[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
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