Coded Transmit Macrodiversity: Block Space-Time Codes over Distributed Antennas

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Coded Transmit MacrodiversityBlock Space-Time
Codesover Distributed Antennas

• Yipeng Tang
• and
• Matthew Valenti
• Lane Dept. of Comp. Sci. Elect. Engg.
• West Virginia University
• mvalenti_at_wvu.edu

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Overview

• Block Space Time Codes
• Coded Transmit Macrodiversity
• Space-time codes with widely separated antennas
• Simulation Results
• 2-antenna case
• 3-antenna case
• Both 60 and 120 degree sectorized antennas

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Block Space Time Codes

• The original STC were trellis based.
• Tarokh 1998.
• Shifted burden of diversity from receiver to
  transmitter.
• Rather complex.
• Block space time codes later emerged as a lower
  complexity alternative.
• Block STC has no memory.
• Symbol-in, symbol out.
• Simple decoding structure.

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Advantages and Disadvantages of Block STC

• Advantages
• Space-time block coding utilizes multiple
  antennas to create spatial diversity, this allows
  a system to have better performance in a fading
  environment.
• Good performance with minimal decoding
  complexity.
• Can achieve maximum diversity gain equivalent to
  space-time trellis codes.
• Receivers that use only linear processing.
• Disadvantages
• Does not have as much coding gain as space-time
  trellis codes.
• Can not always achieve the maximum data rates
  allowed by the general theory of space-time
  codes.

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Diagram of Block STC Transmission
Fading ?
c
Data
y
STC encoder
x
r
STC decoder
Modulation
AWGN ?
Encoder matrix
Rate 1/2
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Block STC decoder

• Each symbol in a block is decoded separately by
  minimizing the metric
• The decoder outputs the hard-decisions on the
  data.
• The more TXs and RXs the system has, the better
  performance the system can achieve.

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Decoding Block STC
The received signals are
In order to minimize
it is equivalent to minimize
By using
we have
and
Since x1x2 (PSK), we can get
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Performance of Block STC
Two transmit antenna system has around 18 dB
coding gain while three transmit antenna system
has around 25 dB coding gain. Most of diversity
has been achieved just by two transmit antenna.
back
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Coded transmit Macrodiversity

• Previous research has only considered the case
  that all of the transmit antennas are located in
  the same general location.
• At a single base station in a cellular system.
• Microdiversity.
• Spatial correlation is an issue.
• We consider combination of macrodiversity with
  block STC.
• Macrodiversity Antennas are far apart.
• The array consists of the antennas of adjacent
  base stations.
• For edge excited cells with 120 or 60 degree
  sectorized antennas, can use the three base
  stations at the corner of the cell.

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Assumptions

• Power control.
• We assume that each of the antennas transmits
  with the same power.
• TX power is controlled by closest base station.
• However, due to different path lengths, the
  received signal powers will be different.
• Depends on geometry and path loss exponent (n).
• The best way to allocate power is an open
  problem.
• Synchronization.
• The received signals are not phase synchronized.
• However, we assume the signals are time
  synchronous.
• i.e. aligned in time.
• This may not be an accurate assumption.

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Two antenna system
Normalized distances
Average received power at mobile station is
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Performance Two transmit antenna system
When the mobile station is close to either of the
base station (to achieve the same BER), it
requires more signal power than if it is halfway
between the two transmit antennas. Performance is
the best at location 0.5 which is exactly halfway
between the two transmit antennas. Why? The
center has the best diversity advantage.
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Three transmit antenna systemEdge-excited Cell
For the downlink, space time codes are
transmitted from three base stations and received
by one receive antenna in the mobile
station. Encoder matrix is
Edge-excited
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120-degree sectorized antenna

• Each antenna transmits over a 120-degree sector.

A O
C B
Three base stations are located in A, B and C,
where the highest required SNR values are. The
center of the cell is O, where the lowest
required SNR is. The BER is 10-3.
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120-degree system Performance
Shown is the received SNR required to achieve a
BER of 10-3 Again performance is best at center
of cell. -Best diversity advantage. -Highest
total SNR.
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60-degree sectorized antenna

• Three base stations are located on the corners of
  an equalateral triangle, and the block STC with
  encoder matrix G3 for the downlink communications
  is simulated.

A O B
C
Three base stations are located in A, B and C,
where the highest SNR values are. The center of
the cell is in O, where the lowest SNR is.
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60-degree system performance
Shown is the received SNR required to achieve a
BER of 10-3 Again performance is best at center
of cell. Performance remains good at the midpoint
between any two antennas.
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Performance Comparison

• 120-degree system has better trunking efficiency,
  and less frequent handoff.
• 60-degree system has better energy efficiency,
  and larger system capacity.

• Why?
• Consider mobile at location x
• With 120-degree sectorization is served by A-B-C
• With 60-degree sectorization is served by A-B-D
• D is closer than C.

D
A
B
C
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Conclusion

• Theres no reason that the antennas in a STC
  system must all be in the same general location.
• Spread the antennas out!
• The antennas could be located at different base
  stations.
• Macrodiversity not microdiversity.
• This improves coverage in areas that are far from
  base stations.
• Could use three base stations and either 120 or
  60 degree sectorized antennas.
• Future work
• Optimal power allocation strategies.
• Impact of channel estimation.
• Coping with signals that are not
  time-synchronous.