An efficient technique for evaluating direct-sequencespread-spectrum multiple-access communications

A technique is presented for obtaining bounds on the average probability of error for direct-sequence spread-spectrum multiple-access (DS/SSMA) communications. The technique is of interest because it yields arbitrarily right bounds, involves a small amount of computation, avoids numerical integrations, and applies to many types of detection. As an illustration, the technique is applied to binary DS/SSMA communications, an additive white Gaussian noise channel, and a coherent correlation receiver. It is assumed that all the signature sequences are deterministic. Each transmitter is assumed to have the same power, although the approach can accommodate the case of transmitters with unequal powers. Expressions are given for the density functions of the random variables that model the multiple-access interference. These expressions are used to obtain arbitrarily tight upper and lower bounds on the average probability of error without making a Gaussian approximation or performing numerical integrations to incorporate the effects of multiple-access interference     

Median-prefiltering-based robust acquisition of direct-sequencespread-spectrum signals in wide-band pulse jamming

We propose robust acquisition schemes for chip time-synchronous direct-sequence spread-spectrum (DS/SS) signals in wide-band pulse jamming. To mitigate the performance degradation due to the impulse-like wide-band jamming signal, the received signal is preprocessed by employing simple order statistic filters. We consider the use of two types of median filtering schemes: a running median filter operating at a chip rate and a block median filter processing the received signal sampled at an oversampling rate. For analytic design, the partial correlation of median filtered pseudonoise (PN) signals is approximated by a piecewise linear model using an upper bound. The analytic design is verified by computer simulation. Finally, the acquisition performance of the proposed schemes is compared to that of conventional schemes     

Code-aided interference suppression for DS/CDMA communications. I.Interference suppression capability

A code-aided technique for the simultaneous suppression of narrow-band interference (NBI) and multiple-access interference (MAI) in direct-sequence code-division multiple-access (DS/CDMA) networks is proposed. This technique is based on the linear minimum mean-square error (MMSE) algorithm for multiuser detection. The performance of this technique against MAI has been considered previously. In this paper, its performance against NBI as well as combined NBI and MAI is addressed. Specifically, the performance of this technique against three types of narrow-band interferers, namely, multitone interferers, autoregressive (AR) interferers, and digital interferers, is analyzed. The NBI suppression performance of this method is then compared with performance bounds of the linear and nonlinear estimator-subtracter NBI suppression techniques. It is seen that this method outperforms all of the previous linear or nonlinear methods of NBI suppression, while simultaneously suppressing the MAI     

Threshold decision for PN code acquisition in direct-sequencespread-spectrum packet radio systems

A threshold decision for code acquisition in direct-sequence spread-spectrum packet radio systems is described. It is shown that this scheme achieves lower false alarm probability than the acquisition scheme based on the Neyman-Pearson criterion, giving comparable performance for the detection probability     

A low-power digital matched filter for direct-sequencespread-spectrum signal acquisition

This paper presents a low-power 128-tap dual-channel direct-sequence spread-spectrum (DSSS) digital matched-filter chip. Design techniques used to reduce the power consumption of the system include latch-based register file filter structure, a high-rate compression scheme, optimized compressor cells, and semicustom layout design. To further reduce the power consumption and the hardware requirement of the clock tree, a double-edge-triggered clocking scheme is adopted. The proposed chip is fabricated using a 0.8-μm standard CMOS process. As the experimental results of the chip indicate, the matched filter can operate at 50 MHz and dissipates 184 mW at 5-V supply voltage. The supply voltage can be scaled down to 2 V for lower speed applications. As a consequence, the proposed design has low power consumption and can be used for code acquisition of DSSS signals in portable systems     

A comparison of slow-frequency-hop and direct-sequencespread-spectrum communications over frequency-selective fading channels

Spread-spectrum modulation can provide protection from the selective fading that is typically encountered in mobile radio networks. Because the methods of combating frequency-selective fading are quite different for slow-frequency-hop (SFH) and direct-sequence (DS) spread spectrum systems, these two types of modulation perform very differently. The purpose of this paper is to compare the performance of SFH and DS systems under identical conditions for several models of the wide-band fading channel. Each system has the same bandwidth, transmits over the same frequency-selective Gaussian wide-sense-stationary uncorrelated-scattering channel, and uses error-correction coding to combat thermal noise and fading. The probability of bit error at the output of the decoder is determined for each system by a combination of analysis and simulation. Results are presented for systems with a single transmitter-receiver pair and for networks with multiple simultaneous transmissions (i.e., multiple-access communications). The multiple-access network is distributed, so that control of power by a central terminal, such as a base station, is not possible. The results illustrate the tradeoffs in performance between SFW and DS spread-spectrum systems as a function of the parameters of the signals and the channel model. The performance of the SFH system is shown to be less sensitive to the exact characterization of the channel delay spectrum than the DS system. For most of the channels considered in this paper, SFH spread spectrum gives better performance than DS spread spectrum     

Selection of spreading sequences for direct-sequencespread-spectrum communications over a doubly selective fading channel

Error probabilities are evaluated for direct-sequence spread-spectrum communications over channels with doubly selective fading. The error probability for such a system depends on the spreading sequence, the autocorrelation function of the fading process, and the receiver signal-to-noise ratio. The focus of this paper is on the effect of the spreading sequence on the performance of differentially coherent detection of binary direct-sequence spread-spectrum signals using a correlator receiver. It is shown that significant performance differences result from different choices of spreading sequence. It is also shown that, given a moderate range of delay and Doppler spreads, sequences can be found which yield low bit error probabilities over the entire range. These are found to be robust with respect to a variety of shapes for the channel autocorrelation function and the full range of signal-to-noise ratios     

Variable dwell-time code acquisition for direct-sequencespread-spectrum systems on time-variant Rayleigh fading channels

Variable dwell-time code acquisition based on multiple-dwell or sequential linear tests is investigated for direct-sequence spread-spectrum systems on time-variant Rayleigh fading channels. Unlike in the conventional additive white Gaussian noise channels, the channel memory incurred by fading renders the exact analysis of the acquisition systems extremely difficult, if not impossible. In this paper, a novel method is developed to evaluate the mean acquisition time of the acquisition systems very accurately. The effects of Rayleigh fading are evaluated, and comparisons are made between double-dwell and sequential linear tests. Numerical results show that Rayleigh fading may result in 1-4-dB loss in performance, and the sequential linear test can outperform double-dwell test by a margin of 1-2 dB. The analytical results are verified by computer simulations     

Interference rejection in digital wireless communications

The growth in wireless communications necessitates more efficient utilization of spectrum. The increased sharing of spectrum translates into a higher likelihood of users interfering with one another. Interference rejection techniques allow a high capacity of users within the available spectrum. This overview comprises a literature review of published papers pertaining to single-channel adaptive interference rejection in digital wireless dating primarily from 1980 to the present. Though previous overviews are referenced and summarized, the focus is on advances not covered by previous overviews (consequently, some papers are included that predate 1980 because they are not covered in previous overviews). An organizational chart (divided into spread spectrum and nonspread spectrum techniques) is shown which outlines the types of techniques covered. Interference rejection is important for several reasons. Cellular capacity is inherently interference limited, particularly by co-channel interference (CCI) and adjacent-channel interference (ACI). One solution to combat CCI and ACI is to split cells and decrease the power, but cell-splitting is expensive. Interference-rejection techniques often represent a less expensive alternative to cell-splitting. In addition, as newer communication technologies supersede older technologies, interference-rejection techniques are important in helping to facilitate compatibility during transitions between the old and new technologies     

查处"电力变压器"对移动通信干扰引出的思考

0引言最近,我们在处理一起"电力变压器"对移动通信G网的干扰时,遇到了一些新的情况,得到了不透明结论。虽然最终消除了干扰,但排查过程值得思考。     

Interference rejection techniques in spread spectrum communications

It is argued that the ability of a spread-spectrum system to withstand interference, both intentional and unintentional, is probably its greatest asset. Any spread spectrum receiver can only suppress a given amount of interference; if the level of interference becomes too great, the system will not function properly. Even under these latter circumstances, however, other techniques, which enhance the performance of the system over and above the performance improvement that comes automatically to systems from using spread spectrum, are available for use. These techniques typically involve some type of additional signal processing and are examined here. Two general types of narrowband interference suppression schemes are discussed and an overview is presented for several other techniques. The two classes of rejection schemes emphasized are (1) those based on least-mean-square estimation techniques, and (2) those based on transform-domain processing structures     

Characteristics of motorist aid communications systems

The nature and magnitude of the demand for emergency communications generated by users of rural freeways is described. A classification diagram of potential communications systems is presented and a preliminary selection of candidate systems is made on the basis of overall technical and economic considerations. The operating efficiency of the selected systems is analyzed in terms of their estimated detection efficiency and detection time performance. This analysis when combined with system cost projections demonstrates that a one-mile interval terminal design providing two-way voice communication between the motorist and an aid dispatch center provides the best benefit-cost ratio consistent with a satisfactory level of service. The operational characteristics of the selected system are described in some detail.     

Interference suppression factor in DS-CDMA systems

In a wireless DS-CDMA system, the interference power is suppressed by a factor greater than the spreading factor of the spreading code. An investigation into the interference suppression factor is presented and its simple frequency domain expressions for other-user interference from the same system and other systems are derived. Some numerical results are presented for two spectrally overlapped systems with different raised cosine Nyquist filters     

Interference suppression by biased nonlinearities

It is shown that, when the input signal-to-interference ratio (SIR) is small, a biased nonlinearity that has a dead zone below some threshold value can provide a large enhancement in output SIR and signal-to-interference-plus-intermodulation ratio (SIIMR) if the threshold is set equal or close to the amplitude of the strong interference. In the absence of noise, the output SIR and SIIMIR are virtually independent of the input SIR, however low the latter may be. It Is shown that, for a dead-zone limiter, the output SIIMR in this case is -4.93 dB regardless of the input SIR. Under strong interference, any noise present in the input reduces the SIR and SIIMR improvement, but a biased nonlinearity can still provide an output SIR and SIIMR superior to the input SIR, i.e. an output SIIMR 6.35 dB below the input signal-to-noise ratio (SNR) instead of 6.02 dB below the input SIR as is the case with hard limiting     

Soft output interference suppression in TDMA wireless communications

We present a novel symbol-by-symbol Bayesian multiuser detector for cochannel interference cancellation in TDMA cellular communications. To begin with, we present the usual composite signal model consisting of the desired signal, interferer, channel parameters and channel noise. Then, we derive the soft output interference cancellation algorithm (SICA) to compute fixed-lag maximum a posteriori (MAP) estimates of the data symbols of the desired user, using a fixed lookahead of D observations. In the process, the interferer symbols are treated as nuisance parameters and averaged out. The complexity of the SICA is exponential in the length of the combined channel at the receiver. Subsequently, we present a simple, yet effective, technique for complexity reduction without significant performance degradation. We apply the algorithm to some typical scenarios, using the IS-136 TDMA standard, to demonstrate its interference suppression capabilities. In this paper, we assume that all relevant channel parameters are known. Hence, our results should be viewed as representing the best possible performance that can be obtained with the SICA in the scenarios considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Beam interference suppression in multi-cell millimeter wave communications

Due to the increase in the number of users, beam switching is used for suppressing interference, which leads to higher computational complexity in multi-cell millimeter wave communications. In order to resolve this problem, a beam interference model is introduced, and a lower complexity beam interference suppression algorithm based on user grouping is proposed. The proposed algorithm operates beam switching and multi-cell cooperative transmission for a part of the users when there exists beam interference due to high user density. In particular, considering the distinct interference suffered by each user, the proposed dual-threshold user grouping method can effectively solve the frequent switching problem at the base station caused by multi-cell cooperative transmission in multi-cell environments. Simulation results show that the proposed algorithm can reduce the computational complexity of beam switching and approach ideal system capacity, compared with conventional interference suppression algorithms that do not involve grouping of users.     

TD-LTE室分建设中的干扰抑制方法

TD-LTE室内分布系统建设中遇到的干扰问题可分为系统间干扰和系统内干扰两部分。系统间干扰主要包括杂散干扰、互调干扰、阻塞干扰三种,在室分系统中三种干扰会同时影响网络性能;系统内干扰主要是采用OFDM技术带来的多邻区同频干扰。文章主要对TD-LTE室内分布系统建设中遇到的干扰问题进行分析并提出了初步的解决方案。     

Narrowband interference suppression in time-hopping impulse radio ultra-wideband communications

Ultra-wideband (UWB) technology has been considered an innovative solution for future short-range high-speed wireless communications. Interference suppression is important for the UWB devices to operate over spectrum occupied by narrowband systems. In this paper, the use of a notch filter in time-hopping impulse radio (TH-IR) for UWB communication is considered, where a Gaussian monopulse is employed with pulse position modulation. Lognormal channel fading is assumed, and a complete analytical framework is provided for the performance evaluation of using a transversal-type notch filter to reject narrowband interference (NBI). A closed-form expression of bit-error probability is derived, and the numerical results show that the use of a notch filter can improve the system performance significantly. Furthermore, a performance comparison between TH-IR and multicarrier code-division multiple-access (MC-CDMA) UWB systems is made under the conditions of the same transmit power, the same data rate, and the same bandwidth. It is shown that in the presence of NBI, the TH-IR system and MC-CDMA system achieve similar performance when both use a notch filter.     

Interference suppression for DS/CDMA

A direct-sequence spread spectrum (DS/SS) receiver for suppressing multiple-access interference in direct-sequence code-division multiple-access (DS/CDMA) communication systems is introduced. The proposed receiver does not require knowledge of other users' spreading codes, timing, or phase information. Moreover, the receiver allows the number of taps to be chosen independently of the processing gain and, hence, is easily applicable to CDMA systems employing either a small or a large processing gain. Performance analysis, including average probability of error and signal-to-noise ratio, is provided, and results are presented for systems varying from lightly loaded (for example, eight user/255 chip) to heavily loaded (for example, 50 user/200 chip). Performance results indicate that the proposed receiver outperforms the linear correlation receiver and, in many cases, it does so by a considerable margin     

Interference suppression via operating frequency selection

Spectral analysis of the environment, often called sniffing, is used to find a clear operating band in radar and communication systems. The authors show that the sniffing technique can be used to null multiple broad-band sidelobe jammers by exploiting the transfer function of the antenna. Analysis and computer simulation are used to demonstrate the performance