On The Research Of The Adaptive OFDM System With Adaptive Guard Interval Length And Modulation Scheme

To fulfill the dream of the next generation wireless communication system, challenges exist at various layers of the system. At the application layer, more efficient source compression algorithm, which demands less bandwidth and adapts better to the dynamicly available bandwidth, should be developed. Given the variety of multimedia services with different Qos requirement to support and the limited bandwidth physical layer provides, a more efficient RRM (Radio Resource Management) scheme in the link layer should be provided. To transmit with faster speed, higher spectrum efficiency and finer channel granularity is the requirement to the physical layer technologies.OFDM (Orthogonal Frequency Division Multiplexing) is one of the promising candidates for the next generation wireless communication system, because it has fine channel granularity, the key to support services with various Qos requirement, and can easily cope with frequency-selective fading, the challenge to transfer services with high-speed demand such as video stream. But the spectrum efficiency of the traditional OFDM system is limited due to variance of channel parameters such as channel response and RMS delay spread. To increase the spectrum efficiency of the OFDM system, a novel AOFDM (Adaptive OFDM) system, which can change its modulation type and length of guard interval according to the estimated channel information, is designed. As any other adaptive system, the following three steps are essential to our AOFDM: estimating channel, choosing the right modulation parameters and signaling the chosen parameters.At first the key technologies for the fulfillment of the next generation wireless system are introduced. As the background of our work, the advances of this field are presented. The major works and contributions of this paper are given.At chapter 2, the configuration of the AOFDM system, which is decided after comparing various duplexing schemes and signaling methods, is discussed in detail. The advantages and drawbacks of the traditional OFDM systems are fully analyzed. Hoping to overcome the drawbacks, the AOFDM system is designed. The concept of variable guard interval is introduced into AOFDM system for the first time. The data flow, frame structure and diagram of the transceiver is also given.Two categories of pilot based channel estimation are examined in chapter 3: ML criteria based and MMSE criteria based. The performance and implementation complexity of these two methods are analyzed and compared. The best position of pilot tones and some implementation issues are also discussed. A simple but effective RMS delay spread estimation method is also presented. Finally, the joint estimation of channel response and RMS delay spread is discussed.How to choose the modulation parameter is the main concern of chap 4. Supposing that channel estimation is complete, a closed form relation between modulation switchinglevels and target BER is first derived. Then the effects of the more practical issues such as estimation error and the estimation delay are considered, which results in a method that could be used when channel estimation is incomplete. The effects, caused by RMS delay spread estimation, on choosing the right guard interval length is also discussed. Simulation results are given as well.