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Wireless Communication Fundamentals II

Wireless Communication Fundamentals II. David Holmer dholmer@jhu.edu. Review. Wireless = electro-magnetic waves Path-loss over distance Multi-path reflections Modulation. Symbol Rate & Bandwidth. Modulation allows transmission of one of several possible symbols (two or more)

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Wireless Communication Fundamentals II

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  1. Wireless Communication Fundamentals II David Holmer dholmer@jhu.edu

  2. Review • Wireless = electro-magnetic waves • Path-loss over distance • Multi-path reflections • Modulation

  3. Symbol Rate & Bandwidth • Modulation allows transmission of one of several possible symbols (two or more) • Data stream is encoded by transmitting several symbols in succession • Symbol rate ≈ bandwidth • Throughput (bits/sec) • Spectrum usage (Hz) • Inter-symbol interference (ISI) occurs unless delay spread << symbol time

  4. Thermal Noise • Ever-present thermal noise in wireless medium • Sums with any wireless transmission • Potentially causes errors in reception (digital) or degradation of quality (analog) • Effectively limits transmission range when transmitting signal strength falls below noise floor • -174 dBm/Hz

  5. Thermal Noise Calculation • Depends on channel bandwidth • About 25 MHz for 802.11b or 802.11a channel • = -174dBm/Hz + 10log(bandwidth in Hz) • So for 802.11 • Noise Floor is about -100 dBm • -100 dBm = 10log( .0000000000001 Watts )

  6. Noise Limits Transmitting Distance Signal to Noise Ratio (SNR) Short range transmission (low path loss) + = High Long range transmission (high path loss) = + Low

  7. Physical Channel Properties Review • Wireless signal strength • Transmit power • Loss over distance (falls off by d2) • Shadowing (e.g. absorption by walls) • Multi-path (e.g. bouncing off of metal objects) • Noise • Thermal noise floor • Environmental noise (e.g. microwave ovens) • Channel quality • Related to signal to noise ratio

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