Transcript No Slide Title

CSNDSP ‘06
PATRAS, July 19-21, 2006
The Adaptive Potential of
Receiver Space Diversity Techniques
Authors:
Ligia CHIRA, Tudor PALADE
• Context: a study of adaptive radio techniques
• Focus: physical and link layer adaptation,
spatial diversity - receiver combining
• Goals: link availability, reliability, efficient use
of radio resources
• Action: implementation and performance
analysis of receiver combining techniques for
indoor multipath scenarios, in the particular
case of an 802.11a PHY
July 19-21, 2006
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BWA Challenges and current solutions
BWA Challenges
Technical solutions
Link reliability
OFDM (Orthogonal Frequency Division Multiplexing)
Adaptive power control
Adaptive modulation
Sub-channelization
Directional antennas
Link availability
Link quality
Coverage
Adaptive FEC and ARQ
Tx and Rx diversity techniques
Smart antennas
Spectral efficiency
Capacity,
throughput
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STC (Space-Time Coding)
AAS (Adaptive Antenna Systems)
MIMO (Multiple Input Multiple Output)
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Benefits of using adaptive techniques
Adaptive Techniques
Improvements
Adaptive modulation
12-16dB SNR increase
Adaptive power control
Efficient use of radio resources
Adaptive subcarrier
allocation
Allocation of a lower modulation scheme for
the low SNR subcarriers
Adaptive bandwidth
10 times smaller bandwidth -> a received SNR
increase of 10 dB
QoS increase, BER decrease
Adaptive user allocation
Average signal power increase of 3-5dB
Bite rate and SNR increase, BER decrease
Higher capacity achieved by exploitation of
frequency dispersive fading, by allocating the
peaks in frequency response
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The adaptive potential
Adaptive combinations
Potentially adaptive techniques
Receiver
diversity
techniques
ThC, SDC,
MRC, EGC
Significant BER decrease,
SNR/bit rate increase
4-16dB diversity gain increase
Adaptive threshold selection
Combined with the rosette
architecture
Rosette
architecture
Vertical extension
Combined with adaptive antenna
techniques: beam forming, nullsteering, Tx and Rx diversity
techniques
Multiple
antenna
systems MIMO
Repeaters
Dynamic change of active
configuration
Global system capacity increase
Pathloss decreases with 7dB
when using 2 repeaters (indoor),
and with 20dB when using 8
repeaters
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Adaptive modulation +
adaptive user
allocation
The necessary transmit power
is reduced by 5-10dB
Adaptive modulation +
OFDM + beamforming
no publicly available results
Adaptive user
allocation + adaptive
bandwidth + adaptive
modulation
Rosette architecture +
beam forming, nullsteering, Tx and Rx
diversity techniques
High spectral efficiency
High QoS
Adaptive receiver
combining
Adaptive Threshold
Combining
BER decrease – link reliability
SNR and bit rate increase
Maximize bit rate, minimize
BER
Patras, CSNDSP 2006
BER decrease – link reliability
Interference mitigation
Efficient radio resource
utilization
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Receiver Combining Techniques
•
•
•
•
Threshold Selection Combining (ThC)
Selection Diversity Combining (SDC)
Maximum Ratio Combining (MRC)
Equal Gain Combining (EGC)
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Threshold Combing - ThC
• Single receiver
•
•
•
•
received signals are scanned in a sequential
order
the first signal with a SNR level above a
certain threshold is selected
and used as long as its SNR is higher than
the threshold value
SNR level ->below the threshold
->the selection process is reinitiated
• Two Branches
• SSC (Switch and Stay Combining)
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Selection Diversity Combining
• SDC
• Selects the maximum SNR branch
• Does not require co-phasing
• Independent branches
(spatial + polarization
diversity, d>λ/3)
Select the maximum SNR antenna
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Maximum Ratio Combining
• MRC
• Separate receivers
a means of combining the signals from all
receiver branches, so that signals with a
higher received power have a larger influence
on the final output
• Digital:
Co-phasing and summing
RF – BB conversion on each branch
• Analog:
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RF summing
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MRC - OFDM
• Subcarrier MRC - Post-DFT MRC or subcarrier
combining receiver
rm,k received signal at the
mth antenna on the
kth subcarrier
gm,k complex weights allocated
proportionally to the signal power on each branch
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Simulation results for reference
scenarios (2 to 12 propagation paths)
As the number of propagation paths is
increased the overall performance
degrades: higher PERs and the
non-zero PER values tend to last longer
Simulation results for three
combining techniques
Receiver combining
techniques improve
performance in terms of PER,
thus ensuring a higher link
availability and reliability
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The improvement in PER when using
one of the receiver combining
techniques.
[%]
SDC and MRC are the schemes that
ensure the lowest PER
[Mbps]
SDC advantages:
selects the actual best signal in terms of received
power, and not merely the first one that meets a
certain criterion.
ensures a low PER
Disadvantage: moderate bit rates (compared to
other techniques)
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ThC performance function of the number of receive
antennas and value of the SNR threshold
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Compared simulation results
receiver combining
techniques improve
performance in terms of
PER, thus ensuring a higher
link availability and
reliability.
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Mean Bit Rate
Mean bit rate function of SNR
threshold value for 3-path
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Mean PER
Mean PER function of SNR
threshold value
for 3-path Threshold Combining
Threshold Combining
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Adaptive Threshold Combining
Observations made on the time variation graphics of PER values in the case of ThC, lead to the
establishment of different PER ranges corresponding to different SNR thresholds.
The next step was to establish PER threshold values at which the SNR threshold of the combiner to be
switched to a lower level. The algorithm is similar to the ones used in adaptive coding or adaptive
modulation.
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Conclusions I
• Performance of diversity combiners increases with the
number of antennas, but not linearly, and will eventually
stop growing beyond a certain number of antennas
• Receiver combining techniques improve performance in
terms of PER, thus ensuring a higher link availability and
reliability
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Conclusions II
• Reported results - for 8 Rx antennas:
• ThC - is reported to provide a diversity gain of up to 2
dB - corresponding to a 20% range improvement.
• SDC - the gain lies between 7-9 dB, meaning 50%
system range improvement.
• MRC - 12-16 dB depending on the chosen data rate,
and the system range is improved with up to 100%.
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Conclusions III
• Already established hierarchy, based on overall diversity
gain: MRC > EGC > SDC > ThC
• SDC and MRC are the schemes that ensure the lowest
PER. When using ThC all the monitored parameters are
influenced by the selected SNR threshold value
• MRC - more efficient for non-dominant-path scenarios
- more suited for uplink implementation in the BS Rx.
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Conclusions IV
• SDC - better results for dominant-path scenarios
- ensures a low PER but moderate bit rates, compared to those
provided by ThC, because SDC performs the calculations on low
SNR branches too, and does not cancel them like ThC does. Still
SDC out performs ThC in terms of overall diversity gain and
coverage.
• ThC - choosing the threshold value is a compromise between bit rate
and PER values
- for a dominant-path scenario the SNRth value should be
chosen very carefully, otherwise we risk to cancel the dominant path
- a lower threshold enables both high bit rates and low PERs
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Conclusions V
• We have also established the premises for building an
adaptive threshold combiner.
• Means of implementing it vary from “if-then-else”
blocks to neural networks and genetic algorithms.
• Get all the advantages – the highest bit rates and the
lowest packet error rates
• Integrate it onto a platform that already uses these
implementation solutions for other blocks as well.
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References
[1] Martin Clark, MATLAB Central model: IEEE 802.11a WLAN PHY, The MathWorks
[2] L. Chira, T. Palade, “Adaptive Radio Techniques at 5 GHz”, Acta Tehnica Napocensis,
Vol.46., No.2, Technical University of Cluj-Napoca, pp.1-6, Mediamira 2005
[3] Eric Phillip Lawrey Be (Hons), Adaptive Techniques for Multiuser OFDM, Thesis submitted
in December 2001, School of Engineering, James Cook University
[4] Andreea Goldsmith, Wireless Communications, Stanford University, Cambridge Univ.
Press, 2005
[5] IEEE Std 802.11a-1999 Telecommunications and information exchange between systems.
LANs and MANs. Specific requirements Part 11: Wireless LAN MAC and PHY specifications
High-speed PHY in the 5 GHz Band
[6] Oskar Bexell, “Antenna diversity gain in the wireless local area network standards
HiperLAN/2 and IEEE 802.11a”, Lulea University of Technology, 2002
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