Transmitter device i (1, 2, 44) for a secure channel training method applied to a power domain non-orthogonal multiple access system comprising a plurality of transmitters and receivers, wherein the transmitter device i (1, 2, 44) is configured to: estimate the channel (6, 9, 48) between receiver device j (3, 45) and transmitter device i (1, 2, 44); estimate the channel (6, 9, 48) between receiver device j + 1 (4, 46) and transmitter device i (1, 2, 44); characterized by further apply a channel processing block (19, 18, 55) to encode the estimated channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receiver devices j (3, 45) and j + 1(4, 46); transmit a signal of said encoded channels (6, 7, 9, 10, 48, 49) to receiver device j + 1 (4, 46) containing information regarding the estimated channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receiver's devices j (3, 45) and j + 1(4, 46); wherein the transmitter device i (1, 2, 44) is further configured to apply said channel processing block (19, 18, 55) to the estimated channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receiver devices j (3, 45) and j + 1 (4, 46), by applying the following encoding operation: x F i k = F i p H h j + 1, i k, h j, i k, r k = p H h j + 1, i k h j, i k − 2 e j θ j, i k − 2 θ j + 1, i k − r k 2 π M where F i (.) in equation (Eq. 39) defines the processing function to be applied, h j, i ( k) is the small-scale channel (6, 9, 48) fading coefficient between transmitter device i (1, 2, 44) and receiver device j (3, 45), h j +1, i ( k) is the small-scale channel (6, 9, 48) fading coefficient between transmitter device i (1, 2, 44) and receiver device j + 1 (4, 46), | h j,i ( k)| is the magnitude of the channel (6, 9, 48) between transmitter device i (1, 2, 44) and receiver device j (3, 45), | h j +1, i ( k)| is the magnitude of the channel (6, 9, 48) between transmitter device i (1, 2, 44) and receiver device j + 1 (4, 46), θ j, i ( k) is the phase of the channel (6, 9, 48) h j,i ( k) between transmitter device i (1, 2, 44) and receiver device j (3, 45), θ j +1,i ( k) is the phase of the channel (6, 9, 48) h j +1, i ( k) between transmitter device i (1, 2, 44) and receiver device j + 1(4, 46), x F ( i) ( k) is the signal to be sent by transmitter device i (1, 2, 44) containing the encoded channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receivers devices j (3, 45) and j + 1(4, 46), p H corresponds to a power allocation coefficient allocated to receiver j + 1 (4, 46), r(k) is defined as 0 in case that a cooperative jammer is present or otherwise as an integer random variable defined in the interval [1, M] wherein M is the order of a digital modulation for which the system is configured, and k defines the system realization index. estimate the channel (6, 9, 48) between receiver device j (3, 45) and transmitter device i (1, 2, 44); estimate the channel (6, 9, 48) between receiver device j + 1 (4, 46) and transmitter device i (1, 2, 44); characterized by further apply a channel processing block (19, 18, 55) to encode the estimated channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receiver devices j (3, 45) and j + 1(4, 46); transmit a signal of said encoded channels (6, 7, 9, 10, 48, 49) to receiver device j + 1 (4, 46) containing information regarding the estimated channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receiver's devices j (3, 45) and j + 1(4, 46); wherein the transmitter device i (1, 2, 44) is further configured to apply said channel processing block (19, 18, 55) to the estimated channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receiver devices j (3, 45) and j + 1 (4, 46), by applying the following encoding operation: x F i k = F i p H h j + 1, i k, h j, i k, r k = p H h j + 1, i k h j, i k − 2 e j θ j, i k − 2 θ j + 1, i k − r k 2 π M where F i (.) in equation (Eq. 39) defines the processing function to be applied, h j, i ( k) is the small-scale channel (6, 9, 48) fading coefficient between transmitter device i (1, 2, 44) and receiver device j (3, 45), h j +1, i ( k) is the small-scale channel (6, 9, 48) fading coefficient between transmitter device i (1, 2, 44) and receiver device j + 1 (4, 46), | h j,i ( k)| is the magnitude of the channel (6, 9, 48) between transmitter device i (1, 2, 44) and receiver device j (3, 45), | h j +1, i ( k)| is the magnitude of the channel (6, 9, 48) between transmitter device i (1, 2, 44) and receiver device j + 1 (4, 46), θ j, i ( k) is the phase of the channel (6, 9, 48) h j,i ( k) between transmitter device i (1, 2, 44) and receiver device j (3, 45), θ j +1,i ( k) is the phase of the channel (6, 9, 48) h j +1, i ( k) between transmitter device i (1, 2, 44) and receiver device j + 1(4, 46), x F ( i) ( k) is the signal to be sent by transmitter device i (1, 2, 44) containing the encoded channels (6, 7, 9, 10, 48, 49) between transmitter device i (1, 2, 44) and receivers devices j (3, 45) and j + 1(4, 46), p H corresponds to a power allocation coefficient allocated to receiver j + 1 (4, 46), r(k) is defined as 0 in case that a cooperative jammer is present or otherwise as an integer random variable defined in the interval [1, M] wherein M is the order of a digital modulation for which the system is configured, and k defines the system realization index.