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Reference symbols are added to the downlink transmissions for:
Channel quality measurements
Channel estimation and equalisation over the frequency band to allow demodulation of the received signal
Hence the reference symbols are distributed in time and frequency.
If a second antenna is used (MIMO operation) this will transmit reference signals in the alternate resource elements.
In the example above, we assume frame structure type 1 and a short cyclic prefix so there are 7 symbols in the timeslot