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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