Eustace K. Tameh

Hotspot wireless LANs to enhance the performance of 3G and beyond cellular networks

At present, WLANs supporting broadband multimedia communications are being developed and deployed around the world. Standards include HIPERLAN/2 defined by ETSI BRAN and the 802.11 family defined by the IEEE. These systems provide channel adaptive data rates up to 54 Mb/s (in a 20 MHz channel spacing) over short ranges up to 200 m. The HIPERLAN/2 standard also specifies a flexible radio access network that can be used with a variety of core networks, including UMTS. It is likely that WLANs will become an important complementary technology to 3G cellular systems and will typically be used to provide hotspot coverage. In this article the complementary use of WLANs in conjunction with UMTS is presented. In order to quantify the capacity enhancement and benefits of cellular/hotspot interworking we have combined novel ray tracing, software-simulated physical layer performance results, and optimal base station deployment analysis. The study focuses on an example deployment using key lamppost mounted WLAN access points to increase the performance (in terms of capacity) of a cellular network.

Joint Shadowing Process in Urban Peer-to-Peer Radio Channels

For multihop and ad hoc networks, a conventional 1-D channel model cannot capture the spatial correlation of the shadowing processes. This paper investigates the joint spatial correlation property of the shadowing process for peer-to-peer (P2P) radio links in urban environments. When a fixed base station is assumed, statistical analysis reveals that the shadowing process is mainly a result of spatial displacement at the mobile station (MS). Furthermore, the joint correlation property of the MS-MS channel shows that MS displacements at each end of the P2P link have an independent and equal effect on the correlation coefficient. A sum-of-sinusoids simulation model is proposed to generate the joint correlation shadowing process for urban P2P radio channels. The performance of the proposed channel simulator is analyzed in terms of the autocorrelation and joint correlation function of the simulated shadowing process. Simulations illustrate that the proposed model is able to output deterministic shadowing with a normal distribution (in decibels) and the desired correlation properties. It is thus suitable for use in system-level simulations, such as the evaluation of routing and radio resource management algorithms in ad hoc or mesh networks.

Modelling and performance prediction for multiple antenna systems using enhanced ray tracing

Multiple antenna systems can enhance wireless communication links by improving their capacity and/or reliability. Multiple input multiple output (MIMO) communications are the most common exploitation of this property. When applied to deterministic ray modelling, the computational cost of MIMO predication is a major drawback. Two MIMO modelling approaches are investigated. Both methods make use of an enhanced deterministic ray-tracing propagation model. The first method relies on point-to-point prediction for each of the multiple element-to-element links. The second approach estimates the MIMO link matrix from a single point-to-point ray tracing study. A comparison of normalized capacity and path loss is performed for the two methods in an outdoor city centre environment. For a single input multiple output (SIMO) case, the two modelling approaches are presented and compared with measured array data. Result show that the single point-to-point approximation works well and can significantly reduce run-time when compared with full element-to-element ray tracing.

Efficient Multielement Ray Tracing With Site-Specific Comparisons Using Measured MIMO Channel Data

In this paper, an advanced site-specific image-based ray-tracing model is developed that enables multielement outdoor propagation analysis to be performed in dense urban environments. Sophisticated optimization techniques, such as preprocessing the environment database using object partitioning, visibility determination, diffraction image tree precalculation, and parallel processing are used to improve run-time efficiency. Wideband and multiple-input-multiple-output (MIMO) site-specific predictions (including derived parameters such as theoretic capacity and eigenstructure) are compared with outdoor site-specific measurements at 1.92 GHz. Results show strong levels of agreement, with a mean path-loss error of 2 dB and a mean normalized-capacity error of 1.5 b/s/Hz. Physical-layer packet-error rate (PER) results are generated and compared for a range of MIMO-orthogonal frequency-division-multiplexing (OFDM) schemes using measured and predicted multielement channel data. A mean Eb/N 0 error (compared to PER results from measured channel data) of 4 and 1 dB is observed for spatial-multiplexing and space-time block-code schemes, respectively. Results indicate that the ray-tracing model successfully predicts key channel parameters (including MIMO channel structure) and thus enable the accurate prediction of PER and service coverage for emerging MIMO-OFDM networks such as 802.11n and 802.16e

Path Loss Models for Air-to-Ground Radio Channels in Urban Environments

This paper provides new statistical models for air-to-ground channels in an urban environment. The model is derived to operate at frequencies from 200 MHz to 5 GHz. Issues such as path loss and shadowing are evaluated as a function of the elevation angle to the airborne platform, rather than the more usual separation distance used for terrestrial mobile communications. Results demonstrate the advantages of an air-to-ground channel for urban communication, and relayed peer-to-peer links in particular

Statistical peer-to-peer channel models for outdoor urban environments at 2 GHz and 5 GHz

The paper provides a new statistical propagation model that addresses a number of gaps in the area of peer-to-peer radio channel modelling in an urban microcellular environment. The paper focuses on the 2.1 GHz (UMTS) and 5.2 GHz (HIPERLAN/2 and 802.11a/e/h) bands and makes use of a detailed three-dimensional ray-tracing tool. Propagation analysis reveals that the standard deviation of shadowing is actually a function of the separation distance between the transmitter and receiver. Path loss increases with lower terminal heights, as does the probability of a line-of-sight. Statistical channel models are derived that combine standard parameters, such as separation distance, operating frequency and terminal height, with more advanced and innovative parameters, such as distance dependent shadowing and LOS probability.

The use of measurement data to analyse the performance of rooftop diffraction and foliage loss algorithms in a 3-D integrated urban/rural propagation model

This paper describes a 3-D integrated propagation model, which considers propagation effects in both urban and rural environments. It uses a variable resolution digital terrain map (DTM) as well as building and foliage databases. Scattering off terrain pixels and building walls is modelled, as are off-axis terrain and rooftop diffraction contributions. The scattered power is estimated from radar cross-section analysis of the illuminated pixels and walls. The effects of foliage attenuation are also fully considered. The model predicts signal strength and time dispersion and provides fading and arrival angle information for the propagation channel. Narrowband measurements in both urban and rural areas show very good agreement with the predicted results. The accuracy of different foliage-loss and diffraction-loss models has been assessed, with the ITU-R foliage-loss model and the UTD model giving the best results. It is shown that large errors can result if the effects of foliage are ignored in the modelling process.

A 3-D integrated macro and microcellular propagation model, based on the use of photogrammetric terrain and building data

This paper describes a fully three-dimensional deterministic propagation model based on a radar cross-section analysis of terrain pixels and building walls. The model utilises a variable resolution terrain database as well as a buildings database to predict signal strength, time dispersion and fading information, in addition to providing spatial information (e.g. arrival angles) for the propagation channel. It considers scattering from illuminated terrain pixels and building walls as well as considering off-axis terrain and rooftop diffraction contributions. The model can be particularly useful for integrating macrocells and microcells e.g. predicting interference in a microcell due to a nearby macrocell or vice versa. Signal strength predictions from the model show good agreement with narrowband measurements for a rural macrocell. The effects of buildings and foliage in the propagation channel are investigated, and reveal that large errors can result if these effects are ignored. The accuracy improvement of the 3-D analysis over the simpler 2-D approach is also demonstrated.

MIMO–OFDM WLAN Architectures, Area Coverage, and Link Adaptation for Urban Hotspots

This paper considers the suitability of a range of multi-input-multi-output (MIMO) orthogonal frequency-division multiplexing architectures for use in urban hotspots. A ray-tracing propagation model is used to produce realistic MIMO channel data. This information is used to determine the expected throughput and area coverage for various physical (PHY) layer schemes. Site-specific throughput predictions are generated in a city-center environment. Link adaptation (LA) is shown to play a key role in the choice of space-time algorithm, the use of adaptive modulation and coding, and the number of antennas employed at both ends of the radio link. No single PHY layer scheme is suitable to cover the entire coverage area. Results demonstrate the need for MIMO LA under a wide range of channel conditions. For the area under test, 2% of covered locations selected a spatial multiplexing (SM) scheme, 50% selected a space-time block coding (STBC) scheme, and 48% selected a hybrid SM/STBC scheme. With suitable power control and LA, for the scenario under consideration, high peak capacities and good geographic coverage were achieved.

A mixed-cell propagation model for interference prediction in a UMTS network

With the support of mixed cells and hierarchical cell structures in UMTS, propagation information (coverage and interference) between different cell types is required. A mixed-cell propagation model is presented which is suitable for predicting interference in mixed macrocellular and microcellular environments. The main features of the model are described together with examples of interference prediction including C/I and most-likely-server plots. Comparisons between model predictions and measurements for a mixed cell environment are also presented.