This week we have a very interesting guest contribution from Mike Motes, Chief Engineer at Channel D Solutions. His firm designs accessories for the Tempest line of professional intercoms, and consults on com system installations all over the country. We normally think of multipath reflections as harmful byproducts of wave propagation. But in this post, Mike describes how he uses radio wave reflections to sculpt ideal coverage patterns in difficult indoor environments by using directional antennas to make the most of bad situations. He cites a recent install in mid-town Manhattan that used a pair of RF Venue's very own 2.4 GHz CP Beam antennas.
What is multipath interference?
Simply put, “multipath” is a term used to describe reflections of the same RF signal being reflected from different and multiple surfaces. Since it is much more pronounced in digital communications systems, we will limit this discussion to digital comms.
The RF signal is a digital representation of an analog signal. In order to properly convert the digital information back into the original signal, we must have all of the correct information in digital format. While encoding schemes have some sort of error correction included in the “packets” of data, the error correction has its limits as to what it can recover from. Lose too much data, it cannot be properly be decoded.
If you have an area where too many reflections are arriving, even if they are close in time and/or phase, you will still have data corruption. An example of this is a closed-dome stadium, where there is a lot of metal used in the dome. It provides for a near-perfect environment for multipath to take place, especially if the intended coverage area is wide open to both the original signal and to the reflections. More often than not, this results in poor decoder performance and equipment operation. One way of dealing with this is to use antennas with a narrow aperture of radiation, reducing the signal strength in the vertical plane towards the dome and increasing the signal strength towards the area you are attempting to cover.
But what about using this phenomenon to our advantage?
Consider a theater or studio-type of environment consisting of many RF absorbing materials (concrete, human bodies, and other types of obstacles the RF signal must face). Deployment of a digital wireless intercom system in this type of environment can actually benefit from multipath signals. For example, you may have adequate coverage for people on wireless comms on stage left and stage right, but due to several stage backdrops, the main signal is unable to penetrate directly to people behind the stage. However, there is metal rigging above the stage. Any signals being reflected off of the “rigging” may actually find their way behind the RF absorbing materials, and reach those people located in the dead zones. Since there is no main signal for these reflections to interfere with, the multipath signal in this case, becomes the main signal. Another example would be a main area that is being covered, and an adjoining hallway that needs coverage, but just barely has any past the entrance. By having a reflective surface at the main entrance, the signal can, by means of multipath, actually obtain much better coverage in the hallway.
Multipath can also be of benefit in an open, outside environment. In this scenario, the client needed coverage at the talent entrance which is located on a side street, that had no coverage from the main antennas covering the front of the building. A directional antenna (in this case, RF Venue’s 2.4 GHz CP Beam circularly polarized antenna) was placed behind a billboard on the second floor of the building, and aimed directly down towards the street above the entrance. The billboard was supported by a metal grid, and provided the perfect framework for multipath reflections. The main signal from the antenna provided error-free decoding at the entrance, and the incident reflections allowed for coverage well past the entrance away from the main street, and also towards the main street, where the front antennas picked up the users. This provided the seamless coverage that was requested by the client.
Not every venue is suited for using multipath to your advantage. But for those that are, it can often reduce antenna count, and exceed the coverage requirements specified by the client. Always use a high quality coaxial cable that is 100 percent shielded in connecting the antennas. We have seen some people using less than optimum cable that radiates signal from the coax. That can also mimic multipath interference, and be difficult to diagnose. Finally, there is no set rule in determining if a venue is or isn’t suited for taking advantage of multipath. While experience can be used as a guide, there is no guarantee of success. ALWAYS test with actual equipment to determine suitability. Above all, don’t be afraid to “think outside of the box”. You (and your client) may be pleasantly surprised!