Comparisons Between Code- and Time-Division-Multiplexing for MIMO Sonar Signal Processing

Abstract

Multiple-input multiple-output (MIMO) sonar systems enhance angular resolution by using travel-time information from all transmitter–receiver pairs compared to conventional single-input multiple-output (SIMO) systems. When dealing with multiple transmitters in a MIMO system it is crucial to separate pulses properly at the receiver side, which is possible by using multiplexing methods such as code division multiplexing (CDM), time division multiplexing (TDM), or frequency division multiplexing (FDM). The latter is not suitable for MIMO systems due to spatial ambiguities and image artifacts it introduces and is therefore not considered within this study. CDM enables simultaneous pulse transmission, however, it suffers from crosstalk caused by a non-zero cross-correlation of the pulses, which limits its performance. On the other hand, TDM allows for perfect pulse separation and thus a reduction of the crosstalk. However, during a prolonged overall ping period changes in the acoustic channel may occur which lead to artifacts after coherent processing. This work investigates the performance of MIMO sonar systems which implement CDM, TDM, and their combination with a focus on the reduction of crosstalk in a short range application. The pulse separation methods are studied through simulations and experimental validation in a harbour environment using various pulse types and number of transmitters. The results show that the generalised integrated sidelobe level (GISL) as a measure for the crosstalk can be reduced by the integration of TDM