Band-limited DS-CDMA using exponentially weighted despreadingsequence

The signal-to-interference ratio (SIR) performance of a band-limited DS-CDMA system using an exponentially-weighted despreading sequence (EWDS) in the receiver is addressed. It is shown that tuning a parameter of the EWDS helps to flatten the in-band spectrum of the received coloured signal, leading to a better performance when the transmission bandwidth is limited. With the constraint of the system transmission bandwidth equal to and twice the chip rate, numerical results show that the SIR of the proposed system can be increased up to 28 and 94%, respectively, when compared with a conventional receiver using a rectangular despreading sequence     

A DS-CDMA system using despreading sequences weighted by adjustablechip waveforms

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDSs) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDSs outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for bandlimited spreading signals is also provided to reveal the practical implications of the proposed technique     

Performance of coherent receiver with weighted despreading sequencefor DS-CDMA

The performance of a DS-CDMA system using coherent receivers with the despreading sequences weighted by adjustable chip waveforms is presented. A new expression for the signal to interference plus noise ratio (SINR) of the decision variable is derived for the proposed receiver. When the system parameters are given, the derived SINR expression is determined only by one parameter of the weighted despreading sequence. As a result, the parameter can be simply tuned in practice for multiple access interference (MAI) rejection     

Performance Analysis of Heterodyne-Detected Coherent Optical CDMA Using a Novel Prime Code Family

In this paper, a novel prime code, which is hereby referred to as ldquodouble-padded modified prime code,rdquo is introduced and employed in a synchronous optical code division multiple access for both spreading and despreading operations. We have analyzed this system signal-to-noise-ratio penalty as a function of the number of simultaneous users that is accommodated to maintain an appropriate value of the bit-error rate. In the analysis, the coherent binary-phase-shift-keying modulation is used, and at the receiver, signals are heterodyne detected and processed. In addition, in the analysis, both the receiver noise and multiple-access interference are investigated.     

Improvement and Analysis of Carrier-Interferometry Orthogonal Frequency Division Multiplexing System

1IntroductionOrthogonal Frequency Division Multiplexing(OFDM) isknown to be with high spectrumefficiency and effectivein coping with such wireless channel i mpairments asmultipath fading and Inter-Symbol Interference(ISI)[1]. Moreover ,OFDMsystems have become prac-tical toi mplement using Fast Fourier Transform(FFT)techniques[2]thanks to advances in digital signal pro-cessing. Up to the present , OFDMhas been employedin several digital transmission systems , such as AD-SL[3], HIP…     

A novel prefiltering technique for downlink transmissions in TDD MC-CDMA systems

We discuss a prefiltering technique for interference mitigation in the downlink of a time division duplex (TDD) multicarrier code-division multiple access (MC-CDMA) system. The base station (BS) is equipped with multiple transmit antennas, and channel state information (CSI) is obtained at the transmitter side by exploiting the channel reciprocity between uplink and downlink transmissions. The prefiltering coefficients are designed so as to minimize a proper cost function that depends on the signal-to-interference-plus-noise ratios (SINRs) at the mobile terminals (MTs). The resulting scheme allows using a simple despreading receiver, thereby eliminating the need for channel estimation and equalization. Numerical results show the advantages of the proposed scheme over some existing solutions.     

Two-stage Wiener filter based cancellation receivers for DS-CDMA

The authors present the two-stage Wiener filter based cancellation receiver for DS-CDMA, i.e. a receiver using Wiener filters to regenerate the interferers and to make the final despreading. Theoretical calculation as well as simulation results show that the two-stage Wiener filter performs considerably better than any other two-stage receiver     

Rate-Distortion Optimized Video Transmission Over DS-CDMA Channels with Auxiliary Vector Filter Single-User Multirate Detection

In this paper, we consider the rate-distortion optimized resource allocation for video transmission over multi-rate wireless direct-sequence code-division-multiple-access (DS-CDMA) channels. We consider the performance of transmitting scalable video over a multipath Rayleigh fading channel via a combination of multi-code multirate CDMA and variable sequence length multirate CDMA channel system. At the receiver, despreading is done using adaptive space-time auxiliary-vector (AV) filters. We propose a new interference cancelling design that uses just a single AV filter for single-user mutirate despreading. Our experimental results show that the proposed interference cancelling design has excellent performance in scalable video transmission over DS-CDMA systems that use a combination of multicode multirate and variable processing gain multirate CDMA. The proposed design takes advantage of the fact that single user's video data is transmitted using two spreading codes, one for the base layer and one for the enhancement layers, and of the fact that these spreading codes can have different processing gains. The proposed interference cancelling design is compared with two conventional single-user multirate CDMA receiver configurations, however now we use an AV filter rather than a simple matched filter. We also propose a resource allocation algorithm for the optimal determination of source coding rate, channel coding rate and processing gain for each scalable layer, in order to minimize the expected distortion at the receiver.     

正交循环码M进制扩频接收机的研究

该文提出了一种基于正交循环码的M进制扩频接收机方案,发端将一条原型扩频码循环移位构成M个扩频码,实现M进制扩频,接收端利用时频变换域正交匹配滤波器实现了非相干解扩解调。该方案不仅减少了需要的扩频码数目,而且有效地降低了接收机的计算复杂度。该文对所提出的系统在单用户和多用户条件下的误码性能进行了理论分析和仿真,并比较了3种M进制解扩方法的运算量。结果表明:正交循环码M进制扩频系统的计算复杂度和误码特性均优于传统的M进制系统。     

无线体域网低复杂度重复码解扩频方法研究

为了提高数据传输的可靠性,降低窄带接收机的复杂度,无线体域网标准IEEE 802.15.6采用了基于重复码的扩频方法。通过设计一种根据动态阈值选择性进行量度运算的解扩频结构,并结合硬判决阈值特性利用简化的最大比合并以及广义最小距离原理,提出了硬阈值—合并法和硬阈值—删除法,计算复杂度较小,同时解扩频性能近似最优。理论分析及仿真结果表明,与基于最大似然方法的解扩频相比,其扩频增益接近理论值,在高信噪比环境下,计算复杂度可降低86%以上。     

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 blind receivers for MC‐CDMA communication systems

In this paper, we propose three adaptive blind algorithms for multiuser multicarrier code division multiple‐access systems in multipath fading channels. The proposed adaptive blind receivers are based on the property of the discreteness of the input data symbol and are updated in every symbol interval. We also use the concept that the variance of the output signal approaches to the variance of the desired signal to get the cost function. The three proposed receiver structures are the traditional finite impulse response (FIR) structure, the despreading (DES) filter structure and the generalized sidelobe canceller (GSC) structure. The advantage of the FIR filter is that the length of the filter weights does not have to be the same length as the spreading code. For the DES filter, the combination of the adaptive weight and the despreading code has the simplest structure than the other two proposed receiver structures. The constrained GSC filter is superior to the other two proposed receiver structures in the environments dominated by multiple‐access interference. By this constraint, the blind GSC filter can guarantee to converge to the desired solution. Simulation results are given to show the effectiveness and comparison of the proposed adaptive blind receivers. Copyright © 2012 John Wiley & Sons, Ltd.     

An improved design of chip waveforms for band-limited DS-CDMA systems

This paper introduces an efficient and improved design of chip waveforms to minimize the multiple-access interference in band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. For ease of implementation, the DS-CDMA system employs a time-limited chip waveform, whereas its band limitation is ensured by the low-pass filters at both the transmitter and receiver ends. The design uses sinusoids to synthesize the time-limited chip waveform so that the portion of its spectrum across the specified bandwidth is as flat as possible. It is shown that by using a simple series expansion (with only a few terms) the synthesized chip waveforms significantly outperform the spreading/despreading waveforms previously proposed, particularly for large values of the chip duration-bandwidth product.     

SINR estimation and orthogonality factor calculation of DS‐CDMA signals in MIMO channels employing linear transceiver filters

A discrete‐time model of DS‐CDMA signaling using multiple transmit and receive antennas employing linear transceiver filters is derived. For each link, connecting a base‐station to a wireless mobile user, the downlink signal to interference plus noise ratio (SINR) after despreading is derived analytically, and as a by‐product an exact closed‐form solution of the orthogonality factor is obtained. The orthogonality factor is derived for any linear receiver structure that is implemented by a bank of correlators and for any linear combining techniques (such as MRC and MMSE), and for any number of transmit and/or receive antennas that utilize transmit (pre‐RAKE) and receive (post‐RAKE) filtering. The MIMO DS‐CDMA model is derived using a filter representation, and is extended to a vector/matrix formulation that permits a systematic and efficient way of computing the SINR in a radio network simulator. Copyright © 2006 John Wiley & Sons, Ltd.     

基于码匹配滤波的数字化直扩接收机

推导出了一种基于码匹配滤波的数字直扩接收机模型，该方法通过在解扩之前对本地扩频码进行匹配滤波实现对变带宽扩频信号的最佳接收。由于不需直接对输入基带信号进行滤波，而对本地扩频码的滤波又可以通过查表的方式实现，因此与传统的扩频接收机相比，占用的硬件资源大大减少。文中还提出了一种易于实现的次最佳方案。仿真结果表明，次最佳接收机的误码率性能和最佳接收机的误码率性能基本一致。     

Weighted Discriminator Function Based Unambiguous Tracking Method for Dual-Frequency Constant Envelope Modulated Signals

The dual-frequency constant envelope modulated (DCEM) signals, represented by alternative binary offset carrier signal, have obvious advantages in code tracking accuracy and anti-multipath performance compared with traditional binary phase shift keying signal, and are attracting more and more attention in the field of global navigation satellite system signal design. However, DCEM signal has a multiple-peaked auto-correlation function, which would lead to the tracking ambiguity problem. To deal with this problem, this paper proposes an unambiguous tracking method based on weighted discriminator function (WDF). This method uses the discriminator functions of noncoherent early minus late power (NELP) and sub carrier phase cancellation (SCPC) to generate a linear WDF without no false-lock points. In addition, a simplified version of the proposed WDF method is also provided for interoperable application. The theoretical code tracking error in thermal noise is derived out and verified. The performance of the proposed WDF is evaluated and compared with SCPC and pseudo correlation function methods with the help of simulation. Results show that our method has a significant improvement in code tracking accuracy. Not only the ambiguity threat is efficiently removed, but also the outstanding performance of DCEM signal is maintained.     

MIMO-assisted space-code-division multiple-access: linear detectors and performance over multipath fading channels

In this contribution, we propose and investigate a multiple-input-multiple-output space-division, code-division multiple-access (MIMO SCDMA) scheme. The main objective is to improve the capacity of the existing direct-sequence (DS)-CDMA systems, for example, for supporting an increased number of users, by deploying multiple transmit and receive antennas in the corresponding systems and by using some advanced transmission and detection algorithms. In the proposed MIMO SCDMA system, each user can be distinguished jointly by its spreading code signature and its unique channel impulse response (CIR) transfer function referred to as spatial signature. Hence, the number of users might be supported by the MIMO SCDMA system and the corresponding achievable performance are determined by the degrees of freedom provided by both the code signatures and the spatial signatures, as well as by how efficiently the degrees of freedom are exploited. Specifically, the number of users supported by the proposed MIMO SCDMA can be significantly higher than the number of chips per bit, owing to the employment of space-division. In this contribution, space-time spreading is employed for configuring the transmitted signals. Three types of low-complexity linear detectors, namely, correlation, decorrelating, and minimum mean-square error (MMSE) are considered for detecting the MIMO SCDMA signals. The bit-error rate performance of the MIMO SCDMA system associated with these linear detectors are evaluated by simulations, when assuming that the MIMO SCDMA signals are transmitted over multipath Rayleigh-fading channels. Our study and simulation results show that MIMO SCDMA assisted by multiuser detection is capable of facilitating joint space-time despreading, multipath combining, and receiver diversity combining, while simultaneously suppressing the multiuser interfering signals.     

Superposition Coding with Unequal Error Protection for the Uplink of Overloaded DS-CDMA System

The overloaded CDMA schemes exploited in direct sequence CDMA (DS-CDMA) systems are mainly to accommodate a greater number of users than the available spreading factor N. In this paper, a superposition coding CDMA (SPC-CDMA) with unequal error protection (UEP) is proposed as one of the overloaded CDMA schemes for the next generation mobile communication systems. It exploits the available power control in most base stations to adapt the transmitted power of active users in the uplink channel. In this scheme, the active users are divided into G groups and each group consists of K users. The K users share the same spreading sequence and are distinguished by different received power levels. At the receiver side, the system first performs despreading for group detection followed by multiuser receiver to estimate the K user signals in each group. It is shown through simulations that better performance are achieved compared to the conventional DS-CDMA and existing overloaded collaborative spreading CDMA (CS-CDMA) schemes, in additive white Gaussian noise (AWGN) and fading channels. Hence, the proposed scheme maximizes the system capacity K-fold compared to conventional DS-CDMA system without requiring extra spreading codes, with average signal to noise ratio (SNR) cost of only 1dB and 2 dB over AWGN and fading channels respectively at BER of 10^?3. On the other hand, for the same N, K and power constraints, SPC-CDMA scheme achieves twofold increase in data rate with 0.7 and 4 dB gains over AWGN and fading channels respectively, compared with overloaded CS-CDMA scheme in the same system capacity. In addition, the proposed scheme can also attain different levels of UEP for different users?? requirements by adjusting their fractions of transmitted powers.     

Delay-time=code division multi-access in frequency-selective channel

A new hybrid multi-access technique, called delay-time/code division multi-access (DT/CDMA), is proposed. Multiplexing is accomplished in both the delay-time domain and code domain. Delay-time division multiplexing is achieved by assigning different cyclic time delays to the same spreading sequence. At a receiver, frequency-domain equalisation, despreading, and demultiplexing are performed simultaneously in the frequency domain. The bit error rate performance when using the proposed DT/CDMA in a frequency-selective Rayleigh fading channel is evaluated by computer simulation.