  A pilot spoofer can paralyze the channel estimation in multi-user orthogonal
frequency-division multiplexing (OFD- M) systems by using the same
publicly-known pilot tones as legitimate nodes. This causes the problem of
pilot authentication (PA). To solve this, we propose, for a two-user
multi-antenna OFDM system, a code-frequency block group (CFBG) coding based PA
mechanism. Here multi-user pilot information, after being randomized
independently to avoid being spoofed, are converted into activation patterns of
subcarrier-block groups on code-frequency domain. Those patterns, though
overlapped and interfered mutually in the wireless transmission environment,
are qualified to be separated and identified as the original pilots with high
accuracy, by exploiting CFBG coding theory and channel characteristic.
Particularly, we develop the CFBG code through two steps, i.e., 1) devising an
ordered signal detection technique to recognize the number of signals
coexisting on each subcarrier block, and encoding each subcarrier block with
the detected number; 2) constructing a zero-false-drop (ZFD) code and block
detection based (BD) code via k-dimensional Latin hypercubes and integrating
those two codes into the CFBG code. This code can bring a desirable pilot
separation error probability (SEP), inversely proportional to the number of
occupied subcarriers and antennas with a power of k. To apply the code to PA, a
scheme of pilot conveying, separation and identification is proposed. Based on
this novel PA, a joint channel estimation and identification mechanism is
proposed to achieve high-precision channel recovery and simultaneously enhance
PA without occupying extra resources. Simulation results verify the
effectiveness of our proposed mechanism.
