Digital Signal Processing Reference
In-Depth Information
Symbol time is synchronized with the digital length
D
of the system
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
1, 00
0, 00
-1,00
7, 5
0, 0
-7,5
0,60
0,45
0,30
0,15
0,00
0
25
50
75
100
ms
-250
-150
-50
0
50
100
150
200
250
Hz
No synchronization of symbol time and period length
D
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
1, 00
0, 00
-1, 00
7, 5
0, 0
-7, 5
0,60
0,45
0,30
0,15
0
25
50
75
100
12
0,00
ms
-250
-150
-50
0
50
100
150
200
250
Hz
Illustration 275:
OFDM and digital period length T
D
The symbol length is regarded as a time reference in connection with OFDM. In addition, its synchronism
with the digital period length T
D
of the signal processing overall system is of particular importance. Only
then is the spectrum - the sum of all carrier oscillations - represented or reconstructed correctly. This
Illustration shows a random selection of the four possible phases of the individual carriers.
The bottom half of the Illustration shows the consequences of non-synchronism of symbol length and the
digital period length T
D
of the overall system as simulated by DASYLab. This is why the block length (in
this case 2048) does not quite correspond with the scanning rate (in this case 2040).
Symbol length and digital period length T
D
of the overall system thus need to have an integer relation
towards each other.
A special problem with OFDM-systems is therefore the exact time synchronization of all the transmitters
with regard to interference and synchronized reception of adjacent transmitters.
