WO1999040687A1

WO1999040687A1 - Wireless communication system having frequency scanning based directivity

Info

Publication number: WO1999040687A1
Application number: PCT/US1999/002815
Authority: WO
Inventors: John R. Sanford
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-02-10
Filing date: 1999-02-10
Publication date: 1999-08-12
Anticipated expiration: 2000-08-10

Abstract

A wireless communication system (5) utilizing a frequency scanner (23) to perform frequency scanning to direct communication data. Base station and mobile unit (20) embodiments are presented as are interference suppression techniques, TDMA, ATM operation, separate receive (11) and transmit (13) antennas and related features. A holographic beam forming wireless communication system is also disclosed.

Description

- 1 -

WIRELESS COMMUNICATION SYSTEM HAVING FREQUENCY SCANNING BASED DIRECTIVITY

CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No. 60/074,219, filed February 10, 1998, and having the same title and inventors as above .

FIELD OF THE INVENTION The present invention relates to wireless communication systems and, more specifically, to improving spectral efficiency in such systems.

BACKGROUND OF THE INVENTION Current standards for wireless communication include frequency division multiple access (FDMA) , time division multiple access (TDMA) and code division multiple access (CDMA) , and these standards are directed towards improving the efficiency by which the available wireless communication frequency spectrum is allocated amongst multiple users. Each of these standards utilize two degrees of freedom, time and frequency, in providing multiple user access. Accordingly, these standards or systems are considered to be two dimensional.

Subsequent attempts at improving spectral efficiency have attempted to exploit the spatial variable in conjunction with time and frequency. Since most wireless users are on or close to the surface of the earth, the space variable can be considered to be two dimensional - 2 -

(latitude and longitude) . While combining these two spatial dimensions with time and frequency would provide four dimensions or degrees of freedom in designing a wireless communication system, optimally exploiting all four dimensions has proven to be difficult.

In general, antennas that exploit three or more dimensions for increasing spectral efficiency are termed "adaptive" or "smart" antennas. A characteristic of these antennas is that they can automatically change their radiation pattern in response to their signal environment. Adaptive antennas utilize beam formers to combine the signals from a phased array of antenna elements. A composite antenna pattern is produced which can be controlled by adjusting the amplitude and phase with which the individual element signals are combined. This enables the antenna to act as a spatial filter that enhances or rejects signals based on their direction of arrival .

The use of adaptive antennas in a base station of a cellular radio system allows individual beams to be generated for each mobile user or group of mobile users. A beam composed of a single channel forms a communication link with a wireless unit. Simultaneously, the adaptive antenna may form nulls in the direction of several interference sources.

Known adaptive antenna systems include spatial diversity combining, switched/multiple beam arrays, RF phased arrays and digital beam forming. Spatial diversity combining techniques include selection combining, maximum ratio combining and equal gain combining. Antenna elements are maintained at a predefined distance and used to determine the directivity of uplink signals from a user. Disadvantageous aspects - 3 - of spatial diversity combining systems include that no significant antenna gain is provided and adaptive nulling is not supported.

Switched/multiple beam array systems utilize a Butler matrix power distributor that is combined with an array antenna. While basal systems exhibited undesirably low trunking efficiency, more sophisticated systems have utilized frequency hopping to increase trunking by reusing a frequency within a standard cell. Adaptive nulling is not employed in switched/multiple beam, but the directionality of the antenna beam does provide a degree of interference rejection. Disadvantageous aspects of switched/matrix beam array system include limited interference rejection, significant loss in the combining network (i.e., the Butler matrix), complicated beam processing and beam formation in discrete locations that may not have peaks where mobile users are.

Phased array systems employ a conventional phased array for beam forming and require RF phase shifters to direct the beam towards mobile users and null out interference sources. Such systems are most applicable to TDMA schemes without frequency division. Disadvantageous aspects of phased array systems include that all channels are in a single beam which significantly increases the cross channel interference.

In a digital beam forming system, signals received at each element are mixed down and digitized. The digitized signals are then combined in some optimal fashion in order to provide the best communication channel. Each channel excites the antenna elements independently from the other channels. Therefore, an independent beam can be formed for every channel. Antenna nulls are generated towards co-channel interference - 4 - sources, improving signaled interference ratios. An example of a digital beam forming system is disclosed in U.S. Patent No. 5,515,378, issued to Roy III, et al . , and entitled Spatial Diversity Multiple Access Wireless Communication Systems. This system utilizes a complex computer algorithm to control channel formation and optimize directivity. While this system provides desired performance, it is disadvantageously expensive and complicated. Other relevant background information includes frequency scanned radar. The structure of frequency scanned radar is such that it permits a user to pan through a particular azimuth range by continuously varying the frequency of the transmitted signal. The frequency of a reflected signal generally indicates a location of an object (a plane, a ship, etc., within the particular azimuth range). U.S. Patent No. 3,017,630, issued to Begovich et al., for a Radar Scanning System, in January, 1962, is representative of a frequency scanning radar system. Frequency scanning radar techniques have not been applied to wireless communication systems.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a wireless communication system that provides a high level of performance, yet is low in cost.

It is another object of the present invention to provide a wireless communication system that utilizes high gain antennas and thus has a wide coverage range.

It is another object of the present invention to provide a wireless communication system that provide interference suppression. - 5 -

It is another object of the present invention to provide a wireless communication system that utilizes frequency scanning antennas.

It is another object of the present invention to provide base stations and mobile units that operate in a wireless communication system that utilizes frequency scanning.

It is also an object of the present invention to provide a holographic wireless communication system. These and related objects of the present invention are achieved by use of a wireless communication system having frequency scanning based directivity as described herein.

The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram of a frequency scanning wireless communication system in accordance with the present invention.

Fig. 2 is a block diagram of a mobile unit for a frequency scanning wireless communication system in accordance with the present invention.

Fig. 3 is a diagram of an antenna operating in asynchronous transmission mode (ATM) in accordance with the present invention. Figs. 4A-4B are diagrams of an antenna arrangement with separate receive and transmit antennas and a guard band therebetween, respectively, in accordance with the present. - 6 -

Fig. 5 is a power v. frequency diagram 90 (such as that output by a spectrum analyzer) for signals in a frequency scanned antenna system in accordance with the present invention. Fig. 6 is a power v. frequency diagram for two closely spaced users SIM, SIN.

Figs. 7A-7C are diagrams illustrating interference suppression or adaptive nulling in accordance with the present invention. Fig. 8 is a diagram of a holographic beam forming wireless communication system in accordance with the present invention.

DETAILED DESCRIPTION Referring to Fig. 1, a diagram of a frequency scanning wireless communication system 5 in accordance with the present invention is shown. Fig. 2 illustrates an embodiment of a mobile unit for use it system 5 in accordance with the present invention. System 5 uses a frequency scanning antenna 10 to achieve directional communication. A frequency scanning antenna is disclosed in U.S. Patent No. 3,017,630 issued to Begovich et al . Antenna 10 includes a plurality of antenna elements 12 that are serially connected by a transmission line 16.

The spacing of the antenna elements (the length of transmission line 16 between two elements) is preferably a wavelength (or a multiple thereof) of the frequency that achieves signal transmission normal to the plane of the antenna elements. When a signal of lower frequency than the normal frequency is input to antenna 10, the output of the antenna elements add in-phase (i.e., add constructively) in a direction below the normal. - 7 -

Similarly, when a signal having a frequency above the normal frequency is input to antenna 10, the output of the antenna elements add in-phase in a direction above the normal. In this manner, the direction of signal transmission is determined by the frequency of the input signal .

Accordingly, to send a signal to a desired mobile user 20, that signal is input to antenna 10 with a frequency that corresponds to the location of the mobile user. The reciprocal process is utilized in reception. Each mobile units 20 transmit at a frequency that is optimum for its individual location (the mobile units and the manner in which they determine an optimum frequency are discussed in more detail below) . In general, a signal received by antenna 10 is output to receive circuit 30 where it is demodulated and processed for identification (e.g., telephone number (s) ) and tracking. Receive circuit 30 functions in conjunction with control circuit 40. The output of receive circuit 30 is fed to a conventional switching network 60 and from there to a conventional wide area network (WAN) 70 such as a telephone system where it is routed and metered. The functions of switching network 60 and WAN 70 are known in the art. On the transmit side, signals from WAN 70 are propagated to switching network 60 and then to transmit circuit 50. Transmit circuit 50 also works in conjunction with control circuit 40 and provides signal identification, location matching and modulation functions. The output of transmit circuit 50 is fed to antenna 10 for transmission to a desired mobile user 20.

Referring more specifically to the receive and transmit circuits 30,50, there are at least two "types" - 8 - of communications that occur in system 5. The first is for optimum frequency (and hence location) determination and the second is for the transfer of user information. Signals for optimum frequency determination may be distinguished from user information by placing them in a sub-band, for example, the first 1-5 KHz, of a 5MHz user information channel or by conventional techniques for distinguishing communication link protocol data. The optimum frequency for communication is determined by a handshaking protocol that is conducted between the base station (i.e., antenna 10) and each mobile unit 20. The handshaking protocol is preferably as follows, though other methods of determining optimum frequency are contemplated. At appropriate intervals (similar to those of conventional cellular phone locator signals) a mobile unit 20 transmits a low power, broadband signal in the sub-band mentioned above that includes the frequencies within a particular azimuth. Receive circuit 30 includes a spectrum analyzer 32 that is tuned to receive these broadband locator signals and logic which determines from which mobile unit a broadband signal was sent. Spectrum analyzer 32 determines which frequency transmitted the most powerful signal and that frequency is designated as the initial optimum frequency (IOF) . The IOF designation is passed by control circuit 40 to confirmation propagation logic 52 in transmit circuit 50. Confirmation propagation logic 52 generates a signal at the IOF and propagates this signal to antenna 10 for transmission. The recipient mobile unit (as discussed with reference to Fig. 2) contains logic that examines the carrier to interference ratio of the incoming IOF. The mobile unit preferably returns a figure of merit to the - 9 - base station that includes the carrier to interference ratio and acknowledgment of communication at the IOF. If the carrier to interference ratio is accepted, the IOF becomes the designated optimum frequency (OF) and the OF and mobile unit identifier are time stamped and stored in memory 45. A history of the time-stamped OF (i.e., location) of each mobile unit is preferably compiled in memory 45 and this information may be used in the selection of a transmission frequency that is most likely to be directed to a desired user. This "tracking" information may also be used in the process of "handing off" a user from one sector to another or from one cell to another.

While the above protocol is preferred, it should be recognized that there are other known methods of determining an optimum frequency and they may be employed herein. These include, but are not limited to, measuring signal strength for all channels, looking at potential interference sources (e.g., though one frequency may have a weaker signal than another, the potential interference may be sufficiently less as to make that a better or equal choice) , and considering the frequency envelope as a whole (e.g., looking at available parameters and making an intelligent decision based on that information) . It should also be recognized that the logic for determining optimum frequency may be provided in the mobile unit, the base station or both. It should further be recognized that OF determination information and user information may have different codings and separate demodulation- modulation logic can be provided for processing these codings .

With respect to user information, this information is propagated from antenna 10 to demodulate logic 34 (on - 10 - receive) . Demodulation logic 34 performs known demodulation processing including removal of carrier frequencies and conversion of the input signal from analog to digital. The output of demodulation logic 34 is preferably a serial stream of digital data which includes an identification (e.g., telephone number) of the mobile unit from which the signal was transmitted. This identification is read or "stripped-off" by identification (ID) logic 36 which is known in the art. The digital data stream is then output to the switching network and WAN.

It should be recognized that demodulation logic 34, in conjunction with control circuit 40, may be configured to listen for a "best" possible signal (i.e., a multipath signal) from a mobile unit. For example, a multipath signal from a mobile unit (based on reflections, etc.) may produce three signal spikes. Control circuit 40 and demodulation logic 34 can be configured to provide three oscillators that each mix one of the received signals. The mixer products are then preferably compared and the corresponding time delay is determined. The time delay between the signals is then corrected for to form a single stronger signal. This is a scheme to correct for multipath. As an alternative to multiple oscillators, one oscillator could be used to convert the three received signals to a lower frequency and the lower frequency signals could be fed directly into a computer and processed (i.e., delay correction) . For example, signals under 56K can be fed directly into a computer using a standard modem.

With respect to incoming calls (i.e., communication data for transmission to a mobile unit) , incoming calls are propagated from WAN 70 to switching network 60 and - 11 - then to transmit circuit 50. Upon entry to transmit circuit 50, the identification of the recipient mobile user 20 is stripped off by identification (ID) logic 56. The identification is compared to a table in memory 45 to determine the location (corresponding frequency) of the desired mobile unit within azimuth 13.

In response to the comparison, a code indicative of the frequency that corresponds to the location of the identified mobile user is propagated from control circuit 40 to modulation logic 54. At modulation logic 54, the incoming signal from which the identification was stripped is appropriately modulated to have the desired transmission frequency. From modulation logic 54, this signal is output to antenna 10 for transmission. Modulation logic 54 preferably includes the mixing of IF and LO signals as is known in the art. The appropriate IF signal is preferably provided by control circuit 40. Filtration of one of the mixer products is preferably provided as is known. If desired, history information stored in memory 45 may be used to estimate the current position of a moving mobile user. Previous locations and their time stamps can be used to determine a general trajectory and rate of movement. A code for the frequency corresponding to the estimated location is then propagated from control circuit 40 to modulator 54.

Control circuit 40 preferably includes a microprocessor 42 or other logic (ASIC, PLA, etc.) that provides similar functions. Suitable processors are commercially available and techniques for programming these processors to achieve the functions described herein are known in the art. Components of the receive and transmit circuits such as the modulator/demodulator - 12 - and identification logic and other conventional circuitry/logic that is not specifically described herein are available commercially from such sources as Motorola (Schaumburg, IL) or Gray (Silicon Valley, CA) . Referring to Fig. 2, a block diagram of a mobile unit for a frequency scanning wireless communication system in accordance with the present invention is shown. Mobile unit 20 preferably includes a conventional cellular phone antenna 21; mobile receive circuit 22 that includes a carrier to interference ratio detector 19, a frequency scanner 23 and demodulation logic 17 (similar to demodulation logic 34) ; mobile control logic 24; mobile transmit circuit 25 that includes modulation logic 15 (similar to modulation logic 54) ; and user input/output (10) 26 that includes audio 10 27, a keypad 28 and electronic 10 29, amongst other related features.

Frequency scanner 23 preferably scans a predefined frequency range for signals that have a sufficient power level. Carrier to interference ratio (C/I) detector 19 determines the C/I of a received signal. If the C/I is acceptable, the mobile unit selects the frequency that provided the highest power. This determination is made by control logic 24 that preferably includes programmable processing logic. Transmit circuit 25 contains conventional cellular phone transmission circuitry and modulator 15 that modulates the output signal at the frequency indicated by control logic 24 (which is preferably communicated by confirmation logic in transmit circuit 50 as discussed elsewhere herein) . The user 10, keypad and electronic 10 (e-mail, Internet, etc.) are as known in the art .

The frequency scanning antenna system illustrated in Figs. 1-2 is similar in some aspects to a switched beam - 13 - system. Rather than having a discreet set of beams each containing a set of channels, however, the frequency scanning system of the present invention has a continuous spectrum of beams as indicated by azimuth 13 of Fig. 1. A relative advantage of the frequency scanning system disclosed herein is that it is not necessary to combine beams to fill in rejection nulls.

Other advantages of the present invention include that frequency scanned antennas (1) can achieve high gain thus facilitating a large cell size and lower interference (fewer dropped calls) and (2) have very low losses. High gain is achieved because antenna elements 12 are arranged serially and not in a dispersed or attenuating arrangement as is the case in other antenna arrangements, for example, the Butler matrix excited array system. The frequency scanning antenna systems are also relatively inexpensive to construct (and in some instances significantly less expensive) .

Referring to Fig. 3, a diagram of antenna 10 operating in asynchronous transmission mode (ATM) in accordance with the present invention is shown. Fig. 3 illustrates antenna 10 as a singular antenna having a plurality of frequency bands extending therefrom each at an angle, θ, to the normal. Through the use of ATM, receive and transmit can occur in the same bands. ATM is known in the art .

Fig. 3 also illustrates that the outer beams, e.g., f(θ₅) are slightly larger than the beams closer to normal to compensate for the angle at which they are propagated. Referring to Fig. 4A, a diagram of antenna 10 with separate receive and transmit antennas 11,13, respectively, in accordance with the present is shown. Fig. 4A illustrates that as an alternative to ATM - 14 - operation, the receive and transmit operations can be provided by two separate antennas that are preferably disposed with respect to one another such that the same locations corresponding to different frequencies in the azimuths of the two antennas. In this manner, for example, the receive signals 7 could be sent at 800-890 MHz and the transmit signals 8 could be sent at 910-1000 MHz, but the antennas could be oriented such that the azimuths overlap (e.g., 860 MHz in receive is the same location as 970 MHz in transmit) . This scenario also provides a guard band 9 of 20 MHz (see Fig. 4B) .

Referring to Fig. 5, a power v. frequency diagram 90 (such as that output by a spectrum analyzer) for signals in a frequency scanned antenna system in accordance with the present invention is shown. Diagram 90 plots power v. frequency for a plurality of mobile units that are spaced about azimuth 13.

Referring to Fig. 6, a power v. frequency diagram for two closely spaced users SIM, SIN is shown. When this occurs, time division multiple access (TDMA) techniques are preferably utilized to divide access to the subject frequency temporally amongst the users. TDMA techniques and accompanying logic are known in the art. TDMA normally includes compressing voice data into smaller packets (shorter time periods) and then serially interleaving packets intended for different recipients. TDMA logic is represented in the receive circuit, transmit circuit and mobile unit by reference number 38, 58 and 18, respectively. Referring to Figs. 7A-7C, diagrams illustrating interference suppression or adaptive nulling in accordance with the present invention are shown. By way of example, when a mobile unit 20' on channel 1 emits a - 15 - signal, some radiation (from the sidelobes) is input to channels 2 and 3. The same is true when the base station transmits to unit 20' . Fig. 7B illustrates the spectrum analysis plot of this transmission. To suppress the transmission in channels 2 and 3, cancellation signals 95 (Fig. 7C) can be sent that have the same modulation but opposite phase, such that on mixing they cancel the interference. This is one method of interference suppression. Other methods will be apparent to those skilled in the art and may depend on processing power. One other method of interference suppression includes predistorting an emitted signal for anticipated interference. In this method, it is intended that the predistortion counteracts the effects of the anticipated interference so that the resultant signal is transmitted to a desired destination with desired characteristics.

With respect to other considerations, antenna 10 is preferably designed such that it scans 1/2 a 3 dB beamwidth or less over the channel bandwidth. Thus the beamwidth is preferably narrow, but not so narrow that it has more than approximately 1 dB of loss from scanning over the channel bandwidth.

Referring to Fig. 8, a diagram of a holographic wireless communication system in accordance with the present invention is shown. The system of Fig. 8, also termed a holographic beam forming (HBF) system, is similar to very ling baseline interferometry (VLBI). Differences include that in an HBF system (1) the type of signal that is being looked for, i.e., the particular modulation (frequency, code, etc.) is known and (2) there is an interaction (handshaking) between the source and the receiver.

Holographic Beam Forming (HBF) of the -present - 16 - invention serves to increase the spectral efficiency and range of a wireless communications network. It works on a premise similar to very long baseline interferometer (VLBI) . Several antenna are located many wavelengths apart. Therefore, they are in very different multipath environments. When a source, which may be a fixed or mobile radio, transmits a signal it is received at each of the antennas with varying time delays and powers. The link the source makes with each of the antennas is referred to as the channel. The source may or may not use the same channel frequency to communicate with each of the antennas. The propagation characteristics may dictate that a different frequency bands are used to form the links with the different antennas. In this case the source would send a number of frequency channels simultaneously. Whether the source broadcast a single channel to all the antennas or different channel to each, the concept remains the same. Each antenna receives the signal and mixes it to either base band or an intermediate frequency (IF). Next, the signals from each antenna are combined with the appropriate time delay such that all of the signals add in phase. Because the antennas are spaced so widely, the noise at each antenna is decorrelated and adds randomly. Further, by simple triangulation, the position of the source is known. This is analogous to the method employed in global positioning systems (GPS) .

On transmit the antennas each send the information with a time delay that causes it to add coherently in the radio antenna. It can be considered to form a local maximum, or standing wave, at the radio. Hence, the radio receives the maximal energy possible. Correction data for the time delays are obtained by a feed back from the - 17 - radio. As before, different frequencies may be used to obtain the best link with each radio. In this case there is not a standing wave at the carrier frequency however there is still a standing wave formed at the IF or base band frequency.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.

Claims

- 18 -CLAIMS

1. An antenna apparatus for frequency scanning based communication, comprising: a frequency scanning antenna; a communication data input; and a modulation circuit coupled between said frequency scanning antenna and said communication data input that modulates communication data received at said input and propagates the modulated communication data to said antenna for transmission; wherein said modulation circuit modifies said communication data such that said modulated communication data has a frequency associated therewith that determines a direction at which the modulated communication data will be transmitted from said antenna.

2. The apparatus of claim 1, wherein said input communication data is digital communication data.

3. The apparatus of claim 1, wherein said input communication data includes a unique recipient identifier.

4. The apparatus of claim 1, further comprising control logic having memory associated therewith that is coupled to said modulation circuit and communicates to said modulation circuit an indication of a frequency that should be associated with the modulated communication data to cause the modulated communication data to be routed in a desired direction from said antenna.

5. The apparatus of claim 4, further comprising identification logic coupled to said control logic that - 19 - reads a unique identifier of a receiving unit for input communication data and propagates said unique identifier to said control logic, said control logic matching said unique identifier to a frequency that corresponds to an approximate location of that uniquely identified unit relative to said antenna.

6. The apparatus of claim 1, further comprising time division multiple access (TDMA) logic coupled to said communication data input that processes multiple communication data for unique receiving units based on temporal separation.

7. The apparatus of claim 1, wherein said antenna apparatus is designed for communication with a plurality of mobile units and wherein said apparatus further comprises confirmation logic that communicates to each of a plurality of mobile units a frequency at which to propagate communication signals.

8. The apparatus of claim 1, wherein said antenna apparatus functions in asynchronous transmission mode.

9. The apparatus of claim 1, further comprising demodulation logic that demodulates a communication signal received by said antenna.

10. The apparatus of claim 1, further comprising logic that determines which of a plurality of mobile units transmitted a received signal.

11. The apparatus of claim 1, further comprising logic that determines which of a range of frequencies - 20 - sent by a given unit is the preferred frequency for receiving communication data from that unit.

12. A mobile unit for use in a frequency scanning based wireless communication system, comprising: a transmission circuit; a reception circuit; logic that causes a broadband signal to be emitted by said transmission circuit; and logic that causes said mobile unit to transmit communication data at an indicated frequency within said broadband that is received from a base station.

13. A holographic beamforming wireless communication sytem.