Spectrally efficient high capacity wireless communication systems

Description of equivalent WO 9622662 A1

SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS

0002Background of the Invention

0003This invention relates to wireless communication systems and more particularly, to using antenna ar¬ rays and signal processing to dramatically increase the capacity and performance of wireless communication systems

0004Wireless communication systems can be used to complement and in some instances replace conventional wired communication systems in areas where conventional wire-line systems are unavailable unreliable or excessively expensive Examples of such areas are rural areas with a small number of widespread users underdeveloped areas with little or no current infrastructure, reliability sensitive applications in areas where wired infrastructure is unreliable, and political environments where monopolistic wired service providers maintain artificially high prices Even in metropolitan areas and highly developed countries wireless communication systems may be used for low-cost ubiquitous communication, new flexible data services and emergency communication systems In general wireless communication systems may be used for voice communications just like conventional telephone systems, and for data communications in a radio-based wide area or local area network as well

0005Wireless users access wireless communication systems using remote terminals such as cellular tele¬ phones and data modems equipped with radio transceivers Such systems (and in particular the remote terminals) have protocols for initiating calls receiving calls, and general transfer of information The infor¬ mation transfer can be performed in real-time such as is the case for circuit-switched voice conversations and faxes or in a .tore-and-forward manner such as is often the case for electronic mail, paging and other similar message transfer systems

0006Wireless communication systems are generally allocated a portion of the radio frequency spectrum for their operation The allocated portion of the spectrum is divided up into communication channels These channels may be distinguished by frequency by time, by code, or by some combination of the above Each of these communication channels will be referred to herein as conventional channels Depending on the available frequency allocations, the wireless system might have from one to several hundred communication channels To provide full-duplex communication links typically some of the communication channels are used for communication from base stations to users' remote terminals (the downlink), and others are used for communication from users' remote terminals to base stations (the uplink)

0007Wireless communication systems generally have one or more radio base stations, each of which provide coverage to a geographic area known as a cell and often serve as a point-of-presence (PoP) providing connection to a wide area network such as a Public Switched Telephone Network (PSTN) Often a pre¬ determined subset of the available communication channels is assigned to each radio base station in an attempt to minimize the amount of interference experienced by users of the system Within its cell a radio base station can communicate simultaneously with manv remote terminals by using different conventional communication channels for each remote terminal As aforementioned, base stations can act as PoPs, providing connection to one or more wired commu¬ nication systems. Such systems include local data networks, wide area data networks, and PSTNs Thus, remote users are provided access to local and/or wide area data services and the local public telephone system Base stations can also be used to provide local connectivity without direct access to a wired net¬ work such as in local area emergency and mobile battlefield communication systems Base stations can provide connectivity of various kinds as well In the aforementioned examples, point-to-point communica¬ tions where roughly equal amounts of information flow in both directions between two users were assumed In other applications such as interactive television, information is broadcast to all users simultaneously , and responses from many of the remote units are to be processed at the base stations

0008However, conventional wireless communication systems are comparatively spectrally inefficient. In conventional wireless communication systems, only one remote terminal can use any one conventional channel within a cell at any one time If more than one remote terminal in a cell attempts to use the same channel at the same time, the downlink and uplink signals associated with the remote terminals interfere with each other Since conventional receiver technology can not eliminate the interference in these combined uplink and downlink signals remote terminals are unable to communicate effectively with the base station when interference is present. Thus, t he total capacity of the system is limited by the number of conventional channels the base station has available, and m the overall system, by the way in which these channels are re-used among multiple cells Consequently, conventional wireless systems are unable to provide capacity anywhere near that of wired communication systems

0009Summary of the Invention

0010Accordingly, an object of the present invention is to use antenna arrays and signal processing to separate combinations of received (uplink) signals. Another object of the present invention is to transmit spatially multiplexed downlink signals The result is a dramatic increase in spectral efficiency capacity signal quality, and coverage of wireless communication systems Capacity is increased by allowing multiple users to simultaneously share the same conventional communication channel within a cell without interfering with one another, and further by allowing more frequent reuse of the same conventional channel within a geographic area covering many cells. Signal quality and coverage area are improved through intelligent processing of signals received from and transmitted by multiple antenna elements. Moreover a further object of the present invention is to provide capacity gains by dynamically allocating conventional channels among base stations and remote terminals

0011Briefly, the invention comprises antenna arrays and signal processing means for measuring, calculating storing, and using spatial signatures of receivers and transmitters in wireless communication systems to increase system capacity, signal quality, and coverage, and to reduce overall system cost. The antenna array and signal processing means can be employed at base stations ( PoPs) and remote terminals Generally there can be different processing requirements at base stations where many signals are being concentrated than at remote terminals where in general only a limited number of communication links are being managed

0012As an example, in a wireless local loop application, a particular base station might serve as a PoP for many remote terminals and employ the antenna array and signal processing described herein Additionalh remote terminals could employ antenna arrays and signal processing to further improve their capacity and sienal quality over simpler remote terminals that handle fewer communication links Herein, the distinction between base stations and remote terminals is that base stations generally act as concentrators connectinc. to multiple remote units simultaneously, possibly providing a high capacity connection to a wide area network While for the sake of clarity much of the ensuing discussion is couched in terms of simple remote terminals that do not employ antenna arrays, nothing herein should be interpreted as preventing such an application Thus, while hereafter spatial signatures will be associated primarily with remote terminals when antenna arrays are employed at remote terminals, base stations will have associated spatial signatures as well

0013Briefly, there are two spatial signatures associated with each remote terminal/base station pair on a particular frequency channel, where for the purpose of this discussion it is assumed that only base stations have antenna arrays Base stations associate with each remote terminal in their cell a spatial signature related to how that remote terminal receives signals transmitted to it by the base station s antenna array, and a second spatial signature related to how the base station s receive antenna array receives signals transmitted by the remote terminal In a system with many conventional channels, each remote terminal/base station pair has transmit and receive spatial signatures for each conventional channel

0014The receive spatial signature characterizes how the base station antenna array receives signals from the particular remote unit in a particular conventional channel In one embodiment, it is a complex vector containing responses (amplitude and phase with respect to a reference) of each the antenna element receivers, J t . for an m-element array, a.r = [a6r( l ), a»_r(2), , a._kr(m)]^T , ( 1 ) where β(,_r(ι) is the response of the ^• receiver to a unit power transmitted signal from the remote terminal Assuming that a narrowband signal s_r(t) is transmitted from the remote terminal, the base station receiver outputs at time t are then given by z_t(t) = a_6rs_r(r - r) + n_t(t), (2) where r accounts for the mean propagation delay between the remote terminal and the base station antenna array, and n.(t ) represents noise present in the environment and the receivers

0015The transmit spatial signature characterizes how the remote terminal receives signals from each of tht antenna array elements at the base station in a particular conventional channel In one embodiment, it is a complex vector containing relative amounts (amplitude and phase with respect to a reference) of each the antenna element transmitter outputs that are contained in the remote terminal receiver output ι t for an m-element array, a_r. = [a_r4( l ), a_r.(2), a_rB(m)], (3) where a_rj(2) is the amplitude and phase (with respect to some fixed reference) of the remote terminal receiver output for a unit power signal transmitted from the ι'^h element m the base station array Assuming that a vector of complex signals s& = [s.(l), , s*(m)]^τ were transmitted from the antenna array, the output of the remote terminal receiver would be given by z_r(t) = a_ris₆(< - τ) + n_r(t), (4) where n_r{t ) represents noise present in the environment and the receiver These spatial signatures are calculated (estimated) and stored at each base station for each remote terminal in its cell and for each conventional channel For fixed remote terminals and base stations in stationary environments the spatial signatures can be updated infrequently. In general, however, changes in the RF propagation environment between the base station and the remote terminal can alter the signatures and require that they be updated Note that henceforth, the time argument in parentheses will be suppressed, integers inside parentheses will be used solely for indexing into vectors and matrices

0016In the previous discussion, temporally matched receivers and transmitters were assumed If there are differences in the temporal responses, these can be equalized using temporal filtering techniques as is well- known Furthermore, the channel bandwidths were assumed to be small compared to the center frequency of operation Large bandwidth channels may require more than one complex vector to accurately describe the outputs as is well known

0017When more than one remote terminal wants to communicate at the same time, the signal processing means at the base station uses the spatial signatures of the remote terminals to determine if subsets of them can communicate with the base station simultaneously by sharing a conventional channel In a system with m receive and m transmit antenna elements, up to m remote terminals can share the same conventional channel at the same time

0018When multiple remote terminals are sharing a single conventional uplink channel the multiple antenna elements at the base station each measure a combination of the arriving uplink signals and noise These combinations result from the relative locations of the antenna elements, the locations of the remote termi¬ nals, and the RF propagation environment The signal processing means calculates spatial demultiplexing weights to allow the uplink signals to be separated from the combinations of uplink signals measured by t he multiple antenna elements

0019In applications where different downlink signals are to be sent from the base station to the remote terminals, the signal processing means computes spatial multiplexing weights that are used to produce multiplexed downlink signals which when transmitted from the antenna elements at the base station result in the correct downlink signal being received at each remote terminal with appropriate signal quality

0020In applications where the same signal is to be transmitted from the base station to a large number ( more than the number of antenna elements) of remote terminals, the signal processing means computes weights appropriate for broadcasting the signal, covering the area necessary to reach all the remote terminals

0021Therefore, the signal processing means facilitates simultaneous communication between a base station and multiple remote terminals on the same conventional channel The conventional channel may be a frequency channel, a time slot in a time division multiplexed system, a code in a code division multiplexed system, or any combination of the above

0022In one embodiment, all elements of a single antenna array transmit and receive radio frequency signals while in another embodiment the antenna array includes separate transmit antenna elements and receive antenna elements The number of transmit and receive elements need not be the same If they are not the same, the maximum number of point-to-pomt links that can simultaneously be established in one conventional channel is given by the smaller of the two numbers

0023The invention and objects and features thereof will be more readily apparent from the following detailed description together with the figures and appended claims

0024Brief Description of The Drawings

0025Figure 1 is a functional block diagram of a base station in accordance with the invention Figure 2 is a functional block diagram of multichannel receivers in the base station Figure 3 is a functional block diagram of a spatial demultiplexer in the base station.

0026Figure 5 is a functional block diagram of a multichannel transmitter in the base station.

0027Figure 6 is a functional block diagram of a spatial processor in the base station.

0028Figure 7 is a functional block diagram of a remote terminal with a transponder switch.

0029Figure 8 is a functional block diagram of a remote terminal.

0030Figure 9 is a schematic diagram of a network system comprised of three base stations and a multiple base station controller.

0031List of Reference Numerals

00321. base station

00332. base station communication link

00343. base station controller

00354. demodulated received signal

00365. spatially separated uplink signals

00376. received signal measurements

00387. demultiplexing weights

00398. data to be transmitted directionally

00409. modulated signal to be multiplexed for transmission

004110. modulated, spatially multiplexed signals to be transmitted

004211. calibration signals to be transmitted

004312. multiplexing weights 13- spatial processor

004414. multichannel transmitters

004515. multichannel receivers 16a. multichannel receiver

004616m. multichannel receiver

004717a. multichannel transmitter 17m. multichannel transmitter

004818a. transmit antenna 18m. transmit antenna

004919a. receive antenna 19m. receive antenna

005020. spatial demultiplexer

005121. adder 22a. multipliers

005222m. multipliers

005323. spatial multiplexer

005424. signal modulator

005525. signal demodulator 26a. multipliers

005626m. multipliers

005727. spatial control data

005828. spatial parameter data

005929. common receiver oscillator

006030. receiver control data

006131. transmitter control data

006232. common transmitter oscillator 33 spatial processor controller 34. active remote terminal list

006335 channel selector

006436 remote terminal database

006537 spatial weight processor

006638 spatial signature processor remote terminal antenna remote terminal duplexer remote terminal duplexer output remote terminal receiver remote terminal received signal remote terminal received calibration signal remote terminal demodulator remote terminal demodulated data remote terminal keyboard and keyboard controller remote terminal keyboard data remote terminal display data remote terminal display and display controller remote terminal modulator remote terminal data to be transmitted remote terminal modulated data to be transmitted remote terminal transmitter remote terminal transmitter output remote terminal transmitter control data remote terminal receiver control data remote terminal microphone remote terminal microphone signal remote terminal speaker remote terminal speaker signal remote terminal central processing unit remote terminal transponder switch remote terminal transponder switch control

006765 wide area network

006866 multiple base station controller 7a cell boundary 7b cell boundary 7c cell boundary

006968 high speed message link

007069 remote terminal Descnptton of Invention

0071Figure 1 depicts the preferred embodiment of a base station 1. A base station controller 3 acts as an interface between base station 1 and any external connection via a base station communication link 2, and serves to coordinate the overall operation of base station 1. In the preferred embodiment, base station controller 3 is implemented with a conventional central processing unit and associated memory and programming.

0072Incoming or uplink radio transmissions impinge on an antenna array composed of a number, m, of receive antenna elements 19(a,. . . ,m) each of whose outputs is connected to one of m multichannel receivers in a bank of phase-coherent multichannel receivers 15. Multichannel receivers 15 have well- matched amplitude and phase responses across the frequency bands of interest, or. as is well known, correction filters are implemented to account for any differences.

0073The illustrative embodiment describes a conventional frequency division multiple access system. Each multichannel receiver is capable of handling multiple frequency channels. The symbol Λ'-_e will be used to reference the maximum number of conventional frequency channels that can be handled by the receivers Depending on the frequencies allocated for the operation of the wireless communication system and the bandwidths chosen for particular communication links. N_cc could be as small as one (a single frequency channel) or as large as thousands. In alternate embodiments, multichannel receivers 15 might instead handle multiple time slots, multiple codes, or some combinat ion of these well known multiple access techniques.

0074In each conventional channel, receive antenna elements 19(a,. . . ,m) each measure a combination of the arriving uplink signals from the remote terminals sharing this conventional channel. These combinations result from the relative locations of the antenna elements, the locations of the remote terminals, and the RF propagation environment, and for narrowband signals are given by equation (2).

0075Figure 2 depicts individual multichannel receivers 16(a m) with antenna element connections, common local receiver oscillators 29, one for each conventional frequency channel to be used at that base station, and received signal measurements 6 Common local receiver oscillators 29 ensure that the signals. from receive antenna elements 19(a m) are coherently down-converted to baseband: its Λ^',- frequencies are set so that multichannel receivers 16(a m) extract all N_e- frequency channels of interest. The frequencies of common local receiver oscillators 29 are controlled by a spatial processor 13 (figure 1 ) via receiver control data 30. In an alternate embodiment, where multiple frequency channels are all contained in a contiguous frequency band, a common local oscillator is used to downconvert the entire band which is then digitized, and digital filters and decimators extract the desired subset of channels using well known techniques.

0076The illustrative embodiment describes a frequency division multiple access system. In a time division multiple access or code division multiple access system, common oscillators 29 would be augmented to relay common time slot or common code signals respectively from spatial processor 13. via receiver control data 30. to multichannel receivers 16(a, . . . ,m). In these embodiments, multichannel receivers 16(a m) perform selection of conventional time division channels or conventional code division channels in addition to down conversion to baseband.

0077Referring again to Figure 1. multichannel receivers 15 produce received signal measurements 6 which are supplied to spatial processor 13 and to a set of spatial demultiplexers 20. In this embodiment, received signal measurements 6 contain m complex baseband signals for each of Λ--_r frequency channels Figure 6 shows a more detailed block diagram of spatial processor 13 Spatial processor 13 produces and maintains spatial signatures for each remote terminal for each conventional frequency channel, and calculates spatial multiplexing and demultiplexing weights for use by spatial demultiplexers 20 and spatial multiplexers 23 In the preferred embodiment, spatial processor 13 is implemented using a conventional central processing unit Received signal measurements 6 go into a spatial signature processor 38 which estimates and updates spatial signatures Spatial signatures are stored in a spatial signature list in a remote terminal database 36 and are used by channel selector 35 and spatial weight processor 37, which also produces demultiplexing weights 7 and multiplexing weights 12 A spatial processor controller 33 connects to spatial weight processor 37 and also produces receiver control data 30 transmitter control data 31 and spatial control data 27

0078Referring again to Figure 1. spatial demultiplexers 20 combine received signal measurements 6 accord¬ ing lo spatial demultiplexing weights 7 Figure 3 shows a spatial demultiplexer 20 for a single conventional channel In this embodiment, arithmetic operations in spatial demultiplexer 20 are carried out using gen¬ eral purpose arithmetic chips In figure 3, z&(t) denotes the ι^,h component of received signal measurement vector b for a single conventional channel, and w^_x ( i ) denotes the complex conjugate of the ι^th component of the spatial demultiplexing weight vector 7 for a remote terminal using this conventional channel

0079For each remote terminal on each conventional channel, the spatial demultiplexer 20 computes the inner-product of the spatial demultiplexing weights 7 for the conventional channel with the received signal measurements 6 w;_rz₆ = w;_x(l)z_t( l) + - w;_r(m)z»( ), (5) where ( )" indicates complex conjugation, numbers inside parentheses indicate element number ( g . w_rr (ι) is the ι^th component of the vector _rr ), the multiplication is performed by multipliers 22(a, ,m), and the addition is performed b\ adder 21 For each remote terminal on each conventional channel, the output of adder 21 given by equation ( 5) comprises the spatially separated uplink signals δ

0080Referring again to Figure 1 the outputs of spatial demultiplexers 20 are spatially separated uplink signals 5 for each remote terminal communicating with the base station Spatially separated uplink signals 5 are demodulated by signal demodulators 25. producing demodulated received signals 4 for each remote terminal communicating with the base station Demodulated received signals 4 and corresponding spatial control data 27 are available to base station controller 3

0081In embodiments where channel coding of the signals sent by remote terminals is performed, base station controller 3 sends the demodulated received signals 4 to spatial processor 13 which, using well known decoding techniques estimates Bit-Error-Rates (BERs) and compares them against acceptable thresholds stored in the remote terminal database 36 If the BERs are unacceptable, spatial processor 13 reallocates resources so as to alleviate the problem In one embodiment, links with unacceptable BERs are assigned to new channels using the same strategy as adding a new user with the exception that the current channel is not acceptable unless the current set of users of that particular channel changes Additionally, recalibration of the receive signature for that remote terminal/base station pair is performed when that conventional channel is available

0082For transmission siεnal modulators 24 produce modulated signals 9 for each remote terminal the base station is transmitting to and a set of spatial multiplexing weights 12 for each remote terminal are applied to the respective modulated signals in spatial multiplexers 23 to produce spatially multiplexed signals to be transmitted 10 for each of the m transmit antennas 18(a, ,m) and each of the Λ'-- conventional channels

0083In the illustrative embodiment the number N_ec of downlink conventional channels is the same as the number Λ -_<- of uplink conventional channels In other embodiments, there may be different numbers of uplink and downlink conventional channels Furthermore, the channels may be of different types and bandwidths as is the case for an interactive television application where the downlink is comprised of wideband video channels and the uplink employs narrowband audio/data channels

0084Additionally, the illustrative embodiment shows the same number of antenna elements m for transmit and receive In other embodiments the number of transmit antenna elements and the number of receive antenna elements may be different, up to and including the case where transmit employs only one transmit antenna element in an omnidirectional sense such as in an interactive television application

0085Figure 4 shows the spatial multiplexer for one remote terminal on a particular conventional channel Arithmetic operations in spatial multiplexer 23 are earned out using general purpose arithmetic chips The component of modulated signals 9 destined for this remote terminal on this conventional channel is denoted by s& and _tr( .) denotes the ι^th component of spatial multiplexing weight vector 12 for this remote terminal on this conventional channel

0086For each remote terminal on each conventional channel the spatial multiplexer 23 computes the product of its spatial multiplexing weight vector (from the spatial multiplexing weights 12) with its modulated signal s_\, (from the modulated signals 9) w_t'_x(l)s_b w, ,.s*„* == (6) w^*_r(m)s_t where ( )^* indicates complex conjugate (transpose) and the multiplication is performed by multipliers 26(a, m) For each conventional channel, equation (6) is evaluated by the spatial multiplexer 23 for each remote terminal that is being transmitted to on this conventional channel Corresponding to each remote terminal is a different multiplexing weight vector and modulated signal For each conventional channel spatial multiplexer 23 adds the spatially multiplexed signals for each remote terminal being transmitted to on this conventional channel, producing modulated and spatially multiplexed signals 10 that are the signals to be transmitted for each conventional downlink channel from each antenna

0087Modulated and spatially multiplexed signals 10 are inputs to a bank of m phase coherent multichannel transmitters 14 Multichannel transmitters 14 either have well-matched amplitude and phase responses across the frequency bands of interest, or as is well known correction filters are implemented to account for any differences Figure 5 depicts multichannel transmitters 17(a m) with antenna connections common local transmitter oscillators 32 and digital inputs 10 Common local transmitter oscillators 32 ensure that the relative phases of spatially multiplexed signals 10 are preserved during transmission b\ transmit antennas 18(a ,m) The frequencies of common local transmitter oscillators 32 are controlled by spatial processor 13 (see figure 1 ) via transmitter control data 31

0088In an alternate embodiment, spatial multiplexer 23 uses well known baseband multiplexing techniques to multiplex all the calculated conventional channel signals to be transmitted into a single wideband signal to be upconverted and transmitted bv each of the multichannel transmitters 17(a in ) The multiplexing can be performed either digitally or in analog as appropriate The illustrative embodiment shows a system with multiple frequency channels In a time division multiple access or code division multiple access system, common oscillators 32 would be augmented to relay common time slot or common code signals respectively from spatial processor 13 via transmitter control data 31, to multichannel transmitters 17(a, . ,m)

0089Referring again to figure 1 , in applications where transmit spatial signatures are required, spatial processor 13 is also able to transmit predetermined calibration signals 11 for each antenna on a particular conventional downlink channel Spatial processor 13 instructs multichannel transmitters 17(a, .m), via transmitter control data 31. to transmit predetermined calibration signals 11 in place of spatially multiplexed signals 10 for a particular conventional downlink channel This is one mechanism used for determining the transmit spatial signatures of the remote terminals on this conventional downlink channel

0090In alternate embodiments where well known channel coding techniques are used to encode the signals to be transmitted to remote terminals, remote terminals employ well known decoding techniques to estimate BERs which are then reported back to the base station on their uplink channel If these BERs exceed acceptable limits corrective action is taken In one embodiment, the corrective active involves reallocating resources by using the same strategy as adding a new user with the exception that the current channel is not acceptable unless the current set of users of that particular channel changes Additionally, recalibration of the transmit signature for that remote terminal/base station pair is performed when that conventional channel is available

0091Figure 7 depicts the component arrangement in a remote terminal that provides voice communication The remote terminal's antenna 39 is connected to a duplexer 40 to permit antenna 39 to be used for both transmission and reception. In an alternate embodiment, separate receive and transmit antennas are used eliminating the need for duplexer 40 In another alternate embodiment where reception and transmission occur on the same frequency channel but at different times, a transmit/receive (TR) switch is used instead of a duplexer as is well known Duplexer output 41 serves as input to a receiver 42 Receiver 42 produces a down-converted signal 43 which is the input to a demodulator 45 A demodulated received voice signal 61 is input to a speaker 60

0092Demodulated received control data 46 is supplied to a remote terminal central processing unit 62 (CPU) Demodulated received control data 46 is used for receiving data from base station 1 during call setup and termination, and in an alternate embodiment, for determining the quality (BER) of the signals being received by the remote terminal for transmission back to the base station as described above

0093Remote terminal CPU 62 is implemented with a standard microprocessor Remote terminal CPU 62 also produces receiver control data 57 for selecting the remote terminal's reception channel transmitter control data 56 for setting the remote terminal's transmission channel and power level, control data to be transmitted 52, and display data 49 for remote terminal display 50 Remote terminal CPU 62 also receives keyboard data 48 from remote terminal keyboard 47

0094The remote terminal's voice signal to be transmitted 59 from microphone 58 is input to a modulator 51 Control data to be transmitted 52 is supplied by remote terminal CPU 62 Control data to be transmit¬ ted 52 is used for transmitting data to base station 1 during call setup and termination as well as for transmitting information during the call such as measures of call quality ( e g bit error rates ( BERs) ) The modulated signal to be transmitted 53. output by modulator 51 is up-converted and amplified <img file="WO9622662A1_D0001.tif" /> a trans¬ mitter 54. producing a transmitter output signal 55 Transmitter output 55 is then input to duplexer 40 for transmission bv antenna 39 In an alternate embodiment, the remote terminal provides digital data communication Demodulated received voice signal 61, speaker 60, microphone 58, and voice signal to be transmitted 59 are replaced by digital interfaces well-known in the art that allow data to be transmitted to and from an external data processing device (for example, a computer)

0095Referring again to figure 7, the remote terminal allows received data 43 to be transmitted back to base station 1 via switch 63 controlled by remote terminal CPU 62 through switch control signal 64 In normal operation, switch 63 drives transmitter 54 with modulated signal 53 of modulator 51 When the remote terminal is instructed by base station 1 to enter calibration mode, remote terminal CPU 62 toggles switch control signal 64, which instructs switch 63 to drive transmitter 54 with received data 43

0096Figure 8 shows an alternate embodiment of the remote terminal calibration function Switch 63 of figure 7 is no longer used Instead, the output of receiver 42 is supplied to remote terminal CPU 62 by data connection 44 In normal operation remote terminal CPU 62 ignores data connection 44 In calibration mode, remote terminal CPU 62 uses data connection 44 to compute the remote terminal's transmit spatial signature, which is transmitted back to base station 1 through modulator 51 and transmitter 54 as control data to be transmitted 52

0097In an alternate embodiment, special calibration procedures in the remote terminal are not required In many conventional wireless protocol standards, remote terminals regularly report received signal strength or receive signal quality back the base station In this embodiment, the received signal strength reports are sufficient to compute the remote terminal's transmit spatial signature, as described below

0098Operation of Invention

0099General Principles — Base Station

0100In many respects, the spectrally efficient base station shown in figure 1 behaves much like a standard wireless communication system base station The primary distinction is that the spectrally efficient base station supports many more simultaneous conversations than it has conventional communication chan¬ nels The conventional communication channels may be frequency channels, time channels code channels or any combination of these. The spatial multiplexer/demultiplexer increases the system capacity by al¬ lowing multiple spatial channels on each of these conventional channels Moreover, by combining signals from multiple receive antennas, the spatial demultiplexer 20 produces spatially separated uplink signals 5 that have substantially improved signal-to-noise. reduced interference, and improved quality in multipath environments compared to a standard base station

0101In the illustrative embodiment, a wireless communication system comprised of multiple remote termi¬ nals and base stations incorporating antenna arrays and spatial signal processing is described Such systems have application, for example, in providing wireless access to the local PSTN Information transfers (or calls) are initiated by either a remote terminal or by communication link 2 through base station controller 3 Call initialization takes place on a downlink and uplink control channel as is well known in the art In the present embodiment, the downlink control channel is transmitted using transmission antennas 18(a, .m ) In an alternate embodiment the downlink control channel is broadcast from a single, omnidirectional an¬ tenna. Base station controller 3 passes the identification of the remote terminal to be involved in the call to spatial processor 13 which uses the stored spatial signatures of that remote terminal to determine which conventional communication channel the remote terminal should use The selected channel may already be occupied by several remote terminals however spatial processor 13 uses the spatial signatures of all of the remote terminals on that channel to determine that they can share the channel without interference. In a system with m receive and m transmit antenna elements, up to m remote terminals can share the same conventional channel. More generally, the number of point-to-point full-duplex communication links that can occupy the same conventional channel at the same time is given by the smaller of the number of receive and transmit elements.

0102Spatial processor 13 uses calculated spatial multiplexing and demultiplexing weights for the selected channel and the remote terminal in question to configure spatial multiplexer 23 and spatial demultiplexer 20 Spatial processor 13 then informs controller 3 of the selected channel. As in a conventional base station, controller 3 then commands the remote terminal (via the downlink control channel) to switch to the selected channel for continued communications. In the event that the remote terminal has power control capabilities, as is well known in the art. controller 3 also commands the remote terminal to adjust its power to an appropriate level based on parameters such as the power levels of the other remote terminals sharing the same conventional channel and the required signal quality for each link as discussed below At the termination of communications, the remote terminal returns to its idle state where it monitors the downlink control channel awaiting its next call. This frees up that "spatial channel" for another remote terminal.

0103Spatial Processing — Base Station

0104Figure 6 shows a block diagram of spatial processor 13. It is controlled by spatial processor controller 33, which interfaces to base station controller 3 via link 27. Spatial processor controller 33 controls the gain and frequency settings of multichannel transmitters 14 and multichannel receivers 15 by control lines 31 and 30.

0105Spatial processor 13 maintains an active remote terminal list 34 that catalogs which remote terminals are currently using each conventional communication channel as well as their current transmit power levels Other parameters of the remote terminals such as modulation formats currently used, receiver noise levels in current frequency channels, and current signal quality requirements are stored as well. Spatial processor 13 also maintains a spatial signature list in the remote terminal database 36. which in alternate embodiments includes remote terminals^' power control levels, allowed conventional frequency channels for receive and transmit, and list of modulation formats understood.

0106The spatial signature list in the remote terminal database 36 contains a transmit spatial signature, a_rt, and a receive spatial signature, aj,_r, for every frequency of operation for each remote terminal. In another embodiment, estimates of the quality ( e.g., estimate error covariances) of the spatial signatures are stored as well. As aforementioned, the transmit spatial signature, a_r|_>, for a particular remote terminal and a particular frequency channel is defined as the vector of relative complex signal amplitudes that would be seen arriving at that particular remote terminal as a result of identical (equal amplitude and phase) unit power narrow band signals, at that particular frequency, being transmitted through multichannel transmitters 14 and transmission antennas 18(a, m). The transmit spatial signature includes the effects of the propagation environment between the base station and the remote terminal, as well as any amplitude and phase differences in multichannel transmitters 14. antenna cables, and transmission antennas 18(a. . . . ,m ). The receive spatial signature. aj,_r , for a particular remote terminal and a particular frequency channel, is defined as the vector of complex signal amplitudes that would be measured at the outputs of multichannel receiver 16 given a single unit power narrow band signal being transmitted by that particular remote terminal, at that particular frequency

0107When the base station controller 1 forwards a call initialization request for a particular remote terminal via link 27. a channel selector 35 searches active remote terminal list 34 to find a conventional communication channel that can accommodate the remote terminal In the preferred embodiment, there is a receive active remote terminal list and a transmit active remote terminal list which are used by channel selector 35 in forming both a multiplexing and a demultiplexing spatial signature matrix for each conventional channel For each conventional channel, the columns of the demultiplexing and rows of the multiplexing spatial signature matrices are the stored receive and transmit spatial signatures of each of the remote terminals currently active on (using) that channel plus one more column containing the appropriate spatial signature of the remote terminal requesting a communication channel

0108The multiplexing spatial signature matrix for each channel, A_rt, _p (where p denotes the conventional channel number) is formed using transmit spatial signatures as shown in equation (7)

0109<img file="WO9622662A1_D0002.tif" /> where a'_{rb p} is the transmit spatial signature for ι*^Λ remote terminal assigned to channel p and n_p is the total number of remotes on conventional channel p

0110The demultiplexing spatial signature matrix, A_{br p} , is formed using receive spatial signatures as shown in equation (8)

0111Abr — [aj_{r p} , a_{4r p} , _>^a_{iP p}J, (8) where aj,_p is the receive spatial signature for *"* remote terminal assigned to channel p

0112Channel selector 35 calculates functions of these signature matrices to assess whether or not com¬ munication between the base station and the new remote terminal can be successfully carried out on the selected conventional channel In the preferred embodiment channel selector 35 first calculates spatial multiplexing and demultiplexing weights for that remote terminal and then uses these weights to estimate link performance

0113In the illustrative embodiment, spatial multiplexing weights are the rows of a matrix \\ _z given in equation (9)

0114\V_tτ =- Sb(ArbA_τ'_br ¹A_rb , (9) where ( )^{- 1} is the inverse of a matrix, ( )^* is the complex conjugate transpose of a matrix, A_rb is the multiplexing spatial signature matrix -4_r._,p associated with the relevant conventional channel, and Sb is a (diagonal) matrix containing the amplitudes of the signals to be transmitted The amplitudes to be transmitted. S are calculated in the preferred embodiment using the (diagonal) matrix of remote terminal receiver mean-square noise voltages (N) and the diagonal matrix of minimum desired signal qualities (SNRj_e, ) as given in equation ( 10)

0115S_b = (SNR_de, x N)^l/- ( 10)

0116Now channel selector 35 calculates the average mean-square voltage (power) P to be transmitted from each element as the sum of squares of the elements in each row of \Y_tτ ι t ft = dιag{W'._IW7_x} ( 11 ) 14 and the peak square voltage (power) P_b^pta to be transmitted from each element as the square of sum of the magnitude of the elements in each row of W_tx, i.e.,

0117P_b^pea" = diιg{ abs(W_tr ) I abs(W_t'_x)} , ( 12) where I is a matrix of all ones of the appropriate size and abs( ) is elementwise absolute value. Channel selector 35 compares these values against the limits for each of the transmitters for each of the elements. If any of the average or peak values exceed the acceptable limits, the remote terminal in question is not assigned to the candidate channel. Otherwise, the ability to successfully receive from the remote terminal is checked. In an alternate embodiment, the transmitter limits are used as inequality constraints in an optimization algorithm for calculating transmit weights that meet the specifications given and that also result in the minimum amount of transmitted power possible. If transmit weights satisfying the constraints can not be found, the remote terminal in question is not assigned to the candidate channel. Such optimization algorithms are well known.

0118To test the uplink, channel selector 35 calculates spatial demultiplexing weights W_rι using ,4._r , the demultiplexing spatial signature matrix A_t,_r - associated with the relevant conventional channel, as given for the preferred embodiment in equation (13):

0119W„ = ( Abr PrA'_br + Rnn )- ^l Atr P_r , ( 13 ) where P_r is a (diagonal) matrix of mean-square amplitudes (powers) transmitted by the remote terminals and R_nn is the base station receiver noise covariance. Then, the expected value of the normalized mean- squared error covariance is calculated in one embodiment as follows:

0120WE = PJ^{1 /2}(( I - w;_xA_br)Pr(i - w;_xA_iτγ + w;_xR_nnw_rx)p-'^/2 ( 14)

0121The notation (-)~^*'² indicates complex conjugate transpose of the square root of the matrix. The inverse of MSE is an estimate of the expected Signal-to-lnterference-plus-Noise Ratio (SINR) at the output of the spatial demultiplexer:

0122SINR = MSE . ( 15)

0123If all of the diagonal elements of SINR are above the desired thresholds based on the signal quality required to be received from each remote terminal, the remote terminal is allowed access to the channel. If the candidate remote terminal is below its threshold and has the ability to increase its output power, the same computations are again performed for increasing remote terminal power output until either the maximum output power for that remote terminal is reached and the SINR is still insufficient, another remote terminal

0124SINR falls below its threshold in which case its power is increased if possible, or all thresholds are exceeded. If acceptable remote terminal transmit powers can be found, the remote terminal is granted access to this particular conventional channel, otherwise it is denied access and another conventional channel is checked.

0125In an alternate embodiment, the calculation of demultiplexing weights is performed using well known optimization procedures with the objective of minimizing remote terminal transmit powers subject to estimated signals at the base station meeting or exceeding their minimum desired SINR's.

0126Also, in an alternate embodiment, in the case that no conventional channel can be found to accom¬ modate the remote terminal, channel selector 35 calculates whether some rearrangement of the existing remote terminals among the conventional channels would allow the remote terminal to be supported on some conventional channel In this case the remote terminal will only be denied communication at this time if no rearrangement of existing users allows the remote terminal to be accommodated

0127In an alternate embodiment employing frequency division duplexing (FDD), remote terminals are not restricted to being assigned a fixed conventional channel pair for transmit and receive A sufficiently flexible system architecture is employed where channel selector 35 may choose to assign a particular remote to transmit and receive conventional channels separated by different frequency duplex offsets in order to minimize overall system interference levels

0128Spatial multiplexing and demultiplexing weights for remote terminals already using a conventional channel must be recalculated because adding a new remote terminal to that conventional channel may change them significantly In the preferred embodiment channel selector 35. having already done the nec¬ essary calculations, sends the new spatial multiplexing and demultiplexing weights to the spatial weight processor 37 for use in setting up the spatial multiplexer 23 and demultiplexer 20 In an alternate embod¬ iment, spatial weight processor 37 uses the spatial signature matrices sent to it by channel selector 35 to calculate different sets of spatial multiplexing and demultiplexing weights for all of the remote terminals on that conventional channel

0129Spatial weight processor 37 then sends the new spatial demultiplexing weights to spatial demultiplex¬ ers 20 and the new spatial multiplexing weights to the spatial multiplexers 23 for this conventional channel updates the active remote terminal list 34. and informs spatial processor controller 33 which in turn informs base station controller 3 of the selected channel Base station controller 3 then transmits a message to the remote terminal using the downlink control channel that instructs the remote terminal to switch to the desired conventional channel

0130It can be shown from equation (9) that the multiplexing weight matrices W_tx have the property

0131A_riW-_χ = S_i ( 16)

0132This means that at the ^•"^* remote terminal, the signal intended to be sent to that terminal is received with a sufficient (positive real) amplitude Sb( ι, ι) The fact that Sj has zero off-diagonal elements means that at the *"^• remote terminal, none of the other signals being transmitted are received by that remote terminal In this manner, each remote terminal receives only the signals intended for it at the necessary power levels to ensure proper communications In alternate embodiments, uncertainties in the estimates of Ar_t, are incorporated in setting base station transmit power levels and calculating weights so as to minimize the effect of errors and/or changes in A_rb

0133Similarly, at the base station the particular demultiplexing weight matrices given in ( 13) have the property that conditioned on the knowledge of the receive spatial signatures and the transmitted voltages (powers) from the remote terminals, the estimated signals 5 given by s = \v;_xz_b, (17) are the most accurate in the sense of least mean-squared error In particular, they most closely match the signals transmitted by the remote terminals given the measurements made at the base station by the multiple antenna elements

0134Equations (9) and ( 13) represent only one way to calculate spatial multiplexing and demultiplexing weights There are other similar strategies that demonstrate properties similar to those shown in equation ( 16) and described in the previous paragraph Other well known techniques for calculating weight matrices W_tχ and W„ account for uncertainty in spatial signature matrices A_rb and A _τ, for wide bandwidth conventional channels, and can incorporate more complex power and dynamic range constraints.

0135Determining Spatial Signatures

0136As shown in figure 6. spatial processor 13 also contains a spatial signature processor 38 for finding the spatial signatures of the remote terminals. In the illustrative embodiment, spatial signature processor 38 uses the calibration techniques described in copending U.S. patent application 08/234,747.

0137In the illustrative embodiment, each remote terminal is capable of entering a calibration mode in which the signal received by the remote terminal 43 is transmitted back to base station 1. Referring to figure 7, this function is provided by switch 63 controlled by remote terminal CPU 62 through switch control signal 64.

0138To determine the transmit and receive spatial signatures of a remote terminal, spatial signature pro¬ cessor 38 commands the remote terminal to enter calibration mode by transmitting a command to it on the downlink channel. This command is generated by base station controller 3. based on a request from spatial processor controller 33. and modulated by signal modulators 24. Spatial signature processor 38 then transmits predetermined calibration signals 11, on the conventional channel occupied by the remote terminal, by instructing multichannel transmitters 17(a m) via transmitter control data 31 and spatial processor controller 33. In the present embodiment, the m signals (for each antenna) among the prede¬ termined calibration signals 11 are different frequency complex sinusoids. In another embodiment, the predetermined calibration signals 11 are any known, distinct, signals.

0139The remote terminal shown in figure 7 transmits back the signal received at the remote terminal. This transponded signal is received by multichannel receivers 15 in base station 1 shown in figure 1 and supplied to spatial signature processor 38 shown in figure 6. In one embodiment described in patent application 08/234,747. spatial signature processor 38 computes the receive and transmit spatial signatures of the remote terminal from the received signal measurements 6 and predetermined calibration signals 11 as follows. Time samples of the received data are stored in an m by n data matrix Z which in the absence of noise and parameter offsets is given by <img file="WO9622662A1_D0003.tif" /> where S is the m by n matrix of predetermined calibration signals and ifc is a known amount by which the signal is amplified in the remote terminal before transmission back to the base station. The receive spatial signature is proportional to the singular vector (uj ) corresponding to the largest singular value (σ_max o the data matrix Z. Transmission of a unit power signal from the remote terminal and received by the base station at antenna element 1 provides the necessary scaling g _r for the receive spatial signature a»_r = j*irUj uι( l ), ( 19) where Uι( l) is the first element of uj . Once a-._r is known. a_rj is calculated by a_rt = λ-- ^I (_i?6ru₁ /u. ( l ))^t ZS^t . (20) where B* is the well known Moore- Penrose pseudo-inverse of the matrix B satisfying BB ' = / ( the identity matrix) for full-rank matrices B having more columns than rows. B^ B = I for full-rank matrices B havin more rows than columns. In alternate embodiments also described in copending application 08/234.747 well known techniques are used to account for noise present in the system and parameter variations such as oscillator frequency offsets

0140Spatial signature processor 38 stores the new spatial signatures in remote terminal database 36 Upon completion, spatial signature processor 38 commands the remote terminal to exit calibration mode by transmitting a command to it on the downlink channel

0141In one alternate embodiment, computation of remote terminal transmit spatial signatures can be per¬ formed directly by the remote terminals This embodiment of the remote terminal is shown in figure 8 In calibration mode, spatial signature processor 38 transmits predetermined calibration signals 11. on the conventional channel to be calibrated by the remote terminals, as before Remote terminal CPU 62 uses received calibration signals 44 and the known transmitted waveforms to compute the remote terminal^'s transmit spatial signature using the same techniques used by spatial signature processor 38 in the previous embodiment The computed transmit spatial signature is transmitted back to base station 1 through mod¬ ulator 51 and transmitter 54 as control data to be transmitted 52 When received by base station 1. spatial signature processor 38 stores the new transmit spatial signature in remote terminal database 36 Since each remote terminal performs the transmit spatial signature calculation independently, this arrangement allows multiple remote terminals to compute their own transmit spatial signature simultaneously on the same conventional channel In this embodiment, remote terminal receive spatial signatures are computed by spatial signature processor 38 in the same manner as in the previous embodiment

0142Using these techniques, spatial signature processor 38 can measure a remote terminal transmit and receive spatial signatures for a particular channel any time that channel is idle The efficiency of these calibration techniques allow spatial signature processor 38 to update the spatial signatures of numerous remote terminals for a particular channel while occupying that channel for only a short time

0143Many other techniques for obtaining spatial signatures of remote terminals are also available In some RF environments, spatial signatures for remote terminals can be determined using well-known techniques that depend upon knowledge of the geometric arrangement of the reception antennas 19(a m ) and their individual directivity patterns (element gain and phase with respect to a reference a.s a iunction ol angle-of-arπval). and the direction from the base station to the remote terminal Furthermore techniques such as ESPRIT (U S. patents 4.750.147 and 4,965,732) can be used to estimate directions in applications where they are not known a priori

0144Similarly, as is well known, knowledge of any predetermined modulation format parameters of the underlying signals being transmitted by the remote terminals (for example, knowledge of certain training or preamble sequences, or knowledge that the signals are constant modulus) can also be used to determine the receive spatial signatures for remote terminals. A further example is decision-directed feedback techniques also well known in the art. where receive data is demodulated and then remodulated to produce an estimate of the original modulated signal These techniques allow receive spatial signatures to be estimated even when multiple remote terminals are occupying a single conventional channel

0145In some RF environments, transmit spatial signatures for remote terminals can be calculated explicitly as is well known, using knowledge of the remote terminal locations and the locations and directivity patterns of the base station transmit antennas This requires no special capability on the part of the remote terminal

0146If the remote terminal has the ability to measure and report the strength of the signal it is receiving the system can use this information to derive transmit spatial signatures albeit in a less efficient manner than the embodiment shown in figure 7 where the remote terminal has full transponder capabilities or the embodiment shown in figure 8 where the remote terminal directly computes its transmit spatial signature The transmit spatial signature is determined based solely on received signal power reports from the remote terminal as follows First, spatial signature processor 38 transmits identical unit power signals from two of the m antenna elements at a time. Spatial signature processor 38 then changes the amplitude and phase of one of the two signals until the remote terminal reports that it is receiving no signal The set of complex weights for antenna elements 2 through m required to null a unit power signal from element 1 are changed in sign and inverted to produce the transmit spatial signature for the remote terminal

0147In yet another embodiment, the system can be designed to continuously update the spatial signatures of the remote terminals in a "closed loop^" manner This is done to account for the time variation of spatial signatures due to. for example, motion of the remote terminal or changes in the RF propagation conditions To do this, both the base station and the remote terminal periodically transmit predetermined training sequences Each remote terminal currently active on a particular channel is assigned a different predetermined training sequence and is given the training sequences for all other remotes currently active on that particular channel In one embodiment, the different training sequences are orthogonal in the sense that the inner product of any two of the training sequence waveforms is zero Each time t he training sequences are transmitted, each remote terminal calculates how much of each training sequence it has received using well known techniques, and transmits this information to the base station

0148In the illustrative embodiment, the base station uses the receiver outputs and knowledge of the trans¬ mitted waveforms to calculate the remote terminal recene spatial signatures In another embodiment, the base station calculates how much of each remotely transmitted training sequence has come through on each output of the spatial demultiplexer, expressed as a complex vector of coupling coefficients Knowledge of these coupling coefficients allows the currently active receive and transmit spatial signatures to be corrected so as to reduce mutual interference using well know techniques

0149Finally in systems that use time division duplexing (TDD) for full-duplex communications as is well known in the art. the transmit and receive frequencies are the same In this case usine the well known principle of reciprocity, the transmit and receive spatial signatures are directK related Thus this embodiment determines only one of the signatures for example the receive spatial signature and tin other in this case the transmit spatial signature, is calculated from the first ( receive) spatial signature and knowledge of the relative phase and amplitude characteristics of multichannel receivers 15 and multichannel transmitters 14

0150Network Level Spatial Processing

0151In the embodiment illustrated herein the spatial processor for each base station in the cellular-like wireless communication system operates independently to maximize the number of communication channels in the immediate cell However, significant system capacity improvements can be realized if t he spatial processor from each base station communicates with and coordinates us efforts with the spatial processors from other nearby cells A specific embodiment is shown in figure 9

0152A multiple base station controller 66 acts as the interface between the wide area network 65 through link 68 and base stations 1 (a,b,c ) via base station communication links 2 (a.b c ) Each base station is responsible for providing coverage to a number of remote terminals In one embodiment, each remote terminal is assigned to only one base station thus defining cell boundaries 07 ( a. c i w ithin w hich all remotes attached to a particular base station are located Users equipped with remote terminals 69 are identified by a boxed "R" in the figure

0153Each spatial processor contained in base stations 1 (a b,c) measures and stores the spatial signatures of the remote terminals in its cell and also of the remote terminals in adjacent cells The determination of spatial signatures of the remote terminals in adjacent cells is coordinated by multiple base station con¬ troller 66 through base station communication links 2 (a b c) Through base station communication links 2 (a.b.c) and multiple base station controller 66, spatial processors in base stations 1 (a,b.c) from adjacent cells inform each other of which remote terminals they are communicating with on which conventional channels Each spatial processor includes the spatial signatures of remote terminals that are currently active in adjacent cells to form extended transmit and receive spatial signature matrices A_rb and A_br which are sent to all the adjacent base stations The channel selectors in each base station using these extended spatial signature matrices, jointly assign remote terminals to each conventional channel in each of base stations 1 (a,b,c)

0154The resultant weight matrices ,V_1r and l.'_rr for each base station are then calculated using extended spatial signature matrices A_r and A_br In calculating the weights the objective is to minimize the signal transmitted to and received from the adjacent cell's active remote terminals thereby allowing manv more remote terminals to simultaneously communicate

0155In an alternate embodiment multiple base station controller 66 assigns remote terminals requesting access to base stations dynamically using a list of active remote terminal/base station/conventional channel links, the associated remote terminal databases, and the particular requirements for the link to be assigned Additionally, remote terminals can employ multiple (directional) transmit and receive antennas to facilitate directive links to multiple nearby base stations as instructed by multiple base station controller 66 to further increase system capacity

0156Advantages

0157The apparatus and method in accordance with the invention provides a significant advantage over thf prior art in that it allows many remote terminals to si ultaneously share the same conventional commu¬ nication channel In particular for a system with n. receive and m transmit antenna elements up to w remote terminals can share a single conventional communication channel Moreover, signals received from and transmitted to the remote terminals have substantially improved signal-to-noise, reduced interference and improved quality in multipath environments compared to a standard base station

0158Thus a wireless communication system can support many times more conversations or have a much greater data throughput with the same amount of spectrum Alternatively a wireless communication system can support the same number of conversations or data throughput with much less spectrum

0159Alternate Embodiments

0160In one alternate embodiment transmission antennas 18(a, ,m ) and reception antennas 19(a m ) at base station 1 are replaced by a single array of m antennas Each element in this array is attached to both its respective component of multichannel transmitters 14 and its respective component of multichannel receivers 15 by means of a duplexer In another alternate embodiment, signals on the uplink control channel may be processed in real time using the spatial processing described in copending patent application 07/806.695 This would allow multiple remote terminals to request a communication channel at the same time

0161In yet another embodiment for applications involving data transfer of short bursts or packets of data no separate uplink control channel is required and the system may service requests for communication and other control functions during control time intervals that are interspersed with communications intervals

0162As stated above, many techniques are known for measuring the spatial signatures of the remote terminal radios and using these spatial signatures to calculate multiplexing and demultiplexing weights that will allow multiple simultaneous conversations and/or data transfers on the same conventional communication channel

0163While the above description contains many specificities these should not be construed as limitations on the scope of the invention, but rather as an exemplification of one preferred embodiment thereof Many other variations are possible Accordingly the scope of the invention should be determined not by the illustrated embodiments but by the appended claims and their legal equivalents