This post covers WIRELESS COMMUNICATIONS AND NETWORKS by William Stallings.

Basic Ideas

• The Spread Spectrum- Fundamental System Design
  • Modulation is the process of encoding information from a message source in a manner suitable for transmission.
  • It generally involves translating a baseband message signal (called the source) to a bandpass signal at frequencies that are very high when compared to the baseband frequency.
  • The bandpass signal is called the modulated signal and the baseband message signal is called the modulating signal.
  • Demodulation is the process of extracting the baseband message from the carrier so that it may be processed and interpreted by the intended receiver (also called the sink).
  • This topic describes various modulation techniques that are used in mobile communication systems
  • A large variety of modulation techniques have been studied for use in mobile radio communications systems, and research is ongoing
  • Given the hostile fading and multipath conditions in the mobile radio channel, designing a modulation scheme that is resistant to mobile channel impairments is a challenging task.
  • Since the ultimate goal of a modulation technique is to transport the message signal through a radio channel with the best possible quality while occupying the least amount of radio spectrum, new advances in digital signal processing continue to bring about new forms of modulation and demodulation
• Since bandwidth is a limited resource, one of the primary design objectives of all the modulation schemes detailed thus far is to minimize the required transmission bandwidth.

• Spread spectrum techniques, on the other hand, employ a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth.

• Assume we wish to transmit a 56-kbps data stream using spread spectrum. Find the channel bandwidth required when SNR = 0.1,0.01, and 0.001. In an ordinary (not spread spectrum) system, a reasonable goal for bandwidth efficiency might be 1 bps/Hz. That is, to transmit a data stream of 56 kbps, a bandwidth of 56 kHz is used. In this case, what is the minimum SNR that can be endured for transmission without appreciable errors? Compare to the spread spectrum case.

The concept of Spread Spectrum

• Input is fed into a channel encoder that produces an analog signal with a relatively narrow bandwidth around some center frequency.

• This signal is further modulated using a sequence of digits known as a spreading code or spreading sequence.

• Typically, but not always, the spreading code is generated by a pseudonoise, or pseudorandom number, generator.

• The effect of this modulation is to increase significantly the bandwidth (spread the spectrum) of the signal to be transmitted.

• On the receiving end, the same digit sequence is used to demodulate the spread spectrum signal.

• Finally, the signal is fed into a channel decoder to recover the data. Several things can be gained from this apparent waste of spectrum

• We can gain immunity from various kinds of noise and multipath distortion.

• The earliest applications of spread spectrum were military where it was used for its immunity to jamming.