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
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