A spatial-temporal decorrelating receiver for CDMA systems withbase-station antenna arrays

We investigate multiuser signal detection with a base-station antenna array for a synchronous DS-CDMA uplink using nonorthogonal codes in Rayleigh fading channels. We have developed a new formulation for a spatial-temporal decorrelating detector using the maximum-likelihood criteria. The detector is shown to be near-far resistant. We propose to implement the spatial-temporal decorrelating receiver iteratively by applying the space-alternating generalized expectation-maximization (SAGE) algorithm. Simulation results show that the SAGE-based decorrelating receiver significantly outperforms the conventional single-user receiver and with performance close to that of a spatial-temporal decorrelating receiver with known channel parameters. We have observed that adding base-station antennas can actually improve convergence of the proposed iterative receiver     

The performance of optical fiber direct-sequence spread-spectrummultiple-access communications systems

Because of the random nature of the photodetection process and the multiple-user interference, an exact analysis of avalanche photodiode (APD)-based optical code division multiple-access (CDMA) communications systems is intractable and quite often, Monte Carlo (MC) simulations which yield exact estimates of system performance in terms of bit error rates (BERs) require a prohibitive computational burden. A quick and accurate MC method for simulating APD-based optical CDMA systems is presented. In particular, a performance analysis of optical CDMA systems employing optical orthogonal and prime sequence codes is undertaken     

Throughput of unslotted direct-sequence spread-spectrummultiple-access channels with block FEC coding

An analysis of unslotted random-access direct-sequence spread-spectrum multiple-access (DS/SSMA) channels with block forward error correction (FEC) coding is presented. Extending a methodology that was introduced in an earlier paper on unslotted packet code-division multiple access (CDMA) without coding, a procedure for calculating the error probability of an L-bit packet in the variable message length, FEC-coded, DS/SSMA environment is described. This procedure is then used in conjunction with appropriate flow equilibrium traffic models to compute channel throughput. Using BCH block coding as an example, the analytical model is exercised to obtain throughput versus channel traffic curves over a range of code rates, leading to an assessment of maximum achievable throughput and the associated optimum FEC code rate. The results show that the use of block FEC coding provides a significant improvement in the bandwidth-normalized channel throughput (utilization), approaching values competitive with those for comparable narrowband ALOHA channels     

Phase weighting for linear antenna arrays

A novel method for introducing an amplitude taper onto a linear array is described, using equal amplitude elements with randomised phase reversals to create an effective taper.     

Deterministic approaches for blind equalization of time-varyingchannels with antenna arrays

In this paper, we study the blind equalization problem of time-varying (TV) systems where the channel variations are too rapid to be tracked with conventional adaptive equalizers. We show that using a finite Fourier basis expansion, a TV antenna array system can be cast into a time-invariant multi-input, multi-output (MIMO) framework. The multiple inputs are related through the bases, thereby allowing blind equalization to be accomplished without the use of higher order statistics. Two deterministic blind equalization approaches are presented: one determines the channels first and then the equalizers, whereas the other estimates the equalizers directly. Related issues such as order determination are addressed briefly. The proposed algorithms are illustrated using simulations     

MLSE receiver for direct-sequence spread-spectrum systems on a multipath fading channel

To accommodate high-speed data transmissions, it may be necessary to substantially reduce the processing gain of a direct-sequence spread-spectrum (DSSS) system. As a result, intersymbol interference effects may become more severe. In this paper, we present a new structure for maximum-likelihood sequence estimation equalization of DSSS signals on a multipath fading channel that performs the function of despreading and equalization simultaneously. Analytical upper bounds are derived for the bit-error probability when random spreading sequences are used, and comparisons to simulation results show that the bounds are quite accurate. The results also show that significant performance improvement over the conventional RAKE receiver is obtained.     

Adaptive antenna arrays for OFDM systems with cochannelinterference

Orthogonal frequency-division multiplexing (OFDM) is one of the promising techniques for future mobile wireless data systems. For OFDM systems with cochannel interference, adaptive antenna arrays can be used for interference suppression. This paper focuses on a key issue for adaptive antenna arrays, that is, parameter estimation for the minimum mean square error (MMSE) diversity combiner (DC). Using the instantaneous correlation estimation approach developed in the paper, an original parameter estimator for the MMSE-DC is derived. Based on the original estimator, we propose an enhanced parameter estimator. Extensive computer simulation demonstrates that the MMSE-DC using the proposed parameter estimators can effectively suppress both synchronous and asynchronous interference in OFDM systems for packet and continuous data transmission     

Synthesis of linear antenna arrays

Synthesis of antenna arrays employing theL_{2}-norm as well as theL_{infty}-norm is discussed. The approximation in theL_{infty}-norm is obtained making use of Lawson's algorithm. A general iterative perturbation technique has been evolved for pattern synthesis for the case when the antenna currents alone are varied as web as for the case when both the antenna currents and the element positions are simultaneously varied. A few illustrative examples are given. The convergence of the iteration and the uniqueness of the solution are discussed.     

An adaptive correlator receiver for direct-sequence spread-spectrumcommunication

This paper presents a novel receiver for direct sequence spread-spectrum signals over channels containing interference and multipath. The receiver employs an adaptive correlator that jointly detects the transmitted data, removes interference, and compensates for multipath. The optimum correlation vector is derived by determining the Wiener vector that minimizes the mean squared error (MSE) between the transmitted data bit and the correlator output. For an additive white Gaussian noise (AWGN) channel, the optimal correlation vector is the spreading sequence used by the transmitter. For interference and multipath channels, the optimal correlation vector will suppress the interference and combine the multipath while optimizing the detection of the transmitted data bit. The paper presents analytical and simulation results which illustrate the bit-error rate (BER) performance of the receiver in multipath and narrowband interference. Additionally, simulation results are presented illustrating the convergence performance of the receiver when the tap weights are adjusted using either the least mean square (LMS) or recursive least squares (RLS) adaptive algorithms     

A receiver of simple structure for antenna array CDMA systems

In code division multiple access systems, the two-dimensional (2-D) RAKE structure had been proposed using multiple antennas to increase the capacity. It consists of multiple fingers of a beamformer and a correlator. In this paper, we consider a simpler receiver structure. We only use one finger, which consists of one pair of spatial and temporal filters to combine signals. It is shown that the performance of the receiver system is comparable with that of the 2-D RAKE receiver. Furthermore, it is observed that the proposed receiver and the 2-D RAKE receiver have limited near-far resistance by the generation of the space domain due to antenna arrays     

一种零中频直接序列扩频数字接收技术

直接序列扩频技术在现代无线通信系统中有着广泛的应用,该信号的数字接收是实现对其侦察截获的关键技术之一。本文介绍了直接序列扩频技术原理及其在cdma2000移动通信系统中的应用,提出一种针对cdma2000直接序列扩频信号的零中频数字接收算法,并详细给出了码捕获及跟踪算法、载波同步算法的数学模型推导。实验结果显示,该算法成功地实现了码同步及载波同步,有较好的解调效果。     

Diversity gain for cdma systems equipped with antenna arrays

If the angular spread is not zero, the diversity gain can be achieved in the antenna-array reception. In order to achieve the diversity gain, we propose a beamforming method that utilizes two beamforming weight vectors and apply this beamforming technique to code-division multiple-access (CDMA) systems equipped with antenna arrays under a time-varying multipath-fading channel environment. In the proposed beamforming method, the channel vector has been estimated using two basis vectors that span the signal subspace. Since the proposed beamforming method utilizes two-dimensional (2D) signal subspace, it provides better performance than the conventional beamforming method, which utilizes one-dimensional signal subspace. Through simulation results, we can see that the performance is improved as the angular spread gets larger.     

A novel linear and planar multiband fractal-shaped antenna arrays with low sidelobes

Fractal antenna arrays are usually used to tune multiband frequencies. However, these types of iteratively constructed antennas are associated with undesirable high sidelobe levels and low directivities. In this paper, an optimization procedure based on the genetic algorithm is used to find the optimal weights of the array elements such that the corresponding array patterns have low sidelobe levels and good directivity. Moreover, the fractal nature in the proposed arrays is maintained regardless of the optimized weights. Thus, the proposed fractal-shaped array maintains its capability in performing multiband frequency operation. These good radiation features make the proposed fractal-shaped array more appropriate for the current and future wireless communication applications. Simulation results confirm the superiority of the presented linear and planar fractal-shaped array structures with compared to the conventional fractal cantor linear array and the standard Sierpinski carpet planar array. For the proposed fractal cantor linear array, the sidelobe level has been reduced to more than −20 dB at different operating frequencies, and the directivity has been improved by more than 8 dB, while the modified Sierpinski carpet planar array has achieved −30 dB depressions in the sidelobe level and 6 dB improvement in the directivity.     

Code-aided blind detection of the transmission rate for multirate direct-sequence CDMA systems

The problem of rate detection for a multirate direct sequence/code division multiple access (DS/CDMA) systems is considered in this paper. It is assumed that, in each data-frame, each user may transmit its data at one out of a set of available data-rates, depending, e.g., on the kind of data to be transmitted, on the desired quality-of-service, and on the channel state. Relying on the knowledge of the spreading codes of the user whose rate is to be detected, rate detection algorithms based on the application of the generalized likelihood ratio test are here proposed. Both the cases where either the rates of the interfering users have been already detected and are known to the receiver or these rates are not known to the receiver are separately considered. With regard to the performance assessment, the effectiveness of the proposed rate detection strategies is validated, for a single-path fading channel, through both computer simulations and, with regard to the case that the interfering users rates have already been acquired, through a Chernoff-bounding technique.     

A comparison of two systems for downlink communication with basestation antenna arrays

We compare the performance of the downlink of two systems with antenna arrays at the base stations. One system uses a third of the available spectrum per cell 1/3, but reuses the channels three times within each cell. The other system reuses all the spectrum in all cells 1/1, but does not reuse channels within a cell. Thus, the maximum number of users in the two systems is the same. In order to account for the interference, the 1/3 system directs antenna pattern nulls against same cell cochannel users, while the 1/1 system directs nulls against cochannel users in other cells. The performances of the two systems are compared as a function of the azimuth angular width (seen from the base) of the multipaths generated at the mobile, using simulations and analytical derivations. The results herein indicate that the system with 1/1 reuse has a higher performance than the system with 1/3 reuse if fast intercell handover is used or if high dynamic range power control, nulling and synchronization is employed     

Genetic design of linear antenna arrays

Discusses a way to optimize both the topology and the numerical parameters of an antenna design. The approach relies on an “antenna language” to define how antennas are constructed, and a genetic algorithm to create new designs using this language. The grammatical rules of a language can be very vague or very specific, depending on the purpose of the designer. With a vague grammar, genetic algorithms search a very large design space, and can occasionally find unexpected solutions to a design problem. Other times, they completely fail to find a reasonable solution because of the vastness of the search space. In this case, including knowledge about the problem into the grammar narrows the search to a region expected to yield good results. This yields more conventional design solutions that usually perform reasonably well. In an example, two languages were used to design a linear antenna array. The general language allowed a wide variety of designs, while the Yagi-log language confined the search to topologies known to perform well. The performance of the antennas produced by both languages was superior to that of a conventional log-periodic design. Further, the Yagi-log design was more fit than the unconventional design from the general language, illustrating the benefits of including knowledge in the grammar     

基于PARAFAC模型的新型DS-CDMA盲接收机

根据平行因子(PARAFAC)模型,研究DS-CDMA盲多用户检测算法。将直接三线性分解算法(DTLD)与三线性交替最小二乘(TALS)算法结合,提出一种新的DTALS-PARAFAC盲接收机,解决了三线性交替最小二乘(TALS)算法中因为初始值估计不当引起的收敛速度差的问题。仿真结果表明,与TALS-PARAFAC接收机相比,DTALS-PARAFAC接收机改善了误码率性能,并且具有更快的收敛速度。     

Analytical array polynomial method for linear antenna arrays with phase-only control

A new analytical synthesis technique is developed for the phase-only control of linear antenna arrays and is based on the Schelkunoff's unit circle. The array factor is formulated as an array polynomial in the complex z-plane and as a product of corresponding subpolynomials such that their roots are located on the unit circle. A constraint is derived such that the expansion of the subpolynomials yields current excitations with unity magnitudes. In general, the constraint of phase-only control forces some element excitations to be zero for an arbitrary number of array elements. The subpolynomial with the largest degree is used to synthesize the main beam characteristics. The null steering for interference suppression is obtained by using only one subpolynomial to steer only one null. The results verify the developed analytical solution to synthesize the prescribed patterns using the phase shifters of the linear antenna arrays.     

A semiblind method for transmit antenna arrays in CDMA systems

Using a transmit antenna array (TAA), the downlink beamforming is able to improve the performance of wireless communication systems. We propose a semiblind downlink beamforming method based on channel reciprocity of the wireless channels for code-division multiple-access (CDMA) systems equipped with TAA. In the proposed semiblind method, it is shown that the feedback delay can result in performance degradation under a time-varying fading channel environment. Hence, the performance of the semiblind method is satisfactory only when the maximum Doppler frequency is low or the feedback delay is not significant. To overcome this difficulty, the channel prediction is considered. Using the channel predictor, the semiblind method can provide a reasonable performance for a moderate maximum Doppler frequency (about a few hundred hertz). For theoretical performance analysis, upper, and lower bounds of the bit error probability under a fading channel environment are derived     

Combined antenna and linear arrays for radiation low-power picosecond pulses

A wire model and a combined antenna for radiation of low-power bipolar pulses with a duration of 200 ps, which is based on this model, are developed. The frequency-domain characteristics of the model and the time- and frequency-domain characteristics of the combined antenna are studied. Linear antenna arrays based on the developed combined antenna are designed and the radiation characteristics of these antennas in the wave-beam scanning mode are analyzed.