Complexity evaluation for the implementation of a pre-FFT equalizer in an OFDM receiver

A pre-FFT equalizer (PFE) has been shown to offer a significant throughput efficiency improvement when applied to an OFDM receiver. Alternatively, the PFE can be used to increase the maximum delay spread conditions under which the OFDM system can operate effectively. Due to the manner of its operation, the PFE requires the use of modified adaptation algorithms if iterative, decision directed, adaptation is required. The computational complexity required to implement a PFE and a suitable adaptation strategy is evaluated. Initially, an LMS adaptation algorithm is investigated and evaluated in terms of its suitability for application in conjunction with the PFE to standards such as ETSI DVB-T and HIPERLAN/2 and IEEE 802.11a. The complexity requirements are found to be high, particularly in the case of DVB-T. The demand for a lower complexity adaptation algorithm is thus identified. As a result, a CSI-based adaptation method is subsequently considered. The complexity requirement of this algorithm is also analyzed and evaluated and is shown to be much lower than that of the LMS algorithm. Thus, it is shown that if the CSI-based adaptation method is used, the dominant complexity requirement is due to the implementation of the equalizing filter and not the adaptation method. Reduced filter complexity requirement is thus shown to be the key to enabling effective application of the PFE. The ATSC 8-VSB standard is identified as a possible source of techniques to reduce or facilitate the high complexity demands for implementation of the PFE filter.