Frequency Diversity Spread Spectrum (FDSS) systems have been evolved as a valuable alternative to traditional direct sequence and frequency hopping systems to combat partial band jamming. In FDSS system, the communication frequency band is partitioned into N disjoint subbands on which N replicas of the signal are simultaneously transmitted. The objective is not to erase the signal replicas hit by the jammer, but rather erase the jammer from the replicas. This work describes the use of beamforming to process the spatial diversity for optimum symbol by symbol detection. The procedure is adaptive and suitable for time varying environments. The beamformer corresponding to each frequency diversity component is updated using a gradient algorithm. This algorithm incorporates automatic gain control and is derived based on the fact that the desired signal is present in every frequency component. The optimum detector for FDSS is described and shown to be fully compatible with spatial processing techniques.

Recent theoretical and experimental work at Bell Laboratories has demonstrated that some widely accepted views about our ability to communicate across a wireless fading channel appear to be incorrect. It now seems that the effects of channel fading, rather than being harmful, can actually be beneficial to wireless communications. BLAST (Bell Labs Layered Space Time) is an example of a multiple- antenna communication link whose capacity in a fading environment grows linearly with the minimum of the number of antennas at the transmitter and receiver, with no increase in bandwidth or transmitted power. BLAST is intended to work in a quasi-stationary environment since it expects the fading coefficients between pairs of transmitter and receiver antennas to change slowly enough for the receiver to track them.

Transmission of multimedia data is the goal of third generation (3G) cellular radio systems necessitating high- speed data links where the multipath delay spread generally encompasses many symbol intervals. A blind multichannel identification scheme is presented along with an attendant space-time equalization algorithm, based on subbanding and the Cross-Relation method (CRM) of Xu, Liu, Tong, and Kailath. The basic approach is to apply the CRM in each subband, using basis functions derived from the symbol waveform and the bandpass filter, to blindly identify the frequency response of each channel in the given subband. A method is presented for proper phasing of each blind subband channel estimate to reconstruct the full channel impulse response associated with each antenna. In the practical case where a small number of training symbols are available, a semi-blind approach is presented that follows the blind procedure by a fractionally-spaced decision feedback equalizer (DFE). The advantage of this semi-blind approach is that the number of taps needed for the DFE need only span the duration of the original symbol waveform, which in the wideband case is much less than the number of taps needed to span the delay spread as required in a conventional DFE. Simulations are presented using parameters drawn from proposed 3G Wideband TDMA standards.

In this paper, we propose a block coded modulation (BCM) technique to reduce the peak to average power ratio (PAPR) in OFDM systems. In the proposed technique, binary blocks are mapped to M-ary blocks and M-ary blocks of small sizes with low PAPR are selected. Large size M-ary blocks with low PAPR are constructed by using the selected small size M-ary blocks. Similar to trellis coded modulation, with this technique variable rates ranging from 1 to a rate much higher than 1 of BCM can be obtained upon different requirements of the random error correction capability in the system, given that the PAPR is below a fixed value. PAPR gain is defined by comparing with the uncoded OFDM system. Optimal coding gain for the BCM given a PAPR gain is also obtained in various cases.

Conventional wireless LAN (WLAN) protocols such as IEEE 802.11 allow only one user to transmit at a time in a frequency band. The Smart Wireless LAN (SWL) system adapts smart antenna systems for WLANs to enable multiple WLAN users to transmit simultaneously in a frequency band. Dynamic slot algorithms are used by SWL to improve transmission quality and maximize capacity by selecting users for time slots based on their unique spatial signatures (SS). These types of algorithms have not been studied much for conventional WLANs because only one user is allowed to transmit per time slot in those systems, eliminating the need for slot assignment. The simulation results, showing the tradeoffs of the various algorithms studied, are presented and analyzed. Experimental results studying the stability of the SS for a stationary transmitter and the variation of the SS with displacement are presented, and show the feasibility of using smart antennas with WLAN systems.

The Smart Wireless LAN (SWL) system integrates the Space- Division-Multiple-Access technique (also called smart antennas) with spread spectrum wireless LAN systems, such as IEEE 802.11 systems, to enable multiple wireless LAN users to transmit simultaneously in a frequency band. This advantage creates a potential problem because the 802.11 standard requires that all terminals use the same spreading code, so a straightforward application of the 802.11 standard could result in the loss of the 10 dB of spreading gain achieved by each terminal. This isn't a problem in a conventional system where only one user may transmit at a time in a frequency band, so the solution to this problem was created specifically for the SWL system. The Timing Synchronization Algorithms studied in this paper enable multiple (up to 11) users to use the same spreading code without a loss of the spreading gain. Two complimentary algorithms, in their sorted and unsorted formats, are studied in this paper and their simulation results are analyzed.

The performance of TCP has been well tuned for traditional networks made up of wired links and stationary hosts. Mobile networks, however, differs from conventional wired computer networks and usually suffer from high bit error rates and frequent disconnections due to handoffs. In this paper, we present simulation results for the performance of various TCP implementations in the presence of a wireless link. To concentrate on the mobility and reliability aspects of the wireless connection, our simulation tests used sufficiently large buffer sizes in the fixed host and the base station of the TCP connection. The results show that throughput of the TCP connection is largely influenced by the link-up period of the wireless link. By varying the link-up and the link- down periods, it is possible to obtain better throughput at higher disconnection probability. For example, the throughput of TCP Reno with disconnection probability of 28.6% and a link-up period of 5 is better than the throughput with disconnection probability of 9% and a link- up period of less than 3. The paper presents timing graphs tracing the movement of packets and acknowledgements between the fixed and mobile hosts. Dropped packets or acknowledgements shown in these graphs are the result of mobile disconnection or wireless bit errors and not because of buffer congestion. Unlike wired networks, Reno TCP was found to perform better than Sack in the wireless mobile environment.

In our Photonics West 98 paper, we presented our study results on using commercially available 860 nm high power laser diodes and high-speed laser driver for free-space laser communication terminal application. We demonstrated the feasibility of a free space laser communication link using a junction-up 860 nm high power laser diode driven by a high current laser driver from Hytek Microsystems up to 622 Mb/s. Recent development in high speed InGaAs/GaAs strained layer quantum well (SLQW) laser at 980 nm has provided an additional design option for a laser communication terminal. The advantages of using the 980 nm laser are: (1) WDM market in the telecom industry has created a volume demand for the 980 nm pump lasers. The future cost of 980 nm lasers is expected to be lower due to the economy of scale. (2) In our previous publications, we have demonstrated CW operation of strained layer QW laser at temperature higher than 200 degree(s)C. There is a potential for this type of laser diode to operate in a much harsher and higher temperature environment, and (3) 980 nm pump laser has output power comparable to high power 860 nm laser diodes. In this paper, we will present the high data rate characteristics of a high-speed 980 nm (SLQW) pump laser. Using commercial-off-the-shelf laser drivers we will demonstrate the laser transmitter system characteristics from 622 Mb/s to 3 Gb/s. Detail experimental results on bit- error-rate measurement for a 980 nm device will be presented.

The problem of multiple source tracking with neural network- based adaptive array antennas for wireless terrestrial and satellite mobile communications is considered to this paper. The Neural Multiple Source Tracking algorithm which is based on an architecture of a family of radial basis function neural networks is introduced. In the first stage a number of RBFNNs are trained to perform the detection phase, while in the second state another set of networks is trained for the direction of arrival estimation phase. The field of view of the antenna array is divided into separate angular sectors, which are in turn assigned to a different pair of RBFNN's. When a network detects one or more sources in the first stage, the corresponding second state networks are activated to perform the direction of arrival estimation step. No prior knowledge of the number of present sources is required. Simulation results are performed to investigate the validity of the algorithm for various angular separations, with sources of random relative SNR and when the system suffers from frequency errors. The aforementioned approach results in substantial reduction of the computational complexity associated with the network training.

Smart antenna systems are becoming practical for indoor applications such as wireless local area networks. However, the challenging indoor propagation environment is one of biggest obstacles for designing smart antenna wireless networks. In order to fully understand and characterize channel propagation characteristics or vector channels of smart antenna systems in indoor environments, experiments are conducted using a 1.8 GHz real-time smart antenna testbed with a uniform linear array. The effects of mobile user motion on the vector channel variation are studied in line-of-sight (LOS) and non-line-of-sight (NOLOS) indoor scenarios. The experimental results on the variation of vector channel parameters such as space-time correlation properties, spatial signatures, direction-of-arrivals, multipath angle spread, and complex path fading are presented. Results show that the vector channel varies more significantly in the NOLOS scenario than in the LOS scenario.

This paper motivates the use of electromagnetic ray tracing for the study of smart antennas in Space Division Multiple Access (SDMA) systems. Whereas past ray tracing studies applied to the study of wireless communications systems have been conducted at low spatial resolution for cell-site design, this study makes use of high resolution data so that fast fading effects could be observed. Uplink diversity gain and downlink signal to interference ratio data is simulated for several mobile user environments to gain insight into the fundamental performance limiting criteria of SDMA systems. Mobile environments were varied from several simple artificial urban environments to a simulation of downtown Austin, Texas. When applicable, all simulation results compare favorably with measurement data taken using the smart antenna testbed at the University of Texas at Austin.

We consider the problem of recovering a spread-spectrum signal in the presence of unknown highly correlated spread- spectrum interference and impulsive noise. In terms of basic system and signal model considerations, we assume availability of a narrowband adaptive antenna array that experiences additive white Gaussian noise in time and across elements, as well as impulsive disturbance that is correlated across elements. The space-time receiver design developed in this work is characterized by the following attributes: (1) Adaptive interference suppression is pursued in the joint space-time domain. (2) An adaptive parametric non-linear front-end offers effective suppression of impulsive disturbances at low computational cost. (3) An adaptive auxiliary-vector linear filter post-processor offers effective, low-complexity suppression of SS interferes that avoids eigen-decomposition and/or matrix inversion operations and leads to superior BER performance under rapid, short-data-record system adaptation. Numerical and simulation comparisons with plain and outlier resistant space-time MVDR filtering procedures are included to illustrate and support the theoretical developments.

The paper presents an adaptive technique to create multiple nulls to suppress multiple jammers. It is demonstrated to be effective and efficient. Computer simulations for a linear array of 32 elements have shown that the optimization technique can create wide as well as deep nulls, which correspond well to jammer widths and strengths. The paper also discusses several microstrip arrays with different feed systems that have been successfully employed in wireless security systems. By applying the optimization technique to the microstrip arrays of 8 elements, it is shown that deep nulls are created in the difference pattern as well as the sum pattern. In all cases the SIR improves substantially and converges quickly, requiring not more than four iterations.

In this paper, we systematically study modulated codes (MC) that are encoded after the binary-to-complex symbol mapping. The main advantage of modulated codes is that their encoding arithmetic operations and the intersymbol interference (ISI) channel arithmetic operations are all defined on the complex field and therefore can be algebraically combined together. With MC, the ISI is not treated as distortion but diversity gain. The performance analysis and simulation results of MC over the ISI channel are presented.

Modulated codes (MC) are error correction codes over the real/complex field, which are used for mitigating the intersymbol interference (ISI). Due to the same arithmetic operations of the MC encoding and the ISI channel, MC can be algebraically combined with the ISI channel. Using MC, a new coded zero-forcing decision feedback equalizer (ZF-DFE) is proposed, which has the superior performance over the conventional uncoded/coded ZF-DFE and the uncoded/coded Tomlinson-Harashima precoding. The computational complexity of the new coded ZF-DFE is, however, similar to the one of the uncoded ZF-DFE. In this paper, we also present the performance analysis for the MC coded ZF-DFE and then present the optimal MC design for a given ISI channel or its statistics especially for the MC coded ZF-DFE. The coding gain is formulated over the uncoded system in the AWGN channel.

In this paper, a new multiuser detection scheme for synchronous DS/CDMA system in AWGN channel is presented. Combining the characteristics of the minimum mean squared error blind adaptive multiuser detector and the group partitioned multiuser detector, this new detector can be viewed as a generalized multiuser detector. Its relationship with other detectors is also discussed. Good performance has been proven by simulations.

The problem of separating multiple, independent code division multiple access (CDMA) sources at a single antenna element receiver in the presence of inter symbol interference, co-channel interference and additive white Gaussian noise is addressed. The channels are not known and separation process is required to be blind and adaptive. The system is assumed to consist of multiple users. Only a single user separation is desired. The CDMA channel is assumed to be a multiple-input single-output dispersive channel. The user's codes are assumed to be orthogonal and known to the receiver. The adaptation process is blind and uses the well-known constant modulus (CM) algorithm. The CM approach uses a modulus-corrected version of the output signal in place of the training signal needed in nonblind equalization techniques. The CM algorithm minimizes the deviation of this modulus from a constant in a mean squared error sense. The minimization is done using an LMS-type algorithm.

As is commonly known, the optimal detector for any waveform in Gaussian background noise is a matched filter. However, HF atmospheric noise is non-Gaussian, necessitating alternate detector designs. The industry standard CCIR 322 model for HF atmospheric noise is a graphical, empirical model based on observations of HF atmospheric noise taken over the course of many years at numerous worldwide receive sites. In this work, it is shown that the CCIR 322 noise model may be approximated by a random process which is a member of the class of non-Gaussian random processes known as spherically-invariant random processes (SIRPs). This analytical, empirical SIRP representation is then shown to be identical to the Hall model of impulsive phenomena. In a departure from the optimal, parametric, coherent detector derived by Hall, a locally optimal, parametric, non-coherent detector is presented. In addition, a means to estimate the parameters of the Hall model is provided and is used as the basis for an adaptive, locally optimum, parametric, non- coherent detector design. Monte Carlo simulations are performed to evaluate detector performance, and comparisons are made with two common, sub-optimal, non-parametric detectors.

Blind identification of discrete nonminimum phase FIR channels is studied here. Exploiting higher order statistics of the unknown channel output, a closed-form solution to the FIR channel impulse response is presented. The algorithm is simple and relies only on a nullspace decomposition. It does not involve global minimization of non-unimodal cost functions nor does it use overparametrization. Its application is broad. It can be used either as the final channel estimate or as a good initial point in non-linear cumulant matching techniques.