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WO2008140344A1 - Procédé et système servant à synchroniser des émetteurs-récepteurs ofdm - Google Patents

Procédé et système servant à synchroniser des émetteurs-récepteurs ofdm Download PDF

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Publication number
WO2008140344A1
WO2008140344A1 PCT/RU2007/000234 RU2007000234W WO2008140344A1 WO 2008140344 A1 WO2008140344 A1 WO 2008140344A1 RU 2007000234 W RU2007000234 W RU 2007000234W WO 2008140344 A1 WO2008140344 A1 WO 2008140344A1
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WIPO (PCT)
Prior art keywords
ofdm
symbols
receiver
symbol
data
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/RU2007/000234
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English (en)
Inventor
Alexander Vasilevich Luzhbinin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Siemens AG
Siemens Corp
Original Assignee
Siemens AG
Siemens Corp
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Filing date
Publication date
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Priority to PCT/RU2007/000234 priority Critical patent/WO2008140344A1/fr
Publication of WO2008140344A1 publication Critical patent/WO2008140344A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2673—Details of algorithms characterised by synchronisation parameters
    • H04L27/2675—Pilot or known symbols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2602—Signal structure
    • H04L27/261—Details of reference signals
    • H04L27/2613—Structure of the reference signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2657—Carrier synchronisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2656—Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2662—Symbol synchronisation

Definitions

  • the invention relates to a method and a system for synchronizing OFDM (Orthogonal Frequency Division Multiplex) - transceivers .
  • Orthogonal Frequency Division Multiplexing is a digi- tal multi-carrier modulation scheme which uses a large number of closely spaced orthogonal sub-carriers. Each sub- carrier is modulated with a modulation scheme, such as quadrature amplitude modulation- at a low symbol rate. OFDM- systems are able to cope with severe channel conditions, for example multi-path and narrow band interference without complex equalization filters.
  • the orthogonality of the sub- carriers results in zero-cross-talk even though they are so close to each other that their spectrum overlaps.
  • the orthogonality of the sub-carriers also allows for a high spe ⁇ - tral efficiency.
  • An OFDM system requires a very accurate frequency synchronization between a transmitter and a receiver. Any deviation causes that the sub-carriers are no longer orthogonal lead- ing to inter-carrier interference ICI, i.e. cross-talk between the sub-carriers.
  • Frequency off-sets are typically caused by mismatched transmitter and receiver oscillators or by a Doppler shift due of movement of the receiver with respect to the transmitter. While Doppler shift alone be com- pensated for by the receiver, the situation is worsened when combined with a multi-path as a reflection will appear at various frequency offsets which are much harder to correct for the receiver. If the receiver is, for instance implemented in a vehicle the inter-carrier interference caused by reflections is increased with increasing speed of the vehicle.
  • the synchronization in OFDM-systems is provided to find and compensate frequency shifts in the transmission channel including transmitter and receiver oscillator frequency shifts and Doppler shifts and to find a start of an OFDM- symbol and the start of a data frame.
  • the invention provides a method for synchronizing transceivers of an OFDM system comprising the steps of
  • modulating sub-carriers of two consecutive OFDM-symbols by a modulator of a transmitter wherein the sub-carriers of both OFDM-symbols are modulated with a predetermined synchronization sequence provided for coding OFDM-pilot symbols
  • transmitting OFDM-symbols from the transmitter to a receiver via a communication medium converting OFDM-symbols received by the receiver to data sequences and multiplying continuously a data sequence with another data sequence which is delayed by a length of an OFDM- symbol to calculate multiplication products, performing a Fourier transformation of the multiplication products to calculate a convolution between the data sequences ; and detecting a presence of an OFDM-pilot symbol in the received OFDM- symbols by comparing the calculated convolution with a calculated energy of the data sequences.
  • the invention further provides a receiver for an OFDM- system comprising
  • a multiplier which continuously multiplies a converted data sequence of received OFDM-symbols with a corresponding data sequence delayed by a length of an OFDM- symbol to calculate multiplication products
  • a transformation unit which performs a Fourier transformation of the multiplication products to calculate continu- ousIy a convolution between the data sequences
  • a comparator for comparing the calculated convolution between the data sequences with an energy of the data sequences to detect a presence of an OFDM-pilot symbol in the received OFDM- symbol.
  • the receiver is switchable between a first mode to detect the presence of an OFDM-pilot symbol and a second mode to detect the beginning of the detected OFDM- symbol .
  • the comparator of the receiver compares in the first mode the calculated convolution between two data sequences with the energy of the data sequences until a ratio between the calculated convolution and the calculated energy exceeds a configurable constant.
  • the receiver if the ratio between the calculated convolution and the calculated energy exceeds a configurable constant, the presence of an OFDM-pilot symbol is detected and the re- ceiver switches to the second mode to detect the beginning of the detected OFDM- symbol.
  • the invention further provides a transmitter for an OFDM- system comprising a modulator for modulating sub-carriers of two consecutive OFDM- symbols with two corresponding synchronization sequences stored in a memory wherein the second synchronization sequence for modulating the second OFDM- symbol is turned with respect to a first synchronization sequence for modulating the first of the two consecutive OFDM- symbols.
  • the. sub-carriers are BPSK-modulated.
  • the sub-carriers are QPSK-modulated.
  • the invention further provides a system for synchronizing OFDM-transceivers comprising
  • Embodiments of the method and system for synchronizing OFDM- transceivers are described with reference to the enclosed figures.
  • Figure 1 shows a block diagram of an OFDM-system according to the present invention
  • Figure 2 shows a block diagram of a possible embodiment of an OFDM-transmitter according to the present invention
  • Figure 3 shows a frame format comprising OFDM-symbols transmitted in the OFDM-system according to the present inven- tion;
  • Figure 4 shows a block diagram of a possible embodiment of an OFDM-receiver according to the present invention
  • Figure 5 shows a diagram for illustrating an OFDM- transceiver according to the present invention operating in a first operation mode,-
  • Figure 6 shows a diagram for illustrating an operation of an OFDM-receiver according to the present invention in a second operation mode
  • Figure 7 shows a flow chart for illustrating a process performed by an OFDM-transceiver according to the present invention.
  • the OFDM-system 1 comprises an OFDM-transmitter 2 transmitting OFDM-symbols via a communication medium 3 to an OFDM-receiver 4.
  • the communication medium 3 can be any communication medium such as air, wire or fiber.
  • the method ac- cording to the present invention is provided for synchronizing OFDM-transceivers, i.e. the OFDM-transmitter 2 and the OFDM-receiver 4.
  • FIG. 2 shows a block diagram of possible embodiment of the ODFM-transmitter 2.
  • the OFDM-transmitter 2 receives input data from a data source.
  • the transmitter 2 comprises a forward error correction coding unit 2-1 which is connected to a multiplexer 2-2.
  • a memory 2-3 a predetermined synchronization sequence is stored.
  • the memory 2-3 is formed for instance by a read-only-memory ROM.
  • the multiplexer 2-2 is connected to an output side of a slicing and mapping unit 2- 4, a Fourier transformation unit 2-5 and a front end 2-6 of the transmitter 2.
  • the Fourier transformation unit 2-5 performs in one embodiment an inverse fast Fourier transmission IFFT.
  • the front end 2-6 of the transmitter 2 can comprise a modulator, an amplifier and a transmission antenna.
  • the synchronization sequence stored in the memory 2-3 contains in one embodiment two pilot OFDM-symbols. In an alternative embodiment, one OFDM-symbol is divided in two parts. The two pilot symbols or two halves of one pilot symbol are processed by the transmitter 2 according to the present invention the same way as regular data supplied by the data source .
  • sub-carriers of two consecutive OFDM-symbols are modulated with the predetermined synchronization sequence provided for coding OFDM-pilot symbols.
  • a cyclic prefix is added and up- conversion and filtering is performed.
  • the sub-carriers of the two OFDM-pilot symbols can be modulated by BPSK or QPSK in the OFDM-transmitter 2.
  • the data used for the modulation is formed by a binary data sequence. Any data sequence with good correlation properties and fitting in one OFDM-frame can be used.
  • QPSK-modulation two synchronization sequences are employed, i. e. one synchronization sequence is used for coding a real part of the signal and a second synchronization sequence is used for coding an imaginary part of the signal.
  • Both OFDM-pilot symbols are modulated with the same binary sequence stored in the memory 2-3.
  • the binary se- quence for the second frame is turned with respect to the first frame. This means that the most significant bit MSB of the binary data sequence is used for modulation of a first sub-carrier in the first OFDM-pilot symbol and the first sub-carrier of the second OFDM-pilot symbol is modulated by the lowest significant bit LSB of the same binary data sequence .
  • Figure 3 shows a frame format of a frame comprising OFDM- symbols transmitted from the transmitter 2 to the receiver 4.
  • Each frame consists of several OFDM-symbols.
  • One OFDM- symbol is a result of a Fourier transformation to which a cyclic prefix CP is added. Accordingly, the length of one OFDM-symbol is a Fourier transformation size plus the length of the cyclic prefix CP.
  • the pilot symbol 1 and the pilot symbol 2 as shown in figure 3 are OFDM-symbols which are formed by a Fourier transformation of the synchronization sequence stored in the read-only-memory 2-3 of the transmitter 3.
  • the pilot OFDM-symbols are used for timing a frequency synchronization.
  • the remaining OFDM- symbols contain transmission data.
  • Figure 4 shows a block diagram of a possible embodiment of the OFDM-receiver 4 according to the present invention.
  • the OFDM-receiver 4 comprises an analogue/digital converter 4-1 which converts the received analog signal into a data sequence stored after optional sample rate conversion in a FIFO input buffer 4-2 of the receiver 4.
  • the output of the buffer 4-2 is connected to a multiplexer 4-3.
  • the other inputs of the multiplexer 4-3 are connected to a correcting frequency generator 4-4 and a memory 4-5 in which the in- verse Fourier transform of same the synchronization sequence is stored as in the memory 2-3 of the transmitter 2.
  • the transceiver 4 further comprises an multiplier 4-6 which continuously multiplies a converted data sequence of the received OFDM- symbols with a corresponding data sequence de- layed by a length of one OFDM-symbol to calculate multiplication products.
  • a transformation unit 7 performs a Fourier transformation of the multiplication products to calculate continuously a convolution between both data sequences.
  • the output of the transformation unit 4-7 can be fed back to the multiplexer 4-3 for calculations in a second operation mode.
  • the receiver 4 comprises a unit 4-8 for performing a conversion of rectangular to polar coordinates.
  • the unit 4-8 calculates a phase shift between two OFDM-pilot symbols.
  • a frequency shift calculation unit 4-11 calculates a frequency shift depending on the phase shift and applies the calculated frequency shift to the correction frequency generator 4-4. After passing an equalizer 4-12, the signal is forwarded to FEC and decision units.
  • Figure 5 shows a diagram for illustrating an operation within an OFDM-receiver 4 according to the present invention in a first operation mode.
  • the OFDM- receiver 4 according to the present invention is switchable between a first operation mode, i. e. a first coarse opera- tion mode and a second operation mode, i. e. a second fine operation mode.
  • the first operation mode is provided for detecting the presence of an OFDM-pilot symbol.
  • the second op- eration mode is provided for finding the start of an OFDM- pilot symbol.
  • the receiver 4 works in the first operation mode to detect a presence of an OFDM-symbol.
  • a complex multiplication product is calculated by means of the multipIicator 4-6 between the last received data sample and data sequence delayed by the length of an OFDM-symbol.
  • a data sequence A and a data sequence B which are separated from each other by the length of one OFDM-symbol in the input buffer 4-2 are multiplied with each other and the generated multiplication product is supplied to the Fourier transformation unit 4-7.
  • the calculation of the multiplication product is performed continuously.
  • the result of Fourier transformation is equal to a convolution between two source synchronization sequences stored in transmitter ROM 2-3.
  • the multiplier 4-6 is in one embodiment a complex multiplier.
  • the OFDM-symbol comprises 32 to 1024 and more sub-carriers.
  • each data sequence A, B as shown in fig- ure 5 comprises also 32 to 1024 samples.
  • the multiplier 4-6 multiplies a sample, e. g. a byte, of the data sequence A with the corresponding data sample of the other data sequence B outputting, for example 32 multiplication products to the Fourier transformation unit 4-7.
  • the receiver 4 further comprises a unit 4-9 for calculating an energy R of the received data signal.
  • the energy term can be calculated as following: n
  • r is the complex valued input data and n is changed from 1 to a number of data samples in one OFDM-symbol.
  • the OFDM-receiver 4 further comprises a comparator 4-10 which compares the continuously calculated convolution between two data sequences of input data with the result of the calculated energy of said data sequences. If the ratio between a convolution term P(d) and the energy term R(d) exceeds a configurable constant C a presence of an OFDM-pilot symbol is detected and the comparator 4-10 indicates that a time synchronization is achieved.
  • the synchronization metric function f is given by:
  • the synchronization convolution term is given by:
  • pi and p2 are two OFDM-pilot symbols stored in the synchronization sequence memory 2-3.
  • the convolution term P(d) is calculated in a possible embodiment by a fast convolution algorithm as following:
  • F is a Fourier transformation operator.
  • the transmitter 2 performs an inverse Fourier transformation of the transmitter input data so that F "1 (pld) and F "1 (p2 d ) are calculated without a requirement of additional hardware. Accordingly, in one embodiment of the system according to the present invention, a part of the corre- lation term is calculated in the transmitter 2 and a part of the correlation term is calculated in the receiver 4.
  • the OFDM-system 1 on the receiver side for cal- culating a convolution term P(d) only a multiplier 4-6 for multiplying two sequences and a Fourier transformation unit 4-7 for calculating a Fourier transform of the multiplication results is necessary. Consequently, the only necessary additional hardware is formed by the multiplier 4-6.
  • the calculated absolute value of the correlation term P(d) is compared with the energy term R(d) output by the energy calculation unit 4-9.
  • the unit 4-7 calculates an arcus tangent of the convolution term P(d) . This indicates a phase shift be- tween two pilot symbols.
  • the Fourier transformation output by the Fourier transformation unit 4-7 is divided by the energy of the signal to achieve a maximum likelihood detection (MLH) . If the ⁇ orre- lation term is higher than the scaled energy term, the presence of an OFDM-pilot symbol is detected.
  • MSH maximum likelihood detection
  • the position of samples with a maximum of magnitudes in an array of Fourier transform result points to a frequency shift in the transmission channel.
  • the shift of this sample on one position is equal to the frequency shift in the channel on reception sample rate. Accordingly, the range of a frequency shift measurement is very wide and limited by the Fourier transformation size.
  • the phase of a sample with a magnitude maximum is equivalent to the frequency shift in the transmission channel. By calculating an angle of this complex vector, it is possible to measure a frequency shift being less than a sample rate.
  • the additional hardware required for synchronization is formed by the multiplier 4-6 and can also be used for frequency correction in a data reception mode. After synchronization is achieved in the coarse first operation mode of the receiver 4, the receivers 4 switch to the second fine reception mode.
  • Figure 6 shows a diagram for illustrating an operation of an OFDM-receiver 4 in the second fine operation mode.
  • the fine operation mode is provided for detecting a start of an OFDM- pilot symbol.
  • the fine operation mode calculation is performed on a block of input data that has been detected in the coarse operation mode.
  • the fine operation mode the convolution is performed in the time domain.
  • the frequency correction by a frequency shift value is measured in the coarse operation mode which is applied on the detected block of data before performing a fine mode calculation execution.
  • the calculation in the second fine operation mode is performed only once, e. g. there is only one calculation cycle.
  • the inverse Fourier transformation is calculated on a block of input data that has been detected to contain a pilot OFDM-symbol.
  • the results of this transformation are multiplied by a synchronization sequence stored in a memory 4-5 and then, on results of this multiplication an forward Fourier transformation is calculated.
  • the receiver 4 performs in the fine operation mode a fast convolution algorithm between input data and a signal image.
  • the position of a first sample of a pilot sym- bol can be calculated from a position of a magnitude maximum in a array of these operation results.
  • the precision of this calculation is always one data sample.
  • the results of the Fourier transformation are also compared with the signal energy to lower synchronization beta errors rate.
  • the receiver 4 in one embodiment switches back to the first coarse operation mode. Otherwise, the synchronization is achieved.
  • One of the two pilot symbols can be used in one embodiment to calculate a channel frequency response in an equalizer. After that, the receiver 4 is synchronized and ready to receive data symbols.
  • the time synchronization between the receiver 4 and the transmitter 2 is achieved with precision of one data sample and the frequency synchroniza- tion is achieved with a precision that depends only on calculation precision.
  • the frequency equalization of the coefficients can also be calculated with the same OFDM-pilot symbols.
  • the OFDM- system 1 according to the present invention can reduce bandwidth and energy expenses dur- ing synchronization.
  • FIG. 7 shows a flowchart of the calculation process in the receiver 4 of the OFDM-system 1 according to the present invention.
  • the energy term calculation of the received data is performed in step Sl by the energy calculation unit 4-9 as shown in figure 5.
  • comparator 4-10 compares the calculated correlation terms received from the Fourier transformation unit 4-7 with the calculated energy term R to decide whether a ratio between the calculated convolution, i. e. a correlation term, and the calculated energy, i. e. an energy term, exceeds a configurable constant C. As long as this is not the case, the process returns to step S3.
  • a convolution term is calculated continuously by calculating multiplication products with the multiplier 4-6 performing a fast Fourier transformation by means of the transformation unit 4-7 and by calculating an absolute value of the FFT- results. If in step S2 it is detected that the correlation term exceeds the energy term by a configurable constant C, the presence of an OFDM-pilot symbol is detected and the receiver 4 switches from a coarse operation mode to a fine op- eration mode. In a further step S4, the position in the input buffer with an OFDM-pilot symbol is calculated and in step S5 an inverse fast Fourier transformation IFFT is calculated of the data at the calculated position in the input buffer.
  • step S6 the IFFT-result calculated in step S5 is multiplied with data stored in the synchronization sequence memory 4-5 of the receiver 4. Then, in a further step S7, a FFT-transformation is performed on the multiplication results and the maximum value of the calculated FFT-results is searched. The position of the maximum peak within the results of the FFT-transformation gives a precise position of time synchronization. After synchronization in step S8, the receiver 4 switches to a normal data reception mode .

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)

Abstract

L'invention concerne un système servant à synchroniser des émetteurs-récepteurs OFDM comprenant un émetteur (2) qui possède un modulateur pour moduler des sous-porteuses de deux symboles OFDM consécutifs, dans lequel les sous-porteuses des deux symboles OFDM sont modulées avec une séquence de synchronisation prédéterminée fournie pour coder des symboles pilotes OFDM, un support de communication (3) pour transmettre des symboles OFDM dudit émetteur (2) à un récepteur (4), un récepteur (4) qui possède un moyen pour convertir des symboles OFDM reçus via ledit support de communication (4) en séquences de données, un multiplicateur pour multiplier en continu une séquence de données avec une autre séquence de données qui est retardée par la longueur d'un symbole OFDM pour calculer des produits de multiplication, une unité de transformation qui réalise une transformation de Fourier des produits de multiplication pour calculer en continu une corrélation entre les séquences de données, et un comparateur pour comparer la convolution calculée entre les séquences de données à l'énergie des séquences de données pour détecter une présence d'un symbole pilote OFDM dans les symboles OFDM reçus.
PCT/RU2007/000234 2007-05-11 2007-05-11 Procédé et système servant à synchroniser des émetteurs-récepteurs ofdm Ceased WO2008140344A1 (fr)

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PCT/RU2007/000234 WO2008140344A1 (fr) 2007-05-11 2007-05-11 Procédé et système servant à synchroniser des émetteurs-récepteurs ofdm

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PCT/RU2007/000234 WO2008140344A1 (fr) 2007-05-11 2007-05-11 Procédé et système servant à synchroniser des émetteurs-récepteurs ofdm

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20020075797A1 (en) * 2000-12-15 2002-06-20 Kilani Mehdi Tavassoli Frame synchronization technique for OFDM based modulation scheme
US6438173B1 (en) * 1997-08-05 2002-08-20 Infineon Technologies Ag Multicarrier transmission system for irregular transmission of data blocks
US20040218695A1 (en) * 2003-02-19 2004-11-04 Matsushita Electric Industrial Co., Ltd. Receiving apparatus and method for digital multi-carrier transmission
US20060224650A1 (en) * 2005-03-11 2006-10-05 Cousineau Kevin S Fast fourier transform processing in an OFDM system
US7145955B1 (en) * 1999-04-23 2006-12-05 Sony Deutschland Gmbh Optimized synchronization preamble structure

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6438173B1 (en) * 1997-08-05 2002-08-20 Infineon Technologies Ag Multicarrier transmission system for irregular transmission of data blocks
US7145955B1 (en) * 1999-04-23 2006-12-05 Sony Deutschland Gmbh Optimized synchronization preamble structure
US20020075797A1 (en) * 2000-12-15 2002-06-20 Kilani Mehdi Tavassoli Frame synchronization technique for OFDM based modulation scheme
US20040218695A1 (en) * 2003-02-19 2004-11-04 Matsushita Electric Industrial Co., Ltd. Receiving apparatus and method for digital multi-carrier transmission
US20060224650A1 (en) * 2005-03-11 2006-10-05 Cousineau Kevin S Fast fourier transform processing in an OFDM system

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
SCHMIDL T M ET AL: "LOW-OVERHEAD, LOW-COMPLEXITY ŸBURST SYNCHRONIZATION FOR OFDM", 1996 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC). CONVERGING TECHNOLOGIES FOR TOMORROW'S APPLICATIONS. DALLAS, JUNE 23 - 27, 1996, IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC), NEW YORK, IEEE, US, vol. VOL. 3, 23 June 1996 (1996-06-23), pages 1301 - 1306, XP000625022, ISBN: 0-7803-3251-2 *

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