Showing papers on "Spread spectrum published in 2007"

Mobile Vehicle-to-Vehicle Narrow-Band Channel Measurement and Characterization of the 5.9 GHz Dedicated Short Range Communication (DSRC) Frequency Band

Abstract: This study presents narrow-band measurements of the mobile vehicle-to-vehicle propagation channel at 5.9 GHz, under realistic suburban driving conditions in Pittsburgh, Pennsylvania. Our system includes differential Global Positioning System (DGPS) receivers, thereby enabling dynamic measurements of how large-scale path loss, Doppler spectrum, and coherence time depend on vehicle location and separation. A Nakagami distribution is used for describing the fading statistics. The speed-separation diagram is introduced as a new tool for analyzing and understanding the vehicle-to-vehicle propagation environment. We show that this diagram can be used to model and predict channel Doppler spread and coherence time using vehicle speed and separation.

Transmission Capacity of Wireless Ad Hoc Networks With Successive Interference Cancellation

Abstract: The transmission capacity (TC) of a wireless ad hoc network is defined as the maximum spatial intensity of successful transmissions such that the outage probability does not exceed some specified threshold. This work studies the improvement in TC obtainable with successive interference cancellation (SIC), an important receiver technique that has been shown to achieve the capacity of several classes of multiuser channels, but has not been carefully evaluated in the context of ad hoc wireless networks. This paper develops closed-form upper bounds and easily computable lower bounds for the TC of ad hoc networks with SIC receivers, for both perfect and imperfect SIC. The analysis applies to any multiuser receiver that cancels the K strongest interfering signals by a factor z isin [0, 1]. In addition to providing the first closed-form capacity results for SIC in ad hoc networks, design-relevant insights are made possible. In particular, it is shown that SIC should be used with direct sequence spread spectrum. Also, any imperfections in the interference cancellation rapidly degrade its usefulness. More encouragingly, only a few - often just one - interfering nodes need to be canceled in order to get the vast majority of the available performance gain.

The Guard Zone in Wireless Ad hoc Networks

Abstract: In ad hoc networks, it may be helpful to suppress transmissions by nodes around the desired receiver in order to increase the likelihood of successful communication. This paper introduces the concept of a guard zone, defined as the region around each receiver where interfering transmissions are inhibited. Using stochastic geometry, the guard zone size that maximizes the transmission capacity for spread spectrum ad hoc networks is derived - narrowband transmission (spreading gain of unity) is a special case. A large guard zone naturally decreases the interference, but at the cost of inefficient spatial reuse. The derived results provide insight into the design of contention resolution algorithms by quantifying the optimal tradeoff between interference and spatial reuse in terms of the system parameters. A capacity increase relative to random access (ALOHA) in the range of 2 - 100 fold is demonstrated through an optimal guard zone; the capacity increase depending primarily on the required outage probability, as higher required QoS increasingly rewards scheduling. Compared to the ubiquitous carrier sense multiple access (CSMA) which essentially implements a guard zone around the transmitter rather than the receiver - we observe a capacity increase on the order of 30 - 100%

COGNITIVE RADIOS FOR DYNAMIC SPECTRUM ACCESS - Dynamic Frequency Hopping Communities for Efficient IEEE 802.22 Operation

Abstract: One of the key challenges of the emerging cognitive radio-based IEEE 802.22 wireless regional area networks (WRANs) is to address two apparently conflicting requirements: ensuring QoS satisfaction for WRAN services while providing reliable spectrum sensing for guaranteeing licensed user protection. To perform reliable sensing, in the basic operation mode on a single frequency band (non-hopping mode), one must allocate quiet times, that is, periodically interrupt data transmission that could impair the QoS of WRAN. This critical issue can be addressed by an alternative operation mode proposed in 802.22 called dynamic frequency hopping (DFH), where WRAN data transmission is performed in parallel with spectrum sensing without interruptions. DFH community, as described in this article, is a mechanism that coordinates multiple WRAN cells operating in the DFH mode, such that efficient frequency usage and reliable channel sensing are achieved. The key idea of DFH community is that neighboring WRAN cells form cooperating communities that coordinate their DFH operations

Asymptotic performance of collaborative spectrum sensing under correlated log-normal shadowing

Abstract: Collaborative spectrum sensing enables opportunistic unlicensed access to the unused portions of the licensed spectrum. We characterize the performance degradation of collaborative sensing due to correlated shadowing by deriving a lower-bound on the probability of missing the opportunities for unlicensed access. Moreover, we evaluate the effective number of collaborating users in terms of the distance spread of the sensing network and the characteristics of the propagation environment. This has practical implications in terms of protocol design as having a few number of users collaborate over a large distance may be more effective than a dense sensing network confined to a small area

Optimal Spectrum Sharing for Multi-Hop Software Defined Radio Networks

Abstract: Software defined radio (SDR) capitalizes advances in signal processing and radio technology and is capable of reconfiguring RF and switching to desired frequency bands. It is a frequency-agile data communication device that is vastly more powerful than recently proposed multi-channel multi-radio (MC-MR) technology. In this paper, we investigate the important problem of multi-hop networking with SDR nodes. For such network, each node has a pool of frequency bands (not necessarily of equal size) that can be used for communication. The uneven size of bands in the radio spectrum prompts the need of further division into sub-bands for optimal spectrum sharing. We characterize behaviors and constraints for such multi-hop SDR network from multiple layers, including modeling of spectrum sharing and sub-band division, scheduling and interference constraints, and flow routing. We give a formal mathematical formulation with the objective of minimizing the required network-wide radio spectrum resource for a set of user sessions. Since such problem formulation falls into mixed integer non-linear programming (MINLP), which is NP-hard in general, we develop a lower bound for the objective by relaxing the integer variables and linearization. Subsequently, we develop a near-optimal algorithm to this MINLP problem. This algorithm is based on a novel sequential fixing procedure, where the integer variables are determined iteratively via a sequence of linear programming. Simulation results show that solutions obtained by this algorithm are very close to lower bounds obtained via relaxation, thus suggesting that the solution produced by the algorithm is near-optimal.

Conducted EMI Reduction in Power Converters by Means of Periodic Switching Frequency Modulation

Abstract: Spread spectrum clock generation techniques were originally developed to reduce electromagnetic interference (EMI) in communications and microprocessor systems working in the range of hundreds of megahertz. Nowadays, the switching frequency of power converters has been increasing up to values that make worthy the application of such switching frequency modulation techniques to reduce EMI emissions in power converters. Although random modulations have been applied before to power converters, periodic patterns can provide some advantages. First, theoretical principles of frequency modulation using three periodic patterns for the modulating function are presented. The influence of some important modulation parameters on the EMI reduction is analyzed and some considerations about the EMI filters design are also presented. The effectiveness of such methods in terms of EMI reduction is demonstrated theoretically and confirmed with experimental results obtained from tests carried out on two converters. The first one is a 2.5 W buck converter that can be switched up to 1 MHz and the second one is a 600 W boost converter switching at 40 kHz. In both cases, attenuations obtained in conducted EMI are evaluated. Finally, special attention has been paid to input current and output voltage ripple in order to evaluate possible undesired side-effects produced by this technique.

Algebraic Constructions of Optimal Frequency-Hopping Sequences

Abstract: Frequency-hopping (FH) spread spectrum and direct-sequence spread spectrum are two main spread-coding technologies. Frequency-hopping sequences are needed in FH code-division multiple-access (CDMA) systems. In this correspondence, three classes of optimal frequency-hopping sequences are constructed with algebraic methods. The three classes are based on perfect nonlinear functions, power functions, and norm functions, respectively. Both individual optimal frequency-hopping sequences and optimal families of frequency-hopping sequences are presented.

Using Antenna Array Redundancy and Channel Diversity for Secure Wireless Transmissions

Abstract: The use of signal processing techniques to protect wireless transmissions is proposed as a way to secure wireless networks at the physical layer. This approach addresses a unique weakness of wireless networks whereby network traffic traverses a public wireless medium mak- ing traditional boundary controls ineffective. Specifically, a randomized array transmission scheme is developed to guar- antee wireless transmissions with inherent low-probability- of-interception (LPI). In contrast to conventional spread spectrum or data encryption techniques, this new method exploits the redundancy of transmit antenna arrays for deliberate signal randomization which, when combined with channel diversity, effectively randomizes the eavesdropper's signals but not the authorized receiver's signals. The LPI of this transmission scheme is analyzed via proving the indeterminacy of the eavesdropper's blind deconvolution. Extensive simulations and some preliminary experiments are conducted to demonstrate its effectiveness. The proposed method is useful for securing wireless transmissions, or for supporting upper-layer key management protocols.

Spread Spectrum Systems for GNSS and Wireless Communications

Abstract: Look to this cutting-edge resource for a modern treatment of spread spectrum (SS) communications, including direct sequence and frequency hopping. The book helps you understand the performance of SS systems under the influence of jamming with and without coding. You find details on the synchronization of SS systems, including initial acquisition and tracking. The book discusses correlation loss to help you determine the impact of filters on the correlation process. Moreover, for the first time in any book, you find details on code acquisition and code tracking with channel filtering. This comprehensive volume presents the principles of design and analysis for all SS systems, and places special emphasis on wireless systems and global navigation satellite systems (GNSS). The book considers all the common coherent and non-coherent modulations, including BPSK, QPSK, DPSK, MSK, MFSK, OFDM, and UWB. Other key topics discussed include multiple access methods for SS, characterization of radio channels, the theory of lock detectors, and low probability of detection (LPD) systems.

Optimal Power Control for Multi-Hop Software Defined Radio Networks

Abstract: Software defined radio (SDR) is a revolution in radio technology that promises unprecedented flexibility in radio communications and is viewed as an enabling technology for dynamic spectrum access. This paper investigates how to support user communication sessions by jointly considering power control, scheduling, and flow routing for an SDR-based multi-hop wireless network. We develop a formal mathematical model for scheduling feasibility under the influence of power control. This model extends existing protocol interference model for wireless networks and can be used for a broad class of problems where power control (and thus transmission range and interference range) is part of the optimization space. We formulate a cross-layer optimization problem encompassing power control, scheduling, and flow routing. Subsequently, we develop an efficient solution procedure based on branch-and-bound technique and convex hull relaxation. Using simulation results, we demonstrate the efficacy of the solution procedure and offer insights on the impact of power control on scheduling feasibility, bandwidth efficiency, and bandwidth-footprint product (BFP).

Ad Hoc Networks: To Spread or Not to Spread?

Abstract: Spread spectrum communication — often called code-division multiple access — has been widely adopted over the years for many types of interference-challenged wireless communication systems including cellular and cordless telephones, wireless LANs and PANs, military applications, and global positioning systems. In this article we explore whether CDMA, in either its frequency hopping (FH) or direct sequence (DS) form, is an appropriate design approach for wireless ad hoc, or mesh, networks. One goal of this article is to help provoke a debate by explaining the main advantages and disadvantages of CDMA in the context of ad hoc networks as exposed by recent research. We argue that CDMA does not inherently improve the spectral efficiency of ad hoc networks; on the contrary, its valued interference averaging effect is not appreciable in ad hoc networks due to the irregular distribution of both the transmitters and receivers. On the positive side, both types (FH and DS) of spread spectrum allow for longer hop distances and a reversal of the usual relationship where the desired transmitter must be closer to the receiver than interfering transmitters. These two facts allow for significant advantages over narrowband systems in terms of energy efficiency and end-to-end delay.

Distributed Spectrum Sensing for Cognitive Radio Systems

Abstract: Cognitive radio is a candidate technology for more efficient spectrum utilization systems based on opportunistic spectrum sharing. Because this new technology does not rely on traditional license-based spectrum allocation policies, it could disrupt existing systems if the spectrum utilization decision is based on unreliable spectral estimation. Distributed sensing methods have the potential to increase the spectral estimation reliability and decrease the probability of interference of cognitive radios to existing radio systems. In this paper, we consider different aspects of the processing and fusion of spectrum sensing information of cognitive radio systems. The use of cyclic feature- based methods for distributed signal detection and classification is discussed and recent results are presented.

Unified Analysis of UWB Transmitted-Reference Schemes in the Presence of Narrowband Interference

Abstract: Transmitted-Reference (TR) signaling, in conjunction with an autocorrelation receiver (AcR), offers a low- complexity alternative to Rake reception in ultrawide bandwidth systems. This paper provides a unified performance analysis of various TR schemes by developing an analytical framework based on the sampling expansion approach. Specifically, we derive the uncoded bit error probability (BEP) of different TR signaling schemes, including TR and differential TR (DTR) signaling valid for a broad class of fading channels. We consider both AcRs and modified AcRs with noise averaging. We further develop a quasi-analytical method as well as an approximate analytical method to extend the BEP analysis to include the effect of narrowband interference (NBI). We show that the approximate analytical method is particularly useful in obtaining BEP expressions that provide insight into the effect of NBI. We quantify the effects of NBI and channel power dispersion profile on the optimum integration interval of an AcR. Finally, we compare TR and DTR signaling in terms of their sensitivity to NBI.

Single-Radio Adaptive Channel Algorithm for Spectrum Agile Wireless Ad Hoc Networks

Abstract: This paper presents the single-radio adaptive channel (SRAC) algorithm which enables dynamic spectrum access in multi-hop wireless ad hoc networks where each node has only one half-duplex radio (transceiver). Designed as a relatively independent module, SRAC can upgrade various existing single-radio legacy medium access control (MAC) protocols to be dynamic spectrum access capable, achieving efficient use of the spectrum, relaxing their operating conditions, and naturally supporting multicast applications. The SRAC algorithm is characterized by three features: (a) dynamic channelization in response to jamming, primary spectrum users and channel load, (b) "cross channel communications", and (c) as-needed use of spectrum. We evaluate the performance of SRAC through analysis and QualNet simulations.

Method and system for data detection in a global positioning system satellite receiver

Abstract: A data detection circuit within a global positioning system (GPS) satellite receiver operates to detect and decode data sent in a spread spectrum signal. The data detection circuit receives input from a radio receiver, the information containing data from a plurality of satellites. The data is supplied to a circular memory device, which determines which data corresponds to which satellite. The memory device sends the received signal to a matched filter, which decodes the signal received from each satellite. This signal is analyzed to determine whether a phase inversion due to data modulation on the received signal is present. The phase inversion can occur at boundaries, known as data epochs, in the received signal, and corresponds to data in the received signal. This data contains information relating to the position of each satellite and is collected by the data detection circuit for use by the GPS receiver.

Cross-layer jamming detection and mitigation in wireless broadcast networks

Abstract: Mobile communication systems are often susceptible to high level of noise injected by adversaries, known as jamming attack. Jamming is difficult to prevent in broadcast networks because a user that can decode a transmission can also jam the transmission. In this paper, we describe a code tree system that helps the physical layer circumvent jammers. This system works with any spread-spectrum communications system. In our system, the transmitter has more information than any single receiver. Each receiver cooperates with the transmitter to detect any jamming that affects that receiver. Our scheme mitigates the jamming attack while allowing the transmitter to transmit on fewer codes than the number of users. We simulated our system in a theoretical setting using MATLAB. The result shows significant improvement over naively transmitting on a single shared code.

Ad Hoc Networks: To Spread or Not to Spread? [Ad Hoc and Sensor Networks]

Abstract: Spread spectrum communication - often called code-division multiple access - has been widely adopted over the years for many types of interference-challenged wireless communication systems including cellular and cordless telephones, wireless LANs and PANs, military applications, and global positioning systems. In this article we explore whether CDMA, in either its frequency hopping (FH) or direct sequence (DS) form, is an appropriate design approach for wireless ad hoc, or mesh, networks. One goal of this article is to help provoke a debate by explaining the main advantages and disadvantages of CDMA in the context of ad hoc networks as exposed by recent research. We argue that CDMA does not inherently improve the spectral efficiency of ad hoc networks; on the contrary, its valued interference averaging effect is not appreciable in ad hoc networks due to the irregular distribution of both the transmitters and receivers. On the positive side, both types (FH and DS) of spread spectrum allow for longer hop distances and a reversal of the usual relationship where the desired transmitter must be closer to the receiver than interfering transmitters. These two facts allow for significant advantages over narrowband systems in terms of energy efficiency and end-to-end delay.

Diversity wireless systems

Abstract: Diversity wireless system, processor, cross-correlator, transmit baseband filter and modulator for processing, cross-correlating, filtering, modulating and providing a first and a second bit rate signal to a transmission system for transmission of the processed, filtered, modulated signal. Transmission system having two transmitters for transmission of the modulated signal. Diversity receiver system with two receivers for receiving the transmitted modulated signal and for providing received signal to a demodulator. Demodulator, receive baseband filter and receive processor for demodulation, filtering and processing received signal. Receive baseband filter for filtering is mis-matched to the transmit baseband filter for filtering of the first bit rate signal and receive processor for processing first bit rate demodulated baseband filtered signal provides received baseband mis-match filtered cross-correlated in-phase and quadrature-phase signal. Processor and transmit baseband filter for processing and filtering a signal into a cross-correlated filtered and into a cross-correlated spread spectrum in-phase and quadrature-phase baseband signal. Processor, first and second modulator for processing and baseband filtering a signal into time division multiplexed (TDM) filtered signal and into Orthogonal Frequency Division Multiplexed (OFDM) signal and for providing processed TDM filtered and processed OFDM signal to two modulators for modulation, transmission and diversity reception.

Bluetooth,internet,multimode TDMA, GSM, spread spectrum, CDMA

Abstract: Bluetooth transmitter and receiver for transmitting and receiving voice signal and for providing received voice signal to a processor for processing demodulated Bluetooth signal. Location finding signal receiver, demodulator and processor for receiving, demodulating and processing a location finder signal into demodulated processed location finder signal and for processing location finder signal with processed and filtered Time Division Multiple Access (TDMA) signal, or with processed and filtered cross-correlated in-phase and quadrature-phase signal, or with processed Orthogonal Frequency Division Multiplexed (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA) or with spread spectrum Code Division Multiple Access (CDMA) signal. Modulator and transmitter for modulation and transmission of processed signal. Receiver for receiving transmitted signal. Demodulator, receiver filter and receiver processor for demodulation, filtering and processing of received modulated cross-correlated signal having filter for filtering of the cross-correlated signal which is mismatched to the transmit filter of the cross-correlated signal. Receive processor provides received mis-match filtered cross-correlated in-phase and quadrature-phase signal. Location finder signal received from one or more satellites and from one or more ground based single or plurality of transmitters. Receiver for receiving transmitted signal has a radio frequency (RF) unit which is located at a remote location from demodulator. OFDM or OFDMA signal is used in a Wireless Local Area Network (WLAN) or a Wi-Fi network and TDMA signal in a cellular network.

An EMI Reduction Technique for Digitally Controlled SMPS

Abstract: A spread spectrum technique and system for reducing average electromagnetic interference (EMI) in low-power digitally controlled dc-dc switch-mode power supplies (SMPS) are introduced. The technique utilizes very simple hardware, where the switching frequency of a SMPS is dynamically varied over a controlled range. This is achieved by changing the supply voltage of a ring-oscillator based digital pulse-width modulator in a pseudo-random fashion, through 128 discrete steps. The change is performed with a 1-b DeltaSigma digital-to-analog converter. Compensator design guidelines for this variable frequency system are provided for obtaining good dynamic response. The technique was tested with a 500-mW, 1.8-V buck converter prototype, whose switching frequency was varied from 1.74 to 2.84 MHz. A reduction of 23 dB in the conducted EMI with an efficiency degradation of less than 0.1 % was obtained, compared to fixed frequency operation.

Performance Evaluation of Bandwidth Allocation in 802.16j Mobile Multi-Hop Relay Networks

Abstract: Being an evolution of IEEE 802.16e, IEEE 802.16j mobile multi-hop relay (MMR) is proposed to gain coverage extension and throughput enhancement. This research conducts theoretical analysis and performance evaluation on the on-demand bandwidth allocation in IEEE 802.16j MMR networks. Theoretical analysis shows that such systems using conventional bandwidth allocation will seriously suffer from low bandwidth utilization. We then develop a new, spectrum efficiency based adaptive resource allocation algorithm. Simulation results verified that, compared with conventional on-demand bandwidth allocation method, the proposed method is more bandwidth efficient and exhibits better fairness in such networks. In terms of throughput, packet loss rate and delay, the performance of SEBARA algorithm is further evaluated in an OPNET-based 802.16j MMR simulation platform. Both theoretical analysis and simulation indicate the novel method provides an attractive solution for bandwidth allocation in 802.16j MMR networks.

Detection, communication and control in multimode cellular, TDMA, GSM, spread spectrum, CDMA, OFDM WiLAN and WiFi systems

Abstract: Detection device for detection and generation of detection device generated signal, processed with remote control device generated remote control signal, with location finder or location tracker or navigation signal and with Modulation Demodulation (Modem) Format Selectable (MFS) communication signal. Processor for processing a digital signal into cross-correlated in-phase and quadrature-phase filtered signal and for processing a voice signal into Orthogonal Frequency Division Multiplexed (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA) signal. Detection device generated signal and location finder signal, with OFDM or OFDMA processed voice signal is used in a Wireless Local Area Network (WLAN) and in Voice over Internet Protocol (VoIP) network. Detection device generated signal and location finder signal with Time Division Multiple Access (TDMA), Global Mobile System (GSM) and spread spectrum Code Division Multiple Access (CDMA) signal is used in a cellular network. Polar and quadrature modulator and two antenna transmitter for transmission of provided processed signal. Transmitter with two amplifiers operated in separate radio frequency (RF) bands. One transmitter is operated as a Non-Linearly Amplified (NLA) transmitter and the other transmitter is operated as a linearly amplified or linearized amplifier transmitter. Two antenna receiver and demodulator. Diversity receiver and demodulator.

Spectral Efficiency of Joint Multiple Cell-Site Processors for Randomly Spread DS-CDMA Systems

Abstract: A chip-interleaved randomly spread direct-sequence code-division multiple-access (DS-CDMA) scheme is considered, employed in two variants of Wyner's infinite linear cell-array model with flat fading. Focusing on the asymptotic setup in which both the number of users per cell and the processing gain go to infinity, while their ratio (the ldquocell loadrdquo) goes to some finite constant, the spectral efficiencies of the optimum and linear minimum mean-squared error (MMSE) joint multicell receivers are investigated. A dramatic performance enhancement as compared to single-cell-site processing is demonstrated. The asymptotic behavior of the two receivers in extreme signal-to-noise ratio (SNR) regimes and in a high cell-load regime are analyzed as well. The impact of chip interleaving versus symbol interleaving is also investigated. Chip-level interleaving is found beneficial in several cases of interests, and is conjectured to be beneficial in general.

Equalization for DS-UWB Systems---Part II: 4BOK Modulation

Abstract: Ultra-wideband wireless transmission has attracted considerable attention both in academia and industry. For high-rate and short-range transmission, direct sequence based ultra-wideband (DS-UWB) systems are a strong contender for consumer market applications. Due to the large transmission bandwidth, the UWB channel is characterized by a long root-mean-square delay spread and the RAKE receiver cannot always overcome the resulting intersymbol interference. We therefore study equalization for DS-UWB systems. This paper is comprised of two parts. In this first part, we consider DS-UWB with binary phase-shift keying (BPSK) modulation, which is the mandatory transmission mode for DS-UWB systems promoted by the UWB Forum industry alliance. We derive matched filter bounds for optimum equalization taking into account practical constraints like receiver filtering, sampling, and the number of RAKE fingers when RAKE preprocessing is applied at the receiver. Our results show that chip-rate sampling is sufficient for close-to-optimum performance. For analysis of suboptimum equalization strategies we further study the distribution of the zeros of the channel transfer function including RAKE combining. Our findings suggest that linear equalization is well suited for the lower data rate modes of DS-UWB systems, whereas nonlinear equalization is preferable for high-data rate modes. Moreover, we devise equalization schemes with widely linear processing, which improve performance while not increasing equalizer complexity. Simulation and numerical results confirm the significance of our analysis and equalizer designs and show that low-complexity (widely) linear and nonlinear equalizers perform close to the pertinent matched filter bound limit.

A robust 3D high precision radio location system

Abstract: For the precise measurement of 3D object coordinates a high precision radio location system is introduced for 3D applications and optimized for very high precision. In different test scenarios the system is evaluated with regards to precision, resolution, reproducibility, and robustness. Based on a direct sequence spread spectrum transmission at 24 GHz the radio location system shows high robustness against interferers as well as a high suppression of reflections. After adequate object tracking using a Kalman filter it offers a very high precision with an uncertainty of 0.1 mm also in very general highly reflective indoor environment.

Channel Uncertainty in Ultra-Wideband Communication Systems

Abstract: Channel uncertainty limits the achievable data rates of certain ultra-wideband systems due to the need to estimate the channel. The use of bursty duty-cycled transmission reduces the channel uncertainty because the receiver has to estimate the channel only when transmission takes place, but the maximum amount of burstiness and hence the possible reduction of channel uncertainty both depend on the spectral efficiency of the modulation scheme used. This general principle is demonstrated by comparing the channel conditions that allow duty-cycled direct-sequence spread spectrum (DSSS) and pulse position modulation (PPM) to achieve the additive white Gaussian noise (AWGN) channel capacity in the wideband limit. We show that duty-cycled DSSS systems achieve the wideband capacity as long as the number of independently faded resolvable paths increases sublinearly with the bandwidth, while duty-cycled PPM systems can achieve the wideband capacity only if the number of paths increases sublogarithmically. The difference is due to the fact that DSSS is spectrally more efficient than PPM and hence allows more bursty transmission

Spread-space spread-spectrum technique for secure multiplexing.

Abstract: A novel technique for multiplexing complex images is proposed in which each image may be demultiplexed only if a set of random encryption keys is known. The technique utilizes the ability of the double random phase encoding method to spread a signals’ energy in both the space and the spatial frequency domains in a controlled manner. To multiplex, images are independently encrypted with different phase keys and then superimposed by recording sequentially on the same material. Each image is extracted by using the particular key associated with it. During decryption the energy from the other images is further spread, making it possible to minimize its effects by using suitable filters. Wigner analysis is applied to the technique, and numerical results are presented. © 2007 Optical Society of America OCIS codes: 070.6020, 090.4220, 070.2580.

Spatial Multiplexing in Cellular MIMO-CDMA Systems with Linear Receivers: Outage Probability and Capacity

Abstract: Even though spatial multiplexing provides a significant spectral efficiency advantage in a single point-to-point noise- limited link, recent studies have shown that this advantage can be lost in cellular MIMO systems unless extra diversity is provided. Spread spectrum is a likely candidate for the extra diversity because spread spectrum can simultaneously provide frequency diversity and robustness to interference. This paper investigates the effectiveness of spatial multiplexing in the forward link of cellular MIMO-CDMA systems with linear receivers. Through the development of new closed-form results on outage probability and capacity for MIMO-CDMA, we show that even MIMO-CDMA loses the spatial multiplexing gain in a cellular context. These results indicate that a practical cellular MIMO system, which will be interference-limited and have a low-complexity receiver, requires new study on techniques to efficiently reduce the impact of the other-cell interference. The developed analytical framework can be used for evaluating other techniques.

Spread spectrum signal detection with inhibiting for known sidelobe locations

Abstract: A method and apparatus for detecting a spread spectrum signal is described. Signals can be detected using correlation techniques. When a signal is detected, declaring of signal presence is inhibited for offsets of signal parameters from the parameters of the detected signal, where the offset corresponds to a known sidelobe of the spreading code. This helps to reduce false detections caused by sidelobes of the spreading code.