A fast ARQ mechanism operated inside 3GPP GERAN mobile radio systems uses short bitmaps, spanning only few octets, in the header of a RLC/MAC block conveying relevant ACK/NACK signalling that is transmitted either in the uplink or downlink direction, indifferently. This short bitmap which may request synchronization between transmitting and receiving frames, is encoded together with the header using coding which is more robust than that used for payload. Correct retransmission of badly received radio blocks is based on the number of RLC/MAC block periods between the instant of transmission of a radio block and the instant of reception of the short bitmap. In addition, the short bitmap is allocated in a new zone placed immediately after the header of the RLC/MAC blocks, and encoded independently of the payload using a more robust coding against the errors than the robustness of the MCS used in the payload data part.