Aerospace and Electronic Systems Magazine May 2018 - 37

Huang and Wang

Figure 4.

Illustration of hopping frequency offsets set for the FDA-OFDM, where
the frequency offsets for each symbol are selected according to the
OFDM coding sequence.

in Figure 4, Nc distinct carriers in each OFDM symbol can be
implemented in different configurations. Note that, if the number
of FDA elements is smaller than the number of OFDM carriers,
namely, M < Nc, some FDA elements can transmit multiple carriers
simultaneously. In contrast, if M > Nc, we can choose only partial
FDA elements to operate for the communication information transmission. That is, the FDA-OFDM system allows flexible operation
modes, which can also be utilized for multi-user communications,
each user adopts one of the specific hopping frequency sets for its
OFDM information transmission.

FDA-OFDM CHIRP WAVEFORM FOR INTEGRATED
NAVIGATION, SENSING, AND COMMUNICATIONS

Figure 3.

Illustrative spectra comparisons between OFDM chirp waveform and
standard OFDM waveform: (a) OFDM chirp waveform spectra. (b)
Standard OFDM waveform spectra.

dard OFDM carrier signal spectra, namely, Twsinc( f Tw − i). Since
sinc( f Tw − i) has the first null at an inverse of the signal duration T,
the spectra of the carriers will be overlapped in time-domain, but
the carriers are mutually orthogonal in frequency-domain.
Suppose i = 1, Figure 3 compares the spectra of the OFDM
chirp waveform and standard OFDM waveform, where the number of carriers (three carriers) is used for illustration purpose only.
It can be noticed that the OFDM chirp basis has lower PAPR,
which is desired for RF hardware transmitter and has an almost
constant modulus like a standard chirp waveform. Therefore, it
is expected that the OFDM chirp waveform provides a potential
solution to waveform diversity for integrated radar and communication systems.
The hybridization of FDA and OFDM chirp waveform can be
interpreted to simultaneously transmit distinct carriers representing a communication information symbol through an FDA antenna towards a desired range-angle position (r, θ). As illustrated
MAY - JUNE 2018

Similar to conventional OFDM communications, cyclic prefix
with a length longer than the desired maximum radar return time
delay should be adopted. For instance, if D range cell paths are
superposed together in the returned signals, the guard interval
length Tg in the analog transmission signal should be Tg = (D
− 1) Ts with Ts being the sampling interval at the receiver. Then
the total OFDM symbol duration should be Tw = T + Tg. A partial cyclic repetition of the time domain signal of the duration
Tg can be used for the cyclic prefix, which is typically at the beginning of the symbol, occupying the interval [0, Tg) within the
total symbol duration Tw. In standard OFDM radar applications,
the transmitted and returned signals are usually separated in time
domain due to the fact that in a monostatic case the transmitter and receiver share the same antenna and cannot transmit and
receive simultaneously. This implies that a reasonable receiving
window is needed between two consecutive pulses. In order to
achieve high transmission throughput for the communications,
the communication-embedded OFDM chirp pulses are transmitted consecutively. Accordingly, separated transmit and receive
antennas are assumed, so that the system can transmit and receive
signals simultaneously.

IEEE A&E SYSTEMS MAGAZINE

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