My brother-in-law recently referred me to an article that spoke about 4G having speeds up to 300Mbps. I can assure you that these are fantasy numbers. Then again, anything you read today about LTE or WiMAX Mobile will usually give you engineering numbers, or marketing's translation of engineering numbers, which is usually the same as "theoretical", or "no way in hell you are going to see those speeds in real life."
I'll give you real world data based on my direct experience with the GSM Alliance Services Working Group and current LTE standards.
To understand the difference between theoretical and actual data rates you need to consider 3 variables:
Terminal Class - There are 5 terminal classes. Terminal Classes range from Voice Only to Peak Data Rate devices. I do not know of any Voice Only or Data Only devices. PC Cards/modems are the only possible example of Data Only, but most of them could support voice anyway because the radio equipment the manufacturers buy is cheaper with voice and data than with data only. GPRS (2G) and EDGE (2.5G) can do voice or data, but only one at a time. If I am downloading a file and get a call, the data session is suspended until the voice call is completed. GSM 3G is the only current technology that supports simultaneous voice and data over the cellular network. If your phone is generating a voice call on the network, it is taking up space that could be used by the data session on the same phone. This means that data is a bit slower when being used simultaneously with a voice call. This is one reason GSM carriers like to put WiFi radios in their 2.5G phones. WiFi is a completely separate radio, so a phone call and a WiFi session can coexist without any loss of performance on either service. If you have a smartphone with WiFi, try to surf the web and make a voice call at the same time. It is cool, and will help promote bluetooth headsets so you can surf and talk at the same time comfortably.
Timeslots (GPRS) or Spectrum Efficiency (3G/4G) - In my standards days, the cellular channels were subdivided into 8 timeslots. Think of these as 8 lanes of traffic on the freeway. The engineers would publish potential data speeds using all 8 timeslots. It made the numbers very impressive, but it was unrealistic for several reasons.
1) It was foolish to assume that you wouldn't have voice traffic on the same channel/freeway. Cellular networks have always been designed to give voice calls priority over data, therefore data never could have had unlimited access to all 8 lanes.
2) A radio that transmitted and/or received on 8 timeslots simultaneously would chew up and spit out a fully charged battery in a couple hours. It would also generate enough heat to melt the plastics in the phone. The fastest devices in that day used only five timeslots at one time, usually 4 downloading and one uploading.
Today 3G/4G uses spectrum efficiency to judge peak data rates. The current 4G peak data rate forecasts are based on 20MHz of radio spectrum. Translated into English, this means that the data device will need to use the entire width of the highway in order to move that much data in so little time. Try downloading a song on your iPhone 3G near a freeway at evening rush hour. It will be SLOW. Try the same download at 2 in the morning and it will zip unencumbered by voice or other data traffic. Since other data traffic does play a factor, a geek trade show will cause your phone to lag where a dog show may not.
As with GPRS, it is unrealistic to give a single user the right to hog all of the width of the highway so they can move more data while everyone else is waiting on an on-ramp for their turn at the data network. Now, LTE standards are written to give 4G data devices access to the whole width of the road, but I expect that this will only play out in the overnight hours and/or in the early stages when 4G traffic is scarce on the new networks.
Forward Error Correction (FEC) - Gizmodo alluded to this. Basically, if the connection is strong and clean, the phone and network can use less space for error correction algorithms. When the data connection is weak and dirty, the phone and network actually use more space to keep the data clean than they do for the data itself. The engineers do all of their preliminary data modeling in a lab, so their results tend to be based on very clean, very strong data connections. The real world ain't so kind.
A fourth consideration that most people overlook is backhaul. Backhaul refers to the amount of bandwidth the carrier has between their towers and their switch. Often times, the backhaul is less than the capacity of the radios on the tower. This means that even though the radios could handle dozens of simultaneous data sessions at a high throughput, the backbone can only support 2/3 or even 1/2 of that. The reason is mainly economics. Most backhaul from a tower is done over another company's lines. It is just plain expensive to buy enough bandwidth to support all the calls that may hit someday, so they budget for normal traffic and let peak traffic suffer. Many people who know the tech behind the networks believe this was the real cause of the iPhone network troubles in the early days. They just didn't have enough pipe to move the amount of data iPhone users were demanding.
Again, the biggest difference is that 3G uses the open space between phone calls on the network, where GPRS used dedicated timeslots. GPRS had a theoretical limit of over 170kbps, but the reality was usually about 30kbps. The newest 3G technology (HSPA) has a theoretical download of over 17Mbps, but performs at about 1Mbps to 1.4Mbps. The LTE standards list a theoretical peak downlink data rate of over 300Mbps, but AT&T is touting a theoretical LTE speed of 100Mbps and an expected real world performance of 10Mbps.
I realize that these numbers put a damper on a lot of the grandiose marketing messages and cool new broadband applications that the wireless companies are promising, but until LTE hits the street, all of the numbers are theoretical. I expect LTE to hit the street at 10Mbps, but improve over the subsequent years to around 100Mpbs.
And, so you don't worry about this merry-go-round ride coming to a stop, LTE Advanced standards are already in development.
Phones will always get faster, but rarely as fast as the wireless evangelists promise as quick as they promise it.