A System is said to be using DSSS if it follows the two basic rules mentioned
•The Bandwidth of the Carrier frequency must be much larger than the Bandwidth of the baseband Signals to be transmitted.
•The same codes that are used for coding the signal must also be used for decoding the signals.
CDMA uses a modulation technique called “spread spectrum” to transport a narrowband voice
signal over a wide bandwidth channel. The wide bandwidth for IS-2000 is 1.23 MHz.
The CDMA modulation technique uses three methods for spectrum spreading:
•FH (frequency hopping)
•TH (time hopping)
Because Lucent systems operate only with DS spreading, it is the only spreading technique
discussed throughout the remainder of this course, so whenever CDMA is mentioned DS
CDMA is implied.
While FH is more popular in CDMA systems used for military purposes in commercial CDMA system DSSS is popularly used.
Spreading
In a spread spectrum system the data information signal, b(t), is multiplied by a wideband
signal, c(t), which is the output signal of the Direct Sequence (DS) generator – a
pseudorandom noise (PN) output signal. The signal which will eventually be transmitted,
y(t)=b(t)c(t), will occupy bandwidth far in excess of the minimum m bandwidth to transmit the
data information.
Tb is the bit interval of the information stream and Tc is the bit interval of the DS
stream. Tc is also called a chip time. It should also be noted that the ratio of Tb to Tc is referred
to as the processing gain.