Coherent Detection Receiver Research Articles

Detection of laser light scattered from aerosols in a bright background using a balanced coherent receiver

We experimentally demonstrate the detection of laser light, which has been scattered from micron-scale atmospheric particulates, using a balanced heterodyne detection system with a non-cooperative tunable laser as a local oscillator source. The signal generated by the coherent detection receiver is provided to a signal processing algorithm designed to discriminate scattered laser light entering the receiver from the incoherent background and internal receiver noise. A receiver operating characteristic quantifies the performance of the receiver and demonstrates that optical coherence can be used as a parameter for laser detection even after the process of optical scattering from a disordered media.

Brillouin Amplification for Enhanced Coherent Communication Applications

Brillouin amplification and its benefit for coherent communication applications with 64-QAM signals is evaluated. This utilizes the narrow gain bandwidth in the tens of megahertz range to enhance the carrier to noise ratio (CNR) of noisy spectral lines applied to either signal carriers at the transmitter before data modulation or pilot tones at the receiver for coherent detection. This expands on the projected capability of 30 dB gain enabling up to ≈24 dB enhancement of CNR and the same reduction of the minimum tolerable pilot to signal power ratio (PSR) for maintaining the same performance than without in case of 48 Gb/s 64-QAM signals. The limit being the narrowness of the Brillouin gain bandwidth (≈30 MHz). Here, the tolerance to larger Brillouin noise and its avoidance with reduced gain of ≈25 dB permitting wider detuning from optimum of the pump frequency and its state of polarization with reduced power oscillations are shown. Extended analysis of carrier recovery for 48 Gb/s 64-QAM signals also indicate a capable low PSR near ≈−40 dB for a bit error rate below the hard decision FEC threshold. This includes with 80 km link transmission and its induced nonlinear signal distortion. Overall, rather than an impediment, Brillouin amplification shows significant performance benefit for wide ranging conditions.

An Ultraefficient Broadband Photonic Channelizer Based on Polarization-Division Multiplexing and Integrated Dual-Polarization Coherent Detection Receiver

We propose and experimentally demonstrate an ultraefficient broadband photonic channelizer. By polarization-division multiplexing the received signal and using dual-output Hartley image-reject receiving topology, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4N$ </tex-math></inline-formula> channels can be generated with only ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N +1$ </tex-math></inline-formula> )-line signal optical frequency comb (OFC) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> -line local oscillator (LO) OFC, and the frequency interval of the OFC is reduced to half of the typical value. With the use of the integrated dual-polarization coherent detection receiver (DP-CDR), the required quadrature optical hybrid (QOH) and the balanced photodetectors (BPDs) are reduced to half. The system size is reduced and stability is improved. Besides, assisted with a digital signal processor (DSP) algorithm, the IQ imbalance can be compensated, and approximately, ideal image rejection can be realized. A proof-of-concept experiment is carried out. A 2–14-GHz wideband signal is successfully divided into 12 channels with 1-GHz bandwidth by applying four-line signal OFC and three-line LO OFC. The image rejection ratio (IRR), channel isolation (CI), and spurious-free dynamic range (SFDR) are also measured and discussed.

Toward Practical Entanglement-Based Satellite FSO/QKD Systems Using Dual-Threshold/ Direct Detection

We propose a new implementation of the BBM92 protocol for satellite continuous-variable quantum key distribution (CV-QKD) systems using the dual-threshold/direct detection (DT/DD) scheme. The proposed system is less complex and therefore possibly cheaper than current discrete-variable (DV) and CV-QKD ones using coherent detection receivers. We model and analyze the performance of the proposed system in the context that a satellite distributes secret keys to two legitimate users. The analytical results are derived by considering channel loss, atmospheric turbulence-induced fading, and receiver noises. The Gaussian beam model is considered to evaluate the impact of geometric spread on the signal received by legitimate users and the possibility of being eavesdropped. Based on the design criteria for the system and analytical results, we investigate the feasibility of a case study for the Japan QKD network using the existing low-Earth orbit (LEO) satellite constellation.

Real-Time Demonstration of Homodyne Coherent Bidirectional Transmission for Next-Generation Data Center Interconnects

We experimentally propose and demonstrate homodyne coherent detection in a short-distance data center interconnect with bidirectional fiber transmission. The transmitter in the proposed system sends the modulated signal and a continuous wave (CW) tone originating from the same laser diode (LD) over the 2 lanes of a duplex fiber to the remote receiver for coherent detection. Thus, this system allows the use of an uncooled LD with a large linewidth and reduces the complexity of digital signal processing (DSP) compared to that in a classical coherent system. A successful real-time demonstration with 600-Gb/s DP-64QAM using uncooled large-linewidth DFB lasers is conducted. Automatic stabilization against polarization fluctuations of the transmitted tone is achieved by the proposed polarization-tracking integrated coherent receiver in compact silicon photonics (SiP), tracking up to 300 rad/s without a performance penalty. The phase noise caused by mismatch and the link loss budget limitations are discussed and analyzed. This study shows that the proposed system is a potentially attractive solution for future 800G and 1.6T intra-data-center optical interconnects.

Enhanced Carrier to Noise Ratio by Brillouin Amplification for Optical Communications

The limits of narrowband Brillouin gain for improving the carrier to noise ratio (CNR) of spectral lines is evaluated for 64-QAM communications. Calculations of the change in CNR for practical gain, noise, and bandwidth parameters indicate the maximum obtainable CNR enhancement factor and output CNR as being a25 dB and a40∼43 dB/0.1 nm, respectively, limited by the narrowness of the Brillouin gain bandwidth and Brillouin amplifier noise, respectively. These limits determined from the relative power suppression of out of band noise contained within a 10 GHz channel bandwidth are relevant to spectral line amplification either before data modulation at the transmitter, or for a local oscillator (LO) at the receiver for coherent detection. A calibration measurement of the bit error rate (BER) of a 48 Gb/s 64-QAM signal versus CNR of a LO loaded with ASE noise is used to extrapolate the BER from the calculated CNR for Brillouin gain. The results predict the BER being minimally degraded for an input CNR of a>20 dB/0.1 nm and falling below the hard decision FEC limit for an input CNR as low as a0 dB/0.1 nm. The estimates are found consistent with 64-QAM signal experiments of pilot tone carrier recovery by Brillouin gain for both single channel and WDM cases. The insight aids the design and estimation of achievable performance improvement for applying Brillouin amplifiers to coherent communications.

Non-Coherent NOMA With Massive MIMO

By accommodating overlapping transmissions over non-orthogonal resources, non-orthogonal multiple access (NOMA) integrated with massive multiple-input multiple-output (MIMO) can boost spectral efficiency and obtain vast diversity for wireless multiuser systems. However, NOMA requires channel state information (CSI) at the receiver for coherent detection, resulting in unaffordable complexity in the channel estimation as the number of users goes large. Add to that, the introduction of massive MIMO makes the channel estimation even more challenging. In this letter, we propose a blind belief propagation (BP) detection for non-coherent NOMA with massive MIMO, where the transmitter of each user first performs differential modulation on PSK symbols, and then spreads its symbols using low-density spreading (LDS); the receiver of base station (BS) employs differential demodulation and then detects all users’ symbols using a blind BP detection without knowledge of CSI. Simulation results show that the proposed blind BP detection can not only support the overloaded users, but also achieve BER performance very close to that of point-to-point systems.

Self-homodyne system for next generation intra-datacenter optical interconnects.

We propose a self-homodyne system for next generation intra-datacenter networking. The proposed system has a higher spectral efficiency for the modulated signal compared to the intensity-modulation/direct-detection (IM/DD) systems and uses digital signal processing of reduced complexity compared to a conventional coherent system. The concept of the proposed system is to send the modulated signal and a tone originating from the same laser over the full-duplex fiber with the aid of circulators to be used remotely at the receiver for coherent detection. The overall system physical complexity approaches the equivalent IM/DD system giving the same target data rate for 400G systems and beyond. We experimentally demonstrate emulation of the proposed system and report data rates of 530 Gb/s, 448 Gb/s and 320 Gb/s on a single wavelength below the KP4 forward error correcting threshold over 500 m, 2 km and 10 km of single mode fiber, respectively.

Pilot Symbol Aided Channel Estimation for Spatial Modulation-OFDM Systems and its Performance Analysis with Different Types of Interpolations

Spatial modulation orthogonal frequency division multiplexing (SM-OFDM) system has been recently proposed as an alternative for multiple-input multiple-output (MIMO)-OFDM systems to increase spectral efficiency by keeping a low-complexity implementation. In the literature, SM-OFDM systems assume a perfect channel state information (P-CSI) available at the receiver for coherent detection and the channel estimation problem is not considered. It is clear that the channel estimation is a critical issue on the performance of SM-OFDM systems. In this paper, a frame structure where pilot symbols are typically interspersed with data symbols among the sub-carriers to aid the channel estimation is considered. Then the pilot symbol aided channel estimation (PSA-CE) technique with different interpolations is proposed for the SM-OFDM systems. It is shown that in the proposed PSA-CE, equidistantly spaced pilot symbols allow to reconstruct the channel response by means of interpolation if the spacing of the pilots is sufficiently close to satisfy the sampling theorem. Linear, nearest neighbor, piecewise cubic Hermite and the low-pass interpolations are investigated to explore trade-off between complexity and performances. We show that the low pass interpolator yields better performance than the other interpolation techniques for selected cases such as higher order modulations. The results validate the potential of the proposed PSA-CE estimation applying to SM-OFDM systems.

Novel MDM-PON scheme utilizing self-homodyne detection for high-speed/capacity access networks.

In this paper, we propose a cost-effective, energy-saving mode-division-multiplexing passive optical network (MDM-PON) scheme utilizing self-homodyne detection for high-speed/capacity access network based on low modal-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX). In the proposed scheme, one of the spatial modes is used to transmit a portion of signal carrier (namely pilot-tone) as the local oscillator (LO), while the others are used for signal-bearing channels. At the receiver, the pilot-tone and the signal can be separated without strong crosstalk and sent to the receiver for coherent detection. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Meanwhile, the self-homodyne detection scheme can effectively suppress laser phase noise, which relaxes the requirement for the lasers line-width at the optical line terminal or optical network units (OLT/ONUs). The digital signal processing (DSP) at the receiver is also simplified since it removes the need for frequency offset compensation and complex phase correction, which reduces the computational complexity and energy consumption. Polarization division multiplexing (PDM) that offers doubled SE is also supported by the scheme. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. Utilizing the proposed scheme, we demonstrate a proof of concept 4 × 40-Gb/s orthogonal frequency division multiplexing (OFDM) transmission over 55-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. Compared with back to back case, less than 1-dB Q-factor penalty is observed after 55-km FMF of the four channels. Signal power and pilot-tone power are also optimized to achieve the optimal transmission performance.

Advanced modulation formats for short reach optical communication systems

The use of advanced modulation formats helps to reduce the number of components required for short reach optical communication modules at 100 Gb/s and beyond. Unlike long haul optical communication systems where externally modulated high order modulation formats with coherent detection receivers are used, direct modulated lasers with direct detection are likely to be employed for short reach system implementation. Here we discuss the performance of pulse amplitude modulation, carrierless amplitude/phase modulation, and direct detection orthogonal frequency- division multiplexing for short reach optical communication systems. Their possible use for future high-capacity short reach optical communication systems is explored.

Digital coherent receiver for orbital angular momentum demultiplexing

We put forward a type of receiver for coherent detection of the photon orbital angular momentum (OAM). A coherent array receiver, consisting of multiple subapertures, with each subaperture coupled to a single-mode fiber, maps the complex optical field in the image plane. Using digital samplers connected to each array element, the local electrical signals resulting from the detection process can be measured coherently, moving the complexity of the full OAM measurement from the optical domain to the digital domain. By computer processing the coherent electrical patterns obtained, one can retrieve full information (amplitude and phase) of the different OAM components that constitute any incoming beam.

Image Transmission over Space Time Coded MIMO-OFDM System with Punctured Turbo Codes

robust progressive compressed image using Punctured Turbo Codes (PTC) is considered while the image is transmitting over Space Time Coded (STC) Orthogonal Frequency Division Multiplexed (OFDM) systems. The Set Partitioning in Hierarchical Trees (SPIHT) technique is used regarding to image coding and compression. SPIHT compression technique fits to be best in this scheme for quality improvement. The BER performance of the PTC over Gaussian noisy channel is first evaluated by assuming perfect channel state information at the receiver for coherent detection. Then this PTC applied to end to end system over wireless fading channel. In simulation results, BER performance of PTC SPIHT system evaluated and Punctured Convolutional Coded (PCC) scheme compared with PTC scheme. Generally, Turbo codes stands to combat the interference of the fading channel. Thus, the simulation results show the effectiveness of our proposed practical image transmission system.

Improved Sparse Channel Estimation for Cooperative Communication Systems

Accurate channel state information (CSI) is necessary at receiver for coherent detection in amplify-and-forward (AF) cooperative communication systems. To estimate the channel, traditional methods, that is, least squares (LS) and least absolute shrinkage and selection operator (LASSO), are based on assumptions of either dense channel or global sparse channel. However, LS-based linear method neglects the inherent sparse structure information while LASSO-based sparse channel method cannot take full advantage of the prior information. Based on the partial sparse assumption of the cooperative channel model, we propose an improved channel estimation method with partial sparse constraint. At first, by using sparse decomposition theory, channel estimation is formulated as a compressive sensing problem. Secondly, the cooperative channel is reconstructed by LASSO with partial sparse constraint. Finally, numerical simulations are carried out to confirm the superiority of proposed methods over global sparse channel estimation methods.

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

We present an optimum diversity receiver called blind ratio combining (BRC), which minimizes the average symbol error probability or maximizes the average output signal-to-noise ratio, where the channels' time delays and the random phases are known while the fading amplitudes are unknown. In contrast to previous works where efforts were made to find a posteriori probabilities at the receiver, the BRC simply calculates the optimum weights, which depend on the channel's statistics, avoiding continuous channel estimation, and thus, it significantly reduces the system's complexity. In nonidentical multipath fading channels with power delay profile (PDP), the BRC receiver performs between maximal ratio combining (MRC) and equal gain combining (EGC), and attains to keep its performance comparable—and in some cases superior—to that of generalized selection combining (GSC), while for large values of the decay factor it approaches MRC. Moreover, in the important practical case of exponential PDP—common in RAKE receivers modeling and adopted for the Universal Mobile Telecommunications System (UMTS) spatial channel modeling by the European Telecommunications Standards Institute-Third Generation Partnership Project (ETSI-3GPP)—the optimum weights can be accurately approximated by simple elementary functions. Furthermore, it is proved that the utilization of these weights ensures an error performance improvement over EGC for arbitrary PDPs. The proposed BRC receiver can be efficiently applied in wireless wideband communication systems, where a large number of diversity branches exists, due to the strong multipath effects.

Progressive image transmission over space-time coded OFDM-based MIMO systems with adaptive modulation

This paper considers a progressive image transmission system over wireless channels by combining joint source-channel coding (JSCC), space-time coding, and orthogonal frequency division multiplexing (OFDM). The BER performance of the space-time coded OFDM-based MIMO system based on a newly built broadband MIMO fading model is first evaluated by assuming perfect channel state information at the receiver for coherent detection. Then, for a given average SNR (hence, BER), a fast local search algorithm is applied to optimize the unequal error protection design in JSCC, subjected to fixed total transmitted energy for various constellation sizes. This design allows the measurement of the expected reconstructed image quality. With this end-to-end system performance evaluation, an adaptive modulation scheme is proposed to pick the constellation size that offers the best reconstructed image quality for each average SNR. Simulation results of practical image transmissions confirm the effectiveness of our proposed adaptive modulation scheme

Performance analysis of optimum receivers for differentially encoded M-PSK in low SNR

A recent paper has proved that the classical receiver for coherent detection of differentially encoded M-PSK in AWGN is optimum for the MAP sequence detection criterion. In this letter, we show that asymptotically, as SNR tends to zero, the MAP symbol detection criterion receiver is equivalent to the classical differentially coherent receiver, for M greater than two. An asymptotic relative efficiency figure of merit is defined in order to compare the performances of the classical coherent receiver and the classical differentially coherent receiver.

Amplifier arrays for CMB polarization

Cryogenic low noise amplifier technology has been successfully used in the study of the cosmic microwave background (CMB). Monolithic millimeter wave integrated circuit (MMIC) technology makes the mass production of coherent detection receivers feasible. We have produced large numbers of MMIC amplifiers for CMB measurements. We have also demonstrated the viability of multi-function multi-chip modules as sensitive receiver front-ends. MMIC integration makes it possible to realize massive arrays of receivers suitable for measurements of the polarization of the CMB. We describe the development of the unit cell of such an array and the development plans for implementation.

Optical Communication in Space Applications

In this paper, importance of optical communication in space applications has been highlighted. Its merits and demerits over the conventional microwave system have been described. Use of an optical preamplifier in direct detection and coherent detection receivers has been discussed. Status of some of the ongoing/future space communication projects has been given.