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Conclusion

By Eric Lawrey, Copyright 1997-2001

The current status of the research is that OFDM appears to be a suitable technique as a modulation technique for high performance wireless telecommunications. An OFDM link has been confirmed to work by using computer simulations, and some practical tests were performed on a low bandwidth base-band signal. So far only four main performance criteria have been tested, which are OFDMís tolerance to multipath delay spread, channel noise, peak power clipping and start time error. Several other important factors affecting the performance of OFDM have only been partly measured. These include the effect of frequency stability errors on OFDM and impulse noise effects.

OFDM was found to perform very well compared with CDMA, with it out-performing CDMA in many areas for a single and multicell environment. OFDM was found to allow up to 2 - 10 times more users than CDMA in a single cell environment and from 0.7 - 4 times more users in a multi-cellular environment. The difference in user capacity between OFDM and CDMA was dependent on whether cell sectorization and voice activity detection is used.

It was found that CDMA only performs well in a multi-cellular environment where a single frequency is used in all cells. This increases the comparative performance against other systems that require a cellular pattern of frequencies to reduce inter-cellular interference.

One important major area, which hasnít been investigated, is the problems that may be encountered when OFDM is used in a multiuser environment. One possible problem is that the receiver may require a very large dynamic range in order to handle the large signal strength variation between users.

This thesis has concentrated on OFDM, however most practical system would use forward error correction to improve the system performance. Thus more work needs to be done on studying forward error correction schemes that would be suitable for telephony applications, and data transmission.

Several modulation techniques for OFDM were investigated in this thesis including BPSK, QPSK, 16PSK and 256PSK, however possible system performance gains may be possible by dynamically choosing the modulation technique based on the type of data being transmitted. More work could be done on investigating suitable techniques for doing this.

OFDM promises to be a suitable modulation technique for high capacity wireless communications and will become increasing important in the future as wireless networks become more relied on.

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