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My preference would be to remove the TLV. In
tracking the downlink, the SS will necessarily synchronize in frequency. In
the high frequency PHY, this parameter is already forbidden. Unless there is
some idiosyncrasy of OFDMA that renders synchronization to the DL insufficient
for UL frequency control, the parameter just adds confusion. Regards, Ken From: Ambroise Popper
[mailto:ambroise@SEQUANS.COM] 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 SEQUANS Communications 101-103 bld tel : +33 1 44 89 48 07 fax : +33 1 44 89 48 06 |