[STDS-802-16] TTG may be too small for the scaleable OFDMA
Dear all,
On P.11 of 802.16-2004, TTG is defined:
"3.63 transmit/receive transition gap (TTG): A gap between the downlink burst and the subsequent uplink
burst in a time division duplex (TDD) transceiver. This gap allows time for the base station (BS) to switch
from transmit to receive mode and SSs to switch from receive to transmit mode. During this gap, the BS and
SS are not transmitting modulated data but simply allowing the BS transmitter carrier to ramp down, the
transmit/receive (Tx/Rx) antenna switch to actuate, and the BS receiver section to activate. Not applicable
for FDD systems."
On P.500 of 802.16-2004, the insertion of TTG and RTG between downlink subframe and
uplink subframe is shown.
On P.659 of 802.16-2004, 8 bits are assigned for specifying TTG and RTG duration in PSs respectively.
On P.643, PS is defined as 4/Fs, and Fs is defined in P.495 as the sampling frequency of the OFDMA mode.
For a 5 MHz OFDMA system, Fs is calculated as specified in P. 495:
Fs=floor(8/7*5e6/8e3)*8e3=5712000.
So the biggest TTG one can specify will be 2^8*4/Fs= 179.2717 µs.
For 10 MHz and 20 MHz systems, TTG is limited to 89.2857 µs and 44.6272 µs respectively.
The maximum TTG one can specify for 5, 10 and 20 MHz systems seems to be too small.
For example, on P. 771 of 802.16-2004, for the 1.25 MHz bandwidth TDD OFDMA profile,
TTG is required to be larger than 200 µs.
One possible solution is to fix the unit used for specifying TTG and RTG.
e.g. for all OFDMA systems, the unit can be fixed at 2.8 µs (which is the current unit (4/Fs) for the 1.25 MHz system).