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Communications Letters, IEEE

Issue 10 • Date Oct. 2001

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Displaying Results 1 - 15 of 15
  • A space-time block-coded OFDM scheme for unknown frequency-selective fading channels

    Publication Year: 2001 , Page(s): 393 - 395
    Cited by:  Papers (32)  |  Patents (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (76 KB) |  | HTML iconHTML  

    We introduce a space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) selective fading channels which does not require channel knowledge either at the transmitter or at the receiver. The decoding algorithm is based on generalized maximum-likelihood sequence estimation. We investigate the performance of the proposed scheme over two-tap Rayleigh fading channels. Simulation results show the performance to be near optimum. View full abstract»

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  • Average level crossing rate and average fade duration of selection diversity

    Publication Year: 2001 , Page(s): 396 - 398
    Cited by:  Papers (43)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (95 KB) |  | HTML iconHTML  

    The average level crossing rate and average fade duration of the output signal envelope of a selection diversity combiner, operating on independent, but nonidentical fading input branch signals are derived. The exact closed-form results are valid for arbitrary diversity order, and are obtained for Rayleigh, Ricean, and Nakagami fading input signals. View full abstract»

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  • A new generic multistep power control algorithm for the LEO satellite channel with high dynamics

    Publication Year: 2001 , Page(s): 399 - 401
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (67 KB) |  | HTML iconHTML  

    LEO satellite CDMA systems, characterized by round-trip delay and high variation in slow fading, call for different considerations on power control algorithms. Based on the criteria of fast response, small over-compensation, oscillation-avoiding and minimum power control error, the transient behavior of the fixed step, adaptive step, and multistep power control algorithms are investigated and compared. A new generic multistep power control algorithm with excellent dynamic performance is proposed for LEO satellite systems. View full abstract»

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  • On frequency offset estimation for flat-fading channels

    Publication Year: 2001 , Page(s): 402 - 404
    Cited by:  Papers (24)  |  Patents (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (81 KB) |  | HTML iconHTML  

    We consider the problem of estimating the frequency offset between the transmitter and receiver in flat-fading communication channels using a training sequence. Kuo and Fitz (1997) and Morelli, Mengali and Vitetta (see IEEE Commun. Lett., vol.2, p.327-30, 1998), proposed two frequency estimators to solve the aforementioned problem. They rely on weighted linear regression for the phases of the correlation sequence or their differences. The method of Kuo et al. needs phase unwrapping and both methods require knowledge of the correlation of the fading process. In this paper, two simple methods which do not require any of these assumptions are proposed. The first is the unweighted version of the method of Morelli et al. The second is drawn from the array processing framework. It is based on a nonlinear least-squares approach that assumes an unstructured correlation of the fading process. The performance of the proposed methods is as good as that of the methods of Kuo et al. and Morelli et al., but the robustness of our methods to modeling errors is much enhanced. View full abstract»

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  • Generalized performance evaluation of low Earth orbit satellite systems

    Publication Year: 2001 , Page(s): 405 - 407
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (59 KB) |  | HTML iconHTML  

    A novel three-dimensional distribution traffic is proposed for low Earth orbit (LEO) satellite systems. The effect of the traffic geographical nonuniformity on the performance of these satellite systems is discussed. View full abstract»

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  • Dynamic spreading code selection method for PAPR reduction in OFDM-CDMA systems with 4-QAM modulation

    Publication Year: 2001 , Page(s): 408 - 410
    Cited by:  Papers (12)  |  Patents (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (55 KB) |  | HTML iconHTML  

    We present a new dynamic spreading code selection technique to obtain low peak-to-average-power ratio (PAPR) of an orthogonal frequency-division multiplexing code-division multiple-access (OFDM-CDMA) system with 4-QAM modulation for the down-link in mobile communication. In this method, initially, each user is assigned a low and a high PAPR spreading code, which allows selecting one when the system is operating. The spreading code of each user is dynamically selected so that total PAPR level of the whole user group present in the system is minimized. With the proposed technique, the PAPR level of a system with 10 users and 64 sub-carriers using 64 chip Walsh-Hadamard (WH) codes, can be limited to 15 dB while the worst case theoretical maximum could go up to 28.1 dB. Although 64 chip WH codes are employed to evaluate the performance it can be generalized to all other sets of spreading codes. View full abstract»

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  • Geo-location in urban areas using signal strength repeatability

    Publication Year: 2001 , Page(s): 411 - 413
    Cited by:  Papers (1)  |  Patents (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (36 KB) |  | HTML iconHTML  

    We propose a new approach to geo-location of cellular mobile subscribers that exploits the structure and repeatability of certain patterns of measured signal strength. With careful selection of the locations of the patterns, and appropriate training measurements, geo-location in urban areas could be based on pattern recognition. Simulation results lead to the conclusion that very reliable geo-location techniques could be developed based on this approach. View full abstract»

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  • Decoding low-density parity-check codes with probabilistic scheduling

    Publication Year: 2001 , Page(s): 414 - 416
    Cited by:  Papers (28)  |  Patents (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (57 KB) |  | HTML iconHTML  

    We present a new message-passing schedule for the decoding of low-density parity-check (LDPC) codes. This approach, designated "probabilistic schedule", takes into account the structure of the Tanner graph (TG) of the code. We show by simulation that the new schedule offers a much better performance/complexity trade-off. This work also suggests that scheduling plays an important role in iterative decoding and that a schedule that matches the structure of the TG is desirable. View full abstract»

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  • Compression of correlated binary sources using turbo codes

    Publication Year: 2001 , Page(s): 417 - 419
    Cited by:  Papers (181)  |  Patents (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (61 KB) |  | HTML iconHTML  

    We propose the use of punctured turbo codes for compression of correlated binary sources. Compression is achieved because of puncturing. The resulting performance is close to the theoretical limit provided by the Slepian-Wolf (1973) theorem. No information about the correlation between sources is required in the encoding process. The proposed source decoder utilizes iterative schemes, and performs well even when the correlation between the sources is not known in the decoder, since it can be estimated jointly with the iterative decoding process. View full abstract»

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  • Analysis and design of interleavers for CDMA systems

    Publication Year: 2001 , Page(s): 420 - 422
    Cited by:  Papers (14)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (87 KB) |  | HTML iconHTML  

    The turbo-principle has been applied to multiuser receivers in CDMA systems. However, the role of interleavers in these turbo-receivers has not been studied yet. In this paper, we give a notion of optimality and of mutual optimality and a way to test it. Moreover, limiting the discussion to congruential interleavers and convolutional codes, we give sufficient conditions to construct a set of mutually optimal interleavers. Also, we provide some simulations to support our results. View full abstract»

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  • Performance comparison between VBR speech coders for adaptive VoIP applications

    Publication Year: 2001 , Page(s): 423 - 425
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (50 KB) |  | HTML iconHTML  

    The introduction of new variable bit rate (VBR) speech coders has opened up new perspectives for the implementation of adaptive voice over IP (AVoIP) systems. The paper compares different VBR speech coding techniques in a scenario in which the rate of the single sources is dynamically adapted to the workload conditions. The coders compared are the AMR, the M/sup 3/ R and the G.729. Using source and rate control mechanism models, performance is evaluated in terms of loss probability, offered throughput and mean CMOS with varying numbers of sources. The use of header compression mechanisms is also evaluated. View full abstract»

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  • On the {m-QAM}2 modulation

    Publication Year: 2001 , Page(s): 426 - 428
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (109 KB) |  | HTML iconHTML  

    The aim of this letter is to analyze a modulation technique called {m-QAM}/sup 2/ in terms of bit error rate (BER). The performance of the {m-QAM}/sup 2/ is compared to the traditional M-QAM modulation scheme, taking into account the same spectral efficiency, where m and M are related by M=2/sup i/ and i=/spl radic/m, ..., 2/spl radic/m-1. Performance curves obtained by the theoretical model and by computer simulation are shown. View full abstract»

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  • Optimization of tap spacings for the tapped delay line decision feedback equalizer

    Publication Year: 2001 , Page(s): 429 - 431
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (101 KB) |  | HTML iconHTML  

    In this letter, we optimize tap spacings in a tapped delay line equalizer. We derive a set of nonlinear equations for the optimum tap spacings and tap weights for the decision feedback equalizer. A recursive numerical technique is used to obtain the optimum settings. With optimized tap spacings, the proposed solutions provide a performance upper bound for the decision feedback equalizer for a given number of taps. View full abstract»

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  • Performance study on a WDM packet switch with limited-range wavelength converters

    Publication Year: 2001 , Page(s): 432 - 434
    Cited by:  Papers (34)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (65 KB) |  | HTML iconHTML  

    We consider a slotted WDM packet switch with limited-range wavelength converters. The performance of this switch is studied using simulations with various types of data traffic. Results show that the slotted WDM packet switch with a small range of wavelength conversion capability can achieve a performance close to that of a switch with the full range of wavelength conversion capability. View full abstract»

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  • Algorithms for routing and wavelength assignment based on solutions of LP-relaxations

    Publication Year: 2001 , Page(s): 435 - 437
    Cited by:  Papers (27)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (66 KB) |  | HTML iconHTML  

    In this letter, we consider the problem of maximizing the number of lightpaths that may be established in a wavelength routed optical network (WRON), given a connection matrix, i.e., a static set of demands, and the number of wavelengths the fiber supports. The problem of establishing all the connections of the connection matrix using the fewest number of wavelengths has been investigated in Banerjee and Mukherjee (1996) and Baroni et al. (1998). We call the former problem max-RWA (problem of maximizing the number of lightpaths) and the latter problem min-RWA (minimizing the number of wavelengths). In this letter, we only consider WRONs with no wavelength conversion capabilities. We formulate the max-RWA problem when no wavelength conversion is allowed as an integer linear programme (ILP) which may be solved to obtain an optimum solution. We hope to solve the ILP exactly for small size networks (few nodes). For moderately large networks (tens of nodes) we develop algorithms based on solutions obtained by solving the LP-relaxation of the ILP formulation. Results obtained for networks such as NSFNET and EONNET are presented. View full abstract»

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George K. Karagiannidis
Aristotle University of Thessaloniki
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