A communications system, comprising: a plurality of communications devices, wherein a communications device, a trainer, generates at least one concatenated training sequence, transmits the at least one concatenated training sequence to train many trainee devices during one training session, and receives one or more response messages from at least one of the rest of the plurality of communications devices; and one or more of the rest of the plurality of communications devices, trainees, each receives the at least one concatenated training sequence, determines the channel characteristics between the trainer and a trainee according to the at least one received concatenated training sequence, embeds channel characteristics information to generate at least one of the response messages, and transmits the response message; the system characterized in that the trainer generates the at least one concatenated training sequence by obtaining at least one training sequence, modulating the training sequence to produce two or more modulated training sequences, and concatenating the modulated two or more modulated training sequences to obtain the concatenated training sequence, wherein each modulated training sequence is modulated by applying different modulation schemes according to a predetermined rule to the training sequence so as to carry different information pertinent to channel characteristics. a plurality of communications devices, wherein a communications device, a trainer, generates at least one concatenated training sequence, transmits the at least one concatenated training sequence to train many trainee devices during one training session, and receives one or more response messages from at least one of the rest of the plurality of communications devices; and one or more of the rest of the plurality of communications devices, trainees, each receives the at least one concatenated training sequence, determines the channel characteristics between the trainer and a trainee according to the at least one received concatenated training sequence, embeds channel characteristics information to generate at least one of the response messages, and transmits the response message; the system characterized in that the trainer generates the at least one concatenated training sequence by obtaining at least one training sequence, modulating the training sequence to produce two or more modulated training sequences, and concatenating the modulated two or more modulated training sequences to obtain the concatenated training sequence, wherein each modulated training sequence is modulated by applying different modulation schemes according to a predetermined rule to the training sequence so as to carry different information pertinent to channel characteristics. the system characterized in that the trainer generates the at least one concatenated training sequence by obtaining at least one training sequence, modulating the training sequence to produce two or more modulated training sequences, and concatenating the modulated two or more modulated training sequences to obtain the concatenated training sequence, wherein each modulated training sequence is modulated by applying different modulation schemes according to a predetermined rule to the training sequence so as to carry different information pertinent to channel characteristics.