WO2002035857A1 - Reseau a acces multiples et procede pour systemes hertziens numeriques - Google Patents

Reseau a acces multiples et procede pour systemes hertziens numeriques Download PDF

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Publication number
WO2002035857A1
WO2002035857A1 PCT/US2001/046692 US0146692W WO0235857A1 WO 2002035857 A1 WO2002035857 A1 WO 2002035857A1 US 0146692 W US0146692 W US 0146692W WO 0235857 A1 WO0235857 A1 WO 0235857A1
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node
user terminal
signals
multiple access
user
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PCT/US2001/046692
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English (en)
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WO2002035857A8 (fr
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Peter Monsen
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Peter Monsen
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Priority to AU2002228827A priority Critical patent/AU2002228827A1/en
Priority to US10/399,706 priority patent/US20040004945A1/en
Publication of WO2002035857A1 publication Critical patent/WO2002035857A1/fr
Publication of WO2002035857A8 publication Critical patent/WO2002035857A8/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0408—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas using two or more beams, i.e. beam diversity

Definitions

  • This invention relates generally to multiple access communication in a network of digital radio systems, and more particularly to improvements in the multiple access communication of fixed remote user terminals and/or mobile user terminals associated with central nodes having antennas with one or more antenna beams.
  • a network of multiple access radio systems provides communication services for fixed remote user terminals and/ or mobile user terminals.
  • Examples of multiple access radio networks include land mobile radio networks, cellular mobile radio networks, and wideband radio networks between fixed subscribers and one or more central nodes, which may use a multibeam antenna for increasing system capacity and improving communications quality.
  • the reverse link or uplink in a multiple access radio system is a communications link between a fixed remote or mobile user terminal and a central node, which can be located at either a fixed location on the Earth in a terrestrial radio system or in a satellite repeater/ ground station in a satellite radio system.
  • a network of multiple access radio systems use many central nodes or base stations in order to cover a large geographical area.
  • the central nodes are connected together with fixed communication links, i.e., telephone lines, fiber optic cable, high speed radio links, etc.; for purposes of coordinating central node operations in the network.
  • the interconnection between central nodes may also be used to support macrodiversity, which is a large scale form of space diversity. For an uplink transmission there are multiple diversity paths to different central nodes.
  • a macrodiversity system selects the best diversity path signal to reduce fading effects based on quality measurements that are transmitted over the fixed communication links.
  • Digital radio systems transmit and receive digital message information, e.g., computer or Internet data.
  • digital radio systems accept analog message information, e.g., voice or video data, and convert this analog information to a digital format during transmission and reception.
  • a fixed remote or mobile user terminal transmits message information in a digital format using an uplink to a central node, where one or more antennas and an associated receiver process received signals to extract user message information.
  • the receiver processing is divided between a satellite repeater and a ground-based station processor.
  • the antenna at a central node may be fixed with either a single or multiple beams providing coverage for one or more beam regions associated with the central node.
  • the antenna at the central node may be multibeam and adaptive thereby producing multiple beam regions that adapt the beam characteristics to the group of user terminal locations associated with the beam region. Because of the overlap of antenna beam patterns, interference can occur between user terminals in different beams of the same central node and between user terminals in beams of different central nodes. In general for each central node and antenna beam there is an associated beam/node region. User terminals located within the same beam/ node region generally avoid mutual interference through the use of some form of multiple access scheme. Conventional multiple access radio services use Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), or some combination thereof.
  • FDMA Frequency Division Multiple Access
  • TDMA Time Division Multiple Access
  • CDMA Code Division Multiple Access
  • FDMA separates users into different frequency subbands
  • TDMA separates users into different time intervals or slots
  • CDMA separates users by assigning different signature waveforms or spreading codes to each user.
  • CDMA spreading codes can be either orthogonal, i.e., there is no interference between synchronized users, or quasi-orthogonal, i.e., there is some small interference between users.
  • FDMA and TDMA are orthogonal multiple access (OMA) schemes because with ideal frequency filters and synchronization there is no mutual interference.
  • OMA orthogonal multiple access
  • Another example of an OMA system is CDMA with orthogonal spreading codes.
  • Quasi-Orthogonal Multiple Access (QOMA) systems include CDMA with quasi-orthogonal spreading codes and FDMA/TDMA with randomized frequency hopping.
  • the multiple access channels are usually assigned by a centralized controller which may make assignments for a group of beam/node coverage regions.
  • the assignments to the user terminals are normally transmitted in time division with downlink message information. After synchronization, user terminals can extract the channel assignment data from the downlink messages.
  • an OMA scheme generally provides a larger system capacity than a QOMA scheme.
  • QOMA schemes for reducing interference between users to acceptable levels.
  • Interference between a user in one beam/node region and users in other beam/node regions is normally reduced by distance and antenna beam discrimination resulting in cross-channel attenuation.
  • cross-channel attenuation usually does not reduce interference enough to allow the reuse of the same orthogonal waveform or channel in adjacent beam/node regions.
  • channel management is typically required for determining when a multiple access channel can be reused in another beam/node region depending on an allowable threshold of the user/beam cross-channel attenuation. This leads to a reuse factor that is less than 1.
  • the reuse factor of a multiple access channel is defined as the number of user terminal assignments to that channel in different beam/node coverage regions divided by the total number of beam/node coverage regions. Because the capacity of a multiple access system is proportional to the average value of the reuse factor with respect to all the multiple access channels, it is desirable to make the reuse factor for each multiple access channel as large as possible subject to interference constraints. Distance, attenuation, and practical limitations on multibea antennas typically cause the reuse factor in cellular OMA systems to vary between 1/3 and 1/ 12. Thus, in known systems, the assignment of channels to users is restricted by distance and attenuation considerations.
  • a multiple access communication system or method not so constrained is characterized herein as "unrestricted".
  • Patent Application PCT/USOO/ 12802 filed 11 May 2000, a system and method is disclosed for achieving a unity reuse factor in an OMA system associated with a central node and its associated multibeam antenna.
  • the unity reuse factor is achieved with a combination of error-correction coding/decoding, interleaving/ deinterleaving, single axis modulation, and interference processing at the central node.
  • Patent Application PCT/US00/ 12802 does not disclose techniques for achieving unity reuse for all beam/node regions in systems with multiple central nodes that may have either omnidirectional or multibeam antennas.
  • the reuse factor can be unity because the combination of user/beam cross-channel attenuation and spreading code interference protection is sufficient to keep mutual interference between users in different beam/node regions to adequately small levels.
  • a QOMA radio system generally has a theoretical capacity that is less than that of an OMA radio system.
  • the reference signal is generated at both the user terminal and the central node and used by the central node receiver for obtaining channel parameters to aid in demodulating a user signal.
  • the insertion of a known reference signal in time multiplex with the transmitted message information is described in "An Adaptive Receiver for Digital Signaling through Channels with Intersymbol Interference", J.G. Proa s and J.H. Miller, IEEE Transactions on Information Theory, vol. IT- 15, No. 4, July 1969 and in U.S. Patent 4,365,338.
  • Error-correction coding adds redundancy to message information in a prescribed manner so that transmission errors may be corrected with a decoder at the receiver.
  • the purpose of the interleaver/deinterleaver is to randomize these transmission errors at the decoder input so as to make the decoder more capable of correcting them.
  • Quadrature transmission is an example of a multisymbol signaling scheme, wherein pluralities of successive binary digits of user data are combined to form symbols to be transmitted. Such multisymbol signaling schemes are typically used to reduce the bandwidth required to transmit the user data.
  • Quadrature amplitude modulation is an example of a general multisymbol signaling scheme, wherein multilevel amplitude modulation is applied separately on each of the two quadrature carriers.
  • Macrodiversity Scheme the uplink signals are detected at each central node and signal decisions are sent to a central point where diversity. combining is accomplished.
  • Macrodiversity Scheme it is recognized that better diversity combining can be achieved with distribution of received signals over the fixed network to the central point but at a cost of larger fixed network traffic.
  • the use of received signals rather than decisions at the central point was considered by Macrodiversity Scheme as "future research", pg. 507, 2 nd paragraph. This future research area would include methods of synchronizing the signals at the central point and methods of combining as well as cancellation of undesired signals.
  • a macrodiversity system that combines received signals must compensate for changes in other user interference delays when user locations change as a result of a modulation-hopping scheme. Moreover the synchronization or lack there of between central node clocks will effect combining techniques in a macrodiversity scheme.
  • Some conventional digital radio systems use adaptive equalizers for combining multipath signals and reducing intersymbol interference.
  • Adaptive equalizers have also been proposed for use with a multibeam receiver for reducing interference from other users.
  • MMSB Equalization of Interference on Fading Diversity Channels Peter Monsen, IEEE Conference on Communications, Conference Record, Vol. 1, Denver, CO, June 1981, pp. 12.2-1-12.2-5, describes an adaptive equalizer that combines multipath signals and reduces intersymbol and other user interference. The total interference is included in an error signal whose mean square value is minimized. Transmission of a time division multiplexed reference that is known at the receiver is also described.
  • US Patent Nos. 4,112,370 and 4,644,562 disclose the cancellation of interference in multibeam antennas as a generalization of the cancellation of interference in dual-polarized antennas. .
  • US Patent No. 5,680,419 discloses adaptive sequence estimation techniques that can be used with a multibeam antenna for canceling interference.
  • Adaptive Equalization and Interference Cancellation for Wireless Communication Systems B.C.W. Lo and K.B. Letaief, IEEE Trans. Comm., vol. 47, no. 4, April 1999, pp. 538-545 discloses in a multiantenna application a maximum likelihood sequence estimation technique that uses a reference signal of the desired user in order to detect a user signal in the presence of intersymbol interference and other user interference.
  • the equalizer is generally preferred because it is not as complex as the sequence estimator.
  • Another object of the invention is to provide a multiple access communication network and method that is orthogonal in each beam/node coverage region and has a channel capacity greater than that of conventional quasi-orthogonal multiple access communication systems.
  • Still another object of the invention is to provide an orthogonal multiple access communication network and method that has a reuse factor of unity where there are multiple central nodes with time clocks that may not be mutually synchronous.
  • a multiple access communication network including a plurality of user terminals, each including a user terminal transmitter; and a plurality of central nodes each including an antenna and associated receiver for receiving digital message information transmitted by the user terminal transmitters.
  • central nodes are connected together with fixed network of communication links, e.g. telephone lines, fiber optic cable, high speed radio links, etc., for purposes of coordinating central node operation and also perforrning remote signal processing.
  • the antenna at the central node may be omnidirectional, i.e. a single beam provides uplink coverage, or multibeam, either fixed or adaptive, resulting in beam/node coverage regions or sectors for that central node.
  • Those user terminal transmitters associated with a beam/node coverage region employ mutually orthogonal multiple access waveforms.
  • User transmitter terminals associated with other beam/node coverage regions can reuse the same orthogonal waveforms from the mutually orthogonal waveform set.
  • the user terminal transmitter includes a coding unit for providing error-correction coding of the digital message information, an interleaving unit for distributing the error-correction coded message information, a multiplexer for multiplexing user reference signals with the error-correction coded message information, and a modulator for modulating the multiplexed signal for subsequent transmission as a respective multiple access signal, wherein each of the respective multiple access signals associated with a beam/node coverage region employ an orthogonal waveform from a mutually orthogonal waveform set.
  • An assignment controller that may be colocated with a central node or distributed in the network provides channel assignments from the mutually orthogonal set. These channel assignments are sent to the user terminal by a downlink radio link.
  • the modulator in the user terminal transmitter may also modify the radio frequency (RF) characteristics corresponding to a group or packet of multiplexed data containing at least one reference signal subblock.
  • RF radio frequency
  • each user is modulation hopped to a new multiple access channel for transmission of the packet.
  • the radio frequency modification can be omitted because user terminal motion provides a similar effect.
  • an uplink receiver includes an antenna for receiving respective multiple access signals from the user terminal transmitters in each of the one or more antenna beams resulting in a plurality of beam/node signals for the present central node, one or more link receivers for receiving beam/node signals received at an adjacent central node and retransmitted over the fixed network connecting the central nodes, an adaptive processor for each user that processes beam/node signals and the reference signals to combine the beam/node signals and reduce other user interference, and a deinterleaver and decoder for each user to recover the digital message information from the combined signal.
  • a delay compensator adjusts the delays of beam/node signals from the present central node and beam/node signals from the adjacent central nodes to time align the received signal packets.
  • Modulation hopping randomizes the other user interference, which after the deinterleaving/ decoding operation averages the effects of other user interference.
  • the error-correction coded information is interleaved before transmission so that after deinterleaving at the receiver, the other user interference associated with successive error-correction coded symbols is different.
  • the interleaving thus improves the error-correcting capability of the decoder and increases the interference protection.
  • every user may employ a unique reference signal so that the receiver can extract channel information for that user by generating a replica of the unique reference signal and processing it with the received beam/node signals. Since users within the same beam/node coverage region are assigned channels from a mutually orthogonal set, these users may employ the same reference signal.
  • the adaptive processor is a multiuser decision-feedback equalizer (MDFE) that minimizes a mean square error by solving a set of simultaneous equations for each received data group. The solution of these equations provides the processor settings for that received group.
  • the user reference signal for. a respective bea /node coverage region and user reference signals corresponding to interfering beams are employed to determine the adaptive processor parameter settings in this embodiment.
  • the MDFE provides diversity combining of signals and reduction of other user interference. In doing so it compensates for both multipath effects between user terminals and antennas as well as path delay differences due to changing interference effects.
  • the MDFE can be used with both synchronous and asynchronous central nodes.
  • FIG. 1 is a diagram of beam/node regions in a multiple access network according to the present invention with multiple central nodes some of which contain multibeam antennas;
  • FIG. 2 is a diagram of a packet for transmission in the communication network according to the present invention;
  • FIG. 3 is a functional block diagram of a central node connected to an assignment controller according to the present invention.
  • FIG. 4 is a functional block diagra of a user terminal transmitter in the communication network of the present invention;
  • FIG. 5 is a functional block diagram of a multiuser detector at a central node according to the present invention; and
  • FIG. 6 is a diagram of received packet intervals for multiple central nodes with asynchronous timing clocks.
  • FIG. 1 shows an uplink multiple access network with a plurality of beam/node regions 11 within which user terminal transmitters 12 are assigned to central nodes 13, i,e, base stations, for purposes of transmitting message information from a user terminal to its assigned central node 13.
  • Each central node 13 may have one or more associated beam/node regions.
  • Omnidirectional coverage and a single beam region per central node are shown in FIG. 1 as region HE, node 13B and region 11D, node 13C.
  • the use of multibeam antennas can provide multiple beam/node regions or sectors, HA, 11B, and 11C associated with a single central node 13A.
  • the beam/node coverage regions typically overlap because antenna beams do not have ideal cutoff transitions.
  • a user terminal transmitter 12 is usually assigned to a user terminal beam/node coverage region 11 depending on geographic location and antenna beam response characteristics.
  • the central nodes 13 are connected together with fixed communiation links 14 that are generally used to coordinate central node operations in the network. These links may be dedicated telephone lines, fiber optic cable, or high speed radio circuits, all of which can generally support high transmission rates.
  • a fixed communication link 14AB connects central nodes 13A and 13B
  • link 14BC connects nodes 13B and 13C
  • link 14AC connects nodes 13A and 13C.
  • Central nodes 13 contain timing clocks (not shown) that are used to provide timing for digital transmission of information.
  • the central node clocks are maintained in mutual synchronism. This is a preferred choice for this invention because it minimizes receiver complexity at the central node.
  • an embodiment of the invention is described wherein the achievement of a unity reuse factor is obtained.
  • user terminal transmitters 12 with the same channel assignment but in different beam/node regions will interfere with each other at central node 13 locations.
  • user terminal transmitter 12A in sector HA will have an uplink transmission to central node 13A but this transmission may be interfered with by transmissions to other central nodes, e.g. 13B and 13C, and transmissions to other beams at central node 13A, e.g. from sectors HB and HC.
  • user terminal transmitters 12C and 12D are in the same beam/node region so they are assigned mutually orthogonal channels.
  • user terminal transmitters 12A, 12B, 12D, 12E, and 12F in the present invention can all be assigned the same orthogonal channel even though there may be significant cross interference.
  • interference from 12E and 12F (indicated by dotted lines) is reduced by distance effects and interference from 12B and 12D is reduced by antenna discrimination. In conventional systems this interference reduction is usually insufficient to allow this set of user terminal transmitters to all use the same orthogonal channel.
  • FIG. 2 shows the transmission format for a packet 15 of user message information to be sent over a reverse link or an uplink in a multiple access radio system between a fixed remote user terminal or a mobile user terminal and a central node, which may be located at either a fixed location on the Earth in a terrestrial radio system or in a satellite repeater/ ground station in a satellite radio system.
  • the packet 15 includes a user reference signal that is a block of reference data 16, which is inserted into the packet 15 at the user terminal transmitter.
  • the reference data 16 includes a sequence of known data symbols (not shown) that may be inserted into the packet 15 either as a contiguous block as depicted in FIG. 2 or in some distributed manner.
  • the sequence of data symbols and the manner in which they are inserted into the packet 15 are known at the central node for each user in the multiple access radio system whose message information is to be processed at that node.
  • the reference data 16 is used for determining adaptive processor parameter settings at the central node, as described in further detail below.
  • the user reference signal could also be multiplexed with the user message information as a separate signal such as a pilot signal rather than as a time division multiplexed component of a packet.
  • the packet 15 also includes interleaved and coded data 17, which is representative of processed user message information in digital form. Finally, the packet 15 may include other system or user information (not shown) in addition to the reference data 16 and the interleaved and coded data 17.
  • User terminal transmitters 12 associated with the same beam/node coverage region 11 are assigned orthogonal multiple access (OMA) channels from a mutually orthogonal set.
  • OMA orthogonal multiple access
  • Examples of OMA schemes that may be used with the multiple access radio system of the present invention include Frequency-Division Multiple Access (FDMA), Time-Division Multiple Access (TDMA), and Orthogonal- Waveform Code-Division Multiple Access (OCDMA), and various combinations thereof.
  • FDMA Frequency-Division Multiple Access
  • TDMA Time-Division Multiple Access
  • OCDMA Orthogonal- Waveform Code-Division Multiple Access
  • the same multiple access channel may be reassigned to user terminal transmitters in all other beam/node coverage regions for a reuse factor of unity.
  • the assignment of channels to the user terminal transmitter 12 can be accomplished by transmitting assignment data to the user terminal from either the central node or a centralized location that includes multiple central nodes.
  • Assignment data may be time division multiplexed with downlink message transfers or transmitted on a separate channel to the user terminal.
  • the multiple access channel assignments are produced by an assignment controller 18 that is either collocated with the central node 13 or is connected to the central node 13 with a communication link (indicated as a double-arrow line in FIG. 3) to transfer assignment data to an assignment processor 19.
  • the assignment processor 19 formats assignment data for downlink transfer to user terminals via a downlink transmitter 20 and it receives status information from the uplink receiver 21 to be described subsequently. This status information is passed on to the assignment controller 18 by the assignment processor 19.
  • the uplink receiver 21 downconverts one or more antenna beam signals from radio frequency to digitized beam/node signals, one for each antenna beam in this present central node.
  • These digitized signals are provided to link transmitters 22 in a fixed network as shown as an example as fixed communication links 14 in FIG.1 connecting together central nodes so that digitized signals are sent to adjacent central nodes.
  • the digitized beam/node signals are also provided to a multiuser detector 23 to be described subsequently that recovers uplink message information.
  • Link receivers 24 receive digitized beam/node signals from adjacent central nodes via the fixed network and provide these signals to the multiuser detector 23 as inputs to be used in interference cancellation.
  • the fixed network could also route the digitized signals to some central point as suggested in Macrodiversity Scheme for subsequent signal processing rather than performing the signal processing at the present central node as described herein in connection with the preferred embodiment.
  • Each user terminal transmitter 12 associated with a particular beam/node coverage region is assigned a multiple access channel from a mutually orthogonal set.
  • user terminal transmitters 12C and 12D associated with the same beam/node coverage region do not interfere with each other under ideal transmission conditions.
  • multiple access channels can be reused in beam/node regions so for example, the user terminal transmitters 12 A, 12B, 12D, 12E, and 12F shown in beam/node coverage regions HA, HB, HC, HD, and HE, respectively, may be assigned the same multiple access channel by the assignment controller 18.
  • the present invention includes conventional communication system components, e.g., error-correction coder/ decoder, interleaver / deinterleaver, multiplexer/ demultiplexer, modulator/ demodulator, and adaptive processor, which perform tasks related to the transmission and/or reception of user message information.
  • conventional communication system components e.g., error-correction coder/ decoder, interleaver / deinterleaver, multiplexer/ demultiplexer, modulator/ demodulator, and adaptive processor, which perform tasks related to the transmission and/or reception of user message information.
  • error-correction coder/ decoder e.g., error-correction coder/ decoder, interleaver / deinterleaver, multiplexer/ demultiplexer, modulator/ demodulator, and adaptive processor, which perform tasks related to the transmission and/or reception of user message information.
  • adaptive processor e.g., a preferred construction is identified below.
  • FIG. 4 shows a preferred embodiment of a user terminal transmitter 12 according to the present invention.
  • User message information to be transmitted on an uplink to a uplink receiver 21 may initially be in either analog or digital form. However, the user message information is preferably converted, if necessary, into digital form before being provided as a digital input to a coder 25, which adds redundancy in the form of an error- correction code, thereby causing the digital transmission rate of coded data at the output of the coder 25 to be greater than the digital input rate at the input of the coder 25.
  • the type, the subclass, and the parameters related to the error-correction code are not critical to the present invention. In a binary communication system an example of an error-correction coding technique would be the rate 1/2, constraint length 7, convolutional code with generators 133,171.
  • the coder 25 provides the coded data to an interleaver 26, which distributes the coded data among multiple packets in a predetermined manner.
  • the coded data is distributed among the multiple packets as follows. If there are N symbols per packet, then the N symbols are evenly distributed over N packets; e.g., symbol 1 goes in packet 1, symbol 2 goes in packet 2, and so on, until symbol N goes in packet N, and then the process is repeated until all N packets are full.
  • the interleaver 26 may distribute the coded data into the multiple packets in other ways and still achieve a reuse factor of 1 in the uplink of the OMA system.
  • a reference generator 27A locally produces the sequence of known data symbols included in the reference data 16 mentioned above, and then provides the reference data 16 to a packet multiplexer 28.
  • the block of reference data 16 that is inserted into packet 15 can be unique to each user or it may be the same for each user in a beam/node coverage region 11 but unique relative to users in other beam/node coverage regions 11 in the multiple access radio network.
  • the interleaver 26 provides the interleaved and coded data 17 to the packet multiplexer 28, which then generates packets having the general form shown in FIG. 1.
  • each packet may include other system or user information in addition to the reference data 16 provided by the reference generator 27A and the interleaved and coded data 17 provided by the interleaver 26.
  • the packet multiplexer 28 sequentially provides the generated packets to a modulator 29, which converts the packetized data to a multiple access signal suitable for transmission over an uplink to an uplink receiver 21 using a radio frequency (RF) channel.
  • the packets generated by the packet multiplexer 28 are converted by the modulator 29 to use only one of two quadrature carriers, e.g., cos ⁇ c t or sin ⁇ c t.
  • the modulator 29 may use, e.g., pulse amplitude modulation (PAM) for applying multilevel amplitude modulation of the user data on one of the two quadrature carriers.
  • PAM pulse amplitude modulation
  • a known pseudo- noise code can be applied to the modulation so that it has the transmit characteristics of a quadrature signal.
  • the known PN code may be removed at the uplink receiver 21.
  • the bit error rate (BER) performance in this multibeam application is expected to be better for PAM because the user terminal transmitted signal is more resistant to interference when adaptive interference reduction is accomplished at the multiuser detector 23.
  • This increased resistance to interference for PAM over QAM is more pronounced in unity reuse systems where there is a single central node 13 and a multibeam antenna, for example, the system and method disclosed in patent application SN/USOO/ 12802.
  • An alternative embodiment for the user terminal transmitter 12 therefore includes a modulator 29 with quadrature axis modulation.
  • the modulator 29 preferably uses a different multiple access channel for each transmitted packet, i.e. modulation hopping transmissions.
  • Modulation hopping is a generalized version of frequency hopping as used in FDMA/TDMA systems. In these systems the radio frequency channel assignment is changed for each user in each time slot. These hopping changes can be realized so as to maintain orthogonality between users in a beam/node region.
  • the change to a different CDMA channel for each user in each time slot in a CDMA/TDMA system is another example of modulation hopping.
  • the use of modulation hopping by each user terminal transmitter 12 insures that the interference from other users assigned to the same modulation-hopped channel changes for every packet.
  • the modulator 29 provides the multiple access signal to an antenna 30 for transmission over an uplink to an uplink receiver 21 using an RF channel.
  • While the modulation hopping of users provides the potential for other user interference averaging, it also produces a random delay variation in the other user interference.
  • the new user is in a different location.
  • the transmission paths between the new user and the antennas may change significantly relative to this previous user. These changes in delay may be as long as a few symbol durations and must be compensated at the multiuser detector 23.
  • FIG. 5 shows a preferred embodiment of a multiuser detector 23 according to the present invention.
  • the multiuser detector 23 accepts digitized beam/node signals associated with an assigned multiple access channel from the uplink receiver 21 as shown in FIG. 3.
  • the beam/node signals includes multiple access signals on one or more antenna beams for the present central node, each beam including a set of users assigned to respective OMA channels.
  • the uplink receiver 21 segregates user terminal signals by exploiting the mutual orthogonality of the multiple access channels.
  • the multiuser detector 23 simultaneously produces recovered information signals for a plurality of users that are assigned the same multiple access channel and are in different beam/node regions.
  • the digitized signal from the uplink receiver 21 for each assigned multiple access channel contains B beam/node signals.
  • An adjacent central node provides its digitized beam /node signals via the fixed communication network to a link receiver 24 as depicted in FIG. 3.
  • the beam/node signals from each of the adjacent central nodes are provided along with the B beam/node signals from the present central node to a delay compensator 31. Beam/ node signals from adjacent central nodes are delayed relative to the present central node because of processing and transmission delay. Moreover adjacent central nodes may not be synchronous with the present central node.
  • the delay compensator 31 adjusts the delays of beam/node signals from neighbor central nodes so that the beam/node signals corresponding to the assigned multiple access channel are approximately synchronized for the duration of the received packet. Since processing and transmission delay are known in advance, the delay adjustment due to these effects can be fixed. Compensation for slow timing changes due to asynchronism between central nodes requires delay measurements and adjustment of delay. This measurement is accomplished by correlation of user reference data 16 and beam/node signals in the adaptive processor 32. Delay adjustments are computed in the adaptive processor 32 and are provided to the delay compensator 31 as shown in FIG. 5. It should be understood that the delay compensation may also be accomplished in a distributed manner, for example, a synchronism correction at each central node and a processing/ delay compensation at the multiuser detector. -
  • the antenna at the present central node has B beams and B beam/node signals and there are N beam/node signals from adjacent central nodes, there are D beam/node signals with significant mutual interference, wherein D ⁇ B+N.
  • the choice of D is dependent upon the required communications quality and the required complexity of the multiuser detector 23.
  • the selected D beam/node signals from the delay compensator 31 are provided to a D- dimensional adaptive processor 32 for beam combining and other user interference reduction. For example in FIG.l, there may be significant mutual interference from sectors 11A,11B and 11C and from macrodiversity cells 1 IE and 1 ID so that D is five.
  • the adaptive processor 32 processes multiple beam/node signals and possibly previously detected digital symbols from K users, K81, and generates K combined signals with reduced mutual interference from the other users.
  • the adaptive processor 32 may take the form of an adaptive equalizer that minimizes some error criterion, or an adaptive sequence estimator that finds the most likely transmitted digital symbol sequence for the user set assigned to the same multiple access channel, or some combination of both.
  • Examples of adaptive equalizers that might be used in this multibeam application include linear minimum mean square error (MMSE) receivers, decorrelation detectors, and MMSE decision-feedback equalizers.
  • the preferred embodiment is an adaptive equalizer that is based on the multiple diversity Decision-Feedback Equalizer (DFE) in "MMSE Equalization of Interference on Fading Diversity Channels", P.
  • DFE multiple diversity Decision-Feedback Equalizer
  • the combining for each of the K desired users is accomplished by using the replica of the reference signal of the desired user and replicas of reference signals for interfering users.
  • the reference generator 27B provides a user-identifying replica of the reference signal, e.g., reference data 16, to the adaptive processor 32.
  • the adaptive processor 32 exploits the user-identifying replicas to adapt its parameters and then generates K combined signals that correspond to users associated with a subset of the D beam/node signals.
  • the invention is first described for the condition that the central node clocks are mutually synchronized.
  • the packets are approximately time aligned by the operation of the delay compensator 31 because all users regardless of their destination antenna, time their transmissions to arrive at the antennas simultaneously.
  • the interference After alignment of the beam/node signal components by the delay compensator 31 the interference must be equalized for time delay variations in order to effectively cancel or reduce the other user interference effect.
  • MDFE Multiuser Decision-Feedback Equalizer
  • the representing of the transmittance in terms of a fraction of the symbol period is used in fractional-tapped adaptive equalizers in order to reduce equalization degradation in a bandlimited application.
  • a typical value of Q is 2 as a compromise between reducing degradation and receiver complexity.
  • the equalizer weights in the forward and backward filter vectors of the MDFE can be determined from a compound matrix equation for K users, D antennas, L intersymbol interferers, Q fractional taps per symbol, NQ forward filter coefficients per antenna and L backward filter coefficients per user.
  • the forward filter signal can be expressed as a NQxl vector, i.e.,
  • the forward Adk and backward Bdk transmittance matrices are a function of the channel transmittance coefficients
  • the MDFE is adapted by estimating for each received signal packet the above channel transmittance coefficients.
  • the packet duration is sufficiently short that these coefficients are approximately constant. For longer packet durations it may be necessary to use multiple subpackets with either multiple subreference groups or a decision-directed tracking algorithm based on the estimates from the first reference group.
  • the coefficients are estimated in the adaptive processor 32 by correlating the kth user reference signal with the dth diversity signal with a delay separation corresponding to the delay parameters specified in Eq. 4.
  • B ⁇ B dk ⁇ is DQ ⁇ xKL
  • b ⁇ bk ⁇ is KLxl
  • the noise variance ⁇ 2 is estimated in the adaptive processor 32 during periods of signal absence as a calibration procedure or continuously in the presence of signal in an adaptive averaging technique that measures the minimum received power.
  • the optimum MDFE includes a matched filter whose output vector is
  • the noise vector v has covariance matrix H.
  • the detected user combined signal value is
  • ⁇ jk if j ⁇ k and 0 otherwise.
  • the matrix M is symmetric for PAM and Hermetian for QAM so that the solution for the forward filter equalizer weights, represented in (8a), can be obtained by a known Cholskey decomposition, e.g., as described in numerical computational texts such as "Least Square Estimation with Application to Digital Signal Processing" by A.A. Giordano and F.M. Hsu, John Wiley and Sons, New York, NY, 1985, Chapter 3.3.
  • the matrix M is decomposed into G'G where G'is the complex conjugate transpose of G and G is a lower diagonal matrix, i.e. all the elements in the matrix above the diagonal are zero.
  • this MMSE solution Eq. 8 reduces to a solution that cancels all the interference, i.e. a zero-forcing solution.
  • the cancellation of previous detected symbols in the K user MDFE can be augmented by linear equalization of an additional KL users.
  • Each of these additional users is associated with one of the D beam/mode signals but the D beam/mode signals do not contain all of the beam/mode signals necessary for inclusion of these additional users into the multiuser detection set.
  • This combination of decision-feedback and linear equalization is accomplished by augmenting the first term of Eq. 2 to include KL additional interferers.
  • the packets of beam/node signals are approximately aligned after the delay adjustment of the delay compensator 31.
  • an interference user associated with a different central node may change its multiple access channel in the middle of the packet time of the desired user group in the present cental node.
  • FIG. 6 An example of asynchronous operation with three central nodes is shown in FIG. 6.
  • the packet A, 36A represents the received beam/node signals corresponding to a desired signal group of K users after delay compensation from all central nodes for which multiuser detection is to be employed.
  • the packets B and C, 37, and packets D and E, 38 are interference signal packets from two neighbor central nodes after delay compensation.
  • FIG.6 shows that the interference packets 37 and 38 from neighbor central nodes result in three separate interference epochs in an interval 36B associated with the packet of the desired signal group.
  • the MDFE during this epoch would be a K multiuser detector with linear equalization of the interference produced by the additional KLB + KLD users.
  • the embodiment of the invention for independent timing clocks at the central nodes includes the identification of these multiple epochs by the delay compensator 31 and the application of transmittance estimation and equalization as described above but applied separately in the multiple epochs.
  • the number of epochs is equal to the number of macroscopic cells.
  • a typical multicell configuration would be a combination of three macrocells each containing one or more beam sectors that would require a three epoch MDFE solution.
  • the example given in FIG.1 requires such a three epoch solution. It is anticipated that the continuing increase in signal processing capability will result in network solutions where multiple realization of the MDFE may be preferable to mutually synchronizing central node clocks over a large geographical area.
  • the packet demultiplexer 33 provides the combined signal without the reference data to a deinterleaver 34, which reverses the interleaving of the coded data performed by the interleaver 26.
  • the deinterleaver 34 provides the deinterleaved data to a decoder 35, which performs error-correction decoding, thereby producing a digital output that is representative of the transmitted digital message information of the kth user terminal.
  • Subsequent processing, e.g., digital-to-analog conversion (not shown), of the digital output may be required to obtain analog message information, e.g., voice signals, of the particular user.

Abstract

L'invention concerne un réseau de communication hertzien numérique à accès multiples et un procédé de liaisons montantes de communication entre des émetteurs d'un terminal d'utilisateur et des noeuds centraux connectés par des liaisons de communications fixes. Les émetteurs du terminal d'utilisateur sont affectés à des zones de faisceaux/noeuds correspondant à un noeud central associé et à leur position dans la zone de couverture du faisceau d'antenne du noeud central. Les émetteurs du terminal d'utilisateur affectés à une zone de couverture de faisceaux/noeuds utilisent des voies d'accès multiples mutuellement orthogonales pour transmettre les informations de messages numériques. Ces voies d'accès multiples sont réutilisées dans les zones de couverture adjacentes et dans d'autres zones de couverture de faisceaux/noeuds. Le codage de correction d'erreur (25), le désentrelacement (26), et un modulateur (29) sont utilisés dans l'émetteur d'utilisateur pour accroître la résistance des émetteurs du terminal d'utilisateur aux interférences potentielles dans d'autres zones de couverture de faisceaux/noeuds. Dans un noeud central, un récepteur de liaison montante (21) fournit des signaux de faisceaux/noeuds numérisés à la fois à un détecteur (23) à utilisateurs multiples et, via les liaisons de communication fixes, à des détecteurs à utilisateurs multiples dans les noeuds centraux adjacents.
PCT/US2001/046692 2000-10-23 2001-10-22 Reseau a acces multiples et procede pour systemes hertziens numeriques WO2002035857A1 (fr)

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US5563610A (en) * 1995-06-08 1996-10-08 Metawave Communications Corporation Narrow beam antenna systems with angular diversity
US5596533A (en) * 1995-04-07 1997-01-21 Hyundai Electronics Industries Co., Ltd. Method and apparatus for reading/writing data from/into semiconductor memory device

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US5513215A (en) * 1993-09-20 1996-04-30 Glenayre Electronics, Inc. High speed simulcast data system using adaptive compensation
US5596533A (en) * 1995-04-07 1997-01-21 Hyundai Electronics Industries Co., Ltd. Method and apparatus for reading/writing data from/into semiconductor memory device
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