US20020001316A1 - Modulation methods and structures for wireless communication systems and transceivers - Google Patents
Modulation methods and structures for wireless communication systems and transceivers Download PDFInfo
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- US20020001316A1 US20020001316A1 US09/817,657 US81765701A US2002001316A1 US 20020001316 A1 US20020001316 A1 US 20020001316A1 US 81765701 A US81765701 A US 81765701A US 2002001316 A1 US2002001316 A1 US 2002001316A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L5/00—Arrangements affording multiple use of the transmission path
- H04L5/14—Two-way operation using the same type of signal, i.e. duplex
- H04L5/143—Two-way operation using the same type of signal, i.e. duplex for modulated signals
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L5/00—Arrangements affording multiple use of the transmission path
- H04L5/02—Channels characterised by the type of signal
- H04L5/023—Multiplexing of multicarrier modulation signals
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- Wireless communication systems have typically selected like transmission processes for both upstream and downstream carrier signals. In these systems, therefore, a single selection is chosen as a trade-off between cost and effectiveness and such trade-offs have not generally realized optimum utilizations of existing communication technologies.
- the present invention is directed to wireless communication methods and structures that enhance communication robustness while reducing cost. These goals are realized by communicating downstream data with orthogonal frequency division multiplexing (OFDM) transmission processes and upstream data with single carrier transmission processes.
- OFDM orthogonal frequency division multiplexing
- This combination of transmission processes is configured with various signal modulations (e.g., quadrature phase shift keying (QPSK), m-ary phase shift keying (MPSK) and n-quadrature amplitude modulation (QAM)) to provide lower cost upstream communication from customer services equipments (CPEs) and more robust downstream communication from headends than has been achieved in conventional communication systems.
- various signal modulations e.g., quadrature phase shift keying (QPSK), m-ary phase shift keying (MPSK) and n-quadrature amplitude modulation (QAM)
- signal diversity is enhanced by receiving communication signals with multiple antennas that are spatially separated and potentially have different polarizations to thereby enhance signal diversity.
- Signal diversity is further enhanced by combining the received signals in ways that maximize the ratio of desired to undesired signals.
- FIG. 1 is a block diagram of a wireless communication system of the present invention
- FIG. 2 is a block diagram of a headend in the communication system of FIG. 1, and
- FIG. 3 is a block diagram of a CPE in the communication system of FIG. 1.
- the present invention is directed to wireless communication methods and systems that combine OFDM transmission processes for downstream data communication and single carrier frequency hopping transmission processes for upstream data communication.
- OFDM is relatively expensive to realize but is particularly effective for non-line-of-sight communication applications because of its relative disadvantageity to multipath phase and amplitude effects and its ability to realize cost effective methods for correcting these effects at the receiver.
- single carrier frequency hopping is more cost effective, it has often been limited to line-of-sight applications.
- the present invention employs these processes in embodiments that reduce costs and enhance communication robustness. A detailed investigation of these communication methods is preceded by the following description of a system embodiment.
- a communication system 20 is shown in FIG. 1. It includes a headend 22 and a plurality (e.g., M) of customer premises equipments 24 that are associated with each of a plurality (e.g., N) hub transceivers 26 that relay communication signals between the headend and the CPEs.
- the headend 22 receives program data 27 from program sources 28 (e.g., internet data via Ethernet protocol and television data via satellite and cable).
- the headend modulates the program data onto OFDM downstream carrier signals 30 which are generally relayed via the hub transceivers 26 to the CPEs 24 .
- the CPEs generate CPE data and modulate it onto upstream single carrier signals 32 which are relayed via the hub transceivers 26 to the headend 22 .
- the headend 22 is illustrated in FIG. 2 which shows that it includes conventional termination equipment 38 and a headend transceiver 40 that receives the program data 27 into a media access controller 41 whose output is processed through an OFDM modulator 42 , a downstream upconverter 43 and a power amplifier 44 .
- the program data is thus modulated onto OFDM downstream carrier signals 30 which are radiated from an antenna 46 .
- the headend also includes a plurality of receive antennas 48 that are spatially separated and which are configured to receive upstream single carrier signals 32 with different polarizations (e.g., vertical and horizontal polarizations) as indicated by + and ⁇ polarization symbols.
- the output of these receive antennas is downconverted in upstream downconverters 50 and coupled through an adaptive equalizer and combiner 52 to a demodulator 54 which passes upstream data 55 through a forward error corrector 56 to the media access controller 40 .
- an exemplary CPE 24 includes a CPE transceiver 60 .
- the downstream carrier signals 30 are received within this transceiver by a plurality of receive antennas 61 which are spatially separated and configured to receive different signal polarizations as indicated by + and ⁇ polarization symbols.
- a selected receive antenna 61 S is preferably shared with upstream signals by steering its respective downstream signal through a diplexer 62 (as an example, the shared antenna 61 S is configured with two polarizations).
- Signals from the antennas are downconverted in downstream downconverters 63 and combined in a diversity combiner 64 before being demodulated in an OFDM demodulator 66 .
- the demodulated downstream data signals 67 are processed through a media access controller 68 and a physical layer device (PHY) 70 to a CPE data interface 71 (e.g., a personal or network computer).
- PHY physical layer device
- Upstream data signals 72 are originated by CPE users and are processed through the physical layer device 70 and the media access controller 68 to a modulator 74 where they are modulated onto an upstream single carrier signal that is upconverted in an upstream upconverter 76 and amplified in a power amplifier 78 .
- the single carrier signal 32 is steered through the diplexer 62 to be radiated from the shared antenna 60 S.
- the media access controller 41 of FIG. 2 oversees and controls various communication functions, e.g., demodulation, modulation, frequency and bandwidth selection, power ranging, program source allocation and signal combinations in the CPE diversity combiner 64 of FIG. 3.
- bundling of program data 27 at the headend 22 by the media access controller 41 presents data in a form that is usable by the CPE media access controller 68 of FIG. 3.
- Output signals from the media access controller 41 of FIG. 2 are processed in the OFDM modulator 42 with inverse Fourier transforms and error correction coding.
- the OFDM modulator provides various communication functions which include generating a variable number of subcarriers, sending continual scattered pilot signals that contain training sequences for channel estimation, providing variable guard bands and selecting various modulations (e.g., QPSK, MPSK and QAM) in each subcarrier.
- Intermediate frequency data from the OFDM modulator is then upconverted (e.g., to the 2500 megahertz range) in the downstream upconverter 43 and amplified by the power amplifier 44 which is preferably backed off its maximum amplifying capability by a significant amount to enhance its linearity.
- the upconverted and amplified downstream carrier signals 30 are then broadcast via the antenna 46 .
- the downstream carrier signals are received, at each CPE 24 with the receive antennas 61 of FIG. 3 that are spatially separated and have different polarizations.
- the spatial differences provide reception time differences and, thereby, phase differences in the received downstream signals. These factors (i.e., polarization and phase) are the dominant elements in achieving decorrelation in received signals that enhances signal quality of the final received signal.
- the downstream carrier signals are then downconverted in FIG. 3 by each antenna's respective downconverter 62 and combined in the diversity combiner 64 which scales (amplifies or attenuates), delays, and adds the downconverted signals in a way that maximizes the ratio of desired to undesired signals for the signal to be demodulated and thus minimizes the error rate.
- the demodulated signal will be passed to the media access controller 68 by the OFDM demodulator 66 .
- the diversity combiner 64 also processes continual and scattered pilot signals that contain training sequences for channel estimation.
- the OFDM demodulator 66 processes the downconverted signals with fast Fourier transforms and forward error correction.
- Conventional OFDM modulators are capable of demodulating a variable number of subcarriers, working with variable channel guard bands and providing the necessary demodulation (e.g., QPSK, MPSK or QAM) in each subcarrier.
- downstream data is channel processed in the media access controller 68 and output to an appropriate PHY 70 which further transforms the downstream data 67 into a format and voltage level that is usable by the CPE data interface 71 .
- the OFDM transmission processes in the downstream communication of FIGS. 1 - 3 essentially divide a given channel (e.g., a 6 megahertz channel) into a large number (e.g., 200 to 8000) of subchannels. Each subchannel contains a fraction of the original channel information and is isolated by temporal guard bands (e.g., several kilohertz or ⁇ fraction (1/32) ⁇ to 1 ⁇ 4 of the data symbol rate) from subsequent symbols to thereby decrease multipath effects on the downstream communication.
- temporal guard bands e.g., several kilohertz or ⁇ fraction (1/32) ⁇ to 1 ⁇ 4 of the data symbol rate
- This CPE data is directed through the PHY 70 to the media access controller 68 which bundles the data in a form that is usable by the headend transceiver ( 22 in FIG. 2).
- the media access controller 68 performs other upstream data communication functions, e.g., overseeing and controlling demodulation, modulation, frequency and bandwidth selection, and power ranging.
- the CPE data is then modulated (e.g., with QPSK or QAM) by the modulator 74 which preferably operates in a frequency hopping single carrier transmission mode.
- the frequency is hopped between channels under control of the media access controller 68 to reduce dispersive effects of the weather and terrain (e.g., rain and foliage).
- the intermediate frequency signals are upconverted (e.g., to the 2500 megahertz range) in the upstream upconverter 76 and amplified in the power amplifier 78 . Because single frequency modulation is used for upstream communication, this power amplifier need not be as linear (and therefore not as expensive) as is preferred for the OFDM of the downstream communications. Finally, the diplexer 62 couples the power amplifier 78 to the shared antenna 61 S so that the upstream single carrier signal 32 is broadcast to the head end transceiver ( 22 in FIG. 2) via a respective hub transceiver ( 26 in FIG. 1).
- the upstream single carrier signals 32 are received in the receive antennas 48 that are spatially separated and configured to receive signals having different polarizations.
- the upstream single carrier signals are then combined in the adaptive equalizer and combiner 52 which varies the order of equalizing and combining to that which is the most effective for error reduction and which will scale (amplify or attenuate), delay, and/or add the downconverted signals to maximize the ratio of desired to undesired signals.
- the adaptive equalization methods of the adaptive equalizer and combiner 52 are directed by the media access controller 41 to minimize the probability of data errors by the use of different algorithms (e.g., decision-directed equalization, filter output computation based upon training and transversal filter storage, transversal filter coefficient adaptation, zero-forcing equalization which starts with the sinc function (sin ⁇ t/ ⁇ t) and solves n simultaneous equations, least mean squares in which transversal filters are gradually adjusted to converge to a filter that minimizes the error between the equalized data word and a stored reference header word, decision feedback equalization and recursive least squares).
- the adaptive equalizer is preferably programmed in a digital signal processor or similar flexible state machine architecture that is optimized for implementing algorithms.
- the signal is demodulated (e.g., with QPSK or QAM demodulation) to baseband by the single carrier demodulator 54 .
- the frequency selection for the frequency hopped carrier signal 32 is controlled by the media access controller 41 to minimize dispersive effects of the weather and terrain (e.g., rain and foliage).
- the CPE data is then processed with various algorithms (e.g., Viterbi and Reed-Solomon) in the forward error corrector 56 to further reduce errors and is then coupled to the media access controller 41 for realizing various customer needs.
- various algorithms e.g., Viterbi and Reed-Solomon
- the downstream data is communicated with the aid of OFDM transmission processes and the upstream data is communicated with the aid of single carrier transmission processes.
- the present invention thus utilizes the better non line-of-sight capabilities of OFDM processes for headend and hub transmissions where relatively few equipment installations are required and single carrier transmission processes for the far more numerous CPE installations.
- the system thus limits the number of expensive installations because it has only one headend ( 22 in FIG. 1) and its hub transceivers ( 26 in FIG. 1) are also of limited quantity.
- the expensive linear power amplifiers ( 44 in FIG. 2) that are required for OFDM processes are used only in the headend and the hubs.
- the receiver portion of the headend transceiver can also be configured to be more sensitive (and thus more expensive) because it is only used once in the communication system 20 .
- the single carrier for upstream communication is preferably frequency hopped for each transmission burst and received at the headend with multiple antennae that differ spatially and have different polarizations to thereby compensate for the conventional lack of robustness of this method.
- the degrading effects of various link characteristics e.g., multipath and frequency fading are thereby mitigated.
- the CPE transceiver ( 60 in FIG. 3) communicates to the headend (e.g., via acknowledgement handshaking protocols) which of a set of substantially separated frequency channels will receive the downstream data of various bandwidth requirements.
- the CPE transceiver also communicates which of a set of substantially separated frequency hopping channels it will use to transmit the upstream data (which may also have various bandwidth requirements) to thereby achieve frequency diversity by means of frequency hopping across a substantially separated set of available frequency channels.
- frequency diversity is realized downstream with channel selection across a set of substantially separated frequency channels and is realized upstream by frequency hopping across a substantially separated set of available channels.
- the available frequency is divided such that the downstream and upstream channels assigned to each CPE are interleaved with those assigned to CPEs in other sectors or cells.
- some communication channels are not available to the CPEs of a particular sector or cell.
- initial communication contact begins with a predetermined order of available channels, so that communication links can be established for initial contact between the headend and the CPEs.
- the order and sequencing is preferably part of a look-up table that is programmed into signal computers of the headend and CPE media access controllers ( 41 in FIG. 2 and 68 in FIG. 3).
- Communication components that have been described above (e.g., media access controllers, diversity combiners and physical layer devices) are conventional and easily obtained in varying degrees of complexity. Modulation methods of the invention have been disclosed above to include n-quadrature amplitude modulation. Exemplary values for n are 4, 16, 32, 64, 128, 256, 512 and 1024.
Abstract
Description
- This application claims the benefit of U.S. Provisional application Ser. No. 60/214,894 filed Jun. 29, 2000.
- Wireless communication systems have typically selected like transmission processes for both upstream and downstream carrier signals. In these systems, therefore, a single selection is chosen as a trade-off between cost and effectiveness and such trade-offs have not generally realized optimum utilizations of existing communication technologies.
- The present invention is directed to wireless communication methods and structures that enhance communication robustness while reducing cost. These goals are realized by communicating downstream data with orthogonal frequency division multiplexing (OFDM) transmission processes and upstream data with single carrier transmission processes.
- This combination of transmission processes is configured with various signal modulations (e.g., quadrature phase shift keying (QPSK), m-ary phase shift keying (MPSK) and n-quadrature amplitude modulation (QAM)) to provide lower cost upstream communication from customer services equipments (CPEs) and more robust downstream communication from headends than has been achieved in conventional communication systems.
- In system embodiments, signal diversity is enhanced by receiving communication signals with multiple antennas that are spatially separated and potentially have different polarizations to thereby enhance signal diversity. Signal diversity is further enhanced by combining the received signals in ways that maximize the ratio of desired to undesired signals.
- The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings
- FIG. 1 is a block diagram of a wireless communication system of the present invention;
- FIG. 2 is a block diagram of a headend in the communication system of FIG. 1, and
- FIG. 3 is a block diagram of a CPE in the communication system of FIG. 1.
- The present invention is directed to wireless communication methods and systems that combine OFDM transmission processes for downstream data communication and single carrier frequency hopping transmission processes for upstream data communication. OFDM is relatively expensive to realize but is particularly effective for non-line-of-sight communication applications because of its relative impunity to multipath phase and amplitude effects and its ability to realize cost effective methods for correcting these effects at the receiver. Although single carrier frequency hopping is more cost effective, it has often been limited to line-of-sight applications. The present invention employs these processes in embodiments that reduce costs and enhance communication robustness. A detailed investigation of these communication methods is preceded by the following description of a system embodiment.
- A
communication system 20 is shown in FIG. 1. It includes a headend 22 and a plurality (e.g., M) ofcustomer premises equipments 24 that are associated with each of a plurality (e.g., N)hub transceivers 26 that relay communication signals between the headend and the CPEs. Theheadend 22 receivesprogram data 27 from program sources 28 (e.g., internet data via Ethernet protocol and television data via satellite and cable). - With its termination equipment and transceivers, the headend modulates the program data onto OFDM
downstream carrier signals 30 which are generally relayed via thehub transceivers 26 to theCPEs 24. The CPEs generate CPE data and modulate it onto upstreamsingle carrier signals 32 which are relayed via thehub transceivers 26 to theheadend 22. - The
headend 22 is illustrated in FIG. 2 which shows that it includesconventional termination equipment 38 and aheadend transceiver 40 that receives theprogram data 27 into amedia access controller 41 whose output is processed through anOFDM modulator 42, adownstream upconverter 43 and apower amplifier 44. The program data is thus modulated onto OFDMdownstream carrier signals 30 which are radiated from anantenna 46. - The headend also includes a plurality of receive
antennas 48 that are spatially separated and which are configured to receive upstreamsingle carrier signals 32 with different polarizations (e.g., vertical and horizontal polarizations) as indicated by + and − polarization symbols. The output of these receive antennas is downconverted inupstream downconverters 50 and coupled through an adaptive equalizer and combiner 52 to ademodulator 54 which passes upstreamdata 55 through aforward error corrector 56 to themedia access controller 40. - As shown in FIG. 3, an
exemplary CPE 24 includes aCPE transceiver 60. Thedownstream carrier signals 30 are received within this transceiver by a plurality of receiveantennas 61 which are spatially separated and configured to receive different signal polarizations as indicated by + and − polarization symbols. A selectedreceive antenna 61S is preferably shared with upstream signals by steering its respective downstream signal through a diplexer 62 (as an example, the sharedantenna 61S is configured with two polarizations). - Signals from the antennas are downconverted in
downstream downconverters 63 and combined in a diversity combiner 64 before being demodulated in anOFDM demodulator 66. The demodulateddownstream data signals 67 are processed through amedia access controller 68 and a physical layer device (PHY) 70 to a CPE data interface 71 (e.g., a personal or network computer). -
Upstream data signals 72 are originated by CPE users and are processed through thephysical layer device 70 and themedia access controller 68 to amodulator 74 where they are modulated onto an upstream single carrier signal that is upconverted in anupstream upconverter 76 and amplified in apower amplifier 78. Thesingle carrier signal 32 is steered through thediplexer 62 to be radiated from the shared antenna 60S. - Having described the basic structures of the
communication system 20 of FIGS. 1-3, attention is now directed to operation of the system. In downstream data communication, themedia access controller 41 of FIG. 2 oversees and controls various communication functions, e.g., demodulation, modulation, frequency and bandwidth selection, power ranging, program source allocation and signal combinations in the CPE diversity combiner 64 of FIG. 3. In addition, bundling ofprogram data 27 at theheadend 22 by themedia access controller 41 presents data in a form that is usable by the CPEmedia access controller 68 of FIG. 3. - Output signals from the
media access controller 41 of FIG. 2 are processed in theOFDM modulator 42 with inverse Fourier transforms and error correction coding. The OFDM modulator provides various communication functions which include generating a variable number of subcarriers, sending continual scattered pilot signals that contain training sequences for channel estimation, providing variable guard bands and selecting various modulations (e.g., QPSK, MPSK and QAM) in each subcarrier. - Intermediate frequency data from the OFDM modulator is then upconverted (e.g., to the 2500 megahertz range) in the
downstream upconverter 43 and amplified by thepower amplifier 44 which is preferably backed off its maximum amplifying capability by a significant amount to enhance its linearity. The upconverted and amplifieddownstream carrier signals 30 are then broadcast via theantenna 46. - After relaying by hub transceivers (26 in FIG. 1), the downstream carrier signals are received, at each
CPE 24 with the receiveantennas 61 of FIG. 3 that are spatially separated and have different polarizations. The spatial differences provide reception time differences and, thereby, phase differences in the received downstream signals. These factors (i.e., polarization and phase) are the dominant elements in achieving decorrelation in received signals that enhances signal quality of the final received signal. - The downstream carrier signals are then downconverted in FIG. 3 by each antenna's
respective downconverter 62 and combined in the diversity combiner 64 which scales (amplifies or attenuates), delays, and adds the downconverted signals in a way that maximizes the ratio of desired to undesired signals for the signal to be demodulated and thus minimizes the error rate. The demodulated signal will be passed to themedia access controller 68 by theOFDM demodulator 66. The diversity combiner 64 also processes continual and scattered pilot signals that contain training sequences for channel estimation. - The
OFDM demodulator 66 processes the downconverted signals with fast Fourier transforms and forward error correction. Conventional OFDM modulators are capable of demodulating a variable number of subcarriers, working with variable channel guard bands and providing the necessary demodulation (e.g., QPSK, MPSK or QAM) in each subcarrier. - Finally, the downstream data is channel processed in the
media access controller 68 and output to anappropriate PHY 70 which further transforms thedownstream data 67 into a format and voltage level that is usable by theCPE data interface 71. - The OFDM transmission processes in the downstream communication of FIGS.1-3 essentially divide a given channel (e.g., a 6 megahertz channel) into a large number (e.g., 200 to 8000) of subchannels. Each subchannel contains a fraction of the original channel information and is isolated by temporal guard bands (e.g., several kilohertz or {fraction (1/32)} to ¼ of the data symbol rate) from subsequent symbols to thereby decrease multipath effects on the downstream communication.
- Attention is now directed to the upstream data communication of the
communication system 20 of FIGS. 1-3 which begins with user-generatedCPE data 72 at aCPE data interface 71 of eachCPE 24. - This CPE data is directed through the
PHY 70 to themedia access controller 68 which bundles the data in a form that is usable by the headend transceiver (22 in FIG. 2). In addition, themedia access controller 68 performs other upstream data communication functions, e.g., overseeing and controlling demodulation, modulation, frequency and bandwidth selection, and power ranging. - The CPE data is then modulated (e.g., with QPSK or QAM) by the
modulator 74 which preferably operates in a frequency hopping single carrier transmission mode. In particular, the frequency is hopped between channels under control of themedia access controller 68 to reduce dispersive effects of the weather and terrain (e.g., rain and foliage). - After the data is modulated, the intermediate frequency signals are upconverted (e.g., to the 2500 megahertz range) in the
upstream upconverter 76 and amplified in thepower amplifier 78. Because single frequency modulation is used for upstream communication, this power amplifier need not be as linear (and therefore not as expensive) as is preferred for the OFDM of the downstream communications. Finally, thediplexer 62 couples thepower amplifier 78 to the sharedantenna 61S so that the upstreamsingle carrier signal 32 is broadcast to the head end transceiver (22 in FIG. 2) via a respective hub transceiver (26 in FIG. 1). - In the
headend transceiver 22 of FIG. 2, the upstream single carrier signals 32 are received in the receiveantennas 48 that are spatially separated and configured to receive signals having different polarizations. The upstream single carrier signals are then combined in the adaptive equalizer andcombiner 52 which varies the order of equalizing and combining to that which is the most effective for error reduction and which will scale (amplify or attenuate), delay, and/or add the downconverted signals to maximize the ratio of desired to undesired signals. - In particular, the adaptive equalization methods of the adaptive equalizer and
combiner 52 are directed by themedia access controller 41 to minimize the probability of data errors by the use of different algorithms (e.g., decision-directed equalization, filter output computation based upon training and transversal filter storage, transversal filter coefficient adaptation, zero-forcing equalization which starts with the sinc function (sinπt/πt) and solves n simultaneous equations, least mean squares in which transversal filters are gradually adjusted to converge to a filter that minimizes the error between the equalized data word and a stored reference header word, decision feedback equalization and recursive least squares). To facilitate this algorithm processing, the adaptive equalizer is preferably programmed in a digital signal processor or similar flexible state machine architecture that is optimized for implementing algorithms. - After equalization, the signal is demodulated (e.g., with QPSK or QAM demodulation) to baseband by the
single carrier demodulator 54. If frequency hopped, the frequency selection for the frequency hoppedcarrier signal 32 is controlled by themedia access controller 41 to minimize dispersive effects of the weather and terrain (e.g., rain and foliage). - The CPE data is then processed with various algorithms (e.g., Viterbi and Reed-Solomon) in the
forward error corrector 56 to further reduce errors and is then coupled to themedia access controller 41 for realizing various customer needs. - In operation of the
communication system 20 of FIGS. 1-3, therefore, the downstream data is communicated with the aid of OFDM transmission processes and the upstream data is communicated with the aid of single carrier transmission processes. The present invention thus utilizes the better non line-of-sight capabilities of OFDM processes for headend and hub transmissions where relatively few equipment installations are required and single carrier transmission processes for the far more numerous CPE installations. - The system thus limits the number of expensive installations because it has only one headend (22 in FIG. 1) and its hub transceivers (26 in FIG. 1) are also of limited quantity. In an exemplary cost reduction, the expensive linear power amplifiers (44 in FIG. 2) that are required for OFDM processes are used only in the headend and the hubs. The receiver portion of the headend transceiver can also be configured to be more sensitive (and thus more expensive) because it is only used once in the
communication system 20. - The single carrier for upstream communication is preferably frequency hopped for each transmission burst and received at the headend with multiple antennae that differ spatially and have different polarizations to thereby compensate for the conventional lack of robustness of this method. The degrading effects of various link characteristics (e.g., multipath and frequency fading) are thereby mitigated.
- These link characteristics determine how well a particular communication signal is received. In a method of the invention, the CPE transceiver (60 in FIG. 3) communicates to the headend (e.g., via acknowledgement handshaking protocols) which of a set of substantially separated frequency channels will receive the downstream data of various bandwidth requirements.
- The CPE transceiver also communicates which of a set of substantially separated frequency hopping channels it will use to transmit the upstream data (which may also have various bandwidth requirements) to thereby achieve frequency diversity by means of frequency hopping across a substantially separated set of available frequency channels. Thus frequency diversity is realized downstream with channel selection across a set of substantially separated frequency channels and is realized upstream by frequency hopping across a substantially separated set of available channels.
- In a sectorized or cellular embodiment of the invention, the available frequency is divided such that the downstream and upstream channels assigned to each CPE are interleaved with those assigned to CPEs in other sectors or cells. In this system embodiment, some communication channels are not available to the CPEs of a particular sector or cell.
- In a method of the invention, therefore, initial communication contact begins with a predetermined order of available channels, so that communication links can be established for initial contact between the headend and the CPEs. The order and sequencing is preferably part of a look-up table that is programmed into signal computers of the headend and CPE media access controllers (41 in FIG. 2 and 68 in FIG. 3).
- Communication components that have been described above (e.g., media access controllers, diversity combiners and physical layer devices) are conventional and easily obtained in varying degrees of complexity. Modulation methods of the invention have been disclosed above to include n-quadrature amplitude modulation. Exemplary values for n are 4, 16, 32, 64, 128, 256, 512 and 1024.
- The preferred embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.
Claims (25)
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