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

This paper derives a Karhunen-Loève (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.published_or_final_versio

Impact of power control and lognormal shadowing on the mean transmit power of Bluetooth devices

This letter analyzes Bluetooth's power-control algorithm with a goal to study the mean transmit power required in the presence of lognormal shadowing. The following results are found. 1) A smaller power-control step size yields a lower mean transmit power. 2) When the standard deviations of lognormal shadowing are 3 and 6 dB, respectively, a Bluetooth device needs to consume 11.1 dB and 15.0 dB more in the transmit energy than the minimum one required in the absence of shadowing. 3) The transmit energy consumption varies by around 6 dB among Bluetooth devices as a result of the ±6 dB tolerance in the Golden Receive Power Range, which has a nominal size of 20 dB.published_or_final_versio

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

This paper proposes a mathematical model for simulating a Nakagami-m 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 statistical properties of the samples generated from the proposed model are close to the required ones. Index Terms - Fading channels, HF channels, simulation, stochastic processes. © 2000 IEEE.published_or_final_versio

Effects of carrier frequency accuracy on quasi-synchronous, multicarrier DS-CDMA communications using optimized sequences

It is known that the multiple access interference (MAI) of a quasi-synchronous (QS) multicarrier direct signal code division multiple access (DS-CDMA) system can be substantially reduced by using signature sequences having optimized cross correlation at small shifts around the origin. This paper shows that the time-frequency cross correlation function rather than the usual (time-domain) cross correlation determines the MAI when the system is operated in the presence of carrier frequency offset (CFO), which arises due to the frequency-accuracy limit of the oscillator. Several known sets of sequences having optimized time-domain cross correlation are investigated for their MAI-minimization capabilities in the presence of CFO. It is found that: i) a system using Walsh codes or Suehiro-Hatori polyphase sequences can be driven into outage as a result of significant worst-case MAI and ii) it is possible to minimize the MAI for systems using preferentially phased Gold codes cyclic-shift m-sequences or Lin-Chang sequences only if the product of chip period and maximum frequency deviation is less than around 0.01. Implications of these findings to practical implementation of systems are discussed.published_or_final_versio

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.published_or_final_versio

A low-complexity multitone-CDMA communication technique

Digital implementation of multitone-(MT-) CDMA systems often involves considerable complexity. Assuming the spreading-sequence length, N, is a multiple of the number of subcarriers, M, this paper proposes a new digital MT-CDMA technique that achieves low-implementation complexity. In particular, for the case of rectangular chip waveform, it is shown that the MT-CDMA technique proposed earlier and the technique proposed in this paper require α + αlog2 αN and 1 + (M/N)log2 M multiplications per chip, respectively, where α ≥ 1 is the number of samples per chip. The reduction in complexity is particularly considerable when N is large.published_or_final_versio

Transmit-power reduction for class-1 Bluetooth-enabled indoor cordless phones

Class-1 Bluetooth devices support a transmission range of about 100m and are useful for indoor cordless telephony with advantages of wider coverage, greater user mobility, and more convenience. To minimize the transmit power of class-1 devices, feedback power control specified in the Bluetooth specification can be used. This paper shows that further transmit-power reduction is possible by reducing the Golden Receive Power Range (GRPR) from the specified value of 20dB. For typical indoor log-normal-shadowing channels, more than 4dB reduction in the mean transmit power can usually be obtained by reducing the GRPR to 10dB. However, using a smaller GRPR increases the frequency of making power-adjustment requests through the Link Manager Protocol (LMP), thereby pre-empting more voice packets and affecting the voice quality. We compute the overhead cost due to power control, defined as the percentage of the total number of packets used for power-adjustment requests, when the GRPR is set at 10dB ± 6dB, wherein 6dB is the tolerance allowed in implementation. It is found that the overhead cost is less than about 1% but becomes close to 1% as the GRPR approaches 4dB, indicating that the link performance would become barely acceptable for voice transmission in some situations. We also consider utilizing the reserved byte in LMP power-control commands to convey the preferred number of power-adjustment steps to the transmitter in order to reduce the overhead cost. With this arrangement, the (worst-case) overhead cost is reduced to about 0.3%, so that the voice quality can be maintained acceptable even if the GRPR is reduced to 10dB ± 6dB for transmit-power reduction.published_or_final_versio

Performance sensitivity of quasi-synchronous, multicarrier DS-CDMA systems due to carrier frequency disturbance

The multiple access interference (MAI) of a quasi-synchronous, multicarrier DS-CDMA system can be substantially reduced by using sequences having low crosscorrelation at small shifts around the origin. This paper shows that the time-frequency crosscorrelation rather than the usual (time-domain) crosscorrelation determines the MAI when the system is operated in the presence of carrier frequency offset (CFO) which arises due to frequency-accuracy limit of the oscillator. Analysis on the time-frequency crosscorrelation properties reveals that (i) a system using Walsh codes or Suehiro-Hatori polyphase sequences can be driven into outage in the presence of CFO as a result of significant worst-case MAI, and (ii) it is possible to minimize the MAI for systems using preferentially phased Gold codes, cyclic-shift m-sequences or Lin-Chang sequences only if the product of chip period and maximum frequency deviation is less than around 0.01.published_or_final_versio

Reducing the required transmitted power for personal communication devices in indoor-video spread-spectrum transmission

This paper analyzes the use of a coded multicode spread-spectrum technique to reduce radio power for personal communication devices in indoor video transmission. The performance of indoor video transmission is measured by the outage probability, which is the probability that the instantaneous bit error rate (BER) exceeds an acceptable level. We first derive the outage probability. The result is used to characterize the power reduction. It is found that the amount of power reduction depends on the required BER level. For indoor video transmission, which typically requires a BER of 10-5, a substantial power reduction exceeding 3dB can be obtained. If outer coding is used in order that the BER requirement can be relaxed, results show that it is preferred to maintain the required BER to be less than 10-4 for more-effective utilization of the power-reduction capability.published_or_final_versio

A simulation model for HF channels that are characterized by Nakagami m-fading, m<1

Experimental results have indicated that the Nakagami (1960) m-distribution with the fading parameter m in the range 0.5<m<1 can be used to model the fading characteristic of a HF channel when its fading is more severe than Rayleigh fading. In this paper, we propose a mathematical model for simulating a HF channel that is characterized by m-fading, m<1. The procedure for implementation is given. Numerical results demonstrate that statistical properties of the samples generated from the proposed model are close to the required ones.published_or_final_versio