1. Coex RIFS Discussion Date: 2006-11-03 Authors: November 2006
doc.: IEEE /1679r1
November 2006
Coex RIFS Discussion
Date:
Authors:
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Jim Petranovich, Conexant Systems, Inc.
Jim Petranovich, Conexant Systems, Inc.

2. November 2006
doc.: IEEE /1679r1
November 2006
Abstract
The presentation is to motivate discussion regarding resolution of these comments from LB84 (referenced by CID)
713, 4432, 7905, 8044, 10036, 10295, 3449, 7521, 10037
Jim Petranovich, Conexant Systems, Inc.
Jim Petranovich, Conexant Systems, Inc.

3. November 2006
CID Breakdown
713, 4432, 7905, 8044, , 10295, 7521
All more or less argue that transmitting RIFS is optional and ask for wording change to clarify this
10037
Points out that PHY-RXEND indication is not appropriate for defining receiver RIFS timing
Jim Petranovich, Conexant Systems, Inc.

4. Key Text from D1.05 November 2006
RIFS (Reduced Interframe Spacing) is a means of reducing overhead and thereby increasing network efficiency.
RIFS may be used in place of SIFS to separate multiple transmissions from a single transmitter, when no SIFS-separated response transmission is expected. The value of RIFS is defined by the aRIFS PHY characteristic. See Table n88 (MIMO PHY characteristics) and (Reduced interframe Space (RIFS)).
There are additional restrictions regarding when RIFS may be employed defined in 9.14 (Reverse Direction protocol) and 9.15 (PSMP Operation). See also (RIFS protection).
Reduced Interframe Space (RIFS)
The transmitter shall be able to transmit a packet 2 usec after PHY-TXEND.confirm is asserted. RIFS timing accuracy is  10%.
Reduced Interframe Space (RIFS)
The receiver shall be able to decode a packet if it starts 2 usec after PHY-RXEND.indication is generated for the previous packet. RIFS timing accuracy is  10%.
Jim Petranovich, Conexant Systems, Inc.

5. SIFS Text From REVma 9.2.3.1 November 2006
The SIFS shall be used prior to transmission of an ACK frame, a CTS frame, the second or subsequent MPDU of a fragment burst, and by a STA responding to any polling by the PCF. The SIFS may also be used by a PC for any types of frames during the CFP (see 9.3). The SIFS is the time from the end of the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the air interface. The valid cases where the SIFS may or shall be used are listed in the frame exchange sequences in 9.12.
The SIFS timing shall be achieved when the transmission of the subsequent frame is started at the TxSIFS Slot boundary as specified in An IEEE implementation shall not allow the space between frames that are defined to be separated by a SIFS time, as measured on the medium, to vary from the nominal SIFS value by more than ±10% of aSlotTime for the PHY in use.
Jim Petranovich, Conexant Systems, Inc.

6. Intent of Draft (IMO) Transmissions of RIFS is intended to be optional
November 2006
Intent of Draft (IMO)
Transmissions of RIFS is intended to be optional
There is no situation defined in the draft where RIFS-separated PPDUs must be transmitted
Reception of RIFS is intended to be mandatory
When two PPDUs are separated by a RIFS spacing, they must be separated by aRIFSTime  10% (not more or less)
This constraint is to facilitate receiver AGC design by allowing AGC to re-acquire if no packet is received within the RIFS time limit.
Jim Petranovich, Conexant Systems, Inc.

7. November 2006
Problems with draft
Section implies that a transmitter must be capable of sending a RIFS packet
I could propose rewording section to indicate that a STA may only transmit a packet within aRIFSTime  10% or within a SIFS time, more or less as suggested in comments
The transmitter may either transmit a packet aRIFSTime  10% after PHY- TXEND.confirm is asserted.
However, this introduces the problem of how to mandate the PHY does not transmit other times.
So I prefer to delete and add a further limit to at the end of the second paragraph
When RIFS transmission is used, PHY-TXSTART.request must be asserted aRIFSTime  10% after PHY-TXEND.confirm is asserted for the previous PPDU.
Issues
Unfortunately, these MAC primitives are not tight enough time contraints
In the case of short GI, the actual gap between the last symbol of one PPDU and the first symbol of the next could be up to 3.6 usec longer than aRIFTS due to 4 usec slot spacing
Is this a problem?
There is no indication of whether RIFS can be used with non-HT frames.
I don’t think it should be used except with HT frames.
Jim Petranovich, Conexant Systems, Inc.

8. Proposed Solution My proposal is to borrow text from REVma
November 2006
doc.: IEEE /1679r1
November 2006
Proposed Solution
My proposal is to borrow text from REVma
Delete
add a further limit to at the end of the second paragraph
The RIFS is the time from the end of the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the air interface. An IEEE implementation shall not allow the space between frames that are defined to be separated by a RIFS time, as measured on the medium, to vary from value by more than aRIFSTime±10%. Both frames separated by RIFS must each be HT format.
Jim Petranovich, Conexant Systems, Inc.
Jim Petranovich, Conexant Systems, Inc.

9. November 2006
Receiver problem
referenced PHY-RX.end, which may not be asserted until the previous PPDU has been decoded
I propose to reword this to:
The receiver shall be able to decode a packet that was transmitted by a STA with a RIFS separation from the previous packet in accordance with section
Jim Petranovich, Conexant Systems, Inc.