Will We Run Out of Spectrum?


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If we apply the Law of Conservation of Energy to spectrum, it might read: “Spectrum can neither be created nor destroyed. It can only be reused, repurposed or shared.” It’s physics, after all. The Radio Spectrum is well-defined in the 3kHz to 300 GHz range in the electromagnetic spectrum.

With next generation wireless, the question is whether there is enough spectrum to meet the demands of high-speed data throughput, coverage, and latency needed to support the myriad of use cases and applications that are envisaged.

The short answer is yes, for the foreseeable future. That is not to say there are swaths of spectrum sitting idle and available for the pickings. On the contrary, almost all bandwidth within the Radio Spectrum is already in use. Freeing up specific frequency bands and associated bandwidth will take a lot of regulatory flexibility to reassign spectrum and release new bands for future wireless communications. At the same time, vendors need to deliver new radio and antenna designs capable of meeting the expected performance in those bands. 

The main driver, of course, is mobile data that grows exponentially. There is correlation between bandwidth, channel capacity and data throughput. In general, data rate depends on available signal bandwidth for transmission, channel size within the signal bandwidth, signal-to-noise ratio, and receiver sensitivity. The greater the transmitted signal bandwidth, the higher the data-carrying capacity. A host of transmission factors limit data throughput: transmit frequency, eradiated power, modulation, distance, coverage, terrain, interference, and ultimately, the radio receiver capability.

Early cellular systems at 850 MHz band used narrow channels but covered wide areas for voice and text messaging. Cellular networks reuse the same frequencies in alternate cells or in other areas. Higher frequency 1900 MHz used wider channels to accommodate low-speed data and internet access over large coverage areas with the same frequency reuse capability.  

Repurposing low-band 700 MHz from prior analog TV channel usage along new mid-band AWS (1700/2100 MHz) opened up more spectrum for 4G LTE use and greater data-handling capacity. The 600 MHz ‘repack’ is another reuse example; TV broadcasters agreed to move to other frequency bands to make room for high-speed 4G LTE/5G services. Satellite downlink channels in the 3.5 GHz C-band also are targeted for 5G. At the same time, the accumulated 2.5 GHz mid-band spectrum makes available 194 MHz of signal bandwidth in very wide channel assignments.

Licensed frequency operation assures the licensee of unencumbered transmission within the licensed area. Sharing is becoming a hallmark for use of unlicensed bands for commercial wireless services. Unlicensed WiFi and fixed wireless access (FWA) applications offer cost-effective connectivity but are highly susceptible to interference. Managed sharing is a longer-term trend. FCC-approved spectrum access system (SAS) administrators facilitate spectrum sharing among multiple users ensuring quasi-dedicated access and no interference. 

Under the U.S.’s Citizens Broadband Radio Service (CBRS) plan, SASs protect 3.5 GHz incumbent access users from interference by priority access license (PAL) holders and general authorized access (GAA) unlicensed users who all want to share CBRS’ 150 MHz capacity. Dynamic Spectrum Sharing (DSS) is a new technology that accommodates 4G and 5G on the same spectrum without dedicating blocks of existing 4G spectrum to 5G.

Along with low-band 600/700 MHz and mid-band 2.5/3.5 GHz, millimeter wave (mmW) high-band frequencies at 24, 28, 37, 39, and 47 GHz support 5G on channels of 50 MHz or more. mmW transmission must be short range to be effective, however. Low-band frequencies achieve wide area coverage but not the data throughput or latency specs. Mid-band 2.5 and 3.5 GHz bands seem to offer the right balance among coverage, throughput and latency, and are highly attractive for 5G operation.

We have spectrum. It all depends how we use it.

By John Celentano, Business Editor, Inside Towers

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