L4_multiplex-freqplanning - EEC173B/ECS152C Spring 2010...

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Page 1 EEC173B/ECS152C, Spring 2010 Fundamentals of Wireless Communications #4: Spread Spectrum #5: Multiplexing #6: Frequency Reuse (Cellular Concept) Case Study: Wireless cellular networks #7: Handoff Acknowledgment: Selected slides from Prof. Schiller Quick Review of Last Lecture Radio Propagation Model Signal coverage, maximum data rate, channel fluctuation Path loss, shadow fading, multipath propagation, mobility SNR, SINR Modulation Energy efficiency Power efficiency 3 How do we overcome limitations imposed by the radio channel? Flat fading countermeasures Fade Margin Diversity Coding and Interleaving Adaptive Techniques Delay Spread Countermeasures Equalization Multicarrier Spread Spectrum Antenna Solutions 4 #4: Spread Spectrum Technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code Protection against narrow band interference Detection at receiver interference spread signal signal spread interference ff Power Power 5 Spreading and frequency selective fading frequency channel quality 1 2 3 4 5 6 narrow band signal guard space 2 2 2 2 2 frequency channel quality 1 spread spectrum narrowband channels spread spectrum channels 6 Spread Spectrum Spread spectrum increases the transmit signal bandwidth to reduce the effects of flat fading, ISI and interference. SS is used in all wireless LAN products in the ISM band Required for operation with reasonable power Minimal performance impact on other systems IEEE 802.11 standard There are two SS methods: direct sequence and frequency hopping Direct sequence multiplies the data sequence by a faster chip sequence. Frequency hopping varies the carrier frequency by the same chip sequence
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Page 2 7 Direct Sequence Spread Spectrum (DSSS) XOR of the signal with pseudo random number (chipping sequence) Many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages Reduces frequency selective fading In cellular networks •Base stations can use the same frequency range • Several base stations can detect and recover the signal •Soft handover Disadvantages Precise power control necessary user data chipping sequence resulting signal 01 011 0 10 1 0 10 1 1 XOR 0 01 0 1 0 1 = t b t c t b : bit period t c : chip period 8 DSSS X user data chipping sequence modulator radio carrier spread spectrum signal transmit signal transmitter demodulator received signal radio carrier X chipping sequence lowpass filtered signal receiver integrator products decision data sampled sums correlator 9 Frequency Hopping Spread Spectrum (FHSS) Discrete changes of carrier frequency Sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages
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This note was uploaded on 04/29/2010 for the course ECS 152 taught by Professor Mr. during the Spring '10 term at University of Great Falls.

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L4_multiplex-freqplanning - EEC173B/ECS152C Spring 2010...

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