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Abstract-This paper details on the design of OFDM receivers. Special attention is paid to the OFDM-specific receiver functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. In Part I of the paper, the effects of nonideal transmission conditions have been thoroughly analyzed. To show the impact of the synchronization algorithms-which are most critical in OFDM-on system performance and complexity we consider the design of a complete receiver consisting of symbol synchronization, carrier/sampling clock synchronization and channel estimation. The performance of the algorithms is analyzed and a qualitative estimate of the resulting complexity is given. This allows to draw conclusions concerning the achievable system performance under realistic complexity assumptions.

I IntroductionMulti carrier systems such as orthogonal frequency division multiplexing (OFDM) are well known to allow a bandwidth-efficient transmission over strongly frequencyselective channels at a moderate implementation effort [1]. A coherent system based on this method has just been selected for the new European digital terrestrial TV standard (DVB-T) [2] and OFDM-based systems are considered for broadband wireless indoor systems [3].The estimation of the frame start position determines the alignment of the FFT-window in the receiver with the useful portion of the OFDM symbol. A false estimate leads to ISI which may disturb the orthogonality of the system and cause essential degradation due to inter channel interference (ICI). In the case of a coherent system in addition the a priori assumptions made for channel estimation may be violated which results in a degradation of the channel estimate. It is useful to analyze these effects to find a proper measure for the performance of synchronization algorithms. II System ModelWe consider an OFDM System having a FFT length of N ff t where a total of N u subcarriers are used for transmission. The transmitted signal s(t) is generated via an inverse FFT operation of the subcarrier symbols a n;l and zeros representing the virtual carriers. To prevent ISI a guard interval of length 1 is placed in front of the useful portion T u of the signal according to figure 2. If 1 is chose bigger than the maximum delay of the channel impulse response (cir) the orthogonality of the system is preserved

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