Channels

The channels contained in this library come from various experiments. The experiments differ by geographical location, transmission distance, water depth, transmitter/receiver mobility, acoustic bandwidth, and the size of the recording array. The channels are color-labeled, and a detailed description of relevant parameters is given below for each. The channels are available for download from Zenodo:

(https://doi.org/10.5281/zenodo.21287414)

Each channel is estimated from the experimental data and stored as a tensor of complex-baseband impulse responses evolving over time, with dimensions [delay, receiver, time]. If a recording array is available, the tensor holds one matrix for each array element. Each column of this matrix is an instantaneous channel response as a function of delay, that is, the channel vector $\hat{\underline{\mathbf{h}}}[n]$ at time index $n$. Successive columns correspond to different, equi-spaced instants in time. Any motion-induced delay drift is suppressed in the channel matrix, leaving the drift-free response $\hat{\underline{h}}(\tau, t)$, which compresses efficiently for storage. A separate vector, the phase estimate $\hat{\varphi}(nT_s)$ stored as phi_hat, is provided that contains the uncompressed time-varying channel phase, which is directly related to the delay drift. Here $T_s$ is the sampling interval along the delay axis. To fully reconstruct the channel, the stored matrix needs to be decompressed, and the phase/delay needs to be imparted to re-introduce the phase shift and the delay drift. Details of this process, along with the ready-to-use code, are given in the User’s Guide. The stored variable names, and the alternative convention in which only the phase $\hat{\theta}(nT_s)$ is tracked (theta_hat) and the delay drift is left in the taps, are documented in the channel file format specifications.

Column key: d_T, d_R, and d_w are the transmitter depth, the receiver depth, and the water depth; d is the transmitter-receiver distance; f_c is the center frequency; R is the symbol rate of the transmitted probe signal; M is the number of array elements; ℓ is the inter-element spacing. Where two values are listed for d_w, they are the water depths at the transmitter and at the receiver. An "x" marks a value that was not recorded.

For the Green channel, M denotes the number of time-diversity channels formed from repeated transmissions on a single hydrophone, and ℓ denotes the inter-transmission interval.