Guard time and cyclic extension
 OFDM
is the most efficient way to deal with multipath delay
8
spread. By dividing the input datastream in N s subcarriers, the
symbol
duration is made N s times, which also reduces the
6
8
relative multipath delay spread.
4
6
2
4
Single carrier
0
2
Single carrier
-2
0
-4
-2
-6
-4
OFDM technique
-6
-8
Wireless Communication Technologies 2.5.2
1
Guard time and cyclic extension
 To eliminate inter-symbol interference (ISI) almost completely,
a guard time is introduced for each OFDM symbol.
 The guard time is chosen larger than the expected delay
spread , such that multipath components from one symbol
cannon interfere with the next symbol.
 The guard time could consist of no signal at all. In that case,
however, the problem of intercarrier interference (ICI) would
arise.
 ICI is crosstalk between different subcarriers, which means
they are no longer orthogonal.
Wireless Communication Technologies 2.5.2
2
Guard time and cyclic extension
 The ICI effect is illustrated in the following figure.
 Note that when the receiver tries to demodulate the first
subcarrier, it will encounter some interference from the second
subcarrier, because within the FFT interval, there is no integer
number of cycles
difference between
subcarrier 1 and 2.
3
Guard time
FFT integration time = 1/Carrier spacing
Subcarrier #1
2
1
0
ICI
Delay Subcarrier #2
-1
-2
-3
OFDM symbol time
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wireless Communication Technologies 2.5.2
2
3
Guard time and cyclic extension
 To eliminate ICI, the OFDM symbol is cyclically extended in
the guard time, as shown in figure.
 This ensures that delayed replicas of the OFDM symbol
always have an integer number of cycles within the FET
interval.
1
Cyclic Prefix
0
300
-1
200
100
-2
0
-3
-100
-4
-200
-5
-300
0
0.05
Guard Time
0.1
0.15
0.2
OFDM symbol time
0.25
Guard time / cyclic
prefix
0.3
FFT integration time = 1/carrier spacing
OFDM symbol time
-6
ZPS
-0.5
0
0.5
Wireless Communication Technologies 2.5.2
1
4