OFDM
(ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING)
Transmitter
An OFDM carrier signal is the sum of
a number of orthogonal sub-carriers, with baseband data on each sub-carrier
being independently modulated commonly using some type of quadrature amplitude
modulation (QAM) or phase-shift keying (PSK). This composite baseband signal is
typically used to modulate a main RF carrier.
s[n] is a serial stream of binary
digits. By inverse multiplexing, these are first demultiplexed into N parallel
streams, and each one mapped to a (possibly complex) symbol stream using some
modulation constellation (QAM,
PSK,
etc.). Note that the constellations may be different, so some streams may carry
a higher bit-rate than others.
An inverse FFT
is computed on each set of symbols, giving a set of complex time-domain
samples. These samples are then quadrature-mixed to passband in the standard way. The real and imaginary
components are first converted to the analogue domain using digital-to-analogue converters (DACs); the analogue signals are then used to modulate cosine and sine
waves at the carrier
frequency, fc, respectively. These signals are then summed to give the
transmission signal,s(t).
The receiver picks up the signal r(t),
which is then quadrature-mixed down to baseband using cosine and sine waves at
the carrier frequency. This also creates signals centered on 2fc, so low-pass
filters are used to reject these. The baseband signals are then sampled and
digitised using analog-to-digital converters (ADCs), and a forward FFT
is used to convert back to the frequency domain.
This returns N parallel streams,
each of which is converted to a binary stream using an appropriate symbol detector.
These streams are then re-combined into a serial stream,s^[n], which is an
estimate of the original binary stream at the transmitter.
Summary
of advantages
- High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc.
- Can easily adapt to severe channel conditions without complex time-domain equalization.
- Robust against narrow-band co-channel interference.
- Robust against intersymbol interference (ISI) and fading caused by multipath propagation.
- Efficient implementation using Fast Fourier Transform (FFT).
- Low sensitivity to time synchronization errors.
- Tuned sub-channel receiver filters are not required (unlike conventional FDM).
- Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.
Summary
of disadvantages
- Sensitive to Doppler shift.
- Sensitive to frequency synchronization problems.
- High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.
- Loss of efficiency caused by cyclic prefix/guard interval.
- The double sideband modulation of each sub-carrier causes lower spectral efficiency and higher transmitter power requirements for equivalent coverage as compared to VSB modulation
In OFDM, the sub-carrier
frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and
inter-carrier guard bands are not required. This greatly simplifies the design
of both the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-channel is not
required.
