What is 3G?
The G of course means generation. Cell phones and systems are classified by the generation they belong to. Third generation (3G) phones were developed in the late 1990s and 2000s. The goal was to improve the data capability and speed. 3G phones were defined by the Third Generation Partnership Project (3GPP) and later standardized by the ITU-T. Generally known as the Universal Mobile Telecomunications System (UMTS), this 3G system is based on wideband CDMA that operates in 5 MHz of bandwidth and can produce download data rates of typically 384 kb/s under normal conditions and up to 2 Mb/s in some instances. Another 3G standard, cdma2000, was developed by Qualcomm. It uses 1.25 MHz bands to produce data rates to 2 Mb/s. Another version of cdma2000 is an improved IS-95 version. It is a 3GPP2 standard. It can transmit data at a rate to 153 kb/s and up to 2 Mb/s in some cases.
3G phone standards have been expanded and enhanced to further expand data speed and capacity. The WCDMA phones have added high speed packet access (HSPA) that use higher level QAM modulation to get speeds up to 21 or 42 Mb/s downlink (cell site to phone) and up to 7 and/or 14 Mb/s uplink (phone to cell site). AT&T and T-Mobile use HSPA technology. The cdma2000 phones added 1xRTT as well as Rev. A and Rev B modifications that boost speed as well. Verizon and Sprint use cdma2000 3G standard technology. Virtually all standard and smartphone models and most tablets still use some form of 3G.
Long Term Evolution (LTE)
LTE is what most are calling 4G but in reality it is just an advanced 3G standard that some call 3.9G. LTE uses a completely different radio technology. Instead of CDMA, it uses orthogonal frequency division multiplexing (OFDM) and OFDM access. This modulation technique divides a channel usually 5, 10 or 20 MHz wide into smaller subchannels or subcarriers each 15 kHz wide. Each is modulated with part of the data. The fast data is divided into slower streams that modulate the subcarriers with one of several modulation schemes like QPSK or 16QAM.
LTE also defines multiple input multiple output (MIMO) operation that uses several transmitter-receiver-antennas. The data stream is divided between the antennas to boost speed and to make the link more reliable. Using OFDM and MIMO lets LTE deliver data at a rate to 100 Mb/s downstream and 50 Mb/s upstream under the best conditions.
Verizon and AT&T are using LTE but it is not widespread yet. Sprint and partner Clearwire use WiMAX. They call WiMAX 4G but by definition, WiMAX is also 3G. Sprint and Clearwire have plans to switch to LTE in the future.
Incidentally, keep in mind that 3G and 4G technologies are data only. Voice is not carried by this technology. Instead, all voice calls are still via 2G GSM or cdma2000. All 3G/4G handsets contain multiple radios, some for voice and some for data. Eventually voice will be carried over LTE but it is not there yet.
Despite the use of a completely different technology, the 3GPP and ITU-T still call LTE a 3G technology. If that is the case…
What is 4G?
The fourth generation has been defined but we are not in it, yet. Yes, many if not most of the mobile carriers and the various phone and equipment manufacturers actually advertise 4G now. The formal definition of 4G as declared by the 3GPP and the ITU-T is something called Long Term Evolution-Advanced (LTE-A). The standard has not been fully completed but basically it is an improved and enhanced version of LTE that uses wider bandwidth channels and a greater number of MIMO antennas. The theoretical upper data rate is 1 Gb/s. That remains to be seen in practice.
As for what the various companies are calling 4G, Verizon says that their LTE network is 4G. AT&T promotes their LTE and HSPA networks as 4G. T-Mobile indicates that their HSPA+ networks are 4G. Furthermore Sprint and Clearwire say that their WiMAX network is 4G. As mentioned, WiMAX is actually defined as a 3G technology by ITU-T like LTE.
Since both LTE and WiMAX are completely different technologically from previously defined WCDMA/cdma2000 3G, it seems appropriate to say LTE and WiMAX are 4G. Both are a step up in data speed. It all depends on your interpretation and acceptance of what 4G is. If the carriers are calling LTE 4G now, will they call LTE-Advanced 5G. If it is good marketing, they may.