Coexistence between operators of TDD systems in the same band is provided by inter-operator synchronization, in order to avoid interference between downlink and uplink transmissions of different operators. This means that all operators need to have the same downlink/uplink configurations and frame synchronization, not in itself an RF requirement, but it is implicitly assumed in the 3GPP specifications. RF requirements for unsynchronized systems become very strict.
• Release-independent frequency-band principles. Frequency bands are defined regionally and new bands are added continuously. This means that every new release of 3GPP specifications will have new bands added. Through the “release independence” principle, it is possible to design terminals based on an early release of 3GPP specifications that support a frequency band added in a later release





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