Figure 25: Power spectrum of OFDM signal.

To make the spectrum shaping specification easier, the last sides of the spectrum are specified to zero as that have been used in the specification of IEEE 802.11a. These sub-carriers work as a frequency guard and some also refers as virtual guides.In order to prevent problems of converting in Digital to Analogue and Analogue to Digital and to prevent DC, sub-carriers falling at DC is not used at all. Figure 26 shows the power spectrum of that kind of system. Here length of sub-carriers that are specified to zero at the sides of the spectrum was 11.

Figure 26: Power spectrum density of OFDM signal

10.1.1 Cyclic extension of OFDM symbol

Time domain OFDM signal is cyclically expanded in order to evade the result of time spreading. In order to avoid ISI [23, 24] the size of cyclic prefix must go above to the highest excess delay of the channel. The general theme is that The basic idea here is to copy part of the OFDM time-domain character from behind to the start to make a guard period. Figure 27 shows this idea. It also shows how cyclic prefix avoids the ISI.

Figure 27: Illustration of cyclic prefix extension.

There are certain reasons to use the cyclic prefix;

1. In order to keep the receiver time synchronization that because a long period of silence can produce synchronization to be lost.

2. In order to transform the linear convolution of the signal and channel to a circular channel.

3. It is much easier to create in FPGAs.

11. Conclusion

Here in our thesis work we first investigate evolution of cellular communications. We briefly describe about first-generation (1G), second-generation (2G), third-generation (3G) and fourth-generation (4G) systems. Countries like Bangladesh, India, and Pakistan are still depend on 2G (EDGE, GPRS) due to insufficient infrastructure and low bandwidth. Whereas Japan (WCDMA), United States (cdma2000) others European countries (UMTS) are now using 3G systems and proceed to develop 4G systems. After this we discuss three types of multiple access techniques: TDMA, FDMA and CDMA. These are the three main multiple access technique that has been designed so far to accommodate various users to share usable bandwidth in radio communication. There are many extensions, and hybrid techniques for these methods, such as OFDM, and hybrid TDMA and FDMA systems. An understanding of these three major methods is required for developing of any extensions to these methods. Then we go through some points about Wideband CDMA. We discuss physical layer of WCDMA, Multiple User Data Transmission, and some performance enhancing schemes. We also briefly discuss about air interface procedure, handover and power control in WCDMA. WCDMA still continue to evolve based research and new innovations. Release 2000 (release 4) specifications will give efficient IP support enabling services through an all-IP core network. Technical problems solved by include header compression and Quality of Support (QoS). Then the next 3GPP releases will give higher data rates