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Reverse Path Troubleshooting: Evaluating TDMA Return Path Signals

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quadrature modulation are used to increase the available modulation

bandwidth so that a TDMA cell-phone sends the same information in one-

third the time of an AMPS cell-phone. Through time-interleaving, three cell-

phones now occupy the same frequency bandwidth that one occupied just a

few years ago.

Another area of rapid growth using TDMA is CATV reverse path. Today’s

CATV system includes many different signal and modulation types. Each

advanced service can have one or many different TDMA signals assigned to

a particular frequency. To improve the efficiency of bandwidth usage in

cases where more than one signal is assigned to a frequency, a method of

varying the transmit time allotted to each signal based on need—variable

time slot TDMA—was developed.

Variable Time Slot TDMA

When multiple users are accessing a network via modems, an enormous

amount of bandwidth would be occupied if each modem were assigned one

frequency channel. The majority of time the user is hooked up to a network

is spent idle. Most of the bandwidth would be wasted. For example, the ratio

of authoring a typical email to transmitting a typical email is at least a

hundred to one. One possible answer would be to use a low transfer rate.

This system would work if no large files need to be transmitted. A better

answer is variable time TDMA. Variable time TDMA gives more bandwidth

when needed and takes just a little bandwidth the rest of the time. More

flexibility can be achieved on cable modems because of the high fidelity

connection. Therefore, unlike IS-54B, the talk times can be variable. Fig. 5-4

illustrates that user ‘a’ is sending more data than users ‘b’ and ‘c’.

Fig. 5-4 Variable Time Slot TDMA

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