1. In a communication system comprising a sender and a target, the sender sending at least a set of input vectors to the target, the set of input vectors comprising a primary input vector and a secondary set of input vectors, wherein the sender detects errors in the set of input vectors, a method for the sender to indicate to the target that the set of input vectors contains errors, the method comprising the steps of: a) encoding the secondary set of input vectors to produce a secondary set of encoded vectors; b) modulating, by the sender, the secondary set of encoded vectors with a predetermined input vector to produce a secondary set of modulated vectors; c) sending, by the sender, the secondary set of modulated vectors and a predetermined codeword to the target; d) upon receiving the secondary set of modulated vectors and the predetermined codeword, decoding the predetermined codeword to produce a decoded predetermined codeword; and e) demodulating, by the target, the secondary set of modulated vectors using the decoded predetermined codeword to produce a secondary set of demodulated vectors, the secondary set of demodulated vectors having a sufficient number of errors to indicate to the target that the set of input vectors contains errors.