Digital Cellular Systems (CDMA)

The idea behind spread spectrum is to transform a signal with bandwidth Bs into a noise-like signal of much larger bandwidth Bss. This is ilustrated in the fig 1, where the vertical axis is power spectral density (watts/hertz) and the horizontal is the frequency axis (hertz). It can be assumed that the total power transmitted by the spread-spectrum signal is the same as that in the original signal.

Fig 1: Spectra of a signal before and after spreading

In this case, the power spectral density of the spread-spectrum signal is Ps(Bs/Bss); where the ratio Bss/Bs is called the processing gain, and it is denoted by N. Typically N ranges between 10-30 dB. Thus, the power of the radiated spread spectrum signal is spread over 10-1000 times the original bandwidth. This convenient feature gives the spread spectrum signal the characteristic of causing little interference to a narrow-band user; and is the key point of proposed overlay systems that operate spread-spectrum concurrently with existing narrow-band systems.

Another requirement of the spread-spectrum signal is that it should be noise-like. That is, each spread-spectrum signal should behave as if it were uncorrelated with every other spread signal using the same band.