Cell Network Physics

Physics plays quite a large role when designing and operating a mobile network, considering the effects of radio signal frequency, reach, data capacity, modulation technique, cell size and cell use patterns. 

Since the time of first-generation analog cellular services, the amount of spectrum devoted to mobile service has both increased and moved higher in frequency, as this chart illustrates. All of us are familiar with at least some elements of higher-frequency spectrum. 

The 800-MHz radio waves used to support 1G networks were pretty good at penetrating walls. By the time we get to 4G, using spectrum up around 2 GHz, signals have limited ability to get through walls. In the 5G era, using resources up to about 4 GHz to 6 GHz, signals are directional, and will not get through a plant leaf. 

source: Keysight Technologies 

So all sorts of signal processing has to be used to create multiple paths for those signals, to get around the line-of-sight propagation pattern. 

Another issue is that radio signals at higher frequencies, using power levels common for cell networks, will not travel as far as they do when launched at similar power levels but using lower frequencies. 

That means smaller cells must be used, and that accounts for the increasing importance of optical fiber distribution networks, a trend that will continue as we continue to move up in frequency for subsequent mobile generations. 

The other beneficial aspect of higher-frequency radio waves, however, is their capacity. Radio signal frequency and capacity are directly related: the higher the frequency, the greater the capacity. 

So both smaller cells and higher frequencies mean the amount of bandwidth a mobile network can supply will increase dramatically. Looking only at frequency, frequencies in the 24-GHz range have two orders of magnitude more physical capacity than 800-MHz signals. 

source: IEEE 

Modulation schemes add more capacity. Looking only at quadrature amplitude modulation, the number of bits we can encode per symbol increases capacity, all other things being equal (channel size, for example). 

source: Microwave Link 

Smaller cells also help, by intensifying the degree of frequency reuse possible in any geographic area. All other things remaining equal, shrinking cell radius by 50 percent quadruples the total number of cells. 

source: Slideshare 

But those are techniques used on mobile networks alone. These days, mobile data traffic also can be offloaded to fixed networks when users connect their mobile devices to Wi-Fi. In the 5G era, perhaps as much as 70 percent of mobile data traffic actually is offloaded to fixed networks in the form of Wi-Fi access. 

source: Spectrum Futures

The other interesting angle is that mobile network traffic shows a Pareto distribution. About 75 percent of total traffic occurs on just about 30 percent of cell locations. 

source: Spectrum Matters