Spread spectrum communications was originally used in the military for the purpose of interference rejection and enciphering. In digital cellular communications, spread spectrum modulation is used as a multiple-access technique. Spectrum spreading is mainly performed by one of the following three schemes.
Direct sequence (DS): Data is spread and the carrier frequency is fixed.
Frequency hopping (FH): Data is directly modulated and the carrier frequency is spread by channel hopping.
Time hopping (TH): Signal transmission is randomized in time.
The first two schemes are known as spectral spreading, and are introduced in this chapter. Time hopping is known as temporal spreading, and will be introduced in Chapter 20. Spectrum spreading provides frequency diversity, low PSD of the transmitted signal, and reduced band-limited interference, while temporal spreading has the advantage of time diversity, low instantaneous power of the transmitted signals, and reduced impulse interference.
CDMA is a spread spectrum modulation technology in which all users occupy the same time and frequency, and they can be separated by their specific codes. For DS-CDMA systems, at the BS, the baseband bitstream for each MS is first mapped onto M-ary symbols such as QPSK symbols; each of the I and Q signals is then spread by multiplying a spreading code and then a scrambling code. The spread signals for all MSs are then amplified to their respective power, summed, modulated to the specified band, and then transmitted.