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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

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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

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

mrc issues1
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

equalization
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

diversity
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

channel coding
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

mrc improvement
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

equalization1
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

equalization2
Equalization

Rx Signal Spectrum

(w/ Tx Spectrum overlay)

Before Equalization

After Equalization

Bc

Bc

f

f

fc

fc

ECE 4730: Lecture #16

equalization3
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

adaptive equalizer
Adaptive Equalizer

ECE 4730: Lecture #16

adaptive equalizer1
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

adaptive equalizer2
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

adaptive equalizer3
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

generic adaptive equalizer
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

generic adaptive equalizer1
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

generic adaptive equalizer2
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

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