This paper explores the integration of line-of-sight (LoS) multiple-input multiple-output (MIMO) RF and optical wireless links to establish high-capacity satellite feeder links via a High-Altitude Platform Station (HAPS). Our approach connects the ground station to the HAPS using a LoS MIMO RF link and to the satellite via an optical link, addressing challenges such as atmospheric effects, antenna spacing constraints, and capacity imbalances. We identify the minimum antenna spacing required at both the ground station and HAPS to maximize the LoS MIMO RF link capacity, considering various configurations and frequency bands. Additionally, we analyze the impact of HAPS movement, demonstrating that certain antenna configurations can sustain maximum capacity even when the HAPS moves along a circular path above the ground station.

Asynchronous Scrambled Coded Multiple Access (ASCMA) technology presents several features that make it well suited for IoT and mMTC type applications over state-of-the-art terrestrial wireless networks. These include low-rate error correction coding combined with low complexity receiver processing and an ability to transmit data asynchronously using single as well as multiple-streams from a user terminal. The resulting advantages in terms of spectral efficiency, low-latency, and reduced power consumption have been quantified previously using link-level simulations. In this paper, we evaluate the system level performance of a 5G New Radio (NR) mobile network employing ASCMA for uplink mMTC type applications. Using an accurate system level simulator, we evaluate two different scheduling schemes for ASCMA, the first assumes a grant-based scheduling of user terminals over available network resources and the second assumes random access or grant-free scheduling of the same. A novel abstraction method is developed to aid the system level evaluations. Using extensive simulations, we demonstrate the ability of ASCMA technology to provide significant improvements to key network performance indicators such as packet loss rate, average retransmission rate, average cell throughput and cell-edge throughput.

Aggressive frequency reuse alleviates the frequency-spectrum shortage in multibeam satellite systems targeting multi-Terabits-per-second throughput but creates harsh co-channel interference (CCI) environment. For forward-link, state-of-the-art receivers at user terminals assume memoryless CCI to avoid exponential increase in complexity. However, memory effects of CCI are inevitable, arising when co-channel signals combine asynchronously. First, we provide analytical characterization of CCI accounting for memory, a formulation that captures synchronization impairments. Then, low-complexity receivers, capable of realizing full frequency reuse, are developed to compensate for spatial and temporal CCI profiles, with computational complexity increasing linearly in the memory span. This is achieved by our novel soft-in soft-out, iterative divide-and-conquer (IDAC) paradigm, decomposing interference into smaller sets depending on intensity. A set is created from weak interferers which are not decoded but considered as thermal noise. The interferers which are decoded are further split into a set that subtracts strong ones and another set addressing intense interferers in optimal-Bayesian fashion. Extensive numerical studies reveal that IDAC architecture offers lossless compensation of dominant CCI with memory for beam-edge terminals, when rate coordination is guided by network information theory. Further, its performance is superior compared with approaches using iterative subtractive cancellation, commonly employed for return-link, random-access satellite applications.

Asynchronous Scrambled Coded Multiple Access (A- SCMA) has been shown to offer high spectral efficiencies on Random Access (RA) channels. By allowing unscheduled data transmission, latency is reduced and power can be conserved, making such techniques well suited for IoT type applications. In this paper multi-stream (multi- layer) transmission from a single terminal is demonstrated which enables higher per-terminal spectral efficiency. The application of ASCMA with single and multi-stream on both Additive White Gaussian Noise (AWGN) as well as terrestrial fading channels is presented.

A family of new non-orthogonal multiple access techniques named Scrambled Coded Multiple Access (SCMA) is described. It allows different users of a communications system to efficiently share the same bandwidth by using specific scramblers to separate user bursts. SCMA technology is designed for satellite communications with extremely small antenna terminals for potential applications such as asset tracking, sensor networking, and smart grid; and also for latency reduction. The latest variation, Asynchronous Scrambled Coded Multiple Access (A-SCMA) on the other hand, not only removes the need for timeslot synchronization and precise power control, but also offers an unprecedented system throughput of more than 4 bits/symbol at a modest per user signal to noise ratio (SNR).

Orthogonal frequency-division multiplexing (OFDM), special form of multicarrier modulation, is the cornerstone of broad range of current standards. In addition, fifth-generation (5G) terrestrial wireless networks, advancing at accelerated speed, continue to use OFDM air interface. The first part of this paper endeavors to apply OFDM-like signaling for broadband satellite transmission in the forward direction, from gateway to terminals. Two layers of multicarrier operation are invoked. One allows multiple signals to share single on-board high-power amplifier. The other layer permits transmitted symbols from individual signals to modulate multiple narrowband OFDM subcarriers, followed by interpolating filters to suppress interference into adjacent signals and limit out-of-band emission to be compatible with satellite uplink. The second part of this paper proposes novel nonlinear compensation techniques, applied at the transmitter or receiver, and combination of both. Two performance assessments are provided: one exploiting information-theoretic framework using ideal channel coding; another based on total degradation using practical forward-error correction codes. We show that performance loss from nonlinear distortion can be significantly reduced when adopting the proposed correction methods. Furthermore, it is possible to use OFDM-like signaling over satellite, while being competitive, and in some cases surpassing, traditional systems with single-carrier modulation employing state-of-the-art enhanced receiver architecture.

To satisfy the aggressive demand for higher satellite throughput, industry trend is moving toward sharing the transponder amplifier by multiple carriers, each employing high-order modulations that are spectrally compact. This trend, in conjunction with the inherently nonlinear nature of the amplifier when driven efficiently closer to saturation, creates significant intermodulation distortion that needs to be appropriately compensated. This paper presents a powerful compensation technique that is capable of mitigating the nonlinear intermodulation distortion and is placed at the transmitter or gateway. It is a novel multicarrier data predistortion technique that successively modifies the transmitted symbols to drive multicarrier distortion vector toward zero. This distortion vector results from passing the transmitted symbols from the multiple carriers, intrinsically accessible at the gateway, through the nonlinear satellite channel model. The novel successive predistortion technique and methods of estimating the distortion are described in detail. It is demonstrated using extensive computer simulations that the proposed multicarrier predistortion technique is capable of achieving near-optimum performance, even when only a simple linear receiver is employed and no exchange of data is assumed between receivers.

We introduce analytical characterization of the nonlinear interference that results when passing more than one high-order modulation carrier through the same nonlinear transponder high-power amplifier (HPA). A Volterra filter is proposed which is novel in its implementation of this analytical characterization and modeling of intersymbol interference (ISI) and adjacent channel interference (ACI). Our focus is on adaptive algorithms with pilot-based training so that the solutions are completely blind to unknown transponder HPA characteristics, and can rapidly respond to varying operating back-off level. Furthermore, two families of adaptive solutions are provided to compensate for nonlinear ISI and ACI. The first set performs adaptive channel inversion and then applies equalization. The second set of solutions performs adaptive channel identification and then applies cancellation. The effectiveness of the proposed analysis and techniques is demonstrated via extensive simulations for high-order QAM and APSK modulations. Also included is the coded performance with selected LDPC codes designed for the DVB-S2 standard. Finally, computational complexity is assessed and performance impact is quantified when complexity is reduced by decreasing the number of Volterra coefficients.

This paper presents a methodology for determining the capacity of coherently detected continuous- phase frequency shift keying (CPFSK) modulation under the constraints of binary coding and an ergodic channel. Building upon the capacity results, the coded CPFSK parameters of code rate, alphabet size, and modulation index are jointly optimized by using capacity as a cost function. From this optimization, it is possible to determine the minimum epsivb/No required for bit-interleaved coded CPFSK to achieve an arbitrarily low error rate as a function of spectral efficiency. Results showing this minimum epsivb/No are presented for a range of spectral efficiencies and several alphabet sizes in an AWGN channel.

This paper investigates a capacity-based approach to parameter optimization for energy and bandwidth efficient communication systems. In particular, non-coherently detected, bit-interleaved coded, M-ary Gaussian frequency shift keying (GFSK) is considered. Non-coherent detection is accomplished using a sequential, soft-output (SO), soft-decision differential phase detector (SDDPD). The capacity of the proposed system under modulation, channel and detector design constraints is calculated. For a wide range of spectral efficiencies, the most energy efficient combination of GFSK parameters and code rates is identified using information theoretic bounds on reliable signaling. The information outage probabilities under modulation and detector design constraints are calculated for the block fading channel. Select results reveal that the capacity-based approach also helps in identifying the combination of modulation parameters and code rates with the lowest outage probabilities in block fading

This paper investigates an energy efficient, noncoherent system design for Bluetooth. We propose using a soft decision differential phase detector (SDDPD) along with the (15, 10) shortened Hamming channel (SHC) code. Unlike previous work, where SDDPD with ML detection gives hard estimates of the modulated bits, we develop a sequential, soft-in/soft-out (SISO) SDDPD which exchanges soft information with the channel decoder. The proposed soft decision based receiver gives significant improvements in energy efficiency and throughput over the benchmark Bluetooth receiver using limiter discriminator integrator (LDI) detection with hard decision channel decoding. It also outperforms SDDPD with ML detection followed by hard decision channel decoding. Additional gains are obtained by extending our proposed system to perform bit-interleaved coded modulation (BICM). The effect of bit-interleaving on the performance of LDI detection and SDDPD with hard decision decoding is studied. The capacity of our system under BICM is evaluated and extrinsic information transfer (EXIT) chart analysis is used to analyze convergence behavior of the proposed receiver

This paper proposes and analyzes a new cross-layer protocol for ad hoc and sensor networks that unifies the concepts of geographic random forwarding (GeRaF) and hybrid-ARQ. The protocol is given the descriptive name hybrid ARQ-based intra-cluster geographically informed relaying (HARBINGER). Like GeRaF, HARBINGER assumes that each node knows its own position and that messages are addressed by location. As is common for sensor networks, the nodes cycle on-and-off according to a sleep schedule. Unlike GeRaF, which returns to the initial transmission state If no active node is within range, the nodes in HARBINGER combine transmissions thereby achieving an additional time-diversity benefit. With HARBINGER, a lower density of active nodes achieves almost the same delay and energy efficiency as GeRaF, implying that a lower duty cycle sleep schedule could be used to prolong the useful lifetime of the network. The paper gives detailed analysis of a version of the protocol (Fast-HARBINGER) whereby the sleep states of the network are synchronized with the data packet transmission rate of the network.