One of the key requirements for optimum demodulation of OFDM signals is that there is accurate frequency and timing synchronisation.
Fortunately good OFDM timing and frequency synchronisation is relatively easy to obtain as accurate signals are easy to derive.
Poor timing and frequency synchronisation result in significant levels of degradation to the signal, and as a result this aspect of the signal chain is key to optimum performance.
OFDM synchronization basics
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.
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.
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.
When using OFDM it is necessary to ensure that the synchronisation for both timing and frequency is accurate. By ensuring accurate synchronisation, it is possible to perform the optimum demodulation of the signal. Any misalignment causes the receiver to start to pick up the unwanted interference signals. Fortunately it is relatively easy to obtain accurate synchronisation signals as these are available from the network, and short term synchronisation can be generated internally.
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