多码CDMA系统中联合并行干扰抵消与迭代信道估计方法

本文针对多码CDMA系统提出了一种联合并行干扰抵消与迭代信道估计方法。该方法首先通过导频符号对信道进行估计，然后使用RAKE接收机后的软信息对信道参数进行修正，最后通过并行干扰抵消(PIC)去除多码干扰(MCI)。仿真结果表明，经过多次迭代后，该方法可显著地降低多码CDMA系统的误比特率平台。本文同时还给出了信道估计质量、码道数和PIC次数对多码CDMA系统性能的影响。     

Downlink transmission of broadband OFCDM Systems-part II: effect of Doppler shift

The orthogonal frequency and code division multiplexing (OFCDM) system with 2-D spreading (time- and frequency-domain spreading) is becoming a promising candidate for future broadband wireless communication systems. OFCDM is more attractive than orthogonal frequency-division multiplexing (OFDM) both by introducing frequency-domain spreading for frequency diversity provision and time-domain spreading for flexible data rate provision. To provide high-speed mobile services, multicode transmission is employed in conjunction with OFCDM. In a Gaussian or flat-fading channel, multicode channels are orthogonal. However, in a realistic wireless channel, the orthogonality no longer maintains. Thus, multicode interference (MCI) is caused. This paper focuses on the investigation of the effect of Doppler shift on the downlink transmission of high-speed mobile OFCDM systems. A practical channel estimation algorithm based on a code-multiplexed pilot channel is employed to track the variations of fading channels. Hybrid MCI cancellation and minimum mean-square error (MMSE) detection proposed by the authors is employed as an efficient way to eliminate the MCI in the frequency domain. The system performance is analytically studied with imperfect channel estimation to show how it is affected by parameters such as the window size in the channel estimation, Doppler shift, the number of stages of the hybrid detection, the power ratio of pilot to data channels, spreading factor, and so on.     

Tentative chip decision-feedback equalizer for multicode wideband CDMA

We investigate a chip-level minimum mean-square-error (MMSE) decision-feedback equalizer (DFE) for the downlink receiver of multicode wideband code-division multiple-access systems over frequency-selective channels. First, the MMSE per symbol achievable by an optimal DFE is derived, assuming that all interchip interference (ICI) of the desired user can be eliminated. The MMSE of DFE is always less than or at most equal to that of linear equalizers (LE). When all the active codes belong to the desired user, the ideal DFE is able to eliminate multicode interference (MCI) and approach the performance of the single-code case at high signal-to-noise ratio (SNR) range. Second, we apply the hypothesis-feedback equalizer or tentative-chip (TC)-DFE in the multicode scenario. TC-DFE outperforms the chip-level LE, and the DFE that only feeds back the symbols already decided. The performance gain increases with SNR, but decreases with the number of active codes owned by the other users. When all the active codes are assigned to the desired user, TC-DFE asymptotically eliminates MCI and achieves single-user (or code) performance at high SNR, similarly, to the ideal DFE. The asymptotic performance of the DFE is confirmed through bit error rate simulation over various channels.     

Downlink transmission of broadband OFCDM systems-part IV: soft decision

In this paper, the performance of turbo-coded orthogonal frequency and code-division multiplexing (OFCDM) systems is investigated with soft multicode interference (MCI) cancellation and minimum mean-square error (MMSE) detection for downlink transmission in future high-speed wireless communications. To regenerate the soft interference signal, the conventional turbo decoding algorithm must be modified to provide log-likelihood ratio (LLR) values for all coded bits. Based on the LLR outputs of turbo decoder, two soft-decision functions are proposed, called LLR-soft-decision and Gaussian-soft-decision functions. The Gaussian assumptions used for deriving these two soft functions are verified by simulation results, and simple methods are proposed to estimate parameters used in the soft functions in practical systems. By means of computer simulations, the performance of soft MCI cancellation is studied extensively and compared with that of hard ones. It is shown that in a highly frequency-selective channel, soft MCI cancellation and MMSE detection can significantly improve the performance of turbo-coded OFCDM systems. Two iterations in turbo decoding are sufficient for both hard and soft-decision functions. The proposed soft-decision functions outperform the hard-decision function with various channel conditions and system parameters, such as the channel correlation, the quality of channel estimation, the number of iterations in turbo decoding and the frequency-domain spreading factor (N/sub F/). Furthermore, the Gaussian-soft-decision function provides better performance than the LLR-soft-decision function. Finally, although frequency diversity gain is saturated for large channel correlation when N/sub F/ is large as in , the gain increases further with increasing N/sub F/ for small channel correlation even when N/sub F/ is large.     

Successive interference cancellation algorithms for downlink W-CDMAcommunications

In this paper, successive intracell interference cancellation (IIC) of the wideband-code division multiple access (W-CDMA) signal at the mobile unit is considered. Three new interference cancellation techniques suitable for the downlink of any CDMA system with orthogonal spreading are proposed. No prior knowledge of users' spreading codes or even their spreading factors are required for interference cancellation. A new term, effective spreading code, has been introduced, which is defined as the interfering user physical code as seen by the desired user within the desired user symbol duration. The mobile receiver estimates the effective spreading codes of the interfering users regardless of their spreading factors using fast Walsh transform (FWT) correlators (instead of the regular correlators) and uses this information to suppress the intracell multiuser interference. Three different interference-suppressing techniques are studied: subtraction; combined interfering signal projection; and separate interfering signal subspace projection. The complexity of the proposed techniques is low compared to conventional interference cancellation techniques. For a W-CDMA system and the IMT-2000 vehicular channel model, a capacity increase of up to 150% of the original (without IIC) system capacity is shown     

Blind decorrelating RAKE receivers for long-code WCDMA

The problem of blind and semiblind channel estimation and symbol detection is considered for long-code wideband code division multiple access (CDMA) systems, including systems with multirate and multicode transmissions. A decorrelating matched filter, implemented efficiently in state-space, eliminates multiaccess interference and produces a bank of vector processes. Each vector process spans a one-dimensional (1-D) subspace from which channel parameters and data symbols of one user are estimated jointly by least squares. A new identifiability condition is established, which suggests that channels unidentifiable, in short-code CDMA systems are almost surely identifiable when aperiodic spreading codes are used. The decorrelating matched filter is implemented efficiently based on time-varying state-space realizations that exploit the structure of sparsity of the code matrix. The mean square error of the estimated channel is compared to the Cramer-Rao bound, and a bit error rate (BER) expression for the proposed algorithm is presented.     

Performance and optimization of multistage partial parallel interference cancellation for wideband CDMA systems in multipath fading channels

In this paper, a closed-form bit error rate (BER) of hard-decision multistage partial parallel interference cancellation (PPIC) having perfect and imperfect channel estimation for complex spreading code-division multiple access (CDMA) over multipath Rayleigh channels was derived. The effects of receiving BER, power ratio of pilot to data channels, multiple-access interference (MAI) and multipath interference, additive white Gaussian noise, and other factors affecting channel estimation error are analyzed. The expression determining the optimal interference cancellation weight at path level for PPIC is first derived. Then, the oscillatory behavior of user-level-optimized PIC and the convergence of perfect channel estimation PIC and path-level-optimized PIC are validated. Results indicate that all factors determining the optimal weight certainly affect the BER performance of PPIC and that the lower BER limit of multistage PPIC depends on the adjustable factors, which include power ratio, observation length, and signal-to-noise ratio, and an unchangeable factor, i.e., number of users. In addition, the variance of MAI for complex spreading sequence is obtained, and the analytical expression can also be employed to predict the performance of multicode CDMA.     

Channel estimation and multiuser detection in long-code DS/CDMAsystems

The problems of channel estimation and multiuser detection for direct sequence code division multiple access (DS/CDMA) systems employing long spreading codes are considered. With regard to channel estimation, several procedures are proposed based on the least-squares approach, relying on the transmission of known training symbols but not requiring any timing synchronization. In particular, algorithms suited for the forward and reverse links of a single-rate DS/CDMA cellular system are developed, and the case of a multirate/multicode system, wherein high-rate users are split into multiple virtual low-rate users, is also considered. All of the proposed procedures are recursively implementable with a computational complexity that is quadratic in the processing gain, with regard to the issue of multiuser detection, an adaptive serial interference cancellation (SIC) receiver is considered, where the adaptivity stems from the fact that it is built upon the channel estimates provided by the estimation algorithm. Simulation results show that coupling the proposed estimation algorithms with a SIC receiver may yield, with a much lower computational complexity, performance levels close to those of the ideal linear minimum mean square error (MMSE) receiver, which assumes perfect knowledge of the channels for all of the users and which (in a long-code scenario) has a computational complexity per symbol interval proportional to the third power of the processing gain     

Blind Channel Estimation in Multicode CDMA Using Multiantenna Receiver

In this paper blind channel estimation methods for long code CDMA system with multicode transmission are proposed. By allocating multiple codes, higher data rates may be provided to a user. Blind channel estimation allows reducing pilot signaling. Hence, higher effective data rates may be achieved and larger sample support may be obtained by using both the information symbols and known pilot symbols for channel estimation. Algorithms are extended to multi-antenna receiver which further improves the performance. The performance of the algorithms is studied in extensive simulations using WCDMA system model. The effects of code selection, interference and noise, number of antennas, antenna correlation are considered using frequency selective channel models. The proposed methods are more robust in the face of interference and code selection than the well-known subspace method in [7]. The computational complexity is lower as well.     

The Other-Cell Interference Calculation of Forward Link Channel in Multicode CDMA Systems

This paper analyzes the other-cell interference of forward link channel in multicode CDMA system supporting a voice and data service. In this system, voice communication can be served by a basic code and data service can be supported by multiple codes. Using this analysis, we can get the approximated capacity of forward link. We describe the effect of data user activity and received power level ratio between a data service and a voice service. Also, this paper describes the forward link capacity and the power ratio of data power to voice power according to the change of FER threshold for data service.     

Improved coding techniques for preceded partial-response channels

A coset of a convolutional code may be used to generate a zero-run length limited trellis code for a 1-D partial-response channel. The free squared Euclidean distance, d_free², at the channel output is lower bounded by the free Hamming distance of the convolutional code. The lower bound suggests the use of a convolutional code with maximal free Hamming distance, d_max(R,N), for given rate R and number of decoder states N. In this paper we present cosets of convolutional codes that generate trellis codes with d_free ²>d_max(R,N) for rates 1/5⩽R⩽7/9 and (d _free²=d_max(R,N) for R=13/16,29/32,61/64, The tabulated convolutional codes with R⩽7/9 were not optimized for Hamming distance. Instead, a computer search was used to determine cosets of convolutional codes that exploit the memory of the 1-D channel to increase d_free² at the channel output. The search was limited by only considering cosets with certain structural properties. The R⩾13/16 codes were obtained using a new construction technique for convolutional codes with free Hamming distance 4. Newly developed bounds on the maximum zero-run lengths of cosets were used to ensure a short maximum run length at the 1-D channel output     

Downlink transmission of broadband OFCDM systems-part III: turbo-coded

Orthogonal frequency and code-division multiplexing (OFCDM) systems have been introduced for high-speed data transmission in future wireless mobile communications. In this paper, a hybrid multicode interference cancellation (MCI) and minimum mean-square error (MMSE) detection scheme is presented for the turbo-coded OFCDM systems. Channel estimation based on a code-multiplexed pilot channel is employed. The weights of the hybrid detection are derived theoretically and an effective method to generate the weights in practical applications is proposed. By means of computer simulation, the effects of system parameters on the performance are studied extensively. It is shown that the hybrid detection outperforms pure MMSE detection in various channel conditions, especially for high-level modulation schemes. To carry out interference regeneration for the hybrid detection, the conventional turbo decoding algorithm which only decodes systematic bits should be extended to decode parity bits as well. Moreover, two iterations in turbo decoding are sufficient to provide good performance for the system with the multistage hybrid detection. Finally, given time-domain spreading factor N/sub T/, the system performance improves with frequency-domain spreading factor N/sub F/. However, the frequency diversity gain is saturated when N/sub F/ is large (i.e., N/sub F//spl ges/16).     

Interference Cancellation in CDMA Systems Employing Complementary Codes Under Rician Fading Channels

The ideal correlation properties of single user code division multiple access (CDMA) systems with complementary codes (CC) are lost by multiple access interference and near–far effects in downlink frequency selective fading channels. This severity in interference has motivated to develop ways to overcome the threats with suitable suboptimal interference cancellation schemes. In this paper, we propose successive interference cancellation (SIC) for downlink CDMA systems with CC to improve the system capacity and reduce error rate under near–far situations. Theoretical study and extensive simulations were conducted to verify the effectiveness of proposed SIC under frequency selective Rician fading channels in achieving frequency diversity gain, close to theoretical lower bound in complementary coded CDMA (CC-CDMA) systems.     

Implementing interference cancellation to increase the EV-DO Rev A reverse link capacity

This article provides the principles and practice of how interference cancellation can be implemented on the EV-DO Rev A reverse link. It is shown that applying interference cancellation to CDMA achieves the multiple access channel sum rate capacity for either frame synchronous or asynchronous users. The per user SINR gain from space-time interference cancellation translates directly into a CDMA capacity gain of the same factor, allowing EV-DO Rev A to support more users with higher data rates. We demonstrate how interference cancellation can be added to base station processing without modifying user terminals, EV-DO standards, or network coverage. We present commercially viable receiver architectures for implementing interference cancellation with the asynchronism and H-ARQ of EV-DO RevA, and explain why closed loop power control can operate the same way it does today. Network level simulations over a wide range of channels confirm that interference cancellation offers significant capacity gains for all users, while maintaining the same link budget and system stability.     

Code-spread CDMA with interference cancellation

The performance of an asynchronous code division multiple access (CDMA) system (uplink) employing very low-rate maximum free distance codes for combined coding and spreading is analyzed when successive or parallel interference cancellation is applied. An analytical approach to the evaluation of the bit error rate is presented and shown to give results close to simulations. Our results show that the code-spread system outperforms the conventionally coded and spread system. Without interference cancellation the single-user bound is never reached (except for one user). With two stages of parallel interference cancellation, a code-spread system with a load only slightly less than 1 bit/chip can obtain a bit error rate very close to that of a single-user system     

On the comparisons of system performance and capacity of asynchronous STBC MC‐CDMA multirate access schemes

Three multirate access schemes, multicode, variable spreading gain (VSG), and spectral overlaid multiple‐symbol‐rate (MSR), for asynchronous space‐time block coded (STBC) multicarrier code division multiple access (MC‐CDMA) systems are proposed. The three possible spectral overlaid configurations for MSR systems are also investigated. The expressions to evaluate the multiple access interferences, bit error rate (BER) performances, and system capacities of a antenna STBC MC‐CDMA using the three multirate access schemes are obtained. Transmit power allocation is adjusted according to the service rates and the number of active users in each service class to maintain the link quality and to improve the system capacity. Our numerical results show that systems with multicode access scheme using orthogonal Gold spreading codes and with VSG access scheme have similar system performance and capacity, and both perform in general better than systems with MSR access scheme of any spectrum configurations. In case when non‐orthogonal Gold codes are used, multicode access scheme shows degradation in the system capacity as compared to VSG, due to the presence of larger self‐interference (SI) among the codes used by each user. The achievable capacities for the three spectral overlaid configurations of MSR multirate systems are also compared. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

BER performance comparison of single code and multicode DS/CDMAchannelization schemes for high rate data transmission

The bit error rate (BER) performance of single code and multicode channelization schemes for high rate data transmission in direct-sequence code division multiple access (DS/CDMA) systems is compared. The multipath interference (MPI) effects, which become significant as the data rate increases, are accurately included in the analysis. It is shown that notable performance improvement can be achieved by using multicode scheme in the multipath fading channel     

A product-coded WDM coding system

Forward error correction is a feasible approach for reducing the bit error floor of lightwave systems, arising from the fact that a simple single-error-correcting code can reduce the error floor from O(P _e) to O(P_e²). We propose a novel wavelength demultiplexer (WDM) coding system using a product code to improve the performance of WDM systems with an error floor caused by fiber dispersion or system noise     

The error performance of CD900-like cellular mobile radio systems

The symbol error performance of CD900-like digital cellular mobile radio systems over narrowband and urban wideband transmission channels was investigated. The basic performance is presented for Gaussian, flat-fading Rayleigh, and log-normal channels in the presence of selection and ratio combining space diversity schemes. For wideband channels having more than one resolvable fading path, a CD900-like system without diversity reception suffers from large residual symbol error probabilities P_R(≈10^-1). The introduction of adaptive correlation diversity (ACD) mitigates the effects of multipath, yielding a P_R of 6×10^-5. Although this P_R value is relatively low, the probability of symbol error (P_e) versus signal-to-noise ratio (SNR) is significantly poorer than for the Gaussian channel. By combining the ACD scheme with space diversity, the P_R is eliminated by P_e >10^-5, and the channel SNR is within 5 dB of the Gaussian channel performance when P_e is 10^-10