13-09-2013, 02:12 PM
Code Division Multiple Access (CDMA)
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INTRODUCTION TO CDMA
Code Division Multiple Access (CDMA) is a radically new concept in wireless
communications. It has gained widespread international acceptance by cellular radio
system operators as an upgrade that will dramatically increase both their system
capacity and the service quality. It has likewise been chosen for deployment by the
majority of the winners of the United States Personal Communications System spectrum
auctions. It may seem, however, mysterious for those who aren't familiar with it. This site
is provided in an effort to dispel some of the mystery and to disseminate at least a basic
level of knowledge about the technology.
CDMA is a form of spread-spectrum, a family of digital communication techniques that
have been used in military applications for many years. The core principle of spread
spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths
much wider than that required for simple point-to-point communication at the same data
rate. Originally there were two motivations: either to resist enemy efforts to jam the
communications (anti-jam, or AJ), or to hide the fact that communication was even
taking place, sometimes called low probability of intercept (LPI). It has a history that
goes back to the early days of World War II.
Direct Sequence Spread Spectrum
CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly
on 64 kbit/sec digital signals. These signals can be digitized voice, ISDN channels,
modem data, etc.
Figure 1 shows a simplified Direct Sequence Spread Spectrum system. For clarity, the
figure shows one channel operating in one direction only.
Signal transmission consists of the following steps:
1. A pseudo-random code is generated, different for each channel and each
successive connection.
2. The Information data modulates the pseudo-random code (the Information data
is “spread”).
3. The resulting signal modulates a carrier.
4. The modulated carrier is amplified and broadcast.
Signal reception consists of the following steps:
1. The carrier is received and amplified.
2. The received signal is mixed with a local carrier to recover the spread digital
signal.
3. A pseudo-random code is generated, matching the anticipated signal.
4. The receiver acquires the received code and phase locks its own code to it.
5. The received signal is correlated with the generated code, extracting the
Information data.
Pseudo-Noise Spreading
The FEC coded Information data modulates the pseudo-random code, as shown in
Figure 2a. Some terminology related to the pseudo-random code:
• Chipping Frequency (fc): the bit rate of the PN code.
• Information rate (fi): the bit rate of the digital data.
• Chip: One bit of the PN code.
• Epoch: The length of time before the code starts repeating itself (the period of the
code). The epoch must be longer than the round trip propagation delay (The
epoch is on the order of several seconds).
Figure 2b shows the process of frequency spreading. In general, the bandwidth of a
digital signal is twice its bit rate. The bandwidths of the information data (fi) and the PN
code are shown together. The bandwidth of the combination of the two, for fc>fi, can be
approximated by the bandwidth of the PN code.
Processing Gain
An important concept relating to the bandwidth is the processing gain (Gp). This is a
theoretical system gain that reflects the relative advantage that frequency spreading
provides.
Transmitting Data
The resultant coded signal next modulates an RF carrier for transmission using
Quadrature Phase Shift Keying (QPSK). QPSK uses four different states to encode each
symbol. The four states are phase shifts of the carrier spaced 90_ apart. By convention,
the phase shifts are 45, 135, 225, and 315 degrees. Since there are four possible states
used to encode binary information, each state represents two bits. This two bit “word” is
called a symbol. Figure 3 shows in general how QPSK works.
First, we’ll discuss Complex Modulation in general, applying it to a single channel with no
PN-coding (that is, we’ll show how Complex Modulation would work directly on the
symbols). Then we’ll discuss how we apply it to a multi-channel, PN-coded, system.