Performance Evaluation of STBC Codes With Increased Order Of Pseudo-random Number Generator

Wireless network devices have quickly become a part of everyday life. Wireless LANs, cell phone networks and personal area networks are few examples of widely used wireless networks. However, wireless devices are range and data rate limited. To overcome these limitations, Multiple- Input Multiple Output (MIMO ) links are used, wherein data is encoded using higher order space-time block code, which is then split into n streams and simultaneously transmitted using n transmitting antennas. The received signal is a linear superposition of the n transmitted signals perturbed by noise. Maximum likelihood decoding is achieved in a simple way through decoupling of the signals transmitted from different antennas rather than joint detection. In this paper, we have evaluated the performance of space time block codes with increased order of pseudo random number generator and derived an analytical expression for the SNR and Bit Error Rate , showing that higher order STBC scheme provides full diversity and effective BER for particular values of SNR gain, for which the results have been verified through Matlab simulations. Keywords—Space time codes, Transmit diversity, Rayleigh fading channel, Higher order STBC codes, MIMO. I. INTRODUCTION With the increase in demand for the wireless multimedia, current wireless broadband services fall short in terms of the quality of service (QoS), data rate, coverage and the number of users which can be supported. Multiple antennas (1) at both the transmit (Tx) and receiver (Rx) sites seem to meet these demands through improved higher data rate due to spatial multiplexing and link reliability due to diversity. The multiple antennas in MIMO systems can be exploited in two different ways. One is the creation of a highly effective antenna diversity system,the other is the use of the multiple antennas for the transmission of several parallel data streams to increase the capacity of the system. Antenna diversity is used in wireless systems to combat the effects of fading (2),(3). Open loop full rate system enables full diversity with linear processing at the receiver, without need for feedback information (2) If multiple, independent copies of the same signal are available, we can combine them into a total signal with high quality—even if some of the copies exhibit low quality. The different signal copies are linearly combined, i.e., weighted and added. The resulting signal at the combiner output can then be demodulated and decoded in the usual way. The optimum weights for this combining are matched to the wireless channel (maximum ratio combining (MRC)). If we have N receiving antenna elements, the diversity order, which describes the effectiveness of diversity in avoiding deep fades is N. In other words the diversity order is related to the slope of the signal-to-noise ratio (SNR) distribution at the combiner output. The multiple antennas also increase the average SNR seen at the combiner output. When the channel is known to the transmitter, we can again ―match‖ the multiple transmitted signal copies to the channel, resulting in the same gains as for receiver diversity. If the channel is unknown at the transmitter other strategies like delay diversity or space-time-coding have to be used. In that case we can gain high diversity order but not improvement of average SNR. The logical next step is the combination of transmit and receive diversity. It has been demonstrated that with Nt transmit and Nr receive antennas, a diversity order of NtNr can be achieved. An alternative way of exploiting the multiple antenna elements is the so-called ―spatial multiplexing‖ or ―BLAST‖ approach. Different data streams are transmitted (in parallel) from the different transmit antennas. The multiple receive antenna elements are used for separating the different data streams at the receiver. We have Nr combinations of the Nt transmit signals. If the channel is well-behaved so that the Nr received signals represent linearly independent combinations, we can recover the transmit signals as long as Nt ≤ Nr. The advantage of this method is that the data rate can be increased by a factor Nt without requiring more spectrum