As cell phones became more accessible, and
analog cellular networks began to reach their capacities, several digital
technologies have been introduced to alleviate problems associated with
increased traffic on the airwaves. One digital transmission scheme that is
currently being highly researched is Code Division Multiple Access (CDMA).
In this exploration of CDMA, we modeled the
entire CDMA transmission scheme using Matlab.
The forward link
refers to signal transmission between the transmission tower and the user
phone. This was modeled using 8 bit Walsh codes.
The reverse link
refers to signal transmission between the user phone and the transmission
tower. This was modeled using 21 bit psuedo-random Gold codes.
In addition, we also discovered some
interesting features of CDMA that indicate potential for it to be a more
efficient means of wireless transmission than FDMA or TDMA, two other means of
transmission.
In general, the transmission process works by multiplying the data
signal with a digital code, or a "chip", that consists of a series of
positive and negative pulses and is of a higher frequency than the
data signal.
The encoded nature of CDMA requires that the receiver be synchronized
with the transmitted signal. However, since it is almost impossible to
synchronize signals coming from multiple phones, the transmission
implementation is different for each direction.
One interesting aspect of
CDMA is the asymmetric nature of the forward and reverse links.
©2001 Kyle Bryson, Alison
Chen, and Allen Wan