Code Division Multiplexing Using AI Based Custom Constellation Scheme – Efficient Modulation for High Data rate Transmission

To achieve high bit rate in transmission over wireless channels frequency reuse is an encouraging concept . Rather than dividing allocated frequency spectrum into narrow band width channels , one for each user information is transmitted over a very wide frequency spectrum using the same carrier frequency within same frequency band . In this paper we propose code division multiplexing scheme in which Custom QAM modulator itself is used as code . The propose system is simulated and tested in Matlab 7.4 Key wordsCDM, OFDM ,Artificial Intelligence ,QAM 1.Introduction Future wireless services including internet and real-time video will continue to increase the required data rates for wireless communication systems. This paper investigates modulation formats for high data rate applications in wireless environments. In particular, we consider situations where the delay spread of the channel is long compared to the bit duration. Highly dispersive channels cause significant inter-symbol interference (ISI), but they also offer a substantial potential for performance improvement through diversity. Traditional single carrier modulation formats such as phase shift keying (PSK), frequency shift keying (FSK), and quadrature amplitude modulation (QAM) require significant equalization to perform adequately in a highly dispersive channel. In this paper, we present code division multiplexing (CDM) as an alternative modulation format to the above mentioned single carrier and multicarrier formats. To motivate this technique, we draw an analogy to multiuser systems .In multiuser systems, the channel is typically shared by one of three multiple access schemes, frequency-division multiple-access (FDMA), time-division multiple-access (TDMA), or codedivision multiple-access (CDMA). The OFDM modulation format is analogous to an FDMA system where ISSN: 0975-5462 2377 K. Seshadri Sastry et. al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 2377-2383 users share the channel by using orthogonal frequency slots. In the case of OFDM, it is not different users sharing the channel but rather different data bits being transmitted in parallel. The single carrier modulation formats are analogous to TDMA systems where different users transmit their bits in orthogonal time slots. With PSK, FSK, and QAM, the different bits are transmitted in orthogonal time slots as well This modulation format which we propose is not new. It is a special case of parallel combinatory spread spectrum multiple access [ l ] and also a special case of multi-code CDMA [2], [3]. In these cases, the technique is used to enhance CDMA systems, whereas in our work, we are considering CDM as a singleuser modulation format. A similar technique is used in [4], but we will demonstrate a much higher spectral efficiency than was achieved in that work This paper is organized as follows Section 2 explains Proposed scheme section 2.1 explains AI block ,section 3 explains results and section 4 concludes paper. 2. Proposed Scheme Proposed scheme uses custom constellation ordering of QAM modulator as code . It uses 8 QAM , the same carrier frequency within same frequency band can be used for four wireless systems. In the proposed scheme four custom constellation orderings which were shown in Fig 1 , 2, 3 4 are used . Every constellation ordering differs from other in arrangement of quadrature and phase component. For every wireless system a fixed code is given. ( i.e. for system A ordering A is employed , for system B ordering B is employed , for system C ordering C is employed , for system D ordering D is employed ) as shown in table 1 . Block diagram of proposed system is shown in Fig 5.