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Many OFDM wireless protocols (e.g. 802.11, DVB-T2, LTE, etc.) utilize pilot tones to provide channel state information for the receiver. Within the transmission, the pilot tones generally assume a BPSK or QPSK modulation depending on the protocol.

Why do specifications have the pilot tones' phase follow a known psuedorandom sequence across frequency and time as opposed to utilizing the same phase or a straightforward rotation for simplicity?

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Advantage of pilot pseudo-random sequences in standards (with examples are taken from LTE) :

  • Interference from other sources with different identities (UE ID, Cell ID, ...) are mitigated if detection by correlation operation is used (cell search for example).
  • Allow including identity in sequence by changing the value of sequence initialization. For example, pilot sequence in LTE is 31-length Gold code which is reinitialized at the start of each OFDM symbol with a value that depends on cell id.

For info, pilots are QPSK because it is constant enveloppe waveform that can reduce significantly PAPR.

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