Introduction

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).

Project Abstract

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.

FDMA v. TDMA v. CDMA

Sending and Receiving in CDMA

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.