OFDM
OFDM spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. OFDM is sometimes called multi-carrier or discrete multi-tone modulation.
The OFDM transmission scheme has the following key advantages:
· Makes efficient use of the spectrum by allowing overlap
· By dividing the channel into narrowband flat fading subchannels, OFDM is more
· resistant to frequency selective fading than single carrier systems are.
· Eliminates ISI and IFI through use of a cyclic prefix.
· Using adequate channel coding and interleaving one can recover symbols lost due to
· the frequency selectivity of the channel.
· Channel equalization becomes simpler than by using adaptive equalization
· techniques with single carrier systems.
· It is possible to use maximum likelihood decoding with reasonable complexity
· As discussed in OFDM is computationally efficient by using FFT techniques to
· implement the modulation and demodulation functions.
· In conjunction with differential modulation there is no need to implement a channel estimator.
· Is less sensitive to sample timing offsets than single carrier systems are.
· Provides good protection against co channel interference and impulsive parasitic noise.
In terms of drawbacks OFDM has the following characteristics:
· The OFDM signal has a noise like amplitude with a very large dynamic range; therefore it requires RF power amplifiers with a high peak to average power ratio.
· It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.
It is used for wireless as well as wireless access.
Some of the Major Tecnologies using OFDM : 802.11a WLAN, WiMax, LTE
Cyclic Prefix (CP)
In telecommunications, the term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes.
- As a guard interval, it eliminates the intersymbol interference from the previous symbol.
- As a repetition of the end of the symbol, it allows the linear convolution of a frequency-selective multipath channel to be modelled as circular convolution, which in turn may be transformed to the frequency domain using a discrete Fourier transform. This approach allows for simple frequency-domain processing, such as channel estimation and equalization.
The intersymbolic interference is almost completely eliminated by introducing a guard time for a each OFDM symbol. The guard time is chosen larger than the expected delay spread such that multipath components from one symbol cannot interfere with the next symbol. This guard time could be no signal at all but the problem of intercarrier interference (ICI) would arise. Then, the OFDM symbol is cyclically extended in the guard time. Using this method, the delay replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard time. Multipath signals with delays smaller than the guard time cannot cause ICI.
In order for the cyclic prefix to be effective (i.e. to serve its aforementioned objectives), the length of the cyclic prefix must be at least equal to the length of the multipath channel. Although the concept of cyclic prefix has been traditionally associated with OFDM systems, the cyclic prefix is now also used in single carrier systems to improve the robustness to multipath.
LTE Bandwidth/Resource Configuration (for normal CP – 7 OFDM symbols)
Channel Bandwidth [MHz]
|
1.4
|
3
|
5
|
10
|
15
|
20
|
Number of resource blocks (N_RB)
|
6
|
15
|
25
|
50
|
75
|
100
|
Number of occupied subcariers
|
72
|
180
|
300
|
600
|
900
|
1200
|
IDFT(Tx)/DFT(Rx) size
|
128
|
256
|
512
|
1024
|
1536
|
2048
|
Sample rate [MHz]
|
1.92
|
3.84
|
7.68
|
15.36
|
23.04
|
30.72
|
Samples per slot
|
960
|
1920
|
3840
|
7680
|
11520
|
15360
|
