With the release of any new generation, or “G,” in the cellular world, the goal is always to outperform the previous generation when it comes to spectral efficiency—that is, how many bits you can pack into your slice of airwaves. To telecom nerds, this is expressed as bits per second per hertz (bps/Hz). Going from 3G to 5G, peak spectral efficiency skyrockets from 1.3 bps/Hz with 3G, to 16 bps/Hz with 4G LTE , to 30 bps/Hz with LTE-A, and to a truly eye-watering 145 bps/Hz with 5G (in the lab).

And it makes sense: Spectrum is an expensive and limited resource. Operators pay billions for every MHz they can acquire.

Not What It Seems

Unfortunately, the reality of spectral efficiency in deployed mobile networks is far less stratospheric. A 2017 study pegged spectral efficiencies for a live LTE network at roughly 1 bps/Hz on average with a peak of about 5.5 bps/Hz. So where did all that spectral efficiency go?

The short answer is that it ran smack into a wall. Literally! In 2016, ABI Research Director Nick Marshall said that “more than 80 percent of all traffic [is] originating or terminating indoors,” and we serve the vast majority of that traffic with outdoor cells.

The Inertia of Tradition

In the push toward 5G, we hear a lot about network densification. So far, given the amount of effort going into changing the siting rules, it sounds like the plan is to deploy more outdoor cells to help increase spectral efficiency in 5G networks. In a recent RCR Wireless News article, the headline read “US outdoor small cell antenna shipments to grow by 75% in 2018: Study” citing a study by EJL Wireless Research.

Putting aside the immense issues facing the economics of that approach (more on that in the next blog post covering our TCO analysis), it still relies on an architecture of deploying outdoor cells to handle a largely indoor traffic load. It still puts literal barriers in the way of increased spectral efficiency.

Airtime Perspective

Let’s quantify this issue a bit to make sure we have a shared perspective on the system capacity impact of using outdoor cells to handle indoor traffic because it’s a big deal.

Sending a typical video packet from an outdoor cell to an outdoor user takes 33 resource blocks, whereas sending that same frame to a deep indoor user can take 209 resource blocks (1500B IP packet, I_TBS 3 vs I_TBS 19, TM2 with 2TRx)! On average, it takes seven times more airtime resources to serve an indoor user than an outdoor user.

Given the inefficiency, why are we still trying to cross the walls?

User Behavior

It’s probably not news to anyone that indoor penetration is costly. A common industry view says that when a user is indoors, his or her data should be served by Wi-Fi to offload the burden on the cellular network. Industry reports are produced every year showing that large amounts of traffic from mobile devices are offloaded to Wi-Fi networks (e.g., ~80 percent in 2017).

However, as the industry moves toward unlimited data plans, and as mobile speeds increase, the incentives for seeking out Wi-Fi for offload are diminishing. A recent CableLabs Strategy Brief (CableLabs membership login required) provides empirical data showing that Wi-Fi data offload is declining as adoption of unlimited data plans increases. The trend, across all age groups, shows increased cellular data usage. So as demand for cellular data is going up, an increasing portion is going to be crossing the walls.

There are a number of long-held complaints about the Wi-Fi user experience. I won’t enumerate them here, but I’ll point out that as the incentives to offload data to Wi-Fi are weakened, even the slightest hiccup in the Wi-Fi user experience will drive a user away from that offload opportunity at the expense of your cellular system capacity.

Introducing Low-Cost Femtocells

There’s a growing breed of operator that has both cellular operations and traditional cable hybrid fiber coax (HFC) infrastructure—a big wired network and a big wireless network (Note: here I am talking about full MNOs with HFC/DOCSIS networks, not MVNOs. MVNOs with HFC/DOCSIS networks will have different goals in what optimizing looks like). For these operators, the carrots of convergence dangle in all directions.

Over the past couple of years, CableLabs has ramped up efforts to solve the technology issues that have traditionally hindered convergence. Latency concerns for backhaul or vRAN fronthaul can be resolved by the innovative Bandwidth Report project. CableLabs leadership in the TIP vRAN Fronthaul project is making latency-tolerant fronthaul protocols a reality. Timing and synchronization challenges presented by indoor deployments are months away from commercialization, thanks to CableLabs’ new synchronization spec.

The summation of these projects (and more on the way) provides a suite of tools that converged operators can leverage to deploy mobile services over their HFC/DOCSIS network.

Enter the femtocell deployment model. Femtocells aren’t new, but with the new technologies developed by CableLabs, for the first time, they can be done right. Gone are the days of failed GPS lock, poor handover performance, and interference issues (topics of our 3^rd blog in this series). From a spectral and economic viewpoint, femtocells over DOCSIS are poised to be the most efficient deployment model for 4G evolution and 5G cellular densification.

Wi-Fi Precedence

Take Wi-Fi as a guide to how femtocells can improve spectral efficiency. Modern Wi-Fi routers—even cheap home routers—regularly provide devices with physical link rates approaching 10 bps/Hz. That is a huge gain over the sub-1 bps/Hz achieved using an outdoor cell to serve an indoor user. In such a scenario, the benefits are myriad and shared between the user and the operator: The user experience is dramatically improved, the operator sees huge savings in outdoor system capacity, and it all occurs with more favorable economics compared to traditional small cell strategy.

When selectively deployed alongside home Wi-Fi hotspots, indoor femtocells give the converged operator the chance to capture the majority of indoor traffic with an indoor radio, freeing the outdoor radio to better serve outdoor traffic.

More Discussion to Come

In this post, I talked about the spectral efficiency problems of traditional outdoor small cell deployments and how a femtocell deployment model can address them. Next time, I’ll discuss a total cost of ownership (TCO) model for femtocells over a DOCSIS network, both full-stack and vRAN-based solutions.

And don’t take my word for it! Stay tuned to the CableLabs blog over the next couple months for more discussions about cellular deployments over a DOCSIS network.