In a Direct-Sequence/Spread-Spectrum (DS/SS) system, a RAKE receiver is used to improve a bit error rate (BER) performance. The RAKE receiver can collect more signal energy through independent paths and achieve path diversity. The RAKE receiver obtains further diversity gain through fractional sampling. However, the power consumption of the RAKE receiver increases in proportion to a sampling rate and does not always maximize the signal-to-noise ratio (SNR). Therefore, sampling rate selection schemes have been proposed to reduce the average sampling rate without degrading the BER. These schemes select the tap positions and the sampling rate depending on channel conditions and the power consumption can be reduced. In this paper, sampling rate selection schemes for the DS/SS system are investigated through an experiment since there have been no numerical results through an experiment. Numerical results show that the power consumption can be reduced even through the experiment without the degradation of the BER.

In this paper, sampling rate selection diversity (SRSD) scheme for Direct-Sequence/Spread-Spectrum (DS/SS) is proposed. In DS/SS communication systems, oversampling may be employed to increase the signal-to-noise ratio (SNR). However, oversampling enlarges the power consumption because signal processing of the receiver has to be carried out at a higher clock rate. Higher sampling rate does not always maximize the SNR. In the proposed SRSD scheme, the power consumption can be reduced by selecting the optimum sampling rate depending on the characteristics of the channel. The proposed SRSD scheme can also reduce the BER more than the conventional oversampling scheme under certain channel conditions.

In this paper, three new ultra wideband (UWB) communication systems with quadrature-phase shift keying (QPSK) impulse modulation are proposed. First, direct-sequence (DS) multiple-access scheme is applied. The second proposed system is based on time-hopping (TH) multiple-access scheme. The last proposed system applies TH multiple-access scheme with QPSK impulse modulation and pulse position modulation (PPM). The conventional UWB communications as TH scheme with PPM modulation and DS scheme with binary-phase shift keying (BPSK) are used to compare. The simulation results show that all proposed UWB communication systems can provide obviously better performances compared with the conventional TH-PPM and DS-BPSK UWB communication systems. The comparisons in aspects of transmission bit rate and the number of users are also investigated.

In this paper, a dual level access scheme is proposed to provide two levels of access to the broadcast data; one to video signals protected for authorized users, another to extra information e.g. advertisements provided for the remaining users in the network. In the scheme, video signals in MPEG format are considered. The video contents are protected from unauthorized viewing by encrypting the DC coefficients of the luminance component in I-frames, which are extracted from the MPEG bit-stream. An improved direct sequence spread spectrum technique is used to add extra information to non-zero AC coefficients, extracted from the same MPEG bit-stream. The resultant MPEG bit-stream still occupies the same existing bandwidth allocated for a broadcast channel. At the receiver, the extra information is recovered and subtracted from the altered AC coefficients. The result is then combined with the decrypted DC coefficients to restore the original MPEG bit-stream. The experimental results show that less than 2.9% of the size of MPEG bit-stream was required to be encrypted in order to efficiently reduce its commercial value. Also, on average, with a 1.125 Mbps MPEG bit-stream, an amount of extra information up to 1.4 kbps could be successfully transmitted, while the video quality (PSNR) was unnoticeably degraded by 2.81 dB.

This paper conducts performance evaluation and performs simulation for a code division multiple access (CDMA) system when channel bands of multiple neighboring CDMA/DSSS are overlapped in time domain. It is assumed that all systems adopt direct-sequence spread-spectrum (DSSS) technique and are BPSK modulated by the different carrier frequencies. Automatic power control (APC) is also applied in the interfered system such that the receiver gets the same power from all users. Without loss generality, an additive white Gaussian noise (AWGN) channel is also assumed during analysis. In this paper, the analytic solution of the signal to noise ratio (SNR) is first derived in which both CDMA systems are modulated by different carrier frequencies. We have the results by simulation with Δ f = 0 and Δ f = 1 MHz, respectively. This analysis is good for general cases; and the results show an excellent computational performance. In particular, the result is very close to Pursley's result, when the systems have the same code length with no carrier difference.

2.4-GHz-band mid-speed (1- to 2-Mbit/sec) wireless LAN systems are being widely used in offices and factories. Electromagnetic interference can occur between these systems because they use the same frequency range. In this paper, we investigate the characteristics of the interference between wireless LAN systems that use direct-sequence (DS) systems and frequency-hopping (FH) systems. The interference characteristics were measured for three DS systems and one FH system that meet the IEEE 802.11 and RCR standards and that use different modulation methods. Our results indicate that throughput depends on the system and the modulation method. We have also developed a model that can be used to calculate the interference characteristics between DS and FH systems by considering the bandwidth of their transmission signals, the dwell time of the FH system, and the time that the DS system needs to transmit a data frame. We used this model to calculate the bit error rate (BER) characteristics of the systems used in our experiment, and the results indicate that BER characteristics depend on the modulation method. The throughput characteristics of the systems used in our experiment were also calculated, and agreed with the experiment results within +/- 5 dB. The throughput characteristics of wireless LAN systems based on IEEE 802.11 were also calculated when the signal level was higher than the receiver noise level. The results show that FH systems require a D/U ratio about 7 or 8 dB higher than the ratio required in DS systems because the parameters in the standard differ between FH and DS systems.

This paper conducts performance evaluation for a code division multiple access (CDMA) system when channel bands of multiple neighboring CDMA/DSSS are overlapped in frequency domain. It is assumed that all systems adopt direct-sequence spread-spectrum (DSSS) technique and are BPSK modulated by the different carrier frequencies. Automatic power control (APC) is also applied in the interfered system such that the receiver gets the same power from all users. Without loss generality, an additive white Gaussian noise (AWGN) channel is also assumed during analysis. In this paper, the analytic solution of the signal to noise ratio (SNR) is first derived in which both CDMA systems are modulated by different carrier frequencies. This analysis is good for general cases; and the result shows an excellent computational performance. In particular, the result is very close to Pursly's result, when the systems have the same code length with no carrier difference.

The minimum mean-squared error (MMSE) linear detector has been proposed to successfully suppress the multiple access interference and mitigate the near-far problem in direct-sequence code-division multiple access communication systems. In the presence of unknown or time-varying channel parameters, the MMSE linear detector can be implemented by the blind Griffiths' algorithm, which uses the desired signal vector instead of a training sequence of symbols for initial adaptation. In this paper, a variable step-size (VSS) Griffiths' algorithm is proposed for accelerating the convergence speed, especially in the presence of strong interference. Numerical results show that the convergence properties of the VSS Griffiths' algorithm are robust against the wide eigenvalue-spread problem of the correlation matrix associated with the received signal vector compared to the Griffiths' algorithm using a fixed step-size.

In this paper, we study and analyze the overall acquisition performance of the combined acquisition-tracking synchronization loop for direct-sequence spread-spectrum (DS-SS) signals in the presence of Doppler shift. We consider both the change of effective search rate and the impact on the detection probability due to Doppler for the acquisition loop. We also determine the acquisition behavior of the digital delay lock loop (DDLL) in the presence of code Doppler. As a result, the influence of the DDLL's acquiring capability on the complete acquisition process is investigated and some numerical results are presented to demonstrate the acquisition performances of this combined loop which are quite different from the previous reports.

Considering that the application of the direct-sequence slotted spread ALOHA communications system to access/control channel is effective in communications systems with traffic channels operating at CDMA mode, the spread ALOHA communications system is discussed in terms of the system configuration and transmission efficiency. The transmission efficiency of the spread ALOHA communications system using a unified spread code is derived by means of two methods. One is based on the simulation of demodulation algorithm, and the other is based on the approximation by modeling. It becomes obvious from quantitative evaluation in terms of the probability of packet success and the throughput performance that the approximated results coincide with the simulated results, and that the modeling is very effective to estimate the transmission efficiency of the spread ALOHA communications system.

Recently, the low earth orbit satellite communications has been attracting much attention. These communications have many strong features, however, the communication performances are influenced by carrier frequency offset (CFO) and, particularly, it is hard to acquire the synchronization. A large number of publications have so far been made on the synchronization acquisition of DS/SS systems under CFO and most of them make use of the maximum likelihood decision in finding the maximum values of Fourier transform outputs. However, the implementations of Fourier transforms usually require high cost and large space. In this paper, we propose a new simple acquisition scheme using half-symbol differential decoding technique for DS/SS systems under CFO. This scheme makes use of the addition and subtraction of baseband signals and their delayed versions, (omitting Fourier transforms), together with integrations by recursive integrators, and thus resulting in much simpler implementation. In general, it is shown that the proposed scheme can acquire the code synchronization under carrier frequency offset with much smaller computational complexities and the sacrifice of longer acquisition time.

The intermodulation distortion (IMD) due to laser diode (LD) nonlinearity of an asynchronous direct-sequence code division multiple access (DS/CDMA) system in optical transmission is analyzed. A third-order polynomial without memory is used to present LD nonlinearity. In DS/CDMA systems, only one harmonic of the third-order inter-modulation term falls on the signal frequency band and influences the system performance. The average distortion is derived with only the information of autocorrelation functions. The results are useful for CDMA system design and performance analysis. With LD nonlinearity it is necessary to select an optimal modulation index that provides a maximum signal-to-noise ratio (SNR). The analytical method is applicable to other general nonlinearities in CDMA systems.

This paper discusses the performance of asynchronous direct-sequence spread-spectrum multiple-access systems using binary or quaternary phase-shift keyed signals with the strict bandwidth-limitation by Nyquist filtering. The signal-to-noise plus interference ratio (SNIR) at the output from the correlation receiver is derived analytically taking the cross-correlation characteristics of spreading sequences into account, and also an approximated SNIR of a simple form is presented for the systems employing Gold sequences. Based on the analyzed result of SNIR, bit error rate performance and spectral efficiency are also estimated.