*2.6.1 OFDM transmitter*

*Multiplexing*

munication network.

• It is scalable.

each user.

*2.5.2 Disadvantages*

*2.5.1 Advantage of CDM*

The receiver also wants to know the same code used at the transmitter side in order to decode the signal at the receiving side. Here different random sequences correspond to different communication channels from different stations. To separate other channels, CDM assigns each channel with its own code. The main advantage of CDM is protection from interference and tapping because only the sender and

CDM is widely used in second-generation and third-generation wireless com-

the receiver know the spreading code (**Figure 11**).

• The CDM does not require any synchronization.

• It is well-matched with other cellular technologies.

• Efficient utilization of fixed frequency spectrum.

• The system is more complicated.

**2.6 Orthogonal frequency division multiplexing**

• In CDM more number of users can share the same bandwidth.

• Interference is reduced because different code words are allocated to

• As the number of users increases, the overall quality of services decreases.

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

• More complex system and primarily it is used in wireless transmission.

**18**

modulated wideband signal.

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 shift keying (PSK).

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 communication systems (**Figure 13**).
