This article originally appeared in the September 2018 issue of Pro Sound News. Innovations is a monthly column in which different pro audio manufacturers are invited to discuss the thought process behind creating their products of note.
While only one of a number of extremely important factors, range truly is the holy grail of wireless products. “How far can I go?” is almost always the first question on everyone’s list. This is especially true for professional wireless intercom systems, where increased coverage area can often mean the difference between success and failure in challenging production environments.
At first thought, just turning up the transmit power might sound like an easy and practical method of boosting range, but it is much easier said than done. To see why, let’s take a look at range as it relates to RF power. The distance an RF signal will propagate is governed by the Inverse-Square Law, which states that the intensity of a radio wave is inversely proportional to the square of the distance from its source. Said another way: To go twice as far, you have to increase RF power by four times! Expressed in dB, it takes a 6 dB increase or decrease in power to achieve double or half range. This simply isn’t practical for a number of reasons, including regulatory, battery life, spectral efficiency and user density.
If cranking up the power isn’t a practical technique for increasing range, what is? This is one of the most important questions we had to answer as we developed our new CrewCom wireless intercom system, which involved defining what range actually is. Manufacturers commonly quote range under “ideal conditions.” This is essentially how far the receiver can be from the transmitter with no obstructions and no interference, taking the RF power of the transmitter, the receiver sensitivity, the antenna gains and the calculated path loss at the given operating frequency into account.
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Calculating range using the ideal conditions method can achieve a large number, but it really isn’t helpful in understanding practical-use conditions. There are many other factors that limit range in day-to-day production activity. For instance, the radio pack is worn on the body, which acts as an RF absorber. Depending on the body size and shape, and the specific frequency in use, anywhere from approximately 9 to 18 dB of RF power can be lost. Remembering that a 6 dB reduction in power results in half the range, an 18 dB reduction in power from body-shielding would result in an eight times reduction from the ideal conditions calculation (half range three times).
It seems that a good working definition of range is how far radio packs can get from the transmission source under actual use conditions for the system. Therefore, there are many factors that must be considered in addition to RF power. In today’s modern digital transmission schemes, which utilize the spectrum far more efficiently, one of the biggest factors in maximum practical range is how well the system handles intersymbol interference as a result of multipath propagation.
Digital transmission schemes actually transmit the desired information (in this case, intercom audio) as a series of ones and zeros—referred to as bits or symbols. The speed at which these symbols are sent determines the data rate (or bit rate). Since more data is almost always desirable, higher data rates are used. Since symbol rate is inversely proportional to symbol width, narrower symbols are the result. As we will see, narrow symbols are more susceptible to intersymbol interference.
Intersymbol interference is different from traditional multipath fading, which occurs when one or more reflected signals arrives at the receiver out of phase with the primary, direct signal and they cancel each other out. With intersymbol interference, one or more reflected signals arrive at the receiver in phase with the primary signal, but delayed in time. In this case, both signals are received and they overlap each other. The problem occurs when the secondary signal arrives more than approximately 20 percent of the symbol width later than the direct signal. When this happens, the receiver cannot reliably distinguish a one from a zero in the bit stream, resulting in garbled, unusable audio.
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There are many techniques available to combat intersymbol interference, each with its specific strengths and weaknesses. In the design of the CrewCom radio scheme, we chose to utilize a technique commonly associated with OFDM transmission schemes. We split the required data among multiple RF carriers. In doing so, we were able to reduce the data rate applied to each carrier, thus increasing the symbol width and greatly reducing the potential for intersymbol interference in typical production environments. This technique also ensures that enough data can be sent over multiple carriers to achieve high-quality audio and support efficient user density.
Mitigating the harmful effects of intersymbol interference is critical to achieving excellent functional range, but that in and of itself does not expand the area that may be covered. To do this, CrewCom enables a “put the RF where you need it” approach. CrewCom uses a distributed architecture and a proprietary network to allow radio transceivers to be deployed in close proximity to any desired coverage area. Radio packs then seamlessly roam from one transceiver to another as the user moves from one coverage zone to another, all the while keeping the distance between the radio pack and the radio transceiver as short as possible.
Shortening the distance between the transmitter and the receiver ensures that interference, body-shielding, structure-related RF attenuation or a host of other detrimental RF factors have no significant impact on system operation. This is the most effective way to greatly expand a given coverage area without adversely affecting battery life, pack size, spectrum efficiency or user density.
To review: Increasing RF power is not a practical way to increase operational range. The theoretical or “ideal” conditions range of an RF system does not accurately reflect the practical operating distance under real conditions. Intersymbol interference plays a large factor in effective range of a digital RF system in reflective environments. CrewCom utilizes unique, proprietary technology to overcome the harmful effects of intersymbol interference. Ultimately the best way to greatly increase operational range is to put the RF where you need it. CrewCom’s distributed architecture and proprietary network allows system administrators to do just that.
Tom Turkington is vice president of technology at Pliant Technologies.
Pliant Technologies • www.plianttechnologies.com