Receiver Structures

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Receiver Structures

• Jörgen Nordberg
• (School of Engineering)
• Blekinge Institute of Technology

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Outline

• Wireless Communication
• ISI
• Multipath Channel
• Co-channel Interference
• Receiver structures
• Temporal Receivers
• Spatial Receivers
• Spatio-Temporal Receivers

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Wireless Communication(Intersymbol Interference)

• Strict system bandwidth requirements
• Intersymbol Interference.

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Wireless Communication(Multipath Propagation)

• Reflections will cause the receiving antenna to
  receive multiple version of the transmitted
  signal, i.e. multipath propagation.

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Wireless Communication(Multipath Propagation)

• The multipath propagation is illustrated by the
  channel impulse response

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Wireless Communication(Multipath Propagation)

• Important channel characteristic
• Coherence time
• Coherence bandwidth
• Doppler Spread
• Delay Spread

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Wireless Communication(Co-Channel Interference)

• Co-Channel interference is caused by several
  users transmitting on the same frequency.

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Wireless Communication(Co-Channel Interference)

• Avoided or reduced by frequency planning in 1G
  and 2G systems (reuse distance)
• Introduced on purpose in 3G systems, all users in
  each cell transmits on the same bandwidth
  (code-planning)

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Wireless Communication(Interference)

• Frequency leakage from other systems, that do not
  control their bandwidth usage, i.e. RF background
  noise, huge problem for radio-physics (deep space
  exploration)
• Using a system operating in the unlicensed
  frequency bands, i.e. Bluetooth.

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Receiver Structures
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Receiver Structures(Contents)

• Why?
• Temporal Receivers
• Spatial Receivers
• Spatio-Temporal Receivers

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Receiver Structures(Why?)

• To reverse or reduce the effects of the wireless
  channel some kind of receiver structure must be
  used.

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Receiver Structures(Temporal Receivers)

• Using temporal information/diversity to recover
  the original transmitted signal
• Channel Equalization
• (Inverse filtering)
• Rake Receivers
• (Multipath alignment)

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Receiver Structures(Channel Equalization)

• In channel equalization the wireless-channel is
  modeled as a Finite Impulse Response (FIR) filter
  and the receiver tries to recover the original
  transmitted signal through inverse channel
  filtering.

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Receiver Structures(Channel Equalization)

• There are several inherit problems with inverse
  filtering
• Noise amplifications (solved by doing FIR
  estimation)
• Even a few multipath components results in
  estimations of high order filter, i.e. hundreds
  of taps.
• Computationally complex
• Convergence problems

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Receiver Structures(Temporal Receivers)

• Reducing the computationally complexity in the
  receiver by using multirate (or subband)
  techniques.
• The received signal is split up into M different
  frequency bands and decimated with a factor D.
• The inversing filtering problem is then solved in
  each subband.
• There are two basically two multirate approaches

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Receiver Structures(Temporal Receivers, Channel
Equalization, Subband Technique 1)

• Complexity reduction
• Straightforward implementation
• Aliasing problems gt complicated filter bank
  design
• Large processing delay

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Receiver Structures(Temporal Receivers, Channel
Equalization, Subband Technique 2)
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Receiver Structures(Temporal Receivers, Channel
Equalization, Subband Technique 2)

• Complexity reduction
• Complicated implementation
• Parallel processing
• Less aliasing problems gt moderate filter bank
  design complexity
• Small processing delay

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Receiver Structures(Temporal Receivers, Adaptive
Channel Equalization)

• Adaptive Channel Equalization
• (Time variant channel)
• Non-blind Techniques, uses pilot/training
  sequences to estimated the channel.
• (LMS, RLS)
• Blind Techniques, using pre-know signal
  characteristics to restore the original
  transmitted signal.
• (CMA)

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Receiver Structures(Temporal receivers, Rake
Receivers)

• Using multipath propagation to get diversity
  gain.
• Each path is time aligned
• The signal is multiplied with the complex
  conjugate of the channel coefficient .
• The different paths are then summed together.
• Require good estimates of the both path delay and
  amplitude attenuation, i.e. good channel
  estimation.

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Receiver Structures(Temporal receivers, Rake
Receivers)
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Receiver Structures(Spatial Receivers)

• Spatial Receivers uses the fact that the target
  signal and interfering signals are transmitted
  from different spatial points.
• Uses multiple receiving antennas in order to
  suppress interfering signals.
• Spatial Diversity
• Sector Antennas
• Adaptive Antennas
• Signal Separation (blind/non-blind)

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Receiver Structures(Spatial Receivers, Spatial
Diversity)

• The power of the received signal will vary with
  time (signal fading) due to reflections,
  shadowing etc.
• The fading minima in the received signal can be
  avoided, by appropriate antenna separation.

Antenna 1
Antenna 2
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Receiver Structures(Spatial Receivers, Sector
Antennas)

• Another way to exploit the spatial domain is to
  weight the different received signals together, a
  beamforming approach.
• By using beamforming the antenna array can be
  steered to listen in a certain direction and
  ignore signals from other directions.

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Receiver Structures(Spatial Receivers, Sector
Antennas)

• Each sector area is covered by a beam/lobe.
• Beam/lobe handover

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Receiver Structures(Spatial Receivers, Sector
Antennas, PIERS)
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Receiver Structures(Spatial Receivers, Signal
Separation)

• A signal separator is a unit that uses M antennas
  to separate a mixture of N independent signals (N
  lt M).
• The signal separator can be viewed as a
  beamformer that creates a beam towards each
  source signal.
• There exist both blind and non-blind signal
  separation
• Least Squares, RLS, LMS using training
  sequences
• ICA using higher order statistics

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Receiver Structures(Spatial Receivers, Signal
Separation)
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Sector Antennas combined with Signal Separation
Beam-former
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Receiver Structures(Spatial Receivers, Adaptive
Antennas)

• In Adaptive Antennas (or smart antennas)
  solution, the user is handled by one beam only
  during the users stay in the cell.
• The beamformer must be able to track the user, in
  order to steer the beam towards the user, i.e.
  Direction Of Arrival (DOA) estimation.
• NO beam-handoffs !!!
• Signal separation can be viewed as a kind of
  smart antennas.

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Receiver Structures(Spatio-Temporal Receivers)

• Temporal receivers, can reverse the effect of
  multipath propagation but not the effects of
  co-channel interference.
• Spatial Receivers can take care of co-channel
  interference but cannot handle the effects of
  multipath propagation.
• By combining the two approaches in a Spatio
  Temporal Receiver both co-channel interference
  and multipath propagation effects can be dealt
  with.

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Receiver Structures(Spatio-Temporal Receivers)

• Combining spatial diversity and channel
  equalization results in a spatio-temporal channel
  equalizer.

Equalizer
Equalizer
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Spatial Channel Equalization( The NEWTEST
project)
W1
W2
Estimation Algorithm
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Spatial Channel Equalization (Results from the
NEWTEST project)

• By using just one extra antenna at the receiver a
  6-7 dB gain was achieved in Symbol Error rate
  (SER) performance over the one antenna case.
• By using a fractionally spaced (oversampled)
  spatial equalizer the SER performance was
  improved with 4 dB compared to the symbol rate
  case.

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Receiver Structures(Spatio-Temporal Receivers)

• Adding a temporal domain to the signals
  separation approach.

Signal Separation
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Signal Separation using Multi-rate Signal
Processing (ICOTA)
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Signal Separation using Multi-rate Signal
Processing (Results from ICOTA)