Spread spectrum R. Z. Ziemer, Colorado Springs, CO
Outline I. History of Spread Spectrum II. Spread Spectrum System Model III. Spread Spectrum Classification IV. Spread Spectrum Techniques
Introduction to Spread Spectrum • “Spread” radio signal over a wide frequency range • Several magnitudes higher than minimum requirement • Gained popularity by the needs of military communication • Proved resistant against hostile jammers • Ratio of information bandwidth and spreading bandwidth is identified as spreading gain or processing gain • Processing gain does not combat white Noise
Offers the following applications: • able to deal with multi-path • multiple access due to different spreading sequences • spreading sequence design is very important for performance • low probability of interception • privacy • anti-jam capabilities
Spread Spectrum Applications • Interference • Prevents interference at specific frequencies • E.g. other radio users, electrical systems • Military • Prevents signal jamming • Scrambling of ‘secret’ messages • Wireless LAN security • Prevents ‘eavesdropping’ of wireless links • Prevents ‘hacking’ into wireless LANs • CDMA (Code Division Multiple Access) • Multiple separate channels in same mediumusing different spreading codes
System Model: Spread Spectrum Transmission
Spread Spectrum Criteria A communication system is considered a spread spectrum system if it satisfies the following two criteria: • Bandwidth of the spread spectrum signal has to be greater than the information bandwidth. (This is also true for frequency and pulse code modulation!) • The spreading sequence has to be independent from the information. Thus, no possibility to calculate the information if the sequence is known and vice versa.
Spread Spectrum Classification
Direct Sequence Spread Spectrum • Information signal is directly modulated (multiplicated) by • a spreading sequences (see next slide) • Spreading sequence consists of chips each with a • duration of tchip • A set of chips represent a bit; the exact number of chips • per bit equals the spreading gain • Near far effect • Require continuous bandwidth
Direct Sequence Spread Spectrum Example R. Z. Ziemer, Colorado Springs, CO
Direct Sequence Spread Spectrum: Transmission Technique
Direct Sequence Spread Spectrum Transmitter R. Z. Ziemer, Colorado Springs, CO
Direct Sequence Spread Spectrum Receiver R. Z. Ziemer, Colorado Springs, CO
Direct Sequence Spread Spectrum Using BPSK Example R. Z. Ziemer, Colorado Springs, CO
ApproximateSpectrum of DSSS Signal R. Z. Ziemer, Colorado Springs, CO
Frequency Hopping Spread Spectrum • The information signal is transmitted on different frequencies • Time is divided in slots • Each slot the frequency is changed • The change of the frequency is referred to as slow if more than • one bit is transmitted on one frequency, and as fast if one bit is • transmitted over multiple frequencies • The frequencies are chosen based on the spreading sequences
Frequency Hopping Spread Spectrum (FHSS) R. Z. Ziemer, Colorado Springs, CO
Frequency selection in FHSS R. Z. Ziemer, Colorado Springs, CO
FHSS cycles R. Z. Ziemer, Colorado Springs, CO
Frequency Hopping Spread Spectrum
Bandwidth sharing R. Z. Ziemer, Colorado Springs, CO
Time Hopping Spread Spectrum • Time divided into frames; each TF long • Each frame is divided in slots • Each wireless terminal send in exactly one of these slots per • frame regarding the spreading sequence • No near far effect
Comparison of different Spread Spectrum Techniques SS Technique advantage disadvantage best behavior in multi path rejection no synchronization simple implementation difficult to detect Direct Sequence near far effect coherent bandwidth no need for coherent bandwidth less affected by the near far effect Frequency Hopper complex hardware error correction needed high bandwidth efficiency less complex hardware less affected by the near far effect Time Hopper error correction needed
The Idea In MCM , we split the data in to different streams and transmit using separate Sub Carriers.
DS-CDMA Figure: The Principle of DS-CDMA
