overview of the timing and frequency synchronization required for
the successful use of OFDM, orthogonal frequency division multiplex.
While OFDM has been successfully deployed in many different radio
communications systems, one of the main problems that needs to be
overcome is that if OFDM synchronization.
Effective OFDM synchronization enables the data error rates to be
kept to a minimum, whereas if the system is not accurately synchronized,
then errors will result and the system will become less effective.
The need for OFDM synchronization
OFDM offers many advantages in terms of resilience to fading,
reflections and the like. OFDM also offers a high level of spectrum
efficiency. However to reap the rewards, it is necessary that the OFDM
system operates correctly, and to achieve this, it is necessary for the
OFDM synchronization to be effective.
There are a number of areas in which the OFDM synchronisation is critical to the operation of the system:
- OFDM synchronization in terms of frequency offset: It is necessary that the frequencies are accurately tracked to ensure that orthogonality is maintained.
- OFDM synchronisation in terms of clock accuracy:
It is necessary that the sampling occurs at the correct time interval to
ensure that the samples are synchronized and data errors are minimised.
In order to ensure that the OFDM system works to its optimum, it is
necessary to ensure that there are schemes in place to ensure the OFDM
synchronization is within the required limits.
Frequency offset OFDM synchronization
It is particularly important that the demodulator in an OFDM receiver
is able to synchronize accurately with the carriers within the OFDM
signal. Offsets may arise for a number of reasons including any
frequency errors between the transmitter and the receiver and also as a
result of Doppler shifts if there is movement between the transmitter
and receiver.
If the frequency synchronisation is impaired, then the orthogonality
of the carriers is reduced within the demodulation process and error
rates increase. Accordingly it is essential to maintain orthogonality to
reduce errors and maintain the performance of the link.
First look at the way that sampling should occur. With the
demodulator in synchronisation, all the contributions from the other
carriers sum to zero as shown. On this way all the carriers are
orthogonal and the error rate is at its minimum.
An OFDM signal where demodulation is in synchronisation
If a situation is encountered where the OFDM synchronisation for the
frequency aspects are poor, then the demodulator will centre its samples
away from the peak of the signal, and also at a point where the
contributions from the other signals do not sum to zero. This will lead
to a degradation of the signal which could in turn lead to an increase
in the number of bit errors.
An OFDM signal where demodulation has poor synchronisation
Clock offset OFDM synchronization
It is also necessary to maintain OFDM synchronization in terms of the
clock. Gain if the clock synchronisation is not accurate, sampling will
be offset and again orthogonality will be reduced, and data errors will
increase.
When looking at OFDM synchronization with regard to the clock offset,
the carrier spacing used within the receiver for sampling the received
signal will be based upon the internal clock rate. If this differs from
that used within the transmitter, it will be found that even if the
first carrier within the multiplex is correct, then there will be a
growing discrepancy with each carrier away from the first one. Even
small levels of discrepancy will cause the error rate to increase.
OFDM synchronization problem with clock offset problem
