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.