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This paper considers the joint design of source-relay precoders in OFDM systems where power constraint is specified for a contiguous block of subcarriers. The design criteria considered are the popular Arithmetic sum of average Bit Error Rate(ABER) and Arithmetic sum of average Mean Square Error(AMSE) criteria. The proposed algorithm optimizes the precoders for a block of subcarriers. For the ABER criterion, it gives a performance improvement of about 3 dB at an ABER of 10-3 when compared to a system with equal power allocation on all subcarriers. However, the same approach for the AMSE criterion does not give any significant improvement in performance. This behavior is reasoned by plotting the performance surface of ABER and AMSE criteria.