Adaptive cancellation for wireless repeaters

[0002] This application is a continuation-in-part of prior application Ser. No. 09/357,032, filed Jul. 20, 1999 entitled “Side-to-Side Repeater”.

[0003] This invention is directed generally to repeater systems for wireless communications and more particular to a method and apparatus for improving the isolation between donor and null antennas in a repeater system.

[0004] In existing wireless technologies, such as Cellular, PCS (personal communication services), MMDS (multi-user multipath distribution system), WLL (wireless local loop), or the like, repeaters are used to extend coverage of cell sites (base stations) to areas that have low signal reception power, and therefore low signal to noise ratios. One of the principle technology drivers is that of system gain. That is, the higher the gain, the greater the range (distance) and coverage area for the system.

[0005] A limiting characteristic for repeaters is that of the feedback loop, or conversely, the isolation between the two opposing antennas (or sensors). That is, the total front to back (F/B) ratio for the system, or isolation, must be higher than the desired gain. Usually, repeaters employ at least one “link” or “donor” antenna which communicates with the base station and at least one broadcast, coverage, or “null” antenna which communicates with the remote stations or subscribers. Generally speaking, the isolation between link and null antennas is equal to the total gain plus some margin, typically around 10 to 15 dB. Therefore, the system gain will in general be less than the isolation minus the margin. For example, if the isolation between antennas is around 60 dB, then the maximum system gain allowed will be about 45 dB. For PCS frequencies, these figures result in a repeater range of less than 100 feet.

[0006] In a scattering environment, which is common in PCS, every 6 dB of additional system gain will double the coverage distance. Thus, obtaining an additional 24 dB of isolation between the two antennas, will allow the range to double 4 times, to 1600 feet. For conventional repeater systems, where the two antennas and repeater electronics are in three separate enclosures, and locations, the link (donor) antenna (to the base station) and broadcast (null) antenna (to the desired coverage area), are separated in space by (usually) more than 10 feet. This distance adds over 50 dB to the isolation between antennas, generating a total isolation value of well over 100 dB. Therefore, with a 15 dB margin, this type of system can utilize a total gain of up to 85 dB or more, which results in fairly large range and coverage.

[0007] For the side-to-side repeater technologies (as described in the above-referenced prior application), where the opposing antennas are in the same enclosure, and separated in space often by less than a few inches, the F/B ratio (or isolation) is typically limited to a value below 80 dB or so. This therefore allows a total system gain of no more than 65 dB, which limits the system range to a few hundred feet or less.

[0008] The invention provides a method and apparatus for improving isolation between donor and null antennas in a repeater system.

[0009] The described embodiment of the invention provides an adaptive cancellation approach which removes a significant portion (between 10 dB and 40 dB), of the feedback signal power, therefore increasing the total system isolation by the same amount (10 to 40 dB). This additional isolation can be used to achieve greater system gain, and therefore significantly extend the range of the system. This is especially useful in the side-side repeater technology approach mentioned above and described in the above-referenced application. The cancellation scheme uses digitally processed information to generate a signal, which, when added to the original input signal, cancels the feedback signal.

[0010] In the drawings:

[0011] FIG. 1 is a block diagram of one form of a conventional repeater system;

[0012] FIG. 2 is a diagram of a typical repeater tower, with the conventional repeater system of FIG. 1;

[0013] FIG. 3 is a block diagram of another form of a conventional system (similar to FIG. 1), with four distinct input/output ports for separate uplink and downlink antennas;

[0014] FIG. 4 shows a tower-top arrangement, using the four-port system of FIG. 3;

[0015] FIG. 5 is a block diagram of one signal path through a repeater system;

[0016] FIG. 6 is a block diagram of one signal path through a repeater system, as in FIG. 5, adding an adaptive cancellation circuit;

[0017] FIG. 7 is a block diagram (high level) of a (digitally) adaptive cancellation circuit to in accordance with one embodiment of the invention;

[0018] FIG. 8 is a block diagram (high level) of the (digitally) adaptive cancellation circuit of FIG. 7 which shows the technique in further detail;

[0019] FIG. 9 is a block diagram of a repeater system, similar to that of FIG. 1, using the adaptive cancellation (AC) circuit of FIGS. 7 and 8;

[0020] FIGS. 10aand 10bshow the directional characteristics of the AC blocks, for the downlink path (FIG. 10a) and the uplink path (FIG. 10b);

[0021] FIGS. 11 and 13 show two examples of side-to-side repeaters; and

[0022] FIGS. 12 and 14 show block diagrams of the AC approach of the invention, applied to the side-to-side repeater systems of FIGS. 11 and 13, respectively.

[0023] FIG. 1 shows a block diagram of a conventional repeater system, showing three distinct elements: the link (or “donor”) antenna, the coverage (or “null”) antenna 22, and the repeater electronics 24. The typical repeater electronic system 24 incorporates bandpass filters 26, for the uplink and downlink bands, as well as power amplifiers 28 and frequency diplexers 30. The filters 26 and amplifiers 28 are arranged as a pair of signal transmission circuits 31 (downlink) and 33 (uplink). In each circuit 31, 33, a first filter 26 attenuates the bandwidth of incoming communications signals, an amplifier 28 amplifies the signals from the first filter, and a second filter 26 located at the output of the amplifier protects the amplifier 28 from signal power in the other branches of the circuits 31, 33. The illustrated system in FIG. 1 is based on a “straight through” design, in which the signals go in a single direction through each branch or path (i.e., uplink and downlink) of the system.

[0024] FIG. 2 shows a diagram of a typical repeater tower 25, with the system of FIG. 1, including the link (or “donor”) antenna 20, the coverage (or “null”) antenna 22, and the repeater electronics 24. This system could also be installed on the top of a building, or within a building (for in-building coverage).

[0025] FIG. 3 shows a similar system to that of FIG. 1, but with four distinct input/output ports. In this system, the signals from each path are not combined by frequency diplexers. Additionally, this system requires four, rather than two, antennas. Accordingly, FIG. 3 shows two donor antennas 20aand 20b,designated as the downlink and uplink to the base station (BS). Similarly, on the broadcast or coverage side, two null antennas 22a,22bare shown, one functioning as the uplink and one as the downlink antenna relative to the remote units or subscriber equipment 35. Since separate antennas are used respectively for the uplink and downlink between the base station and the remote units, separate paths through the electronics 24a,each including their own filters 26 and amplifier 28 are provided, and diplexers such as the diplexers 30 of FIG. 1 are not utilized.

[0026] FIG. 4 shows a tower arrangement, using the four-port system of FIG. 3. In FIG. 4, the respective downlink and uplink antennas 20a,22aand 20b,22bbetween the base station and the remotes are illustrated on the tower 25, with the electronics 24aof FIG. 3 being located at the base of the tower 25.

[0027] FIG. 5 shows a block diagram of one path through a repeater system. The input signal, S(t), either from the base station (for the downlink path), or from the remote (for the uplink path), is received via an antenna, bandpass filtered, amplified (with active gain=G), filtered again, and finally transmitted by an antenna. Some of the transmitted signal energy couples back (through space, or through the electronics) into the receive antenna. This is denoted in FIG. 5 as the feedback signal, f(t), which is simply a delayed version (attenuated) of the original signal, S(t). Therefore, the composite signal, S(t)+f(t), is fed into the amplifier, with output G(S(t)+f(t)). Assume for example, that the antennas have 0 dBi gain, then the new feedback signal it G f(t). The propagation of this signal, back to the input antenna, will incur attenuation, H. Therefore, the amplified, attenuated signal at the input antenna will be H G f(t). If this signal is comparable in power to the original signal S(t), then the amplifier 28 will go unstable, and oscillate (ring). This oscillation will cause severe distortion in the desired signal.

[0028] FIG. 6 shows the same circuit as FIG. 5; however, adding an adaptive cancellation circuit 40. The goal of this circuit 40 is to create a inverse f(t) signal−f(t) (a 180 degree shifted f(t) signal), and sum it with the input signal; including the feedback signal, f(t) at a summing junction 45, and thereby remove the feedback signal f(t).

[0029] FIG. 7 shows a general block diagram (high level) of one form of the adaptive cancellation circuit 40. In this approach, the input (RF) signal is summed at the junction 45 with a modulated signal constructed via a digitally adaptive process, to destructively interfere with the feedback signal embedded in the input composite signal. After the summation, the composite signal, S(t)+f(t), is digitally sampled and digitally processed via a digital signal processor (DSP) 50, which computes an intermediate signal for a modulator 52. The modulator 52 takes the intermediate signal, and a sample of the amplified (output) signal, and creates a near copy of the correct inverted f(t) signal−f(t). This process will work with many, if not most, of the digitally adaptive algorithms for feedback control. Additionally, this methodology does not require a injected signal (training or pilot tone, or wideband noise), for the adaptive process. The entire circuit shown in FIG. 7 may be designated as an “AC block” 55.

[0030] FIG. 8 shows the circuit 40 in further detail. The DSP 50 is a combination of an RF downconverter 62 to shift the signal to an intermediate frequency that allows digital sampling, an analog-to-digital (A/D) converter 62 which digitizes the analog signal, and a processor 64 which performs the required operations to compute the intermediate signal. The modulator 52 is a combination of a controllable attenuator 66, and an I/Q modulator 68. Additional details shown in FIG. 8 include respective couplers 72 and 74 which couple the signals from the signal path to and from the adaptive cancellation circuit 40, a first coupler 72 being interposed between the summation junction 45 and the filter 26 and the second coupler 74 being at the output of the power amplifier 28. In addition to the couplers 72 and 74 respective delay lines 82 and 84 may be employed at either end of the RF path, one just prior to the summing junction 45 and one subsequent to the coupler 74.

[0031] FIG. 9 shows a block diagram of a repeater system, similar to that of FIG. 1, however using adaptive cancellation (AC) circuit blocks 55, details of which are shown in FIGS. 10aand 10b,as well as in FIGS. 7 and 8. The “direction” of the circuit (AC block) 55 in each RF path has been taken into account. In this system, each (uplink, downlink) path has a separate AC circuit block 55.

[0032] FIGS. 10aand 10bshow the directional characteristics of the AC circuit blocks 55, whether for the downlink or uplink path. The blocks are “mirror images” of one another, differing by the direction of the desired signal, with the arrows 75 denoting the directionality of each circuit 55, in FIGS. 9, 10aand 10a.

[0033] FIGS. 12 and 14 show block diagrams of the adaptive cancellation (AC) approach, applied to side-to-side repeater systems, such as the ones shown and described in the above-referenced prior application. FIG. 11 shows a side-to-side repeater having a body or housing 92 having opposed flat surfaces. To each of these opposed flat surfaces, there is mounted a single patch antenna element 94, 96, respectively comprising the null and donor antennas. An equivalent circuit diagram is shown in FIG. 12. It will be understood that the circuit components of FIG. 12, including the adaptive cancellation (AC) circuits, may be carried in the body or housing 92.

[0034] Similarly, FIG. 13 shows a side-to-side repeater structure 90ahaving a similar body or housing 92awhich mounts separate uplink and downlink transmit (Tx) and receive (Rx) patch antenna elements for each of the null and donor antennas. The respective Tx and Rx null antennas are designated by reference numerals 94aand 94b,while the respective Tx and Rx donor antenna elements are designated by reference numerals 96aand 96b.The circuits shown in FIG. 14 may be mounted to (in) the body (housing) 92a.

[0035] As indicated above, the electronics, i.e., the adaptive cancellation circuits, may be carried on/in the body/housing 92, 92aof the antenna element in the side-to-side repeater structures of FIGS. 11 and 13, permitting a tower-top modular repeater installation, in addition to the other advantages, and/or use in other applications as more fully discussed and described in the above-referenced prior application.

[0036] As indicated above, FIGS. 11 and 12 show the case for a single antenna (element) on each side, including frequency diplexers to separate each path (or frequency band). FIG. 13 and 14 show the approach when using separate Tx and Rx antennas, and therefore separate circuits, for each (uplink, downlink) path.

[0037] The above-described approach may be used in a number of applications, including: Cellular Coverage (null fill, in-building systems), PCS, MMDS, WLL and LMDS.

[0038] While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.