1. doc.: 11-03-0888-01-000n Jeff Gilbert Atheros Communications
Nov 2003
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Nov 2003
Simulation Methodologies and Radio Impairment Modeling for Fair TGn Proposal Comparisons
Jeff Gilbert
Atheros Communications
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

2. Both can dramatically affect comparison of proposals
Nov 2003
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Nov 2003
The Problem
Well specified:
Usage models
Also specifies some basic PHY characteristics
Channel models
Currently unspecified:
Radio impairment models
PHY abstraction used in MAC / system simulations
Both can dramatically affect comparison of proposals
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

3. Radio Impairments Several categories of impairments to consider:
Nov 2003
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Nov 2003
Radio Impairments
Several categories of impairments to consider:
Link margin components (Tx power, ant gain, Rx NF)
Synthesizer phase noise and frequency offset
Non-linearities (PA primarily)
Baseband impairments (I/Q mismatch etc.)
We need to decide as a group which to include and assure that all proposal simulations are consistent
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

4. Radio Impairments I This group affects link margin
Nov 2003
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Nov 2003
Radio Impairments I
This group affects link margin
Output power – some combination of:
Per chain PA limits – 17-20dBm?
EIRP (w/ any beamforming gains) limits
Beamformed EIRP increases w/ NumAnts2 allowing large gains
Individual antenna gain
Just use 0 dBi? 3 dBi? Omni directional, any polarization
Receiver noise floor
Specify as PSD or NF (as in usage models) – not noise floor
These could dramatically scale obtainable ranges – must be specified for fair comparisons
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

5. Radio Impairments II Phase noise (synthesizer) Frequency offset
Nov 2003
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Nov 2003
Radio Impairments II
Phase noise (synthesizer)
Primary motivator for pilots and pilot tracking
Could also limit constellation size
Typically specified as shaped noise spectrum
Independent per-chain or common – affects pilot schemes
Frequency offset
Same as 11a or 11g is likely fine (specify one…)
Without phase noise and frequency offset, pilot and freq offset detection schemes cannot be fairly evaluated and we could select an inferior or unimplementable proposal
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

6. Radio Impairments III Non-linearities (PA, mixer)
Nov 2003
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Nov 2003
Radio Impairments III
Non-linearities (PA, mixer)
This will limit the power usable for high-order constellations
If the PHY models are used to compute spectral mask for interference / capacity then this could be needed since it affects spectral regrowth
While nonlinearity is a critical implementation issue, it will likely not affect the implementability of proposals (beyond Tx power limit). If spectral mask is important then may be needed.
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

7. Radio Impairments IV Baseband impairments Timing offset I/Q mismatch
Nov 2003
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Nov 2003
Radio Impairments IV
Baseband impairments
I/Q mismatch
A/D converter resolution
Baseband noise floors
While these are critical implementation issues, likely they will not affect the implementability of proposals
Timing offset
Signal detection included in proposals?
At least require randomized sample timing
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

8. PHY Modeling in System Sims
Nov 2003
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Nov 2003
PHY Modeling in System Sims
How TGn differs from other task groups
MAC & PHY w/throughput measured at MAC SAP
Time-varying channel creates time-varying PERs
Time-varying channel could affect systems w/feedback
PHY abstraction used in MAC / System simulations
Including full PHY model in system sims not feasible
However time-varying PER affects delay jitter and error rate application sees – thus modeling is critical
Limits specified in usage models subject to variations in methods used to measure them
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

9. Possible Modeling Solutions
Nov 2003
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Nov 2003
Possible Modeling Solutions
Modeling time-varying channels
PHY simulations per distance and get average PER
Use fixed time and number of packets
Could include fixed fading penalty instead
Could use full Markov model
Feedback systems need explicit modeling
PHY level simulations should incorporate interval between channel sounding and transmission incorporate the effect of channel variations
TGn should specify how to generate consistent results but not own simulation or require specific packages
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

10. Modeling Interference
Nov 2003
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Nov 2003
Modeling Interference
Interference model impacts large enterprise usage model
Cellular reuse implies that co-channel and adjacent-channel interference will impact performance and system capacity
Several options for modeling interference
Simplified system-capacity reduction factor model given specified # channels (Cramer)
Per-connection PHY model using desired and interferer distance in 2-D table
Can be per-packet or per-symbol interference granularity
Solicit input from ?
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

11. Layer 3 and above Parameters
Nov 2003
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Nov 2003
Layer 3 and above Parameters
TCP stack parameters for TCP streams
TCP Window size will affect how much aggregation is possible
Aggregation is a key technique that most proposals will need to use to obtain the desired performance
Others?
Jeff Gilbert, Atheros Communications
Jeff Gilbert, Atheros Communications

12. What to do? Various implementation options:
Nov 2003
What to do?
Various implementation options:
Separate group / document referenced by FRCC?
Just add to Usage Models
Just add to Functional Requirements / Comparison Criteria
Just add to Channel Models
Some combination of the above
Jeff Gilbert, Atheros Communications

13. Time spent now will be saved later!
Nov 2003
Conclusions
802.11n has taken great steps towards a smooth proposal comparison process
Still some ambiguity in radio impairments and PHY / MAC co-simulation
Key is balancing accuracy and effort
Simple (perfect) radio and PHY / MAC interface can be used – but it is critical that this is specified and uniform
Time spent now will be saved later!
Jeff Gilbert, Atheros Communications