The automatic recognition of the modulation format of a detected signal, the intermediate step between signal detection and demodulation, is a major task of an intelligent receiver, with various civilian and military applications. Obviously, with no knowledge of the transmitted data and many unknown parameters at the receiver, such as the signal power, carrier frequency and phase offsets, timing information and so on, blind identification of the modulation is a difficult task. This becomes even more challenging in real-world scenarios with multipath fading, frequency-selective and time-varying channels. With this in mind, the authors provide a comprehensive survey of different modulation recognition techniques in a systematic way. A unified notation is used to bring in together, under the same umbrella, the vast amount of results and classifiers, developed for different modulations. The two general classes of automatic modulation identification algorithms are discussed in detail, which rely on the likelihood function and features of the received signal, respectively. The contributions of numerous articles are summarised in compact forms. This helps the reader to see the main characteristics of each technique. However, in many cases, the results reported in the literature have been obtained under different conditions. So, we have also simulated some major techniques under the same conditions, which allows a fair comparison among different methodologies. Furthermore, new problems that have appeared as a result of emerging wireless technologies are outlined. Finally, open problems and possible directions for future research are briefly discussed.
An improved discrete Fourier transform (DFT)-based channel estimation for orthogonal frequency division multiplexing systems is proposed. Conventional DFT-based channel estimations improve the performance by suppressing time domain noise. However, they potentially require information on channel impulse responses and may also result in mean-square error (MSE) floor due to incorrect channel information such as channel delay spread. In contrast, our purposed channel estimation can improve the performance by deciding significant channel taps adaptively without requiring any channel statistical information: Significant channel taps are detected on the basis of a predetermined threshold. The optimal threshold to reduce the MSE of the estimation is also derived, and it is confirmed by computer simulation. Simulation results demonstrate that the proposed algorithm can improve the MSE performance similar to 6.5 dB compared with the conventional DFT-based estimation, and the MSE floor is not observed in any channels.
An overview of the developments in optical wireless systems viewed from the traditional communications viewpoint of transmitter, channel and receiver is presented. The trends in modu- lation formats that match information to the optical wireless channel are considered. This is fol- lowed by the discussion of recent transmitter and receiver innovations, particularly the utilisation of diversity transceivers. As a preliminary to the following treatment, the nature and modelling of the optical wireless channel are introduced, with particular emphasis on its unique features in terms of transmitted power constraints and non-negativity. From the examination of modulation formats, on-off-keying remains the format of choice for basic binary transmission, whereas pulse-position modulation and its derivatives are preferred for more sophisticated require- ments. The recent introduction of techniques from radio systems employing subcarriers is seen to be the most promising development in modulation techniques at present. In receiver technology, quasi-diffuse systems employing multispot diffusion and angular diversity are significant develop- ments. They offer lower path loss and less multipath dispersion, at a lower transmission power compared to 'conventional' wide-angle diffuse systems, while providing a high level of user mobi- lity compared to line-of-sight transmission. These developments are helping optical wireless systems to fulfil their promise by adopting a philosophy inspired by the radio domain to accommo- date operation within a hostile channel.
Mobile ad hoc networks (MANETs) are unpredictable by nature. Providing any kind of reliability for quality of service (QoS) in such networks is challenging. Quantifying available resources accurately, avoiding interference with ongoing QoS traffic and adapting to QoS violations caused by nodes' mobility are the main concerns for the design of an efficient admission control protocol in MANETs. Adaptive admission control (AAC), a novel admission control protocol which uses robust and accurate resource estimation and prediction techniques for relevant admission decisions has been proposed. Furthermore, AAC uses statistical QoS provision to counteract the QoS threatening mobility. Through simulations, we show that our proposed scheme outperforms existing approaches in terms of correctness and overall performance.
Traditionally, the frequency spectrum is licensed to users by government agencies in a fixed manner where the licensee has exclusive right to access the allocated band. However, with increasing demand for the spectrum and scarcity of vacant bands, a spectrum policy reform seems inevitable. Meanwhile, recent measurements suggest the possibility of sharing spectrum among different parties subject to interference-protection constraints. In order to enable access to an unused licensed spectrum, a secondary user has to monitor licensed bands and opportunistically transmit whenever no primary signal is detected. Spectrum-sharing between a primary licensee and a group of secondary users has been studied. The structure of an asymptotically optimum detector based on the measurements of all secondary users is derived and the effect of the quantisation error in such a system is evaluated. Also, it is shown that by using the proposed detector in a sequential detection structure, it is possible to shorten the decision time needed by the detector. The results show the superiority of the proposed detector to other schemes.
Linear precoding (LP) techniques for a multiuser multiple-input-multiple-output broadcast channel is investigated and analytical results of the achievable sum capacity and error performance for zero forcing (ZF) linear precoders is presented. It is shown that the detection signal-to-noise ratio of ZF-LP can be accurately approximated by a shrinking Chi-square distribution. The symbol error rate and its achievable diversity gain of ZF-LP are given. Then, an improved linear precoder based on the minimum mean-square error (MMSE) criterion is derived. Its error performance and sum capacity are analysed and compared with that of the ZF-LP. It is shown that the MMSE-LP can achieve much better error performance and a high sum capacity than the existing ZF-LP.
Low complexity channel estimation for single-carrier block transmission systems over multiple-input multiple-output time varying frequency-selective channels is investigated. A time slot structure that uses Golay complementary sequences with perfect periodic autocorrelations as two-sided pilot blocks is presented. Employing this strucutre, optimal least square estimate in the minimum mean square error (MMSE) sense is achieved. Furthermore, a computationally efficient algorithm which is named as fast periodic Golay correlation is proposed based on the specific generator and the properties related to circulant matrices. Finally, the simulation results show the MMSE performance of the proposed scheme and algorithm.
Radio networks of universal mobile telecommunication system (UNITS) need accurate planning and optimisation, and many factors not seen in second generation (2G) networks must be considered. However, planning and optimisation of UMTS radio networks are often carried out with static simulations, for efficiency and to save time. To obtain a good trade-off between accuracy and computational load, link-level performance factors need to be taken into account. The authors propose a mathematical model for UMTS radio network planning taking into consideration fast power control, soft handover and pilot signal power in both uplink and downlink. Optimisation strategies are investigated based on three meta-heuristics: genetic algorithm, simulated annealing (SA) and evolutionary-SA. The base station location problem is modelled as a simplified P-median problem, and parameter tuning of these meta-heuristics are presented. Extensive experimental results are used to compare the performance of different algorithms in terms of statistical measurements.
The fact that ad hoc networks are required to support mobility of individual network nodes results in problems arising when routing data. These problems include route loss, poor longevity of established routes and asymmetric communications links. Mobility of nodes also increases the control traffic overhead and affects the performance of the protocol. Mobility can, however, be exploited to improve route longevity when establishing the route. In some situations, the source of information is not available (e.g. GPS information in the underground), hence protocols relying on this information will fail to operate correctly. In such situations, alternative 'self-content' information should be available to perform the needed task of routing. Three novel schemes that make use of such information, the heading direction angle, to provide a mechanism for establishing and maintaining robust and long-lived routes are presented. The results show that these schemes reduce the overhead and increase the route longevity when compared with the AODV protocol. The schemes described can operate as a standalone mechanism or can be adopted by other routing protocols in order to improve their performance.
The first complete measurement of the Chinese Internet topology at the autonomous systems (AS) level based on traccroute data probed from servers of major ISPs in mainland China is presented. Both the Chinese Internet AS graph and the global Internet AS graph can be accurately reproduced by the positive-feedback preference model with the same parameters. This result suggests that the Chinese Internet preserves well the topological characteristics of the global Internet. This is the first demonstration of the Internet's topological fractality, or self-similarity, performed at the level of topology evolution modelling.
Multicast is an efficient paradigm for transmitting data from a sender to a group of receivers. According to the IEEE 802.11 standard, the multicast service is defined as an unreliable service, that is, it does not include the use of ACK frames. Furthermore, different to the unicast service, the multicast service makes use of a single rate out of the various rates included in the basic service set defined by the IEEE 802.11 standard. Even though various proposals have recently appeared in the literature addressing these issues, none of them has come out with a structured set of control mechanisms taking into account the varying conditions characterising the wireless channels as well as the requirements of various applications. A novel cross-layer auto rate selection multicast mechanism for multi-rate wireless LANs, namely auto rate selection for multicast, capable of adapting the data transmission to the varying conditions of the channel and taking into account the characteristics of various applications, is introduced. The simulation results show that our proposal outperforms the IEEE 802.11 standard and the mechanisms recently proposed in the literature.
Diffusionless Lorentz equations a simplified one-parameter version of the well-known Lorentz model. Also, it was attained in the limit of high Rayleigh and Prandtl numbers, physically corresponding to diffusionless convection. A simplified, one-parameter version of the Lorentz model called diffusionless Lorentz is proposed, which is suitable for chaotic synchronisation and masking communication circuits using Matlab-Simulink and PSpice programmes. It is also suitable for a real electronic experimental circuit.
Time-selective and frequency-selective fading of propagating channels degrades the performance of multiple-input multiple-output/orthogonal frequency-division multiplexing (MIMO/OFDM) systems extensively by introducing double convolutions in both time-domain and frequency-domain. The author addresses the problem of linearly time-varying (LTV) channel estimation of MIMO/OFDM systems. Firstly, the time-varying coefficients of LTV channel are modelled by complex exponential basis expansions. Secondly, LTV channel is estimated and optimal pilot symbols are derived following the minimum mean square error criterion. It is shown that the optimal pilot strategy is to group consecutive pilot tones together as a pilot cluster and to distribute uniformly all pilot clusters in frequency-domain. Furthermore, LTV channel estimation is improved by imposing a window function on received signals. Finally, through simulations, it is shown that new channel estimator can provide considerable performance improvement in estimating MIMO-LTV channels, especially for the rapidly time-varying channel of a large Doppler frequency.
The use of multiple antennas at both transmitter and receiver is a promising technique for significantly increasing the capacity and spectral efficiency of wireless communication Systems. In particular, spatial multiplexing techniques provide a means of increasing the data rate of the system without having to increase the transmitter power or the bandwidth. In recent years, special attention has been paid to the sphere decoder (SD) to detect spatially multiplexed signals. It provides optimal maximum likelihood (NIL) performance with reduced complexity, compared to the maximum likelihood detector (MLD). An analysis of the performance of the SD in the presence of spatially correlated multiple-input multiple-output (MIMO) channels is presented. Analytical and simulation results show that, compared to suboptimal linear and non-linear MIMO detectors, the SD suffers a complexity increase when correlation exists between the antennas at the transmitter or the receiver. In addition, a novel low-complexity channel ordering technique is introduced to reduce the complexity of the SD.
The paper develops a new method for estimating and correcting the carrier frequency offsets (CFO) of orthogonal frequency division multiple access (OFDMA) systems at the uplink receiver. Multiple CFO of the different users in the OFDMA system may violate the orthogonality among subcarriers and induce inter-carrier interference (ICI) and inter-user interference (IUI). The IUI provides some extra challenges to CFO estimation not addressed in OFDM systems with only a single CFO. By significantly reducing the IUI, the paper presents an improved CFO estimator which can provide a higher accuracy than the existing approaches. After having estimated the CFO, the author also proposes a new method for CFO correction or compensation at the uplink receiver. Through simulations, it is shown that the new CFO estimator and the correction method outperform the existing approaches.
Two approaches for reducing peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) are proposed that are relied on a set of cyclically shifted phase sequences (CSPS) and implemented using the time domain circular convolution. After multiplying CSPS with the frequency domain data, the signal candidates can be expressed as weighted sum of the circularly shifted OFDM time domain data in the first method, which is called CSPS method. In the second method, weighted coefficients for generating the signal candidates in CSPS method are optimally selected to improve its performance; thus, the second method is referred to as optimised CSPS (OCSPS) method. The performances of the CSPS and OCSPS methods are evaluated using simulated data and compared with those of selective mapping (SLM) and partial transmit sequences (PTS). The simulation results show that both the CSPS and OCSPS methods can reduce the PAPR effectively, and that the OCSPS performs even better than the CSPS. The OCSPS can achieve the same performance as compared to the PTS. A distinct feature of the proposed methods is that only one inverse discrete Fourier transform is needed, and thus, the candidates can be calculated in time domain directly.
Nonlinear Tomlinson-Harashima precoding (THP) is an attractive solution for a scenario where the transmission system employs multiple antennas at transmitter and multiple users with a single antenna at the receiver, so that the cooperation among the receive antennas are impossible (downlink scenario). THP solution based on zero forcing (ZF) and minimum mean square error (MMSE) criteria is one of the important techniques to achieve near multiple input multiple output channels capacity with reasonable complexity. In this paper, the effect of channel imperfection on THP is considered. At first, the achievable rate of THP with respect to ZF criterion in an imperfect channel state information (CST) scenario is calculated. Moreover, based on MMSE criterion, a new robust solution is derived which provides a significant improvement with respect to the conventional optimisation method. Then, the effect of channel estimation error on THP is considered as an improved optimisation where THP filters are optimised together with a channel estimator. Spatial power loading is found to be important to the THP performance. This loading for robust/joint optimisation of MMSE THP is developed by minimum average symbol error rate sense. Simulation results show the capacity loss, the performance advantage attained by the robust/joint optimisation and the power loading in an imperfect CST scenario.
Researches and development efforts in wireless networking and systems are progressing at an incredible rate. Among them, measurement and analysis of performance achieved at network layer and perceived by end users is an important task. In particular, recent advances concerning IEEE 802.11b-based networks seem to be focused on the measurement of key parameters at different protocol levels in a cross-layered fashion, because of their inherent vulnerability to in-channel interference. By adopting a cross-layer approach on a real network set-up operating in a suitable experimental testbed, packet loss against signal-to-interference ratio in IEEE 802.11b-based networks is hereinafter assessed. Results of several measurements aimed at establishing the sensitivity of IEEE 802.11b carrier sensing mechanisms to continuous interfering signals and evaluating the effects of triggered interference on packet transmission.
Automatic repeat request (ARQ) retransmission in user cooperative networks, which use amplify-and-forward as a relaying strategy is dealt with. In contrast to the conventional environments, where the source responds in a retransmission demand, in cooperative systems this requirement can also be satisfied by a relay node. In addition, if a relay node can be selected according to the instantaneous channel conditions, the source is not always the optimal responder. The problem under consideration here is to find the retransmission combination which optimises the performance under a given delay quality-of-service constraint. It will be shown that the optimal combination depends on the number of available relays, the total number of retransmissions and the average signal-to-noise ratio. We provide an analytical framework for the definition of the optimal combination in function of these three system parameters. Since the practical ad hoc networks are not centralised and do not have an external control, a distributed truncated ARQ protocol is further proposed to apply the decided retransmission combination.
A simple and efficient virtual-source ray-tracing technique for the simulation of indoor wideband radio and optical propagation channels is proposed. The parametric deterministic model considers the room geometry, transceiver locations, material properties and probe signal types. It is applied to the indoor ultrawideband channel in the FCC-allocated 3.1-10.6 GHz band, and a range of novel results are presented to illustrate several possible applications. The channel small-scale fading statistics and spatial variability are examined by synthesising a densely sampled aperture. Multiple-antenna array systems are simulated to evaluate multiple-input multiple-output performance. The multipath angular characteristics are analysed from the simulated azimuth-delay profile. The simulation results closely match previous channel measurement studies and statistical models, validating the proposed technique. It is shown that specular reflection is dominant, and power convergence is achieved with three reflections in a typical indoor environment. Thus, it is demonstrated that despite its simplicity, the model yields reliable and accurate results, and can therefore be a useful tool for indoor wireless network planning and performance prediction.