Ravinder David Koilpillai

We present a modified w idely l inear s ingle-tap b lind e qualizer f or t he joint multi-impairment compensation of polarization mixing, IQ Imbalance and transceiver phase noise, analyze its performance through simulation and experiments for 32Gbaud PM-16QAM transmission.

We study the use of nonlinear semiconductor optical amplifier (SOA) for generating optical phase conjugate towards compensation of distortions in short distance optical fiber transmission due to Kerr nonlinearity and chromatic dispersion in coherent multi-carrier lightwave signals. We experimentally demonstrate the effectiveness of the SOA-based phase conjugator to improve the link budget with a 100 km standard single mode fiber link for 20 GHz coherent OFDM signals, with QPSK and 16QAM modulations and a corresponding net bit-rate of 40 Gbps and 80 Gbps respectively. Mid-span spectral inversion scheme is employed where the optical phase conjugate is generated through a partially degenerate four-wave mixing process in a nonlinear SOA. We demonstrate a bit error rate performance within $2\times 10^{-2}$ for an average launched power of up to 12 dBm (9 dBm) for QPSK (16QAM) coherent OFDM signals, in a 100 km fiber link. We also investigate the possible improvement in link budget using numerical simulation for 16QAM and 64QAM CO-OFDM signals with the proposed scheme.

We present a modulation transparent geometric parameter extraction method to compensate for the receiver IQ imbalance. We evaluate the algorithm's performance for various imbalance values for 32-GBd 16/64QAM signals in both simulations and through experiments.

In this paper, a 2 inverted U-slot integrated rectangular patch antenna array is proposed that can be used for S band applications i.e., radar applications, communication and wireless networks. In the proposed antenna design 2 inverted U slots are introduced in the patch to improve bandwidth providing a bandwidth of 150 MHz and gain of 4.08 dBi resonating at 3.3 GHz. Furthermore, 4 X 4 antenna array is fabricated on 1.6 mm thick FR4 substrate with dielectric constant 4.4 providing a gain of 14.2 dBi. Ansys HFSS is used for antenna simulation and a prototype is fabricated and tested.

We present a multi-tap filter for residual intersymbol interference correction which is tolerant to the transmitter IQ imbalance by adapting the reference constellation simultaneously. We evaluate the algorithm's performance for various imbalance values for 32-GBd 16QAM signals through experiments.

We experimentally demonstrate 25.33 Gbps digital power division multiplexed DD-OFDM transmission in 6.5 GHz bandwidth through 5.3 km few mode fiber to enable higher split-ratio and better spectral efficiency in short-haul links.

Modulation schemes tolerant to large delay spreads and high Doppler shifts are needed for reliable underwater acoustic communication (UAC). Due to the sensitivity of orthogonal frequency division multiplexing (OFDM) to Doppler shifts, two alternatives, namely, orthogonal time frequency space (OTFS) and orthogonal chirp division multiplexing (OCDM) are studied to evaluate their performance in UAC chanenls. The OTFS and OCDM schemes are seen to achieve significantly lower bit error rates (BERs) compared to OFDM in the ideal scenario, i.e., when the channel information at the receiver is perfect. Even with estimated channel knowledge at receiver, time-variations of the channel between the estimation phases, and reduced guard intervals, OTFS and OCDM outperformed OFDM. A linear channel interpolation of the estimated channel gains is proposed to account for the time-variations of the channel and is seen to improve the BERs of OTFS and OCDM schemes.

We experimentally demonstrate the use of a pilot-free phase noise mitigation algorithm for a gain-switched comb-based 480 Gbps 64QAM CO-OFDM superchannel system using weighted-blind estimation and Bayesian filter with 18% potential improvement in spectral efficiency.

Performance bench-marking of underwater acoustic modems manufactured by various vendors are not straightforward due to the unavailability of standard channel models for Underwater Wireless Acoustic channels (UWA). Recognizing the difficulty for accurately modeling the UWA channels, channel replay simulation is often resorted to as a realistic simulation strategy. In this approach, Time-Varying Impulse Response (TVIR) of the channel is measured continuously for a duration using channel sounding technique. Such databases are created for benchmark sites and the recorded channel TVIR are used for channel replay simulation. One such database of TVIR developed by Norwegian Defence Research Establishment (FFI) is named as underWater AcousTic channEl Replay benchMARK (WATERMARK) is adopted by many authors as a benchmarking tool for acoustic modem physical layer technologies. Popular modulation in communication literature (such as BPSK and QPSK etc.) will not work in UWA channels without the support of additional processing blocks such as complex adaptive equalizers, synchronization schemes, and Doppler compensation techniques due to the double-spread and wide-band characteristics of UWA channels. In this paper, we have demonstrated Chirp Slope Keying (CSK) as a simple and robust modulation scheme which will work on almost all UWA channels encountered in practice by suitably choosing the data rate, without the support of additional processing blocks such as complex adaptive equalizers, synchronization schemes and Doppler compensation techniques. Hence CSK provides a "practical performance benchmark for channel replay simulation"against which the performance of other advanced physical layer schemes can be compared.

We experimentally demonstrate 608 Gbps CO-OFDM superchannel transmission over 25 km using optical carrier generated from an externally injected gain-switched comb source with linewidth ≈19 kHz. We show the BER performance is within the HD-FEC limit.