A deep echo transmission array is used to bounce a subspace signal between layers of subspace to increase the effective range. It is similar to how certain radio signals bounce off of the ionosphere of a planet and overcome line of sight restraints allowing such radio broadcasts to be heard literally across the surface of planet.
Operation
In this case a powerful and carefully modulated subspace signal is broadcast deep at very high power into subspace where it has an echoing effect from the energy interacting with the different layers of subspace. The signals then bounce back up into normal space across an incredibly large area of space. This allows the signals to be heard for hundreds of lightyears in any direction but making pinpointing the origin of the signals nearly impossible. Any properly tuned subspace radio is capable of receiving the signals if the radio was to be precicely tuned. Because the echos bounce back into normal space at random points deep in subspace, tracking a signal to its original broadcast point is nearly impossible. The transmission array requires enormous amounts of power and requires to be tuned to broadcast in specific conditions in space - mobile transmitters of this type are difficult to operate requiring hours of special tuning if moved from one region of space to another.
Rumor and Speculation
As a point of interest, signals have been long detected across vast stretches of space that very likely use this kind of array. To date no public or private agency has admitted to being the origin of such signals. Although much like the terrestrial Number Stations of Earth it is widely accepted that such signals are covert in nature. Some signals are simply repeating numbers, others are a series of tones in a repeating pattern. Still others have a clear carrier wave but no content. In fiction throughout Federation space such signals are widely speculated at to sensational and dramatic ends within a story.