How to minimize the effects of time-delay interference
Last week, we began a discussion of time-delay interference (TDI), which affects every simulcast system, regardless of whether it is analog or digital. In this article, we conclude by examining ways to minimize TDI’s effects, which will cripple system performance — even when excellent RF coverage exists.
Most radio receivers can handle a negligible amount of delay and this figure varies with manufacturer designs. Be sure that you know the maximum delay that your receiver can handle and design your system accordingly. If your system is an existing system and you suspect TDI, examine your link budgets and hire a qualified consultant to conduct a new propagation study to verify where these TDI areas should be, given your antenna types, effective radiated power, site location and antenna height and azimuth. You can work the process in reverse by verifying station output power, sweeping transmission lines and verifying system timing.
In a given design, even when the RF signal strength is high, the user will experience reduced DAQ in areas of excessive TDI. If you are designing a simulcast system, remember that you can have a strong signal, say –80 dBm and above. At –80 dBm, a DAQ of 3.4 or better easily is achievable. However, TDI can still be present in these areas and reduce your DAQ to 2.0 — or worse.
TDI cannot be avoided, but its effect can be minimized by carefully weighting design parameters to maximize the possibility that signals arrive at the center point of the overlapping area, either at the same time or within mere microseconds of each other. Main parameters to consider include:
Site separation. It takes a radio signal approximately 5.4 microseconds to travel one mile, thus TDI is reduced when sites are closer together.
Control of overlapping coverage. The extent of the overlapping area can be controlled by adjusting transmit power, azimuth and down-tilt of the transmitted signals. In general terms, the goal is to use directional antennas with high front-to-back ratios and narrow beam-widths to provide greater control of signals. Typically, the outer and most distant sites may require directional antennas. Use caution, as too much vertical gain from a transmit antenna may cause the site to "overshoot" the users operating close-in to the site. Lower antenna gain and down-tilt will help in this regard.
Antennas can have electrical or mechanical down-tilt, and can be used to reduce extended coverage from a site and to increase usable signal to the subscribers within the required coverage area. Remember, higher gain antennas “squash the doughnut” and radiate more energy towards the horizon. When optimizing a simulcast system, this may not be desirable depending on the site locations and their distance from each other.
Alignment. Signal synchronization can be achieved by delaying the launch of one or more transmitters. In digital simulcast systems, GPS is used to synchronize the launch of signals. Once TDI is suspected, signals can be delayed so that signals of the same strength arrive at the same time. This timing delay adjustment is known as "system alignment." Remember, any alignment and optimization should be performed using the same piece of test equipment. Even though a piece of test equipment may be calibrated, there still are variances in multiple pieces. A single service monitor ensures identical measurements at each site. In a simulcast system, consistency is crucial.
Antenna height. Remember, height is king. As a system designer, you can get far better results from antenna height than from RF power. The higher the frequency, the more line-of-sight the signal becomes. If multiple vertical tower locations are available, model the chosen antennas at various heights and use elevation and your operating frequency band to your advantage.
Without question, simulcast systems are indispensable to agencies that need superior coverage with minimal frequencies. Time-delay interference can cripple these systems and leave the end-users believing they are using a system that delivers poor coverage, when in reality the system's non-capture areas are not optimized. Proper simulcast optimization will turn a mediocre-operating radio system into a well-functioning design. TDI is one design parameter to watch out for.
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