OFDM vs f-OFDM | difference between OFDM and f-OFDM
This page compares OFDM vs f-OFDM (Filtered OFDM) and mentions difference between OFDM and f-OFDM (Filtered OFDM) techniques. OFDM stands for Orthogonal Frequency Division Multiplexing.
OFDM Modulation
OFDM is a multicarrier modulation scheme known as Orthogonal Frequency Division Multiplexing. In this densely packed low frequency subcarriers are assigned with some complex data symbols before RF up conversion. Each subcarriers carry data. In this OFDM modulation, each subcarried is demodulated successfully in the absense of ISI and ICI. This is achieved with the concept of cyclic prefix (CP). CP is choosen to have duration greater than delay spread of the channel between transmit and receive end.
The addition of CP results into reduction of spectral efficiency and hence
it is not favourable for multicarrier uplink as well as cognitive radio system.
Refer following links on OFDM.
OFDM vs CDMA >>,
OFDM vs OFDMA >>,
FDM vs OFDM >> and
OFDM Matlab code >>.
f-OFDM | Filtered OFDM
The figure-2 depicts f-OFDM (Filtered OFDM) transmitter and receiver. As shown after IFFT/CP insertion block, filtering module is used. In f-OFDM, a filter is applied to time domain OFDM symbol in order to improve out of band radiation of sub-band signal, while at the same time complex domain orthogonality of the OFDM symbols is being maintained.
Following filters can be used in f-OFDM technique.
Soft-Truncated Sinc Filters either using Hann window or RRC window
Equi-ripple filter using Remez exchange algorithm
Difference between OFDM and f-OFDM
Following section and table describes difference between OFDM and f-OFDM techniques.
➨In LTE system, maximum bandwidth occupancy is 90% of allocated bandwidth.
In 5G, f-OFDM is used and 90% limit is not applied. 5G uses filter to meet
Out Of Band Emission (OOBE) requirements using technique known as filtered OFDM or f-OFDM.
The figure-3 depicts PSD of OFDM and f-OFDM both.
➨The f-OFDM leverages all the benefits of OFDM technique.
In addition it helps in efficient spectrum utilization by meeting OOBE requirements.
➨In filtered-OFDM (f-OFDM), the entire assigned bandwidth is first divided into several sub-bands.
The different types of services are accommodated in these different sub-bands using most suitable waveform/numerology.
This leads to improvement in spectrum utilization.
➨The f-OFDM provides throughput gain compare to conventional OFDM technique.
➨The f-OFDM offers significant reductions on usage of guard band. This leads to more efficient spectrum utilization.
| Specifications | OFDM | f-OFDM |
|---|---|---|
| Full Form | Orthogonal Frequency Division Multiplexing | Filtered Orthogonal Frequency Division Multiplexing |
| Filter Granularity | Whole Band | Sub-band |
| Typical Filter Length | <= CP Length | <=(1/2)*Symbol Duration |
| Time Orthogonality | Orthogonal | Non-orthogonal |
| Frequency Orthogonality | Orthogonal | Quasi-Orthogonal |
| Out Of Band Emission | Bad | Better |
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