It was just over ten years ago when a professor at USF explained in a graduate class that 3G was mature enough to be deployed commercially, if only there were some “killer app” to provide a reason someone would want that much data in a phone. Web browsers that ran on phones were crude at the time. Apps for Android and iOS became that “killer app” just a few years later. This led to an explosion in mobile data. The predictions of growth in mobile data are alarming.
Providers deal with this by using smaller cells, lower towers with smaller coverage areas. When the AMPS system was new, one base station often covered a large part of the city. The base station coverage areas were divided into hexagonal cells of equal size that fit together like a puzzle. Handoffs were “hard handoffs” that broke the current connection before connecting to the next base, causing the call to cut out for a second as it handed off from one cell to the next. These cutouts were infrequent, though, because the cell sizes were so large. 
As more users adopted mobile phones, providers used smaller cell sizes. Some bases are designed to cover just part of a building (picocells) or a single room (femto cells).
The result is a heterogenous network of large cells with smaller cells irregularly overlapping them, as in the diagram above showing multiple small cells inside larger cells serviced by the large red dots. This presents difficulties of choosing when to handoff and avoiding wasting time and bandwidth handing over to a cell the mobile device will only be in for a few seconds.
The limitations of the mobile network leads to users switching to a Wi-Fi internet connection when one is present. Businesses that provide Wi-Fi Internet access to their customers often require a login to prevent non-customers from automatically switching to their Wi-Fi access point whenever they're in range. People still use them, despite the hassle, to get higher throughput and avoid using up their allotment of mobile data.
This started as a potential threat to mobile providers , but recently mobile providers have been putting up their own Wi-Fi hotspots and working with owners of hotspots to encourage their users to switch to a Wi-Fi connection when possible to take load off the mobile data network.
The 2.4GHz Wi-Fi band is crowded and supports a maximum bandwidth of 20 MHz. (The 802.11(n) spec supports 40MHz channels in conditions with low congestion, but I have never seen it implemented.) The 5GHz bands have much more space, are currently less crowded, and the Wi-Fi (ac) spec supports 40- and 80-MHz channels in these bands. Throughput can be further increased using MIMO if channel conditions permit.
Wi-Fi's channel access scheme works amazingly well. It works by doing a clear-channel assessment before transmitting. Each data packet is acknowledged (ACKed) by the receiver. If the sender does not receive an ACK for a packet, it re-sends it at a random point during a “contention window” of possible starting points. With each retry, the duration of contention window of possible starting point doubles. After a number of failed transmissions (the number is usually set to four), it the transmitter begins requesting a clear to send (CTS) from the receiving node. All nodes that hear either side of this exchange know to relinquish the channel for a period of time. 
As a result of this system, you can have an environment with several networks, some with protocols other than Wi-Fi, transmitting a few Mbps each in the same area, and they share the available bandwidth efficiently.
A typical Wi-Fi connection with a strong signal can get 100Mbps actual throughput over a TCP connection. (The underlying data rate may be several hundred Mbps, but the actual throughput the user sees is much lower. ) A typical mobile data connection provides a few Mbps, so a single Wi-Fi channel can offload a good deal of traffic.
Although Wi-Fi is usually deployed in links under 100 meters, if the antenna is high enough to allow line-of-sight, it works very reliably over several miles. The trouble is if there are a large number of users with line-of-sight to the access point, the channel access scheme becomes inefficient. The main problem is once you get enough nodes to have frequent collisions you start wasting a lot of time in the contention window. As a result of this, Wi-Fi is complementary to the mobile data network, but it's far from being similar.
Implementations
Comcast is configuring the modem/routers they provide cable customers to act as Wi-Fi access points for any of its customers to connect to. The modem/APs provide separate network IDS for the cable subscriber and subscribers connecting to it as a hotspot. Premium subscribers can connect to any of the Comcast hotspots. Others must pay a fee.
Cable subscribers can opt out of running the Wi-Fi network for other users, but only 1% of customers do.
There's no requirement that subscribes place the modem in an upstairs location, but adding the hotspots to all modems, even though some go in basements, increases the number of places where users can access the Internet through Wi-Fi.
Reasons for Offloading to Wi-Fi
- Spectrum is free and a Wi-Fi AP is less costly than a mobile base station. I am amazed at how the unlicensed ISM bands are put to good efficient use in the absence of regulation and haven't followed the path of the 11m CB band.
- Extraordinarily high throughputs are possible with a reasonable number of users.
- 5GHz Wi-Fi bands aren't crowded, yet.
- A lower-frequency band may be opening in the “whitespace” where TV channels used to be. Some articles have given this fanciful names such as “Wi-Fi on Steroids” because it's at a lower frequency and therefore has lower propagation loss. The lower propagation loss buys 10 dB of link margin, but the main benefit is just more space.
- Wi-Fi roaming has improved allowing user to go from one AP to another without interruption.
By 2018, Cisco predicts that there will be more data traffic passing through Wi-Fi APs affiliated with the mobile data providers than on the mobile data networks themselves.
5G
Standards bodies are still working out the specifications of 5G. It is possible that some that 5G support for device-to-device communication and using different base stations for uplink and downlink will reduce the growth of Wi-Fi data.
The current practice of offloading half of mobile data to Wi-Fi access points doesn't seem an elegant solution, but it still may end up being the best solution, partly because Wi-Fi resides on unlicensed bands. I would like to see regulators open up more unlicensed bands because users have put the existing ISM bands to such good use.
