The first generation of communication system is the analog cellular system, which means the carrier is analog. we just send waveforms primarily with voice. it is continuous in amplitude and in time. the variation of a signal is disruptive over long distances. This analog system was introduced in the 70s. NMT, AMPS, TACS and CDPD technologies were used in 1G. It uses the FDMA technique to call a country. All analog systems used circuit-switched technology and were designed only for voice communication and not for data. The cost of system and voice transfer is also high in 1G. The problems with 1G are poor voice quality, short battery life, no security, phone size is large, and calls are frequent. The second generation is digital cellular and started sending 1s and 0s with the air. it is a circuit-switched, narrow-band technology. It is a low-band digital cellular signaling system. Digital is more spectrally efficient and uses technologies such as TDMA, GSM, CDMA, iDEN, PDC, PHS. It uses digital modulation for better audio quality. GSM technology has both FDMA and TDMA techniques. The number of GSM system users worldwide is estimated to be 248 million and it is currently used in 140 countries. In addition to this, PDC (Personal Digital Communication) technology based on TDMA is introduced. After the introduction of this CDMA technology, it divides speech into small segments and is coded to identify calls using spread spectrum. While GSM and TDMA systems are the most important in 2G wireless technology, CDMA technology has proven to provide better voice quality, low noise, few call drops, good security and capacity high network. Transmission of data, faxes and messages at up to 9.6 kbps is possible over 2G, but is not acceptable for multimedia and web browsing applications....... middle of paper .... .. spectrally efficient modulation schemes which cannot be applied to 3G infrastructure. The typical speed of 3G is limited to 2 Mbps, while the speed of 4G can reach 100 Mbps and also eliminates the interference caused by the high-speed signal. 4G technology also provides uninterrupted voice, navigation and data services. This speed is obtained by the OFDM modulation technique. Some of the other techniques used are scheduling, power control and adaptive modulation, wireless and smart millimeter antennas. 4G Requirements The advantages of 4G are high speed and it also has greater bandwidth, it uses multi-hop and Adhoc network and the spectral efficiency of 4G is high, it provides service portability and access worldwide, it is a low cost IP based mobile system. We've never seen a 3G phone working on a 4G network. some operators having both 3G and 4G network like Verizon.