Roshdy H. M. Hafez

Improved transmit null steering for MIMO-OFDM downlinks with distributed base station antenna arrays

Space-division multiple-access (SDMA) is a communication technique that enables a base station to communicate with several mobile users simultaneously. The ability of the base station to spatially separate several users depends on the pairwise cross correlations between the channel matrices of the users (the inter-user correlation). In this paper, we propose an improved null steering downlink multiple-input-multiple-output-orthogonal frequency-division multiplexing (OFDM) system that reduces both the inter-user correlation and the near-far problem resulting in a significant enhancement in system performance. In this system, several base station multiantenna arrays are distributed in a given area. Each array communicates with the base station via optical fiber links, and all transmitter signal processing is performed at the base station. Multiantenna users are spatially separated such that only a subset of the users is served by each tone of the OFDM symbol. The served users are selected based on an algorithm that reduces the inter-user correlations. Distributing the arrays around the users also balances the channel matrix leading to significant reduction in the effect of the near-far problem. The channel matrix of each user is assumed correlated and Ricean distributed. Several data symbols can be spatially multiplexed to each user over each OFDM tone with high reliability and with good total system capacity.

Scheduling schemes for multimedia service in wireless OFDM systems

Scheduling schemes play a key role in the system performance of broadband wireless systems such as WLANs/WMANs. Maximal SNR and round robin are two conventional scheduling strategies that emphasize efficiency and fairness, respectively. The proportional fair scheme provides a trade-off between efficiency and fairness, and has been well studied in TDMA and CDMA systems. In this article we extended the PF scheduling scheme to OFDM-based BWSs (OPF). In addition, we propose three variations: adaptive OPF (AOPF), multimedia AOPF (MAOPF), and normalized MAOPF (NMAOPF) in order to meet the QoS requirements for multirate services in multimedia systems. The adaptive modulation and coding schemes in time varying and frequency selective fading are considered. The system performances of the algorithms are compared in terms of efficiency (throughput and mean packet delay) and fairness (user satisfaction rate and average user rate). Joint physical and media access control layer simulation results show that AOPF and MAOPF can improve throughput at the cost of fairness, and NMAOPF can provide the highest throughput without losing fairness.

Improving throughput and fairness by improved channel assignment using topology control based on power control for multi-radio multi- channel wireless mesh networks

Multi-radio multi-channel (MRMC) wireless mesh networks (WMNs) achieve higher throughput using multiple simultaneous transmissions and receptions. However, due to limited number of non-overlapping channels, such networks suffer from co-channel interference, which degrades their performance. To mitigate co-channel interference, effective channel assignment algorithms (CAAs) are desired. In this article, we propose a novel CAA, Topology-controlled Interference-aware Channel-assignment Algorithm (TICA), for MRMC WMNs. This algorithm uses topology control based on power control to assign channels to multi-radio mesh routers such that co-channel interference is minimized, network throughput is maximized, and network connectivity is guaranteed. We further propose to use two-way interference-range edge coloring, and call the improved algorithm Enhanced TICA (e-TICA), which improves the fairness among flows in the network. However, the presence of relatively long links in some topologies leads to conflicting channel assignments due to their high interference range. To address this issue, we propose to utilize minimum spanning tree rooted at the gateway to reduce conflicting channels, and in turn, improve medium access fairness among the mesh nodes. We call the improved algorithm e-TICA version 2 (e-TICA2). We evaluate the performance of the proposed CAAs using simulations in NS2. We show that TICA significantly outperforms the Common Channel Assignment scheme in terms of network throughput, and e-TICA and e-TICA2 achieve better fairness among traffic flows as compared to TICA. It is also shown that e-TICA2 leads to improved network throughput, as compared to TICA and e-TICA.

Adaptive rate controlled, robust video communication over packet wireless networks

Video transmission over wireless packet networks is gaining importance due to the concept of universal personal communication. Further, it is considered an important step towards wireless multimedia. The challenge however is to achieve good video quality over mobile channels, where typically the channel conditions vary due to signal fading. Hence this paper investigates adaptive rate controlled video transmission for robust video communication under packet wireless environment. A combination of mobile and an ATM backbone network is assumed in this work. An error resilient design for the video coder, as proposed in Rajugopal et al. (1996) is employed here. This video coder comprises wavelet transform (WT), multi-resolution motion estimation (MRME) and a robust design for zero tree quantization. Two configurations, one employing MRME and the other using 1D-WT for temporal analysis, are considered for the video coder. Adaptive dynamic rate control is required to adapt the video communication to the channel conditions. It provides more channel protection when the channel is severe and improves the source rate and hence the performance when the conditions are favorable. An algorithm for dynamic rate control under varying channel conditions is proposed in this paper. It is evaluated under narrowband and broadband channel conditions. From the results, it is concluded that the dynamic rate control is very effective in optimizing the quality under varying mobile channel conditions. It was observed that the dynamic rate control provides at least an acceptable video quality under severe channel conditions and a good video quality when the channel conditions are favorable.

Throughput Improvement in Multi-Radio Multi-Channel 802.11a-Based Wireless Mesh Networks

Single-radio mesh routers operating on a single channel suffer from low throughput due to collisions. Equipping mesh routers with multiple radios operating on non-overlapping channels can significantly improve the throughput. However, the assignment of channels to radios in a multi-radio mesh network is a challenging task. In this paper, we propose a channel assignment algorithm, TICA (Topology-controlled Interference-aware Channel-assignment Algorithm), which significantly improves network throughput by minimizing interference within the mesh network using a novel approach of controlling the network topology based on power control before intelligently assigning the channels to the multi-radio mesh routers, as well as guaranteeing network connectivity.

On the impact of interference models on channel assignment in multi-radio multi-channel wireless mesh networks

We study the impact of three different interference models on channel assignment in multi-radio multi-channel wireless mesh networks, namely the protocol model, the signal-to-interference ratio (SIR) model and the SIR model with shadowing. The main purpose is to determine the minimum number of non-overlapping frequency channels required to achieve interference-free communication among the mesh nodes based on a realistic interference model. We propose novel, effective, and computationally simple methods for building the conflict graph based on the SIR model with shadowing, and for finding channel assignments from the resulting conflict graph. We find that channel assignment using a realistic interference model (SIR model with shadowing) requires more frequency channels for network throughputs at different node-degree constraints as compared to using simpler interference models.

Novel Scheduling Algorithms for Multimedia Service in OFDM Broadband Wireless Systems

Scheduling algorithms play a key role in overall system performance of broadband wireless systems (BWS) such as WLAN/WMAN. Maximal SNR (MaxSNR) and Round Robin (RR) are two conventional scheduling strategies which emphasize efficiency and fairness respectively. Proportional Fair (PF) algorithm provides tradeoff between efficiency and fairness and it has been well studied in TDMA and CDMA systems. In this paper, we apply the PF scheduling algorithm to IEEE 802.16a OFDM based BWS and call it OPF. In addition, we propose three algorithms for multimedia services: (1) Adaptive OPF (AOPF), (2) Multimedia AOPF (MAOPF) and (3) Normalized MAOPF (NMAOPF). Adaptive modulation and coding schemes are applied to combat the time varying nature of the wireless channels. System performances of all six algorithms are compared in terms of efficiency and fairness.. Joint PHY and MAC layer simulation results show that the proposed schemes provide better tradeoff between efficiency and fairness than conventional algorithms.

Broadband indoor wireless communications in the (20-60)GHz band: Signal strength considerations

Future wireless communication systems designed for indoor environments are expected to handle integrated voice, data, and video services. The use of millimeter waves in the 20GHz to 60GHz band for future indoor wireless communications appears to be a favorable choice as it provides sufficient bandwidth to support broadband services, current sparse usage, small RF device dimensions and favorable regulatory environment. This work is aimed to provide insight into the capabilities and limitations of the (20-60)GHz band in supporting future indoor wireless communications requirements from a signal strength perspective. This study is based on the currently available information about the (20-60)GHz band, such as: the attenuation rate of different building materials, the distance attenuation factors, the ambient noise, and the power requirements. The results were obtained through the simulation of indoor environments with propagation parameters typical of the MM-wave propagation.

Two level access control strategy for multimedia CDMA

In this paper we propose a new technique for access control in an outdoor CDMA cell supporting multimedia traffic. The proposed scheme controls the flow of the traffic at both packet and call level. Delay scheduling at packet level is employed to minimize the variance of intra-cell interference while a call admission control based on a modified equivalent bandwidth technique exploits the minimized interference and reduces the call blocking probability. It is shown that the proposed scheme allows efficient integration of voice and low bit rate video in a CDMA cell with imperfect power control.

Channel Requirements for Interference-Free Wireless Mesh Networks to Achieve Maximum Throughput

In a multi-radio multi-channel wireless mesh network, a channel assignment that is based on a fixed number of available frequency channels may cause co-channel interference, which degrades the network throughput. We address this problem by ensuring interference-free communication among the mesh nodes. The main purpose of this work is to determine the minimum number of non-overlapping frequency channels required for interference-free channel assignment in order to achieve the maximum network throughput while maintaining fairness among the multiple network flows, given the location of the mesh nodes and the number of their half-duplex radio interfaces. To minimize the number of channels required, we apply our Select x for less than x Topology Control Algorithm to build the connectivity graph instead of using the classical approach based on maximum power (MP). We show that our approach outperforms the MP-based approach in terms of the number of channels required as well as the links to channels ratio for all node-degrees.