TTG, RTG
In the standard 802.16 -2004 I found that TTG must be > to 200µs and RTG > 5µs.
Must in reality, how long, in average, are RTG and TTG?
Transition Gap
Transmit/receive transition gap (TTG)
- A gap between the downlink burst and the subsequent
uplink burst in a TDD transceiver
- During TTG, BS switches from transmit to receive mode
and SSs switch from receive to transmit mode ( TDD switching timing: ( 13µs <> with SOFDMA modulation
Receive/Transmit transition gap (RTG)
- A gap between the uplink burst and the subsequent
downlink burst in a TDD transceiver
- During RTG, BS switches from receive to transmit mode
and SSs switch from transmit to receive mode ( TDD switching timing 13µs <> with SOFDMA modulation
- The gap is an integer number of PS durations and
starts on a PS boundary
The IEEE specifications define TTG and RTG in terms of Physical Slots. A Physical Slot (PS) is a duration calculated as:
PS = 4 / Fs
Where Fs is the sampling frequency, which can roughly be calculated as Fs = n x BW. Where n is the sampling factor and BW is the channel bandwidth. The values of n are available in the IEEE specifications as well.
The WiMAX profiles released by the WiMAX Forum have a list of TTG and RTG values (in terms of PS) for different channel bandwidths. See http://www.wimaxforum.org/technology/documents/WiMAX_Forum_Mobile_S...
According to the profiles,
TTG = 296 PS for 10 MHz, 218 PS for 8.75 MHz, 376 PS for 7 MHz, 148 PS for 5 MHz and 188 PS for 3.5 MHz
RTG = 168 PS for 10 MHz, 186 PS for 8.75 MHz, 120 PS for 7 MHz, 84 PS for 5 MHz and 60 PS for 3.5 MHz
And, both should be at least 5 micro sec.
These values, of course, depend on the Frame Duration as well, which is set to 5 ms by the Profiles.
The effect of TTG and RTG durations on coverage, or rather cell coverage limit, comes from the fact that in TDD systems, if the propagation time delay between the base station and the receiver is higher than the lowest of the two values, i.e., Lowest(TTG, RTG), the downlink and uplink subframes from different base stations will overlap creating uncorrectable UL-DL interference. And, the receiver will no longer be able to differentiate between the useful data it's getting in DL from its base station, and the interference it is receiving on the UL from nearby mobiles. Plus, the base station will not be able to get the UL transmission from this mobile in time, i.e., it will receive the UL transmission from the mobile during the DL subframe.
The coverage limit of cells with respect to the TTG and RTG durations can be calculated as:
Maximum Coverage Range (m) = Lowest(TTG, RTG) x 300000 / 2
Where TTG and RTG are in ms, 300000 m / ms is the speed of electromagnetic waves (speed of light, you can also use 299458.792 if you want ;-), and the division by 2 takes into account the round-trip time.
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