Up- gradation from 2G (second generation) to 3G (third generation) and now current 4G

With the consistent growth in the mobile users, technically mobile (cellular) companies were facing a huge increase in MS Traffic (increase in Erlangen). The more traffic among all these was of data; speaking on the side of the mobile users is INTERNET.

INTERNET or data services has incredibly hold the world of internet on the move, for the fulfillment of the demand 3gpp (third generation participation project) has taken a step forward from the previous 2G. This step was required because the limited trough-put (data rate) of the air interface of 2G. This limitation was because of the limited access of the allotted band and the modulation scheme of the air interface although on the side of the base station BS was never limited with the dedicated E1. This has shown that transmission to the BS was with the capabilities to hold high through put but air-interface was not.

While 3G was in progress for launch, it was for the first time the concept of TDMA (time division multiplexing) was left behind with the new technique of WCDMA which was an enhancement over CDMA. While developing 3G standards before its all release, the concept was to somehow make the air-interface more widen to achieve high throughput to meet the commercial demand. In 3G modulation scheme of high symbol rate were introduced, with QPSK to QAM. In 3G SSM (spread spectrum modulation) was introduced for the first time in mobile communication this has widen the bandwidth associated to a user, providing him higher data-rate for internet access.

Although 3G has provided higher data-rate for internet access including voice but still the service we call DATA ON THE MOVE (DOTM) was behind and people still prefer to have fixed line services for connecting to internet. Fixed line service including DSL lines, OFC, Leased Lines and PTP (micro wave short distance). The weight on the arms of 3G was still high to compete the market commercially and considering all these 3gpp has taken a next step with the release of HSUPA+ and LTE which has triggered the market with more catches of Traffic and has now becoming a wide application part for VPN, a foot step into business.

OFDM TRANSMITTER AND RECEIVER

                          OFDM TRANSMITTER        

An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, with baseband data on each sub-carrier. These sub-carriers are being independently modulated commonly using some type of modulation technique may be  Quadrature amplitude modulation (QAM) or phase shift keying (PSK). This  baseband signal is typically used to modulate a main RF carrier.

 is a serial stream of binary digits. By inverse multiplexing , these are first demultiplexed into  parallel streams, and each one mapped to a  symbol stream using some modulation technique.  The modulation techniques may be different, so some streams may carry a higher bit-rate than others.

An inverse FFT is computed on each set of symbols, giving a set of complex time-domain samples. These samples are then quadrature mixed to passband in the standard way. The real and imaginary components are first converted to the analogue domain using DACs ; the analogue signals are then used to modulate cosine and sine waves at the carrier  frequency, , respectively. These signals are then summed to give the transmission signal, 

                     OFDM RECEIVER

The receiver picks up the signal  , which is then quadrature-mixed down to baseband using cosine and sine waves at the carrier frequency. This also creates signals centered  on , so low-pass filters are used to reject these. The baseband signals are then sampled and digitised using ADCs , and a forward FFT is used to convert back to the frequency domain.

This returns  parallel streams, each of which is converted to a binary stream using an appropriate symbol detector. These streams are then re-combined into a serial stream ,  , which is an estimate of the original binary stream at the transmitter.

                             

USAGE:

OFDM is used in ADSL connections that follow the ITU G.992.1 standard, in which existing copper wires are used to achieve high-speed data connections.

Long copper wires suffer from attenuation at high frequencies. The fact that OFDM can cope with this frequency selective attenuation and with narrow-band interference are the main reasons it is frequently used in applications such as ADSL modems. However, DSL cannot be used on every copper pair; interference may become significant if more than 25% of phone lines coming into a central office are used for DSL.

 OFDM is exclusively used in LAN and MAN applications.           

ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS (OFDMA) - SINGLE-CARRIER FDMA (SC-FDMA)

ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS

It is a multi user version of digital modulation scheme.it works by assigning subsets of subcarriers to individual users.This technique has following advantages over CDMA:

1. It can combat multipath interference and it is less complex.
2. It can achieve a higher MIMO spectral efficiency.

Principle—  because of feedback information about the channel conditions, user to subcarrier assignment can be achieved. In this different number of subcarriers can be assigned to different users in view to support Quality of service that means to control data rate and error probability for each user.This is achieved by combining OFDM with time division multiple access. In this constant and shorter delay can be achieved. It is considered to be highly suitable for broadband wireless networks due to its advantagesincluding scalability and MIMO and it ability of channel frequency selectivity.It is used in Wimax and 3GPP LTE. High speed OFDM packet acess (HSOPA) now named as Evolved umts terrestrial radio access (E-UTRA).

  SINGLE-CARRIER FDMA (SC-FDMA)

It is a frequency- division multiple access scheme. It deals with the assignment of multiple users to a shared communication resorce.  It has drawn great attention as an attractive alternative to OFDMA. It has been adopted as the uplink multiple access scheme in 3GPP LTE or E-ULTRA ( evolved ultra). The major advantage of SC-FDMA is low PAPR (peak-to-average power ratio).

Transmission of SC-FDMA is very similar to OFDMA. In this, for each user the sequence of bits transmitted is mapped in symbols (BPSK,QPSK).

In SC-FDMA multiple access among users is made possible by assigning different users different set of sub carriers. The distinguishing feature  of SC-FDMA is that it leads to single carrier transmit signal in contrast to OFDMA which is multi carrier transmission scheme.

Advantages:

Low PAPR, low sensitivity to carrier frequency offset, greater robustness against spectra nulls, less sensitive to non linear distortion and hence, it allows the use of low cost power amplifiers.