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Communications, IEEE Transactions on

Issue 4 • Date April 2003

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Displaying Results 1 - 25 of 25
  • Single-user sparse channel acquisition in multiuser DS-CDMA systems

    Page(s): 682 - 693
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (688 KB) |  | HTML iconHTML  

    Single-user channel estimation in multiuser DS-CDMA systems for the case of sparse channels with large delay spreads is addressed. In addition, practical pulse shapes are considered. In sparse channels, the efficient way to estimate the parameters is to estimate the continuous delays of each path, instead of using the typical discrete tapped delay-line model. Due to the facts that the desired delays are not drawn from a simple finite set and that band-limited pulse shapes are employed, the resulting methods require numerical optimization techniques. To facilitate estimation, it is proposed to optimize the spreading code employed during the training, or estimation, phase. The optimal single-path spreading code is derived and extended for estimation in the multipath scenario. Both single-path and multipath channel estimation are considered. The proposed algorithms are evaluated through simulation and via the determination of the Cramer-Rao lower bound on the estimation variance. Analytical approximations of key performance measures are also derived and are seen to be tight for a variety of scenarios. View full abstract»

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  • An efficient implementation of a maximum-likelihood detector for space-time block coded systems

    Page(s): 521 - 524
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (317 KB) |  | HTML iconHTML  

    We investigate maximum-likelihood (ML) sequence estimation for space-time block coded systems without assuming channel knowledge. The quadratic form of the ML receiver in this case does not readily lend itself to efficient implementation. However, under quasi-static channel conditions, the likelihood function reduces to a simple form similar to the classical correlation receiver in matrix notation. It also allows the development of a recursive expression that can be easily implemented by a Viterbi-type algorithm with a reasonable complexity. Although the receiver is suboptimum for the nonstatic case, its performance is close to the optimum for a range of signal-to-noise ratios. View full abstract»

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  • Two-dimensional differential demodulation for OFDM

    Page(s): 580 - 586
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (522 KB) |  | HTML iconHTML  

    The possibility of combining differential demodulators in time direction and frequency direction for orthogonal frequency-division multiplexing (OFDM) frames is investigated. The proposed algorithm uses not only the direction in which the information is modulated, but also joins it with other symbols in time or frequency direction of the OFDM frame, resulting in an additional second direction that is differentially demodulated as well. In the special case that is investigated here, the second direction does not carry any modulated information. Nevertheless, it is shown that it can be used for the two-dimensional demodulation process without changing the transmitter. Therefore, the proposed algorithm is applicable for existing digital transmission systems, for example, digital audio broadcasting, as well as new systems. View full abstract»

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  • Analysis of transmit-receive diversity in Rayleigh fading

    Page(s): 694 - 703
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (762 KB) |  | HTML iconHTML  

    We analyze the error performance of a wireless communication system employing transmit-receive diversity in Rayleigh fading. By focusing on the complex Gaussian statistics of the independent and identically distributed entries of the channel matrix, we derive a formula for the characteristic function (c.f.) of the maximum output signal-to-noise ratio. We use this c.f. to obtain a closed-form expression of the symbol error probability (SEP) for coherent binary keying. The method is easily extended to obtain the SEP for the coherent reception of M-ary modulation schemes. View full abstract»

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  • The PAM decomposition of CPM signals with integer modulation index

    Page(s): 543 - 546
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (318 KB) |  | HTML iconHTML  

    This article presents the solution for decomposing continuous-phase modulated (CPM) signals with integer modulation index into pulse-amplitude modulated components. The notion of main complex pulse is also introduced. A simplified demodulator for CPM signals with integer modulation index is proposed as an application example and simulation results using a quaternary 2 raised cosine (RC) scheme are given. View full abstract»

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  • Wavelength converter placement under different RWA algorithms in wavelength-routed all-optical networks

    Page(s): 607 - 617
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (551 KB) |  | HTML iconHTML  

    Sparse wavelength conversion and appropriate routing and wavelength assignment (RWA) algorithms are the two key factors in improving the blocking performance in wavelength-routed all-optical networks. It has been shown that the optimal placement of a limited number of wavelength converters in an arbitrary mesh network is an NP-complete problem. There have been various heuristic algorithms proposed in the literature, in which most of them assume that a static routing and random-wavelength assignment RWA algorithm is employed. However, the existing work shows that fixed-alternate routing and dynamic routing RWA algorithms can achieve much better blocking performance. Our study further demonstrates that the wavelength converter placement and RWA algorithms are closely related in the sense that a well-designed wavelength converter placement mechanism for a particular RWA algorithm might not work well with a different RWA algorithm. Therefore, the wavelength converter placement and the RWA have to be considered jointly. The objective of this paper is to investigate the wavelength converter placement problem under the fixed-alternate routing (FAR) algorithm and least-loaded routing (LLR) algorithm. Under the FAR algorithm, we propose a heuristic algorithm called minimum blocking probability first for wavelength converter placement. Under the LLR algorithm, we propose another heuristic algorithm called weighted maximum segment length. The objective of the converter placement algorithms is to minimize the overall blocking probability. Extensive simulation studies have been carried out over three typical mesh networks, including the 14-node NSFNET, 19-node EON, and 38-node CTNET. We observe that the proposed algorithms not only outperform existing wavelength converter placement algorithms by a large margin, but they also can achieve almost the same performance compared with full wavelength conversion under the same RWA algorithm. View full abstract»

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  • Probability distributions for the number of radio transceivers which can communicate with one another

    Page(s): 676 - 681
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (422 KB) |  | HTML iconHTML  

    This paper considers points which can communicate with one another and which are uniformly and randomly distributed on the plane, assuming an inverse power law for attenuation and log-normal shadowing. First, a probability distribution is obtained for the distance between a pair of such points. Communication is assumed to be possible if the attenuation of a signal transmitted from one point, say a mobile phone, does not exceed some specified value by the time it reaches the other, say a base station. The probability distribution of the number of base stations which can hear such a signal from a given mobile is found. An example shows how the probability that a mobile is in a handover region between two or more base stations can be evaluated. The analysis is extended to the case where the power law changes at a specified distance from the mobile, and also to consider the effect of an area within the plane containing a greater density of mobiles and/or base stations (a hot spot). A final result gives the conditional distribution for the number of base stations within some specified area of the plane, given the number within some larger area containing that specified area. The results also apply to the number of mobiles within range of a base station and, in an ad hoc network, to the number of nodes within range of a given node. View full abstract»

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  • UCHT-based complex sequences for asynchronous CDMA system

    Page(s): 618 - 626
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (602 KB) |  | HTML iconHTML  

    The use of orthogonal spreading codes has attracted much attention due to their ability to suppress interference from other users, compared with the nonorthogonal sequences in the synchronous case. In this paper, new sets of orthogonal sequences derived from the unified complex Hadamard transforms (UCHTs) are investigated. Various correlation properties of the sequences are mathematically derived and analyzed. It is shown that some UCHT sequences provide better autocorrelation properties than orthogonal Walsh-Hadamard sequences. Performance comparisons between UCHT sequences, Gold, small set of Kasami, and m-sequences show that some UCHT sequences outperform these well-known spreading sequences under simulation of systems in the presence of multiple access interference and additive white Gaussian noise. View full abstract»

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  • Sensitivity to timing errors in EGC and MRC techniques

    Page(s): 530 - 534
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (421 KB) |  | HTML iconHTML  

    The effect of imperfect timing is analyzed in equal gain combining (EGC) and maximal ratio combining (MRC) techniques over Rayleigh and Nakagami-m fading channels with binary phase-shift keying modulation. In the case of EGC, the bit-error probability is derived, while in the case of MRC, error rate bounds are presented. Theoretical results are justified by computer simulation. Numerical results demonstrate that both EGC and MRC are fairly sensitive to timing errors, and comparatively, MRC is more sensitive. View full abstract»

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  • Adaptive MAC protocols for broadcast networks with bursty traffic

    Page(s): 553 - 557
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (270 KB) |  | HTML iconHTML  

    An adaptive medium access control protocol for broadcast networks, which is capable of operating efficiently under bursty traffic conditions, is introduced. According to the proposed protocol, the station which grants permission to transmit at each time slot is selected by taking into account the network feedback information. In this way, the number of idle slots is minimized and the network performance is significantly improved. Furthermore, the portion of the bandwidth assigned to each station is dynamically adapted to the station's needs. View full abstract»

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  • Fast unbiased echo canceller update during ADSL transmission

    Page(s): 561 - 565
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (290 KB) |  | HTML iconHTML  

    This paper presents an efficient method for updating echo canceller (EC) coefficients in asymmetric digital subscriber line transceivers during transmission that removes the dependency of updates on the far-end signal. The new method improves on the frequency-domain EC architecture by Ho et al. (1996), in that it converges faster, improves the stability to perturbation, and is not prone to bias from pilot tones. The new EC updating system architecture is presented for both the central office and remote terminal systems and is fully compliant with the ITU-T G.DMT standard. View full abstract»

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  • High performance, high throughput turbo/SOVA decoder design

    Page(s): 570 - 579
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (444 KB) |  | HTML iconHTML  

    Two efficient approaches are proposed to improve the performance of soft-output Viterbi (1998) algorithm (SOVA)-based turbo decoders. In the first approach, an easily obtainable variable and a simple mapping function are used to compute a target scaling factor to normalize the extrinsic information output from turbo decoders. An extra coding gain of 0.5 dB can be obtained with additive white Gaussian noise channels. This approach does not introduce extra latency and the hardware overhead is negligible. In the second approach, an adaptive upper bound based on the channel reliability is set for computing the metric difference between competing paths. By combining the two approaches, we show that the new SOVA-based turbo decoders can approach maximum a posteriori probability (MAP)-based turbo decoders within 0.1 dB when the target bit-error rate (BER) is moderately low (e.g., BER<10-4 for 1/2 rate codes). Following this, practical implementation issues are discussed and finite precision simulation results are provided. An area-efficient parallel decoding architecture is presented in this paper as an effective approach to design high-throughput turbo/SOVA decoders. With the efficient parallel architecture, multiple times throughput of a conventional serial decoder can be obtained by increasing the overall hardware by a small percentage. To resolve the problem of multiple memory accesses per cycle for the efficient parallel architecture, a novel two-level hierarchical interleaver architecture is proposed. Simulation results show that the proposed interleaver architecture performs as well as random interleavers, while requiring much less storage of random patterns. View full abstract»

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  • Radial basis function-assisted turbo equalization

    Page(s): 664 - 675
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (779 KB) |  | HTML iconHTML  

    This paper presents a turbo equalization (TEQ) scheme, which employs a radial basis function (RBF)-based equalizer instead of the conventional trellis-based equalizer of Douillard et al. (1995). Structural, computational complexity, and performance comparisons of the RBF-based and trellis-based TEQs are provided. The decision feedback-assisted RBF TEQ is capable of attaining a similar performance to the logarithmic maximum a posteriori scheme in the context of both binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) modulation, while achieving a factor 2.5 and 3 lower computational complexity, respectively. However, there is a 2.5-dB performance loss in the context of 16 quadrature amplitude modulation (QAM), which suffers more dramatically from the phenomenon of erroneous decision-feedback effects. A novel element of our design, in order to further reduce the computational complexity of the RBF TEQ, is that symbol equalizations are invoked at current iterations only if the decoded symbol has a high error probability. This techniques provides 37% and 54% computational complexity reduction compared to the full-complexity RBF TEQ for the BPSK RBF TEQ and 16QAM RBF TEQ, respectively, with little performance degradation, when communicating over dispersive Rayleigh fading channels. View full abstract»

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  • Closed form and infinite series solutions for the MGF of a dual-diversity selection combiner output in bivariate Nakagami fading

    Page(s): 539 - 542
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (353 KB) |  | HTML iconHTML  

    Using a circular contour integral representation for the generalized Marcum-Q function, Qm(a,b), we derive a new closed-form formula for the moment generating function (MGF) of the output signal power of a dual-diversity selection combiner (SC) in bivariate (correlated) Nakagami-m fading with positive integer fading severity index. This result involves only elementary functions and holds for any value of the ratio a/b in Qm(a,b). As an aside, we show that previous integral representations for Qm(a,b) can be obtained from a contour integral and also derive a new, single finite-range integral representation for Qm(a,b). A new infinite series expression for the MGF with arbitrary m is also derived. These MGFs can be readily used to unify the evaluation of average error performance of the dual-branch SC for coherent, differentially coherent, and noncoherent communications systems. View full abstract»

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  • Interference rejection for frequency-hopping communication systems using a constant power algorithm

    Page(s): 627 - 633
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (479 KB) |  | HTML iconHTML  

    Partial-band interferences are known to have a deleterious effect on the receiver performance in frequency-hopping communication systems. We consider the rejection of such interferences using an array of sensors. A simple yet effective method is proposed based on a constant power algorithm. The principle behind this method is that partial-band interferences contribute to power variations in the received signal, hence, the idea to constrain the output power of the array to be constant in order to reject interferences. It is shown that the algorithm converges in a reasonably low number of hops. Additionally, it achieves a nearly optimal signal-to-interference-plus-noise ratio without requiring any information about the location of the desired user. Finally, its robustness to synchronization errors is illustrated via numerical simulations. View full abstract»

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  • Baud rate timing recovery scheme for filter bank-based multicarrier transmission

    Page(s): 652 - 663
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (770 KB) |  | HTML iconHTML  

    This paper is devoted to the design and the steady-state performance analysis of a data-aided or decision-directed timing recovery scheme for filter bank-based multicarrier transmission with equalization. The symbol synchronizer is obtained from a Mueller and Muller approach extended to multiband transmission. A simple synchronizer is first proposed which can be derived from an approximate likelihood function. Then, possible improvements are discussed. Timing jitter variances are computed and discussed for high bit-rate transmission over copper wires. View full abstract»

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  • Power-distortion optimized mode selection for transmission of VBR videos in CDMA systems

    Page(s): 525 - 529
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (361 KB) |  | HTML iconHTML  

    A power-distortion (P-D) optimized coding mode-selection scheme is proposed for variable bit rate videos in wireless code-division multiple-access (CDMA) systems. Unlike the conventional rate-distortion (R-D) optimized mode-selection methods, the proposed scheme takes into account time-varying channels. We show that the proposed optimization problem can be represented as the conventional R-D mode-selection problem, in which the Lagrangian multiplier depends on the channel conditions. In the proposed scheme, as the link gain of a user increases, the optimum mode for the user tends to be intramode rather than intermode, and vice versa. Numerical results show that, in CDMA systems, the proposed scheme consumes much less power at the same image quality compared with conventional R-D schemes. View full abstract»

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  • A robust rank estimation algorithm of group-blind MMSE multiuser detectors for CDMA systems

    Page(s): 547 - 552
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (437 KB) |  | HTML iconHTML  

    We propose an adaptive rank-estimation method for the additive white or colored Gaussian noise model. The main contribution of this paper has three parts. (1) We investigate the rank mismatch problem in the group-blind multiuser detector of Wang and Host-Madsen (see IEEE J. Select. Areas Commun., vol.17, p.1971-1984, Nov. 1999), and find that underestimating the rank causes significant performance degradation, whereas, rank overestimation can achieve performance gain in the low signal-to-noise-ratio (SNR) region. However, rank overestimation can lead to inaccurate channel estimation, which degrades the detector performance significantly in the high SNR region. (2) We propose a heuristic criterion for initial rank estimation which is robust for nonwhite noise cases. (3) In order to mitigate the rank mismatch problem, we introduce a hypothesis testing criterion for rank and signal subspace decisions, which selects the most probable rank for the group-blind detectors. Simulation results show that the performance of the group-blind detector using this adaptive rank estimation algorithm is comparable to the group-blind with perfect knowledge of the rank, and even better in the low-to-medium SNR region. View full abstract»

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  • Determination of LTPB parameters to guarantee message deadlines

    Page(s): 558 - 560
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (226 KB) |  | HTML iconHTML  

    A bandwidth allocation scheme for a linear token passing multiplex data bus is proposed based on balancing message transmission time with traffic load to each node. It is proved to provide a network worst case achievable utilization of 50%. Under this allocation scheme, closed-form expressions for the initialized value of token holding timers and token rotation timers in each node are obtained. View full abstract»

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  • Analysis of true jitter arising from pulse-stuffing schemes

    Page(s): 597 - 606
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (709 KB) |  | HTML iconHTML  

    In synchronous digital hierarchy and plesiochronous digital hierarchy networks, it is frequently necessary to recover a data clock from a gapped clock derived from stuff information present at the desynchronizer. In this paper, a comprehensive analysis of the timing jitter resulting from phase-locked loop-type desynchronizers is presented. This analysis is different from the conventional analysis where the jitter is represented using a phase-error sequence. It is shown that such a simplified approach cannot accurately describe the jitter at the output of the desynchronizer. From the detailed analysis, it is also shown how the use of threshold modulation at the synchronizer reduces the low-frequency jitter at the desynchronizer. It has been demonstrated in the paper that when threshold modulation is used at the synchronizer, the dominating low-frequency jitter terms cannot be explained by the conventional jitter analysis methods. Therefore, in future networks, where tighter jitter performances are to be imposed on the synchronizers, jitter characterization using the proposed true jitter analysis technique would be very useful. View full abstract»

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  • Average bit-error probability performance for optimum diversity combining of noncoherent FSK over Rayleigh fading channels

    Page(s): 566 - 569
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (344 KB) |  | HTML iconHTML  

    We derive the performance of the optimum noncoherent frequency-shift keying diversity combining receiver and compare it with that which implements the more traditional noncoherent postdetection equal gain combining scheme to reveal the degree of suboptimality of the latter. View full abstract»

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  • Trellis-coded differential unitary space-time modulation over flat fading channels

    Page(s): 587 - 596
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (686 KB) |  | HTML iconHTML  

    Coding and modulation for multiple-antenna systems have gained much attention in wireless communications. This paper investigates a noncoherent trellis-coded scheme based on differential unitary space-time modulation when neither the transmitter nor the receiver know the channel. In a time-varying flat Rayleigh fading environment, we derive differentially noncoherent decision metrics and obtain performance measures for systems with either an ideal interleaver or no interleaver. We demonstrate that with an ideal interleaver, the system performance is dominated by the minimum Hamming distance of the trellis code, while without an interleaver, the performance is dominated by the minimum free squared determinant distance (a novel generalization of the Euclidean distance) of the code. For both cases, code construction is described for Ungerboeck-type codes. Several examples that are based on diagonal cyclic group constellations and offer a good tradeoff between the coding advantage and trellis complexity are provided. Simulation results show that, by applying the soft-decision Viterbi decoder, the proposed scheme can achieve very good performance even with few receive antennas. Extensions to trellis-coded differential space-time block codes are also discussed. View full abstract»

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  • On the error probability of decision-feedback differential detection

    Page(s): 535 - 538
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (370 KB) |  | HTML iconHTML  

    We give a tight approximation for the bit-error rate (BER) of decision-feedback differential detection (DF-DD). The influence of error propagation is modeled by a Markov chain. A simple state reduction method is proposed to limit computational complexity. Our results show that error propagation strongly depends on the chosen feedback filter. In particular, the popular assumption that error propagation increases BER by a factor of two is not always justified. View full abstract»

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  • Iterative detection for multicarrier transmission employing time-frequency concentrated pulses

    Page(s): 641 - 651
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (802 KB) |  | HTML iconHTML  

    We consider multicarrier transmission schemes in which the elementary signal pulses relate to the elements of a Weyl-Heisenberg system, i.e., resulting from a prototype function shifted in time and frequency. The overlapping of the information-bearing signal parts at the output of doubly dispersive channels and the resulting interference are confined by utilizing a prototype function whose energy is concentrated in both time and frequency. We derive a symbol detector which first calculates a sufficient statistic for the unknown data symbols from the linearly combined output signals of a filter bank, and second, performs an iterative maximization of the likelihood function. The presented receiver takes full advantage of the confined pulse overlapping to limit the computational effort. An analysis of the computational complexity and bit-error rate performance of the iterative detection scheme is provided for wide-sense stationary uncorrelated scattering channels. View full abstract»

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  • Cramer-Rao bound on timing recovery of linearly modulated signals with no ISI

    Page(s): 634 - 640
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (542 KB) |  | HTML iconHTML  

    A new Cramer-Rao lower bound for symbol timing recovery of linearly modulated (quadrature amplitude modulation) signals is presented. Contrary to some other works on the subject, the transmitted data is assumed to be unknown at the receiver. The bound is derived from a likelihood function that includes the symbol randomness. For large number of symbols, the bound is achievable at any signal-to-noise ratio. The separation of symbol timing recovery and phase recovery schemes is investigated using the new results. It is shown that the separation of these operations causes a degradation of less than 0.3 dB compared to joint phase and timing recovery. The bound is derived for symbol shaping limited to a single symbol length (i.e., no intersymbol interference.) Simulations for longer pulse shapes demonstrate that the new results provide better performance prediction than other known techniques. View full abstract»

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Aims & Scope

IEEE Transactions on Communications focuses on all telecommunications including telephone, telegraphy, facsimile, and point-to-point television by electromagnetic propagation.

 

 

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Meet Our Editors

Editor-in-Chief
Robert Schober
University of British Columbia