Pairwise Coding Research Articles

Polarization Multiplexing Based UOWC Systems Under Bubble Turbulence

High-speed and turbulence-tolerant transmission links are required to meet the growing demand for underwater optical wireless communication (UOWC). However, most previous works were implemented in statically simulated underwater channels with uniform temperature and salinity gradients irrespective of bubble motion. This paper presents an experimental demonstration of a polarization multiplexing (PolMux) based UOWC system considering bubble-induced turbulence. A theoretical transmission model is built and experiments are carried out to investigate the impact of bubble turbulence on PolMux transmission. The general rules of bubble impairments are summarized by manipulating different bubble flow rates, positions and bubble-effected region thicknesses. Moreover, to mitigate these effects, subchannel pairwise coding is implemented to address the subchannel signal-to-noise-ratio (SNR) imbalance issue, and subcarrier pairwise coding is utilized to overcome the subcarrier SNR imbalance, reaching a sum data rate of 8.58 Gbps. This work evaluates the performance of PolMux-UOWC links in the presence of air bubbles and demonstrates the feasibility of pairwise coding to achieve high-speed transmission in UOWC systems with bubble turbulence.

Enhancing underwater VLC with spatial division transmission and pairwise coding.

In this paper, we propose and evaluate two spatial division transmission (SDT) schemes, including spatial division diversity (SDD) and spatial division multiplexing (SDM), for underwater visible light communication (UVLC) systems. Moreover, three pairwise coding (PWC) schemes, including two one-dimensional PWC (1D-PWC) schemes, i.e., subcarrier PWC (SC-PWC) and spatial channel PWC (SCH-PWC), and one two-dimensional PWC (2D-PWC) scheme are further applied for signal-to-noise ratio (SNR) imbalance mitigation in the UVLC systems using SDD and SDM with orthogonal frequency division multiplexing (OFDM) modulation. The feasibility and superiority of applying SDD and SDM with various PWC schemes in a practical bandlimited two-channel OFDM-based UVLC system have been verified through both numerical simulations and hardware experiments. The obtained results show that the performance of SDD and SDM schemes are largely determined by both the overall SNR imbalance and the system spectral efficiency. Moreover, the experimental results demonstrate the robustness of SDM with 2D-PWC against bubble turbulence. Specifically, SDM with 2D-PWC can obtain bit error rates (BERs) under the 7% forward error correction (FEC) coding limit of 3.8 × 10-3 with a probability higher than 96% for a signal bandwidth of 70 MHz and a spectral efficiency of 8 bits/s/Hz, achieving an overall data rate of 560 Mbits/s.

An Empirical Comparison of Modulation Schemes in Turbulent Underwater Optical Wireless Communications

This paper presents an empirical comparison of different modulation schemes in still and turbulent water conditions. Using an underwater channel emulator, it is shown that pulse position modulation (PPM) and subcarrier intensity modulation (SIM) have an inherent resilience to turbulence induced fading with SIM achieving higher data rates under all conditions. Finally, the signal processing technique termed pair-wise coding (PWC) is applied to SIM in underwater optical wireless communications for the first time. The performance of PWC is compared with the, state-of-the-art, bit and power loading optimisation algorithm. Using PWC, a maximum data rate of 5.2 Gbps is achieved in still water conditions.

Pairwise Coded mCAP with Chaotic Dual-Mode Index Modulation for Secure Bandlimited VLC Systems

In this paper, for the first time, we propose and experimentally demonstrate a novel pairwise coding (PWC)-based multiband carrierless amplitude and phase (mCAP) modulation with chaotic dual-mode index modulation (DM) for secure bandlimited visible light communication (VLC) systems. The combination of mCAP and DM can sustain a higher spectral efficiency (SE) compared with mCAP with conventional index modulation (IM), while PWC can be employed to efficiently mitigate the signal-to-noise ratio (SNR) imbalance caused by the low-pass frequency response of light emitting diodes (LEDs). Moreover, the DM is enhanced by a two-dimensional (2D) chaotic encryption scheme to guarantee the security of the useful information in VLC systems. Simulation and experimental results successfully verify the superiority of the proposed PWC-based mCAP-DM scheme with two-level chaotic encryption over other benchmark schemes.

Pairwise Coding for MIMO-OFDM Visible Light Communication

This work proposes a dual pairwise coding (DPWC) technique for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) systems. DPWC is a technique designed to mitigate the signal-to-noise ratio (SNR) imbalance created in the received information symbols due to the light emitting diodes’ bandwidth limitations and the strong correlation of typical indoor VLC MIMO channels. With singular value decomposition precoding, the MIMO VLC channel is decomposed into independent parallel sub-channels. The DPWC technique is then applied to further improve the system performance by pairing and interleaving OFDM subcarriers and MIMO sub-channels with imbalanced SNR. The error performance of the proposed DPWC technique is investigated in detail via simulation and experimental demonstration. Its effectiveness with different degrees of bandwidth-limitation (data rate) and channel correlation is presented, and the results are compared with other variants of the pairwise coding technique. DPWC provides performance improvement with a significant reduction in the SNR required to achieve a specific bit error rate (BER).

Performance-enhanced NOMA-VLC using subcarrier pairwise coding

To alleviate the error propagation issue of non-orthogonal multiple access (NOMA) based visible light communication (VLC) systems, we propose and experimentally demonstrate the subcarrier pairwise coding (PWC) based NOMA-VLC. Without extra coding overhead, the computation-efficient subcarrier PWC can significantly improve the overall system performance by pairing the subcarriers of high signal-to-noise ratios (SNRs) with those of low SNRs. Under different transmitted baud rate from 150 MSa/s to 225 MSa/s, the bit error rate (BER) performance with and without PWC is investigated in a NOMA-VLC system. Experimental results show that with the optimized power ratio between the NOMA users, up to 3.3-dB Q2-factor improvement can be achieved by the proposed scheme.

An Enhanced Adaptive Scheme With Pairwise Coding for OFDM-VLC System

For a visible light communication (VLC) system, different modulation formats and coding schemes are considered as the effective way to improve system performance under the limited bandwidth of light emitting diode. In this letter, we propose and experimentally demonstrate an enhanced adaptive scheme with pairwise coding (PWC) for an orthogonal frequency-division multiplexing (OFDM)-VLC system. Due to its no coding overhead and low complexity, it can be used to enhance the performance of the OFDM-VLC system. Through pairing the good and bad subcarriers, the signal-to-noise ratio (SNR) imbalance of this pair subcarrier is mitigated. Hence, it can lower the required SNR for a certain bit error ratio (BER). In the enhanced adaptive scheme with PWC, the low-frequency subcarriers (SCs) are modulated by 16 quadrature amplitude modulation (16QAM), while the high-frequency SCs are modulated by 4QAM, and in the midst of them, 32QAM is used. The experimental results show that, it can achieve 1.1 dB Q 2 factor improvement compared with the conventional adaptive scheme and the BER can be up to 1.66×10 -5 using the proposed adaptive scheme with PWC.

Subcarrier Pairwise Coding for Short-Haul L/E-ACO-OFDM

Dispersion-induced power fading in intensity modulated direct-detection systems degrades the higher frequency subcarriers’ signal qualities. In layered/enhanced asymmetrically clipped optical OFDM (ACO-OFDM), which is a spectrally efficient version of ACO-OFDM, the error propagation in the iterative decoding can further degrade the performance, especially when a low bias current is used for the directly modulated laser. In this letter, we propose using pairwise coding within each layer to improve the bit error rate. We transmitted ~5-GBb 16-QAM OFDM signals over a 19.8-km single mode fiber, and found a 3.6-dB improvement in the $Q^{2}$ -factor.

Improved polarization dependent loss tolerance for polarization multiplexed coherent optical systems by polarization pairwise coding.

Polarization dependent loss (PDL) causes imbalanced optical signal to noise ratio (OSNR) of the two polarizations, thus remains one of the major bottlenecks for next-generation polarization-division-multiplexed (PDM) coherent optical transmission systems. In this paper, we investigate Pairwise Coding for adaptive PDL mitigation in PDM coherent optical systems. By pre-coding across two polarizations, the PDL-induced performance degradation can be largely mitigated without any coding overhead. We present details of the coding and de-coding design, and also derive the analytical symbol/bit error rate of the Polarization Pairwise Coding scheme, which can be used to predict the performance gain as well as for optimal rotation angle calculation. Simulation results verify that Pairwise Coding achieves substantial system performance gains over a wide range of PDL values. Compared with other digital coding techniques, Polarization Pairwise Coding shows improved performance than Walsh-Hadamard transform since it maximizes the coordinate diversity; and also Pairwise Coding is computationally much simpler to decode compared with the Golden and Silver Codes, therefore is practical for current 100-Gb/s and future 400-Gb/s and 1-Tb/s digital coherent transceivers.

Coding-Based Data Broadcasting for Time-Critical Applications With Rate Adaptation

In this paper, we dynamically select the transmission rate and design wireless network coding to improve the quality of services such as delay for time critical applications. In a network coded system, with low transmission rate and hence longer transmission range, more packets may be encoded, which increases the coding opportunity. However, low transmission rate may incur extra transmission delay, which is intolerable for time critical applications. We design a novel joint rate selection and wireless network coding (RSNC) scheme with delay constraint, so as to maximize the total benefit (where we can define the benefit based on the priority or importance of a packet for example) of the packets that are successfully received at the destinations without missing their deadlines. We prove that the proposed problem is NP-hard, and propose a novel graph model to mathematically formulate the problem. For the general case, we propose a transmission metric and design an efficient algorithm to determine the transmission rate and coding strategy for each transmission. For a special case when all delay constraints are the same, we study the pairwise coding and present a polynomial time pairwise coding algorithm that achieves an approximation ratio of 1 - 1/e to the optimal pairwise coding solution, where e is the base of the natural logarithm. Finally, simulation results demonstrate the superiority of the proposed RSNC scheme.

Adaptation shapes spike train correlations: theory and application to tinnitus

weakened adaptation reported in neurons from animals with behavioral evidence of tinnitus both increases firing rate and spike train correlations. Our work shows that the biophysical correlates of tinnitus are consistent with the spike train correlates of tinnitus, and that this link is not obvious from past theories linking firing rates and spike train correlations because of the presence of SFA. These results reveal that the biophysical mechanisms giving rise to hyperactivity in tinnitus and other neural pathologies have specific implications for the pairwise statistics and population coding of the affected neurons.

MACSE: Multiple Alignment of Coding SEquences Accounting for Frameshifts and Stop Codons

Until now the most efficient solution to align nucleotide sequences containing open reading frames was to use indirect procedures that align amino acid translation before reporting the inferred gap positions at the codon level. There are two important pitfalls with this approach. Firstly, any premature stop codon impedes using such a strategy. Secondly, each sequence is translated with the same reading frame from beginning to end, so that the presence of a single additional nucleotide leads to both aberrant translation and alignment.We present an algorithm that has the same space and time complexity as the classical Needleman-Wunsch algorithm while accommodating sequencing errors and other biological deviations from the coding frame. The resulting pairwise coding sequence alignment method was extended to a multiple sequence alignment (MSA) algorithm implemented in a program called MACSE (Multiple Alignment of Coding SEquences accounting for frameshifts and stop codons). MACSE is the first automatic solution to align protein-coding gene datasets containing non-functional sequences (pseudogenes) without disrupting the underlying codon structure. It has also proved useful in detecting undocumented frameshifts in public database sequences and in aligning next-generation sequencing reads/contigs against a reference coding sequence.MACSE is distributed as an open-source java file executable with freely available source code and can be used via a web interface at: http://mbb.univ-montp2.fr/macse.

Is rate adaptation beneficial for inter-session network coding?

This paper considers the interplay between rate adaptation and inter-session network coding gains in wireless mesh or ad hoc networks. Inter-session network coding opportunities at relay nodes depend on packets being overheard by surrounding nodes ? The more packets nodes overhear, the more opportunities relays have to combine packets, resulting in a potential increase in network throughput. Thus, by adapting its transmission rate, a node can increase the range over which its packets are overheard, enabling additional opportunities for coding and increased overall throughput. This paper considers inter-session coding, restricted to a single relay (bottleneck) node, or star network topology. Even for such simple topologies the optimal joint rate adaptation and network coding policy is known to be NP-hard. Optimal rate vector selection is a combinatorial optimization problem, which is NP-hard, and finding optimal coding scheme turns out to be a clique partitioning problem which is also NP-hard. So, we provide heuristics to find a suboptimal rate vector and coding scheme. Additionally, we provide a linear programming formulation for network coding when only pairwise intersession coding is allowed. We evaluate the averaged throughput in two different scenarios, in which relays have different access opportunities, giving some intuition on the impact of rate adaptation in lightly and heavily loaded systems. The gains of joint rate adaptation and network coding are marginal when the relay has a higher access opportunity than other nodes, or when the MAC operates ideally it ranges from 9% to 19% as compared to a network without network coding and 4% over a network using regular network coding. While, when the relay has equal access opportunity as other source nodes, which is more typical of todays MAC protocols under heavy loads, the gain ranges from 40% to 62% as compared to the standard relaying case and is upto around 20% as compared to a network with regular pairwise network coding. This can further be increased to a gain of 40% to 120% by replacing pairwise coding with sub-optimal general network coding scheme.

Closing the Gap in the Capacity of Wireless Networks Via Percolation Theory

An achievable bit rate per source-destination pair in a wireless network of n randomly located nodes is determined adopting the scaling limit approach of statistical physics. It is shown that randomly scattered nodes can achieve, with high probability, the same 1/radicn transmission rate of arbitrarily located nodes. This contrasts with previous results suggesting that a 1/radicnlogn reduced rate is the price to pay for the randomness due to the location of the nodes. The network operation strategy to achieve the result corresponds to the transition region between order and disorder of an underlying percolation model. If nodes are allowed to transmit over large distances, then paths of connected nodes that cross the entire network area can be easily found, but these generate excessive interference. If nodes transmit over short distances, then such crossing paths do not exist. Percolation theory ensures that crossing paths form in the transition region between these two extreme scenarios. Nodes along these paths are used as a backbone, relaying data for other nodes, and can transport the total amount of information generated by all the sources. A lower bound on the achievable bit rate is then obtained by performing pairwise coding and decoding at each hop along the paths, and using a time division multiple access scheme