1 / 16

MRC Issues

MRC Issues. Mobile Radio Channel (MRC) Impairments: 1) ACI/CCI  system generated interference 2) Shadowing  large-scale path loss from LOS obstructions 3) Multipath Fading  rapid small-scale signal variations 4) Doppler Spread  channel conditions change rapidly

iliana-burris
Télécharger la présentation

MRC Issues

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. MRC Issues • Mobile Radio Channel (MRC) Impairments: 1) ACI/CCI  system generated interference 2) Shadowing  large-scale path loss from LOS obstructions 3) Multipath Fading  rapid small-scale signal variations 4) Doppler Spread  channel conditions change rapidly due to motion of mobile unit • All can lead to significant distortion or attenuation of Rx signal • Degrade BER of digitally modulated signal ECE 4730: Lecture #16

  2. MRC Issues • Three techniques to improve Rx signal quality and/or lower BER: 1) Equalization 2) Diversity 3) Channel Coding • Used independently or together ECE 4730: Lecture #16

  3. Equalization • Equalization  Primary goal is to reduce ISI by compensating for finite BW of MRC • MRC is frequency selective when Bc < Bs • Some signal frequencies undergo significant fading • In time domain  multipath signals cause significant ISI when time delay (st) > 0.1 Ts • Equalizers compensate for selective fading by enhancing certain frequencies of Rx signal • Must be adaptive since MRC is time-varying !! ECE 4730: Lecture #16

  4. Diversity • Diversity  Primary goal is to reduce depth & duration of signal fades in flat fading channel • Flat fading Bs < Bc no ISI from multipath ! • Spatial or antenna diversity  most common • Use multiple Rx antennas in mobile or base station • One antenna with signal null while another antenna may have signal peak • Small antenna separation (l) changes phase of signal  constructive /destructive nature is changed • Other diversity types  polarization, frequency, & time ECE 4730: Lecture #16

  5. Channel Coding • Channel coding  Primary goal is to reduce BER by detecting & correcting some (or all) bit errors • Correction data bits are added to original (source) Tx data stream • Data errors are corrected in Rx after demodulation • Coding results in: 1) Lower data rate for same signal BW OR 2) Higher signal BW for same data rate • Reduced spectral efficiency ECE 4730: Lecture #16

  6. MRC Improvement • All 3 techniques improve mobile radio link performance • Effectiveness of each varies widely in practical wireless systems • Cost & complexity are also important issues • Mobile vs. base station implementation • Some techniques may be implemented in base station or mobile only ECE 4730: Lecture #16

  7. Equalization • Equalization Fundamentals • ISI is the major obstacle to high speed data transmission over MRC  “Darth Vader” • Time domain perspective  caused by multipath interference • Frequency domain perspective  frequency selective fading • Combat ISI by “equalizing” frequency response of MRC  transform to flat fading channel ECE 4730: Lecture #16

  8. Equalization Rx Signal Spectrum (w/ Tx Spectrum overlay) Before Equalization After Equalization Bc Bc f f fc fc ECE 4730: Lecture #16

  9. Equalization • Combat ISI by “equalizing” frequency response of MRC • Reduce ISI  increase Bc increase allowable signal BW (Bs)  increase data rate • Equalization actually done @ baseband (after demodulation) • Figure 7.1, pg. 358 ECE 4730: Lecture #16

  10. Adaptive Equalizer ECE 4730: Lecture #16

  11. Adaptive Equalizer • Define: • x(t) : baseband source data • f(t) : impulse response of Tx + MRC + Rx • heq(t): impulse response of equalizer • : output response of equalizer (want = x(t) !) • Now and for = x(t) ECE 4730: Lecture #16

  12. Adaptive Equalizer • In Frequency Domain: •  unity at all frequencies • Ideal frequency response • Cannot be achieved in practice due to practical limitations • Heq ( f ) is inverse filter of MRC • Enhances weak frequency components • Attenuates strong frequency components • Yields “flat” output frequency response ECE 4730: Lecture #16

  13. Adaptive Equalizer • MRC is time-varying due to motion of mobile or nearby objects • Heq (f)must vary in time as well • Adaptive equalization • Can’t use R/C/L analog filters • Must implement using DSP in time domain • FIR filters • Time-varying discrete filter  Fig. 7.2, pg. 359 ECE 4730: Lecture #16

  14. Generic Adaptive Equalizer Time-varying discrete filter Discrete time = k N Delay Elements N+1 taps N+1 variable filter coefficients or “weights” wN k ECE 4730: Lecture #16

  15. Generic Adaptive Equalizer • Adaptive algorithm • Update filter coefficients (weights) continuously • Two basic operating modes: 1) Training 2) Tracking • Training • Fixed length known sequence sent by Tx • Typically a PN sequence • Flat spectral response at Tx output • Rx input shows frequency selective fading • Equalizer adjusts weights to obtain flat output • Weights near optimal value for reception of user data at end of training sequence ECE 4730: Lecture #16

  16. Generic Adaptive Equalizer • Tracking • Tx data follows training sequence • Adaptive algorithm tracks changing channel and updates weights to compensate • Periodic retraining required for effective ISI cancellation since MRC response is time-varying • Very common in digital communication systems where data is segmented in time blocks • TDMA systems use fixed-length time blocks with training sequence contained in each block ECE 4730: Lecture #16

More Related