Unjeng Cheng

Spread-spectrum code acquisition in the presence of Doppler shift and data modulation

A spread-spectrum code acquisition technique for a direct-sequence (DS) system in the presence of Doppler effect and data modulation is investigated. Both the carrier-frequency offset and code-frequency offset due to severe Doppler effect are considered. The code-chip slipping during the correlation process caused by code-frequency offset can degrade the acquisition performance significantly. However, this issue can be alleviated by compensating code-frequency offset in an appropriate manner. Results are presented for the cases with and without data modulation. Coherent detection is considered when there is no data modulation. If data modulation is present, the authors partition the correlation time into subintervals and the integration results in these subintervals are square-law noncoherently combined for detection. The implementation of this code acquisition technique using the fast Fourier transform (FFT) algorithm is described. The use of theoretical results to estimate the hardware complexity of an actual system is illustrated step by step, showing that implementation is feasible with existing technology. The tradeoff between hardware complexity and acquisition performance is discussed. >

Performance of a class of parallel spread-spectrum code acquisition schemes in the presence of data modulation

An acquisition procedure for data-modulated direct-sequence spread-spectrum systems is investigated. The correlation time is partitioned into subintervals, and the integration results in these subintervals are noncoherently combined for detection. The tradeoff between noncoherent combining loss and data modulation degradation guides the optimum choice of the number of subintervals. Two forms of data modulation are considered, namely, the alternate-data and random-data cases. The parallel acquisition schemes discussed allow multiple code-phase offsets to be examined at each test. The circular state diagram approach is used to analyze the performances of these schemes. The theory presented is valid for a class of such parallel schemes. >

Optimum detection of slow frequency-hopped signals

Optimum detectors have previously been derived for fast frequency-hopped (FFH) signals with M-ary frequency-shift-keyed (MFSK) data modulation received in additive white Gaussian noise (FFH here implies that a single MFSK tone is transmitted per hop), The present paper extends that work to the more analytically complex category of slow frequency-hopped (SFH) signals with multiple MFSK tones per hop. A special subset of the SFH/MFSK format that receives particular attention in the paper is the case of continuous-phase modulation (CPM) for which the carrier phase is assumed to be constant over the entire hop. A fundamental conclusion is that SFW/CPM modulation is advantageous not only to the communicator but also to a sophisticated noncoherent detector. By applying techniques developed in the paper to exploit the continuous-phase characteristic, an intercept receiver of reasonable complexity will perform appreciably better than traditional channelized detectors. >

Statistical models for evaluating the performance of noncoherent slow frequency-hopped M-FSK intercept receivers

Exact and approximate statistical models (analytical and simulation), based on average- and maximum-likelihood ratio tests, are studied and compared for the purpose of establishing an accurate assessment of the performance of noncoherent SFH M-FSK intercept receivers. Both continuous and discontinuous phase M-FSK cases are considered with important differences between the two. >

The performance of the all-digital data transition tracking loop using nonlinear analysis

This paper describes the performance of the all digital data transition tracking loop (DTTL) with coherent and noncoherent sampling using nonlinear theory. The effects of few samples per symbol and of non-commensurate sampling and symbol rates are addressed and analyzed for perfectly square pulses as well as filtered pulses. Their impact on the probability density and variance of the phase error are quantified through computer simulations. It is shown that the performance of the all-digital DTTL approaches its analog counterpart when the sampling and symbol rates are noncommensurate (i.e., the number of samples per symbol is irrational). The phase error variance for an even number of samples per symbol is also shown to degrade compared to an odd number of samples per symbol. >

Novel network test-bed simulators

The novel network testbed simulators are described: the request protocol simulator and the link protocol simulator. These testbeds are useful for evaluating delay-throughput performances of demand-assisted multiple-access (DAMA) protocols in mobile fading environments. The request protocol testbed simulates the multiple-access scheme for making connection requests in the DAMA protocol. The link protocol testbed simulates the link-layer protocol for data transmissions. In both testbeds a physical layer simulator is utilized to perform channel encoding and decoding, interleaving and deinterleaving, modulation and demodulation, and fading channel propagation. The authors present the architecture and some simulation results of both testbed simulators.<<ETX>>