Technical Definition
In March 2008, the International Telecommunications Union-Radio communications sector (ITU-R) specified a set of requirements for 4G standards, named the International Mobile Telecommunications Advanced (IMT-Advanced) specification, setting peak speed requirements for 4G service at 100 megabits per second (Mbit/s) for high mobility communication (such as from trains and cars) and 1 gigabit per second (Gbit/s) for low mobility communication (such as pedestrians and stationary users).
Since the first-release versions of Mobile WiMAX and LTE support much less than 1 Gbit/s peak bit rate, they are not fully IMT-Advanced compliant, but are often branded 4G by service providers. On December 6, 2010, ITU-R recognized that these two technologies, as well as other beyond-3G technologies that do not fulfill the IMT-Advanced requirements, could nevertheless be considered "4G", provided they represent forerunners to IMT-Advanced compliant versions and "a substantial level of improvement in performance and capabilities with respect to the initial third generation systems now deployed".
Mobile WiMAX Release 2 (also known as WirelessMAN-Advanced or IEEE 802.16m') and LTE Advanced (LTE-A) are IMT-Advanced compliant backwards compatible versions of the above two systems, standardized during the spring 2011, and promising speeds in the order of 1 Gbit/s. Services are expected in 2013.
As opposed to earlier generations, a 4G system does not support traditional circuit-switched telephony service, but all-Internet Protocol (IP) based communication such as IP telephony. As seen below, the spread spectrum radio technology used in 3G systems, is abandoned in all 4G candidate systems and replaced by OFDMA multi-carrier transmission and other frequency-domain equalization (FDE) schemes, making it possible to transfer very high bit rates despite extensive multi-path radio propagation (echoes). The peak bit rate is further improved by smart antenna arrays for multiple-input multiple-output (MIMO) communications.
The term "generation" used to name successive evolutions of radio networks in general is arbitrary. There are several interpretations of it, and no official definition has been made despite the large consensus behind ITU-R's labels. From ITU-R's point of view, 4G is equivalent to IMT-Advanced which has specific performance requirements as explained below. But according operators, a generation of network refers to the deployment of a new non-backward-compatible technology. This usually corresponds to a huge investment with its own depreciation period, marketing strategy (if any), and deployment phases. It can even be different among operators. From the end user's point of view, only performance and cost makes sense. It is expected that the next generation of network performs better and cheaper than the previous generation, which is not that simple to state. Indeed, while a new generation of network arrives, the previous one can keep evolving to a point where it outperforms the first version of the new generation. In many countries, GSM, UMTS and LTE networks still coexist. It is thus much less ambiguous to use the name of the technology/standard, possibly followed by its version number, than a subjective arbitrary generation number which is destined to be challenged endlessly.
Read more about this topic: Beyond 3G
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