Low complexity reconfigurable physical layer for the l-band digital aeronautical communications system

Abstract

Exponential increase in global air traffic and demands to support heterogeneous services ranging from data to multimedia have compelled communication the- orists and engineers to explore an alternative to existing very high frequency (VHF) band (118-137 MHz) based air-to-ground communication system. After various studies and experiments, one promising solution of utilizing multiple 1 MHz frequency bands between incumbent distance measuring equipment (DME) signals in L-band (960-1164 MHz) is proposed and it is referred as L-band Digital Aeronautical Communication System (LDACS). An efficient utilization of such narrowband non-contiguous spectrum is challenging and hence, various activities to invent breakthrough technologies that provide higher spectral efficiency are being encouraged. Given the history of more than a century of wireless innovation, redesign of physical layer (PHY) is critical to improve the vacant spectrum utilization. Success of orthogonal frequency division multiplexing (OFDM) in cellular and WiFi networks makes it the popular choice for LDACS PHY. LDACS also applies a raised cosine slope with a roll-off factor of 0.107 within a window time of 12.8 μs for better spectrum containment and less out-of-band radiation. It is assumed that DME does not use narrow-band filters at their receiver input. With that, DME might be susceptible to interference even if the transmit spectrum of the potential interferer is strongly contained. Therefore, the existing LDACS PHY to utilizes a fixed transmission bandwidth of only 498 kHz kHz over in between the 1 MHz channel grid of DME. Furthermore, fixed transmission bandwidth limits its usefulness to support heterogeneous services demanding distinct bandwidths and dynamic control over PHY parameters such as OOB emission, transmission bandwidth etc. The overall objective of this thesis is to study and develop spectrum efficient, re-configurable and low complexity LDACS PHY, analyze its performance in real radio environment and feasibility on system-on-chip (SoC). This thesis mainly focuses on the FL transmission of the LDACS. The first contribution of this thesis is to design the PHY frame structure for the LDACS comprising of data, null and reference signals to support tunable bandwidth. The proposed frame structure maintains the complete compatibility with existing LDACS and offers transmission bandwidth from 186 kHz to 732 kHz compared to 498 kHz in existing LDACS. The tunable bandwidth makes LDACS more flexible and allows to adjust to the local DME interference situation. To support such frame structure, we develop a reconfigurable filtered OFDM (Ref-OFDM) based LDACS PHY and it is the second contribution of this thesis. The Ref-OFDM augments conventional OFDM with dynamic scheduler to support tunable bandwidth and reconfigurable linear phase fixed-coefficient multi-band finite impulse response (FIR) filter to enable on the-fly control over transmission bandwidth. It also supports multi-band filtering for simultaneous transmission in multiple narrow frequency bands. In addition to the mathematical and performance analysis using synthetic data, we demonstrate the functionality of the proposed Ref-OFDM PHY in a real radio environment on universal software radio peripherals (USRPs) based testbed. The results show that proposed approach offers higher throughput due to wider bandwidth, at least 32 dB reduction in interference to incumbent L-band users and improved BER due to appropriate filtering of legacy user OOB emission compared to existing LDACS. Most of the ongoing works are focused on the theoretical analysis and multiple antenna extensions of LDACS PHY. From architecture perspective, the performance analysis of LDACS PHY on fixed-point hardware in the presence of various RF impairments and wireless channels/interference is critical and hence, the need to efficiently map LDACS PHY on SoC is the motivation behind the third contribution of this thesis. We design and implement existing as well as proposed LDACS PHY on heterogeneous Zynq SoC (ZSoC) platform, consisting of field programmable gate array (FPGA) as programming logic (PL) and Advanced RISC Machines (ARM) as processing system (PS). The PHY is integrated with the programmable analog front-end to validate its functionality in the presence of various RF impairments and wireless channels and interference specific to the LDACS environment. We propose a novel Hardware-Software co-design approach and explore various PHY configurations by dividing it into PL and PS. Such analysis offers the flexibility to choose the appropriate configuration, as well as the word-length (WL) for a given OOB emission, BER, chip area, delay, and power constraints. Though Ref-OFDM PHY offers superior performance with tunable bandwidth, it incurs significant penalty in terms of resource utilization (27 % higher DSP48 based embedded multiplier) and power consumption compared to OFDM and Windowed-OFDM based LDACS. The use of filtering leads to increase in chip area, power and delay complexity and hence, design of low complexity reconfigurable filters is the focus of the forth contribution of this thesis. We propose the design of low complexity FIR filter based Ref-OFDM (LRef-OFDM) using a multi-stage filter. The proposed filter is carefully designed to meet the stringent non-uniform spectral attenuation requirements of LDACS using novel interpolation and masking approach. We show that the LRef-OFDM based LDACS offers identical OOB and BER performance to Ref-OFDM with 14.14 % less power and fewer resources (12.78 % DSP48). The L-band spectrum is a scarce resource and, thus spectrum sensing is important for future communication systems. In our fifth and final contribution, we propose a sub-Nyquist Sampling (SNS) sensing based L-band spectrum characterization. The proposed approach replaces high-speed wide bandwidth analog-to-digital converter (ADC) with multiple low-speed narrow bandwidth ADCs. We consider two SNS approaches: 1) modulated wideband converter (MWC) and 2) finite rate of innovation (FRI). SNS is followed by spectrum reconstruction via orthogonal matching pursuit (OMP). To the best of our knowledge, this is the first work exploring SNS for LDACS. We shows that the FRI offers better probability of detection, higher throughput (at lower number of ADCs) and fewer number of sensing failures than MWC and requires lower number of ADCs due to non-contiguous sensing. In this thesis, we propose a low complexity reconfigurable LDACS FL PHY augmented with SNS based wideband sensing. The proposed work improves the spectrum utilization efficiency of existing LDACS and extend it to support heterogeneous services. In-depth performance analysis, feasibility on SoC and backward compatibility with existing LDACS makes it an attractive alternative for next generation LDACS system.