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Abstract

The demand for high-speed mobile wireless communications is rapidly growing.
OFDM technology promises to be a key technique for achieving the high data capacity and spectral efficiency requirements for wireless communication systems of the near future. This work presents an investigation into methods for maximizing the spectral efficiency of Orthogonal Frequency Division Multiplexing (OFDM) systems. The current standards such as IEEE 802.11a in USA and Hyper LAN/2 in Europe are all based on OFDM in their PHY layer. First, we show that in order to be free of both inter channel interference and inter block interference, wireless OFDM has to occupy a bandwidth wider than the Nyquist rate and use insufficient statistics in symbol demodulation. Thus, the conventional OFDM gains computational efficiency by using Discrete Fourier Transform (DFT) in demodulation at the cost of low efficiency of bandwidth usage and degradation in symbol error performance.

INTRODUCTION
Digital multimedia applications as they are getting common lately create an ever increasing demand for broad band communication systems. Although the technical requirements for related products are very high the solutions must be cheap to implement since we are basically talking about consumer products.

The OFDM projects mostly, is concerned with the development and evaluation of receiver structures for OFDM. Whereas so far we have focused on the broadcasting scenario attention now is switching to other applications including wireless ATM and fourth generation transmission techniques. In an OFDM system, the entire channel is divided into many narrow sub channels, which are utilized in parallel transmission, thereby increasing the symbol duration and reducing Inter Symbol Interference. Therefore, OFDM is an effective technique for combating multipath fading and for high-bit-rate transmission over mobile wireless channels.
Most WLAN systems currently use the IEEE802.11b standard, which provides a maximum data rate of 11 Mbps. New WLAN standards such as IEEE802.11a and Hyper LAN2, are based on OFDM technology and provide a much higher data rate of 54 Mbps. However systems of the near future will require WLANs with data rates of greater than 100 Mbps, and so there is a need to further improve the spectral efficiency and data capacity of OFDM systems in WLAN applications.

In addition, an OFDM system can achieve adaptive allocation of transmission load in different sub channels to achieve optimum transmission rate. Besides, because of the longer duration of symbols, the OFDM system can alleviate the effect of impulse noise. When an OFDM system is designed such that there is neither ICI nor ISI, the computationally efficient FFT can be applied to decouple sub channels and the channel equalization is accomplished simply by a complex scalar for each sub channe

CHANNEL IMPAIRMENTS
Wireless channel is always very unpredictable with harsh and challenging propagation situations. Wireless channel is very different from wire line channel in a lot of ways. Multipath reception is the unique characteristic of wireless channels. Together with multipath, there are other serious impairments present at the channel, namely propagation path loss, shadow fading, Doppler spread, time dispersion or delay spread, etc.

Multipath Scenario
Multipath is the result of reflection of wireless signals by objects in the environment between the transmitter and receiver. The resultant is random signal fades as the reflections destructively (and/or constructively) superimpose one another, which effectively cancels part of signal energy for a brief period of time. The severity of fading will depend on delay spread of the reflected signal, as embodied by their relative phases and their relative power.
A common approach to represent the multipath channel is channel impulse response which gives us the delay spread of the channel. Delay spread is the time spread between the arrival of the first and last multipath signal seen by receiver. In a digital system, delay spread can lead to ISI. In Figure 1, delay spread amounts to max. It is noted that delay spread is always measured with respect to the first arriving component.

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