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?