Antenna Manufacturers respond to LTE frequency requirements - By: Mobile Mark

LTE frequency requirements mean additional frequencies need to be covered US cellular networks are moving beyond the established 850 & 1900 MHz bands to include the LTE frequency requirements in the 700 MHz band and the new AWS frequency requirements in the 1.7 & 2.1 GHz bands. Although different operators have access to different combinations of frequencies, many modems and antennas will need to cover all of the new frequency bands.

The following chart shows the Cellular and LTE frequency requirements for the wireless systems currently available in the US.

System : Band reference : Exact frequencies
US AMPS : 850 MHz : 824-894 MHz
US GSM/CDMA : 850 & 1900 MHz : 824-894 & 1850-1990 MHz
European GSM : 925 & 1800 MHz : 870-960 & 1710-1880 MHz
UMTS 2100 MHz : 1920-2170 MHz : WiMAX & LTE TDD 2.5 GHz 2496-2690 MHz
AWS : 1700 & 2100 MHz : 1710-1755 & 2110-2155 MHz
LTE 700 MHz band : 700 MHz : 694-804 MHz
LTE 800 MHz band : 800 MHz : 806-824 & 851-869 MHz
Nextel PCS (future FD-LTE): 1900 MHz :1910-1915 & 1990-1995 MHz
LTE FDD (LightSquared): 1600 MHz :1525-1559 MHz & 1598-1605 MHz
The initial challenge came when US Cellular antenna designers needed to add the 1900 MHz band to the original 850 MHz band. The dual-band designs were difficult to achieve but designers overcame the obstacles and were able to present good designs.

Device manufacturers then began receiving modems which were not only designed for the U.S. Cellular frequencies, but also included the European bands. To make the designs work, the designers started using wideband elements instead of just dual-band designs. With a wideband element the antenna resonates with an acceptable VSWR at all frequencies within the frequency range covered. A wideband design also diminishes the rejection of spurious emissions that is sometimes found in dual-band designs. However, in some cases a wide-band element is not as efficient as the dual-band design which might cause certification issues.

The next challenge was the licensing of the AWS in the US and the UMTS in Europe. Since both of these bands go up to 2100 MHz, the total bandwidth coverage area was expanding. The frequency band ranged from 850 MHz to 2100 MHz. This band was already quite wide and that was before the additional LTE frequency requirements.

The latest challenge is the opening up of the 700 MHz band. The LTE frequency requirements for this band are 694 to 806 MHz. The lower frequency, and therefore the longer wavelength, of this band provide the greatest design challenge so far, but antenna designers were able to develop innovative antennas to cover this range. Many manufacturers are releasing antennas to cover all the Cellular, AWS and LTE frequency ranges in single designs. Because of the bandwidth and LTE frequency requirements, especially in the 694 to 960 MHz range, antenna performance is more difficult to maintain.

Certification with the very wideband antennas can sometimes be a challenge due to lower antenna efficiency. Maintaining the size of the antennas is more of a challenge because of the longer wavelength, while the applications that require the use of these services are getting smaller. The quest for a high efficiency, small antenna that can meet Cellular, AWS and LTE frequency requirements will challenge the designers even more. The modem manufacturers where able to very quickly add this band to their products and antenna designers are responding.

The latest news of a new addition to the LTE frequency requirements list is the 800 MHz band used by Sprint. This band was formerly used for the iDEN technology (e.g. Nextel’s Push-to-Talk radios) but reports say that Sprint is reallocating its iDEN customers to 3G service on the 900 MHz band which will free up the 800 MHz band to meet additional LTE frequency requirements.

Many had expected that Sprint would use the Clearwire frequencies at 2.5 GHz as a platform for LTE, but that appears they will continue to use of for the existing WiMAX networks but will hold on any moves to use if for LTE frequency requirements. Meanwhile, Clearwire will move ahead with its own LTE-Advanced strategy on the 2.5 GHz band.

Sprint has also announced its intention to use LightSquared frequency bands to add to its LTE offering. LightSquared currently has access to 1525-1559 MHz for the downlink and 1598-1605 MHz for the uplink. This is pending FCC approval and is controversial because many in the high-precision GPS community are concerned about possible interference at GPS frequencies. LightSquared has announced they will start with the 1526-1536 MHz portion of the lower band rather than the originally planned 1550-1555 MHz, but have held out the possibility of using the rest of the band at a later date. Portions of the upper band may also interfere with other satellite systems, such as GLONASS. The issue is still under discussion at the FCC.

Identifying the different frequencies used by the major networks
Although roll-outs of the new systems have begun, there will be significant geographic differences as the rate and locations of the roll-outs will vary across the country. In addition, there will be significant differences among the service operators as they do not all have access to the same frequency bands.

The following chart shows the use of specific Cellular & LTE frequency requirements used by the major US Carriers.

Carrier : Frequencies used
AT&T : 850 & 1900 MHz, 700 MHz
T-Mobile : 850 & 1900 MHz, 1700 & 2100 MHz
Verizon : 850 & 1900 MHz, 700 MHz
Sprint : 800 MHz, 1600 MHz, CDMA 1900 MHz, Nextel 1900 MHz
Clearwire/Sprint : 2500 MHz
US Cellular : 850 & 1900 MHz
MetroPCS : 1700 MHz, 1900 MHz

The new systems offer wider bandwidth and quicker speeds. Many new applications will be possible for both end-user consumers and for commercial/industrial users. Certainly there will be plenty of video downloading by the public, but there will also be more use of wireless for applications such as digital signage.

There will still be plenty of applications that do not need the faster/larger capabilities. Some of the Cellular M2M (machine-to-machine) applications that transmit small quantities of data may continue to operate on the legacy systems, if available. As a result, there will be differences among the types of modems offered and used in practice. The antenna needs to be compatible with the specific single-band or combination of multi-bands required by the modem in use.

Network designs, equipment manufacturers and wireless users will all play a part in determining when multi-band coverage is most critical. It may be application driven (speed or capacity requirements) or geographic driven (pace and location of roll-outs), but in the end it is likely to be driven by the need to stay flexible and available for all possible Cellular & LTE frequency requirements.

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