Chirp waveform plays a significant role in radar and communication systems for its ability of pulse compression and spread spectrum. This paper presents a principle of multiplexing a bank of orthogonal chirps, termed orthogonal chirp-division multiplexing (OCDM) for high-speed communication. As Fourier transform is the kernel of orthogonal frequency-division multiplexing (OFDM), which achieves the maximum spectral efficiency (SE) of frequency-division multiplexing, Fresnel transform underlies the proposed OCDM system, which achieves the maximum SE of chirp spread spectrum. By using discrete Fresnel transform, digital implementation of OCDM is introduced. According to the properties of Fresnel transform, the transmission of OCDM signal in linear time-invariant channel is studied. Efficient digital signal processing is proposed for channel dispersion compensation. The implementation of the OCDM system is discussed with the emphasis on its compatibility to the OFDM system; it is shown that it can be easily integrated into the existing OFDM systems. Finally, the simulations are provided to validate the feasibility of the proposed OCDM. It is shown that the OCDM system can efficiently exploit multipath diversity and thus outperforms the OFDM, and that it is more resilient against the interference due to insufficient guard interval than single-carrier frequency-domain equalization.

In this paper, we propose an orthogonal chirp division multiplexing (OCDM) technique for coherent optical communication. OCDM is the principle of orthogonally multiplexing a group of linear chirped waveforms for high-speed data communication, achieving the maximum spectral efficiency (SE) for chirp spread spectrum, in a similar way as the orthogonal frequency division multiplexing (OFDM) does for frequency division multiplexing. In the coherent optical (CO)-OCDM, Fresnel transform formulates the synthesis of the orthogonal chirps; discrete Fresnel transform (DFnT) realizes the CO-OCDM in the digital domain. As both the Fresnel and Fourier transforms are trigonometric transforms, the CO-OCDM can be easily integrated into the existing CO-OFDM systems. Analyses and numerical results are provided to investigate the transmission of CO-OCDM signals over optical fibers. Moreover, experiments of 36-Gbit/s CO-OCDM signal are carried out to validate the feasibility and confirm the analyses. It is shown that the CO-OCDM can effectively compensate the dispersion and is more resilient to fading and noise impairment than OFDM.

We show, for the first time, dense WDM $(8\times 20\mathrm{Gbit}/\mathrm{s})$ transmission at 2 μm enabled by advanced modulation formats (4-ASK Fast-OFDM) and the development of key components, including a new arrayed waveguide grating (AWGr) at 2 μm. The AWGr shows -$12.8\pm 1.78\mathrm{dB}$ of excess loss with an 18-dB extinction ratio and a thermal tunability of 0.108 nm/°C.

Orthogonal chirp-division multiplexing (OCDM) was recently proposed as a promising modulation technique for high-speed fiber-optic communications due to its resilience to transmis-sion impairments. By exploiting the pulse-compression property of the chirped waveforms, in this paper we propose a channel estima-tion algorithm compatible with the coherent optical (CO) OCDM system. In the proposed algorithm, a chirp from the orthogonal basis of the Fresnel transform is employed as the pilot signal. At the receiver, noise-rejection windowing is adopted in the Fresnel domain to remove the excessive noise after pulse compression, and then dispersions can be effectively compensated by efficient single-tap equalizers. It is shown that the proposed estimator is unbiased as long as the width of the noise-rejection window is wider than the maximum excess delay of the system. Moreover, the optimal win-dow function achieving the minimum mean square error is derived in closed form. With these desired properties, it is shown that the proposed estimator outperforms conventional estimators, such as the intra-symbol frequency-domain average estimator, especially when the dispersion is severe. Finally, numerical results are pro-vided to confirm the advantages of the CO-OCDM system with the proposed channel estimation algorithm.

Channel cannot be compensated by single-tap equalizers in fast orthogonal frequency-division multiplexing (OFDM) easily unless the channel impulse response (CIR) is symmetric. In this letter, we propose a low-complexity fast OFDM scheme, which enables efficient single-tap equalization for generic linear channels without the symmetric limitation. Simulations under both wireless multipath fading and multimode fiber channels are provided to verify the feasibility. Compared with other schemes, the proposed technique is simpler and avoids net data rate loss. It is shown that this scheme exhibits better performance than the frequency-domain-equalization-based fast OFDM scheme under the wireless frequency-selective fading channel.

In multiple-input multiple-output (MIMO) systems, channel estimation is of crucial importance to guarantee reliable recovery of ultra-high-speed MIMO signals. This paper proposes a novel channel estimation algorithm for the emerging MIMO-based orthogonal chirp-division multiplexing (OCDM) systems by utilizing the unique features of OCDM signals. In the proposed algorithm, a set of pilot signals is designed based on the Fresnel basis, which is essentially a family of orthogonal linear chirps. The pilots are assigned to different antennas for transmission occupying the same time slot and bandwidth. According to the convolution-preservation theorem of the Fresnel transforms, the transfer matrices of MIMO-OCDM systems can be readily estimated at the receiver without any inter-antenna interference, even if the pilots overlap in both the time and frequency domains. The proposed algorithm avoids bandwidth waste in conventional channel estimators, in which silent pilots will be required in time and/or frequency to ensure the received MIMO pilots separable. We show that the proposed algorithm is unbiased for the unique OCDM pilots and has better estimate accuracy and system performance. Finally, analysis and numerical results are provided to validate its advantages as a promising algorithm for emerging wireless access technology based on MIMO-OCDM.