OFDM-based Digital Array Radar with Frequency Domain Mode Multiplexing John P. Stralka Northrop Grumman Electronic Systems Baltimore, MD USA john.stralka@ngc.com Abstract—Improvements in RF and digital technology have made digital array radar (DAR) feasible. The combination of orthogonal frequency-division multiplexing (OFDM) as a wideband pulse compression modulation with a DAR architecture allows time dispersion effects to be mitigated for electrically-long antenna arrays. This concept can be extended to the simultaneous operation of multiple radar modes. Each mode is allocated some number of OFDM subcarriers. The subcarriers corresponding to a particular mode are then phase- shifted to create an element-to-element phase shift across the antenna array to steer the full-aperture antenna beam for that particular mode. This concept multiplexes the modes in the frequency domain while the OFDM-based DAR allows each mode to experience the full-aperture gain on transmit and receive while being electronically steered to an independent spatial position. I. INTRODUCTION Modern radars must meet challenging requirements. Desired radar system features include true simultaneous modes, transmit spectrum flexibility, mode-dependent variable range resolution, electronic protection (EP), and the ability to operate as a radar and a communications node. To fulfill these needs, researchers have proposed the use multicarrier modulations, such as orthogonal frequency-division multiplexing (OFDM), for radar [1, 2]. OFDM is a wideband, multicarrier digital communications modulation technique in which high-rate data is transmitted in parallel at a slower rate over a frequency-selective channel via multiple closely-spaced narrowband orthogonal subcarriers [3]. With a large number of OFDM subcarriers, each frequency segment, or subband, in the signal spectrum spanned by a single modulated subcarrier can be viewed as being a frequency-flat, as opposed to a frequency-selective, channel [4]. Time and frequency synchronization is crucial in OFDM communications systems to preserve subcarrier orthogonality. For radar, however, sensitivity to synchronization is beneficial since the radar receiver uses a stored version of the transmit signal to measure time-delay (range) and frequency (Doppler) offsets between the transmitted signal and the received echo to derive target parameters [1]. The unique multicarrier nature of OFDM allows solutions to radar problems that are frequency-selective in nature. In addition, more degrees of freedom are available to create diverse waveform sets with favorable correlation properties. Improvements in RF and digital technology have made digital array radar (DAR) feasible [5]. In a DAR each antenna array element (or subarray) has a digital-to-analog converter (DAC) for transmit and an analog-to-digital converter (ADC) for receive. Such architectures allow signals to be both digitally distributed to, and combined from, the array elements (subarrays). The combination of OFDM with DAR allows unique capabilities. Research conducted on OFDM-based radar signals resulted in a software-defined radar architecture that is capable of wideband OFDM and Costas coded waveforms, along with conventional single-carrier phase-coded waveforms [2]. This architecture was extended to create a wideband digital phased array architecture that is cable of transmitting and receiving wideband pseudo-randomly coded pulses with minimal dispersion effects [6]. As previously mentioned, a desired radar system feature is the ability to operate with true simultaneous modes. An example would be doing both search and dedicated track dwells simultaneously. This capability would maintain a desired search frame time while providing the necessary accuracy and revisit rates on time-critical targets in track. Previous designs achieved near simultaneous modes, by time interleaving modes down to a coherent processing interval (CPI). A radar with a subapertured phased array antenna is capable of true simultaneous modes if a subset of the subapertures is dedicated to a particular mode, while the other subapertures are dedicated to another. Each mode’s transmit beam could conceivably be electronically steered to a different angular direction, or line-of-sight (LOS). The problem with this approach is that each mode only uses a portion of the antenna resulting in reduced antenna directivity. An OFDM-based DAR can mitigate this dilemma by frequency-multiplexing waveforms for true simultaneous