CDMA TECHNOLOGY

CDMA TECHNOLOGY

History of CDMA

The Cellular Challenge
The world's first cellular networks were introduced in the early 1980s, using analog
radio transmission technologies such as AMPS (Advanced Mobile Phone System).
Within a few years, cellular systems began to hit a capacity ceiling as millions of new
subscribers signed up for service, demanding more and more airtime. Dropped calls and
network busy signals became common in many areas.
To accommodate more traffic within a limited amount of radio spectrum, the
industry developed a new set of digital wireless technologies called TDMA (Time
Division Multiple Access) and GSM (Global System for Mobile). TDMA and GSM used
a time-sharing protocol to provide three to four times more capacity than analog systems.
But just as TDMA was being standardized, an even better solution was found in CDMA.
Commercial Development
The founders of QUALCOMM realized that CDMA technology could be used in
commercial cellular communications to make even better use of the radio spectrum than
other technologies. They developed the key advances that made CDMA suitable for
cellular, then demonstrated a working prototype and began to license the technology to
telecom equipment manufacturers.
The first CDMA networks were commercially launched in 1995, and provided
roughly 10 times more capacity than analog networks - far more than TDMA or GSM.
Since then, CDMA has become the fastest-growing of all wireless technologies, with
over 100 million subscribers worldwide. In addition to supporting more traffic, CDMA
brings many other benefits to carriers and consumers, including better voice quality,
broader coverage and stronger security.
The world is demanding more from wireless communication technologies than ever
before. More people around the world are subscribing to wireless services and consumers
are using their phones more frequently. Add in exciting Third-Generation (3G) wireless
data services and applications - such as wireless email, web, digital picture
taking/sending and assisted-GPS position location applications - and wireless networks
are asked to do much more than just a few years ago. And these networks will be asked to
do more tomorrow.
This is where CDMA technology fits in. CDMA consistently provides better capacity
for voice and data communications than other commercial mobile technologies, allowing
more subscribers to connect at any given time, and it is the common platform on which
3G technologies are built.
CDMA is a "spread spectrum" technology, allowing many users to occupy the same
time and frequency allocations in a given band/space. As its name implies, CDMA
assigns unique codes to each communication to differentiate it from others in the same
spectrum.
Brief Working of CDMA
CDMA takes an entirely different approach from TDMA. CDMA, after digitizing
data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on
each other on the channel, with each assigned a unique sequence code. CDMA is a form
of spread spectrum, which simply means that data is sent in small pieces over a number
of the discrete frequencies available for use at any time in the specified range.

In CDMA, each phone's data has a unique code.
All of the users transmit in the same wide-band chunk of spectrum. Each user's signal is
spread over the entire bandwidth by a unique spreading code. At the receiver, that same
unique code is used to recover the signal. Because CDMA systems need to put an
accurate time-stamp on each piece of a signal, it references the GPS system for this
information. Between eight and 10 separate calls can be carried in the same channel
space as one analog AMPS call.
Spread Spectrum Communications
CDMA is a form of Direct Sequence Spread Spectrum communications. In general,
Spread Spectrum communications is distinguished by three key elements:
1. The signal occupies a bandwidth much greater than that which is necessary to send the
information. This results in many benefits, such as immunity to interference and jamming
and multi-user access, which we’ll discuss later on.
2. The bandwidth is spread by means of a code which is independent of the data. The
independence of the code distinguishes this from standard modulation schemes in which
the data modulation will always spread the spectrum somewhat.
3. The receiver synchronizes to the code to recover the data. The use of an independent
code and synchronous reception allows multiple users to access the same frequency band
at the same time.
In order to protect the signal, the code used is pseudo-random. It appears random, but
is actually deterministic, so that the receiver can reconstruct the code for synchronous
detection. This pseudo-random code is also called pseudo-noise (PN).
Three Types of Spread Spectrum Communications
Frequency hopping.
The signal is rapidly switched between different frequencies within the hopping
bandwidth pseudo-randomly, and the receiver knows before hand where to find the signal
at any given time.
Time hopping.
The signal is transmitted in short bursts pseudo-randomly, and the receiver knows
beforehand when to expect the burst.
Direct sequence.
The digital data is directly coded at a much higher frequency. The code is generated
pseudo-randomly, the receiver knows how to generate the same code, and correlates the
received signal with that code to extract the data.
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.
Implementing CDMA Technology
The following sections describe how a system might implement the steps illustrated in
Figure 1.
Input data
CDMA works on Information data from several possible sources, such as digitized voice
or ISDN channels. Data rates can vary, here are some examples:

The system works with 64 kBits/sec data, but can accept input rates of 8, 16, 32, or 64
kBits/sec. Inputs of less than 64 kBits/sec are padded with extra bits to bring them up to
64 kBits/sec.
For inputs of 8, 16, 32, or 64 kBits/sec, the system applies Forward Error Correction
(FEC) coding, which doubles the bit rate, up to 128 kbits/sec. The Complex Modulation
scheme (which we’ll discuss in more detail later), transmits two bits at a time, in two bit
symbols. For inputs of less than 64 kbits/sec, each symbol is repeated to bring the
transmission rate up to 64 kilosymbols/sec. Each component of the complex signal
carries one bit of the two bit symbol, at 64 kBits/sec, as shown bel

Generating Pseudo-Random Codes
For each channel the base station generates a unique code that changes for every
connection. The base station adds together all the coded transmissions for every
subscriber. The subscriber unit correctly generates its own matching code and uses it to
extract the appropriate signals. Note that each subscriber uses several independent
channels.
In order for all this to occur, the pseudo-random code must have the following properties:
1. It must be deterministic. The subscriber station must be able to independently generate
the code that matches the base station code.
2. It must appear random to a listener without prior knowledge of the code (i.e. it has the
statistical properties of sampled white noise).
3. The cross-correlation between any two codes must be small (see below for more
information on code correlation).
4. The code must have a long period (i.e. a long time before the code repeats itself).
Code Correlation
In this context, correlation has a specific mathematical meaning. In general the
correlation function has these properties:
• It equals 1 if the two codes are identical
• It equals 0 of the two codes have nothing in common
Intermediate values indicate how much the codes have in common. The more they have
in common, the harder it is for the receiver to extract the appropriate signal.
There are two correlation functions:
Cross-Correlation: The correlation of two different codes. As we’ve said, this should
be as small as possible.
Auto-Correlation: The correlation of a code with a time-delayed version of itself. In
order to reject multi-path interference, this function should equal 0 for any time delay
other than zero.
The receiver uses cross-correlation to separate the appropriate signal from signals

Figure 2a. Pseudo-Noise Spreading

Figure 2b. Frequency Spreading
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.Figure 3a. Complex Modulator

Figure 3b. Complex Modulation





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.