Three of the more common transmission
schemes include FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple
Access), and CDMA (Code Division Multiple Access).
To better understand CDMA and how it is
different, let's compare the three transmission schemes.
FDMA divides the given
spectrum into channels by the frequency domain. Each phone call is allocated
one channel for the entire duration of the call. In the figure above, each band
represents one call.
Time Division Multiple Access
TDMA enhances FDMA by
further dividing the spectrum into channels by the time domain as well. A
channel in the frequency domain is divided among multiple users. Each phone
call is allocated a spot in the channel for a small amount of time, and
"takes turns" being transmitted. In the figure above, each horizontal
band represents the channel divided by the frequency domain. Within that is the
vertical division in the time domain. Each user then takes turns occupying the
channel.
Unlike FDMA and TDMA, CDMA
transmission does not work by allocating channels for each phone call. Instead,
CDMA utilizes the entire spectrum for transmisson of each call. Each phone call
is uniquely encoded and transmitted across the entire spectrum, in a manner
known as spread spectrum transmission. In the figure above, each brightly
colored pattern represents the encoded phone call being transmitted across the
spectrum.
©2001 Kyle Bryson, Alison Chen, and Allen
Wan