Direct Sequence Spread Spectrum Cdma

This paper addresses and implements a Direct Sequence Spread Spectrum (DSSS) transceiver for Visible Light Communication (VLC) systems based on FPGA. A transceiver was implemented including a transmitter capable of driving an array of light emitting diodes (LED) and a Pseudo-Noise (PN) matched decorrelator. The receiver architecture uses a discrete FIR correlator for data synchronization and acquisition. In this paper, a novel and simple PN code with a 10 bit sequence length is developed. This code ...

Spread-spectrum modulation techniques have been adopted for many current and future military communication systems to accommodate high data rates with high link integrity, even in the presence of significant multipath effects and interfering signals. A more recent synergistic combination is a direct-sequence spread-spectrum (DSSS) signaling with the use of integrally coordinated frequency hopping (FH) and/or time-hopping (TH) modulation, generically dubbed hybrid spread-spectrum (HSS). A highly useful form of this transmission scheme for many types of command, control, and sensing applications is the specific code-related combination of standard DSSS modulation with "fast" frequency hopping (FFH), wherein multiple frequency hops occur within a single data-bit time. In this paper, detailed error-probability analyses are performed for a hybrid DS/FFH system over standard Gaussian and fading-type channels, progressively including the effects from wide-band, partial-band, and follow-on jamming, multi-user interference and/or varying degrees of Rayleigh and Rician fading. In addition, a simulation-based study of the DS/FFH performance is performed and compared to several forms of existing standard DSSS and FHSS wireless networks. The parameter space of HSS is also explored to further demonstrate the adaptability of the waveform for varied harsh RF signal environments.

The work aims to investigate into the modern concept of cognitive radio and details about its functioning. The ideas that led to its invention and the thoughts that led to this modern technology of using white spaces in the most efficient way. It aims to take a dig into the earlier radios namely the conventional radio and the software defined radio and later on modifications of those that ultimately led to cognitive radio. Not only that but it deals with every nuance between each of them. Various persons including the person who was the propagator of the concept of cognitive radio technology Dr. Joseph Mitola and various organizations have tried to define cognitive radio and the discussion considers all those definitions to let the reader get a clear picture regarding their understanding as far as defining cognitive radio is concerned. The requirements of cognitive radio in various fields and the future scope of the technology are profoundly dealt with. A network of cognitive radios known as cognitive radio network (CRN) is also the matter of discussion in the latter half of the work. Various components of the CRN and certain details regarding it are included with appropriate diagrams as per requirements

A Simulink® model for Direct Sequence Spread Spectrum (DSSS) receiver is presented. Using data-path size as an additional parameter in the model blocks made it possible to do the hardware optimization of the receiver on the block-by-block basis. The effect of the round-off errors to the bit-error rate (BER) of the receiver is quantified for each block in the model. Different functional parts of the receiver, such as analog-to-digital converter (ADC), digital down-converter (DDC), matched filter, are shown to have different sensitivity to the data-path size. Using multivariable optimization, one can use the model to minimize the hardware implementation with respect to the BER. Keywords Digital down-converter, matched filter, analog-to-digital converter round-off error.

We propose and demonstrate an advanced optical modulation format that makes use of both spectral and temporal phase encodings (2-D) for applications requiring exceptional security. The method combines modulation techniques used in direct-sequence spread-spectrum coding, spectral-phase encoding, and M-ary phase-shift keying with codes generated using cryptographically secure pseudorandom number generators. The wideband transmission signal is very difficult for an eavesdropper to record or analyze. Signal-to-noise ratio limitations imposed by quantum effects enhance the security further. The properties of the transmitted signal make it especially useful for physics-based key expansion systems. We have successfully used this setup to transmit encrypted 155 Mb/s data over 70 km of fiber with a BER value of 4E-5.

Wideband measurements were performed using a direct-sequence spread-spectrum measurement system in the air-to-ground radio environment to characterize propagation between an airborne transmitter and a ground-based receiving antenna array at a center frequency of 2.05 GHz. The transmitter was flown along constant-radius arcs at low altitudes around the receiver location to obtain measurement results for 7.5, 15, 22.5, and 30 degree elevation angles. An 80 megachip per second (Mcps) modulating PN sequence was transmitted by the airborne station. The receiver was located in a campus environment of four- to six-story buildings and rolling terrain. The receiver used a four-element antenna array and sampled received signals at 1 gigasample per second (Gsps) per channel. Power-delay profiles that approximated channel impulse responses were used to measure magnitude, phase, and delay of multipath signal components received at each element. Characterization parameters produced from the measurements include RMS delay spread, excess delay spread, multipath fading CDFs, antenna diversity gain, and gain achieved through spatial-temporal combining. The measurements presented here support analysis of wireless systems for intentional transmissions, such as data communications between ground nodes and low-altitude aircraft. In addition, these measurements support investigations into interference from ground sources to low-altitude aircraft (e.g., on instrument approaches) or interception of signals originating from ground sources.

The performance of a single-user ultra-wideband (UWB) communication system employing binary block-coded pulse-position modulation (PPM) and suboptimal receivers in multipath channels is considered. The receivers examined include a RAKE receiver with various diversity combining schemes and an autocorrelation receiver, which is used in conjunction with transmitted reference (TR) signaling. A general framework is provided for deriving the performance of these receivers in multipath channels corrupted by additive white Gaussian noise (AWGN). By employing previous measurements of indoor UWB channels, we obtain numerical results for several cases which illustrate the tradeoff between performance and receiver complexity.

The Sequential Estimation (SE) technique introduced by Ward was designed for the acquisition of pseudo-noise (PN) signals. It proves to be an effective model in medium SNRs as long as inverted PN sequences are not encountered. Recognizing that negative data bits invert PN sequences in spread spectrum transmissions, Chiu and Lee developed an Improved Sequential Estimator (ISE) that processes both types of sequences. The ISE design, however, erroneously despreads the incoming signal by inverting it during negative bit acquisitions. In some applications, this could be a problem, hence a Dual Correlating Sequential Estimator (DCSE) is proposed in this paper. The DCSE detects modulation errors by gathering correlation measurements from resident SE and ISE subsystems and corrects for them with an inverting mechanism. Key performance parameters such as mean timing and the variance of acquisition for the sequential estimator designs are derived via the generalized signal flow graph method and verified through simulation. In an additive White Gaussian noise (AWGN) environment, the DCSE design is shown to outperform the SE for every tested signal-to-noise ratio (SNR).