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2G systems use telephone like circuit switching for voice communication in an efficient way but were unable to transmit data with higher data rates due to its spectral inefficiency. An evolution of 3G system is due to the fact of desire to have higher data rates which is observed in the characteristics of International Mobile Telecommunication 2000(IMT 2000) standard. The expectation of IMT 2000 with 3G systems had 2G compatibility, higher capacity, multimedia support and high data rates (up to 2Mb/s for stationary, 384kb/s for pedestrian and 144kb/s for high moving environment). During the evolution from 2G systems to 3G systems, many intermediate systems, some based on TDMA while other on CDMA, were introduced and termed as 2.5G systems. 
Initially the IMT-2000 union - Radiocommunication (ITU-R) sector unify all wireless system including satellite networks, cellular network, WLANs, and fixed wireless links in the same frequency band and on ATM based protocols . But after some period of time, as the wireless operates better at higher frequency and WLAN can achieve higher rates at unlicensed band, the fixed and WLANs are discarded from the group leaving only satellite and cellular systems .
After a very long wait and number of proposals, in the year of 2000, IMT-2000 recommended five modes of operation which includes Direct Spread (IMT-DS), Multi-Carrier (IMT-MC), Time Code (IMT-TC), Single Carrier (IMT-SC), and Frequency Time (IMT-FT) . Some popular systems comprises 3G technology includes CDMA2000 and Wideband CDMA (W-CDMA).The W-CDMA based upon Universal Mobile Telecommunication System (UMTS) manage by the 3rd generation partnership project (3GPP) which uses GSM-MAP core network while CDMA 2000, which built upon the ANSI/TIA/EIA-41 core network, is manage by 3GPP2 organisation .
Coding technique used in 3G:
The main reason of the evolution of 3G technology lies in the need of very high data rate at minimum BER. Hence, 3G system requires a coding technique that can improve the performance in term of data rate closer to the Shannon limit which can be achieved with Turbo codes. Hence, many 3G system quickly adopted Turbo coding . But later on it was found out that the use of the turbo code increases delay in the system due to interleaving and iterated decoding processing . Hence, it is used only in the case of data packet transmission and those applications where time delay is not a concern .
Modulation Scheme used in 3G:
Turbo code, employed in the 3G system, enables the use of different and higher order modulation scheme for higher data rates. 3G system employs 2-ary QAM to 64-ary QAM modulation scheme in which phase and amplitude are varied to generate symbols. 
Multiple Access technique used in 3G:
Supporting huge number of users with higher data rates, 3G systems adopt CDMA technique, which consider being more robust than TDMA or FDMA on the basis of multi path fading, provides higher capacity, good coverage with less cell sites, better frequency reuse, to provide multiple access for wide number of users. The CDMA based systems includes Wideband CDMA (W-CDMA) and CDMA 2000 and proved to be more popular and robust communication system than the TDMA systems including IMT-SC and IMT-FT. 
Towards Fourth Generation (4G):
The next era of communication technology will merge the cellular as well as wireless communication technologies together and give rise to a new technology called as 4G Systems. 
Though the 4G system is not yet standardised, but it is already well under way in the many wireless . operator's networks. Some providers already started claiming the systems which they are providing is 4G systems by proving high data rate from 1Mb/s to 5Mb/s while others claim the systems as 3.5G or 3.9G .
Systems like WiMax networks (Worldwide Interoperability for Microwave access) and LTE (Long Term Evolution) initiates 4G evolution. LTE, UMTS standard, is now been standardised as 3rd Generation Partnership Project. And the 3GPP2 has standardized Ultra Mobile Broadband (UMB) to improve CDMA for 4G applications and requirements .
Figure: Evolution towards 4G 
Candidates service beyond 3G:
High speed Downlink Packet Access (HSDPA):
As per its name, HSDPA stands for the technology of packet data transmission  and basically an upgraded version of UTRA FDD and this is reason why is sometimes termed as 3.5G. .
HSDPA uses adaptive modulation and coding (AMC), short packet size, multi-code operation and fast hybrid automatic repeat request (HARQ) to provide higher data rates .
Worldwide Interoperability of Microwave Access (WiMax):
IEEE 802.16e is a part of ITU's IMT - 2000 family . According to IEEE 802.16-2004, there are three PHY levels includes 256-dot FFT (FFT - Fast Fourier Transform) OFDM, SC (Single-Carrier, which brings this standard near CDMA) and 2048 OFDMA (Orthogonal Frequency Division Multiple Access)  of which 256 dot FFT OFDM is mandatory. Along with OFDM, various types of modulation schemes are used in WiMax depending upon the channel quality . And like the latest cellular communications networks, digital wireless networks pay much attention to QoS, which allows  prioritizing traffic and also includes cryptographic algorithm .
Later on, an extension to mobile applications is being pursued under 802.16e .Recently, ITU delegates declare WiMax as an IMT2000 technology which based upon IP-OFDMA. 
Though the technology WiMax, evolves from the technology Wi-Fi, but never became a competitor rather it became competitor to cellular communication. 
3GPP LTE (Long Term Evolution):
The 3GPP LTE develops Universal Mobile Telecommunication System (UMTS) mobile phone standard to manage the requirements in future wireless systems . LTE is a flat IP-based network architecture designed to replace the GPRS Core Network and ensure support for, and mobility between, some legacy or non-3GPP systems, for example GPRS and WiMAX respectively . Objectives include improving efficiency, lowering costs, improving services, making use of new spectrum opportunities, and better integration with other open standards . Networks based on 3G LTE are expected to be available by the 2010-11 timeframe, enabling a convenient entry for 4G in the subsequent years .
UMB (Ultra Mobile Broadband):
The 3GPP2 has standardized Ultra Mobile Broadband (UMB) to improve CDMA for 4G applications and requirements . It is based on CDMA2000 and formally known as EV DO Rev C. The UMB system was designed to deliver both voice and high speed data services based on all-IP network architecture. 
IEEE 802.20 is a proposed WMAN standard, sometimes referred as Mobile-Fi or Mobile Broadband Wireless Access (MBWA), to provide the WiMAX capabilities to a wide area, mobile user .
Below table compares the characteristics and performances of above candidates:
4G System Enabling Requirements while moving from 3G:
The requirements to achieve to enable 4G are:
Support a wide variety of spectrum bands
Develop cost-effective high-performance cell-site solutions 
Enable higher capacity for hot-spot deployment
Support VoIP applications
Provide a cost-effective 4G migration path from 2G and 3G 
4G Enabling Technology:
To meet the requirements mentioned above, a number of types of technology need to used i.e. related to coding, modulation scheme, and multiple access which includes smart antennas concept to combat against multi path or fading, adaptive modulation, and OFDM-TDMA, VSF-OFCDM, MC-CDMA for multiple access.
Smart Antenna Concepts or MIMO:
SISO systems, i.e. without diversity, can experience deep fade due to fading, which eventually increases BER and reduces the data rate of the system. By using multiple antennas at the transmitter as well as receiver side, diversity can be achieved which reduces the effect of the fading and results in higher data rates with better performance . MIMO is the smart antennas concept which incorporates multiple antennas at the input and output separated spatially and provides high spectral efficiency as the data streams are transmitted over the same path simultaneously . MIMO is the key technique for 4G evolution which can offer 100Mb/s or higher rates if employed .
Figure -MIMO implementation 
Advanced Coding techniques:
Advanced coding techniques are also a key to enable 4G evolution which maximizes the performance of the system by achieving the capacity of the system near to the Shannon limit. It includes scheme like concatenated (based on two or more codes and interleaving process) and LDPC coding (can outperform turbo code used in 3G system). 
OFDM and OFDMA:
In the evolution of cellular communication, OFDM plays very important role in reducing the effect of fading which eventually which results in higher data rates. OFDM is the orthogonal multicarrier transmission modulation technique which converts a frequency-selective channel into set of flat fading channels. But for fading channel, frequency -selective channel is desirable as it provides diversity which is removed by OFDM since the channel for each subcarrier is flat. Hence, with OFDM, channel coding is also employed across the subcarriers to employ multi path diversity and also known as coded OFDM (COFDM) . Adaptive modulation with different schemes are also employed which selects the constellation size for each subcarrier according to the associated SNR for a particular required BER .
OFDMA is a modulation and multiple access scheme which possess the advantages of both TDMA as well as FDMA schemes. In this scheme, a transmission is packed, frame size is fixed and each user share frame with every other user which increase efficiency .
Figure : OFDMA 
Higher data rates can be achieved by using adaptive modulation technique which adapts to the particular scheme depending upon the quality of Channel which uses higher order modulation scheme for higher rate. This technology includes M-PSK, M-QAM, M-ary continues phase modulation (CPM), M-ary multi hop phase coded modulation (MHPM) and GMSK for varying bit duration bandwidth product of the pre-modulation Gaussian filter .
Multiple Access Scheme:
This topic is highly debated technology of all and will be treated as very important technology which will help evolving 4G systems . On this multiple access scheme, some can be used to achieve the requirement of 4G systems:
CSMA-OFDM: Transmission is performed using three way handshake .
TDMA-OFDM: Each user transmits using all the subcarriers in allocated fixed time slots and found to be more efficient than CSMA-OFDM .
FDMA-OFDM: Similar to TDMA-OFDM that creates orthogonal resources by dividing the available subcarriers into fixed sets where each set is used by user .
MC-CDMA: It spreads the symbol over time slots or subcarriers .
SDMA-OFDM: Using multiple antennas and beam-forming, SDMA creates orthogonal resources .
OFDMA: It combines the advantages of TDMA and FDMA schemes .
VSF-OFCDM: It uses a VSF in conjunction of OFCDM. It adapts the OFCDM frequency and temporal domain's spreading factor based on channel condition, delay spread and Doppler frequencies .
Cognitive Radio & Cooperative Communication:
Cognitive Radio & Cooperative Communication is also a key to enable 4G evolution which improves spectral efficiency of the system .
MIMO-OFDM combine diversity gain advantages of MIMO and simple equalization advantage of OFDM together to improve the performance of the system which is also consider one of the key enabler of 4G communication .
Ultra Wideband Signalling (UWB):
Ultra wideband (UWB) is also destined to be a key enabler of 4G communications systems, promising to offer high indoor wireless channel and wireless PAN (WPAN) data rates specified by IEEE 802.15.3a (over 110 Mb/s at 10.0 meters and over 480 Mb/s below 4.0 meters) . UWB is an interesting 4G enabler because, unlike all other mentioned signals herein, this signaling scheme utilizes time hopping with ultra-short pulses .
The drive to 4G is being fuelled by the promise of target peak data rates of approximately 1Gbit/s for low mobility and approximately 100Mbit/s for high mobility. These data rates are part of the International Telecommunication Union (ITU) long-term vision (2010-2015) for 4G, as outlined in the ITU's IMT-Advanced requirements . The 4G system not only about the increase in the data rate, but will offer seamless, high bandwidth network connectivity, with access to any network regardless of location by working on the all IP network for the first time .