The realities of spectrum--Page 3.
Exploratory Band Searching
In addition to optimizing networks that already exist, there’s a lot of work going on to identify how well-suited particular potions of the spectrum are to providing new services.
The techniques in this case are more open-field configuration than the optimization of existing infrastructure.
When considering new bands, some of the challenges are in the different performance based on the physics of the frequency being used, the effects of overlaying new modulations adjacent to or on top of other signals, and impacts to user and infrastructure equipment of using a specific band. When authorizing a new band, frequency is one of the major points of interest.
All else being equal, lower frequencies will travel a farther distance than higher frequencies due to signal wavelength.
For cellular planning, this propagation distance is a very important consideration as it directly relates to the infrastructure costs of deploying a network and impacts many of the optimization criteria.
Higher frequencies, in addition to shorter propagation distances, are also generally more “line-of-sight” propagation and are more impacted by obstacles such as buildings and rain.
Depending on what the intended use is, shorter propagation distances can be considered a plus for high data rate services in that for newer modulation styles like WiMAX and LTE, the denser reuse pattern (and higher equipment cost) is considered more beneficial because once the infrastructure is invested in more user throughput per kilometer (when using equal power) and the ability to use smaller antennas in the end-user equipment.
Overlaying is consideration of the impact of new modulations to existing signals that are expected to remain in existence near or in the band being considered.
This is a very complex area, which requires that all existing uses of the spectrum be considered.
For very small area networks that use lower power, this is much less of a concern and the licensing is much less constrained.
For this reason, the Industrial, Scientific, and Medical (ISM) bands have been so successful in bringing small area networks to market.
Designated in ITU-R 5.138, 5.150, and 5.280 and initially intended for international use in the use of radio frequency (RF) energy for industrial, scientific and medical purposes other than communications.
Recently, many of these bands however have begun to be used for communication protocols such as WiFi (2.4 GHz), cordless phones (900 MHz, 2.4 GHz, and 5.8 GHz), ZigBee, 5 GHz WiMAX, and many others.
In order to evaluate the suitability of a specific frequency to be used for communication especially for large areas, significant studies are generally conducted to estimate the impact of adding the new modulation style permitted by the license in the bands and frequencies being considered.
Extensive theoretical research is done, as is specific simulation and testing.
The need to simulate use of the network before licensing has gained even more importance due to the numerous issues with Light-Square’s attempted use of the 1.5 GHz band (1525 – 1559 MHz) for LTE signals causing unforeseen interference with GPS equipment.
Modeling did not foresee all of the problems which fundamentally came down to GPS radios receiving signals in the GPS L1 band (1559 - 1610 MHz) not being sufficiently immune to interference generated in an adjacent band.
The equipment used includes much of the equipment discussed above in optimization, with the additional of signal simulators and transmitters which can be repositioned to test locations and simulate in a controlled way possible signal patterns and modulations being used.
Increased demand for wireless services at greater speeds is driving a huge increase in the demand for spectrum, which is valuable, but limited natural resource.
Meeting this demand requires overcoming numerous regulatory, financial, and time constraints for both wireless providers and government.
Because of this, there is a strong need to make the best and most efficient use of providers’ existing spectrum licenses.
Employing advance network testing and measurements tools and techniques has been demonstrated to be effective to this end.
In addition, careful consideration is required for adopting and optimizing new wireless technologies to newly-allocated frequency bands.
About the Authors
Ray Babineau is Vice President of Engineering at QRC Technologies where he oversees the development of the company’s products. He has a BSEE and an MSEE, is an expert in spectrum management automation and has worked in numerous roles including as an RF Engineer, Software Engineer, Product Manager, and Program Manager. He can be reached by email at Ray.Babineau@QrcTech.com.
Tom Callahan PMP is the general manager and CTO of QRC Technologies, where he has grown the organization to a top provider of Network Discovery Cellular Test and Measurement tools for Military and Government use. He has a BSEE and MBA, is an expert in the air interfaces for cellular protocols, and has worked in numerous roles including as DSP Engineer, Software Engineer, Systems Engineer, and is a Certified Program Management Professional (PMP). He can be reached by email at Tom.Callahan@QrcTech.com.