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Reverse Path Troubleshooting: Evaluating TDMA Return Path Signals

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Troubleshooting TDMA

Current reverse path troubleshooting strategies are not capable of detecting

problems inside of a TDMA modem’s frequency allocation. While checking

the level at each frequency is a good start for characterizing the reverse path

performance, a spectrum graph only looks at each frequency for a small

percentage of the total time. This leads to two uncertainties:

• When the spectrum analyzer is at the TDMA frequency will the carrier be

there?

• When a signal is seen in the spectrum graph, the question of origin

remains. Was that a carrier or some ingress?

While installing and characterizing your system, you realized that certain

frequencies were going to be unusable because of the frequent,

uncontrolled ingress. The spectrum around these frequencies is wasted.

Why spend time measuring these frequencies instead of looking at the

frequencies the system is using? What you really want to know is, is there

anything wrong with my modem signals?

TDMA signals have two modulation techniques—QPSK or 16 QAM. This

modulation includes overhead and data at the approximate rate of 2 to 4

times the bandwidth allocated. The second modulation is pulse modulation.

Pulse modulation allows the modem to turn off the signal when no

transmission is required. This allows multiple modems to transmit at the

same frequency. There are two states to these TDMA signals:

• When all modems are idle, there should be low noise at the modem’s

frequency.

• When a modem is active the power level should be at a predetermined

level.

Both of these states must be met for optimum operation of the modem.

Another state occurs in some of the older technology. This is called a

collision. Collisions occur when two or more modems try to access the same

frequency at the same time. When a collision occurs all messages are lost

and must be retransmitted.

Zero Span Measurements

The best way to look at a TDMA signal is using a Zero Span on a spectrum

analyzer. Zero Span uses the spectrum analyzer’s tuner in a special way.

Instead of providing a frequency versus level graph, Zero Span provides a

time versus level graph at a particular frequency. Zero Span can be thought

of as a tuned oscilloscope, displaying the level fluctuations of only frequency

you're interested in. It is very important to have rising edge triggering when

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