David Haley

Cooperative Intelligent Transport Systems: 5.9-GHz Field Trials

The mobile outdoor radio environment is challenging for vehicular communications. Although multipath propagation offers diversity and benefits in non-line-of-sight (NLOS) conditions, simultaneous multipath and mobility results in a doubly-selective fading channel. In practice, this means that the channel parameters vary significantly in both time and frequency within the bandwidth and typical packet durations used in 802.11p/WAVE standards for short-range vehicular communications. This paper presents the results of extensive field trial campaigns conducted in several countries, totaling over 1100 km. These field trials are scenario based, focusing on challenging low-latency, high-reliability vehicle-to-vehicle (V2V) safety applications including intersection collision warning, turn across path, emergency electronic brake light, do not pass warning, and precrash sensing. Vehicle-to-infrastructure (V2I) applications are also considered. The field trials compared the performance of off-the-shelf WiFi-based radio equipment with a more advanced 802.11p compliant radio employing more sophisticated channel estimation and tracking. Field trial results demonstrate significantly improved performance using the advanced radio, translating into greatly increased driver warning times and stopping distances. In fact the results show that off-the-shelf WiFi equipment fails to provide sufficient stopping distance to avert accidents in some cases. During the field trials, channel sounding data were also captured. Analysis of these channel measurements reveals the critical importance of accurate channel estimation, tracking the channel in both time and frequency within each packet. Delay spread and Doppler spread statistics computed from the channel measurements validate previously reported results in the literature. The results in this paper, however, provide the first instance of channel measurements performed simultaneously to application performance evaluation. The objective is to firmly establish the link between radio channel characteristics and the performance of critical V2V safety applications.

Outdoor Mobile Broadband Access with 802.11

The IEEE 802.11 OFDM physical layer was designed primarily for indoor local area networks. Commercially available 802.11 radios suffer greatly reduced performance, even failing completely, when deployed outdoors, where long delay spreads cause self-interference, and vehicular mobility causes fast variations in the radio channel parameters. This article describes an advanced OFDM receiver that overcomes these problems. It works by combining all useful received energy, accounting for inter-symbol interference, and accurately tracking radio channel variations. Complexity and performance advantages arc gained by splitting the processing between the time and frequency domains. Computer simulations show that even for outdoor urban environments at speeds greater than 140 mph, this receiver delivers performance comparable to a non-mobile, indoor system.

Reversible Low-Density Parity-Check Codes

Low-density parity-check (LDPC) codes may be decoded using a circuit implementation of the sum-product algorithm which maps the factor graph of the code. By reusing the decoder for encoding, both tasks can be performed using the same circuit, thus reducing area and verification requirements. Motivated by this, iterative encoding techniques based upon the graphical representation of the code are proposed. Code design constraints which ensure encoder convergence are presented, and then used to design iteratively encodable codes, while also preventing 4-cycle creation. We show how the Jacobi method for iterative matrix inversion can be applied to finite field matrices, viewed as message passing, and employed as the core of an iterative encoder. We present an algebraic construction of 4-cycle free iteratively encodable codes using circulant matrices. Analysis of these codes identifies a weakness in their structure, due to a repetitive pattern in the factor graph. The graph supports pseudo-codewords of low pseudo-weight. In order to remove the repetitive pattern in the graph, we propose a recursive technique for generating iteratively encodable codes. The new codes offer flexibility in the choice of code length and rate, and performance that compares well to randomly generated, quasi-cyclic and extended Euclidean-geometry codes.

A dual-function mixed-signal circuit for LDPC encoding/decoding

We present a low power, dual-function encode/decode circuit for a class of reversible low-density parity-check codes. The circuit offers a small silicon footprint, by operating as an analog decoder and reusing resources to switch into a digital encode mode. In order to achieve this behaviour from a single circuit we have developed mode-switching gates. These logic gates are able to switch between analog (soft) and digital (hard) computation. Only a small overhead in circuit area is required to transform the analog decoder into a full codec. The encode operation can be performed two orders of magnitude faster than the decode operation, making the circuit suitable for full-duplex applications. The low power and small area of the circuit make it an attractive option for battery powered wireless devices. Circuit simulations indicate a decoding latency of 10@ms with negligible SNR loss with respect to digital sum-product decoders.

Joint estimation of multipath parameters from OFDM signals in mobile channels

We study multipath parameter estimation from orthogonal frequency division multiplex signals transmitted over doubly dispersive mobile radio channels. We are interested in cases where the transmission is long enough to suffer time selectivity, but short enough such that the time variation can be accurately modeled as depending only on per-tap linear phase variations due to Doppler effects. We therefore concentrate on the estimation of the complex gain, delay and Doppler offset of each tap of the multipath channel impulse response. We show that the frequency domain channel coefficients for an entire packet can be expressed as the superimposition of two-dimensional complex sinusoids. The maximum likelihood estimate requires solution of a multidimensional non-linear least squares problem, which is computationally infeasible in practice. We therefore propose a low complexity suboptimal solution based on iterative successive and parallel cancellation. First, initial delay/Doppler estimates are obtained via successive cancellation. These estimates are then refined using an iterative parallel cancellation procedure. We demonstrate via Monte Carlo simulations that the root mean squared error statistics of our estimator are very close to the Cramer-Rao lower bound of a single two-dimensional sinusoid in Gaussian noise.

An analog/digital mode-switching LDPC codec

We present a novel time-multiplexed codec for a class of low-density parity-check codes, which switches between analog decode and digital encode modes. In order to achieve this behaviour from a single circuit we have developed mode-switching gates. These logic gates are able to switch between analog (soft) and digital (hard) computation. Only a small overhead in circuit area is required to transform the analog decoder into a full codec. The encode operation can be performed two orders of magnitude faster than the decode operation, making the circuit suitable for full-duplex applications. Throughput of the codec scales linearly with block size, for both encode and decode operations. The low power and small area requirements of the circuit make it an attractive option for small portable devices.

Game theoretic framework for studying WBAN coexistence: 2-Player game analysis and n-player game estimation

As an emerging technology for anywhere anytime healthcare, Wireless Body Area Networks (WBANs) bring many benefits as well as challenges. One practical issue is the coexistence of multiple networks in a limited space under resource constraints. Game theory, originally developed from economics, is a mathematical tool to analyse decision making among self-interested agents. Due to the heterogeneous, distributed and autonomous nature of WBANs, game theory is believed to be an appropriate tool to study the coexistence issue. This paper formulates a flexible game theoretic framework to study WBAN coexistence. We provide a detailed mathematical analysis for a 2-player game. When the number of WBANs increases in a game, the complexity in utility calculation and simulations grows greatly. We propose an estimation method to model games involving more than two players.

Frequency offset compensation in physical-layer network coding systems

Physical layer network coding (PNC) has the potential to improve the spectral efficiency of wireless relay communications by utilising the superposition of signal propagation. In the ideal case, PNC can double the throughput of a symmetric relay network when compared to conventional time division multiplexing. However, in practice imperfect channel conditions and hardware can lead to time and frequency offsets at the relay receiver that degrade PNC performance.

Energy-efficient adaptive modulation in wireless communication for implanted medical devices

In contrast to conventional wireless communication which takes place over the air, Radio Frequency (RF) communication through the human body poses unique challenges. Studies on RF propagation through human body indicate that the heterogeneous body tissues with different dielectric properties constitute a complicated and lossy environment for signal propagation. This environment also varies with different implant positions, individuals, body shapes and postures. As a result, there is a large variation in the path loss value of the in-body communication channel. In this paper, we first examine the energy efficiency of different digital modulation schemes in a basic wireless implant system. We point out that using a fixed type of modulation does not help to achieve the best energy efficiency in the implant system that has varying channel conditions. We then propose an adaptive communication system model which is suitable for wireless medical implant. Simulations results show that adopting adaptive modulation can provide a considerable amount of energy saving.

A polarimetric line-of-sight channel model for MIMO satellite communications

Spatial multiple-input multiple-output (MIMO) offers increased rate and/or reliability in rich scattering environments such as that of a multipath non-line-of-sight (NLOS) channel. In line-of-sight (LOS) dominated conditions, spatial diversity is reduced. Dual polarisation signalling has been shown to successfully exploit polarisation diversity when LOS is dominant. For satellite systems, such state-of-the-art dual-polarisation techniques do not currently account for the 3D geometry of the link, and specifically the polarisation mismatch from misalignment of the antenna elements. Performance loss due to the mismatch in antenna orientation can be overcome by using 3D polarimetric antennas at both the transmitter and receiver. In this paper, we present a novel polarimetric LOS MIMO channel model by combining satellite geometry, the Friis equation and polarisation mismatch. Our emphasis is on polarisation mismatch and how this varies over the field-of-view (FOV). Our model offers the opportunity for 3D polarimetric signalling techniques to deliver enhanced and more consistent link performance over the FOV.