The Laws Of The Land

In fact, these practical problems are merely symptoms of the fact that LTE and 3G push limits defined in the most fundamental “laws” of communications theory: Shannon’s Law and a newer observation, made by Martin Cooper, sometimes called Cooper’s Law.

Shannon’s Law establishes a minimum required ratio of signal strength to noise spectral density for error-free communication. This ratio is independent of the coding and modulation scheme that is put in place. Where the channel bandwidth is very large compared to the data rate, it reaches an absolute numerical limit of 1.6 dB.

This line of reasoning can be extended to allow communications engineers to establish an absolute maximum data rate for a given channel. Again, this turns out to be independent of the specifics of modulation and coding and dependent only upon the channel bandwidth and signal-to-noise ratio in the channel.

Cooper’s Law observes that the number of radio-frequency conversations that can be concurrently conducted in a given physical region has doubled every 30 months since Marconi’s earliest radio transmissions. Just as important as the bare assertion, however, is the analysis of the driving forces behind this progression.

The 30-month doubling has yielded a 1-million-fold overall increase in capacity in the last 45 years alone. Of this, it is estimated that 25 times is due to using more spectrum; better modulation techniques have brought a fivefold improvement; and frequency division techniques a further fivefold increase. But by far the biggest factor, some 1600 times, is due to spectrum reuse. This principle is at the very heart of “cellular” communications: individual communication interchanges take place within small cells, allowing corresponding interactions in other localities to reuse the same frequencies.

The use of femtocells fits with these “laws of the universe,” established by Shannon and Cooper. Instead of using more powerful transmitters, which themselves increase noise problems for others, femtocells locate the transmitter where it is needed, improving the overall interference environment. They also exploit what Cooper’s law predicts to be the most potent force in expanding capacity, decreasing the average distance between the basestation and the user.

On a technical level, femtocell architectures deliver more than improved coverage and capacity across an individual channel. A typical macrocell might provide an aggregate bandwidth of 30 Mbits/s, designed to be shared between 100 users. Typical smaller cells provide only slightly less total bandwidth, perhaps 10 to 20 Mbits/s, but share it between 20 users at most.

Moreover, the same structural attenuation that limits the in-building coverage and capacity of macrocell networks is a positive benefit for femtocells. The walls of a building provide an effective isolation barrier, reducing interference, not just between terminals, but also between cells.

The superior radio propagation environment within the femtocell makes it more likely that optimum data rates can be attained for any given user. Effectively, the femtocell architecture wins twice. It provides more theoretical bits per second per user when fully loaded, and it offers each user a better chance of attaining the highest data rates.

Even better for a diversity-based technology such as LTE, an indoor environment represents a rich source of multi-path signals, allowing the benefits of MIMO to be maximized. As a result, LTE can work at its highest modulation rates and greatest spectral efficiency in an indoor environment.

What’s Next?

Femtocells also fit with the anticipated usage model of LTE. The transition from 2G to 3G saw a change in the nature of cellular services, from voice only to converged voice and data. LTE offers more data, at faster rates per user and lower cost per bit, but it does not offer the same kind of shift in the user paradigm.

As a consequence, its power will be in offering improved coverage and capacity in a highly targeted fashion—either to help operators surmount capacity problems or to allow them to offer tailored services to small groups of end users. This level of granularity is exactly what femtocells are good at.

It is also worth remembering that when 3G was rolled out, femtocells did not exist. Adding them to an existing network provides benefits. But rolling out a new network architecture like LTE, which is specifically designed to include them, introduces more far-reaching advantages. Not least is the fact that operators can invest their infrastructure dollars exactly where they know a return is most likely—in targeted locations where they have identified a demand for service.

Since it became clear that LTE and femtocells would go hand in hand, infrastructure makers and standards groups have undertaken substantial work to ensure that operators would see the benefits they expect. A common misconception is that femtocells are little more than “stripped-down” macrocells. In fact, nothing could be further from the truth. If they were, they would represent a very poor proposition indeed.