A look at time-delay interference
Does your simulcast radio system seem to exhibit improved range and performance when certain transmitters are turned off? In almost every case, this is an indication of improper system operation and that your system needs on-site optimization.
In a simulcast system there are several parameters that adversely can affect system performance, but one parameter that is sometimes misunderstood is time-delay interference, or TDI. In this article we begin a discussion of TDI and its significance in a simulcast radio system.
Simulcast radio systems are used when a limited number of RF channels are available and a large geographical area needs to be covered. As the name implies, simultaneous broadcast, or simulcast, occurs when two or more sites transmit at the same time on the same frequency; invariably the coverage from these multiple sites will overlap.
Inside of these overlapping zones are areas whereby the RF signals from two or more sites arrive at the same signal level. These areas are referred to as non-capture areas, meaning that the subscriber receiver cannot be "captured" by a stronger signal because the multiple signals arriving at the receiver's antenna are of the same amplitude. “Capture effect” takes place in frequency modulated, or FM, transmissions. You witness this while driving in your car and listening to your favorite FM station. As you crest the next knoll you begin to hear a different station on your same favorite frequency. You hear one station or the other because of capture effect.
As long as the equal amplitude signals arrive inside the non-capture area at the same time, the radio's receiver can receive the carrier with no problem. In fact, the channel can appear much stronger to a given receiver in a simulcast environment inside of a non-capture area. However, when the arriving signals are of equal amplitude and arrive at the receiver's antenna at different times, the receiver may not be able to handle the delay, which is known as TDI. It also is referred to as delay spread.
While it is true that newer generation digital radios can tolerate more delay, system design, system type and expected performance will dictate how much delay spread is acceptable. For example, in a Project 25 C4FM (continuous four level frequency modulation) system, a delay spread of 30us will yield an approximate bit error rate, or BER, of 1.7%. A 40us delay spread will yield around 2.8% BER.
Delivered audio quality, or DAQ, is crucial to subscriber users and typically RF coverage is designed around a minimum DAQ of 3.4 or better for a given percentage of a required coverage area. The higher the delay spread, the higher the BER. The higher the BER, the lower the DAQ. With this stated, a C4FM system will produce a DAQ of 3.4 at 2% BER. At 2% BER, the subscriber receivers can handle a maximum 34us of delay spread. Never forget that TDI, DAQ and BER all correlate.
TDI is present in any simulcast system, whether digital or analog, but its effects are more easily tolerated in an analog environment. This delay will cause subscriber receivers to miss calls or cause the radio traffic to become unreadable. To an analog radio user, this would sound like static and excessive growl in the audio from the speaker. To the digital user, the audio becomes very mechanized, almost robotic, and the radio often drops the call in process.
In trunking systems this problem is worse in that it can cause the subscriber receiver to improperly decode the control channel, which causes the radio to not only miss calls but to show a system "out-of-range" or "control-channel scan" indication — this is the same as the radio being out of service. Critical control channel updates also can be missed, allowing the radio to miss crucial messages and active talk group updates. This will cause many of the radio's features, including scan, to function improperly.
Next week: The discussion continues.
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