In wireless networks, a major source of packet loss is packet collision. Typically when a collision occurs the entire packet is assumed to be lost. Most wireless networks use some form of forward error correcting code to insulate the packet from noise and interference. Side information in the form of erasures is a common technique for improving the performance of forward error correcting codes. We will propose an algorithm that will locate collided bits within packets, and use this information to improve the performance of forward error correcting codes such as Reed-Solomon and convolutional codes.

IEEE 802.11 wireless networks employ the so-called RTS/CTS mechanism in order to avoid data packet collisions. The main design assumption is that all the nodes in the vicinity of a sender and a receiver will hear the RTS or CTS packets, and defer their transmission appropriately. This assumption happens to not hold, in general, even under perfect operating conditions. Often, neighboring nodes are "masked" by other ongoing transmissions nearby and, hence, are unable to receive the RTS or CTS packets correctly. We refer to such nodes as masked nodes. In this paper, we describe the masked node problem and show scenarios leading to data packet collisions. We evaluate the impact of masked nodes through mathematical analysis and real experiments on a small IEEE 802.11 ad hoc network. The analytical and experimental data closely match and reveal that the presence of a masked node in a network can result in an order of magnitude increase in data packet loss compared to a network without masked nodes. These results are further validated by extensive simulations on a large-scale network, which show that masked nodes also significantly affect delay and throughput performance. Therefore, masked nodes severely limit the effectiveness of the RTS/CTS mechanism in preventing performance degradation in wireless LANs.

The RTS/CTS mechanism is widely used in wireless networks in order to avoid packet collisions and, thus, achieve high throughput. In ad hoc networks, however the current implementation of the RTS/CTS mechanism may lead to interdependencies so that nodes become unable to transmit any packets during long periods of time. This effect manifests itself in the form of congestion where, after a certain point, the network throughput decreases with increasing load instead of maintaining its peak value. In this paper, we describe and analyze this problem in detail and provide a backward-compatible solution, called RTS validation. Our simulations show that this solution leads to a 60% gain in the peak throughput in addition to stabilizing the throughput at high load.

We outline the benefits and challenges of using angle diversity in nondirected wireless infrared (IR) communications systems. Multiple transmitter beams and multiple narrow field-of view receivers reduce the path loss, multipath distortion, and background noise of the channel, which leads to improved range. We also discuss practical considerations for multielement angle diversity systems, including channel characterization and suboptimal detection techniques. Maximal-ratio combining provides nearly optimal performance up to 100 Mb/s for the angle diversity systems considered. The design and performance of a prototype angle diversity IR communication system are discussed. The prototype can maintain 70 Mb/s at a P/sub e/ of 10/sup -9/ over a 4-m range.

Wireless mobile networks require efficient bandwidth utilization and reliable link connectivity regardless of terminal mobility. Solutions proposed for bandwidth and location management tend to impose two conflicting requirements on network design, namely (1) maintain maximum resource (bandwidth) utilization and (2) reserve enough bandwidth so that the maximum rate of forced call termination rate due to bandwidth unavailability is kept below an acceptable level. To resolve this conflict, we proposed a predictive guard bandwidth scheme that adapts the resources' access priority via guard bandwidth adjustments, in response to changes in instantaneous traffic load, predicted from terminal mobility tracking. A mathematical formulation has been derived to predict a mobile user's movement behavior in terms of boundary crossing probability and cell residence probability, based on which active mobile probability is obtained to estimate instantaneous traffic load in terms of handoff call arrival rate and call departure rate. For given traffic situation, a dynamic programming approach has been further developed to determine the necessary amount of guard bandwidth to achieve the objective of congestion-related performance. The effectiveness of the proposed predictive guard bandwidth scheme is demonstrated through simulation studies in comparison with fixed guard bandwidth scheme. Simulation results indicate that the proposed scheme is insensitive to traffic load fluctuations caused by changes of user movement behavior, and that it maintains reliable connectivity without sacrificing bandwidth utilization efficiency.

We outline the benefits and challenges of using angle diversity in nondirected wireless infrared communications systems. Multiple transmitter beams and multiple narrow-field-of-view receivers reduce the path loss, multipath distortion and background noise of the channel, which leads to improved range. The design and performance of a prototype angle-diversity infrared communication system are discussed. The prototype can maintain 70 Mb/s at a BER of 10/sup -9/ over a 4 m range.

We show that realistic multipath infrared channels can be characterized well by only two parameters: optical path loss and RMS delay spread. Functional models for the impulse response, based on infrared reflection properties, are proposed and analyzed. Using the ceiling-bounce functional model, we develop a computationally efficient method to predict the path loss and multipath power requirement of diffuse links based on the locations of the transmitter and receiver within a room. Use of our model is a simple, yet accurate, alternative to the use of an ensemble of measured channel responses in evaluating the impact of multipath distortion.

We examine multiple-subcarrier modulation (MSM) schemes for wireless infrared digital communication in the indoor environment. Intensity modulation with direct detection (IM/DD) is employed, which results in equivalent baseband channels with a nonnegativity constraint on the input. The power efficiencies of modulation schemes are compared at 30 Mb/s and 100 Mb/s over an ensemble of experimentally determined multipath channels. Carrier selection and power shaping are examined as methods for improving MSM performance. It is found that MSM schemes can allow operation at higher data rates than single-carrier modulation schemes without equalization. Moreover, MSM schemes can be more bandwidth-efficient and also can provide a simple and flexible method for multiple access to the channel. However, they are not as power efficient as single-carrier schemes, and this will limit their use to applications which are not power limited.

We have experimentally characterized nondirected indoor channels that use intensity modulation and direct detection of an infrared carrier at a wavelength of 832 nm. At several locations in each of five different rooms, we have studied line-of-sight and diffuse link configurations, with and without shadowing, amounting to a total of approximately 100 different channels. We have measured channel frequency responses over the 2-300 MHz range by using a swept-modulation frequency technique, and from these data, we have computed channel impulse responses, path losses and r.m.s. delay spreads. Using channel impulse responses, we have calculated power penalties induced by multipath intersymbol interference in baseband on-off-keyed links operating at bit rates of 10, 30 and 100 Mb/s, considering unequalized links and those employing zero-forcing decision-feedback equalization. Unshadowed line-of-sight configurations generally have smaller path losses, r.m.s. delay spreads and power penalties than their unshadowed diffuse counterparts. Shadowed line-of-sight configurations, however, generally exhibit larger values of all three parameters than the corresponding shadowed diffuse configurations. We show that among the channels measured here, there is a strong correspondence between channel r.m.s. delay spread and multipath power penalty. Finally, we provide an analysis indicating why non-directed infrared channels using intensity modulation and direct detection do not exhibit multipath fading, and justifying their representation as linear, time-invariant systems.>