Wern-Ho Sheen

Effects of multipath fading on delay-locked loops for spread spectrum systems

A renewal process approach is used to analyze noncoherent delay-locked loops (DLLs) for spread spectrum systems operating over frequency selective and frequency nonselective slow fading channels. The effects of multipath fading are evaluated in terms of the mean-time-to-lose-lock (MTLL) and the root-mean-square (RMS) tracking error. For channels that have a specular component, the following results are observed: (i) the effect of multipath fading is more significant for tracking loops that have large loop signal-to-noise ratios (SNRs); (ii) a smaller early-late discriminator offset /spl Delta/, which is bounded by 0 0.5) may result in a higher MTLL, especially if there is a strong multipath effect. From (ii) and (iii), instead of using the popular choice of /spl Delta/=0.5, a larger /spl Delta/ may be chosen for a DLL that works at low SNRs and/or strong multipath fading environments, where MTLL is an important consideration. >

A noncoherent tracking loop with diversity and multipath interference cancellation for direct-sequence spread-spectrum systems

A noncoherent tracking loop with diversity reception and multipath interference cancellation is proposed for direct-sequence spread-spectrum systems on frequency-selective multipath fading channels. The basic idea of the loop is to reproduce the multipath interference and remove it from the received signal, and then a path diversity is embedded in the loop to improve the loop performance. The tracking error performance of the loop is evaluated by linear analyses and computer simulations. Numerical results show that the proposed tracking loop outperforms the traditional delay locked loop and performs very well under all considered fading conditions.

A new tracking loop for direct sequence spread spectrum systems on frequency-selective fading channels

A new tracking loop is proposed for direct-sequence spread-spectrum signaling on a frequency-selective fading channel. By exploiting the inherent multipath diversity of the channel, the new tracking loop overwhelmingly outperforms a traditional noncoherent delay-locked loop (DLL) for all cases under consideration. Linear and nonlinear (based on the renewal process approach) methods are employed to analyze the new tracking loop with perfect channel estimation. Although the analytical methods are developed under the guise of a slowly time-varying channel, the analytical and simulation results show close agreement for channels with a code Doppler shift /spl beta//sub D/ as large as 0.117598 (Doppler shift f/sub D/=83 Hz). Consequently, the analytical methods, especially those based on linear methods, can be easily employed to characterize the new tracking loop. The performance degradation caused by imperfect channel estimation is determined by computer simulations. Over a range of signal-to-noise-ratios (SNRs) of practical interest, the simulation results show a degradation of about 2 dB and 1 dB for /spl beta//sub D/=0.0117598 (f/sub D/=8.3 Hz) and /spl beta//sub D/=0.117598 (f/sub D/=83 Hz), respectively.

Deployment and radio resource reuse in IEEE 802.16j multi-hop relay network in Manhattan-like environment

IEEE 802.16j is an amendment to the IEEE 802.16 broadband wireless access standard to enable the operation of multi-hop relay stations (RS). It aims to enhance the coverage, per user throughput and system capacity of IEEE 802.16e. Compared with base station (BS), RS does not need a wire-line backhaul and has much lower hardware complexity. Using RSs can significantly reduce the deployment cost of the system. In this paper, the RS deployment and radio-resource reuse strategies for IEEE 802.16j multi-hop relay (MR) networks are investigated. Simulation results show that the per-user throughput and system capacity of IEEE 802.16j MR network can be substantially improved by the proposed method.

Resource scheduling with directional antennas for multi-hop relay networks in Manhattan-like environment

Multi-hop relay (MR) networks have been proposed for user throughput improvement, coverage extension and/or capacity enhancement to the traditional mobile cellular networks. In particular, an MR network is being standardized by the IEEE 802.16J Task Group as an amendment to the IEEE 802.16e standard. This paper investigates the issue of resource scheduling for the IEEE 802.16J MR network in the Manhattan-like environment. New scheduling methods are proposed for the MR network with directional antennas equipped at both the base station and relay stations. Simulation results show that the system throughput can be dramatically increased by the proposed methods as compared to the system with omnidirectional antennas

Joint Least Squares Estimation of Frequency, DC Offset, I-Q Imbalance, and Channel in MIMO Receivers

Multiple-input-multiple-output (MIMO) receivers with direct-conversion radio-frequency (RF) architecture are investigated in this paper. Direct conversion is a low-cost RF design that requires fewer external components in chip implementation. Nevertheless, it introduces extra RF impairments such as I-Q imbalance and dc offset in addition to frequency offset that is commonly encountered in all RF architectures. This paper proposes to do the joint least squares (LS) estimation of frequency, dc offset, I-Q imbalance, and a channel in MIMO receivers to improve performance; frequency-dependent and frequency-independent I-Q imbalances are included. Previously, RF impairments were separately estimated in MIMO receivers, which leads to inferior performance. In particular, a receiver architecture that facilitates the joint estimation of the frequency, the dc offset, the I-Q imbalance, and the channel is proposed. The LS criterion is then applied to obtain the joint estimators, with a special training-sequence design to reduce complexity. Simplified estimators on the frequency and dc offset are also proposed with almost no loss in performance. Finally, the estimators are shown through analysis to be unbiased and approach the Cramer-Rao lower bound (CRLB) for signal-to-noise ratios (SNRs) of interest. The analysis is verified by computer simulations.

Downlink Optimization and Performance of Relay-Assisted Cellular Networks in Multicell Environments

Deploying fixed relay stations (RSs) in traditional mobile cellular networks is widely recognized as a promising technology to improve cell coverage, user throughput, and system capacity, to save the transmit power of a mobile station (MS) in the uplink, and to provide a low-cost deployment for coverage extension. One crucial step toward developing such a relay-assisted cellular network is to fully evaluate its performance from both theoretical and practical viewpoints. In the literature, however, the system has only been evaluated for very limited system configurations: with a fixed number of RSs and locations and/or a fixed frequency-reuse pattern. This paper aims to investigate the downlink performance limits of a general relay-assisted network with optimized system parameters in a multicell environment. A genetic algorithm (GA) is proposed for the joint optimization of system parameters, including the number of RSs and their locations, frequency-reuse pattern, path selection, and resource allocation to maximize the system spectral efficiency (SE). Two types of quality of end-user experience (QoE), i.e., 1) fixed-bandwidth allocation (FBA) and 2) fixed-throughput allocation (FTA), are investigated along with two path-selection methods, i.e., 1) SE based and 2) signal-to-interference-plus-noise ratio (SINR) based. Numerical results show that significant improvement on system performance can be achieved with the optimized system parameters.

Variable dwell-time code acquisition for direct-sequence spread-spectrum systems on time-variant Rayleigh fading channels

Variable dwell-time code acquisition based on multiple-dwell or sequential linear tests is investigated for direct-sequence spread-spectrum systems on time-variant Rayleigh fading channels. Unlike in the conventional additive white Gaussian noise channels, the channel memory incurred by fading renders the exact analysis of the acquisition systems extremely difficult, if not impossible. In this paper, a novel method is developed to evaluate the mean acquisition time of the acquisition systems very accurately. The effects of Rayleigh fading are evaluated, and comparisons are made between double-dwell and sequential linear tests. Numerical results show that Rayleigh fading may result in 1-4-dB loss in performance, and the sequential linear test can outperform double-dwell test by a margin of 1-2 dB. The analytical results are verified by computer simulations.

Effects of cell correlations in a matched-filter PN code acquisition for direct-sequence spread-spectrum systems

Rapid pseudonoise (PN) code acquisition with matched-filter correlators has been very popular in direct-sequence (DS) spread-spectrum systems. Conventionally, the analysis of this acquisition method is based on the assumption that the detections among cells are independent. However, there may be strong correlations among cell detections for the case that the cell size is less than a chip duration. In this paper, the mean acquisition time performance of the acquisition method is analyzed with the cell correlations being taken into account. Numerical results show that depending on the threshold value and other system parameters, the effect of cell correlations may be over 20% of the mean acquisition time for signal-to-noise ratios (SNR) of practical interest. The analytical results are substantiated by computer simulations.

Joint Calibration of Transmitter and Receiver Impairments in Direct-Conversion Radio Architecture

Direct-conversion radio architecture is a low-cost, low-power and small-size design that has been widely employed in todays wireless devices. This architecture, however, induces radio impairments such as I-Q imbalance and dc offset that may incur severe degradation in communication performance if left uncompensated. In this paper, a new method is proposed to calibrate simultaneously a transceivers own transmitter and receiver radio impairments with no dedicated analog circuit in the feedback loop. Based on a unified time-domain approach, the proposed method is able to calibrate jointly the frequency-independent I-Q imbalance, frequency-dependent I-Q imbalance and dc offset and is applicable to any type of communication systems (single-carrier, multiple-carrier, etc.). The existing methods in the literature either need a dedicated analog circuit in the feedback loop and/or are applicable only to a particular type of systems with some radio impairments present. The issue of training sequence design is also investigated to optimize the calibration performance, and analytical and simulation results show that the performance loss due to radio impairments can be recovered by the proposed method.