You're confusing baseband symbols with OFDM symbols. The baseband
modulation order (16QAM, 64QAM, etc.) doesn't affect the OFDM timing at
all. It only affects the payload rate.
For example, the BBC DVB-T2 parameters are 8 MHz bandwidth, 32K FFT,
1/128 guard interval, 60 ODFM symbols per frame and 256QAM baseband
modulation for 40.21 Mbps of TS payload rate. If you change to 64QAM,
the only thing that changes is the TS payload rate (to 29.86 Mbps) and
the required S/N ratio.
Also, it's not really the OFDM symbol time that matters, it's the
spacing between OFDM carriers. Shorter OFDM symbols have wider carrier
spacing and are less susceptible to doppler shift. As you go down in
frequency, there's less doppler shift, so you can use longer symbols. A
4K FFT has the same doppler tolerance (in km/Hr) at 500 MHz as a 2K FFT
at 1 GHz.
Ron
On 05/08/2015 04:56 PM, Manfredi, Albert E wrote:
I've been trying to get this point across to you for years. Perhaps since 2011.
The CP is transmitted in the guard interval. In order to optimize OFDM for
mobility, the symbols need to be short. For example, that's why 2K OFDM works
better than 32K OFDM, for mobility in the TV bands. Short symbols necessarily
mean short guard intervals. *That's why* the tower spacing for LTE SFN mode has
to be so tight.
Mark actually puts this differently, although it amounts to exactly the same
thing. Operating in the L band, LTE can credibly use much wider channels than
does TV in the 600 MHz band -- 20 MHz channels vs 6-8 MHz channels. So for a
given constellation, say 64-QAM, the symbol rate for LTE needs to be
considerably higher than it can be in the narrower TV ban channels, to fill up
that 20 MHz channel and achieve the same b/s/Hz as we're used to. That also
results in short GI.
In order to get long guard intervals necessary for wider tower spacing for
SFNs, with 20 MHz channels, the solution is to go with much higher order
constellations, but then that would decrease the symbol rate, which is bad for
mobility. So again, a bad compromise for long range reception, where multipath
distortion is to be expected.
