BMRM/SRM Radio Interface

The radio interface designed in the SIAMtec products provides the special features that are required to maintain excellent mobile radio communication in difficult underground environments.

Mobile radio communications suffer degradation due to the multi-path-fading phenomenon.  This phenomenon has the negative impact of creating dead spots at predetermined distances from the DAS antenna. At those particular locations, the signal level is considerably weaker than at any other location inside the drift.  Real-time communication may be temporarily interrupted while the vehicle antenna is located in a dead spot.  Normal communication resumes as the vehicle moves away from this area (dead spot).

In a teleoperation application, where the vehicle is in constant motion, this interruption of communication may be tragic since the loss of real-time video images can be the cause of accidents.

In order to overcome these difficulties, the following features have been implemented into the SIAMtec radiomodem technology.

Space Diversity:  In order to circumvent the dead spot problem, SIAMtec has introduced a two-antenna modem technology.  The principle is to install two antennas on the vehicle at a predetermined distance from each other.  As the vehicle moves, one antenna may temporarily be located in a dead spot.  In that case, the other antenna will be selected by the system since it is not located in a dead spot and it receives a strong signal.  The “Space Diversity” feature allows the SRM to select the antenna that is receiving the best signal at any given point in time. This dual antenna system allows uninterrupted communication with the central computer (BMRM).

As indicated in the above diagram, the SRM receives the BMRM signal, referred to as F1, from either antenna but the rear one will be selected because of its strong signal.

Frequency Diversity:  A dead spot location varies with the RF signal frequency.  In other words, a dead spot for a frequency F1 is located a few centimeters away from another at frequency F2.  The SRM has been designed to transmit its message via the two antennas but at different frequencies (F1 and F2).  As the vehicle moves, should an antenna be located at a dead spot for a given frequency F1, the probability that the other antenna be located in a dead spot for frequency F2 is negligible.

The “Frequency Diversity” feature allows the BMRM to receive the data without interruptions. The BMRM constantly monitors and selects the antenna port (F1 or F2), which receives the best signal.  This process is repeated for every message received.

Other features:

  • Each SRM radio modem has a unique address selected from 1 to 64.  The system supports broadcast and unicast transmissions. Packets may be sent to a particular SRM or to all simultaneously, depending on the situation.

  • In a “Loading Drive” application, where the vehicle is working at great distances from the DAS antenna, directional antennas (panel type) are installed in the front and at the rear of the vehicle.  A directional antenna improves the communication in long drifts.  A dual antenna SRM will automatically switch from one antenna to the other as the vehicle’s orientation changes with respect to the DAS antenna.

  • Dual antenna SRM’s also provide redundancy at the radio level.  If the antenna or its cable is damaged or becomes inoperative, the other antenna will maintain the communication link with the central computer.  The performance will be equivalent to a single antenna SRM.

Radio link protocol with error control: The information is transmitted in the form of packets.  The complete data string is divided into packets of 256 characters or less.  The integrity of the data is maintained at all times by making use of the error detecting CRC 16 protocol.  The receiver detects the occurrence of a transmission error and requests another transmission of the erroneous packet.  On reception of a valid packet, the receiver sends an acknowledgement message to the transmitter.