We use time periods to classify the generations .the analogue frequency division multiple access systems are 1G,digital voice system which uses time division multiple access are 2G, digital voice/data systems are beyond 2G(B2G),and broadband digital system which use code division multiple access are 3G,wireless local area network/wireless metropolitan area network(WLAN/WMAN) are B3G systems and very-high-speed data rate systems which is based on internet protocol(IP) network are 4G systems.
A major objective in the evolution from 3G to4G is the ability to seamlessly integrate different existing wireless access technologies in a common flexible and expandable platform.the most important technical challenge for the realization of 4g systems is (a) to overcome the severely frequency selective fading channel,and (b) to significantly reduce the transmit power from mobile terminals.
In this report we will discuss about coding,modulation,signal processing and multiple access technologies that underpin the migration from 3G to 4G.
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THIRD GENERATION SYSTEMS:
It is also called as IMT-2000 by ITU. the performance for 3G air interface can be summarized as
>wideband CDMA systems
>spectrum bandwidth 5MHz
>full coverage and data rate of 144kbps for vehicular,384 kbps for for pedestrian and 2Mbps for fixed indoor and pico cell environment.,
>compared to 2G it offers high spectrum efficiency
BEYOND THIRD GENERATION SYSTEMS
The maximum data rate offered by third generation was only 2Mbps which is too slow for most applications.the wireless loop area network (WLAN) including Wi-Fi and WiMAX which is called as B3G(Beyond 3G) offers much higher data speed than 3G.,
IEEE-BASED WIRELESS STANDARD SYSTEMS
IEEE wireless standards use both licensed and unlicensed bands to provide wireless nomadic and mobility service for the customers.
This standard has many specifications like 802.11a, 802.11b,802.11g and 802.11n which is collectively called as Wi-Fi . the goal is to provide wireless lan services that are consistent with 802.3 ethernet network. The MAC layer should be capable to operate with multiple physical layer,each of which exhibits different transmission characteristics,
IEEE 802.11 supports 4 different physical layers for four different transmission technologies: 1.Infrared (IR) ;2.Frequency hopping spread spectrum(FHSS);3.Direct Sequence Spread Spectrum(DSSS) ;4.Orthogonal Frequency Division Multiplexing(OFDM).the physical layer is subdivided into physical layer convergance protocol(PLCP) sublayer which performs clear channel assessment (CCA) and reports the information to the MAC layer and physical medium dependent(PMD) sublayer id responsible for modulation and encoding.,
PULSE POSITION MODULATION(PPM):
Infrared band used PPM which uses short pulses of high peak power and low duty factor as an effective way of combating noise. since all the receiver needs to do is detect the presence of a pulse at the correct time.,
Mostly in order to conserve the frequency spectrum we tend to keep the transmission as narrow as possible but under some circumstances it is advantageous to use spread spectrum techniques where the transmission is spread over wide bandwidth. if the receiver is not tuned to correct frequency it appears as background noise.WLAN use DSSS and FHSS techniques,
Direct sequence spread spectrum
the narrow band signal is multiplied by a wideband pseudonoise(PN) code sequence which is known in advance to both transmitter and receiver.a redundant code bit pattern called CHIP is generated for each data bit that is to be transmitted.the original data can be recovered even if the bits in the chip pattern are damaged but the data throughput is reduced.,
FREQUENCY HOPPING SPREAD SPECTRUM(FHSS):
it uses a narrowband carrier that hops multiple carrier frequencies in a hopping pattern known to both transmitter and receiver.FHSS appears as a short-duration impulse noise for an unintended receiver.it is usually used in the military applications.,
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXIG(OFDM) TECHNOLOGY::imagest.jpg
it is multicarrier technique used to achieve high data rate in a multipath fading environment. the spectrum divided into many subcarriers which are orthogonal to one another so that they can be spaced very closely without individual carrier guardband overhead .different data is transmitted on different subcarriers inorder to reduce the transmission rate and increase the symbol duration to combact multipath fading effects. the intersymbol interference ISI is removed by discarding guard interval resistant to narrowband interference.
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OFDM is more sensitive to frequency and phase noise and it has a relatively large peak-to-average power ratio,which tends to reduce the power efficiency of the RF amplifier.,
It offers up to 54Mb/s using OFDM in 5Ghz band.the system uses 52 subcarriers (48 data+4pilot) that are modulated using BPSK,QPSK,16-QAM or 64-QAM.the table below gives an overview of the combinations of modulation and coding schemes together with the data rates.,
This standard supports a maximum data rate of 11mb/s using 8 chip complementary code keying(CCK) modulation.PHY layer supports 4 data rates 11,5.5,2,or 1 Mb/s.the lower data rates I and 2 Mb/s use 11-chip barker sequence and DBPSK or DQBPSK and for higher data rates,5.5 and 11Mb/s use 8 chip complementry codes needed for CCK modulation,
the below table gives information about the modulation, coding and transmission rates of different ieee 802.11 standards.
IEEE 802.16 is a wireless metropolitan area network(WMAN) standarad which is referred as WiMAX
The table below shows the major differences between the WiMAX varients. It is based on OFDM and supports higher order modulation upto 64-QAM it offers data rate uptp 134Mb/s.the channel bandwidth is restricted to 20 MHz must be an integer multiple of 125Mhz,1.5 Mhz or 1.75 Mhz .one major predictor of WiMAX system data rates is the range from the â€˜hot spotâ€™ antenna .BPSK,QPSK,16-QAM and 64-QAM data modulations are supported and concatenated coding scheme is employed using Reed-solomon block code and inner convolutional code with rates of 1/2,2/3,3/4 and constrain length K=7.
Ieee 802.20 is referred to Mobile-Fi .the goal of this standard is to provide full mobility, high speed broadband data supporting mobility up to 150 Mph with data rate up to 6 Mbps.the operating frequency of 802.20 is below 3.5 GHz and supports 1.25 MHz and 5MHz channel bandwidth. 
4G Communication Systems
4G network use a combination of both WiFi and WiMAX. It is an ip based wireless network replacing the old signaling system 7(SS7) protocol which is considered massively redundant.
Technologies for 4G
Technologies that are used in 4g are SDR,OFDM(as we discussed earlier in wi-fi and wimax ),OFDMA,MIMO,UMTS and TD-SCDMA.
Smart antenna concept : mimo
Mimo is an integral part of 3GPP release 7 which promise to offer 100 mb/s or hidher data rate and it is one of the key enabling technology for 4g.here more than one antennas are used for transmission and reception .different signals can be transmitted from each antenna at the same frequency and stil be recovered at the receiver by signal processing. High spectral efficiency is achieved in rich scattering environment. the spatial diversity and space-time coding gain using mimo yeilds link budget gain greater than 10dB.
it is difficult to incorperate multiple antennas on the mobile terminals Since as the size of the mobile terminals is shrinking every year,so a new approach called virtual antenna array (VAA) technology or cooperative MIMO relaying is used.,
Multiple access techniques:
The key to achieve high level of service delivery is a new air interface OFDMA which is a combination of OFDM+TDMA technology coupled which allows user to transmit OFDM signals in assigned time slots.,
a variable spread factor-orthogonal frequency code division multiplexing technique adapts the the OFCDM frequency temporal domainâ€™s spreading factor which is set 1 in OFDM mode to attain maximum throughput.,
in multi-carrier CDMA,users who transmit identical data over multiple subcarriers are multiplexed using orthogonal coding and several codes are allocated to each user with data spread in time or frequency .MC-CDMA used QPSK data modulation.,
SOFTWARE DEFINED RADIO:
It is a wireless communication in which uses computer at transmitter and receiver to transmit and receive the signal .proper programs must be run by the micro computers to select the desired modulation type.
The main aim of sdr technology is to provide a single radio transcevier capable of playing the roles of wireless fax,e-mail video conferencing ,web browser,global positioning system(GPS) unit and functions operable from any location on the surface of the earth as well as space.,
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ULTRA MOBILE BROADBAND:
UMB is the name for the project 3GPP2 to improve CDMA2000 mobile phone standard for next generation . it employes OFDMA-based air interfacemand frequency division duplex. data speeds over 275 Mbps downstream and over 75 Mbps upstream and supports bandwidth between 1.25-20MHz.it supports mixed cell sizes and ip network architecture supports flat,centralized and mixed topoliges.,
Another key enabler of 4g technology is the coding scheme. some of the coding scheme proposed for 4G are concatenated codes and low density parity check(LDPC) codes.both the codes performance is very close to shannon limit.
In 1966 concatenated codes were introduced which is based on the union of more than one code and interleaving process but the drawback was increased computational complexity.
In 1962 LPDC was proposed by gallager but the idea was dormant for 30 years since LPDC codes were too complex for the technology to support at that time. again in 1990s MacKay reintroduced LDPC and showed that LDPC codes out performes TURBO CODES ehich is used in 3G technology while approaching Shannon limit.,
Adaptive modulation is the main reason for 4G achieving such high data rates.in adaptive modulation based on the channel conditions the transmit power,coding type,data rate and modulation formats are varied. if the channel conditions are good we go for higher order modulation and viceversa.
Several adaptive modulation techniques like M-PSK,M-QAM,M-ary continous phase modulation(CPM),M-ary multi-hop phase coded modulation(MHPM) ,GMSK have been suggested for the use with TURBO-CODED MC-CDMA to achieve 4g data rates.if the number of users is less than 10 QAM and PSK modes perform better but however,M-MHPM modulation with MC-CDMA system is said to provide optimum adaptive modulation performance. ,