David E. Dodds

Acquisition time distribution for spread-spectrum receivers

Serial search spread-spectrum code acquisition is analyzed. A modified flow graph is presented. It permits the use of a generalized lock strategy and allows the false alarm time to be treated as a random variable. The distribution of acquisition time is obtained directly by using an extended generalization of Bernoulli trials. Compared to recent work which uses a fixed penalty false alarm time, the analysis is more general and requires fewer assumptions and approximations. The analysis is suitable for both single dwell and multidwell systems. Examples are presented to show the effect on the acquisition time distribution of sequence length and normalized detection threshold. >

A segmented matched filter for CDMA code synchronization in systems with Doppler frequency offset

This paper presents a segmented matched filter (SMF) for codephase acquisition in direct sequence spread spectrum systems. While conventional matched filters provide fast acquisition in the presence of high co-user noise, they are unable to handle significant carrier frequency offset (Doppler). This problem is alleviated by segmentation with non-coherent summation. The paper develops expressions to approximately relate the matched filter partitioning to the pre-detection filter and dwell time integrator of the conventional non-coherent correlator. It also investigates 1-bit versus 2-bit quantization. A mixed-signal application specific integrated circuit (ASIC) has been fabricated to implement a 512 chip SMF with half chip codephase resolution. The paper presents calculated and measured probability density functions (pdf) for the filter output decision variable for 10, 25, and 50 co-users with 0 to 20 kHz Doppler shift. For the example of a GPS receiver, expected acquisition time is shown as a function of multiple access interference and carrier Doppler shift.

Statistical distribution of PCM framing times

The authors evaluate a robust framing technique which incorporates sufficient memory to simultaneously evaluate all bits in a candidate synchronizing codeword. Three performance measures are identified and the distribution function and mean value are developed in closed form for each measure. Predicted search and maintenance time distributions have been verified by measurements on a laboratory model. The authors present graphs of search time distributions which illustrate the effect of system parameters and variations in the channel error rate. >

A two-phase genetic K-means algorithm for placement of radioports in cellular networks

This paper proposes a two-phase hybrid algorithm for optimal placement of radioports. The placement strategy minimizes the dynamic range of the channel, and thus reduces the peak transmit power needed by a wireless handset. The performance of the proposed algorithm is compared with that of an exhaustive search, as well as with that of a simplex algorithm. Simulation results show that the two-phase hybrid algorithm, although suboptimal, outperforms the simplex algorithm with only a modest increase in computation time. It has the added advantage of being quite insensitive to the seed solution.

Synchronization of Weak Indoor GPS Signals with Doppler Using a Segmented Matched Filter and Accumulation

This paper investigates GPS codephase acquisition with very low SNR signals found within large buildings. Conventional GPS receivers can acquire the GPS codephase relatively easily when in direct line of sight with the satellites. However, acquisition becomes difficult with highly attenuated signals found inside buildings. Recent government regulations aim to improve 911 service for wireless telephones by supplying dispatchers with caller location, however, this requires the development of new technologies. With extensive signal averaging, GPS signals can provide positioning within large buildings. We assume 5dB/floor of signal attenuation so the GPS signal would be -183 dBWor less at the bottom of a 5-story building. We assume 2 MHz system bandwidth and surrounding walls at room temperature of 290 K (17degC) so the Boltzmann noise is -141 dBW. More than 50 dB SNR improvement is provided by an accumulating segmented matched filter that is robust to polarity reversals caused by Doppler frequency offset and by GPS data transitions. Through simulation, performance comparisons are made to a conventional matched filter (CMF) operating without polarity reversals.

A unified time-domain analysis of serial search with application to spread spectrum receivers

The paper presents a time-domain approach to the analysis of the code phase acquisition process in direct sequence spread spectrum receivers. This approach is suitable for a variety of search and verification/lock strategies. The time-domain approach is more direct than the unified z transform-domain approach and permits computation of the acquisition time distribution. The paper also provides equations for calculating the mean acquisition time under various search and verification/lock strategies. The usefulness of the approach is demonstrated by application to a specific nontrivial case of practical significance.

ATM framing using CRC byte

Traditional framing methods search for a unique synchronization word. In this paper, we propose a new framing method for ATM cells which detects the CRC byte of the cell header. We also present an efficient logic design which implements a sliding window CRC check. The synchronization performance of cell delineation using CRC based framing has been predicted through analysis and the circuit operation has been verified in the laboratory.<<ETX>>

Locating water ingress in telephone cables using frequency domain reflectometry

It is important to locate water ingress in telephone cables because water severely attenuates DSL signals used for Internet access. The paper initially reviews the frequency domain reflectometry method and how it measures the distance, magnitude and phase angle of reflections caused by transmission line discontinuities. When water enters a cable it lowers the magnitude and angle of the characteristic line impedance, causing reflections at the points where the water starts and ends. The dry and wet characteristic impedances of a 300-meter section of telephone line were experimentally measured and the reflection coefficient was estimated to have a phase angle of 20deg. Measurements of a properly terminated telephone line with 300 meters of the line immersed in water showed a first reflection with a phase angle of 162deg (180deg-18deg). A second reflection, from the end of the wet section, was measured with a phase angle of 25deg. These reflection angles differ from the 0deg or 180deg typical of bridge taps, open circuits or short circuits and this allows a technician to specifically identify water ingress

Iterative ordered-statistic decoding for LDPC coded modulation systems

In this paper, an iterative demodulation/decoding scheme is proposed for coded modulation systems based on low density parity check (LDPC) codes of short block lengths. Here, the LDPC code is decoded by a soft-input soft-output ordered-statistic (SISO-OSD) decoder. The soft-output of the SISO-OSD is used as the a priori probabilities of the demodulator for the next iteration. Simulation results show that the error performance of the iterative demodulation/decoding ordered-statistic decoder is superior to that of the iterative demodulation/decoding sum-product decoder. The performance of the proposed system closely approaches the union bound of the maximum likelihood (ML) decoding with Gray mapping

Power line communication using timed transmission

This paper reviews established methods of communication using in-building power conductors with particular attention to the 100 kHz to 450 kHz frequency range. It also reviews line characteristic impedance and it presents new measurements of attenuation and noise on the power conductors. Measurements illustrate interfering noise caused by individual electrical loads and also the combined noise at power outlets in an office environment. Using a simple FSK transmission system, bit errors were measured as a function of time and these were found to coincide with noise bursts that occur at regular times during the 60 Hz cycle. The paper concludes by proposing an interference avoidance scheme where transmission is suspended during periods of high interference and then continues during quiet intervals.