Re: [Discuss-gnuradio] Power Line Communications - GNURadio

[Ron Economos]

It's all related by sample rate and IFFT size. For PRIME, it's a 250 kHz 
sample rate and an IFFT size of 512.

Carrier spacing = sample rate / IFFT size = 250,000 / 512 = 488.28125 Hz

OFDM symbol time = IFFT size / sample rate = 512 / 250,000 = 2.048 
milliseconds.

However, only 97 carriers out of the possible 512 are used. 96 carry 
data and one is used as a pilot.

Bandwidth = active carriers * carrier spacing = 97 * 488.28125 Hz = 
47363.28125 Hz. The signal occupies a little over 47 kHz from 42 to 89 kHz.

There is also a guard interval of 48 samples. So an OFDM symbol takes 
2.048 + 0.192 = 2.24 milliseconds to transmit.

So you get to send 96 DBPSK, QQPSK or D8PSK symbols (1 per carrier) for 
each OFDM symbol every 2.24 milliseconds.

This gives an uncoded throughput of 1 / 0.00224 * 96 * 1 = 42847 bps for 
DBPSK

This gives an uncoded throughput of 1 / 0.00224 * 96 * 2 = 85714 bps for 
DQPSK

This gives an uncoded throughput of 1 / 0.00224 * 96 * 3 = 128571 bps 
for D8PSK

Ron

On 04/03/2016 05:00 AM, Paul Creaser wrote:
> With regards to OFDM I have one question.
>
> I have a 8 channel OFDM using 8PSK. To me a single symbol using 8PSK means 8
> bits, hence one byte of data. Since I have 8 channels, that would mean 64
> bytes. This would equate to a single OFDM symbol of 64 bytes?
>
> Going on further a payload of 16 OFDM symbols would mean 16*64 bytes of
> data.
>
> I'm not too sure of the relationship between bits, symbols and OFDM symbols
> even though it appears to be simple.