DS-CDMA RAKE receiver with time-window control loop (TWCL) in multipath fading environment

This paper presents laboratory test measurement results from a post-detection RAKE receiver system with a time-window control loop (TWCL) as the code synchronization scheme under a frequency-selective fading environment. RAKE combining methods, a simple integral demodulation method within a time-window and a recursive least squares (RLS) demodulation method within the time-window that uses the RLS algorithm to realize maximal ratio combining, are investigated. The result shows that the TWCL provides good acquisition and tracking performance under the fading conditions experienced with maximum Doppler frequencies from 5 to 80 Hz. It is also shown that the required E/sub b//N/sub 0/ when the bit-error rate must be 10/sup -3/ under the double-spike frequency-selective Rayleigh-fading model is reduced by 10 dB compared to the non-selective Rayleigh-fading model. The difference of the path-diversity effect between the two RAKE combining methods is only 1 dB.

It has been reported that a smart antenna can improve bit error rate (BER) performance significantly in a mobile communications system (Tanaka, S. et al., IEEE JSAC, vol.18, no.8, p.1495-504, 2000). However, the smart antenna has not been applied and analyzed much for the pseudonoise (PN) code tracking loop, which is one of the important aspects in spread spectrum communications receivers. We present a performance analysis of a first order PN-code tracking loop with a smart antenna under a frequency selective fading environment, and show that the smart antenna can also improve the PN code tracking loop significantly, compared to a single antenna element system. We employ the root mean square (RMS) error and mean time to lose lock (MTLL) of the PN code tracking loop as performance criterion.

The transmission quality of mobile radio communications is strongly affected by fading with rapid fluctuations in the envelope and phase of the received signal, in addition to the effect of thermal noise. This paper considers a multiple access system (DS‐CDMA system) using the spread spectrum technique with direct spread, and presents a method of deriving the approximate average bit error rate in a frequency‐selective fading environment (the n‐ray Rayleigh fading model), with the maximum Doppler frequency, the delay time of the delayed wave, and the number of stations simultaneously utilizing the channel as parameters. The two‐wave model and the IMT2000 model are considered as examples, and the validity of the proposed method is demonstrated by the fact that the results obtained by the approximate formula agree well with the simulation results even if the parameters are varied. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 87(1): 90–100, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1185

MC-CDMA, which is based on OFDM and CDMA, is a major candidate for systems beyond IMT-2000. A time-domain spreading MC-CDMA system that has consecutive pilot symbols can achieve high quality channel estimation in fast fading environments. Furthermore, it is known that time-domain spreading MC-CDMA is suited to the use of AMC (adaptive modulation and channel coding). MC-CDMA based on OFDM generally has a guard interval (GI), which is a cyclic prefix to avoid the intersymbol interference (ISI) and interchannel interference (ICI) caused by delay paths. The receiving characteristics of MC-CDMA are significantly degraded by ISI and ICI when there are delayed paths that exceed the GI. To accurately estimate the transmission characteristics, we need an accurate delay profile model that well reflects actual delay profiles. The existing model is, however, very complicated, so it cannot be used in computer simulations. We propose an extremely simple equivalent path model for time-domain code spreading MC-CDMA; it consists of just two paths but its transmission characteristics are reasonable if the delay profile has paths that exceed GI. Computer simulation results validate the proposed model. Moreover, we consider a parameter to characterize the model with which we transform an arbitrary path model into the equivalent path model.

Recently, space-time block coded OFDM (ST-OFDM), that applies space-time block codes (STBC) to OFDM, has been proposed. Space-frequency block coded OFDM (SF-OFDM) has also been proposed, where the block codes are formed over the space and frequency domain. ST-OFDM and SF-OFDM achieve good performance over multipath fading environments and fast fading environments, respectively. For systems with two transmit antennas, orthogonal conditions are required to separate the received signals: in ST-OFDM, the frequency responses of two consecutive OFDM symbols are almost identical; in SF-OFDM, the frequency responses of two adjacent subcarriers are almost identical. In practical fading environments, however, these orthogonality conditions sometimes cannot be satisfied. In those environments, the received signals cannot be well separated and the performances are degraded. Recently, a new zero-forcing (ZF) diagonalized maximum likelihood decoder (DMLD) was proposed for the space-time block coded single carrier QPSK system to maintain the orthogonality of STBC under fast fading environments and flat fading environments; the channel separation in DMLD is performed by the ZF algorithm using two receive signals at time index 2n, 2n+1 (STC) or two subcarriers (SFC). The matrix generated after the channel separation is not an identity matrix, but a matrix proportional to an identity matrix. We show that ST/SF-OFDM with DMLD outperform ST/SF-OFDM in terms of bit error rate (BER).

In today's DS-CDMA systems, Hadamard-Walsh (HW) codes are employed as spreading codes in the downlink to maintain the orthogonality between users. However, after transmission through multipath fading channels, interference arises between different paths, leading to MPI (multi-path interference) in the DS-CDMA RAKE receiver. This, in turn, degrades the DS-CDMA system's probability of error performance. We propose a set of novel complex spreading codes called carrier interferometry (CI) codes, and we demonstrate how these novel orthogonal spreading codes achieve crosscorrelations independent of the phase offsets between different paths (after transmission over a multipath fading channel). This improved crosscorrelation property (relative to HW codes) leads to higher SIR in the DS-CDMA RAKE receiver, and, as a direct result, better performance in terms of probability of error.

This paper presents a theoretical analysis of subband adaptive array combining the cyclic prefix transmission scheme (SBAA-CP) in a multipath fading environment. The exact expressions for optimal weights, array outputs and the output signal to interference plus noise ratio (SINR) are derived. The analysis shows that use of the cyclic prefix data transmission scheme can significantly improve the performance of subband adaptive array (SBAA). We also show that if the output SINR is plotted versus the delay of the delay path then the output SINR of SBAA-CP is in effect the SINR of SBAA shifted by C samples to the right.

A improved algorithm to perform channel estimation and equalization for multiple input multiple output(MIMO) systems in frequency and time-selective fading environments is put forward. lt contains two steps: training and tracking. In training phase, improved Kalman filtering, namely robust Kalman filtering(RKF), is exploited to estimate channel impulse response(CIR). After that, in tracking stage,the RKF and minimum mean-square error feedback decision equalizer (MMSE-DFE) cooperate to track the time-varying channel. The RKF recursions is presented and a closed-form solution for baud rate MIMO MMSE-DFE under perfect knowledge of CIR and correct past decisions conditions is derived. In addition, it regards unknown dc-offset due to zero intermediate frequency(IF) at the receiver as the mean of measurement noise, which is estimated as a byproduct through robust Kalman filters. Finally, it is compared with well-known ones,such as least mean-square (LMS), recursive least square (RLS), Kalman filtering (KF). All these show that the proposal exhibits better performance.

This paper presents a time-varying stochastic technique to assess conservation voltage reduction (CVR) effects based on load modeling. A time-varying exponential load model is developed to represent voltage dependences of loads. The recursive least square (RLS) method is applied to identify model parameters in a recursive way. CVR factors can be calculated using the identified model parameters. The time-varying stochastic model for CVR effects can then be constructed by the Kolmogorov-Smirnov (K-S) test. The proposed CVR assessment method is applied to one-year measurement data from a utility company. The calculated CVR factors are verified by a Euclidian distance-based comparison method. Stochastic models of CVR effects in each time window are constructed. Compared with previous efforts on assessing CVR effects, the proposed method does not require control groups or assumptions of linear relationships between the load and its impact factors. The probabilistic nature of CVR effects is also fully considered.

With the increasing number of mobile-phone standards there is a demand for a software-based radio for use in base-stations that can adapt to many standards. This paper discusses how to implement a RAKE receiver in a high performance standard DSP that can search out the multiple paths and combine them to demodulate the received data. To perform a complete search for a mobile within a cell there will typically be 8000 potential radio delay paths of which only a small number (3-6) will actually contain any useful data. However as each path contains 5120 chips of data the correlation exercise necessary to find these paths is a very MIPS intensive task. The paper highlights a number of techniques based on rough data on the mobiles whereabouts from the initial random link and information on the relationship between the samples to perform an intelligent simplified search that will still find the paths. This allows a much smaller subset to be searched initially of 512 delay paths and 1536 chips making the task simple enough for a DSP based RAKE receiver implemented on a single TMS320C62xx with a 100 MHz-search engine and receive data using a 6-finger RAKE receiver at 58 MHz.

In this article, a computationally-efficient approach is presented for Bit Error Rate (BER) calculations in multi-user Multiple Input Multiple Output (MIMO) frequency selective fading environments, operating in spatial multiplexing transmission mode. To this end, theoretical expressions for the equivalent Signal to Interference plus Noise Ratio (SINR) per active user and transmission mode, as well as mean BER, are derived. The key idea is that all parameters related to BER calculations can be expressed as sums of identically distributed random variables (RVs). Hence, computational burden can be reduced, since summation formulas take into account the parameters of a standalone RV along with the correlation of an arbitrary pair of RVs. As results indicate, the proposed approach can accurately estimate mean BER in multiuser MIMO orientations with increased reception diversity order, for arbitrary number of transmit/receive antennas, a modulation scheme, and a number of resolvable multi-path components.

We characterize the accuracy of analyzing the performance of a non-orthogonal multiple access (NOMA) system where users are ranked according to their distances instead of instantaneous channel gains, i.e., product of their distance-based path-loss and fading channel gains. Distance-based ranking of users is analytically tractable and can lead to important insights. However, it may not be appropriate in a multipath fading environment where a near user suffers from severe fading while a far user experiences weak fading. Since the ranking of users (and in turn interferers) in an NOMA system has a direct impact on coverage probability analysis, the impact of the traditional distance-based ranking, as opposed to instantaneous signal power-based ranking, needs to be understood. This will enable us to identify scenarios where distance-based ranking, which is easier to implement compared with instantaneous signal power-based ranking, is acceptable for the system performance analysis. To this end, in this paper, we derive the probability of the event when distance-based ranking yields the same results as instantaneous signal power-based ranking, which is referred to as the accuracy probability . We characterize the probability of accuracy considering Nakagami- $m$ fading channels and three different spatial distribution models of user locations in NOMA, namely, the Poisson point process (PPP), the Matern cluster process (MCP), and the Thomas cluster process (TCP). For all these models of users’ locations, we assume that the spatial locations of the base stations (BSs) follow a homogeneous PPP. We show that the accuracy probability decreases with the increasing number of users and increases with the path-loss exponent. In addition, through examples, we illustrate the impact of accuracy probability on uplink and downlink coverage probabilities. Closed-form expressions are presented for the Rayleigh fading environment. The effects of fading severity and users’ pairing on the accuracy probability are also investigated.

Most proposed space-time codec schemes focus on the single-user scenario, which cannot be applied directly in mobile communication systems. Hence, the paper focuses on a multiuser system with multi-transmit-multi-receive antennas in the frequency-selective fading environment. A vertical Bell Labs layered space-time (V-BLAST) coded direct-sequence code-division multiple-access (DS-CDMA) scheme and its blind channel identification algorithm are proposed. By exploiting an ESPRIT-like method and singular value decomposition, the channels between every transmit antenna of every user and every receive antenna of the base station can be estimated with a closed-form solution. The channel identification algorithm includes two steps and reduces the computational complexity dramatically.

Integrating monitor alarms with laboratory test results to enhance patient deterioration prediction

In this paper we developed a framework for resource allocation in multi-class traffic CDMA system using adaptive power control and antenna array multiuser receiver. We devised a new scheme in which both transmit power and receiver filter adapt to the time-varying channel state. An antenna array multiuser receiver that jointly processes received signal vectors from multiple antennas and maximizes the signal-to-interference ratio for each user is proposed. It is combined with adaptive power control based on target SIR for each user. Our scheme can support multi-class traffic with different SIR requirements and guarantee their quality of service in multipath fading environment. A theoretical bound is derived to characterize capacity of CDMA system using adaptive power control and antenna array multiuser receiver in multipath fading environment. Simulations show that actual system capacity in multipath finding environment is close to the theoretical bound at large power constraint. Capacity at various power constraints can be characterized by a notion of bandwidth utility. It is also demonstrated that the antenna array multiuser receiver is quite effective in increasing capacity of users with higher SIR requirement and lower power constraint.