stds-802-16-tg4: Strawman - Sections 2 and 8
Dear All,
I made few minor changes on sections 2 and 8, i.e. "Introduction" and
"Recommended practice for base station synchronization ...". Please find
them at the end of this e-mail and CRITICIZE them.
For section 4 ("Data encoding") I expect several contributions tonight
and tomorrow. Thus I should be able to put everything together within
one-two days.
Best Regards,
Octavian Sarca
Redline Communications Inc.
90 Tiverton Crt. #102
Markham, ON, L3R 9V2
E-mail: osarca@redlinecommunications.com
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2. Introduction
The PHY layer described in this clause is designated for operation in
the unlicensed frequency bands of 5-6 GHz. In order to allow different
deployment scenarios from dense populated areas (where sectorized
environments and the high interference levels may require narrow
channels) to sparse populated areas (where wider channels can help
delivering better services at the same system cost), the PHY layer is
allowed to operate with channel bandwidths of 10 MHz or 20 MHz and
optionally 5 MHz. An 802.16b device must implement at least one of the
10MHz or 20MHz channel bandwidths.
The PHY operation is based on OFDM (Orthogonal Frequency Division
Multiplex) modulation and Time Division Duplexing (TDD). In OFDM the
information is imposed onto the medium by modulating multiple carriers
transmitted in parallel. As the modulation is often implemented using
the FFT algorithm, the modes are designated by the minimum FFT size
(called hereby FFT size), i.e. the next power of 2 above the number of
used carriers. The PHY defines two mandatory modes 64 and 256 -FFT and
one optional mode 1024-FFT. The mandatory modes employ Time Division
Multiple Access (TDMA) while the optional mode employs a combination of
TDMA and Orthogonal Frequency Division Multiple Access (OFDMA). The
modes are summarized in table ??.
Mode Access method FFT size Status
64-FFT TDMA 64 Mandatory
256-FFT TDMA 256 Mandatory
1024-FFT OFDMA 1024 Optional
To be standard compliant, a system must implement both the 64-FFT and
the 256-FFT modes.
Notes: changes to this sections where made to reflect the latest group
decisions on channel bandwidth and FFT sizes.
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8. Recommended practice (optional) for base station frame
synchronization or coordination mechanism in sectorized environments
In sectorized environments, where several base-stations are collocated
in the same hub, the co-channel and adjacent channel interference can be
significantly reduced if all base-stations are frame synchronized, i.e.
they switch between transmit and receive and reverse at the same time.
For synchronization, the base-stations must use the same frame size and
the same uplink/downlink partition. They must also be synchronized to a
common time reference. A hub controller must be able to change the frame
size and the uplink/downlink partition simultaneously in all
base-stations that belong to that hub. Also, to preserve synchronization
over a long period of time, the base-stations should use a common
frequency reference.
It is recommended that the BS should be able to synchronize the start of
the MAC frame with 1 Hz pulses received on an external input. The same
pulses should trigger changes in the frame size and uplink/downlink
partition in all BS's at the same time. In other words, after the hub
controller has updated these two parameters in each base-station,
changes should take place simultaneously in all base-stations at the
next synchronization pulse. To preserve synchronization over a long
period of time, the base-station should be able to lock its reference
frequency to an external 10 MHz reference frequency. It is recommended
that the electrical specifications for the 1 Hz and 10 MHz inputs,
should be compatible with the GPS. However, it is not required to use
GPS for synchronization.
Synchronization can be extended beyond the hub, to the network level if
all hubs use a common time and frequency reference, like the GPS.
However, depending on deployment conditions, different hubs may require
different time offsets from the common time reference to reduce the
overall interference. Thus, if synchronization between hubs is
implemented it is recommended that each hub should be capable to use
distinct delays from the 1 Hz pulse, with all base-stations in a hub
having the same delay.
Notes:
· I modified this section to include opinions that were strongly
debated in Hilton Head. I provided opportunities for synchronization
between hubs (as suggested by Demos) but I made the delay between hubs
adjustable (which should be better than random delay as suggested by
Nico).
· I removed requirements for MAC since this is part of an upper
layer and, as nicely argued by several people, they fall outside the
scope of the standard.
· Anyway the whole section is optional and just recommended
practice.
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