The widespread adoption of smartphones has driven a rapid change in mobile communications. The various wireless technologies known collectively as 3G were optimized to be efficient at handling voice traffic. However, smartphones have shifted this dynamic so that data traffic in various forms now dominates. Add to this that one-in-three tablet computers feature cellular connectivity, and it becomes clear why data revenues for US cellular operators now exceed voice revenues.
While revenues may be roughly comparable, the number of bits that must be transmitted is significantly different. Data traffic is estimated to consume around 85% of the total traffic and is growing at 20% per year, so operators must optimize their networks to accommodate this shift. Fortunately, this transition was anticipated: The technology known as Long Term Evolution (LTE) has been specified by the 3rd Generation Partnership Project (3GPP), and new equipment is rolling out around the world.
LTE has been designed to be far more efficient at transporting bits; it approaches the maximum information rate that can be achieved within a given bandwidth, as defined by Shannon's Law. The original LTE has been superseded already by LTE-Advanced, and “5G” is slated for the end of the decade.
To achieve an even higher throughput, the operators have a number of options. They are allocating more bandwidth to LTE to provide fatter pipes. The target maximum bit rate for a 20 MHz bandwidth is 100 Mbit/s for download and 75 Mbit/s for upload. Operators are also “re-farming” spectrum from older technologies by changing out the equipment to support LTE technology. Globally, there are more than 40 different channels specified for LTE use, which is a major challenge for equipment vendors wishing to serve all the markets. However, there is a finite amount of spectrum, and operators must live or die by their available resources.
Even with the enhanced spectral efficiency from LTE, it became clear that more needed to be done. It was realized that simple networks are no longer appropriate. The older technologies rely on a small number of higher-power cellular towers to provide coverage over a wide area. These are connected to macro base stations, which support multiple users simultaneously.
There are several issues with this setup, including the fact that the signal level reduces by the inverse square of the distance. As a result, the signal-to-noise ratio (SNR) worsens as you move away from the tower. The result is that the data rate experienced by the user will be dependent on the distance to the nearest tower. Operators cannot boost the transmitted power, as this would create interference with adjacent cells. In any event, that would only affect the download data rate, but the users would still be limited by the much lower power that their equipment can output. A further consideration is that — since more of the available bandwidth must be dedicated to each user — the macro base station can support fewer simultaneous users, which is obviously detrimental to the operator.
Exacerbating the problem, a lot of the newly released spectrum is at much higher frequencies than the original 700 to 850 MHz. The higher frequency of up to 2.7 GHz has two effects. First, the signals are more easily absorbed by buildings and trees, so that the effective range of a tower is reduced. Second, this absorption means that signals do not penetrate into buildings, which is where much of the traffic originates.
One solution to these problems is for the operator to reconfigure his cell boundaries and to install more macro base stations that are designed to service a small-cell area. This is not a practical solution. The costs associated with site acquisition, equipment installation, and obtaining planning consent for tall masts are significant. Planning restrictions present a significant barrier in many localities. The unsightly towers are a detrimental change to the landscape, and adverse public reaction to them should not be underestimated.
Again, this scenario has been anticipated by 3GPP, and the LTE specification includes the provision for operators to move to a new cellular arrangement known as heterogeneous wireless networks (HWN). A typical system will still employ a macro base station for wide area coverage, but it also uses a range of small cells to supplement the service.
Vendors are producing micro and metro cells, along with picocells and femtocells. Each has different characteristics in terms of effective range and the number of simultaneous calls that are supported. The small cells sit inside the coverage area of the macro system, and act as off-loads for the macro. Excellent examples of sites that have heavy usage in a very local area are shopping malls and airport terminals. These are ideal for small-cell offloads. In the central business districts of cities, there is heavy demand during the working day contrasted by light traffic at night. The dynamic nature of the load allows the small cells to be powered down to save energy when not required.
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