Re: [STDS-802-16] frequency correction

[Raja Banerjea <RBanerjea@PROXIM.COM>]

Ambroise,

    I agree with you that frequency correction methods described in the IEEE 802.16-2004 standard are confusing. A TDD subscriber should be responsible for adjusting its own frequency. This could be done by tracking the DL transmission. The IEEE 802.16-2004 spec requires the UL transmission be between 2% of the subcarrier spacing.

    For FDD systems however the BS should direct the SS to adjust its UL transmission frequency. It should be clarified that the frequency adjustments refered to in the RNG-REQ message should be applicable only for UL transmission for FDD.

    Therefore I would recommend that frequency adjustment be left in the RNG-REQ message but be modified to suggest that it is applicable only for FDD transmission.

    Also the frequency tolerance of 2% of subcarrier spacing is OK from the ICI point of view for OFDM transmission, but might be too large for UL subchannel reception. The OFDMA profile OFDMA_ProfP1 requires the SS to BS tolerance to be within 2Hz. Should this be adjusted in the OFDM section too ?

Regards,

-Raja

 

 

 

-----Original Message-----
From: owner-stds-802-16@LISTSERV.IEEE.ORG [mailto:owner-stds-802-16@LISTSERV.IEEE.ORG]On Behalf Of Ambroise Popper
Sent: Monday, March 07, 2005 8:37 AM
To: STDS-802-16@LISTSERV.IEEE.ORG
Subject: [STDS-802-16] frequency correction

I think the process of frequency synchronization in the 802.16-2004 standard needs some additional clarification.

 

The text says (sections 8.3.12 and 8.4.14)

"During the synchronization period, the SS shall acquire frequency synchronization within the specified tolerance [2% of subcarrier spacing] before attempting any uplink transmission. During normal operation, the SS shall track the frequency changes and shall defer any transmission if synchronization is lost."

 

This means an SS is fully responsible for adjusting its own frequency. On the other hand, the ranging process enables a BS to send frequency corrections to an SS, using the frequency correction TLV in the RNG-RSP message. These are 2 conflicting processes, because while a BS is estimating the frequency offset, it does not know if the SS is making corrections based on its own tracking. Also, though these corrections are coded with a 1 Hz precision, there is no requirement on the precision of the application of the correction by the SS, so the BS does not know how an SS reacts upon reception of these messages. Having both the BS and SS track the same frequency offset leads to potential instability.

 

A comment (#304 by Yuval Lomnitz) was accepted in the corrigendum last session for the OFDMA section (but the same issue applies to both OFDM and OFDMA) saying:

 

"During the synchronization period, the SS shall acquire frequency synchronization within the specified tolerance before attempting any uplink transmission. During normal operation, the SS shall track the frequency changes by estimating the downlink frequency offset and shall defer any transmission if synchronization is lost. To determine the transmit frequency, the SS shall accumulate the frequency offset corrections transmitted by the BS (for example in RNG-RSP message), and may add to the accumulated offset, an estimated UL frequency offset based on the downlink signal."

 

I think this comment makes things clearer for the SS, but is not satisfactory from a system point of view because the BS still does not know how the SS will behave. A first solution would be to remove the frequency correction TLV from ranging messages. A second solution would be to indicate that once an SS has received a correction from the BS, it should stop making any further corrections itself and rely fully on the BS for its frequency control.

 

Any thoughts?

 

Regards,

 

Ambroise Popper

tel   : +33 1 44 89 48 11

cell : +33 6 60 63 57 20

 

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