DUPLEXER WITH SIGNAL CANCELLATION

Abstract

A wireless communication system has a base station operative to communicate wirelessly with a plurality of customer devices. The base station has a circulator with a first port connected to one or more signal transmission devices, a second port that is connected to one or more antennas, and a third port that is connected to one or more signal reception devices. The circulator communicates transmission signals from the one or more signal transmission devices to the one or more antennas, and communicates received signals from the one or more antennas to the one or more signal reception devices. The plurality of customers devices have cancellation/separation duplexers or circulators.

Claims

1. A wireless communication system comprising: a base station operative to communicate wirelessly with a plurality of customer devices; the base station comprising a circulator having a first port connected to one or more signal transmission devices, a second port that is connected to one or more antennas, and a third port that is connected to one or more signal reception devices, wherein the circulator communicates transmission signals from the one or more signal transmission devices to the one or more antennas, and communicates received signals from the one or more antennas to the one or more signal reception devices; and the plurality of customers devices comprising cancellation/separation duplexers.

2. The wireless communication system of claim 1, wherein the circulator has a power rating sufficiently higher than the maximum uplink power to minimize the affect of the intermodal noise on the low level downlink signals.

3. The wireless communication system of claim 1, wherein the signals using the same frequency for transmit and receive signals.

4. The wireless communication system as in claim 2 wherein the base station communications are encoded with a first and second different encoding schemes for transmit and receive signals.

5. The wireless communication system as in claim 4 wherein the customers equipment comprises a modem operative to encode transmit signals in the second encoding scheme and decoding receive signals in the first encoding scheme.

6. The wireless communication system as in claim 1 wherein the transmit and receive signals are encoded in CDMA.

7. The wireless communication system as in claim 1 wherein one of the ports on the cancellation/separation duplexer is connected to a heterodyne receiver.

8. The wireless communication system as in claim 1, wherein the third port of the circulator is connected to a heterodyne receiver.

9. A method of operating a full duplexer wireless system, the system comprising a base station and a plurality of customer devices in which signals are transmitted and received therebetween using the same frequency, the method comprising the steps of: transmitting signals by the customer devices in a first encoding schemes and receiving signals by the customer devices in a second encoding scheme; and transmitting signals by the base station in a second encoding schemes and receiving signals by the base station in a first encoding scheme.

10. In combination: a circulator having a first port connected to an antenna that receives and transmits wireless signals, a second port, and a third port; a low power capacity filter connected to the second port; and a high power capacity filter connected to the third port.

11. The combination of claim 10, wherein the low power capacity filter is connected to a heterodyne receiver.

12. The combination of claim 10, wherein the circulator has a power rating sufficiently higher than the maximum uplink power transmitted through the high power capacity filter to minimize the effect of the intermodal noise on the low power level signals transmitted through the second port and the low power capacity filter.

13. A circulator comprising: first, second, and third ports connected in a circular arrangement and operative to unidirectionally direct energy received at one of the ports to the next sequential port, wherein the first port is connected to an antenna that receives and transmits wireless signals, the second port is connected to a low power filter and the third port is connected to a high power filter.

14. A wireless communication system comprising: a base station operative to communicate wirelessly with a plurality of customer devices, wherein the base station comprises of high power circulator and the customer devices comprises op amp circulator circuits.

15. The wireless communication system of claim 14, wherein the op amp circulator circuits comprise at least one downlink connected to a heterodyne receiver.

16. Customer devices of a wireless communication system, each customer device comprising an op amp circulator circuit having an uplink connection, an antenna, and a downlink connection, the downlink connection being connected to a heterodyne receiver.

17. The customer devices of claim 16, wherein the uplink connection is connected to a high power filter, and the downlink connection is connected to a lower power filter.

18. The customer devices of claim 16, wherein the circulator has a sufficiently higher power rating compared to the maximum power in the uplink connection to reduce the intermodal noise in the circulator to a negligible level on the downlink signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein:

[0059] FIG. 1 is a generic block diagram of prior art two way communication system;

[0060] FIG. 2 is a block diagram of a prior art two way communication system for customer premises equipment (CPE);

[0061] FIGS. 3a and 3b are block diagrams of a generic wireless two way communication system including a cancellation/separation circuit combination and a single stage Wilkinson adapted to that use respectively;

[0062] FIG. 4 is a block diagram of the customer premises equipment of FIG. 2 adapted to include a cancellation/separation circuit;

[0063] FIGS. 5a, 5b, and 5c are block diagrams of the Wilkinson in various modes of operation;

[0064] FIG. 6 shows op amp circulator circuit;

[0065] FIG. 7 shows circulator with filters; and

[0066] FIG. 8 is a block diagram of the customer premises equipment of FIG. 4 where the cancellation/separation circuit is replaced with a circulator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0067] FIG. 1 is a block diagram of a generic, prior art, two-way communication system. The system comprises a down link 10 and an uplink 11 connected to an antenna 13 via duplexer 14. The uplink and down link are connected to modem 16 via a second duplexer 17, the signals travelling as indicated by down pointing arrow 18 and up pointing arrow 19 in the down link and up link respectively.

[0068] FIG. 2 is a block diagram of prior art customer premises equipment for an illustrative, prior art, two-way communication system for wireless rural broadband service. The equipment comprises an antenna 110. Transceiver 140 is connected between the antenna and a modem 150. Connection to modem 150 is via duplexer 151.

[0069] The transceiver comprises an uplink 160 and a down link 170. Down link 170 and uplink 160 are connected to antenna 110 via duplexer 180. Uplink 160 comprises amplifier 190, up converter (UPC) 200, amplifier 210, and low pass filter 220. Down link 170 comprises amplifier 230. Customer premises equipment herein operates at 743 MHz downstream and 713 MHz up stream and requires a sharp duplexer.

[0070] FIG. 3a is a block diagram of the generic two-way communication system of FIG. 1 utilizing a cancellation/separation duplexer instead of a conventional duplexer. The system comprises a down link 300 and an uplink 301 connected between an antenna 302 and a modem 303 (through duplexer 309) as is the case in the prior art system of FIG. 1. But down link 300 and uplink 301 are connected to antenna 302 via a cancellation/separation duplexer 305.

[0071] FIG. 3b is a diagram of a Wilkinson combiner adapted by the addition of a filter circuit to form the cancellation/separation duplexer herein. The Wilkinson combiner is used to combine two signals into a single output. The Wilkinson combiner has three ports designated P1, P2 and P3. As shown in the FIG. 3b, quarter wave transformers T1 and T2 are connected between ports P1 and P2 and between ports P1 and P3 respectively. A balancing resistor R is connected between port P2 and port P3.

[0072] The Wilkinson combiner with a filter circuit constitutes a cancellation/separation duplexer.

[0073] The cancellation/separation duplexer is connected into any two-way communication system by connecting port P2, in FIG. 3a, to the input to down link 300 and connecting port P3 to uplink 301. As shown in FIG. 3a, downlink 300 and uplink 301 are connected to modem 303, as shown via block 309 in FIG. 3a. Block 309 represents a conventional duplexer.

[0074] FIG. 4 is a block diagram of customer premises equipment (CPE) for an illustrative rural broadband system using the cancellation/separation duplexer 305 of FIG. 3a. The CPE is operative to receive (downstream) signals at 743 MHz and (upstream) transmit signals at 713 MHz. The down link 401 comprises amplifier 402 high pass filter 403 and filter 417 of the duplexer. The uplink comprises low pass filter 410, amplifier 411, up converter 413, sharp filter 414, up converter (UPC) 415 and amplifier 416.

[0075] The uplink includes signal detector 421 connected to amplifier 416 operative to turn amplifier 416 on when a transmit signal is present.

[0076] Sharp filter 414 in the CPE illustratively operates at Intermediate Frequency of about 170 MHz suitable for UHF band (470-860 MHz). The filter can be used for each CPE in communication with a base station. The cost of a different duplexer for each CPE is thus eliminated and the use of identical fixed filters instead permits low cost manufacturing to bring the cost down dramatically.

[0077] As shown in FIG. 4, filter 414 is sandwiched between first and second up converters 413 and 415. These up converters are programmable, are voltage controlled and inexpensive. They are operative to change the frequency supplied by the modem (5-60 MHz) first to some Intermediate Frequency (170 MHz) and then to the 713 MHz transmit frequency as is explained more fully hereinafter.

[0078] Up converter 413 is connected to modem 420 via amplifier 411 and low pass filter 410. Up converter 415 is connected to the antenna via amplifier 416. Signal detector 421 is connected between amplifier 411 and low pass filter 410, and is connected to amplifier 416 via on/off switch (not shown).

[0079] The system of FIG. 4 operates to process transmit and receive signals as follows: The receive signal is received at the antenna 113. The signal goes through quarter wave transformer T1 into filter 417. Filter 417 passes the desired receive signal and attenuates all the other signals. The output of filter 417 is then fed into amp 402 that amplifies the signal. The output is fed into high pass filter 403. The output of the high pass filter 403 is connected to modem 420. Modem 420 receives the receive signal and processes the signal internally.

[0080] The transmit signal is generated by modem 420 and fed into the input of the low pass filter 410. This filter blocks all the receive signal from entering the signal detector 421. The output of the Signal detector is fed into amp 411. The programmable upconverter 413 takes the input signal from amp 411 and translates it to an Intermediate Frequency (170 MHz). The Intermediate Frequency signal is fed into a fixed sharp filter 414. This filter is the same filter used in all the Transceivers regardless of location. The signal is cleaned up by the sharp filter and the output is fed into the programmable upconverter 415 that takes the signal to 713 MHz. This upconverter can be programmed to convert the 170 MHz to any one of the 470-860 MHz UHF band frequencies. The output of the programmable upconverter 415 is fed into amplifier 416. The output of amplifier 416 is only turned on when the signal detector 421 senses an input signal. The output of the amp 416 is fed into port P3 of the Wilkinson combiner 305. The transmit signal fed into port P3 sees 3 dB less at the antenna input. The same transmit signal is seen as being at least 40 dB less at port P21 on the cancellation/separation duplexer. The benefit of this is that even though the transmit signal is large it is seen to be over 40 dB less at port P21 on the Wilkinson combiner that is input port for the receive signal. Effectively using the cancellation/separation duplexer, to connect the antenna cost 3 dB in signal loss for transmit purposes. The Transceiver would have to produce 3 dB additional power to compensate for the loss in the duplexer. The 3 dB loss is easily compensated by outputting additional power on the amp 416 and thereby having the same power input to the antenna. The real benefit is that transmit power seen at input Filter 417 is over 40 dB lower.

[0081] The Intermediate Frequency described herein is conveniently chosen to be intermediate the system transmit frequency (470-860 MHz) and 5-60 MHz supplied by the modem at the customer premises. The Intermediate Frequency, on the other hand, may be any convenient frequency, even above the system transmit frequency. In this case, instead of two up-converters, an up converter converts the modem transmit frequency to the

[0082] Intermediate Frequency and a down converter converts the frequency to the CPE transmit frequency.

[0083] FIGS. 5a, 5b, and 5c show, schematically, the Wilkinson organized as a splitter, a combiner, and as a cancellation/separation device respectively. The arrows in each case represent the direction of data flow. The signals are designated #1, #2, and #3 corresponding to the port designations. It is to be noticed that only FIG. 5c includes a filter which adapts the Wilkinson for use as a duplexer.

[0084] Using the cancellation/separation duplexer as disclosed herein, enables mobile devices to become full duplex systems thereby allowing a doubling of data rates due to fact that they can transmit and receive at the same time.

[0085] The heterodyne receiver is also well know (i.e. a radio tuner) and is used with the duplexer, disclosed herein, to provide a frequency agile transceiver which is programmable and thus obviates the need for different duplexers in CPE systems as noted above.

[0086] The single and multistage Wilkinson combiner also are well known, the latter permitting the frequency band to be made much wider thereby allow the duplexer to operate over a much wider frequency band than is possible with existing duplexers.

[0087] The cancellation/separation duplexer allows the use of the same carrier frequency for both transmit and receive.

[0088] Normally a receive signal is considerably less in power than the transmit signal. Preferably, the power of the transmit signal at the receive port is 50 dB less due to the 180 degree phase shift (signal cancellation) provided by the cancellation/separator duplexer. Consequently, the power of the transmit and receive signals at the receive port is relatively the same.

[0089] To separate the two signals at the receive port, two different modulation techniques are used. Amplitude modulation carrier and frequency modulation carrier were implemented experimentally and provided an additional 20 dB of signal separation, a total effective separation of 70 dB between the signals. It is clear that the same carrier frequency can be used for both transmit and receive by using a Wilkinson combiner/splitter plus a filter to produce the cancellation/separation duplexer. By interfacing to the transmit/receive antenna and by using two different modulation techniques over 70 dB of separation is obtained.

[0090] The 70 dB of separation has also been achieved by tuning the impedance of the transmit and receive ports to exactly match that of the antenna. With exact matched impedance plus the use of two different modulation techniques an extremely robust wireless system using the same carrier frequency is provided.

[0091] CDMA is an example of multiple access where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies. To permit the band sharing without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme where each transmitter is assigned a code. Using CDMA with different coding for the transmit and receive channels the receive code can be separated from the transmit code using a Wilkinson combiner/splitter plus a filter to produce the cancellation/separation duplexer. With exact impedance matching CDMA can be used to provide an extremely robust wireless system using the same carrier frequency for both transmit and receive channels.

Alternative Embodiment

[0092] Circulators are 3-port ferro-magnetic devices that allows energy to be coupled only from port A to port B, from port B to port C, and from Port C to Port A, where no energy can flow in the reverse direction. Referring to FIG. 8, circulator 310 has ports P1, P2 and P3.

[0093] Circulators can be distinguished from duplexers or diplexers. Duplexers are passive frequency-selective three-port devices designed specifically to allow simultaneous transmission and reception on adjacent frequencies on the same antenna and have separate transmitter and receiver ports (connectors) and a common (shared) antenna port. Duplexers are generally carefully tuned to the specific transmit and receive frequencies being used. Typical duplexer specifications might be insertion loss of less than 1 dB and isolation greater than 75 dB. Diplexers are similar in function to the duplexer in that it combines two (or more) ports into a single port. Like the duplexer, the diplexer is frequency selective, but typically broadband. A typical application for a diplexer is to allow two transceivers on two different bands to share a common transmission line. For example, a 2-meter transceiver and a 70-cm transceiver might each be connected a single diplexer which is connected to a single coax. On the other end of the coax might be a dual-band antenna, or another diplexer that separates the signals for connection to two separate antennas.

[0094] By connecting a circulator to an antenna and between a down link and an uplink as shown for the Wilkinson device of FIG. 3(a), or in other words, by replacing the Wilkinson device in duplexer 305 with a circulator 310 as shown in FIG. 8, a duplex operation can be realized.

[0095] FIG. 1 shows a prior art wireless circuit with a downlink 10 and an uplink 11 connected to an antenna 13 via duplexer 14. Duplexer 14 comprises two multistage filters F1 and F2. A circulator may be thought of as being used along with duplexer 14 in an arrangement as shown in FIG. 7. The advantage of thinking about the combination of a circulator 310 and a duplexer 309 is that we see that the filter F1 no longer needs to handle the power of the energy in the transmit uplink. The immediate benefit is a significant drip in the cost of the filter F1. For example, the cost may drop from between $400-$500 to under five dollars. Another benefit is that we gain over 20 dB in isolation, which permits us to operate in a full duplex mode using the same frequency for transmit and receive. As in the case with the Wilkinson cancellation/separation embodiments, the transmit filter F2 is unnecessary. Thus, the circulator plus the downlink filter F1 becomes a complete replacement for a duplexer.

[0096] It was found that, by using a circulator with a significantly higher power rating compared to the maximum power in the uplink, the intermodal noise generated in the circulator became negligible and obviated the additional noise added to the low level downlink signals. For example, it was found that a circulator with a power rating that is at least ten times greater than the power of the uplink gave favorable results and reduced the intermodal noise to negligible levels.

[0097] Unfortunately, such circulators are expensive and bulky. Although it avoids the 3 dB loss, characteristic of the Wilkinson device noted hereinbefore, its use is practical, because of size and cost, only in tower locations or base stations. Nevertheless, active circulators are available which weigh only a few grams and such circulators are usable in subscriber equipment herein. Such active circulators are described by Charles Wenzel of Wenzel Associates Inc., published in RF design awards in 1991 titled Low Frequency Circulator/Isolator Uses No Ferrite or Magnet. While not suggested in this publication, these devices would be more practical in hand held devices. Charles Wenzel's Low Frequency Circulator/Isolator circuit is shown in FIG. 6. In order to use a circulator where the same frequency is used for both transmit and receive, two different encoding schemes need to be used. The following table contains illustrative encoding scheme pairs for transmit and receive signals. CDMA coding also can be used.

TABLE-US-00001 Transmit Signal Receive Signal AM Modulated FM Modulated CDMA signal with first code CDMA signal with second code FM Modulation AM Modulation

[0098] In a first system embodiment a circulator is used in a tower and (Wilkinson type) cancellation/separation duplexers are used in mobile devices and customer premise equipment. In a second embodiment, an active circulator duplexer is used in both tower and subscriber equipment. Other combinations and modifications may be achieved by those skilled in the art.

[0099] The circulator described above may also be used with a heterodyne receiver, which allows the device, either the base station or the CPE, to be provide a frequency agile transceiver that is programmable, and thus obviates the need for different duplexers in order to handle different frequencies. Referring to FIG. 8, the heterodyne receiver (not shown) may be connected up stream of filter 306.

[0100] What has been described is considered merely illustrative of the principles of this invention. Thus, it should be understood that those skilled in the art are capable of producing modifications thereof within the scope of the claims.