5-6
5-2
EVALUATING TDMA RETURN PATH SIGNALS
Time Domain Multiple Access (TDMA) is a term commonly used in
reference to D-AMPS or IS-54B mobile telephone systems. Time
Domain Multiple Access implies a signal assigned to a certain
frequency that has multiple time-slots for multiple transmitters. Our
communications have been TDMA since the dawn of time. The
human ear works in the ~20 to 20 kHz frequency range. Our voices
are limited to 300 to 3000 Hz. We learned to arrange speech so that
only one individual talks at a time. The brain processes the voice
frequency separate from the other frequencies and delivers the
intended message.
TDMA modems are the same. Each modem knows when talk. The
controller deciphers the messages by knowing when each modem is
talking. These modems have specific assigned times to send and
receive information.
In fig. 5-3 you can see that each user has a time slot assigned for its
usage.
Fig. 5-3: Nominal TDMA Timing
TDMA signals were introduced to the commercial electronic world a
few years ago with the advent of digital cellular phones. Data
compression and quadrature modulation are used to increase the
available modulation bandwidth so that a TDMA cell-phone sends
the same information in 1/3
rd
the time of an AMPS cell-phone.