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Home wireless networks are mainly used for data transmission; however, they are now being used in video delivery applications, such as video on demand or wireless internet protocol (IP) television. Off-the-shelf technologies are inappropriate for the delivery of real-time video. In this paper, a packetization method is presented for robust H.264 video transmission over the IEEE 802.11 wireless local area network (WLAN) configured as a wireless home network. To overcome the poor throughput efficiency of the IEEE 802.11 Medium Access Control (MAC), an aggregation scheme with a recovery mechanism is deployed and evaluated via simulation. The scheme maps several IP packets (each containing a single H.264 video packet called a Network Abstraction Layer (NAL) unit) into a single larger MAC frame. Video robustness is enhanced by using small NAL units and by retrieving possible error-free IP packets from the received MAC frame. The required modifications to the legacy MAC are described. Results in terms of throughput efficiency and peak-signal-to-noise ratio (PSNR) are presented for the case of broadcast and real-time transmission applications. Compared to the legacy case, an 80% improvement in throughput efficiency is achieved for a similar PSNR video performance. For fixed physical layer resources, our system provides a 2.5-dB gain in video performance over the legacy case for a similar throughput efficiency. The proposed solution provides considerable robustness enhancement for video transmission over IEEE 802.11-based WLANs. Index Terms--Medium Access Control (MAC), packetization, peak-signal-to-noise ratio (PSNR), video transmission, wireless local area networks (WLANs).

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. Index Terms--Link adaptation (LA), orthogonal frequency-division multiplexing (OFDM), power control, propagation, space-time block codes (STBCs), spatial multiplexing (SM).

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. Index Terms--Ad hoc, channel model, multihop, shadowing, spatial correlation, urban.

In this paper, the capacity of multiple-input multiple-output (MIMO) communication systems is investigated in the presence of a Line-of-Sight (LoS) component. Under this scenario, the channel-response matrix is usually rank deficient due to the high correlation between the LoS responses. Previous studies have shown that this problem can be overcome by the use of specifically designed antenna arrays. The antenna elements are positioned to preserve orthogonality and, hence, maximize the LoS-channel rank. To help in the design of such architectures, we derive a 3-D criterion for maximizing the LoS MIMO capacity as a function of the distance, the orientation, and the spacing of the arrays. The sensitivity of these systems to imperfect positioning and orientation is examined using a geometric MIMO model. The spectral efficiency is also investigated in the presence of scattered signals in the environment using a stochastic channel model and a Monte Carlo simulator. To demonstrate the validity of our predictions, we present the results of two MIMO measurement campaigns in an anechoic and an indoor environment where the measured capacities are compared with the capacities obtained from our models. All experimental results validate our predictions and, hence, confirm the potential for superior MIMO performance (when the developed criterion is applied) in strong LoS channels. Index Terms--Antenna arrays, channel capacity, K-factor, line-of-sight (LoS), multiple-input multiple-output (MIMO), Ricean channel.

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 [E.sub.b]/ [N.sub.o] error (compared to PER results from measured channel data) of 4 and 1 dB is observed for spatial-multiplexing and spacetime 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. Index Terms--Multiple-input-multiple-output (MIMO), propagation, ray tracing, scattering.