Khoa D. Nguyen

Summary: This paper presents an algorithm for iterative joint channel parameter (carrier phase, Doppler shift, and Doppler rate) estimation and decoding of transmission over channels affected by Doppler shift and Doppler rate using a distributed receiver. This algorithm is derived by applying the sum‐product algorithm (SPA) to a factor graph representing the joint a posteriori distribution of the information symbols and channel parameters given the channel output. In this paper, we present two methods for dealing with intractable messages of the SPA. In the first approach, we use particle filtering with sequential importance sampling for the estimation of the unknown parameters. We also propose a method for fine‐tuning of particles for improved convergence. In the second approach, we approximate our model with a random walk phase model, followed by a phase tracking algorithm and polynomial regression algorithm to estimate the unknown parameters. We derive the Weighted Bayesian Cramer‐Rao Bounds for joint carrier phase, Doppler shift, and Doppler rate estimation, which take into account the prior distribution of the estimation parameters and are accurate lower bounds for all considered signal‐to‐noise ratio values. Numerical results (of bit error rate and the mean‐square error of parameter estimation) suggest that phase tracking with the random walk model slightly outperforms particle filtering. However, particle filtering has a lower computational cost than the random walk model‐based method.

Precoding has emerged as a prominent technique to mitigate interbeam interference (IBI) in the forward link of a multibeam high throughput satellite system. In this paper, we consider zero-forcing (ZF) precoding to mitigate IBI. To reduce the channel state information (CSI) feedback from the user terminals, which consequently reduces the latency and signaling overheads, we design a ZF precoder with matrix and vector normalizations using partial CSI and analyze its ergodic SR performance. We consider a practical scenario where the links between the satellite and user terminals are affected by rain fading attenuation and derive an analytical expression, upper and lower bounds, and asymptotic (in terms of transmit power) approximations for the ergodic SR achieved by this ZF precoder. Many important insights are obtained from these results. Also, the robustness of this ZF precoder to CSI estimation errors is numerically analyzed.

Next-generation multibeam high throughput satellite (HTS) systems aim at achieving Terabits/s throughputs with thousands of spot beams. Conventional radio frequency (RF) Ku-/Ka- band feeder links, between gateways and satellites, are unable to achieve such throughputs due to limited spectrum availability in these bands. Free space optical (FSO) feeder links present a promising alternative to RF feeder links and have gained increased attention lately in the satellite community. This article presents performance analysis of multibeam HTS systems where the feeder link operates in the FSO band and the user link, between the satellite and the user terminal, is in the RF Ka-band. We consider that the FSO feeder link is impaired by the scintillation fading due to atmospheric turbulence and the RF Ka-band user link is attenuated by the rain fading. In multibeam HTS systems, frequency reuse in the user link creates interbeam interference (IBI) due to overlapping side lobes of the multibeam antenna radiation pattern. Considering a forward link with a fixed gain transparent satellite payload, we use the zero-forcing precoding technique to mitigate IBI in the user link. We derive exact and approximate statistics of the end-to-end signal-to-interference-and-noise ratio. Also, by capitalizing on these results, we analyze information-theoretic metrics including the outage probability and ergodic capacity. Our results provide various important insights, e.g., 1) outage performance and ergodic capacity is typically restricted by rain fading in the RF link, and 2) outage diversity of rain fading in the RF link is 0, hence outage performance only increases marginally with power at high signal-to-noise ratio.

We consider the problem of tracking, in realtime, an unstable autoregressive (AR) source over a discrete memoryless channel (DMC).We present computable achievable bounds on the optimal tracking error for general DMCs, and we particularise these bounds to the binary erasure, packet erasure and binary symmetric channels. The achievable bounds in this paper are proved using a partially separate source quantisation and channel coding architecture. We do not use complete (or, strict) separation in usual Shannon sense: The quantiser’s resolution is optimised against the error-correction capabilities of the channel code, and the channel code is optimised against an AR Hamming distortion function matched to the source (a weighted Hamming distortion function that provides unequal error protection to different parts of the AR source). The achievability results for general DMCs are proved by combining the AR Hamming distortion function with new realtime (streaming) versions of the random coding union and dependence testing bounds. When applied to erasure channels, these general bounds combine with simple converses to demonstrate that the channel’s cutoff rate plays an important role in realtime tracking.

Anytime codes are known to be useful for tracking and controlling unstable systems over noisy channels. Although practical anytime codes have been developed for point-to-point communication, to date no practical code constructions and tractable analysis of anytime codes are known to exist for relay networks. In this paper, we study the design and theoretical analysis of anytime codes for a three-terminal relay network. We propose bilayer codes for anytime transmission over the relay network and consider two transmission schemes for the relay-destination link. For both schemes, we analytically derive the delay-exponents and show that they closely match the corresponding numerical results obtained from density evolution. Through simulation, we also show the finite-length performance of the proposed bilayer anytime code.

In this paper, we present the performance analysis of a multibeam high throughput satellite (HTS) system with optical feeder link. We consider the forward link, or downlink, with a fixed gain transparent satellite payload, where the optical feeder link, i.e. the link between gateway and satellite, is impaired by turbulence fading and the RF user link, i.e. the link between satellite and user terminals, is degraded by rain fading. In multibeam HTS systems, frequency re-use in the user link creates interbeam interference which needs to be mitigated to avoid performance degradation. For this purpose, we consider our recently proposed novel zero-forcing (ZF) precoder that does not require channel state information at the gateway and compare its performance with the conventional ZF precoder. We derive statistics of the end-to-end signal-to-noise ratio to approximate performance measures including outage probability, average bit error rate and ergodic capacity. These approximations provide valuable insights into the system analysis. Monte Carlo simulations are used to validate the accuracy of our analysis.

Multi-hop relaying is well known to extend the range of wireless communications. However, the end-to-end performance of the multi-hop relaying channel degrades compared to that observed at the first hop. This performance degradation mainly arises due to: 1) half-duplex loss and 2) hopping or relaying loss. In this paper, we analyze the degradation in outage probability and ergodic capacity due to hopping loss in a multiple-input multiple-output (MIMO) dual-hop decode-and-forward relaying network over Rayleigh fading channels. In particular, we characterize analytically the performance degradation by deriving closed-form approximations of ergodic capacity and outage probability in the single-source single-relay single-user, single-source multiple-relay single-user, and single-source single-relay multiple-user topologies. These approximations also provide insights on how to overcome performance degradations due to hopping loss. Moreover, we show that cooperative relaying schemes can recover a significant portion of the hopping loss.