Today’s wireless communications would not be possible without radio signals that are generated and emitted from a sender, propagate through the atmosphere, and are received and interpreted by a receiver. In the area of LBSs, there exist two applications for radio signals. First, as described earlier, they are needed for the wireless communication between a mobile terminal and a fixed network, which is accomplished by manipulating the signal’s parameters depending on the data to be transferred through a process known as modulation. Second, they provide the basis for almost all positioning methods used for locating an LBS target. This is accomplished by utilizing some physical phenomena of radio signals, especially the velocity with which they travel or the attenuation or path loss they experience when traveling from the sender to the receiver.

For an understanding of the working of LBSs in general and positioning in particular, it is therefore useful to get an overview of the basics of wireless communications. This chapter provides an overview of the essentials of radio signals, introduces their representation in time and frequency domain, and shows how to modulate and demodulate data to and from them. Furthermore, it is explained how radio signals propagate and how they are influenced during propagation by certain circumstances. Finally, the chapter gives an introduction into the mechanisms of multiplexing and multiple access, which are needed for subdividing the air interface into a number of independent channels and for coordinating the access to these channels respectively.

Contents

• Signals
  • Modulation
  • Representing Signals in the Frequency Domain
  • Signal Spectrum and Bandwidth
• Propagation of Radio Signals
  • The Electromagnetic Spectrum
  • Antennas
  • Speed of Electromagnetic Waves
  • Attenuation
  • Multipath Propagation
  • Doppler Effect
• Multiplexing and Multiple Access
  • SDM and SDMA
  • FDM and FDMA
  • TDM and TDMA
  • CDM and CDMA
• Conclusion