Kun-Wah Yip

A simulation model for Nakagami-m fading channels, m<1

This paper proposes a mathematical model for simulating a Nakagami-m (1960) fading channel, m<1. It is shown that the m fading process can be expressed as a product of a square-root-beta process and a complex Gaussian process. Extension of the proposed model to simulate correlated diversity channels is also given. Numerical results demonstrate that the statistical properties of the samples generated from the proposed model are close to the required ones.

Partial-encryption technique for intellectual property protection of FPGA-based products

The configuration-data sequence of a field-programmable gate array (FPGA) is an intellectual property (IP) of the original designer. This paper proposes a partial-encryption (PE) technique for IP protection of configuration-data sequences by means of increasing the reverse-engineering cost. The PE technique encrypts a few selected data of the sequence. These data are selected in a judicious way such that, when a rival competitor copies the partially encrypted sequence into a cloned product, the cloned product performs the expected task to a certain degree of correctness but not absolutely error-free. Debugging is required. It is shown that, without an initial knowledge that a reverse-engineering countermeasure is employed, the PE technique outperforms the full-encryption technique in terms of the reverse-engineering cost. This paper describes implementation details of the proposed PE technique. Issues regarding system designs that embed hidden imperfections are also discussed.

Performance tradeoffs between maximum ratio transmission and switched-transmit diversity

Implementation of maximum ratio transmission (MRT) systems is not a trivial task. Alternatively, switched-transmitter diversity (STD) systems are less difficult to implement, but their performance is suboptimal when compared to MRT systems. If the difference is not significant, implementation advantages of STD systems outweigh the minor performance loss so that these systems are desirable for practical implementations. This paper compares the performances between MRT and STD systems under Rayleigh-fading conditions for 2/spl times/1, 2/spl times/2 and 4/spl times/ (transmit antennas/spl times/receive antennas) configurations. It is found that the differences are only about 1 dB for the 2/spl times/1 and 2/spl times/2 configurations, indicating that implementation simplicity of a STD system can be utilized without sacrificing too much performance. However, the difference is around 3 dB for the 4/spl times/ configuration. The considerably larger loss prompts system designers to consider more critically on the tradeoffs between implementation aspects and performances of STD and MRT systems.

Efficient simulation of digital transmission over WSSUS channels

This paper proposes an efficient Monte Carlo method which reduces computation for digital communication simulations over a multipath Rayleigh fading, wide-sense- stationary uncorrelated-scattering (WSSUS) channel. An equivalent discrete-time channel representation, which can be realized by a FIR filter with time-variant tap gains, is employed. In the proposed method, the tap gains are generated from a linear transformation of a set of orthogonal zero-mean complex Gaussian random processes. By the central limit theorem, each random process is approximated by summing a finite number of randomly generated phasors (Monte Carlo approximation). When compared with the tap gain generation method described in earlier literature, which approximates the physical channel by the Monte Carlo approximation first and then generates the tap gain values, the proposed method demonstrates a considerable reduction in the required simulation time as well as improved accuracy under similar conditions.

Karhunen-Loeve expansion of the WSSUS channel output and its application to efficient simulation

This paper derives a Karhunen-Loeve (K-L) expansion of the time-varying output of a multipath Rayleigh fading wide-sense-stationary uncorrelated-scattering (WSSUS) channel. It is shown that under the same mean-squared error condition, the number of terms required by the truncated K-L expansion is less than that of the series expansion obtained by using the discrete-path approximation of the channel so that simulation using the K-L expansion is more efficient. This computational advantage becomes more significant as higher simulation accuracy is required. The derived K-L expansion is applied to develop an efficient simulation technique for digital transmission over a multipath Rayleigh fading WSSUS channel using an optimum receiver. We show that the proposed technique requires shorter computation time than two other known simulation techniques.

Timing-synchronization analysis for IEEE 802.11a wireless LANs in frequency-nonselective Rician fading environments

This paper derives and computes the probability of synchronization failure P/sub fail/ for IEEE 802.11a wireless LANs on frequency-flat Rician fading channels. For a frequency offset within /spl plusmn/232 kHz, it is shown that its effect on the synchronization performance is minor. The E/sub ds//N/sub 0/ ratios required to achieve P/sub fail/=10/sup -3/ and 10/sup -4/ are computed, where E/sub ds/ is the data-symbol energy. We find that E/sub ds//N/sub 0/ ratios over 20 dB are generally required for channels with Rician factors K/spl les/6 dB. In particular, E/sub ds//N/sub 0/ ratios that yield P/sub fail/=10/sup -4/ exceed 30 dB for K/spl les/4 dB.

A unified architecture of MD5 and RIPEMD-160 hash algorithms

Hash algorithms are important components in many cryptographic applications and security protocol suites. In this paper, a unified architecture for MD5 and RIPEMD-160 hash algorithms is developed. These two algorithms are different in speed and security level. Therefore, a unified hardware design allows applications to switch from one algorithm to another based on different requirements. The architecture has been implemented using Alteras EPF10K50SBC356-1, providing a throughput over 200 Mbits/s for MD5 and 80 Mbits/s for RIPEMD-160 when operated at 26.66 MHz with a resource utilization of 1964LC.

Design of multiplierless correlators for timing synchronization in IEEE 802.11a wireless LANs

Timing synchronization for IEEE 802.11a WLANs requires using a correlator to correlate the received signal with a known waveform. Straightforward implementation of this correlator results in the need to perform 320 million complex multiplications per second. This significant requirement can be eliminated by using multiplierless correlators. In this paper, multiplierless correlators are designed based on constraining the real and imaginary parts of correlator coefficients to be sums of powers of two. Sets of coefficients that yield good synchronization performance for simple AWGN channels are first identified; then their goodness for indoor communication environments is verified by simulation for multipath fading channels. Several multiplierless correlators are found. Comparison among these correlators identifies a good one that requires to perform only 26 addition/subtraction operations per correlator output while a similar synchronization performance can be maintained.

Maximum-likelihood symbol synchronization for IEEE 802.11a WLANs in unknown frequency-selective fading channels

Based on the maximum-likelihood principle and the preamble structure of IEEE 802.11a wireless local area network (WLAN) standard, this paper proposes a new symbol synchronization algorithm for IEEE 802.11a WLANs over frequency-selective fading channels. In addition to the physical channel, the effects of filtering and unknown sampling phase offset are also considered. Loss in system performance due to synchronization error is used as a performance criterion. Computer simulations show that the proposed algorithm exhibits better performances than the simple correlation-based algorithms. When compared to the algorithm based on the generalized Akaike information criterion, the proposed algorithm presents comparable performance and exhibits reduced complexity.

Impacts of multipath fading on the timing synchronization of IEEE 802.11a wireless LANs

The timing-synchronization performance of IEEE 802.11a wireless LANs on multipath Rician fading channels is investigated. Simulation results yield the following observations. A higher Rician factor gives a better performance. At low signal-to-noise conditions, the multipath-diversity gain enables an improvement in the synchronization performance but at high signal-to-noise conditions, irreducible probabilities of synchronization failure, which cannot be reduced even if the signal power is increased, occur. A more dispersive channel gives a poorer synchronization performance, and the performance is poor for Rayleigh fading environments.