IT2200 Reader System with Multimode Capability Installation & Maintenance/Service Guide
6-6
Frequency Separation
Review the toll plaza frequency plan that was developed during the IT2200 Reader
System with Multimode Capability installation phase. There are two frequencies for
each reader: downlink and uplink. For the IT2200 Reader System with Multimode
Capability, all readers share the same downlink frequency, which is generally set to
918.75 MHz. Uplink frequencies should alternate between 903.00 MHz and 912.00
MHz in adjacent lanes. For example, a four-lane plaza would have the frequencies
shown in Table 6-1.
RF Power
A good rule of thumb when configuring a toll plaza is to set the RF attenuation at a
lower output and increase the level as needed for optimal system operation (see “Lane
Tag Test” on page 5-6 for procedures to set the RF output). This practice may provide
you with RF attenuation settings at which your reader system can operate with mini-
mal adjustment for cross-lane interference.
Time Division Multiplexing
If after tuning each lane your toll plaza is still experiencing cross-lane interference,
you may want to configure the IT2200 Reader System with Multimode Capability
using time division multiplexing, or TDM. Time division multiplexing offers the
option of isolating each lane in the system so that it operates as a stand-alone reader
for a brief fraction of the transaction time. Time-division multiplexing permits the
antennas in each individual lane to operate apart from the other antennas in the toll
plaza. This results in one lane or alternating lanes transmitting and receiving RF at a
time. Figure 6-2 shows how TDM can delay the reader period and isolate lanes from
each other.
Table 6-1 Frequency Plan for Four-Lane Toll Plaza
Lane Downlink Frequency Uplink Frequency
1 918.75 MHz 903.00 MHz
2 918.75 MHz 912.00 MHz
3 918.75 MHz 903.00 MHz
4 918.75 MHz 912.00 MHz
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