MCI cancellation for multicode wideband CDMA systems

Multicode code division multiple access (CDMA) is a new transmission scheme for flexible and high-speed data communications. The basic idea of multicode CDMA, is to assign multiple channelization codes to any given user. If these channel codes are orthogonal, the self-interference among them can be eliminated in an additive white Gaussian noise (AWGN) channel. However, in a multipath environment, these intrauser signals from different delay paths no longer maintain orthogonality and, thus, cause interference, i.e., multicode interference (MCI), to each other. In high-speed data networks, where the number of users is much less than in the voice networks, the MCI may represent a large portion of the total interference and has a great impact on the system performance. A complex spreading multicode wideband CDMA receiver with RAKE structure and multistage MCI cancellation is studied. By pilot aided channel estimation, the MCI associated with the reference user is regenerated and subtracted from the received signal by a cancellation factor of λ. A complete and consolidated theoretical analysis is presented to show that the system performance is significantly improved by the MCI cancellation. The optimal cancellation factor in the kth stage is approximated by λ_opt^(k)⩽1-2P_e^(k-1) , where P_e^(k-1) the error probability of the (k-1)th stage. The optimal value of each stage can be chosen from 0.5 to 0.85 for a wide range of signal to noise ratios     

An adaptive analog noise-predictive decision-feedback equalizer

In this paper, an adaptive noise-predictive decision-feedback equalizer (NPDFE) is presented. The NPDFE architecture and its implementation are described. The NPDFE consists of an analog finite-impulse-response (FIR) forward equalizer, a recursive analog equalizer for noise prediction, and a decision-feedback equalizer (DFE). The recursive equalizer reduces noise enhancement and improves the signal-to-noise ratio (SNR) at the decision slicer input. The prototype targets a magnetic recording channel modeled by a Lorentzian impulse response. Measured results show that compared to a conventional DFE with FIR forward equalizer, the NPDFE achieves a SNR improvement of about 2 dB with PW₅₀=2.5T. The NPDFE consumes 130 mW at a data rate of 100 Mb/s and occupies 1.3 mm² of die area in a 0.5-μm CMOS process     

A mixed-signal RAM decision-feedback equalizer for disk drives

A mixed-signal RAM decision-feedback equalizer (DFE) that operates at 90 Mb/s is described. In the analog domain, the DFE subtracts intersymbol interference caused by the past four outputs. The equalized signal is fed into a nonuniform flash analog-to-digital converter (ADC) to produce the decision output and error signal used to adapt the RAM contents in the digital domain. With a 5 V supply voltage, the power dissipation is 260 mW during steady-state operation. The active area is 4.5 mm² in a 1 μm CMOS process     

Analog timing recovery for a noise-predictive decision-feedback equalizer

A timing recovery architecture and its CMOS implementation are described for a noise-predictive decision-feedback equalizer (NPDFE). The 0.5-/spl mu/m CMOS prototype includes timing recovery and the NPDFE and operates at 160 Mbit/s. The timing recovery blocks dissipate 27 mW from 3.3 V, occupy 0.2 mm/sup 2/, and achieve a root mean square jitter of 50 ps, which is 0.8% of a bit period.     

Widely linear decision-feedback equalizer for time-dispersive linear MIMO channels

The paper deals with the transmission over multiple-input/multiple-output channels exhibiting time-dispersion. A minimum mean-square error equalizer based on widely linear processing combined with the decision-feedback (DF) strategy is implemented via finite-impulse-response filters. The proposed equalizer provides considerable performance gain at the expense of a limited increase in computational complexity. The performance analysis has been carried out accounting for mismatch conditions always present in practice. The results confirm the stronger sensitivity of the DF-based equalizers with respect to the feedforward-based ones when system parameters are not accurately known.     

Optimal sequences for CDMA with decision-feedback receivers

We consider a symbol-synchronous code-division multiple-access (CDMA) system that is equipped with a multiuser decision-feedback receiver and for which power control is available. The users are each assigned a quality-of-service (QoS) threshold to be guaranteed by the system, and to cover scenarios for which there are multiple classes of users, these are not required to be equal to each other. For an ideal decision-feedback receiver, it is known that with enough power the system can always meet the users' QoS thresholds, so we instead minimize the sum of the users' received powers over system designs (i.e., signature sequences, power-control policy, and decision-feedback receiver) which guarantee the QoS requirements. It is found that the optimal design produces two classes of users, those whose sequences and powers satisfy with equality the generalized Welch bound inequality and those oversized users that are mutually orthogonal to each other and the rest of the users. In terms of power and bandwidth savings, the optimal sequences for the decision-feedback receiver are found to compare very favorably to optimal designs for linear receivers and to random sequences for the decision-feedback receiver.     

用于PMD补偿的判决反馈均衡器的结构研究

文章采用自适应均衡技术对偏振模色散(PMD)进行补偿,给出了判决反馈均衡器(DFE)结构,通过对前向均衡器(FFE)部分的改进,得到了适用于高速光纤通信系统的行波滤波器(TWF)及折叠级联的TWF结构,最后通过ADS仿真软件,得到了系统S参数仿真结果.     

A mixed-signal decision-feedback equalizer that uses a look-aheadarchitecture

A mixed-signal decision-feedback equalizer (DFE) that uses a look-ahead architecture is described. The parallelism in the look-ahead DFE (LA DFE) achieves an increase in the data rate over a conventional DFE with a small increase in area. Fully differential analog circuits perform the convolution operation in the LA DFE, and the coefficient adaption is carried out by digital circuits. The LA DFE occupies 23 mm ² in a 2-μm CMOS process and operates at 50 Mb/s while dissipating 260 mW     

Blind adaptive multiple-input decision-feedback equalizer with aself-optimized configuration

This paper introduces a novel blind adaptive multiple-input decision-feedback equalizer (MI-DFE) which is basically characterized by its ability to self-optimize its configuration, in terms of both structure and criteria, according to the severity of the transmission medium. In the first running mode, the novel equalizer is recursive, linear and “blindly” adapted by criteria leading to a solution closely related to the minimum MSE solution. In the second running mode, it becomes the conventional MI-DFE. From the viewpoints of both robustness and spectral efficiency, this equalizer proves to be very attractive since it avoids pathological behaviors, often encountered with the conventional trained MI-DFE, while requiring no training sequence. Furthermore, its very high speed of convergence renders it competitive in various standard applications, even in the case of burst mode transmission systems. Finally, the novel blind MI-DFE has been successfully tested on underwater acoustic communications signals, in a very severe context. The results are clearly convincing     

A decision-feedback equalizer with pattern-dependent feedback formagnetic recording channels

A new nonlinear equalizer for high-density magnetic recording channels is presented. It has a structure of the decision-feedback equalizer (DFE) with a nonlinear model at the feedback section and a dynamic threshold detector. The feedback nonlinear model is a sequence of look-up tables (LUTs) indexed by time, and each table is addressed by a transition pattern formed by one future and ν past transitions. We call this new nonlinear equalizer the pattern-dependent DFE (PDFE). The feedback nonlinear model cancels the trailing nonlinear intersymbol interference (ISI), and then the data decision is made by considering the precursor nonlinear ISI caused by one future symbol. We propose a tap optimization criterion SNR_d for the PDFE which in effect tries to maximize the output signal to noise ratio, and derive a closed-form solution for the tap values. We compare the detection performance of PDFE with that of the DFE and the RAM-DFE on experimental channels. The RAM-DFE is a DFE with one large LUT at its feedback section. The results show that the PDFE yields a significant performance improvement over the DFE and the RAM-DFE. Also the PDFE derived in this paper achieves a superior performance compared with the PDFE derived by the minimum mean-square-error criterion     

基于分层空时结构的多码CDMA系统中一种新的迭代均衡接收算法的研究

为了提高基于垂直结构的分层空时结构(V-BLAST, vertical bell-laboratories layered space-time)的多码 CDMA 系统的抗干扰能力与系统容量,文章提出了一种将 MMSE 均衡与 Turbo 译码联合运算进行的方法,得到了一种针对该系统的迭代均衡接收算法。该接收算法包含两个连续的软输入软输出模块,首先经过 MMSE 均衡后得到第一个软输出,然后从 Turbo 译码器得到第二个软输出。每一次迭代过程中,从均衡和译码中得到的外赋信息作为第二次迭代的先验信息。仿真结果表明,文章提出的迭代均衡接收算法相比较传统非迭代接收算法,性能有非常可观的改善。     

Mitigating error propagation in decision-feedback equalization for multiuser CDMA

This letter presents a robust decision-feedback equalization design that mitigates the error-propagation problem for multiuser direct-sequence code-division multiple-access systems under multipath fading. Explicit constraints for signal energy preservation are imposed on the filter weight vector to monitor and maintain the quality of the hard decisions in the nonlinear feedback loop. Such a measure protects the desired signal power against the detrimental effect of erroneous past decisions, thus providing the leverage to curb error propagation.     

Interference suppression for space-time coded CDMA via decision-feedback equalization

A single-user receiver structure is proposed for space-time coded code-division multiple-access (CDMA) downlink in a multiuser frequency-selective channel. This structure is a two-dimensional (2-D) decision-feedback equalizer (2D-DFE) whose filters are optimized based on the MMSE criterion to mitigate noise, intersymbol interference (ISI), and multiuser interference (MUI) with a moderate complexity. By modeling the spreading codes of the interfering users as random sequences, system performance was evaluated using the Gaussian approximation. Two models for the desired user's spreading sequence have been considered and compared. Our numerical results show that in both cases the 2D-DFE exhibits significant performance improvement over the standard space-time coded RAKE, especially in interference-limited conditions. It is also observed that the gain obtained by using DFE in a MISO channel is less that in a SISO channel and this problem can be solved by providing diversity at the receiver.     

Capacity estimation for a multicode CDMA system with SIR-basedpower control

Capacity estimation in code division multiple access (CDMA) systems is an important issue which is closely related to power control. Strength-based power control has been assumed in most analyses in which other cell interference was considered as a known and fixed variable. However, in signal-to-interference ratio (SIR)-based power-control systems, power control and other cell interference are closely related to each other and capacity can be obtained by considering this relationship. This study derives the reverse-link capacity of an SIR-based power-controlled multicode CDMA system supporting heterogeneous CBR and on-off traffic in a multiple cell environment. Mean and variance statistics of total other cell interference, and the effects of traffic and propagation parameters on system capacity are investigated     

Medium access protocol design for delay-guaranteed multicode CDMA multimedia networks

To enable multimedia real-time applications over next-generation wireless code-division multiple access (CDMA) packet-switching networks, previous efforts show a proper scheduling policy is the key to provide delay-guaranteed access services to various traffic types with different bit error rate (BER) requirements. Considering the support of the prevailing Internet protocol (IP) with variable-length packets in future mobile networks, we develop a mathematically delay-optimal medium access control (MAC) protocol over multicode CDMA (MC-CDMA) environments under the continuous-time assumption. From our investigations, we suggest that a good MAC protocol should be designed by using a proper single-server scheduling policy to guarantee packet-delay, and controlling the maximal number of simultaneous spreading-code transmissions to maintain the required BER. We further evaluate the performance of some MC-CDMA MAC protocols supporting QoS on BER and packet-delay, and show that MAC schemes conforming to our design rules give better performance on packet-delay when maintaining acceptable BER of various traffic types.     

Sparse chip equalizer for DS-CDMA downlink receivers

We present a low complexity sparse chip equalization scheme for DS-CDMA downlink receivers in which only a small number of filter taps are selected to be non-zero, thereby reducing the computational complexity significantly. Low complexity algorithms for selecting the timings and complex amplitudes of the sparse equalizer taps are proposed. The excellent performance achieved by this scheme combined with its low complexity makes it an attractive choice for the DS-CDMA based cellular downlink receivers.     

Traffic management in a multicode CDMA system supporting softhandoffs

A traffic management scheme is proposed in a multicode code-division multiple-access system supporting soft handoff that uses guard channels and a queue for real-time traffic. Preemptive queue control gives priority to queued handoff calls. Handoff traffic is derived as a function of the new call arrival rate, the size of the soft handoff region, mobile speed, the new call blocking probability, and the handoff failure probability. System performance with K types of calls is analyzed by introducing a concept of effective channel. The effects of the number of guard channels, the number of effective channels, system capacity, and other factors are numerically investigated. The effectiveness of the proposed queue control scheme is also observed in terms of handoff processing delay     

Channel acquisition for wideband CDMA signals

The scenario considered is one where a single new user is to be acquired on the reverse link by the base station, and where the channel parameters of the interfering users are known. Following a minimum mean squared error (MMSE) strategy for suppressing the multiaccess interference, the parameter estimation problem is posed in a maximum likelihood framework, To reduce complexity, the solution is implemented in two stages: first, the estimated tap delays are restricted to be at chip spacings; second, the number of taps is reduced by allowing for arbitrary spacing between them. The performance of the proposed techniques is studied through numerical simulations. It is shown that significant gains can be obtained by exploiting the structure of the interference and acquiring the channel parameters jointly     

Spreading codes for direct sequence CDMA and wideband CDMA cellularnetworks

This article presents an overview of the spreading techniques for use in direct sequence CDMA cellular networks. We review the theoretical background for sequences used in CDMA and wideband CDMA, and discuss the main characteristics of the maximal length, Gold (1967, 1968), and Kasami sequences, as well as variable- and fixed-length orthogonal codes. We also describe different methods of multiple spreading for channelization and scrambling in CDMA and W-CDMA realizations     

Capacities of spectrally overlaid single-code and multicode CDMA systems

This paper is concerned with the capacities of spectrally overlaid narrowband and wideband systems using two code-division multiple-access (CDMA) techniques - single-code and multicode. The capacity is defined here as the maximum achievable total throughput in the narrowband and wideband subsystems. User signal power in the overlay systems affects the performance of the systems. The signal power is characterized as the relative power level of wideband and narrowband users according to the spreading bandwidth, spectral overlay ratio, and the number of spreading codes assigned to a user in single-cell and multiple-cell environments. The capacities of the overlay systems are then analyzed. The results of the analysis show that the single-code technique yields better performance than the multicode scheme in terms of the capacities of the overlay systems. Furthermore, the use of a multicode technique for wideband subsystems and a single-code scheme for narrowband subsystems is desirable for enhancing the performance of CDMA overlay systems.