UCLA Undergraduate Reserch Program in Electrical Engineering

1
UCLA Undergraduate Reserch Program in Electrical
Engineering

• Implementation and Testing of Space Time Block
  Codes
• Daniel N. Liu
• daniell_at_ee.ucla.edu
• Advisor Michael P. Fitz
• fitz_at_ee.ucla.edu

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Presentation Outline

• MIMO Wireless Communications
• General Background
• Block and Data Model
• Related Issues
• Space-Time Block Codes
• A specific example Alamouti Codes
• Used Channel Estimation PSAM
• MRRC scheme
• ML Decoding
• Monte Carlo Simulations
• Alamouti Codes with Perfect CSI under BPSK
  Constellation
• Alamouti Codes with PSAM under QPSK Constellation
• Conclusion
• Final implementation of STBC codes into Narrow
  band Test bed Conclusions

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Background of Wireless Commun. in multipath
fading channel

• Time-varying multipath fading poses a strong
  negative effect on wireless communication
• Multiple delayed version of the transmitted
  signal due to the reflections off buildings,
  roads, and other obstacle
• Each path have different attenuation and time
  delay
• Sometimes signal add constructively, sometimes
  cancel each other
• Fading Makes Wireless Communication a harder
  problem even compared to fiber, coaxial, Line of
  Sight (LOS) microwave even satellite
  communication.

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Background of Diversity Techniques

• Variety of Diversity techniques are proposed to
  combat Time-Varying Multipath fading channel in
  wireless communication
• Time Diversity
• Frequency Diversity
• Space Diversity (mostly multiple receive
  antennas)
• Main intuitions of Diversity
• Probability of all the signals suffer fading is
  less then probability of single signal suffer
  fading
• Provide the receiver a multiple versions of the
  same Tx signals over independent channels
• Time Diversity
• Use different time slots separated by an interval
  longer than the coherence time of the channel.
• Example Channel coding interleaving
• Short Coming Introduce large delays when the
  channel is in slow fading

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Wireless Channel under rapid Rayleigh Fading
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Background of Diversity Techniques (cont.d)

• Frequency Diversity
• Use different frequency carriers separated by a
  distance by the coherence bandwidth of the
  channel
• Short coming No bandwidth efficiency
• Space Diversity
• Use multiple antennas separated wide enough with
  respect to carrier wavelength
• Example Receive diversity, multiple antennas
  deployed at receive side
• Short coming Increase complexity of remote units
  (handsets), in terms of cost, size and power.
• Could we do something better? What else we could
  do?
• Answer Yes. MIMO.

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MIMO Wireless Communications

• Transmission over Multiple Input Multiple Output
  (MIMO) radio channels
• Advantages Improved Space Diversity and Channel
  Capacity
• Disadvantages More complex, more radio stations
  and required channel estimation

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Block and Data Model

• 1X(NP) block of information symbols broadcast
  from transmit antenna i
• Si(d, t)
• 1X(NP) block of received information symbols
  taken from antenna j
• Rj hjiSi(d, t) nj
• Matrix representation

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Related Issues

• How to define Space-Time mapping Si(d,t) for
  diversity/channel capacity trade-off?
• What is the optimum sequence for pilot symbols?
• How to get best estimated Channel State
  Information (CSI) from the pilot symbols P?
• How to design frame structure for Data symbols
  (Payload) and Pilot symbols such that most
  optimum for FER and BER?

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Presentation Outline

• MIMO Wireless Communications
• General Background
• Block and Data Model
• Related Issues
• Space-Time Block Codes
• A specific example Alamouti Codes
• Used Channel Estimation PSAM
• MRRC scheme
• ML Decoding
• Monte Carlo Simulations
• Alamouti Codes with Perfect CSI under BPSK
  Constellation
• Alamouti Codes with PSAM under QPSK Constellation
• Conclusions

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Specific Example of STBC Alamoutis Orthogonal
Code

• Lets consider two antenna i and i1 at the
  transmitter side, at two consecutive time
  instants t and tT
• The above Space-Time mapping defines Alamoutis
  Code1.
• A general frame design requires concatenation of
  blocks (each 2X2) of Alamouti code,

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Estimated Channel State Information (CSI)

• Pilot Symbol Assisted Modulation (PSAM) 3 is
  used to obtain estimated Channel State
  Information (CSI)
• PSAM simply samples the channel at a rate greater
  than Nyquist rate,so that reconstruction is
  possible
• Here is how it works

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Channel State Estimation
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Estimated CSI (cont.d) Block diagram of the
receiver
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Channel State Estimation (cont.d)

• Pilot symbol insertion length, Pins6.
• The receiver uses N12, nearest pilots to obtain
  estimated CSI

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Channel State Estimation Cont.d

• Pilot Symbols could be think of as redundant data
  symbols
• Pilot symbol insertion length will not change the
  performance much, as long as we sample faster
  than fading rate of the channel
• If the channel is in higher fading rate, more
  pilots are expected to be inserted

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Estimated CSI, Space-time PSAM frame design

• The orthogonal pilot symbol (pilots chosen from
  QPSK constellation) matrix is, 4
• Pilot symbol insertion length, Pins6.
• The receiver uses N12, nearest pilots to obtain
  estimated CSI
• Data 228, Pilots 72

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Channel State Estimation (cont.d)MMSE estimation

• Use Wiener filtering, since it is a Minimum Mean
  Square Error (MMSE) estimator
• All random variables involved are jointly
  Gaussian, MMSE estimator becomes a linear minimum
  mean square estimator 2
• Wiener filter is defined as,
  .
• Note, and

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Block diagram for MRRC scheme with two Tx and one
Rx
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Block diagram for MRRC scheme with two Tx and one
Rx

• The received signals can then be expressed as,
• The combiner shown in the above graph builds the
  following two estimated signal

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Maximum Likelihood Decoding Under QPSK
Constellation

• Output of the combiner could be further
  simplified and could be expressed as follows
• For example, under QPSK constellation decision
  are made according to the axis.

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Presentation Outline

• MIMO Wireless Communications
• General Background
• Block and Data Model
• Related Issues
• Space-Time Block Codes
• A specific example Alamouti Codes
• Used Channel Estimation PSAM
• MRRC scheme
• ML Decoding
• Monte Carlo Simulations
• Alamouti Codes with Perfect CSI under BPSK
  Constellation
• Alamouti Codes with PSAM under QPSK Constellation
• Conclusions

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Space-Time Alamouti Codes with Perfect CSI,BPSK
Constellation
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Space-Time Alamouti Codes with PSAM under QPSK
Constellation
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Space-Time Alamouti Codes with PSAM under QPSK
Constellation
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Conclusion

• Spectrum are precious resources
• Space Time Alamouti Code requires no bandwidth
  expension.
• As redundancy is applied in space across multiple
  antennas, not in time or frequency.
• No need for a complete redesign of existing
  systems to incorporate this diversity scheme
• Space-Time Block Codes are constructed from known
  orthogonal designs, and achieves full diversity
• Alamouti is a unique complex orthogonal designs
  of 2 by 2, also it is a rate one code.

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References

• 1 S. M. Alamoouti, A simple transmitter
  diversity scheme for wireless communications,
  IEEE J. Select. Areas Commun., vol. 16,
  pp.1451-1458, Oct. 1998.
• 2 J.-C. Guey, M. P. Fitz, M. R. Bell, and W.-Y.
  Kuo, Signal design for transmitter diversity
  wireless communication systems over Rayleigh
  fading channels, in Proc. IEEE VTC96, 1996, pp.
  136-140.
• 3 J. K. Cavers, An analysis of pilot symbol
  assisted modulation for Rayleigh faded channels,
  IEEE Trans. Veh. Technol., vol. 40, pp. 686-693,
  Nov. 1991
• 4 Zhilin Liu. Design and Implementation of
  Transmit Antenna Diversity in Wireless
  Communication Systems. Masters Thesis, The Ohio
  State University, 2002.
• 5 V. Tarokh, H. Jafarkhani, and A. R.
  Calderbank, Space-Time Block Codes from
  Orthogonal Designs, IEEE Trans. Inform. Theory,
  vol. 45, pp. 1456-1467, Jul. 1999

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Personal Thanks To

• I would like to express my gratitude to my
  advisor and his graduate students. And they are
• Dr. Michael P. Fitz
• Members of Ucla Wireless Reserch and Development
  Laboratory
• David Browne
• Parul Gupta
• Ryan Penrod
• Niket Sourabh
• Weijun Zhu