4G refers to the fourth generation of cellular wireless standards. It is a successor to 3G and 2G families of standards. The nomenclature of the generations generally refers to a change in the fundamental nature of the service, non-backwards compatible transmission technology and new frequency bands. The first was the move from 1981 analogue (1G) to digital (2G) transmission in 1992. This was followed, in 2002, by 3G multi-media support, spread spectrum transmission and at least 200 kbit/s, soon expected to be followed by 4G, which refers to all-IP packet-switched networks, mobile ultra-broadband (gigabit speed) access and multi-carrier transmission. Pre-4G technologies such as mobile WiMAX and first-release 3G Long term evolution (LTE) have been available on the market since 2006[
Some probable standards for the 4G system are 802.20, WiMAX (802.16), HSDPA, TDD UMTS, UMTS and prospect versions of UMTS and proprietary networks from ArrayComm Inc., Navini Networks, Flarion Technologies, and 4G efforts in India ,china and Japan.
The aim is that 4G will be based on OFDM (Orthogonal Frequency Division Multiplexing), which is the key enabler of 4G technology. Other technological aspects of 4G are adaptive processing and smart antennas, both of which will be used in 3G networks and enhance rates when used in with OFDM.
Currently 3G networks still send there data digitally over a single channel, OFDM is designed to send data over hundreds of parallel streams, thus increasing the amount of information that can be sent at a time over traditional CDMA networks.
The 4G data rates will vary depending on the number of channels that are available, and can be used. The channels that can be used will be cleaner thanks to technologies like adaptive processing, which detects interference on a channel and improves reception by actively switching channels to avoid interference.
4G networks will also use smart antenna technology, which is used to aim the radio signal in the direction of the receiver in the terminal from the base station. When teamed up with adaptive techniques, multiple antennas can cancel out more interference while enhancing the signal.
The 4G plans are still years away, but transitioning from 3G to 4G should be seamless for customers because 4G will have evolved from 3G. Users won't even have to get new phones. Digital applications are getting more common lately and are creating an increasing demand for broadband communication systems. The technical requirements for related products are very high but solutions must be cheap to implement since we are essentially talking about consumer products. For Satellite and for Cable; such cost-efficient solutions are already about for the terrestrial link (i.e. original TV broadcasting) the requirements are so high that the 'standard' solutions are no longer an option. Orthogonal Frequency Division Multiplexing (OFDM) is a technology that allows transmitting very high data rates over channels at a comparable low complexity. Orthogonal Frequency Division Multiplexing is the choice of the transmission method for the European digital radio (DAB) and Digital TV (DVB-T) standard. Owing to its great benefit’
s OFDM is being considered for future broadband application such as wireless ATM as well.
How OFDM works
First of all the FDM part - Frequency division multiplexing is a technology that transmits several signals at the same time over a single transmission path, in a medium such as a cable or wireless system. Each signal is transmitted inside its own unique frequency range (the carrier frequency), which is then modulated by the data that is needing to be transmitted.
Orthogonal FDM's spread spectrum technique spreads the data over a lot of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this method which prevents the receivers/demodulators from seeing frequencies other than their own specific one. The main benefit of OFDM is high spectral efficiency, but with OFDM you also get; high resiliency to RF interference, and the multi-path distortion is lower. This is handy because in a standard terrestrial broadcasting situation there are high amounts of multipath-channels (e.g. the signal that was sent arrives at the receiving end using multiple paths of different lengths). Since the various versions of the signal interfere with each other, known as inter symbol interference (ISI) it becomes incredibly hard to extract the original information.
Because an IIFT is used for modulation in OFDM, this spacing of the subcarriers is done in such a way the frequency where we evaluate the received signal all other signals are zero thus allowing the subchannels to overlap. But because of this, for an OFDM system to work using this method, the receiver and the transmitter must be in perfect synch, and there can’t be any multipath fading, which is unusual since finding a fix to this is one of the main goals of OFDM.
Luckily there is an easy way to solve this problem. If a guard interval is used, which is larger than the expected delay spread, which is done by artificially extending the symbol time and then removing this extension at the receiver, the problem is solved but with only a minimal loss in bandwidth.