78 research outputs found
Soft Decoding Techniques for Quantum Key Distribution (QKD) and Weak Energy Optical Communication
No full textThe focus of this research activity is to work on pragmatic information reconciliation applied to QKD schemes based on single photon or weak pulse laser (WPL) sources, so as to use feed-forward techniques which minimize the interaction between transmitter and receiver. The core ideas of the thesis are employing Forward Error Correction (FEC) coding as opposed to two-way communication for information reconciliation in QKD schemes, exploiting all the available information for data processing at the receiver including information available from the quantum channel, since optimized use of this information can lead to significant performance improvement, and providing a security versus secret-key rate trade-off to the end-user within the context of QKD systems. Moreover, as shown by accurate experimental studies, the communication channel used for quantum key exchange is not able to reach high levels of reliability (the Quantum Bit Error Rate -QBER may have a high value), both because of the inherent characteristics of the system, and of the presence of a possible attacker. In order to obtain acceptable residual error rates, it is necessary to use a parallel classical and public channel, characterized by high transmission rates and low error rates, on which to transmit only the redundancy bits of systematic channel codes with performance possibly close to the capacity limit. Furthermore, since the more redundancy is added by the channel code, the more the corresponding information can be used to decipher the private message itself, it becomes necessary to design high-rate codes obtained by puncturing a low-rate mother code, possibly achieving a redundancy such that elements of the secret message cannot be uniquely determined from the redundancy itself, so for that purpose we designed high rate LDPC codes. Using high rate codes increases the security with trade-off to performance. Other low photon number applications have also been considered, such as weak-laser pulses (WLP) communication. For that purpose, a low-complexity photon-counting receiver has been considered which may be employed in long-distance amplification-free classical optical communication schemes, and which is typically modeled as an equivalent Binary Symmetric Channel (BSC). We have developed a time varying Binary Input-Multiple Output (BIMO) channel model for this low-complexity photon-counting receiver, and analyzed its performance in presence of soft-metric based capacity approaching iteratively decoded error correcting codes, such as soft-metric based Low Density Parity Check (LDPC) codes and polar codes. We show that the classical channel capacity of the suggested BIMO model is higher than the capacity of the BSC model, and that the use of the BIMO model allows to feed the channel decoder with soft information, in the form of Log-Likelihood Ratios (LLRs), achieving a significant reduction in Bit Error Rate (BER) and Frame Error Rate (FER) with respect to classical hard-metric-based schemes which should be used in conjunction with a BSC channel mode
FEC coding for QKD at higher photon flux levels based on spatial entanglement of twin beams in PDC
No full textwin beams generated by Parametric Down Conversion (PDC) exhibit quantum correlations that have been effectively used for calibration of single photon detectors and Charge Coupled Device (CCD) cameras [1]. The natural setup of quantization of CCD detection area and measurement of the correlation statistic needed to detect the presence of the eavesdropper Eve leads to a set of QKD parallel channel models that are non-binary Discrete Memoryless Channels (DMC). This work explores Forward Error Correction (FEC) coding for information reconciliation over the resulting parallel DMCs. [1] I. P. Degiovanni M. Genovese M.Gramegna A. Avella, G. Brida and P. Traina. Phys. Rev. A, 82:062309, 2010
LDPC Coding for QKD at Higher Photon Flux Levels Based on Spatial Entanglement of Twin beams in PDC
No full textTwin beams generated by Parametric Down Conversion (PDC) exhibit quantum correlations that has been effectively used as a tool for many applications including calibration of single photon detectors. By now, detection of multi-mode spatial correlations is a mature field and in principle, only depends on the transmission and detection efficiency of the devices and the channel. In [2, 4, 5], the authors utilized their know-how on almost perfect selection of modes of pairwise correlated entangled beams and the optimization of the noise reduction to below the shot-noise level, for absolute calibration of Charge Coupled Device (CCD) cameras. The same basic principle is currently being considered by the same authors for possible use in Quantum Key Distribution (QKD) [3, 1]. The main advantage in such an approach would be the ability to work with much higher photon fluxes than that of a single photon regime that is theoretically required for discrete variable QKD applications (in practice, very weak laser pulses with mean photon count below one are used).The natural setup of quantization of CCD detection area and subsequent measurement of the correlation statistic needed to detect the presence of the eavesdropper Eve, leads to a QKD channel model that is a Discrete Memoryless Channel (DMC) with a number of inputs and outputs that can be more than two (i.e., the channel is a multi-level DMC). This paper investigates the use of Low Density Parity Check (LDPC) codes for information reconciliation on the effective parallel channels associated with the multi-level DMC. The performance of such codes are shown to be close to the theoretical limit
Capacity approaching codes for photon counting receivers
No full textIn [1] a low-complexity photon-counting receiver has been presented, which may be employed for weak-energy optical communications and which is typically modeled through its equivalent Binary Symmetric Channel (BSC) model. In this paper we consider the scheme described in [1], we model it as a time varying Binary Input-Multiple Output (BIMO) channel and analyze its performance in presence of soft-metric based capacity approaching iteratively decoded error correcting codes, and in particular using soft-metric based Low Density Parity Check (LDPC) codes. To take full advantage of such detector, soft information is generated in the form of Log-Likelihood Ratios (LLRs), achieving reduction in Bit Error Rate (BER) and Frame Error Rate (FER) with respect to classical BSC and Additive White Gaussian Noise (AWGN) channel models. Furthermore, we explore the limits of the achievable performance gains when using photon counting detectors as compared to the case when such detectors are not available. To this end, we find the classical capacity of the considered BIMO channel, clearly showing the potential gains that photon counting detectors can provide in the context of a realistic cost-effective scheme from an implementation point of view. Furthermore, we show that from a channel modeling point of view, we can observe that the BIMO channel can be approximated with an AWGN channel for high values of mean photon count Nc, while the AWGN model offers an unreliable result with a low mean photon number Nc, (i.e. with low raw BER). This effect is more evident with lower coding rate
Soft-metric based decoding for photon counting receivers
No full textIn [1] a low-complexity photon-counting receiver has been presented, which may be employed in long-distance amplification-free classical optical communication schemes, and which is typically modeled through its equivalent Binary Symmetric Channel (BSC) model. In this paper, we consider the scheme described in [1], but we model it as a time varying Binary Input-Multiple Output (BIMO) channel, and analyze its performance in presence of soft-metric based capacity approaching error correcting codes. We show that the classical channel capacity of the suggested BIMO model is higher than the capacity of the BSC model, and that the use of the BIMO model allows to feed the channel decoder with soft information, in the form of Log-Likelihood Ratios (LLRs), achieving a significant reduction in Bit Error Rate (BER) and Frame Error Rate (FER) with respect to classical hard-metric-based schemes which should be used in conjunction with a BSC channel mode
Classical Capacity of a Bayesian Inference Quantum Channel Employing Photon Counting Detectors
No full textRecently we investigated the potential improvements in key transmission rate in a Quantum Key Distribution (QKD) scheme whereby photon-counting detectors are used at the receiver. To take full advantage of such detectors, soft information is generated in the form of Log-Likelihood Ratios (LLRs) using a Bayesian estimator of phase of the signal pulse which is used to carry the information. We achieved significant reduction in the residual Bit Error Rate (BER) and Frame Error Rate (FER) using LDPC codes in the information reconciliation process. In this paper we explore the limits of the achievable performance gains when using photon counting detectors as compared to the case when such detectors are not available. To this end, we find the classical capacity of the Bayesian inference channel clearly showing the potential gains that photon counting detectors can provide in the context of a realistic cost-effective scheme from an implementation point of view. While there are binary communication schemes that can achieve a higher capacity for a given mean photon count at the receiver compared to the scheme presented here (e.g., the Dolinar receiver), most such schemes are complex and at times unrealistic from an implementation point of vie
Thermal characterisation analysis and modelling techniques for CubeSat-sized spacecrafts
No full textSimulation and Complexity Analysis of Iterative Interference Cancellation Receivers for LTE/LTE-Advanced
No full textThe paper details the simulation of a single user MIMO receiver operating according to the 3GPP/LTE standard applying a Parallel or Successive Interference Cancellation (PIC/SIC) strategy to a multicarrier (OFDMA/SC-FDMA) scheme. The algorithm details are analyzed and the PIC and SIC cancellation strategies are simulated and compared on random MIMO selective fading channels, considering limited complexities. The best PIC and SIC schemes for a given limited complexity (8 turbo decoding iterations per codeword) are compared for different codeblock lengths and spatial correlation scenarios over an EPA channel model. The 2 cycles SIC scheme shows the best performance over the selected scenarios, offering gains over the non-iterative schemes (measured at BLER values of 0.1) ranging from 1 to 4 dB in the considered cases. Larger gains are obtained with higher spatial correlation and shorter codeblock lengths. Better overall performance are obtained with lower spatial correlation and longer codeblock length
Capacity-approaching Channel Codes for Discrete Variable Quantum Key Distribution (QKD) Applications
No full textSecure communications and cryptography is as old as civilization itself. The Greek Spartans for instance would cipher their military messages and, for Chinese, just the act of writing the message constituted a secret message since almost no-one could read or write Chinese. Modern public key Cryptography until the mid 1980's was founded on computational complexity of certain trap-door one-way functions that are easy to compute in one direction, but very difficult in the opposite direction. To a large extent computational complexity is still the lynchpin of modern cryptography, but the whole paradigm was revolutionized by introduction of Quantum Key Distribution (QKD) which is founded on fundamental laws of Physics. Indeed, to date, QKD is de-facto the most successful branch of Quantum Information Science (QIS) encompassing such areas as quantum computing which is still in its infancy. Modern QKD is fundamentally composed of a series of three steps that shall be explained later in the chapter: 1) data transmission over the error-prone quantum channel; 2) information reconciliation to allow the parties engaged in communication to have two identical copies of a message that may not be as secure as desired; and 3) privacy amplification that ensures the parties possess copies of messages about which the information that could have possibly be gleaned by the eavesdropper is below a desirable threshold. It is this sufficiently private and often much shorter message that can be used as the secret key to allow exchange of longer messages between the legitimate parties. Step-1 must be based on the laws of quantum physics, whereas step-2 and -3 either necessitate the use of quantum error correcting codes which are often complex or as is often done in practice, based on information exchange over a classical public channel. Objective of this chapter is to give a tutorial presentation and evaluation of QKD protocols at the systems level based on classical error-correcting codes. The QKD systems can provide perfect security (from the viewpoint of information theory) in the distribution of a cryptographic key. QKD systems and related protocols, under particular conditions, can use the classic channel coding techniques instead of quantum error-correcting codes, both for correcting errors that occurred during the exchange of a cryptographic key between two authorized users, and to allow privacy amplification, in order to make completely vain a possible intruder attempt. The secret key is transmitted over a quantum, and thus safe channel, characterized by very low transmission rates and high error rates. This channel is safe given the properties of a quantum system, where each measurement on the system perturbs the system itself, allowing the authorized users to detect the presence of any intruder. Moreover, as shown by accurate experimental studies, the communication channel used for quantum key exchange is not able to reach high levels of reliability (the Quantum Bit Error Rate - QBER - may have a high value), both because of the inherent characteristics of the system, and of the presence of a possible attacker. In order to obtain acceptable residual error rates, it is necessary to use a parallel classical and public channel, characterized by high transmission rates and low error rates, on which to transmit only the redundancy bits of systematic channel codes with performance possibly close to the capacity limit. Furthermore, since the more redundancy is added by the channel code, the more the corresponding information can be used to decipher the private message itself, it becomes necessary to design high-rate codes obtained by puncturing a low-rate mother code, possibly achieving a redundancy such that elements of the secret message cannot be uniquely determined from the redundancy itsel
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