Simultaneous Transmission: Some comments from FCC

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Simultaneous TransmissionSome comments from FCC
Jim Lansford Jim.Lansford_at_mobilian.com (405)
377-6170
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Motivation

• Simultaneous Tx is a different view of
  coexistence
• Normally think of Tx-Rx coexistence
• Tx-Tx raises approval issues
• Some companies claim simultaneous Tx is not
  allowed
• This is meant to clarify

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Presentation overview

• Dialog with Tim Harrington
• FCC Labs
• Equipment Authorization Branch
• Addresses approval issues for simultaneous Tx
• Dialog format is
• My question
• My assumed answer
• The FCCs response

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Q1 Simultaneous Tx

• Lq1) Can 802.11b and Bluetooth transmit at the
  same time in the same device?
•  
• La1) Yes, with restrictions. They must have
  separate power amplifiers. If coupled to the
  same antenna, the PA outputs can only be
  passively combined (PA sharing is not allowed).
•  
• FCCa1) Yes for 802.11b and Bluetooth (BT).
  Probably yes for 802.11b or BT with 802.11a.
  Compliance with 15.31(k) for composite devices
  must be addressed. La1 mentions use with same
  antenna. In that case must address RF exposure
  (RFx) and intermod from simultaneous Tx as
  needed.

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Q2 Authorization requirements

• Lq2) If so, are there special requirements for
  equipment authorization for such devices? (SAR,
  etc.)
•  
• La2) A device that does simultaneous transmission
  must be tested to make sure it falls within SAR
  limits for its intended usage. Thus, a TCB
  cannot be used for equipment authorization. Test
  results must be submitted to the FCC for review
  before an FCCID will be granted.
•  
• FCCa2) Some considerations were listed in FCCa1.
  Based on mobile or portable exposure conditions,
  compliance should be evaluated to either MPE or
  SAR limits. See attached notes for other
  comments. At the present time, TCBs are cleared
  to review SAR only for single-Tx Part 22, 24
  handsets.

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Q3 What is same device?

• Lq3) What is the definition of "same device?"
  (For example, are radio modules on opposite ends
  of a PC laptop considered to be in the same
  device, even though they are 25 cm apart?)
•  
• La3) Same device isn't the issue - it's exposure
  to RF energy from two or more sources. If a
  laptop has two transmitters, overall exposure
  from both combined at a pre-determined distance
  (20cm?) is what matters.
•  
• FCCa3) Depending on powers and frequencies
  (near-field effects), two Txs 20 cm apart on a
  laptop computer can possibly be considered
  separately, as long as mutual interactions are
  negligible with respect to EMC and RFx. Mobile
  phones with 1mW BT have been certified, as well
  as laptops with 802.11b and BT. We have not seen
  many 802.11a devices. For any Tx device, if
  persons can approach closer than 20 cm,
  compliance with SAR limits is required.
  Depending on relative powers (levels TBD by FCC
  we will consider industry suggestions), special
  considerations are needed for RFx evaluation of
  multi-Tx devices, including multi-band and probe
  calibrations for simultaneous frequency-band
  tests, and possible alternate on/off and on/on
  testing for each Tx with summation of all RFx
  responses in each applicable operating and
  intermod frequency bands.

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Q4 Antenna Spacing

• Lq4) If two transmitters are located on the same
  IC or PCB, but have separate antennae, are they
  considered to be the same device? Is this
  dependent on antenna spacing?
•  
• La4) See La3. Two or more transmitters in the
  same chip are allowed, but the device must still
  meet the requirements of 15.247, 15.209, and
  other relevant rules in all modes...especially
  the SAR limits.
•  
• FCCa4) See also FCCa3. Per 15.31(k), each
  separate Tx must meet its applicable rule parts.
  For EMC and RFx testing, the devices should
  transmit simultaneously, and emissions due to
  simultaneous operation must not exceed limits for
  the applicable rule parts of each separate Tx.
  Depending on frequencies, aggregate power levels,
  and exposure conditions, routine evaluation for
  compliance with SAR limits is required when used
  in portable configurations (2.1093). Two Tx on
  one board can have one FCC ID, but for example a
  15.247 device with a 15.407 device may require
  application filing under two equipment classes
  with two fees.

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Q5Frequency band

• Lq5) Does it matter if the devices are in the
  same frequency band (for example, Bluetooth and
  GSM1900 are in different bands, while 802.11b and
  Bluetooth share the 2.4GHz ISM band)
•  
• La5) Different band or same band doesn't matter -
  again it's an issue of meeting harmonic and
  spurious specifications as well as SAR limits.
•  
• FCCa5) No. Different bands may wind up having
  different power thresholds. We agree with La5.

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Summary

• FCC doesnt care if devices interfere
• But each needs to follow Part 15.247 restrictions
• Simultaneous Tx poses special approval issues
• Can potentially exceed SAR limits
• Cant use TCB program at present
• BUT
• FCC does allow simultaneous transmission