A wide area mobile communication network with multiple modes of transmission routing is described. This is achieved through the integration of multiple, independent wide area networks at both the fixed base station and the mobile device. Through this network, mobile communication devices have access to multiple rf transmission paths with varying characteristics of signal strength, coverage, data rates and cost. Thereby, a composite network is synthesized, combining the best features of each subnetwork.

The expansion of global networks, caused by growth and acquisition within the commercial sector, is forcing users to move away from proprietary systems in favor of standards-based, open systems architectures. The same is true in the wireless data communications arena, where operators of proprietary wireless data networks have endeavored to convince users that their particular implementation provides the best service. However, most of the vendors touting these solutions have failed to gain the critical mass that might have lead to their technologies' adoption as a defacto standard, and have been held back by a lack of applications and the high cost of mobile devices. The advent of the cellular digital packet data (CDPD) specification and its support by much of the public cellular service industry has set the stage for the ubiquitous coverage of wireless packet data services across the Unites States. Although CDPD was developed for operation over the advanced mobile phone system (AMPS) cellular network, many of the defined protocols are industry standards that can be applied to the construction of a common infrastructure supporting multiple airlink standards. This approach offers overall cost savings and operation efficiency for service providers, hardware, and software developers and end-users alike, and could be equally advantageous for those service operators using proprietary end system protocols, should they wish to migrate towards an open standard.

New error correction protocols and cellular modem pools have significantly improved the reliability and transmission rates of data over the analog circuit-switched cellular network. Analog cellular modems can now match traditional landline modem speeds and make the standard cellular network a viable option for wireless data transmissions.

Deployment of distributed applications in the wireless domain lack equivalent tools, methodologies, architectures, and network management that exist in LAN based applications. A wireless distributed computing environment (KeyWare^TM) based on intelligent agents within a multiple client multiple server scheme was developed to resolve this problem. KeyWare renders concurrent application services to wireline and wireless client nodes encapsulated in multiple paradigms such as message delivery, database access, e-mail, and file transfer. These services and paradigms are optimized to cope with temporal and spatial radio coverage, high latency, limited throughput and transmission costs. A unified network management paradigm for both wireless and wireline facilitates seamless extensions of LAN- based management tools to include wireless nodes. A set of object oriented tools and methodologies enables direct asynchronous invocation of agent-based services supplemented by tool-sets matched to supported KeyWare paradigms. The open architecture embodiment of KeyWare enables a wide selection of client node computing platforms, operating systems, transport protocols, radio modems and infrastructures while maintaining application portability.

Whip type antennas are probably the most commonly used antennas in portable rf systems, such as cordless and cellular phones, rf enabled laptop computers, personal digital assistants (PDAs), and handheld computers. Whip antennas are almost always mounted on the chassis which contains the radio and other electronics. The chassis is usually a molded plastic which is coated with a conducting paint for EMI purposes. The chassis which appears as a lossy conductor to the antenna, has several effects -- detuning, altering the gain of the antenna, and shadowing its radiation pattern. Extensive modeling and measurements must be performed in order to fully characterize the affects of the chassis on the whip antenna, and to optimize antenna type, orientation and position. In many instances, modeling plays a more important role in prediction of the performance of whip antennas, since measurements become difficult due to the presence of common mode current on feed cables. In this paper models and measurements are used to discuss the optimum choice of whip antennas and the impact of the chassis on radiation characteristics. A modeling tool which has been previously described and has been successfully used to predict radiated field patterns is used for simulations, and measured and modeled results are shown.

Achieving reliable, 100 kbps powerline communications on the low voltage, AC powerlines has been realized by combining new techniques in spread spectrum technology with a robust, powerline specific protocol. This approach enables reliable, high speed data networking on the electrically hostile, low voltage powerline. Applications for the new technology include utility DA/DSM (distribution automation/demand side management), intraoffice LANs, powerline based telephony, and industrial data networking applications. This technological advancement was made possible through statistical modeling of the low voltage powerline, and developing unique spread spectrum and protocol techniques specific to the resulting powerline environment. The technology has been implemented as a highly integrated, CMOS chip set, allowing straightforward integration into OEM systems and products.

Mobitex is a mobile data technology standard created by Eritel, now a wholly owned subsidiary of Ericsson, that has been in existence for about a decade. Originally designed as a low speed (1.2 kbps) data system with a voice dispatch overlay, it was significantly enhanced in 1990 for use in North America and the UK. The enhanced system is a data-only system using cellular architecture and multi-channel frequency reuse, store-and-forward capability, and an 8 kbps over-the-air data rate. The mission of RAM Mobile Data USA Limited Partnership ('RAM') is to provide high quality, cost efficient, wireless data communications solutions in its targeted market segments. RAM's Mobitex network is currently one of the two networks providing two way wireless data services nationwide using a long distance service provider of the customer's choice.

Use of direct sequence spread spectrum (DS/SS), in conjunction with TDMA, provides a more cost effective PCS system with better performance in the typical multipath environments encountered, than either CDMA-only or TDMA-only design approaches. This paper describes Omnipoint Corporation's PCS system design approach which utilizes TDMA with a DS/SS overlay and its cost and performance advantages.

A multi-media multi-rate CDMA network is introduced in this paper. Multi-rate CDMA networks not only can transmit information sources with different data rate but they can also accommodate more users by adjusting the transmission rate. The multi-rate CDMA system is designed as a fixed-chip-rate variable-data-rate system differing the multiple-chip-rate CDMA system introduced. The bit error probability of a multi-rate CDMA based on Gaussian approximation is given to obtain the multiple access capacities (MAC) for different data rate. We consider three types of traffic, voice, high-priority data, and low-priority data and two types of transmission rate in the multi-rate CDMA system. A variable-rate admission policy is proposed for voice users and high-priority data users so that the system can accommodate more users. In this way, the average call blocking probability can be fixed by adjusting the number of users operating in the low data rate. Since two sources are competent for available CDMA codes, the code allocations schemes to voice and high-priority data users may yield different call blocking probability. Five allocation schemes are proposed and investigated in this paper. A parallel queuing model is also presented to evaluate exactly the packet delay and throughput for low-priority data users in both forward and reverse links. Since all low-priority data users compete codes remained unused by voice and high-priority data users, four admissions allocation schemes are proposed and analyzed.

Intersatellite crosslinks (between satellites in different orbits) are difficult to implement and manage in nongeostationary orbital constellations. It is possible to alleviate the need for intersatellite crosslinks and yet provide global coverage at a propagation delay of no more than 125 milliseconds. The satellite orbit architecture used in this paper is suitable for future satellite networks applications, such as interactive multimedia, video conferencing, image transfer, audio, voice, and store-and-forward services. The Earth's surface has been divided into spherical hexagonal areas with some overlapping. The key parameters that define the satellite network architecture have been derived in this paper. These parameters include the Earth's central angle, number of satellites per orbit, number of orbits, cell size, maximum propagation delays, and the average number of satellites available for a user (as a function of distance from the Equator). Each of these parameters are plotted vs. satellite altitude and elevation angle. The propagation delay is not sensitive to changes in elevation angle, but sensitive to orbital altitude. The satellite orbit architecture presented herein is currently under investigation for networking aspects and response to traffic loading.

Several constant and quasi-constant envelope modulation techniques are compared to a new modulation technique called SEQAM for the purpose of fulfilling the requirements of PCS cellular radio technology. Constant envelope modulations are desirable for portable digital radios due to the reduced power amplifier backoff needed and consequent increase in power efficiency and decrease in handset dc power requirements and physical size. SEQAM has been adopted by IS-661 for its reduced spectral occupancy, allowing it to efficiently meet FCC emissions requirements and system channel spacing requirements while simultaneously allowing non-linear operation of the power amplifier. It is shown that SEQAM is a flexible modulation allowing for the tradeoff between spectral occupancy and BER performance.

In this paper, a comparison is made between two different modulation/demodulation techniques, capable of offering efficient wireless data transmission. More specifically, M-ary orthogonal modulation with non-coherent demodulation, and M-ary bi-orthogonal differentially encoded modulation with differentially coherent demodulation are considered. These two modulation schemes are compared on a number of features, like: demodulator complexity, bandwidth efficiency, and bit error rate. The advantage of the M-ary bi- orthogonal differentially encoded one prevails.

USA Digital Radio (USADR) is finalizing the development of new state-of-the-art formats for in-band on-channel (IBOC) delivery of digital audio broadcasting (DAB). USADR's IBOC DAB systems are designed for top-notch digital audio delivery as well as enhanced ancillary data transmission capabilities. USADR's AM and FM IBOC DAB systems employ MUSICAM^R source encoding as well as innovative modulation techniques which address the various radio channel impairments characteristic of AM and FM band propagation. The USADR IBOC DAB systems are designed to be backwards compatible with conventional AM and FM broadcasting for a seamless and cost-effective transition to DAB.

The performance of a store-and-forward transmission system for low Earth-orbit (LEO) satellite communications, using noncoherent frequency-shift keying (FSK), is analyzed. The system supports a network in which static and mobile terminals directly access the satellite to transfer messages or data files. The satellite transmission channel is modeled as having flat Rician fading characteristics. Factors that affect transmission performance such as propagation effects (multipath, shadowing, and Doppler), antenna directivity, and satellite elevation are discussed. In order to evaluate the throughput performance of selective-repeat automatic- repeat-request (ARQ), packet success probabilities are derived for both slow and fast fading conditions. It is found, under fast fading conditions, that throughput performance is sensitive to packet length and Rice-factor. However, the opposite is shown under slow fading conditions. It is also shown, under fast fading conditions, that the throughput performance of selective-repeat ARQ is improved with the addition of forward-error-correction (FEC) block codes.

This communication deals with the design and performance analysis of MSK-type pulse formats specially suited for differentially coherent detection. The analysis deals with the conventional delay and multiply demodulator preceded by a bandpass filter. Conventional MSK when using a differentially coherent demodulator presents a performance level well below BPSK, because the output of the bandpass matched filter exhibits intersymbol interference (ISI). In the communication we show that it is possible to design the pulses that achieve the same performance of differential BPSK and simultaneously get a power spectral density (PSD) with significantly sharper roll-off then conventional MSK, thus improving the inter-channel interference performance.

For some applications such as mobile communication or transmission of multimedia data, it is desirable to use modems with variable constellation schemes to adapt to the fast changing channel to accommodate a wide range of data transmission rates, bit error rates and data types. In this research we are interested in designing receivers which can identify the QAM constellation schemes from the received signal with an unknown reference phase. The problem is modeled as an M-ary hypothesis test with each hypothesis corresponding to one of the M possible constellation schemes. We show the performance of BPSK, QPSK, 8-PSK, 16-PSK, V.29-7200 bps, V.29-9600 bps, 16-QAM, 32-QAM, 64-QAM, 128-QAM, and 256-QAM classifiers in numerical experiments.

The time varying nature of wireless channels can cause severe error bursts or dropouts. It is important to recover lost data in coded images in interactive video communication over wireless network. A good spatial interpolation strategy is considered as essential for replenishing missing blocks in still images and video frames. This paper proposes a novel spatial directional interpolation scheme which makes use of the local geometric information extracted from the surrounding blocks. Specifically, statistics of the local geometric structure are modeled as a bimodal distribution. The two nearest surrounding layers of pixels are converted into binary pattern to reveal the local geometric structure. A measure of directional consistency is employed to resolve ambiguity of possible connections of the transition points on the inner layer. The transition lines can be specified within one-pixel accuracy, unlike previous directional filtering schemes which usually filter along only one single direction chosen from a finite candidate set. The new approach produces results that are superior to that of other approaches, in terms of both peak signal-to-noise ratio (PSNR) and visual quality. The computation is also much reduced. It is observed that local structures such as edges, streaks and corners are well preserved in the reconstructed image.

JPEG is a widely used standard for coding still images. The JPEG standard is very sensitive to transmission errors and can be employed successfully only when the transmission channel is almost error-free. Wireless communication channels are characterized by long bursts of data errors and average bit error rates of 10^-3 - 10^-4 are typical in cellular mobile radio channels. In this paper we present an efficient scheme to transmit JPEG images over wireless channels. Also considered are post-processing techniques for error- concealment of JPEG images transmitted over these wireless channels. The effectiveness of these postprocessing techniques is demonstrated for a diverse set of images obtained from indoor radio channel experiments.

In this paper we consider transmitter diversity for a OFDM system. In a OFDM system, each of the sub-carriers experience a different fade, and depending on the coherence band-width, several adjacent sub-carriers may be in a deep fade. If the channel is also varying slowly, these sub-carriers will be in a fade for a long time. To avoid the waste of band-width and errors on the affected sub-carriers, transmitter diversity can be used with the OFDM system so that the sub-carriers over the 'bad' part of the spectrum can still be utilized. OFDM systems which use FFT based demodulation at the receiver are especially unsuitable for Doppler and delay spread channels. In such channels OFDM system performance degrades due to the loss in orthogonality of the sub-carriers and the consequential increase in inter-carrier interference (ICI). Hence, we conclude that an FFT based receiver is not suitable for wireless channels, and we propose the use of multiple demodulators at the receiver, each operating over an individual sub-carrier band. This increases the complexity, however, such an increase is necessary for a wireless channel. In this paper we show how the full band-width available to the system can be utilized using multiple transmitting antennas.

This paper describes a system which assists in the strategic placement of rf base stations within buildings. Known as the site modeling tool (SMT), this system allows the user to display graphical floor plans and to select base station transceiver parameters, including location and orientation, interactively. The system then computes and highlights estimated coverage regions for each transceiver, enabling the user to assess the total coverage within the building. For single-floor operation, the user can choose between distance-dependent and partition- dependent path-loss models. Similar path-loss models are also available for the case of multiple floors. This paper describes the method used by the system to estimate coverage for both directional and omnidirectional antennas. The site modeling tool is intended to be simple to use by individuals who are not experts at wireless communication system design, and is expected to be very useful in the specification of indoor wireless systems.

The cell site in certain PCS systems polls the user terminals to determine their needs for system access. Reflections of the high power interrogation signal can mask the presence of the user terminal's response transmission. Circular polarization, which has been used in radar systems to reduce reflections from rain, is considered as a means of reducing the interference caused by reflections of the polling signal. The paper considers the mechanism of this reduction and establishes the requirements for amplitude and phase balance of the two components of linear polarization as a function of the required level of cancellation at the polling antenna. The mechanisms of depolarization of the reflected signal in both natural and man-made environments are considered. Man-made environments can have very large reflections, but generally these reflections have a low level of cross-polarization. Propagation in urban environments is considered to determine how rapidly the environmental reflections decrease as a function of the time delay between the arrival of the polling signal at the user and the transmission of the user's reply.

High fidelity simulations of communication links operating through frequency selective fading channels require both accurate channel models and faithful reproduction of the received signal. In modern radio receivers, processing beyond the analog-to-digital converter (A/D) is done digitally, so a high fidelity simulation is actually an emulation of this digital signal processing. The 'simulation' occurs in constructing the output of the A/D. One approach to constructing the A/D output is to convolve the channel impulse response function with the combined impulse response of the transmitted modulation and the A/D. For both link simulations and hardware channel simulators, the channel impulse response function is then generated with a finite number of samples per chip, and the convolution is implemented in a tapped delay line. In this paper we discuss the effects of the channel model tap spacing on the performance of delay locked loops (DLLs) in both direct sequence and frequency hopped spread spectrum systems. A frequency selective fading channel is considered, and the channel impulse response function is constructed with an integer number of taps per modulation symbol or chip. The tracking loop time delay is computed theoretically for this tapped delay line channel model and is compared to the results of high fidelity simulations of actual DLLs. A surprising result is obtained. The performance of the DLL depends strongly on the number of taps per chip. As this number increases the DLL delay approaches the theoretical limit.

This paper sets out to review the applications for optical free space links in future broadband networks, covering both long (0.5 to 5 km) and short (< 500 m) external as well as internal systems. The fundamental properties of optical systems -- high bandwidth, minimal regulation and potential low cost, together with the challenges of a relatively low power margin and extreme attenuation in heavy fog -- are evaluated in the light of 5 year's research by the authors.

The major aspects that impair the performance of optical wireless transmission systems are the shot noise induced by the steady ambient light level, transmitted optical power limitations (high path losses), channel bandwidth limitations due to multipath dispersion and the interference produced by artificial light sources. Several modulation and encoding schemes have been proposed for this channel and their performance has been studied and presented by several authors. The work reported in this paper extends the previous analysis by taking into account the optical power penalty induced by artificial light interference. An analytical approach is used to estimate the optical power penalty induced by artificial light interference. In practical systems, the effect of the interference is usually mitigated using electrical high- pass filters. In this paper the combined effect of interference and high-pass filter is evaluated by resorting to simulation. The presented results show that interference produced by fluorescent lamps driven by electronic ballasts induce high power penalties in OOK and PPM systems, even when high-pass filtering is used. For the interference produced by incandescent lamps and fluorescent lamps driven by conventional ballasts, the power penalty induced in OOK systems can be effectively reduced using high-pass filtering, while PPM is very tolerant to that interference even without any high-pass filtering.

In this paper, we present a theoretical study of the dispersion in the optical diffuse infrared channel with complete characterization of the multipath dispersion effect for fixed transmitter and receiver locations. For a NRZ transmitted pulse we obtain the received pulse by modeling the channel impulse response and then an equalizer is employed to eliminate intersymbol interference. We describe a complete realization technique for a filter matched to the equalized pulse shape. The realization technique starts with the time domain specification of the equalized pulse shape and yields a network with a response which closely approximates the response desired. The results indicate the usefulness of the realization technique for determining filters for wireless networks when all that is available is a time domain specification of the received pulse.

This paper describes an analysis for the probability of undetected errors for SDLC/Manchester encoded links. Results of mean time between undetected errors versus probability of line error rate are given. Such graphs are of interest to free space IR link designers offering better insight into the robustness of their design, for 16 or 32 bit CRCs. Coupled with the cost constraints it allows one to decide, with increased confidence, on the suitability of the combination of modulation scheme, framing and choice of CRC for the product under consideration. Furthermore the analysis offers a means for determining the weakest part (most susceptible to noise) of the SDLC framing.

Recent publications have indicated that the effects of propagation delay spread may not be as damaging as first feared when using the Digital European Cordless Telecommunications (DECT) standard in a wireless local loop (WLL) application. It has been suggested that the use of directional antennas in the WLL application is responsible for the surprisingly good performance exhibited in trials conducted using DECT based equipment. This paper proposes a two-path propagation model in two dimensions to theoretically examine the protection offered against excessive delay spread by the use of directional antennas. Results indicate that a range of 1 km, the use of a directional antenna at a multipoint network end can give protection from single reflectors out to a maximum distance of 350 m.

This paper gives special attention to wireless local area networks using infrared technology mainly with respect to the reception techniques and presents the performance evaluation of optical sectored receivers for indoor wireless communication systems in the presence of artificial and natural noise sources. Performance evaluation was extended to four distinct sectored receiver configurations which result in significant gains over a non-sectored optical receiver. A characterization of the ambient light noise distribution due artificial light was performed. Also, the radiation patterns of some directional incandescent lamps were measured and modeled through a generalized Lambertian function. The feasibility of optical sectored receivers in the presence of directional light sources is demonstrated.

In this paper, multipath dispersion associated with the optical diffuse infrared channel is investigated and its effects on optical orthogonal codes (OOCs) are evaluated. A new equalization technique is presented and analyzed in the context of this channel. The method is shown to be effective in reducing the resultant intersymbol interference (ISI). The results indicate that good performance can be achieved with minimum hardware overheads.

In this paper we discuss the design of an optical telepoint receiver. The level of receiver performance required when optical baseband video signals are to be recovered is discussed and a suitable receiver topology is presented. Theoretical and measured results are presented. The effect of ambient light is considered.

A hybrid pulse position modulation code division multiple access (PPM-CDMA) scheme is proposed and studied in the context of the indoor optical wireless system. An original expression is derived for the format power spectral density (PSD) under jitter and results are presented for the extracted chip clock timing variance and the resultant jitter wrong chip error probability, resulting in specifications on the PPM-CDMA order, threshold crossing timing variance, and phase lock loop (PLL) bandwidth.

Optical wireless is a relatively new field, its development encouraged by the ever growing popularity of mobile computing and communications equipment. We present practical results for a 10 MB/s diffused optical wireless Ethernet system which allows a user to freely roam around a room with a laptop computer while maintaining a full Ethernet service. This system has been developed in collaboration with BT labs. Receiver designs and analysis are presented. Also, optical systems have been considered, and practical results presented.

Code division multiple access systems using power control and other methods of controlling the 'near-far problem' have existed for more than 40 years, and have been in production for at least 35 years. (These systems also employed multipath mitigation methods that allowed them to operate in both high speed aircraft and tropo scatter applications). This paper examines the fact that power control CDMA was specifically rejected for use in mobile, aircraft and satellite systems, in favor of other methods that perform better and are more robust in the presence of other systems.