Sergio Morgadinho Rebelo
Z Muhi-Eldeen, M O. Al-Nuaimi, J Richter, L Ivrissimtzis
Due to the rapid progress in wireless communication technologies and the increasing demands for new services; cutting edge techniques and research have developed wireless access delivery of broadband data. Such systems referred to as Broadband Fixed Wireless Access Systems (BFWA) or alternatively, Local Multipoint Distribution Systems (LMDS), are increasingly being regarded as a legitimate challenger to cable and digital subscriber lines (DSL), particularly in markets with poor cable or copper infrastructure. These systems have capabilities that go beyond the current WiFi and WiMAX technologies by providing users with higher bandwidths and faster data rates. The frequency of operation for such networks lies between 28 and 42 GHz according to the spectrum bands allocated for future LMDS networks.
The presented work aims at proposing a generalized stochastic model for the LMDS urban/suburban propagation channel. The model is based on a physical electromagnetic representation of the millimetre wavelength channel, with particular emphasis on fading margins in line-of-sight (LOS) links. The study focuses on the analysis, modelling and measurements of the fading effects of signal scattering caused by building surfaces and the ground on the direct field in an LMDS link at millimetre-waves. Outcomes have been validated against experimental results obtained in realistic propagation scenarios. More than 180 field measurement sets were taken at 40 GHz for model testing and validation purposes.
The main innovation of this work is the solution proposed to address the problem of signal scatter at millimetre-wave lengths. The approach uses high-frequency approximations to the analytical solution given by Kirchhoff’s tangent-plane representation of rough surfaces. The electromagnetic field and signal power at the receiver are considered random and are evaluated using the Physical Optics method over different possible realizations of the surface geometry and building architectural features. This has resulted in novel derivations of the mean field, mean power density and scatter distribution of the scattered field. In comparison to deterministic models, such as ray-tracing that require intensive computations and detailed (millimetric resolution) knowledge of topographical data that are almost impossible to obtain, the proposed model is rather efficient and yields accurate results.
The model has also been employed in analysing the effects of building scatter on the variance of the main desired link and the adjacent- or co-channel interfering link arising from neighbouring cells in cellular LMDS networks. This has enabled predictions of signal-to-interference ratio statistics and distributions for a particular underlying propagation environment.
Overall, comparisons yielded very good agreements between measurements and predictions of the main statistical parameters, thus verifying the main assumptions relating to the received signal strength, as well as the validity of a Rician distribution in describing the signal envelope variability for both the main and interference links in complex millimetre-wave propagation environments.
J Zhang, J Richter , M O. Al-Nuaimi, Leonidas Ivrissimtzis
The project aims to the following tasks:
X Liu, A Hammoudeh, D Scammell
Two European research projects currently exploiting the 60 GHz frequency spectrum are governed by the Advanced Communications Technologies and Services (ACTS) programme and Information System Technologies (IST) programme. These Projects known as MEDIAN and BROADWAY respectively, focus on high speed Wireless Local Area Networks (WLAN )applications.
The IST BROADWAY aims to propose a hybrid dual frequency system based on a tight integration of High Performance Radio Local Area Network – Type 2 (HIPERLAN/2)  Orthogonal Frequency Division Multiplex (OFDM) high spectrum efficiency technology at 5 GHz and an innovative fully ad-hoc extension of it at 60 GHz named HIPERSPOT. This concept extends and complements existing 5 GHz broadband wireless LAN systems in the 60 GHz range for providing a new solution to very dense urban deployments and hot spot coverage. This system is to guarantee nomadic terminal mobility in combination with higher capacity (achieving data rates exceeding 100Mbps). The tight integration between both types of system (5/60 GHz) will result in wider acceptance and lower cost of both systems through massive silicon reuse. This new radio architecture will, by construction, inherently provide backward compatibility with current 5 GHz WLANs standardised by the European Telecommunications Standards Institute as Broadband Radio Access Networks (BRAN) and HIPERLAN/2. The innovations coming out of this project will be a driver for standardisation and spectrum allocation of the next ETSI BRAN HIPERLAN generations .
A 60GHz time domain correlation receiver, as in the block diagrame, will be designed and implemented capable of transmitting digitally modulated data at rates up to 150Mbit/sec. The receiver uses the correlation properties of pseudorandom binary sequences (PRBS) to measure and characterise the bit error rate performance. A highly accurate computer controlled positioning system along with automated data acquisition based on embedded system design and windows programming in Lab Windows/CVI will be key to the automation process of experimental measurements.
The research proposed will provide a novel approach to the characterisation of bit error rate performance of digitally modulated time variant and invariant wideband channels that will contribute to the future design of mobile and static wireless LANs operating in the 60GHz frequency bands. Results will significantly aid in future design and development of wireless LAN infrastructures operating at 60GHz for both indoors and point-to-point outdoor channels.
Measurements will be conducted in a variety of environments and data processed and modelled in order to characterise bit-error-rate performance and establish relationships between data rates and coherence bandwidths.
H Cui, J Richter, M O. Al-Nuaimi
The developments of cellular systems, personal communication system as well as local area networks system are rapidly increasing. Increased demand for both to the number of wireless services and ever increasing data rates requires sophisticated planning tools for radio planners. These tools need to address the need for reliable fast services and spectral efficiency. Planning tools need to accurately predict the influence of obstacles such as buildings, and also vegetation. Many empirical models exist, but lack the ability of fully describing the propagation mechanism involved. The Theory of radiative energy transfer (RET) is a fully analytical description of energy transfer through a scattering medium, which has been adapted to describe radio wave propagation through vegetation. This theory approximates the scattering medium, a forest, as statistically homogeneous scatter objects, like tree trunks, branches, and leaves to be large compared to the wavelength. The RET describes the received signal level inside a vegetation medium, such as forests, with vegetation depths, an excess attenuation additional to free space. It also describes the re-radiation from vegetation into space, the so-called “phase function”. The chapter describes the re-radiation in the direction of the propagation of the incident wave as mainly coherent forward scatter. In all other directions there will be isotropic backscatter. The RET require four input parameters, which need to be established experimentally. They are dependent on the type of vegetation, season, frequency and density of the vegetation. These parameters influence the rate of attenuation with vegetation depth. The phase function is described as function of and alone. A method of measuring the phase function by mean of rotating the receiver antenna inside vegetation has been developed. Initial experiments showed that the receiver antenna pattern has significant influence on the measurement accuracy. The RET assume Gaussian antenna patterns with very narrow beamwidth. The radiation pattern of practical antenna however is non Gaussian and often has considerable beamwidth. The measured phase function is understood to be the result of a convolution of the phase function and the antenna radiation pattern. The aim of this project is to find suitable methods of deconvolution to extract the phase function from the measurement with the knowledge of the antenna radiation pattern.
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