Today life does not seem possible without wireless
in some form or the other. The demands on bandwidth and spectral availability are endless. Consequently, wireless designers face an uphill task of limited availability of radio frequency spectrum and complex time varying problems in the wireless channel, such as fading and multipath, as well as meeting the demand for high data rates. Simultaneously, there is an urgent need for better quality of service (QoS). Orthogonal Frequency Division Multiple Access (OFDMA) is a promising technique for broadband wireless communication because it can combat inter-symbol interference (ISI) caused by the dispersive fading of wireless channels. OFDMA is a multi-user
version of the popular OFDM digital modulation scheme. This paper is one of the footsteps towards technical contribution in the world of wireless communication. OFDM's subcarriers make it very conducive to other technologies, such as multiple input-multiple-output(MIMO) antenna implementations, as well as multiple access (OFDMA) implementations.
[...] In the third phase, various designs and techniques for the improvement of the performance is carried out focusing TABLE 1 SHORT HISTORY OF WIRELESS COMMUNICATION on multi antenna OFDM systems. Performance comparison with other techniques is also done and stressed upon INTRODUCTION The origins of OFDM development started in the late 1960's with the introduction of Frequency Division Multiplexing (FDM) for data communications. In 1966 Chang patented the structure of OFDM and published the concept of using orthogonal overlapping multi-tone signals for data communications. [...]
[...] Figure-5 OFDM Waveform in Time Domain Two Branch Transmit Diversity with Two Receivers It is possible to provide a diversity order of 2M with two transmit and M receive antennas. Figure 9 Two Branch Transmit Diversity Scheme with two Receivers Figure 7 Two Branch Transmit diversity scheme with one receiver Note that an increase in transmit diversity improves the Here, for simulation, parameters taken are Number of Symbol pairs to be transmitted 1000 Modulation Scheme BPSK performance. This is a very important inference from a commercial point of view, because handheld mobiles always pose a lot of problems in achieving antenna diversity at the receiver. [...]
[...] The BER performance of an OFDM signal in a fading channel is much better than the performance of QPSK/FDM which is a single carrier wideband signal. Although the underlying BER of a OFDM signal is exactly the same as the underlying modulation, that is 8PSK is used to modulate the sub-carrier, then the BER of OFDM signal is same as the BER of 8PSK signal in Gaussian channel. But in channels that are fading, the OFDM offers far better BER than a wideband signal of exactly the same modulation. [...]
[...] The BER of an OFDM is only exemplary in a fading environment. We would not use OFDM in a straight line of sight link such as a satellite link. OFDM due to its amplitude variation does not behave well in a non-linear channel such as created by high power amplifiers on board satellites. Using OFDM for a satellite would require a fairly large back off, on the order of 3 dB, so there must be some other compelling reason for its use such as when the signal is to be used for a moving user. [...]
[...] It is obvious that MIMO technique will be effectively used with OFDM based systems for providing mobile multimedia in future with reasonable data rate and quality of service. Two Branch Transmit Diversity with One Receiver The new two-branch transmit diversity scheme is shown which uses two transmit antennas and one receive antenna. The diversity gain is a function of many parameters, including the modulation scheme and FEC coding. Assumptions It is assumed that the total transmit power from the two antennas is the same as the transmit power from the single transmit antenna for MMRC. [...]
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