Simulation and Evaluation of Various Block Assignments

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Simulation and Evaluation of Various Block
Assignments

• Evaluation of multiple carriers deployed in a
  channel block
• 802.20 evaluation criteria section 16 stated the
  channel block size to be evaluated
• 2 x 5 MHz (total 10 MHz)
• 2 x 15 MHz (total 30 MHz)
• For example, if a proposed technology is
  designed for a channel bandwidth that is
  significantly smaller than the channel block size
  in the evaluation, a few carriers may be placed
  adjacent to each other so as to increase the
  spectral efficiency for the evaluated block size.
  
• In addition to the requirement to meet the FCC
  out-of-band spectral emission at the block edges,
  the adjacent channel interference may cause a
  performance degradation, depending on the channel
  spacing

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Adjacent Channel Interference

• Depending on the technology design, different
  levels of adjacent channel interference can be
  tolerated with performance degradation
• The more adjacent channels are packed into the
  channel block, the higher the spectral
  efficiency, provided that the performance
  degradation can be tolerated in the system link
  budget
• A tradeoff exists between channel spacing and
  performance degradation caused by adjacent
  channel interference
• Another important factor causing adjacent
  channel interference is the spectral re-growth
  caused by the power amplifier (PA) non-linearity
• Amount of spectral re-growth and thus adjacent
  channel interference depends on
• signal waveform, e.g., (99.99) peak to average
  power ratio
• Power amplifier characteristics and the
  operating point

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Simulation Model

• A similar simulation model has been discussed in
  Contribution C802.20-04/68r1
• 3 adjacent channels are modeled with the desired
  channel in the center
• Adjacent channels transmit random data using the
  same technology
• Signals from all 3 channels are combined and
  passed through a single PA
• Simulation was repeated for a few values of
  channel spacing
• End-to-end link performance measured in terms of
  probability of bit error
• Both uncoded and coded cases are simulated

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Block diagram for the simulation Model
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Power amplifier model

• The PA model used in this simulation is RAPPs
  model for the AM/AM characteristics
• Model parameter p 2
• Operating point of PA selected such that the
  Output Backoff is about 5 dB

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Power spectrum of transmitted signal - wide
channel spacing

• 3 adjacent channel with channel spacing 1.388
  x Channel Bandwidth

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Power spectrum of transmitted signal - narrow
channel spacing

• 3 adjacent channel with channel spacing 0.992
  x Channel Bandwidth

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Simulation results Uncoded case

• Channel spacing at Df, 0.92 Df, 0.88 Df, 0.82 Df
  and 0.8 Df
• Df Channel Bandwidth
• Error floor appears for channel spacing lt 0.82 Df

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Simulation results Coded case

• R-1/2 convolutional code
• Channel spacing at Df, 0.92 Df, 0.88 Df, 0.82 Df
  and 0.8 Df
• Error floor significantly reduced, e.g., with
  0.82 Df, degradation 0.5 dB at BER 10-4

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Performance degradation caused by adjacent
channel interference

• Based on the simulation results in this example
• If channel spacing is set to 0.88 Df, there is
  no performance degradation in BER performance
  caused by the adjacent channel interference
• But, if the channel spacing is 0.82 Df, there is
  an associated performance degradation
• Even worse, if channel spacing is 0.8 Df,
  degradation at 10-4 BER is 2 dB
• Better spectral efficiency may be achieved in
  the second case, but the performance degradation
  will need to be taken into account in technology
  evaluation, e.g.,
• 0.5 dB degradation at 10-4 BER, 0.8 dB
  degradation at 10-5 BER
• Tradeoff scenario depends on the system design
  and parameters
• Signal waveform characteristics
• Coding rate
• PA characteristics

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Recommendations

• Option 1
• For the purpose of proposal evaluation to enable
  fair comparison between technology, adopt a
  common non-linearity model and methodology for
  simulating the effects of adjacent channel
  interference
• For example, adopt proposed text in
  C802.20-04/68r1 and,
• Replace Figure 1 by the revised simulation model
  in slide 5
• Replace Figure 2 PA model by RAPPs model as
  shown in slide 6
• Refine the proposed text

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Option 1
x.x. Link level simulation model to include the
effects of adjacent channel interference When
multiple frequency channels are deployed in an
assigned channel block, the effect of adjacent
channel interference (ACI) can be modeled and
included in the link level simulation as shown in
Figure 1. A non-linearity model that can be used
in the simulation is shown in Figure 2 TBD.
Link performance of the desired user under the
effect of ACI can be obtained through the
computation of error probabilities at the
receiver. The result of link performance should
then be incorporated into the system level
simulation. The desired signal is generated by
the transmitter model with carrier frequency at
fc. Two interfering signals, which are generated
by similar transmitter models, each of which
represents an adjacent channel centered at Df on
each side of the desired channel that is centered
about fc. Df is the required channel spacing for
the specific proposed technology. A typical and
the worst case scenarios should both be
evaluated. The typical scenario is the one in
which all three channels are transmitting at the
same power. The worst case scenario happens when
the adjacent channels are transmitting at their
maximum power, while the desired channel is
transmitting at the minimum power. The set of
link-level simulation results that need to be
incorporated into the system simulation should
reflect the performance degradation caused by
ACI, based on this simulation methodology. For
the channels that are located at the two edges of
a frequency block, interference from only one
adjacent channel needs to be considered.
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Recommendation Option 2

• Option 2
• Include the following statement in Section 16 of
  the Evaluation Criteria Document V.14-
• In addition, the performance degradation
  associated with the adjacent channel interference
  needs to be stated and included in the system
  level evaluation.
• - Placed at the end of the following paragraph in
  the current text
• A proposal should specify the channel spacing
  out-of-band emission limits

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References

1. IEEE 802.20 Evaluation Criteria document, V. 14,
   March 2005
2. Evaluation of 802.20 proposals with adjacent
   channel interference considerations Description
   text, C802.20-04/68r1, September 2004
3. Models for signal clipping in evaluation of
   MBWA, C802.20-04/83, Sept. 2004
4. Method for capturing noises in digital
   receiver, C802.20-04/81, Nov. 2004
5. Proposed text for methodology for capturing
   noise in digital receivers, C802.20/89r1,
   January 2005.