Uplink carrier frequency offset estimation for WiMAX OFDMA‐based ranging

Ranging is one of the most important processes in the mobile worldwide interoperability for microwave access (WiMAX) standard, for resolving the uplink synchronization and near/far problems. In this paper, we focus on the multi‐user carrier frequency offset (CFO) estimation in both initial ranging and periodic ranging. After the analysis of some existing ranging methods, we propose two algorithms based on the correlation properties of pseudo noise (PN) sequences in time domain and frequency domain respectively. The root mean square error (RMSE) performance is evaluated in both additive white Gaussian noise (AWGN) channel and multi‐path fading channel. Simulation results show that the proposed frequency‐domain cross‐correlation method performs better than the proposed time‐domain cross‐correlation method, and is more robust to multi‐user interference and residual timing offset. Copyright © 2009 John Wiley & Sons, Ltd.     

Co‐channel interference cancelation at the user terminal in multibeam satellite systems

We study the applicability of soft interference cancelation in the forward link of multibeam satellite systems with focus on mobile terminals. We adopt a standard currently used in commercial satellite systems as a reference. The multibeam satellite antenna radiation diagram has been generated using a physical optics reflector model while a widely adopted channel model has been used for the land mobile satellite channel. The interference pattern has been derived using a system simulator developed by the European Space Agency. Starting from the analysis of the interference pattern, we study the application of a low‐complexity soft interference cancelation scheme for commercial applications. Our results show that, under realistic conditions, a two‐colors frequency reuse scheme can be employed while guaranteeing service availability across the coverage and keeping the complexity at the user terminals relatively low. Copyright © 2015 John Wiley & Sons, Ltd.     

Cramer‐Rao lower bound and parameter estimation for multibeam satellite links

Data throughput and availability of multibeam satellite links are limited by interference problems, in particular when the design is determined by an aggressive reuse of frequency bands. Usually, this is mitigated by appropriately selected techniques like precoding or multi‐user detection. Before such methods are applicable, however, the most important transmission parameters have to be recovered successfully even under very challenging conditions caused by (full) frequency reuse. The detailed analysis of the log‐likelihood function characterizing the multibeam scenario shows that a beamwise decoupling of the estimation task is possible, if the following three conditions are met: (i) alignment of all frequency offsets; (ii) symbol synchronicity; (iii) orthogonal synchronization sequences. Based on these assumptions, it turns out that the modified Cramer‐Rao lower bound (MCRLB) for carrier frequency and phase, symbol timing and signal amplitude is the same, no matter whether we are dealing with a single or a multiple beam situation. Furthermore, because a maximum likelihood framework for parameter estimation is not available in closed form, we introduce a sub‐optimal concept of low‐complex algorithms. Finally, it is verified by simulation results that the jitter variance of each recovery scheme is close to the related MCRLB, when the decoupling conditions are satisfied. Copyright © 2016 John Wiley & Sons, Ltd.     

Analysis of multi‐user detection of multi‐rate transmissions in multi‐cellular CDMA

In this paper, we address the issue of multi‐user receiver design in realistic multi‐cellular and multi‐rate CDMA systems based on performance analysis. We consider the multi‐user detection (MUD) technique, denoted interference subspace rejection (ISR), because it offers a wide range of canonic suppression modes that range in performance and complexity between interference cancellers and linear receivers. To further broaden our study, we propose a modified ISR scheme called hybrid ISR to cope better with multi‐rate transmissions. The performance analysis, which is based on the Gaussian assumption (GA) and validated by simulations, takes into account data estimation errors, carrier frequency mismatch, imperfect power control, identification errors of time‐varying multipath Rayleigh channels and intercell interference. This analysis enables us to optimize the selection of the MUD mode for multi‐rate transmissions in different operating conditions. The effectiveness of interference cancellation is indeed investigated under different mobile speeds, numbers of receiving antennas, near‐far situations, channel estimation errors, and out‐cell to in‐cell interference ratios. This investigation suggests that the out‐of‐cell interference, the residual in‐cell interference, the noise enhancement as well as low mobility favor the simplest MUD modes as they offer the best performance/complexity tradeoffs. Copyright © 2008 John Wiley & Sons, Ltd.     

Adaptive array processing MLSE receivers for TDMA digitalcellular/PCS communications

Array processing is a promising approach for improving quality, coverage, and capacity in digital cellular communication systems. By combining array processing with maximum likelihood sequence estimation (MLSE), intersymbol interference (ISI) introduced by multipath propagation can be mitigated as well. Novel symbol-spaced and fractionally spaced adaptive array processing MLSE receivers are developed for both diversity and phased array antenna configurations. The practical issues of synchronization and channel estimation are addressed. A novel approach to automatic frequency error correction (AFC) is proposed and is shown to be critical when cancelling cochannel interference. Performance is evaluated for the reverse link of the IS-136 TDMA-based digital cellular system. Substantial improvements are obtained over conventional antenna configurations for receiver sensitivity (2.5-4 dB) and over traditional antenna combining when cochannel interference is present (0.5-25 dB)     

On the performance of a rapid synchronization algorithm for IR‐UWB receivers

Because of the very low signal duty cycles, synchronization is the most critical issue in ultra wideband (UWB) impulse radio (IR) systems. Some effective synchronization schemes like a symbol‐differential (SD) IR‐UWB receiver have been proposed to synchronize received signals rapidly. Yet, SD IR‐UWB receiver is unsuitable for operation in multi‐user environment because of multiple access interference (MAI). By taking advantage of frame‐differential IR‐UWB receivers, we propose a parallel frame‐differential (PFD) IR‐UWB receiver to do so. Our proposed PFD IR‐UWB receiver manifests better immunity against message passing interface and MAI than the SD IR‐UWB. Based on this PFD IR‐UWB receiver, uncertain (search) regions are limited to one frame duration without any symbol‐level synchronization process. Performance of PFD and SD receivers are compared by computer simulations, showing that the proposed PFD receiver not only achieves significant bit error rate performance but also better and more robust results than the SD receiver in this literature. Copyright © 2012 John Wiley & Sons, Ltd.     

Exploiting code division multiplexing with decentralized multiuser detection in the satellite multibeam forward link

In recent years, the need for increased satellite throughput has been tackled by extending the number of satellite beams thus allowing a higher spectrum reuse and peak throughput. However, the satellite antenna size and the number of simultaneous beams that can be generated onboard over a given coverage region cannot grow beyond certain limits because of payload accommodation constraints. The next step being pursued to increase the system throughput resides in extending the frequency reuse among the loaded beams and to mitigate the effects of the increased co‐channel interference through more advanced digital signal processing. This can be achieved in 2 different ways. The first one, which received a large attention in recent years, is to centrally mitigate the multibeam channel cross talk by exploiting precoding techniques at the gateway. The second approach, less investigated in the past, is to put in place decentralized multiuser detection (MUD) at the user terminal side. The precoding approach has the advantage of concentrating the extra processing complexity at the gateway, but it requires nonstandard payloads or accurate payload calibration techniques and periodic channel estimation reporting from the user terminal. Instead, the decentralized approach can operate in combination with existing payloads and does not require any terminal's periodic channel estimate reporting to the gateway. Only the signal‐to‐noise plus interference variations due to possible fading as for conventional adaptive coding and modulation shall be reported. One of the main barriers to the decentralized MUD approach so far was the demodulator complexity. Some simplified approach for conventional frequency/time division multiplexing schemes has been recently published, and its applicability to the forward link investigated. In this paper, we investigate the possible advantages deriving from the adoption of direct‐sequence code division multiplexing associated with affordable complexity of the MUD at the user terminal side. It is shown that the proposed MUD scheme can be practically implemented and provides sizeable advantages compared to current state‐of‐the‐art when the traffic is not evenly distributed among the beams, ie, when a subset of beams has a higher load than the others.     

Adaptive Radio Resource Allocation for a Mobile Packet Service in Multibeam Satellite Systems

In this paper, we introduce an adaptive radio resource allocation for IP‐based mobile satellite services. We also present a synchronous multibeam CDMA satellite system using an orthogonal resource sharing mechanism among downlink beams for the adaptive packet transmission. The simulation results, using a Ka‐band mobile satellite channel and various packet scheduling schemes, show that the proposed system and resource allocation scheme improves the beam throughput by more than two times over conventional systems. The simulation results also show that, in multibeam satellite systems, a system‐level adaptation to a user's channel and interference conditions according to user locations and current packet traffic is more efficient in terms of throughput improvement than a user‐level adaptation.     

Interference evaluations and simulations for multisatellite multibeam systems

Third generation communication systems will be characterized by full integration between terrestrial and satellite components. To this aim, global coverage along with not severe requirements for user terminals are mandatory for the satellite segment and the use of constellations of satellites in low or medium Earth orbits (LEO or MEO) seems to be a viable solution. Those satellite systems will adopt multibeam antennas to achieve high spectral efficiency and low‐cost terminals. Thus, interference becomes one of the most limiting factors in terms of both link availability and capacity. The paper presents a more complete interference model than previously published in literature. The identification of the interfering users set has been introduced and all the factors impacting interference generation or isolation have been considered in case of both FDMA and CDMA access. This model is suitable for instantaneous analysis of multisatellite constellations. It has been implemented and time‐domain simulations have been performed to evaluate the impairments due to co‐channel interference for different access techniques for LEO configuration. Simulation results on interference and users spatial distribution, on trade‐offs between interference and system capacity will be presented. Finally, the main interference mitigation techniques will be listed and discussed. Copyright © 2002 John Wiley & Sons, Ltd.     

Soft‐output MMSE V‐BLAST receiver with MMSE channel estimation under correlated Rician fading MIMO channels

For wireless multiple‐input multiple‐output (MIMO) communications systems, both channel estimation error and spatial channel correlation should be considered when designing an effective signal detection system. In this paper, we propose a new soft‐output MMSE based Vertical Bell Laboratories Layered Space‐Time (V‐BLAST) receiver for spatially‐correlated Rician fading MIMO channels. In this novel receiver, not only the channel estimation errors and channel correlation but also the residual interference cancellation errors are taken into consideration in the computation of the MMSE filter and the log‐likelihood ratio (LLR) of each coded bit. More importantly, our proposed receiver generalizes all existing soft‐output MMSE V‐BLAST receivers, in the sense that, previously proposed soft‐output MMSE V‐BLAST receivers can be derived as the reduced forms of our receiver when the above three considered factors are partially or fully simplified. Simulation results show that the proposed soft‐output MMSE V‐BLAST receiver outperforms the existing receivers with a considerable gain in terms of bit‐error‐rate (BER) performance. Copyright © 2011 John Wiley & Sons, Ltd.     

Interference analysis and capacity for forward link 3G CDMA network with adaptive antennas

In code division multiple access (CDMA) systems, the capacity of forward link (FL) communication to mobile receivers is limited primarily by co‐channel interference (CCI). Adaptive antenna arrays (AAAs) that use antenna arrays along with advanced signal processing at the base station (BS) have been proposed to mitigate this limitation. For a 3G CDMA cellular network, where each BS equipped with an AAA serves mixture of voice and data users within its coverage, we study FL capacity and investigate the effects of different factors (array topology, multipath angle spread, data rate, and beamforming algorithm) on this capacity under Rayleigh fading channel. By modeling the instantaneous signal‐to‐interference power ratio received at the mobile, we derive the system outage equation that considers blocking of either desired voice or data user. Simulation results show that for the same element spacing and number of antenna elements per cell, the uniform circular array (UCA) topology results in larger capacity than the sectorized uniform linear array (ULA) topology does, and that a larger angle spread or data user rate reduces FL capacity. Copyright © 2007 John Wiley & Sons, Ltd.     

Performance analysis of multiuser selection diversity in SIMO spectrum sharing systems

In this paper, a performance analysis is presented for user selection schemes in a single‐input multiple‐output spectrum sharing system. In the considered system, multiple secondary users try to use the licensed spectrum of a primary user in an opportunistic manner, in which an interference constraint for the primary user is satisfied. In this paper, we first use 2 conventional user selection schemes for single‐input multiple‐output spectrum sharing system and analyze the system performance for each scheme. We then propose a new user selection scheme that can overcome the limitations of those 2 conventional user selection schemes. As for the performance analysis, the average channel capacity, the outage probability, and the bit error rate performances of the system using the presented user selection schemes are analyzed and mathematical closed‐form expressions for the outage probability are derived. The performances of the system are evaluated using the derived mathematical formulas in different cases. In addition, Monte Carlo simulation results are also provided to show the accuracy and correctness of the performed analysis.     

Robust channel estimation and ISI cancellation for OFDM systems with suppressed features

A feature-suppressed orthogonal frequency-division multiplexing (OFDM) system and the corresponding channel estimation and intersymbol interference (ISI) mitigation techniques are investigated in this paper. Cyclic prefix (CP) and pilot tones, which are commonly used in civilian OFDM systems for ISI mitigation and channel estimation, create distinctive waveform features that can be easily used for synchronization and channel estimation purposes by intercepting receivers. As a result, CP and pilot tones are eliminated in the proposed feature suppressed OFDM system to reduce the interception probability. Instead, a set of specially designed OFDM symbols, driven by different pseudorandom sequences, are employed as preambles to avoid unique spectral signature. These preambles are inserted into the OFDM data symbol stream periodically and in a round-robin manner. In addition, a random frequency offset is introduced to each preamble to further mask the multicarrier signature. New challenges arising from these feature suppression efforts are studied, including robust channel estimation and demodulation techniques in the presence of frequency offset and severe interference. Based on our interference analysis, an iterative ISI and intercarrier interference (ICI) estimation-cancellation-based technique is proposed for both channel estimation and OFDM data demodulation. Our channel estimator performs joint frequency offset and channel impulse response estimation based on the maximum-likelihood (ML) principle. To reduce its complexity, we employ a number of techniques, which include approximation of the ML metrics, as well as fast Fourier transform pruning. The performances and feasibility of the proposed feature suppressed OFDM system and the channel estimator are analyzed and verified through numerical simulations.     

A high-speed adaptive antenna array with simultaneous multibeam-forming capability

A new type of adaptive beamforming antenna system architecture is proposed for multichannel wireless communications. Multibeam communication with high data throughput is accomplished using the proposed beamformer architecture. The system consists of analog mixers, a multitone direct digital synthesizer (DDS), and a digital signal processor (DSP) controller. The essential idea of multibeam forming is based on a multitone weighting scheme combined with analog-digital hybrid signal processing. While the real-time multibeam construction is realized by the analog mixer circuits and a DDS, the complicated adaptive beamforming and direction-of-arrival estimation algorithms are carried out by the DSP. In this architecture, only one beamformer circuit is required to handle multiple beams, leading to significant reduction in hardware counts. A 5.8-GHz eight-element adaptive beamforming array successfully demonstrates two-beam simultaneous beamforming with less than three degrees of peak and steering errors and more than 20-dB interference suppression. The test-bed exhibits successful two-channel data recovery at 25-Mb/s data throughput in each channel with binary phase-shift keying modulation, for simultaneous dual-beam reception. The bit-error-rate measurement validates the robustness of the communication quality under strong interferences.     

Partial sampling MMSE interference suppression in asynchronousmulticarrier CDMA system

Linear minimum mean square error (LMMSE) receivers for asynchronous multicarrier code division multiple access (MC-CDMA) system under frequency-selective Rayleigh fading channel is studied. The performance of this LMMSE receiver is evaluated and shown to be superior to that of the other two schemes, equal-gain combining (EQC) and maximum-ratio combining (MRC). However, a perfect timing estimation of the desired user is needed for these receivers, as a misaligned sampling interval of these receivers results in severely self intersymbol interference (ISI) and intercarrier interference (ICI) for the desired signal at the output. In order to remove the timing acquisition requirement of a receiver for an asynchronous MC-CDMA system, we proposed a novel partial sampling MMSE (PS-MMSE) receiver. Numerical result shows that the PS-MMSE receiver without timing knowledge provides significantly stronger interference suppression capability than the LMMSE receiver with known timing. Moreover, a so-called reduced complexity partial sampling MMSE (RPS-MMSE) receiver is proposed to make the number of the receiver's taps independent of the number of subcarriers. Results show that with a proper grouping parameter, a much less complicated RPS-MMSE receiver achieves almost the same performance as the PS-MMSE receiver. Thus, one is able to implement an MMSE receiver without a prior timing requirement to perform multiuser detection for the MC-CDMA system in an asynchronous scenario at the expense of a slight complexity increase     

Iterative space-time soft interference cancellation for UMTS-FDD uplink

Two efficient pilot-aided iterative space-time interference cancellation receivers are studied in order to increase the uplink capacity of the Universal Mobile Telecommunication System (UMTS) in frequency-division duplex (FDD) mode. Both iterative schemes use low-complexity beamforming and path combining techniques associated with soft-input soft-output decoding to mitigate the multiple-access interference in space and time. The difference between the two techniques is in the way they deal with unknown channels: the addition of a space-time channel estimation in each iteration on the one hand and iterative adaptive beamforming and path combining on the other hand. Thanks to the iterative structure, the observation signal used for estimation or adaptation contains less interference from one iteration to the following, and soft estimates of coded bits are available for data-aided estimation or adaptation. A detailed complexity analysis shows that renewing beamforming and path combining in each iteration without a-priori knowledge of the channel has no significant impact on the overall complexity of one iteration. Simulations of true UMTS-FDD uplink communications over a wideband directional channel model reveal that near-single user performance can be obtained for very high system loads, whereas more conventional receivers, such as the interference canceller without beamforming and the two-dimensional RAKE receiver, fail in recovering the transmitted information.     

Link adaptation and SINR errors in practical multicast multibeam satellite systems with linear precoding

The application of linear precoding at the gateway side enables broadband multibeam satellite systems to use more aggressive frequency reuse patterns increasing the overall capacity of future high-throughput satellites (HTS). In any practical precoded system, receivers can estimate only a few coefficients of the channel state information (CSI), while the others, in what is known as nullification, are set to zero. In this paper, the impact of the CSI nullification to the SINR estimation is analyzed statistically and geographically for a multicast multibeam satellite system. The errors in the SINR calculated by the gateway affect to the modulation and coding schemes (MCSs) selection, increasing the rate of erroneous frames or leading to an underutilization of the available resources. Therefore, as countermeasure, a link adaptation algorithm based on an adaptive margin per user is proposed, helping to achieve the error rate constraints of DVB-S2X systems without compromising the throughput, even under severe CSI degradation due to nullification and Rician fading.     

基于循环前缀的OFDM同步算法研究

针对传统基于循环前缀的定时同步算法在多径信道下定时估计精度明显下降的问题,在经典的定时同步算法最大似然估计算法的基础上,研究了一种整体相关的OFDM定时估计算法,最后结合两者提出了一种有效利用截短循环前缀进行定时估计的改进算法,避免了多径环境下ISI带来的定时估计误差。对算法进行了仿真,表明在高斯白噪声信道和多径衰落信道下改进的算法都具有较好的性能。     

Interference Suppression Receivers for the Cellular Downlink Channel

We consider the multi-input multi-output (MIMO) downlink channel in the next-generation cellular networks and propose two improved interference suppression receivers for combating out-of-cell interference. The proposed receivers exploit the fact that the co-channel interference seen on the downlink channel (especially the downlink control channel) has a particular structure, in order to obtain significantly improved performance while ensuring low decoding complexity. The first receiver does not require the user to decode the interference or be aware of the particular inner codes employed by the interfering transmitters. The second receiver decodes and subtracts a subset of interferers in a channel-dependent order before processing the desired signal. Each interferer is decoded at most once and the choice of the ordered subset mitigates error propagation. Simulation results are presented to demonstrate the significant gains obtained by the proposed low-complexity receivers over their conventional counterparts.     

Time‐varying channel estimation for MIMO/OFDM systems using superimposed training and basis expansion models

An approach of superimposed training (ST)‐aided time‐varying (TV) channel estimation for multiple‐input multiple‐output orthogonal frequency division multiplexing systems is presented. By modeling the TV channel with the truncated discrete basis expansion model, a two‐step approach is adopted to estimate the TV channel. In addition, the mean square error (MSE) of the proposed channel estimation is analyzed, and its closed‐form expression is derived, which is a function of the data‐to‐ST power ratio. Using the developed channel MSE, we case the problem of ST power‐allocation by minimizing the lower bound on the average channel capacity. To enhance the performance of channel estimation, a low‐complexity decision feedback mechanism is introduced to iteratively mitigate the unknown data interference. Numerical results verify the performances of the proposed approach. Copyright © 2012 John Wiley & Sons, Ltd.