manuignatius
commented
3 years ago Write up for Multi-channel modems
# Multi-channel modem
The Subnero underwater modems provide options for customization and extension at many levels. One of such customization is when a customer requires multiple synchronous recordings. There are multiple use cases under which such a multi-channel modem is extremely useful. We explain here few such capabilities that can be easily developed.
## Use it as a recorder using multiple hydrophones
It is often a requirement to record synchronized signals from carefully positioned hydrophones. Every time a signal is detected on the main hydrophone/transducer a recording can be triggered on all the hydrophones. In order to fulfill this requirement, we provide an option to integrate multiple preamplifiers and a secondary data acquisition system. A driver is developed for the multi-channel acquisition system. This driver publishes its data to UnetStack using standard baseband recording messages, so that the customer’s software could request these messages. At the event of detection of a signal on the main hydrophone, a baseband reception notification message `RxBasebandSignalNtf` is sent by the modem. The customer’s script can look for the reception of this notification message and request a recording using the secondary data acquisition system.
## Use it to build USBL based navigation system
The ability to record using multiple hydrophones enables us to develop high resolution ultra short baseline (USBL) acoustic navigation system to provide an accurate range and bearing estimates to an underwater target (e.g., an acoustic transponder). The multiple hydrophones can be conveniently positioned to form a desired receiver array geometry. The arrival times (and resulting phase) of the signals on these hydrophones slightly differ based on the array geometry and can be used to estimate the direction of arrival. This technique is used for estimating the bearing of the target/source node such as an acoustic transponder. Note that this system also allows communication with other underwater assets with an acoustic modem since the USBL system is built with just a software customization of the multi-channel modem.
## Use it as a spatial diversity enabled receiver modem
Received signal recordings on multiple hydrophones may differ due to a different channel that each of them might experience. Since the received signals on all hydrophones may differ; an attempt to decode might result in a different result. Some of them might get decoded correctly and some might result in bit errors. It is also possible that all of the received signals get decoded to result in errors which cannot be corrected. However, the power of selection diversity and diversity combining comes into play here. Utilizing the flexibility of UnetStack, we can immediately publish a successfully decoded frame from one of the hydrophones to be consumed by user. And if none of the hydrophones and main transducer of the modem is able to successfully decode the signal, all of them can be optimally combined and fed to the decoder to potentially decode the frame.
Under the "products" page create a 4th section, to add "Acoustic Data Acquisition Systems", which will include aRecorder and multi-channel modems (with and without batteries)