In this thesis, we design and examine the QPSK-modulated codes for combined channel estimation and error protection over frequency-selective channels. We found that when QPSK modulation is considered, phase information of the channel coefficients is more essential than the respective amplitude information, particularly for the blind receiver we are interested in. Under the assumption that the unknown phases are synchronized among different channel taps, we establish a close-form-expressed union bound for the error performance and later use it as a criterion to search for the optimal code design. Our simulations show that the QPSK-modulated codes can provide an acceptable improvement over the BPSK-modulated codes.