Design Challenges Multiply as Frequencies Added

Mon, 09/12/2011 - 9:48am
Jerry Posluszny, Mobile Mark, Inc.

erry Posluszny, Engineering Manager, Mobile Mark, Inc.

Additional frequencies need to be covered
US cellular networks are moving beyond the established 850 & 1900 MHz bands to include addition frequencies such as the 700 MHz band and the new AWS 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 summarizes the frequency bands for the wireless systems currently available in the US.

System                                        Frequency bands covered
US AMPS                                     824-894 MHz
US GSM/CDMA                        824-894 & 1850-1990 MHz
European GSM                         870-960 & 1710-1880 MHz
UMTS                                             1920-2170 MHz
WiMAX/LTE                                2496-2690 MHz
AWS                                                1710-1755 & 2110-2155 MHz
700 MHz/LTE                              694-804 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 finally able to present good designs.

Device manufacturers then began receiving modems which were designed not only for the US Cellular frequencies, but now included the European bands. To make the designs work, the designers started using wideband elements instead of just dual-band designs.

The use of wideband elements that could cover US and European frequencies meant that any rejection of spurious emissions that the original dual-band designs had would be gone. However, in some cases the wideband configurations would not be as efficient as the dualband designs 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 operate up to 2100 MHz, the total bandwidth coverage area was expanding.

The latest challenge is the opening up of the 700MHz band, 694-806 MHz, for LTE. Eventually, some modems will cover the 2.5 GHz band which was initially rolled out for WiMAX but is likely to be used in the future for LTE.

The low frequency range of the 700 MHz LTE band, with a longer wavelength, provides the greatest design challenge so far. Adding frequency coverage at the lower end of a band is more difficult because each additional megahertz added represents a larger percentage of the frequency point.

Maintaining the size of the antenna is a challenge, again due to the fact the wavelength at 700 MHz is longer than the wavelength at higher frequencies, and yet market demands call from ever smaller products.

The modem manufacturers where able to very quickly add new bands, such as the LTE 700 MHz band, to their products and antenna designers are working to respond. The quest is for high efficiency, small antennas that cover all of the Cellular frequency bands, including the new LTE bands. Some antenna designers are developing innovative antennas to cover all the frequency ranges in single designs. Mobile Mark, for example, offers several wideband antenna styles that cover the entire 694 MHz – 2.7 GHz band.

Rate of roll-outs only one aspect of the challenge
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 bands 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                                          1900 MHz, 2100 MHz, 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 greater use of high capacity wireless for commercial 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 legacy systems, if available. As a result, there will likely 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 frequencies.

Multi-element antenna offer different solutions than multiband antennas
Installers and network designers should be prepared to handle unusual requirements before the new systems are fully rolled out. For example, there may be applications which will require two different modems which might involve different frequency coverage. This can be handled with two different antennas, but a multi-element antenna solution in a single radome might make it easier to migrate from one application to the other.

A MIMO (Multiple-Input-Multiple-Output) network also requires multiple antenna elements; but in this case the elements are part of the same network. MIMO networks typically come with 2, 3, 4 or 6 elements, each with a separate feed. The elements may be single, dual-band or wideband.

Certification Test Requirements may change with new frequency bands
Some US Carriers currently require PTCRB approval for Cellular M2M devices that will operate on their networks. At present, the official CTIA test guidelines cover 850 & 1900 MHz, but are likely to cover the additional frequencies in the future. In addition to the design of the modem, the efficiency specification of the antenna design is a key determinant of whether the wireless device will pass certification. With multiband antennas, having high efficiency over the wide frequency ranges can be difficult. Antennas intended submission for testing should be selected with care as not all designs will perform equally well. 

Mobile Mark - RM Series-web1
This high efficiency surface mount antenna from Mobile Mark has been used on numerous systems that have passed PTCRB testing. A new wideband version that covers all Cellular, LTE and WiMAX bands from 694 MHz – 2.7 GHz is now available.

Wideband site antennas face special performance challenges
Traditional antennas used in directional applications have had consistent radiation patterns across the designed frequency. But now with the very wide frequency ranges that are available, consistent radiation patterns are more of a challenge.

Consider antenna radiation from the most basic element, the vertical half wavelength dipole. It is omni-directional in the azimuth plane. To make the basic dipole antenna into a directional antenna, a reflector is added behind the dipole so that some of the radiated energy from the dipole is reflected forward where it adds to the rest of the energy to provide gain. The distance of the reflector is dependent on the wavelength of the antenna.

The reflector might be one quarter wave-length behind the dipole element to provide the optimum gain. One quarter wave-length in the 700 MHz band is approximately 4 inches (10cm) while in the UMTS band (2100 MHz) one quarter wavelength is approximately 1.5 inches (0.6cm). So if the distance is set for the 700 MHz band and the antenna is exhibiting nice patterns, then at the UMTS band the dipole would be much farther than one quarter wavelength and the patterns would be quite different.

Radiation pattern #1
: Typical radiation pattern for a quarterwave antenna at 750 MHz with a reflector placed 4 inches behind it.

Radiation pattern #2
: Typical radiation pattern for a quarterwave antenna at 2100 MHz with a reflector placed 4 inches behind it.

The challenge of designing antennas for such wide frequency ranges is very daunting. Special techniques need to be employed to achieve consistent patterns and gain over the range. Antennas may be characterized with different performance over different sub-bands, or the efficiency of the antenna may suffer, or antennas may be offered that do not cover all of the frequencies.

Fortunately, omni-directional antennas present fewer problems than Directional designs. Directional antennas have elements, such as reflectors, that are very frequency dependent but omni-directional antennas have the ability to use wideband elements stacked vertically to provide gain. With proper phasing and network design the omni-directional designs can easily achieve consistent performance.

Responding to the challenge
Antenna designs should be evaluated based on VSWR, efficiency and the radiation pattern across all bands targeted. The new wideband requirements, which range from a low frequency of 694 MHz to a high frequency of 2.1 GHz or even 2.7 GHz, make it difficult to design an antenna element that performs equally well at all points. Trade-offs are inevitable and there are no simple tricks. Review each manufacturer’s specifications and conduct the proper due diligence testing to ensure the antenna selection will measure up to the task at hand.

Posted by Janine E. Mooney, Associate Editor



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