*2.5.1 Advantage of CDM*


### *2.5.2 Disadvantages*


### **2.6 Orthogonal frequency division multiplexing**

Orthogonal frequency division multiplexing (OFDM) [4, 5] is a multiplexing technique used in broadband communication system. It is a multicarrier modulation scheme. Now it is used in 4G broadband communication system and next-generation systems. OFDM is popular in broadband wireless systems due to its resistance to multipath fading. OFDM has high data rate capability with reasonable computational complexity. OFDM divides a broadband channel into multiple parallel narrowband subchannels, and each channel carries a low data rate stream of signals. Finally these signals are summing and then transmit as a high data rate stream. In an OFDM transmitter, the input signal bits are mapped into a bank of quadrature amplitude modulator which encodes these into complex symbols. This is fed to an inverse fast Fourier transform (IFFT) to ensure the orthogonality of the subchannels. This output is converted into parallel to serial, modulated into a carrier wave, and then transmitted into the air. At the receiver the reverse process is performed for recovering the original signal. The advantages of OFDM are that its low computational complexity because OFDM may be viewed as a many slowly modulated narrowband signals rather than a rapidly modulated wideband signal.

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**Figure 12.** *OFDM transmitter.*

*Multiplexing*

*2.6.1 OFDM transmitter*

shift keying (PSK).

*2.6.3 OFDM receiver*

frequency domain.

*DOI: http://dx.doi.org/10.5772/intechopen.85866*

communication systems (**Figure 13**).

*2.6.2 Block diagram of OFDM transmitter*

The block diagram depicting the OFDM transmitter is shown in **Figure 12**. For OFDM transmitter, a serial stream of binary digits is considered as the input. The input is converted into N parallel streams using inverse multiplexing. The transformation of N parallel streams into the state-space mechanism is performed by means of modulation techniques like quadrature amplitude modulation (QAM) and phase

A digital modulation system for data communication by varying or modulating the phase of the reference signal or the carrier wave signal is known as PSK. A finite number of phases is involved in PSK with each phase having a distinctive pattern of binary digits. An integration of trouble-free AM and simple phase modulation is called QAM in which the large amount of data is transmitted over the same bandwidth due to the synergistic effect of simple amplitude modulation and phase modulation. Hence, QAM increases the efficiency of data transmission for radio

To provide a set of complex time-domain samples, IFFT is calculated for each set of symbols. Later, the time-domain samples are quadrature mixed to passband in the normal way. By the use of digital-to-analog converters (DACs), the real and imaginary components are primarily converted to the analog domain. Such analog signal helps to modulate corresponding cosine and sine waves at the carrier frequency. Finally, those signals are summed up to provide the transmission signal.

At the OFDM receiver's side, the reverse process of transmitter side is performed.

The transmitter-generated signal is further transmitted over the channel for receiving. The receiver receives the baseband OFDM signals, and then it passes through a low-pass filter to remove the unwanted signals. The baseband signals are then sampled and digitized using ADCs, and a forward FFT is used to convert back to the

The block diagram representing the OFDM receiver is shown in **Figure 14**.
