TransCore Amtech IT2200, Инструкция по установке, обслуживанию и ремонту

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IT2200 Reader System with Multimode Capability Installation & Maintenance/Service Guide
6-4
Determining Acceptable Lane Performance
The criteria for optimal lane performance are usually set by the customer and can vary
according to the site requirements. In testing, acceptable lane operation criteria typi-
cally are determined by the length of the RF footprint and the speed of the test vehicle.
Usually, a test vehicle’s speed is limited by the amount of the toll lane that can be used
for starting and stopping distances. Usually, testing speed is limited to 32 kph (20
mph) or less.
An ideally performing toll lane will produce one handshake for every 4 ms of transac-
tion time. At 32 kph (20 mph), the vehicle is using 34 ms to travel through 0.3 m (1.0
ft) of the footprint. If the footprint is 2.4 m (8 ft), this means that the vehicle will
spend approximately 272 ms in the footprint. Based on a vehicle speed of 32 kph (20
mph) and an 2.4m (8-ft) footprint, this yields an ideal maximum number of 68 hand-
shakes. Nulls and voids within the RF footprint will lower this number, as will any
other local sources of RF noise and stray reflections. A rule of thumb for lane perfor-
mance is to have 40 to 60 handshakes within an 2.4m (8-ft) footprint with a test vehi-
cle traveling at 32 kph (20 mph). A system that operates with less than 40 handshakes
should be tested for cross-lane interference.
Identifying Cross-Lane Interference
Cross-lane interference is identified by an area in the RF read zone, or footprint,
which has areas where a tag cannot be read. If a toll lane has been operating satisfacto-
rily and then begins to show a degradation in system performance, that is, an increas-
ing number of missed reads or a spotty read pattern, there is a probability that cross-
lane interference is occurring.
Cross-lane interference can be caused by several factors, including:
•
A downlink antenna transmitting strong RF beyond its lane boundaries
•
Reflection of RF from fixed objects (e.g., toll plazas with low, metal roofs)
•
Reflection of RF from moving objects (e.g., a passing tractor-trailer in an adjacent
lane)
A typical toll lane application encompasses more than a single lane. In some cases a
toll plaza can have more than eight lanes with each lane having separate RF transmit-
ting (downlink) and receiving (uplink) antennas. As shown in Figure 6-1, the RF
transmitted within a lane is not bound by physical dividers such as lane barriers. With
multiple-lane applications, this can create areas of possible cross-lane interference.